Recent Trends of Bioanalytical Sensors with Smart Health Monitoring Systems: From Materials to Applications

Abstract

Smart biosensors attract significant interest due to real‐time monitoring of user health status, where bioanalytical electronic devices designed to detect various activities and biomarkers in the human body have potential applications in physical sign monitoring and health care. Bioelectronics can be well integrated by output signals with wireless communication modules for transferring data to portable devices used as smart biosensors in performing real‐time diagnosis and analysis. In this review, the scientific keys of biosensing devices and the current trends in the field of smart biosensors, (functional materials, technological approaches, sensing mechanisms, main roles, potential applications and challenges in health monitoring) will be summarized. Recent advances in the design and manufacturing of bioanalytical sensors with smarter capabilities and enhanced reliability indicate a forthcoming expansion of these smart devices from laboratory to clinical analysis. Therefore, a general description of functional materials and technological approaches used in bioelectronics will be presented after the sections of scientific keys to bioanalytical sensors. A careful introduction to the established systems of smart monitoring and prediction analysis using bioelectronics, regarding the integration of machine‐learning‐based basic algorithms, will be discussed. Afterward, applications and challenges in development using these smart bioelectronics in biological, clinical, and medical diagnostics will also be analyzed. Finally, the review will conclude with outlooks of smart biosensing devices assisted by machine learning algorithms, wireless communications, or smartphone‐based systems on current trends and challenges for future works in wearable health monitoring.

This review summarizes key issues and offers suggestions for the novel use and integration of smart biosensors in real‐time health monitoring process, as well as provides insight into their material functionality and technological approaches. With smart established systems, bioelectronics can be able to accurately provide capabilities for diagnosing health conditions.

1. Introduction

With the significant development of advanced materials and technologies, the integration of biology and electronics could produce novel bioelectronics and attract interest in response to the current demands of biological or clinical analysis. Fundamentally, although they provide powerful and reliable functionalities for clinical applications, bioanalytical electronic devices are often rigid and bulky.^{[ 1 ]} However, a main disadvantage is assigned to the cumbersome wiring system and poor skin‐integration using these bioelectronics, which probably preclude portable, comfortable, and long‐term monitoring statuses.^{[ 2} , ^{3 ]} Bioanalytical electronic devices have been directly integrated onto wearable substrates to monitor human‐body‐related analyses and external stimuli, which are considered nontoxic, wearable, cost‐effective, and small‐sized bioelectronics for extracting physiological information from electrochemical signals for controlling and treating diseases. Furthermore, mechanical design in wearable technology is still considered conceptually old, although the performance and miniaturization of integrated circuits were significantly advanced.^{[ 4} , ^{5 ]} Alternatively, ideas are proposed for wireless (or portable) technology for biosensors to yield noninvasive control and health monitoring, making their practical application convenient for patients. Advanced strategies in electronic device design, active materials selection, and smart biosensor manufacturing technology could offer directions for establishing health monitoring tools and clinical diagnostics. However, there are still many challenges that need to commercially expand and evolve therapeutic targets.

In this paper, the latest developments in designs, materials, structures, technologies, and integration strategies of bioelectronics that are used as intelligent bioanalytical devices will be reviewed specifically. First, we will give a careful introduction about several scientific keys to bioanalytical sensors to show understand primary requirements and current strategies in the design of bioanalytical sensors. Next, detailed introductions of the functional materials (i.e., carbonaceous, metallic, organic materials, etc.) and technological approaches (i.e., vacuum filtration, drop casting, printing, laser‐scribing, photolithography, etc.) used in the fabrication process of bioanalytical electronic devices will be presented. Subsequently, the bioelectronics assisted by various machine learning algorithms will be discussed, using as smart health monitoring systems. Afterward, recent examples of smart electronic devices for potential applications in biological, clinical, and medical diagnostics, i.e., proteins, DNA, cholesterol, dopamine, cancer markers, diabetes, leukemic disease, myocardial infarction, dengue fever, and H1N1 influenza, will be summarized in detail, at same time that some challenges in development of bioanalytical sensors are also suggested. The review will conclude with outlooks on smart bioelectronics assisted by machine learning algorithms on current trends and challenges for future works on the wireless connection and application‐specific design in wearable health monitoring. Therefore, the readers can perceive the wearable bioelectronics and smart health monitoring systems through this comprehensive review.

2. Scientific Keys and Sensing Mechanism of Bioanalytical Sensors

Basically, a biosensor is considered an analytical device that probably converts a biological recognition element into another signal (i.e., electrical, optical, chemical, or physical signal); from that, this output signal can be measured and quantified in real‐time with proper responses. In the technological approach, a biosensor is concerned as an integrated miniaturized device containing a biosensitive layer that is linked with a conversion system to detect and respond to signals, in which the active layer is generated through immobilizing the biological recognition element onto the biosensor's surface, which probably capture the proper analytes and explicate the biological recognition element (Figure  1 ).

Figure 1.

Schematic illustration of the principle of biosensor operation and key factors: the analyte is detected by the biosensitive layer, immobilized on the transducer, and the biological response is converted to an optical, electrical, or electrochemical signal by the transducer and subsequently further processed to provide the semiquantitative or quantitative information.

For typical classifications of a bioanalytical sensor, biosensors can be divided into electrochemical, physical, or optical ones (Figure  2A), which was based on the biosensitive detection method and transducer system. Among them, a physical transducer system comprises thermometric and piezoelectric biosensors (Figure 2A), in which the piezoelectric type of biosensors mainly involves an alternating potential and creates a standing wave in a crystal at a specific frequency (i.e., an important impaction of the frequency is associated with the possible surface features of a crystal), especially for a biological recognition element‐coated crystal that a target analyte linked to a receptor can offer a change in the resonant frequency. In the meantime, the thermometric type of biosensors is established by coupling enzymes with temperature sensors; from that, the heat of the enzymatic reaction will be determined and calibrated according to the target analyte concentration when this analyte is exposed to the enzyme. For an optical transducer system, these biosensors detect possible changes in the photoluminescence, absorbance, or fluorescence of a suitable indicator, as well as possible changes in the refractive index (Figure 2A). Basically, the optical biosensors are designed to offer an electronic signal regarding the intensity or frequency of the analyte concentration or group to which the biosensing element links.

Figure 2.

A) Typically, types of bioanalytical sensors are based on various transducer systems. B) Measured parameters and curves of bioanalytical sensors according to various electrochemical transducers.

In addition to the above physical and optical biosensors, an electrochemical transducer system applied in a biosensor, i.e., electrochemical biosensors, can be integrated with biosensing devices that yield semiquantitative or quantitative analytical information using a biological recognition element contacted with an electrochemical transduction element. Usually, electrochemical biosensors are classified into amperometry, potentiometry, conductometry, and impedimetry, which are mainly based on the measured parameters, more specifically, i) amperometry–the amperometric transducer determines the electrical current, ii) potentiometry–the potentiometric transducer determines the generated potential, iii) conductometry–the medium conductance is determined by the conductometric transducer, and iv) impedimetry–the medium impedance is determined by the impedimetric transducer,^{[ 6 ]} as shown in Figure 2B. The amperometric biosensors are the most successfully commercialized biosensing devices, at where the current resulting from the oxidation or reduction of an electroactive substance is measured using a constant potential. Concomitantly, another approach to voltammetry is also a method supported by amperometric techniques in which the varying potential is used on the working electrode to observe the change of current. Potentiometric biosensors relate to a measurement of potential or pH variation that is considered a response to applied current, which is often under a low‐amplitude. In the case of conductometric biosensors, they present a particular biological reaction on the surface regarding the electrical conductance measured through the alternating potential under low amplitude. Meanwhile, impedimetric biosensors are used to determine possible changes in impedance as a frequency function that are similar to resistance in current flow; however, the case of resistance takes place in a direct current circuit, whereas that of impedance occurs in an alternating current circuit. Usually, the impedimetric type of biosensors is used to determine affinity interactions between molecules. The pros and cons of various electrochemical techniques using bioanalytical sensors are summarized in Table  1 . Additionally, bioanalytical sensors using electrochemical techniques can also be classified as biocatalytic sensors and bioaffinity sensors regarding the electrochemical recognition process. The biocatalytic sensors involve a catalytic reaction occurring on the surface of sensors and enzymes, while cells and tissues are known as suitable biorecognition elements for this catalytic reaction. The bioaffinity sensors are characterized by the affinity interaction occurring on the electrode surface, i.e., nucleic acids interaction, antigen–antibody interaction, or aptamer interaction.

Table 1.

Pros and cons of various electrochemical techniques using bioanalytical sensors.

Techniques	Pros	Cons
Amperometry or voltammetry	–Fastness, precision –Highest sensitivity –High specificity –Continuous monitoring –The possibility to detect many compounds with different characteristic potentials in one measurement	–Required electroactivity –Current production –Interferences –Effect of the surrounding environment –Long‐term stability –Time‐consuming –Poor selectivity
Potentiometry	–Simple conditioning –Miniaturization –Real‐time monitoring –No current flowing –Limited invasiveness –Electroactivity is not required –Measurement of low‐concentration and turbid samples	–Intrinsic nonspecificity –Very sensitive to temperature changes –Possible ionic buffer interferences –Frequent recalibration is needed –Possibility of false‐negative result by strong buffers
Conductometry	–Use in a living biological system –No reference electrode is needed for easy miniaturization –Measure both electroactive and inactive analytes	–Potential error due to double layer capacitance of nontarget analytes –Low specificity
Impedimetry	–Possibility of label‐free setup –It can be applied in a living biological system –Exert high sensitivity and suitability for miniaturization	–Need nanotechnologies to improve sensitivities –Poor selectivity –Mathematical modeling needed to extract information

Nowadays, multifunctional sensors and smart systems are also attaching lots of attention that is thanks to real‐time health monitoring and disease diagnosis capability. Considering to the complexity of human physiological/biological signals, multiple biosensing information should be measured simultaneously to evaluate human health accurately, in which common types of multifunctional sensors can divided to single sensors with various functions, planar integrated sensors, 3D assembled sensors, and stacked integrated sensors in recent trends and advances. A simple comparison of these common types of multifunctional sensors is described in Table  2 . For the single sensors with multiple functions, this type of multifunctional sensor is often constructed by composite materials or conductive polymers, which exhibit both good stretchability and conductivity. The approach of these single sensors can reveal some advantages of low cost and simple fabrication, in which the coupling of multiple signals can impact to the measurement processing. With the use of single sensors with multiple functions, the biosensing signals can also be separated by specific structural designs, but the sensing functions are still limited. To solve this issue, the use of planar integration is able to be more effective and easy. More particularly, the biosensing device can detect multiple signals concomitantly through integrating various sensors onto a flexible substrate and interconnecting the sensors, but the integration level in this type can be limited by the density, regarding the manufacturing process of single devices and circuits. The use of 3D layout probably improves the integration level, especially for 3D assembling method, in which the functional materials will be rearranged in 3D structures that probably provide novel functions, enhance performance, and improve integration of the sensors. Until now, this type of multifunctional sensors is considered as an emerging approach, so it is still not finalized and should be further explored. Meanwhile, strategies in the stacked integrated sensors can reveal significant potential for high‐density functional integration, of course, this can develop the space in the thickness direction.

Table 2.

Comparison of common multifunctional sensors.

Types	Features	Pros	Cons
Single sensors	Single sensor with multiple functions	–Low cost –Simple fabrication	–Hard to decouple multiple physical signals
Planar integrated	Multiple sensors arranged in the plane	–Simple design –Independent sensing components –Mature fabrication technology	–Low integration
3D assembled	Transform from 2D layout integration to 3D configuration	–Novel functions –High integration –Enhanced performance	–Lack of mature and feasible 3D encapsulation methods/strategies
Stacked integrated	Multilayers integration of multiple electronic components	–High integration density	–High cost –Reduced flexibility –Complex fabrication

Until now, bioanalytical sensors are very important for medical diagnosis, health tracking, and disease treatment; precisely for that, it should meet sensitive and specific detection of expressing biomolecules. Regarding the approach of sensing mechanism, the biosensing devices have been greatly explored for different biological substances (i.e., pathogens, virus, bacteria, lactic acid, glucose, dopamine, etc.), in which quick and portable detection of cancer‐associated biomarkers (i.e., antibodies, proteins, hormones, DNA, etc.) can be concerned as an advanced innovation for intelligent biosensor fields, especially regarding the functional materials and structures.^{[ 7} , ⁸ , ^{9 ]} From that, finding potential materials and structuring strategies are required significantly. Furthermore, the actions’ exertion or the analytes’ adsorption will majorly impact to the possible changes of corresponding signals (i.e., capacitance, current, resistance, etc.) among the designed and constructed structures when the biosensing devices are exposed to external stimulus, which mainly regards to the change of surface carriers, chemical bonds, and local structures.^{[ 10} , ¹¹ , ^{12 ]} More interestingly, the use of micro‐/nanostructuring sensors can develop the reaction active sites and enhance reaction rate, which lead to collecting the output signals and the sensitivity correspondingly and accurately.

Overall, many challenges are still presented in the field of bioanalytical sensors and smart health monitoring systems. With the 3D integration strategy, the function integration can be significantly improved, but the manufacturing process and mass production is quite complex. Moreover, the requirements of stretchability and stability are also essential for smart wearable biosensors, meaning that it relates to the mechanical interaction among the inorganic devices/interconnects and the encapsulation layer through using new materials and structural design. With assists of artificial intelligence and machine learning, the use of bioanalytical sensors with more smart health monitoring systems will be developed to analyze multiple sensing signals to evaluate human health comprehensively and accurately. Besides, low‐cost mass production is still considered as a big challenge to reach lower power consumption, good sensitivity, great density, high robustness, and complicated signal detection of smart and wearable bioelectronics.

3. Functional Materials Using for Bioanalytical Sensors

Recently, advancements in materials and their manufacturing processes have been amazingly exploited, especially for smart bioelectronics, in which detection is very important for healthcare, clinical medicine, environmental monitoring, and food safety.^{[ 13 ]} Therefore, the development of inexpensive and reliable bioelectronics that enables high sensitivity and rapid bioanalytical ability could expand the potential and reliable applications. The key point of effective detection is attributed to signal transduction with selective recognition of the biological features, especially for bioelectronics, which is concerned with a combination of biological recognition and physical/chemical transduction to exhibit advantages in the utilization and commercialization to satisfy the practical demands in the above applications. In the following parts, recent potential materials‐based bioelectronics will be discussed comprising carbon‐based nanomaterials (i.e., single‐/multiwalled carbon nanotubes, graphene) and noncarbon nanomaterials (i.e., metallic nanoparticles/nanowires, porous silica, organic conductive polymers); further details are described in Table  3 and Figure  3 .

Table 3.

Pros and cons of functional materials for ideally constructing bioanalytical sensors.

Materials	Pros	Cons
SWCNT	–Low charge‐carried density –Delocalized π‐orbitals –Large surface area to volume ratio –Electrical conductivity improvements	–Limited surface‐to‐interface with large biological components –Nonspecific adsorption of protein –Hard manipulation during the sensor fabrication process –Unfavorable chemical functionalization
MWCNT	–Outstanding conducting and electrocatalytic properties	–The surface needs to be functionalized to increase biocompatibility –Irreversible agglomerates in aqueous solution
Graphene	–High ratio of surface area/volume –Large active sites –Fast electron transfer –High thermal conductivity –Better mechanical flexibility –Good biocompatibility	–Hard to dissolve in water medium
Metallic nanoparticles	–Efficient electron transfer –Increase in the ratio of surface area/volume –Supplying superior conductivity –Good biocompatibility –Easy functionalization	–Electrical instability in high salt concentration –Inconsistent upon signal amplification
Metallic nanowires	–High ratio of surface area/volume –Rapid response –High electrocatalytic capability and reproducibility –Improvement of the charge transfer and stability	–Decrease in electrostatic potential with distance
Organic conductive polymers	–High‐throughput –Low‐cost –Good flexibility, functionality, solubility, and specificity	–Reproducibility of thin‐film morphology –Chance of physical delamination –High operating voltages –Uncertainties in material stability
Mesoporous silica nanoparticles	–High pore volume and surface area –Good electron transfers and high‐loading capability –Well‐defined surface properties –Tune‐ability of size and shape	–Scattered size distribution –Difficult in preparation of well‐ordered –Formulation of stable‐colloidal suspensions
Indium tin oxide	–Low cost –High transmittance –Good electrical conductivity –Ease of surface modification	–The slow kinetics of electron transfer upon coating the surface with antibodies

Figure 3.

A schematic illustration of bioanalytical sensors using functional materials.

3.1. Carbon‐Based Nanomaterials

Bioelectronics based on carbonaceous nanomaterials, carbon nanotubes (CNTs), and graphene have been widely applied in various types of biosensors, which have well‐exploited potentials of their chemical and physical properties. To date, carbon‐based nanomaterials are very useful for different industrial fields. For example, CNTs are typically classified as single‐ and multiwalled CNT (i.e., SWCNT and MWCNT), which can be adopted as an electrode material, thanks to their large surface area, outstanding mechanical stability, and electrical conductivity caused by sp2–orbital hybridization among adjacent carbon atoms.^{[ 14} , ¹⁵ , ^{16 ]} Meanwhile, another common carbonaceous nonmaterial, namely, graphene, is known as a 2D hexagonal pattern of carbon atoms and is utilized as an electrode material due to its high surface area.^{[ 17 ]} However, this carbonaceous nanomaterial has a low throughput and hydrophobicity, which probably limits its utilization in bioelectronics applications. To overcome this, graphene oxide (GO) and reduced GO (rGO) have been exploited to increase the hydrophilicity of the graphene layer and remove the oxygen groups of GO, obtaining remarkable electrical conductivity and surface modification effectively for immobilizing biomolecules.^{[ 18 ]}

CNT is also considered a promising candidate for next‐generation electronic materials, which possess remarkable electrical and mechanical behaviors that make them ideal for wearable electronic devices, especially for high‐performance biosensors.^{[ 19 ]} These have attracted attention for potential applications in wearable bioelectronics, health monitoring, and clinical analysis. With the integration of biosensors and circuits into CNTs, bioelectronics can become significantly more promising due to the in situ signal processing capability; hence, the interface of circuits should meet a requirement for mechanical‐related soft features to reach appropriate performances, such as wearable and soft bioelectronics. CNTs in biosensors are becoming more widely used, and their industrial demand has become increasingly urgent. To date, there are some available strategies to synthesize CNTs, i.e., laser‐ablation, arc discharge, floating catalyst, chemical vapor deposition (CVD), and plasma‐enhanced CVD methods,^{[ 20 ]} in which the synthetic methods of CNTs have been dramatically improved.^{[ 21 ]}

Of these, the arc‐discharge method (Figure  4A) uses a potential difference of ≈20 V positioned 1–3 mm apart of two graphite‐based electrodes in an inert gas, leading to evaporating graphite from the anode and its condensation at the cathode to yield final CNT products. In particular, MWCNTs are typically formed when using pure graphite electrodes, whereas a metal catalyst (i.e., cobalt–Co, nickel–Ni, or iron–Fe) is used to add into the anode that often forms the final products of SWCNTs. For this arc‐discharge method its main disadvantage involves the inability to control CNTs’ purity. Therefore, this needs further chemical treatments to reach pure CNTs. The laser‐ablation method (Figure 4B) was first successfully reported in 1995,^{[ 22 ]} where a mixture of graphite and metal catalyst is exposed to a laser with a supporting condition of an argon–Ar atmosphere's flow inducing evaporation of graphite, and then CNTs are formed from the resulting condensate. However, this method can produce impure CNTs, requiring further chemical steps.

Figure 4.

Schematic diagrams of A) arc discharge, B) laser‐ablation, C) chemical vapor deposition, and D) electrospinning techniques.

Additionally, CNTs have been confirmed to be a promising material for high‐performance electronic devices, mainly thanks to their potential electrical and mechanical properties and their low‐temperature manufacturing processes. Aside from the abovementioned synthetic methods of CNTs, there are also many other approaches, such as light irradiation, gas/plasma etching, chemical/electrostatic doping, annealing in vacuum/hydrogel, and chemical surface reaction., which have been developed and shown some success by partially or fully removing impurities from CNTs.^{[ 23 ]} More specifically, the CVD method (Figure 4C) can be performed in a dry or solution procedure to effectively prepare pure CNT networks and thin films, in which the dry process uses the CVD approach and then dries the sketching from vertically aligned CNT.^{[ 24} , ^{25 ]} For example, SWCNT films have been well manufactured from this dry‐CVD method that produced ultralong nanotubes bonded to each other through strong connections; thus, SWCNT films possess excellent conductivity to be used as electrode materials in many electronic devices.^{[ 26 ]} In the meantime, in the solution‐CVD approach, several synthetic methods successfully reported consisting of rod/drop coating, vacuum filtration, and printing,^{[ 27} , ²⁸ , ^{29 ]} at which CNTs will be dissolved in aqueous solution or suitable organic solvents. This progress can be conducted to be deposited onto large‐scale soft and glass substrates in the CNT suspension at low temperature and cost. Generally, the CVD method is based on the decomposition of a gas‐derived carbon (i.e., acetylene, ethylene, methane, and carbon monoxide) conducted at the surface of a heated metal catalyst, and then the final products can be SWCNTs and/or MWCNTs.^{[ 30 ]} SWCNTs are quite difficult to control inner/outer diameters, while MWCNTs can be easily adjusted for inner (1–3 nm) and outer (2–20 nm) diameters. Furthermore, another CVD‐related method, i.e., plasma‐enhanced CVD, is known as the incorporation of plasma and CVD processes, at which the gas's activation (i.e., acetylene, ethylene, methane, carbon monoxide) is carried out through electron effects instead of thermal energy impact that probably allows the synthesizing vertically aligned CNTs at a lower temperature range of 400–650 °C.^{[ 31 ]} Hence, plasma‐enhanced CVD becomes more attractive, and the synthesis of CNTs can be well conducted on glass or silicon substrates that do not damage them.

In addition to the use of CNTs in the bioelectronics fabrication process, graphene has also shown its potential applications in fields of wearable bioelectronics and functional nanomaterials, which is mainly thanks to its outstanding characteristics, i.e., Young's modulus (1 TPa), high thermal conductivity (5300 W m⁻¹ K⁻¹), high electron mobility (350 000 cm² V⁻¹ s⁻¹), large specific surface area (2600 m² g⁻¹), and limited thickness (0.34 nm).^{[ 32} , ³³ , ³⁴ , ^{35 ]} Commonly, graphene is known as 2D nanosheets of densely packed honeycomb lattice‐structured carbon atoms, which are capable of well rolling up to produce other final products composed of 0D–fullerene and 1D‐CNT. Each carbon atom will possess a π orbital in the lattice to donate to the electronics’ delocalized network, exhibiting three C─C linkages instead of four linkages as a diamond. Further, the use of external force could contribute to peeling out graphene that, perhaps, may be due to chemical (i.e., composing of electrochemical, ultrasonic, intercalation peeling, and redox exfoliations) or physical (i.e., using tape) exfoliation, at the same time that several synthetic processes of carbon‐containing precursor are also reported more complexly including CVD and chemical synthesis. Mostly, there are three common methods to fabricate graphene (i.e., 0D, 1D, 2D, or 3D) concerning mechanical exfoliation, CVD, and reduction of graphene oxide (GO);^{[ 35 ]} thus, the desired structural morphology of graphene should be concerned with many factors regarding its cost, manufacturing process, and application.

Thus far, graphene can be fabricated into different forms of structural morphology. Increasingly, more graphene‐based electronic devices have been demonstrated that are based on its unique characteristics. More specifically, with the potential assist of laser technique, laser‐scribed/‐induced graphene has received significant attention due to its low cost and fast fabrication process of graphene.^{[ 36} , ^{37 ]} As a result, various morphologies of fabricated graphene have been successfully exploited for wearable applications; at the same time, these approaches assisted laser processing can be flexibly conducted on many different substrates, leading to greatly enriching the raw materials to yield different morphologies of graphene. Moreover, graphene‐based wearable sensors have been widely applied for monitoring physiological, biological, electromechanical, or electrochemical signals and show potential. Importantly, these graphene‐based bioelectronics should be flexible and biocompatible; however, they should have a low‐cost and simple fabrication process and limit the use of rigid substrates.

3.2. Metallic Nanomaterials

Noncarbon nanomaterials, i.e., metallic nanomaterials, can be separated into two types: nanoparticles (NPs) and nanowires (NWs). Here, metallic NPs are considered solid bioelectronic support materials, enhancing the surface‐to‐volume ratio and the transfer efficiency. Gold NPs (AuNPs) are one of the most common metallic NPs applied in biosensor devices due to their special biocompatibility and easy protein functionality. Simultaneously, the use of AuNPs is precisely bound to the sensing electrode as they can exhibit strong electrical signals and a good signal‐to‐background ratio. With the use of a low electrocatalytic electrode, relevant redox reactions are almost slow, except for the mediator's redox reaction. In order to enhance the biosensor signals, AuNPs were mounted on poly(styrene‐co‐acrylic acid) microbeads via the in situ tracing tag method,^{[ 38 ]} in which the AuNPs were triggered to favorably convert neat silver (Ag) into the formation of AgNPs, resulting in a good biological detection using an anodic‐stripping analysis. However, AuNP‐decorated nanosheets were also utilized to tag antiprostate‐specific antiantigen (antiPSA) antibodies and horseradish peroxidase using a new impedimetric immunosensing device for sensitive PSA.^{[ 39 ]} In another report, an Aunanocatalyst label was used to mediate outer‐/innersphere‐response‐philic species, used as an ultrasensitive and incubation‐free electrochemical biosensor to determine the sensitivity of creatine kinase–muscle brain.^{[ 40 ]} Furthermore, AuNPs are also known as electrodes because of their great electrochemical behavior, meaning their free electrons will migrate from the valence to the conduction band. Some polythionine–Au could be used as an electrode using a ratiometric electrochemical method,^{[ 41 ]} with the result that this biosensor obtained could reach a detection limit of 2.2 pg mL⁻¹ with good specificity. Recently, hybrid electrodes containing AuNPs and other active materials have been exploited to improve the bioanalytical performance of electronic devices. For example, silicon dioxide (SiO₂) was used to combine with Au catalysts, resulting in the catalytic activity significantly improved due to the geometrical effects of imperfect sites or the electrical effects of Au particles.^{[ 42 ]}

However, a combination of AuNPs and CNT could produce the final nanocomposite with highly improved performances consisting of simple surface modification, extraordinary electrical conductivity, high sensitivity, and selectivity,^{[ 43 ]} and thus, the AuNPs/CNTs hybrid nanostructures could be used as highly sensitive electrodes.^{[ 44 ]} Furthermore, calcium carbonate (CaCO₃) is also a promising candidate to integrate with AuNP, which is mainly due to its good biocompatibility, better water dispersibility, larger surface area, lower specific gravity, and good enzyme efficiency. When AuNPs were stabilized using sodium citrate, they were able to be well assembled on the surface of the porous carbonate microspheres, and then the hybrid product was produced with better biocompatibility, solubility, and water dispersibility.^{[ 45 ]}

In addition to AuNPs, other metallic NPs, i.e., Ag, copper (Cu), and platinum (Pt), are also potential candidates in the manufacturing of bioelectronics, which can pass electrons more effectively and accommodate more active sites on their surface. However, metallic NPs are often incorporated with other additional materials to possess higher sensitive detection rather than the use of metallic NPs alone. Although its structural defects could be well repaired, the addition of AgNPs dramatically improved the electrical conductivity of rGO.^{[ 14 ]} Zhang et al. have successfully reported a novel nonenzymatic bioelectronic, in which this biosensor contained Ag@Au nanorods and iron(II, III) oxide (Fe₃O₄, or black iron oxide (II, III)) nanospheres, used as a nanoelectrocatalyst.^{[ 46 ]} As a result, the Ag@Au–Fe₃O₄ nanocomposite electrode exhibited better electrocatalytic ability, i.e., hydrogen peroxide (H₂O₂), compared to Fe₃O₄ nanospheres or Ag@Au nanorods, probably thanks to the synergetic effect of catalysts. So far, Pt nanostructures have also been greatly studied in various Pt nanocrystal shapes composed of nanotubes, nanocubes, and dendritic NPs,^{[ 47} , ^{48 ]} in which PtNPs were fabricated by Wang et al.,^{[ 47 ]} and their sizes and shapes were also optimized to improve catalysis and reduce oxygen. Meanwhile, tetrahexahedral Pt nanocrystals with high‐index facets and outstanding electro‐oxidation function have been effectively discovered by Tian et al.,^{[ 49 ]} where the neat Pt single‐crystal surfaces exhibited high‐index planes that related to greater catalytic activity compared with the most typical stable planes. Generally, electrical instability is known to be one of the disadvantages of metallic NPs because they can be aggregated with each other regarding their susceptibility to salt concentrations.^{[ 14 ]} Hence, their chemical and biological characteristics can be appropriately adjusted by modifying the NP surfaces to be applied in humans with a high salt concentration. Unfortunately, the metallic NP‐related signals are often inconsistent with signal enhancement, inducing limitations in reproducibility,^{[ 50 ]} so liquid metal formation is recently suggested to control the NPs’ quality with better performances.

Aside from the NPs, nanowires (NWs) have promising potentials regarding the high surface‐to‐volume ratios of small‐scale, optical, electrical, and magnetic characteristics, which are often more flexible and versatile than the other larger wires; nevertheless, its 1D configuration shows a high ratio of width to length, indicting particular physical properties being comparable to the quantum phenomena.^{[ 51 ]} In particular, its electrical conductivity can be adjusted by fabricating various active materials, i.e., metals (Au, Ag, Ni, Pt, and Cu), metal oxides (zinc oxide–ZnO, tin(IV) oxide – SnO₂ or stannic oxide, iron(II) oxide – Fe₂O₃ or ferric oxide), semiconductors (Si, indium phosphide – InP, gallium nitride – GaN).^{[ 14 ]} AgNWs are considered a common active carrier for electrochemical measurements because of their effective electrocatalytic feature, rapid response, and good reproducibility. For instance, AgNWs‐contained biosensor has been successfully reported by Cao et al.,^{[ 52 ]} while superconductive AgNWs in the sensing probe were well prepared by combining with mesoporous ZnO nanostrawberries using a chemical method. Meanwhile, PtNWs were biocompatible with nucleic acids or proteins and possessed exceptional catalytic properties for the H₂O₂ reaction.^{[ 53 ]} Furthermore, AgNW networks are considered promising alternative materials for preparing a transparent and soft electrode because of their high transparency, conductivity, and thermal, chemical, and mechanical flexibility. With the cost‐effective and low‐temperature processes, many technical approaches in solution coating processes have been exploited to fabricate AgNW electrode materials through reliable, simple, and low‐cost deposition techniques, i.e., spray/rod/spin/dip/slot‐die coating, drop casting, vacuum filtration.^{[ 54 ]} Similarly, other technical methodologies applied for AgNW alignment have also been well performed, i.e., external magnetic/electric fields‐based assembly, rod coating, flow‐enabled, and capillary printing techniques.^{[ 55} , ⁵⁶ , ^{57 ]} Interestingly, a soft contact lens‐integrated smart biosensor system has been presented and used as a wireless device for ocular diagnostics,^{[ 58 ]} where the combination of AgNWs and graphene has been proposed to produce active nanocomposites. Generally, to obtain the outstanding characteristics of AgNW networks, it significantly depends on i) individual NW features, ii) junctions among the connected NWs, and iii) network density. Furthermore, several post‐treatments of the AgNW network will be suggested to investigate for enhancing these characteristics, consisting of mechanical pressing, thermal annealing, and light‐induced plasmonic nanowelding.^{[ 59} , ^{60 ]} Considering the use of metal oxides in the manufacturing of bioelectronic devices, it showed the electron transfer rate favorably and effectively impacted the performance, at the same time that these metal oxide‐contained nanocomposites ensure NWs surfaces, revealing catalytic and biocompatible properties better and enhance their sensing performances correspondingly. A TiO₂ nanowire‐based microelectrode has been introduced to rapidly detect a marker of Gram‐positive bacteria regarding food poisoning outbreaks (i.e., Listeria monocytogenes),^{[ 61 ]} resulting in the degree of change in impedance induced by the antibody–bacteria complex in this NW corresponding to this bacterium's quantity. Additionally, cuprous oxide (Cu₂O) NWs have also been considered an attractive metallic nanomaterial due to good electronic and electrocatalytic characterization. For example, this NW was effectively used to enhance the special optical, electronic, and mechanical characteristics of 2D nanomaterials in label‐free electrochemical biosensors.^{[ 62 ]}

Additionally, liquid metals or alloys are encouraged materials for the fabrication of soft electronic devices because of their outstanding electrical/thermal conductivity and rheological properties, in which gallium (Ga) and its eutectic alloys formed with tin (Sn) and indium (In) are highly conducted thanks to its low vapor pressure, safety, and no pollution, instead of the use of highly toxic mercury (Hg). Typically, liquid alloys possess complex surface structures that are used in electronic devices because of their often adequate softness and deformation, consisting of high stretchability, resolution, and self‐healing ability, limiting failure or circuit fracturing under repeated deformations,^{[ 63 ]} where their surface will create a nanometer‐thick‐amphoteric‐solid‐oxide film in an aerobic medium. This film contained in liquid alloys will impact to the adhesion and shape of the liquid metal to different surfaces.^{[ 64 ]} To date, many technological approaches have been successfully applied to attached liquid metals or alloys on various surfaces to prepare liquid metal‐based patterns, i.e., inkjet‐printing, microchannel injection, transfer writing, and masked deposition.^{[ 65} , ⁶⁶ , ^{67 ]} Of these, stencil print technology is almost concerned as it shows cost‐effective, mask‐free, fast, and mass production. Unfortunately, directly printing the liquid alloy on the flexible substrate is very difficult due to the high surface tension of the liquid alloy and the surficial oxides. Therefore, the selection of suitable transfer templates to contain the liquid alloy or metal is truly important, meaning that it cannot only selectively adsorb this liquid alloy or metal, but it can also be completely transferred on various substrates.^{[ 68 ]} In recent years, lithography technology has reached the final product's high resolution at the sub‐micrometer level.^{[ 63 ]} Briefly, some requirements of liquid alloy patterning in batch, high resolution, and cost‐effectiveness have encouraged strategies in recent years.

3.3. Organic Conductive Materials

In addition to the functional nanomaterials, the use of organic polymers with low mechanical stiffness also demonstrated the most promising platforms for wearable technologies. To date, many flexible and stretchable sensors serve as platforms with capabilities of biological signals converted for health monitoring. Additionally, recently advanced strategies in the selection of functional materials, the design of electronic devices, and the fabrication of biointegrated electronics have offered procedures for establishing clinically relevant diagnostic and monitoring tools.^{[ 69} , ⁷⁰ , ⁷¹ , ^{72 ]} The recent manufacturing of low‐cost and flexible bioelectronics based on organic polymers via advanced techniques is expected to provide advantages for promising biological and chemical detection applications. The rapid development of synthetic methodologies in these organic polymer‐based bioanalytical electronic devices has dramatically exploited new organic materials and an obvious understanding of bioanalytical interactions. Thus, the main characterizations of these electrical devices regarding sensitivity, specificity, stability, and wide‐range detection of electrochemical/biological analysis have been steadily improved.

Among them, these organic conductive polymers are probably attached to the surface of soft or rigid supporting substrates to produce the transistor and electrode. For example, the use of conductive polymers to fabricate a structure of glucose oxidase‐incorporated organic thin‐film transistor structure has been well reported by Elkington et al.,^{[ 73 ]} showing that this thin‐film transistor with poly(3‐hexythiophene) was designed and fabricate to well detect glucose well in saliva, as well as significantly improve the surface uniformity of the films deposited. Particularly, with incorporated organic polymers in biosensors, the generated by glucose oxidation could be dramatically enhanced and determine these biosensors. At the same time, polyaminobenzoic acid was used to provide carboxyl groups on the surfaces of indium tin oxide and created a functioned surface with N‐hydroxysuccinimide ester, implying that this surficial modification can react well with the ─NH₂ groups of antibody. Furthermore, the polyvinylimidazole electrode could also be modified by a mixture of indium tin oxide and Ni²⁺ ions by Choi et al.,^{[ 74 ]} which probably detects a marker of hippuric acid–a toluene metabolite. The transfer of electrons to the electrode is more favorable, meaning that the Ni²⁺ ions can react with the imidazole function and bind to the polymer. Recently, the use of conductive polymers and metal nanoparticles to fabricate potential nanocomposites has received significant attention because of their outstanding optical and electrochemical features and enhanced catalytic stability.^{[ 75 ]} Lu et al.^{[ 76 ]} investigated a new hybrid nanocomposite film containing polypyrrole (PPy) SWCNT, and AuNPs resulting in these active nanocomposites granted well catalytic activity toward the oxidation of uric acids and epinephrine, at the same time that they were able to be used as a high‐performance biosensor for excellent sensitivity, selectivity, and stability.

As is well known, monitoring of electrochemical/biological signals is very helpful in preventing and diagnosing clinical analysis and various diseases, which is favorable in restoring and reconstructing biological functions effectively. In addition to the functional materials used to fabricate bioanalytical electronic devices, porous nanomesh conductive polymers have shown great potential in soft bioelectronics, thanks to lightweight, high water vapor permeability, good skin compatibility, and stability of these prepared porous nanomesh polymers.^{[ 77} , ^{78 ]} An Au/poly(vinyl alcohol) (Au/PVA) nanocomposite nanomesh‐based electrode has been prepared successfully by Miyamoto et al.^{[ 79 ]} and performed well with lightweight, inflammation‐free, gas‐permeable, and stretchable abilities. In the fabrication process of PVA nanomesh, it was dissolved after spraying water, and the nanomesh conductors would quickly stick to the skin closely. This Au/PVA nanomesh electrode could be considered as a tactile sensor and also as a measure of skin impedance.^{[ 80} , ^{81 ]} In another study, another nanomesh‐structured flexible sensor, i.e., a durable strain sensor based on Au/polydimethylsiloxane/polyurethane (Au/PDMS/PU) nanomesh, has been well‐fabricated to monitor the facial tissue movements.^{[ 82 ]} The conductive polymer with porous nanomesh structures can be successfully exploited through various advanced approaches, i.e., natural fiber, photolithography, and electrospinning techniques.^{[ 79} , ^{83 ]} Particularly, electrospinning technology (Figure 4D) was known as an effective and common methodology for preparing the polymer nanomesh (as a structural support) and subsequently functionalized (as a sensing response) by other active nanomaterials to perform the construction of relevant electrochemical/biosensing functions. Moreover, the polymer solution‐contained syringe creates the Taylor cone at the needle because of the combination of the electric field force, the surface tension, and the viscoelastic stress of the solution, which extends then into the uniform filament to deposit on the collector (Figure 4D). These prepared sensors will consist of functionalized and nanomesh materials after a suitable modification among the active materials. In addition to the abovementioned PU and PVA, several other polymers have also been found, i.e., polyvinylpyrrolidone (PVP), poly(styrene–ethylene–butene–styrene), and poly(styrene–butadiene–styrene).^{[ 84} , ^{85 ]} Also, this electrospinning technology can form 2D network polymeric films with micro/nanofibers, resulting in a large specific surface area for the next functional modification.^{[ 86 ]} Briefly, functional modification methods can be divided into two approaches, e.g., one is to add functional materials to the electrospinning solution and form a porous nanomesh‐structured polymer after electrospinning and then finalizes functional modification through following treatment, while another one is to functionalize the surface of the porous nanomesh‐based polymer that was reached from electrospinning process. A direct spraying methodology is commonly used for surface modification, in which functional substances are prepared into solutions and then coated on the nanomesh‐structured polymer by drop coating or spraying, which is only used to modify the nanomesh surface layer. Also, the nanomesh is soaked into the solutions contained functional materials (i.e., carbon nanomaterials, MXene) with ultrasound assist. In metal NPs, the nanomesh is soaked in the precursor solution and then modified using in situ synthesis of metal NPs directly conducted on the nanomesh's surface using chemical reaction, at same time that the metal can also be directly deposited on the nanomesh's surface by sputtering. Additionally, nanomesh‐structured polymers can be modified prior to functional materials’ modification to improve bioelectronics performance.

Another form of organic polymer, i.e., a hydrogel, is considered a hydrophilic 3D polymer network, swells in a water medium, and greatly retains water molecules without totally dissolving. Hydrogels are one of the most commonly used biological materials mainly due to their high water content, good biocompatibility, and controllable physical/chemical properties.^{[ 69} , ^{70 ]} Furthermore, introducing conductive components into the hydrogels can provide electroconductive hydrogels. Currently, these multifunctional hydrogels are hot trends in soft bioelectronics, biosensors, and human–computer interfaces,^{[ 69} , ⁷¹ , ^{87 ]} in which electroconductive hydrogels often consist of conductive materials and soft hydrogel substrates. In various conductive materials, electroconductive hydrogels can be classified electronically (i.e., metal NPs/NWs, CNTs, graphene, MXene, PPy, polyaniline–PANI, poly(3,4‐ethylene dioxythiophene)–PEDOT) and ionically (i.e., ionic salts were dissolved in the hydrogels, NaCl or LiCl) conductive hydrogels;^{[ 88} , ^{89 ]} thus, these composite hydrogels are capable of obtaining the ideal electronic conductivity and holding the enhanced biomechanical advantages of hydrogels. Until now, numerous natural and synthetic polymers have been greatly exploited to reveal the ideal design and manufacturing process of electroconductive hydrogels appropriately and effectively; of course, multifunctional hydrogels have been significantly utilized in implantable and wearable bioelectronics, mainly thanks to their essential skin/tissue‐like features.^{[ 88} , ^{90 ]} For example, Ag flakes were used to embed into polyacrylamide–alginate hydrogel matrix and then partially dehydrated to produce a high‐performance electroconductive hydrogel, i.e., high electrical conductivity (374 S cm⁻¹), high stretchability (250%), and low Young's modulus (<10 kPa).^{[ 91 ]} Besides the introduction of conductive fillers, the hydrogel substrates have also manifested electroconductive hydrogels with ideal adhesive and biomechanical properties; of course, these hydrogels with favorable bioadhesive properties will easily construct appropriate and stable bioelectronic interfaces on the biological surfaces to enhance bioelectrical signal transmission. In other words, the key advantages of electroconductive hydrogels that are used as wearable bioelectronics are probably listed as follows: i) excellent biocompatibility, ii) good elasticity/flexibility, and favorable biomechanical interactions, iii) a broad range of conductivity, iv) effective bioadhesive features at the electrode–skin interfaces, and v) variety and numerous green electronic devices.

3.4. Others

Other active materials applied in manufacturing bioanalytical electronic devices, i.e., mesoporous materials such as indium tin oxide (ITO) and MXene, have also found and received research interest in recent years. Of these, mesoporous materials are known to be crystalline structures with a large surface area, while electron transport can be dramatically improved through ideal designs of the nanoscale mesoporous structure. Based on the size of the pores, porous materials can be divided into i) micropores (<2.0 nm), ii) mesopores (2.0–50 nm), and iii) macropores (>50 nm).^{[ 92 ]} Mesoporous silica NPs were designed to create electrochemical biosensors for measuring prostate‐specific antigens.^{[ 93 ]} In this report, a new controlled release system based on an acidic cleavable linkage was successfully exploited to detect PSA markers, where an acid‐labile‐acetal linker was used to remove the coated AuNPs from mesoporous silica NPs–thionine–Au, leading to releasing the encapsulated thionine. In another work, a novel sandwich‐type sensor has also been prepared to detect Bax protein and Bcl‐2, using as a dual‐signal electrochemical biosensor, in which the rGO substrate was used to immobilize the antibodies of these biomarkers to further capturing target antigens.^{[ 94 ]} Additionally, this biosensor contained a signal probe using mesoporous silica combined with CdSeTe@CdS quantum dots and Ag nanoclusters, which were proportional to the number of antigens of these biomarkers. This combination was due to their individual oxidation peaks being uninterrupted, as well as the easy preparation approach to reach a relatively uniform length. With the good dispersion properties and plentiful functional groups, it was also favorable for the composite hybrid of CdSeTe@CdS quantum dots and Ag nanoclusters to easily more with the biomolecules.

ITO is one of the transparent conductive materials that is efficient and commonly used as an electrode owing to its notable optoelectronic features and outstanding transmittance. ITO often exhibits less flexibility than the use of AgNWs, but its main advantages are that it is cost‐effective and has great electrical conductivity.^{[ 95 ]} With the existence of available hydroxyl groups on the surface of ITO, this conductive material is capable of being functionalized with numerous chemical compounds to produce promising active surfaces containing functional linkages (i.e., carboxylic acid, amine, thiol, aldehyde, and sulfhydryl), which can be considered as self‐assembled monolayers to immobilize captured antibodies (or biomolecules). The use of ITO in the manufacturing of electronic devices could effectively produce the ITO‐based electrochemical biosensor with the point‐of‐care function due to its good conductivity and easier preparation process to form an active thin film.^{[ 14 ]} However, ITO is applied as an electrode that shows several drawbacks; for example, the electron transfer rate of the ITO‐based electrode is dramatically slower than that of the carbon‐ or metal‐based one. In order to overcome this problem, the surficial functionalization of conductive polymers and electron mediators should be minimized to enhance the electron transfer rate of the ITO‐based electrode because once the electrodes are coated with biomolecules, this can increase the negative phenomenon.

So far, MXene is considered a hot trending 2D material, and it is known to be a novel family of 2D carbides, carbonitrides, and nitrides. Its formula can be M_1.33XT _z , M _{n +1}X _n T _z (n = 1, 2 or 3) or M _{n +1}AX _n (i.e., MAX phase); where M is a transition metal, X is N, and/or C, T is ─OH, ─O, and/or ─F, and A is an element of the IIIA or IVA group.^{[ 96 ]} The MAX phase structure has 2D layers of transition metal carbides and/or nitrides attached with an A element, meaning that it is concerned as a group of layered hexagonal materials with P63/mmc symmetry, in which the M layers are almost packed, and the X atoms become full the octahedral sites, and subsequently, the M _{n +1}X _n layers are formed and alternated with the A atoms.^{[ 97 ]} Generally, the MAX phases containing the bonds among the 2D layers are too strong to be fractured under mechanical deformations. Considering the bonds’ characteristics in the MAX phases, the M–A bond has a metallic character, while the M–X bond has a mixed metallic/covalent/ionic one. With the obvious differences in character of these bonds, the M–X bonds are stronger than M–A bonds; therefore, MXene‐based 2D materials can be produced by selectively etching the A element at room temperature, especially for the chemical etching method without damaging the M–X bonds,^{[ 98 ]} using a concentrated hydrofluoric acid (HF) or a mixture of lithium fluoride (LiF) and HF. There are typically two approaches to synthesizing MXene, i.e., bottom‐up and to‐down. For the bottom‐up approach, the CVD method is used to produce high‐quality films on different substrates, but these obtained films are thin films instead of single layers.^{[ 99 ]} Meanwhile, the top‐down approach is assigned to the exfoliation of the layered bulk, i.e., mechanical and chemical exfoliations. The M–A bonds have a strong metallic character, so the use of mechanical exfoliation or classical chemical exfoliation makes it impossible to fabricate MXene. The selective chemical etching of the A element layers in the MAX phases is required to synthesize MXene. Until now, there have been various potentials in the utilization of MXene, composing applications in energy storage, electrocatalysis, electrodes, and bioelectronics,^{[ 100} , ¹⁰¹ , ¹⁰² , ^{103 ]} at the same time that MXene can also be well combined with other nanomaterials to significantly improve malleability. For example, an MXene‐based wearable biosensor has been manufactured to analyze in vitro perspiration through measurement (i.e., glucose and lactate markers).^{[ 104 ]} A piezoresistive sensor has also been designed and produced randomly distributed spinous microstructures, significantly enhancing the sensor performance as well as the conductive channels’ contact area of the conductive channels.^{[ 100 ]} In another work, a capacitive pressure sensor has been proposed to detect and monitor patients’ health through measuring physiological signals, where nanofibrous composite nanofibrous scaffolds have been sandwiched to each other, using as a dielectric layer among PEDOT:poly(styrene‐sulfonate)/PDMS (PEDOT:PSS/PDMS) electrodes.^{[ 105 ]} Furthermore, an optoelectronic spiking afferent nerve with efficient capabilities (i.e., perceptual learning, neural coding, and memorizing) has also been reported to mimic tactile sensing and processing with the use of synthesized Ti₃C₂T _x nanosheets (i.e., MXene), in which the established MXene‐based sensor could be favorable for detecting and converting pressure information into light pulses.^{[ 101 ]} MXene possesses great potential in wearable electronic devices owing to its excellent conductivity, hydrophilicity, mechanical properties, and easily controlled morphology; therefore, the use of MXene materials to combine with other appropriate nanomaterials should be required to generate new biosensing systems with high sensitivity and wide response range.

4. Technical Approaches in Fabrication of Bioanalytical Sensors

Finding technical approaches to fabricating bioanalytical sensors is a considerable requirement for producing high‐performance biosensors. Until now, there are quite a few different techniques that have been well explored, consisting of vacuum filtration, photolithography, nanoimprint‐lithography, drop casting, laser‐scribing, and roll‐to‐roll printing techniques, in which each method exhibits various advantages and disadvantages (Table  4 ). The specific details of technical approaches and recent trends in the fabrication of bioanalytical sensors are discussed in the following parts.

Table 4.

Pros and cons of various technical approaches in the fabrication of bioanalytical sensors.

Techniques	Pros	Cons
Vacuum filtration	–Fast and scalable production –Fabrication using various materials	–It is difficult to ensure homogeneous infiltration of fiber substrates and the resultant morphology. –Poor particle interconnections
Drop casting	–Simple, easy, freedom from complex equipment –Capacity to modify the substrate surface prior to electrode fabrication	–High cost –Control of homogeneity, the amount of nanomaterials, and nanomaterial/additive ratio in suspension –Expensive film deposition equipment
Spin‐coating	–Simple and easy process –Low cost –Fast‐drying times –Without the need for postdeposition heat treatment –Uniform film –The controllable thickness of the thin films	–Material wastage –Use of volatile solvents –Low performance for some particular nanotechnologies due to the fast‐drying times
Screen‐printing	–Low‐cost, simple, large‐scale fabrication –Inks of different materials –Flexibility in template design and size	–Use of templates or masks –Optimization of the ink composition
Inkjet‐printing	–High patternization capacity –Does not require predeposition or templates –Multiple materials can be printed simultaneously using multiple ink cartridges –Large‐scale production –Different substrates can be used –High reproducibility	–Nozzle clogging –High cost –Complex equipment –Need for viscosity and surface tension adjustment of the formulated ink
Laser‐scribing	–Large‐scale device production –Without masks –Fabrication of conductive materials without the use of reagents or synthesis process	–Reproducibility depends on the stability of the applied laser power. –Not compatible with all types of substrates
Photolithography	–High resolution of microfluidic channels –Sharp barriers	–Expensive equipment –Complex steps –Expensive reagents –Brittle and fragile
Nanoimprint‐lithography	–High resolution of nanofluidic channels –Sharp barriers	–Expensive equipment –Complex steps –Expensive reagents –Brittle and fragile
Roll‐to‐roll printing	–Allow direct R2R production in pre‐existing printing machinery –Avoid heat treatment of the printed patterns	–High cost –Complex reagents and templates –The surface requires frequent cleaning to avoid contamination –The quality depends on the smoothness of the substrate surface –Require various printing plates

4.1. Vacuum Filtration and Drop‐Casting Techniques

Vacuum filtration is one of the most common approaches in manufacturing bioanalytical sensors, especially for using 3D substrates; its advantages comprise scalable and fast electrode production and effective utilization of various functional materials. In particular, this technique is commonly used to fabricate electrodes containing 3D fiber‐based 3D substrates and carbonaceous materials. However, it still has two considerable drawbacks: i) these fibers on the paper substrates need to be uniformly filled with the active materials, regarding a coupled effect of filling density and electroactive area on the constructed electrodes, and ii) a mold, mask, or other equipment is essential for patterning the desired layout designed on the platform. For example, a graphene paper‐based device has been developed for pH measurements, in which the graphene film was created on the surface of the paper‐based substrate through the vacuum filtration technique (Figure  5A).^{[ 106 ]} More specifically, graphene material would be directly deposited on the paper substrate with a well‐defined geometry to realize a functional graphene sensor, meaning that a graphene suspension would be poured into the filtration vessel and uniformly filled in the pores of the paper substrate to produce a homogeneous graphene film (Figure 5B). With this setup layout, it enabled to simultaneously produce six sets of electrodes; besides, the hydrophobic barrier has been defined through using a paraffin pen leading to delimiting the detection zone of each electrode. In another work, a coupled AuNPs/SWCNTs hybrid material has also been designed to integrate on the mixed cellulose ester filter paper substrate through the vacuum filtration method,^{[ 107 ]} in which SWCNTs would be deposited first on this substrate, and next step would be the deposition of AuNPs. Interestingly, this new approach in the active materials and fabrication process could have the potential for the rapid manufacturing of hybrid films from various nanomaterials (i.e., carbonaceous and metallic nanomaterials) under ambient conditions; concomitantly, this could also be favorable for easily transferring the active films onto glass or other flexible substrates.

Figure 5.

A) Fabrication process of graphene‐based sensors filled on a paper substrate using the vacuum filtration method. Adapted with permission.^{[ 106 ]} Copyright 2016, Springer Nature. B) Morphological characterization of the graphene paper‐based sensor: (i,ii) SEM images of the sensor surface, indicating the graphene deposited on the paper substrate, as well as the graphene flakes homogenously stacked on the paper substrate with a distinct boundary, and iii) a digital capture of the sensor side view. Adapted with permission.^{[ 106 ]} Copyright 2016, Springer Nature. A schematic illustrations of thin‐film deposition by C) the drop casting and D) the spin‐coating processes.

Another technical approach, i.e., the drop‐casting method, is known as the deposition of active nanomaterials on the electrode surface, which is a simple film‐forming technique and does not further use any supportive equipment; further, the solvent containing the active nanomaterial solution will be evaporated after completing the drop‐casting step of the active nanomaterial on a substrate (Figure 5C). With the use of this technical approach, it is difficult to possess a uniform film and a controllable thickness of the thin films, while this technique is also similar to a spin‐coating method (Figure 5D). Among these technologies, the drop‐casting process does not require substrate spinning compared with the spin‐coating approach, in which a main advantage of the spin‐coating method is less material wastage, and the use of volatile solvents is often favorable for this technique. The film properties and thickness significantly depend on the concentration and volume of the active material dispersion; besides, some factors affect the film structure formation, which can be substrate wetting, evaporation rate, and drying process. With the drop‐casting technique, several aspects should be considered, which can consist of the amount and homogeneity, the ratio of nanomaterial to other compounds, and whether it is necessary to use a supportive mask to produce the desired patterns and control the deposition area. For example, a biosensor containing carbon ink has been modified with active AuNPs from small volumes of tetrachloroauric (III) acid solution,^{[ 108 ]} in which carbon ink suspension in dimethylformamide medium was deposited on a hydrophilic working area of the chromatographic paper substrate patterned with hydrophobic wax, using the drop‐casting method. Through this technical approach, the whole fabrication procedure without the use of a mask was fast and limited the active nanomaterial waste, and the entire area of both carbonaceous electrodes with close contact significantly improved the nanostructuration's precision. In another report on the drop‐casting process, a device containing active AuNPs and a wax‐patterned paper with dual detection was constructed on an office paper platform,^{[ 109 ]} in which electrical contacts were produced from the deposition of AuNPs suspension that was repeated twice through the drop‐casting approach, and the AuNPs‐contained working electrodes were then exposed to an infrared lamp for each repeated time.

4.2. Common Printing Techniques

For common printing techniques, screen printing refers to transferring a stenciled design on a flat surface with active ink, a supportive screen, and a squeegee. In particular, this technical approach can be employed to generate hydrophobic or hydrophilic zones on various substrates and fabricate functional electrodes. For example, a film‐type proximity sensor containing Ag electrodes on a polyethylene terephthalate (PET) film covered by a silicone blanket has been fabricated by using duplex conductive‐ink printing on a film substrate, i.e., screen‐printing approach,^{[ 110 ]} resulting in the fact that this thin and light device could be well mounted at different positions. In other reports, pH sensors containing a two‐electrode configuration of the PANI nanofiber array sensing electrode and the Ag/AgCl reference electrode have been well designed and fabricated using the screen‐printing process (Figure  6A);^{[ 111 ]} similarly, carbon electrodes based on this technique has also been constructed effectively, in which the conductive ink was used to stack layer‐by‐layer with the use of some supportive screen frames.^{[ 112 ]} In this technique, the selection of a suitable mold to be applied in the fabrication process of some active electrodes is truly important to obtain desirable efficiencies of the bioanalytical sensors. In particular, screens are typically materials that are made from solid or transparent films using hand or laser cutters, whereas active inks are almost carbonaceous materials and their derivatives.^{[ 113 ]} In order to produce new functional nanomaterials‐based inks, there are some requirements in the fabrication process of the functional nanomaterials regarding the limiting parameters, i.e., dispersion, viscosity, and agglomeration. So far, AuNPs can be the nanoparticles most concerned with the manufacture of potential metallic inks; concomitantly, other nanoparticle inks (i.e., Ag, Cu, Fe, and Pt) have also been presented.^{[ 114 ]} A main advantage of the screen printing method is the flexible approachability of functional materials. This technique can be applied in mass production due to easily or commercially available patterned screens, and this technique is truly necessary for the fabrication of active electrodes with reliable repeatability. Nevertheless, there are still some limitations regarding the use of active nanomaterial inks in the screen‐printing technique; e.g., the existence of organic solvents in the inks that can impact humidity and temperature, leading to varying the standardized viscosity and concentrations and at the same time that the technique probably shows high ink waste and imprecision because of a low contact between the active ink‐contained patterned screen and the substrate during fabrication process.

Figure 6.

A) A schematic illustration of the fabrication of pH sensors based on the screen‐printing process. Adapted with permission.^{[ 111 ]} Copyright 2019, Springer Nature. B) A schematic illustration of the inkjet‐printing technique. C) Fabrication process of laser‐induced graphene on a PI substrate using the laser‐scribing technique. Reproduced under the terms of the Creative Commons Attribution license 4.0 (CC‐BY). (https://creativecommons.org/licenses/by/4.0/).^{[ 117 ]} Copyright 2021, The Authors, American Chemical Society. D) Graphical step‐by‐step process in the photolithography technique using negative and positive photoresists. Adapted with permission.^{[ 118 ]} Copyright 2023, Elsevier.

A more advanced printing technique, i.e., inkjet printing, is a digital and noncontact type of printing technology that can be employed to fabricate bioelectronics using a commercial inkjet printer. More particularly, inkjet printing is a process in which a liquid phase of active inks is transferred onto a substrate, which means that it allows the release of small ink droplets from the nozzle only when officially required (Figure 6B). With this technical approach, it can contribute to lower the used material consumption and simplifying the processing stages, which can ensure the printing to be cost‐effective. Notably, the subsequence cleaning process in this advanced printing technique is much simpler because of washing solvents of acetone and ethanol replaced instead of strong acids and hazardous solvents. The ink will be jetted through a print head without coming in close contact with the substrate, so this can be favorable for printing the active materials onto various substrates, i.e., paper, silicone, PET, and polyimide (PI). Additionally, this printing technique can enable effective printing of functional inks at low temperatures and viscosity.^{[ 115 ]} In the case of high temperature applied in the printing process, the active materials relating to biological activity (i.e., proteins and nucleic acids) can be dramatically affected by their main function in biosensing ability. Of course, the case of low temperature conducted in the inkjet‐printing approach shows the significant printing flexibility of active ink formulation, including biologically originating active inks. For instance, stability of poly(amidoamine) (PAMAM) dendrimer functionalized magnetic Fe₃O₄ nanoparticles (PAMAM@Fe₃O₄) has been fabricated, characterized, and investigated three substrates (i.e., glass slide, silicon wafer, and graphene paper) using the inkjet‐printing method; besides, the addition of anti‐alpha‐fetoprotein as a model protein conjugated to the dendritic surface has been printed.^{[ 116 ]} From three various substrates used in the nanoparticle ink deposition process, the use of graphene paper showed a more uniform and smoother printed pattern than glass slide and silicon wafer. Thereby, surface energy and roughness of the substrate could play a significant role in the smooth and uniform adhesion of active nanoparticles, and nanoparticle ink suspension would be more stable with appropriate surface tension and viscosity. Concomitantly, the use of antibody‐conjugated nanoparticles to be applied in inkjet‐printing steps revealed the possible applicability of constructing a prototype biosensing device in the future.

4.3. Laser‐Scribing Technique

As is known, laser scribing is a technique that concentrates power at a specific point, enabling precise manipulation of surfaces at the nano–microscale without direct physical contact. This technology can minimize sample damage and create desired patterns with low power. In other words, laser scribing is considered a laser micromachining technique for scanning a laser beam across a substrate to yield scribe lines or blind cuts. Conductive nanomaterials (i.e., carbonaceous or metallic nanomaterials) are generally generated from a nonconductive nanomaterial substrate by using a supportive laser source; at the same time, nano/microsized patterns formed from this technique can improve a surficial contact between the device and human skin without causing discomfort. Interestingly, this technical approach can provide large‐scale device production without the use of supportive molds or masks, without requirements of environmental control or complex equipment, and at the same time, conductive nanomaterials can be favorably fabricated without the use of reagents, as well as an easy‐to‐use setup to change the electrode pattern generated on a substrate. Furthermore, 2D materials with thinner layers can also be fabricated through the laser‐scribing method, in which the laser etching process probably reduces thickness and enhances the conductive characteristics of 2D materials. The laser processing of 2D materials is a prevalent approach aiming to enhance the performances of the fabricated 2D materials, respectively. Particularly, recent advancements in graphene production and patterning, i.e., 2D graphenic materials in 3D superstructures, further increased potential applications;, e.g., laser‐induced graphene (as a 3D porous graphenic material) could be produced from directly laser‐scribing onto carbonaceous precursors to apply in applications of bioanalytical sensors (Figure 6C).^{[ 117 ]} In another report, a carbon paper was decorated with highly uniform NiNPs using the laser‐scribing process, and, for more details, the device was dipped into an Ag colloidal suspension to take up the catalyst agents and subsequently in a Ni‐contained bath for a surface metallization process; afterward, it was washed with acetone, ethanol, and deionized water to effectively release contaminants. As a result, three active electrodes were successfully scribed on the carbon paper with a laser beam,^{[ 119 ]} in which this constructed sensor used for glucose detection (human serum and perspiration samples) possessed a highly electrochemical activity, wide‐linear responses, and low detection limit (2 × 10⁻⁸ mol L⁻¹), this device could offer an excellent sensing platform using as flexible biosensors for tracking physical conditions.

4.4. Photolithography Technique

Photolithography is a standard microfabrication technique capable of patterning high‐resolution features and enabling large‐scale production with a wide selection of patternable materials ranging from metals to organic polymers. Furthermore, it emerges as an indispensable and invaluable tool for manufacturing multilayer structures with an extensive range of dimensions on both hard (i.e., Si wafer) and flexible (i.e., PET, PI, and PDMS) substrates. Moreover, the photolithography technique is built on the light‐sensitive property of the photoresists to transfer the master pattern onto the surface of the flat substrate via a precisely engineered stencil, referred to as a “mask.” In principle, the photoresist‐coated substrate is placed under the optically flat glass or quartz mask and then exposed to UV radiation. After the exposure step, the substrate is treated with a developing solution or a spray developer to transfer the pattern, and it can be either a positive or negative version of the designed mask depending on the type of photoresist used (Figure 6D).^{[ 118 ]} The process is often followed by metal deposition and the lift‐off process in which coated metal on top of the patterned resist can be removed by placing the whole substrate in a bath of solvent that dissolves the resist and leaving behind the metal that was deposited on the bare substrate surface.^{[ 118 ]} Recently, a high‐performance biosensing platform using an interdigitated electrode to detect prostate cancer biomarkers in physiological serum at an early stage was reported.^{[ 120 ]} A single‐masked photolithography process was used to fabricate the interdigitated electrodes by concomitantly spin‐coating a bilayer negative photoresist onto the Si or SiO₂ substrate and then exposed with a chrome mask consisting of interdigital fingers of 10 µm in width and 3500 µm in length with a spacing of 10 µm. Subsequently, the lift‐off process was performed after depositing Ti/Au bilayers (≈20–250 nm) onto the developed photoresists, leaving behind the patterned gold electrodes on the substrate. The SiO₂ gap in between the patterned Au triple microelectrodes was modified with a self‐assembled monolayer. A small potential of −75 mV was applied as a bias voltage for the electrode to repel the aptamer from the Au electrode to reduce the high background noise caused by electrostatic interaction when the negatively charged DNA binds to the Au electrodes. This work demonstrated a sensitive detection and quantification of varying concentrations of PSA antigen, an estimated detection limit of 0.51 ng mL⁻¹ in serum, and a good sensor's reproducibility.

Furthermore, to overcome current challenges in manufacturing Au/Si‐based electronic devices designed at ultramicro scale, i.e., possible damages of single‐band electrode regarding the resistance to the required conditions of the hydrogen bubble template, a proposal for increasing rough‐surface features of the ultramicro electrode in the manufacture of multiplexed biosensing platform has been introduced effectively, resulting in a superior sensitivity of 121 mA/(mA·cm²) and a wide linear range of 0.05 × 10⁻³–22.15 × 10⁻³ mol L.^{[ 121 ]} The devices were manufactured on the SiO₂/Si substrate by first using a metal mask‐1 to create a band sensing electrode pattern and followed by the Ti:Au (10:150 nm or 20:150 nm) metal deposition and lift‐off process. Then, the patterning process of the leads, contacts, counter, and reference electrode areas was carried out using a metal mask‐2, followed by the evaporation of Ti:Ni:Pt or Ti:Au with a lift‐off process. The third metal mask was used to fabricate sensing electrodes, leads, contacts, counter, and reference electrodes using two steps of Au deposition, similar to Ti, Ni, and Pt deposition and lift‐off processes. Afterward, a 500 nm thickness passivation layer of silicon nitride (Si₃N₄) was applied on the wafers, followed by using a passivation mask for photolithography and then etching the windows of six‐sensing electrodes, counter reference electrodes, and contacts. Finally, a protective photoresist layer was coated to cover all wafers, followed by a mechanical dicing step. The fabricated chip consists of six ultramicro single‐band gold sensing electrodes with a width of 1 µm and a length of 45 µm, one platinum reference electrode, and one platinum counter electrode. A 10 mL solution volume containing a miniaturized bubble template was applied in a three‐electrode system consisting of an Au sensing electrode, external Ag/AgCl reference electrode, and external Spiral‐Pt counter electrode. This developed process has paved the way for the fabrication of next‐generation stretchable skin electronic devices. In general, biodegradable sensors play a vital role in alleviating environmental concerns as an eco‐friendly alternative for bioelectronic and implantable medical devices. However, standard microfabrication processes such as photolithography, etching, and deposition are hindered due to the high damage susceptibility of biodegradable polymers to organic solvents, thermal loading, plasma environment, and UV light. Therefore, a high surface roughness, peeling, nonuniformity, and air bubbles within the layers lead to failures in the fabrication.^{[ 122 ]}

4.5. Nanoimprint‐Lithography Technique

The nanoimprint lithography (NIL) technique has emerged as a powerful patterning approach to overcome the practical limitations of the photolithographic method by providing a high‐resolution, high‐throughput, and low‐cost technique for replicating nanoscale patterns below 10 nm.^{[ 124 ]} It is considered a simple nanolithography process, which is due to the fabrication of patterns based on the mechanical deformation of imprint resist cured with supportive factors of heat or UV light during the imprinting process, i.e., thermal NIL and UV NIL (as the most common approaches in the nanoimprint‐lithography technique).^{[ 125 ]} Of these, the advanced UV NIL method at room temperature with a pressure of less than 15 psi demonstrates a 5 nm linewidth for single‐molecule contacts imprinted from a 4 in. SiO₂ mold with features as small as 5 nm.^{[ 124 ]} In contrast, a thermoplastic polymer resin was hardened at high temperatures during the thermal NIL process, which has been demonstrated as a high‐resolution fabrication technique to produce a pattern with 10 nm critical dimensions.^{[ 126 ]} The NIL technique has been widely used for patterning conductive materials with elaborate micro‐/nanostructures, contributing to the development of high‐performance bioelectronic devices. This technique has been considered as an alternative tool to conventional photolithography methods to pattern complex metal electrodes with high resolution and accuracy. For instance, 100 nm AgNWs‐contained transparent devices with different widths and heights using unconventional NIL method has been proposed,^{[ 127 ]} which were demonstrated for ammonia sensing and could be utilized for further biosensing and optical applications. Besides, the utilization of NIL technique in fabrication of conducting polymers is emerging, which is considered one of the most promising techniques for miniaturizing conductive polymer‐based bioelectronic devices, e.g., nanosized active electrodes have been fabricated to integrate onto nanostructured sensing arrays with glassy carbon through using the NIL process, resulting in a significant enhancement of electrochemical characterization in enzymatic redox mediators.^{[ 128 ]} Nanostructured electrodes with a new type of conducting nanopillar array made of multiwalled carbon nanotube doped polymeric nanocomposites (Figure  7A).^{[ 123 ]} First, the NIL process was applied with a PDMS nanowell‐array stamp to fabricate the PU nanopillar‐array template. Then, a perfluoropolyether (PFPE) nanohole array template was obtained by applying NIL for 2 min under a pressure of 0.5 bar with the PU nanopillar array mold treated by TFOCS. Finally, the conducting nanopillar arrays were fabricated on both rigid Si substrates and flexible PET films by imprinting the fabricated PFPE template with uncured MWCNT/NOA83H mixtures. The MWCNT/polymer nanopillar arrays fabricated by scalable NIL have shown outstanding electrical properties with high DC conductivity (≈4 S m⁻¹) and low AC electrochemical impedance (≈104 Ω at 1000 Hz), which pave the way for the development of bioelectrical recording and electrochemical sensing via flexible implantable biosensors, wearable sensing devices, and low‐cost disposable biochips.

Figure 7.

A) A schematic illustration of the technical UV NIL in the fabrication of a nanopillar array‐based device: i) graphical step‐by‐step process of an MWCNT/polymer nanopillar array using the UV NIL approach, ii) digital image of the fabricated (top surface) on a Si substrate, iii) Digital capture of the bent device on a flexible PET substrate, and iv) SEM images of the conductive MWCNT/polymer nanopillar array. Reproduced under the terms of the Creative Commons Attribution‐Noncommercial 3.0 Unported License (CC BY‐NC 3.0) (https://creativecommons.org/licenses/by‐nc/3.0/).^{[ 123 ]} Copyright 2021, Royal Society of Chemistry. B) Graphical principles of the R2R printing technique, including i) gravure printing, ii) flexographic printing, and iii) rotary screen‐printing processes.

In terms of diagnostic devices, Liang et al. developed label‐free plasmonic metasensors fabricated on a wafer‐scale by nanoimprint lithography for ultrasensitive detection of early prostate cancer.^{[ 129 ]} The developed device was performed for the joint detection of serum exosomes and PSA, considering their coexistence in male blood circulation. The metasurface fabrication process is mainly based on NIL and etching process. First, the nanopillar array of the PDMS stamp was obtained by imprinting with a 4 in. Si nanohole array template. The polymer resisted with a thickness of ≈250 nm and was then spin‐coated on a 4 in. Si wafer to concomitantly imprint with the fabricated PDMS stamp through the UV NIL method. The fabricated nanohole array 4 in. wafer was etched with oxygen plasma and followed by the subsequent etching process with SF6 and C4F8 gas. Finally, a magnetron sputtering system was employed to deposit a chromium adhesion layer, followed by an Au layer. This electronic device achieved an ultrahigh sensitivity of 92.3% compared to 58.3% of conventional PSA tests. Additionally, a portable PSA/exosome metasensing system has also been demonstrated, consisting of a biochip integrated biochip, back‐end software, and smart phone with a diagnostic application, which could enable prostate cancer screening for future mobile‐based healthcare monitoring systems. Furthermore, the use of the NIL technique can be a promising tool for patterning cellulose‐based materials, paving the way for a new eco‐friendly bioelectronic device that is likely to overcome the sustainability challenges of modern society.

4.6. Roll‐to‐Roll Printing Technique

Utilizing printing techniques for simultaneous deposition and patterning, roll‐to‐roll (R2R) printing techniques have been developed to mass‐produce printed electronics and bioactive products. The use of the R2R printing technique can fabricate various patterns on the substrates through the pressure forces between the printing rollers, which can be classified into three main methods such as gravure printing, flexographic printing, and rotary screen printing (Figure 7B), which depend on different purposes regarding the viscosity of active ink, processing speeds, and production volumes.

Among them, the gravure printing method is one of the commonly used R2R printing techniques (Figure 7B), which is applied not only for patterning devices but also in our daily lives for printing magazines and high‐volume print runs like catalogs. The gravure printing mechanism is based on the transfer of active ink from a gravure roller patterned with tiny, engraved cavities to the web, which is pressed between the impression roller and gravure roller. The size and shape of printed patterns depend on the tiny cavities engraved on the gravure roller that is continuously filled by the ink bath with a doctor's blade to remove the excess ink. This method can be applied to print with low viscous active inks at a high speed of >15 m s⁻¹, and the printing quality is highly dependent on the printing speed, pressure contact, and ink rheology. Concomitantly, this R2R gravure printing process has been widely used for the mass production of highly conductive patterns on a flexible substrate. For example, the utilization of the R2R gravure printing process has been successfully reported in a mass production of electrochemical sensors on a 150 m flexible PET roll, which was demonstrated to monitor pH values in human sweat during exercise period^{[ 130 ]} (Figure  8A). Three layers of ink including silver, carbon, and insulation layers are deposited for printing electrode arrays on flexible PET substrates with 250 mm wide and 100 µm thick by employing a high throughput the R2R gravure printing system. Two printing units were combined with a custom servomechanism system with a web tension of 5 ± 0.3 kgF for continuous printing. An overlay printing registration accuracy of ±20 µm was maintained during the printing process at room temperature and a humidity of 35% ± 2%. The printed Ag electrodes were passed through the drying chamber at a speed of 0.1 m s⁻¹. The working and counter electrodes were fabricated by printing a carbon layer on top of the printed Ag and then drying using the same strategy. The printed Ag and carbon inks were further cured by rewinding the printed roll to pass through the heating chamber at a speed of 2 m min⁻¹. Finally, the insulator layer was printed on the rewound PET roll at the same speed of 6 m min⁻¹. These demonstrated the high‐throughput capability of our R2R gravure system by completing the three‐layer printing and curing of 150 000 electrodes on a 150 m PET web with a total printing time of 30 min. The printed electrodes on a 150 m roll of PET substrates can be functionalized into sensors for various sensing applications. Custom printed circuit boards can be integrated with our printed sensors as a smart wristband for on‐site real‐time monitoring of analyte profiles in sweat during exercise.

Figure 8.

A) Schematic illustrations and a step‐by‐step graphical process of the R2R gravure printing system for the high‐throughput and low‐cost fabrication of biocompatible electrode array by printing three layers of ink, including silver, carbon, and insulation layers. Adapted with permission.^{[ 130 ]} Copyright 2018, American Chemical Society. B) A schematic illustration of the R2R rotary screen printing system to print Ag paste on PDMS substrate. Reproduced under the terms of the Creative Commons Attribution license (CC‐BY) (https://creativecommons.org/licenses/by/4.0/).^{[ 131 ]} Copyright 2019, American Chemical Society.

Additionally, the R2R flexographic printing method is different from the R2R gravure printing process mainly in the mechanism of active ink transfer that is performed from relief as opposed to engraved cavities (Figure 7B). The final printed patterns come out of a printing plate, which is usually made from a photopolymer or rubber. The R2R flexographic printing system typically consists of a fountain roller to continuously transfer ink to the ceramic anilox roller, which contains engraved cells/microcavities embedded on the exterior. The doctor blade is used to remove excess ink from the anilox roller. The web is pressed between the flexographic plate, and an impression roller enables the final ink transfer to the substrate. The production cost of R2R flexographic printing is significantly lower than that of gravure printing due to the usage of flexible polymer wrapped around a metal cylinder to function as a printing roller. However, plate deformation is the main challenge of this technique in achieving high‐resolution printing patterns. In order to overcome this issue, an engineered nanoporous stamp has been developed to replace conventional elastomeric stamps with carefully controlled porosity, mechanics, and surface chemistry.^{[ 132 ]} Recently, a flexible pH sensor has been fabricated using the R2R flexographic printing process.^{[ 133 ]} First, the precursor solution was prepared by mixing iridium powder with ethyl alcohol and acetic acid. Metal layers of gold and copper were deposited on the flexible PI with a thickness of 90 and 18 nm, respectively, in which a thick carrier foil was used to improve the registration accuracy on the flexible polyimide. The anilox roller was designed with a cell transfer volume of 5 mL m⁻² with a printing speed of 5 m min⁻¹ for the uniformity of the layer. The utilization of a plate size of 25 × 240 mm² enables printing 480 electrodes per sheet with an electrode size of 2 × 15 mm². The R2R flexographic printing methods developed pave the way for a sustainable, cost‐effective, and scalable manufacturing method for disposable and integrated sensors.

In addition to the abovementioned R2R printing approaches, the R2R rotary screen‐printing process (Figure 7B) allows the formation of very thick wet layers and, subsequently, very thick dry films, which can be useful for printed electrodes that require high conductivity. A major advantage of screen printing is that it is highly suitable for R2R manufacturing and high‐throughput processing. In rotary screen printing, the ink is contained within the rotating cylinder with a fixed internal squeegee, which is less exposed to the environment. Since it is a true R2R printing method, rotary screen printing is far superior to flat‐bed screen printing by at least an order of magnitude in speed, edge definition, resolution, and wet thickness (>300 µm). High‐throughput sweat sensor fabrication has been conducted by integrating R2R rotary screen printing of sensing electronics with R2R laser cutting of microfluidic channels.^{[ 134 ]} The sensing layer consists of conductive silver‐based electrodes with a graphite‐based ink‐modified active areas to improve electrochemical sensing. In order to measure sweat rate impedimetrically, parallel spiraling silver lines are placed alongside double‐dielectric polymer layers to insulate the electrode paths and prevent short circuits. Coarser screens with higher ink transfer but poorer resolution were employed to obtain reliable and good ink transfer. Overall, this layer‐by‐layer printed sensing structure enables versatile designs, including spiral and serpentine patterns that are printed together at 60 devices per minute on a 100 m web length. The microfluidic cover sheet and adhesive layer are laser cut to create channels for sweat flow. The tight sealing of the microfluidic channel is performed simply by laminating the double‐sided microfluidic adhesive between the cover sheet and the sensing layer, which allows rapid and uniform printing and assembling of the device layers. Furthermore, using electrodes deliberately positioned within a sweat collection reservoir and microfluidic channel, the assembled patch enables simultaneous electrochemical measurement of sweat analytes and electrical measurement of sweat rate. The wearable sweat patch was used to monitor various body parts to study regional variations in sweat composition and rate. In another report, a mass production of printed Ag conductors used as stretchable electronics has been fabricated on a flexible PDMS substrate using the R2R rotary screen printing method^{[ 131 ]} (Figure 8B). The 100 µm thick PDMS film on a PET carrier was used in the R2R rotary screen‐printing process. The printed traces on PDMS can be detached from the carrier web once the print has fully cured, resulting in a stretched substrate. This finding demonstrated that the feasibility of using mass‐manufacturing methods to produce electrical wiring on highly stretchable and versatile PDMS substrates enables the upscaling of the fabrication of PDMS‐based stretchable electronics. The developed methodology is versatile and can be widely employed to deposit typical materials of electrochemical sensors, such as nanoparticles, polymers, and carbon nanomaterials. Overall, many technical approaches are applied in fabricating step‐by‐step routes of biosensing devices, including traditional and advanced technologies. Depending on research purposes, trends and advances in ideal designs of bioanalytical sensors will be developed appropriately to produce high‐performance bioanalytical sensors.

5. Established System of Smart Monitoring and Prediction Analysis

As is well known, the establishment of smart biosensing devices is increasingly of interest, which can also be considered artificial intelligence biosensors. These smart biosensors are applied to aim for the best precision medicine treatment for individual patients; at the same time, using smart biosensing devices probably allows better patient data acquisition. At present, the appearance of the Internet of Things and big data indicate the usage of bioelectronics devices offers new potential and challenges. Besides terms in material innovation, manufacturing process, biorecognition element, transportation, and signal acquisition, the data processing and smart decision system are also the important factors. Especially, machine learning algorithms have been recently incorporated into biosensors for better data analysis. Smart biosensing devices are worth only when people can utilize data, and understanding in detail what the data means is a prerequisite for utilizing them, i.e., artificial intelligence data. Specifically, this data processing involves learning from the data, which typically composes how to analyze data suitably, offer the proper conclusions, and then identify if the data has been interpreted wrongly. From that, the most common approach, i.e., machine learning, is recommended for the construction of smart health monitoring systems of biosensing devices; this approach exhibits two key sides: i) machine learning probably cuts the data quantity before wireless data transmission to reach ultralow‐powered smart biosensing devices, and ii) machine learning probably solves the data quality issue that consists of consistency of the data, reliability, and accuracy of the monitoring process. Furthermore, this approach consists of evaluating various techniques, methods, and algorithms to learn from the data and offer utility information or predictive models of a phenomenon. Particularly, its widely used paradigms are mainly divided into three types, i.e., supervised learning for labeled observation (i.e., support vector machines (SVMs), decision trees (DTs)), unsupervised learning for unlabeled observation (i.e., principal components analysis (PCA), hierarchical cluster analysis (HCA)), and reinforcement learning for models that learn from the errors to improve accuracy (i.e., Gaming Artificial Intelligence, ChatGPT, Robot Navigation) (Figure  9 ). Among these, both the supervised and unsupervised learning methods have been significantly concerned with design and construction of smart health monitoring of biosensing devices in recent years.

Figure 9.

A graphical illustration of the typical classifications of various machine learning algorithms.

For the supervised learning approaches, SVMs are known as a recent trend of supervised machine learning algorithms to determine the hyperplane optimally separated among two data classes for resolving the identification issue of a two‐class pattern, from which the data can be confirmed through increasing the prediction analysis precision. For example, the use of SVM has been mentioned to evaluate the precision of sensitive detection of glucose levels through volatile organic compounds in breaths,^{[ 135 ]} in which a prediction analysis has been conducted through the characteristic parameters of a coupling assistant of smartphone camera and linear SVM algorithms. Additionally, specific parameters related to the biomolecular sensing behaviors from the effective use of active graphene‐contained nanoelectronics were also analyzed with integrated SVM algorithms to offer the highest precision of monitoring analysis.^{[ 136 ]} Other paradigms, i.e., DTs, are also a common classifier in the supervised learning method and are a supportive tool to effectively resolve the complex features of a big picture. For instance, a prediction analysis regarding papain in a nanosized cubic space existing on a Si‐wafer surface was established with an identified assistant of the DT classifier. This 4‐Si‐surfaces‐contained space was generated to capture free papain; at the same time, the use of tetrapeptides to evaluate with various numbers of atoms was also performed when it was linked to the papain, where the use of DT classifier to favorably create from 18‐tetrapeptides was also suggested to analyze a complex behavior of the papain, resulting that the 12‐tetrapeptides were confirmed for effectively fitting the active sites.^{[ 137} , ^{138 ]}

For unsupervised learning approaches, PCA is commonly used to reduce the degree of dimensionality by replacing available variables with the principal elements,^{[ 139 ]} especially for the extraction of vibrational data from the sensitive detection of glucose and fructose levels using as‐fabricated biosensors with support of surface‐enhanced infrared absorption.^{[ 140 ]} Furthermore, the use of a PCA classifier to separate the spectral features of measured surface‐enhanced resonance Raman scattering into two distinct sets with small similarities for biosensitive detection of nasopharyngeal cancer‐related markers in the nasopharyngeal.^{[ 141 ]} Another unsupervised clustering method, i.e., HCA, is also used to create a hierarchical structure to classify close objects into the same cluster within a data set. More details, this HCA approach has been effectively introduced to determine medical chemistry's classifications by using a bacteriophage‐based colorimetric sensor array.^{[ 142 ]}

To date, deep learning has been considered a potential research direction in machine learning methods, in which it is submachine learning from the human brain for utilizing multilayered neural networks with data models. Of these, artificial neural networks (ANNs) are known as a branch of artificial intelligence that probably extends in the possible simulation of the human brain and are considered as nonlinear models for clarifying types of complex relationships,^{[ 139 ]} for instance, an ANN algorithm has been established to handle complex physical/chemical characterization through distinct points for physiological monitoring.^{[ 143 ]} Meanwhile, convolutional neural networks (CNNs) are a type of neural network that probably uses filters and clustered layers in established networks and which is used with a large enough size of the data set and the modeling images,^{[ 144 ]} e.g., an integration of CNNs and biosensing devices (i.e., femtomolar‐level protein biomarkers) was proposed to determine and quantify the microbubbles in the images by using an assistant of a smartphone camera.^{[ 145 ]}

Additionally, smartphone‐based biosensing systems have attracted particular attention owing to the available conveniences of using smartphones. Smartphone‐based monitoring systems play an important role in establishing smart biosensing devices regarding data processing, storage, sharing, and interaction with the cloud; of course, these are thanks to the effective integrations of various devices and processing and communication functions. Typically, these smart monitoring systems comprise additional biosensing devices and a smartphone, which can be various combinations of Bluetooth, cameras, USB, audio port, fluorescence, electrochemistry, and plasmon resonance to increase accessibility, control the sensitive detection process and then receive the detection data.^{[ 146 ]} Moreover, in wireless data communications, it can be applied to pass the essential information among the biosensing devices and smartphone‐based monitoring systems, in which the wireless technologies often are supportive tools of near‐field communication (NFC), radio‐frequency identification (RFID), and Bluetooth. NFC has been commonly used in smartphones until now as a low‐cost wireless technology in which the NFC system will be coupled with a smartphone‐based reader and a passive NFC tag‐based responder;^{[ 147 ]} normally, miniaturized commercial NFC tags have been well integrated into a circuit board.^{[ 148 ]} Meanwhile, RFID tags are probably applied to wireless data communications among biosensing devices and computers/smartphones.^{[ 149 ]} Bluetooth is currently a potential candidate for artificial intelligence‐biosensor networks owing to its appropriate technical features and highly commercial prospects.^{[ 150 ]} Notably, specific functions of both smartphones and computers are almost the same, i.e., data acquisition, processing, and decision tasks; however, the use of smartphones‐based platforms is as an entertainment device with some possible limitations, as well as the specific functionality of each app in smartphones‐based platforms is independent of each other. Generally, smart monitoring and prediction analysis of the biosensing information processing shows importance of smart biosensors nowadays; thus, the combination of artificial intelligences and biosensing devices has opened advanced trends of smart biosensors, which are well used in personalized medicine, disease management, routine health monitoring, and disease treatment for individuals.^{[ 151 ]}

6. Recent Applications of Bioanalytical Sensors

Biosensors are used as very precise analytical devices and are appropriate for sensitively detecting chemical and biological analytes. They are based on the transformation of a chemical or biological reaction into a measurable signal, meaning that the biorecognition element (i.e., antibodies, enzymes, and genetic materials) will offer a specific binding or catalytic reaction to the analyte and then be directly transformed to a measurable signal through the use of the transducer on its surface. More details, some various applications of bioanalytical sensors in biological, clinical, and medical diagnostics, i.e., proteins, DNA, cholesterol, dopamine, cancer markers, diabetes, leukemic disease, myocardial infarction, dengue fever, and H1N1 influenza, will be specifically presented in the following sections.

6.1. Cancer Diseases

Cancer disease is the most common cause of death; in the approach to cancer diagnosis, it is considered a key to enhancing survival and looking for appropriate and effective treatments for patients. Therefore, detecting cancer‐related markers (i.e., cancer cells, cancer biomarkers, or carcinogenic biomarkers) should be rapid, ultrasensitive, and accurate, which are important requirements in developing bioanalytical electronic devices. For example, a cancer biosensor containing four working electrodes has been proposed,^{[ 152 ]} in which AuNP was used to coat onto a cellulose fiber‐based paper and fabricate a 3D macroporous Au‐paper electrode that served as a working electrode. This Au‐paper electrode indicated a significant enhancement of cancer cell capture capacity (i.e., human acute promyelocytic leukemia cells) and good biocompatibility. Sensitive detection of cancer cells achieved a wide linear range of 5.0 × 10² to 7.5 × 10⁷ cells mL⁻¹ with a detection limit of 350 cells mL⁻¹ to capture cancer cells, which showed good stability and reproducibility. Furthermore, a trimetallic dendritic Au @ PdPt NP hybrid material was applied in the efficient fabrication of a sandwich sensor for the detection of K‐562 cells (i.e., human chronic myelogenous leukemia),^{[ 153 ]} where the Au@PdPt NPs catalyzed an H₂O₂ reduction with the use of electron mediator (thionine) and amplified the electrochemical signals, resulting in a more robust catalytic activity, a wide detection linear range of 1.0 × 10² to 2.0 × 10⁷ cells mL⁻¹ and a detection limit of 31 cell mL⁻¹. More details in the K‐562 cell capture process, the Au@PtPd NPs surface of Au @ PtPd NP was reacted with folic acid through a “click” reaction, while this acid was added to the cell surface's folate receptor with high affinity, resulting that K‐562 cells were probably detected on the Au@ PtPd NP functionalized with folic acid.

Considering the detection of cancer biomarkers, numerous biosensors have also been exploited and developed in recent years. Important requirements composing simple, rapid, and sensitive detection of cancer biomarkers are greatly concerned with designing and constructing potential biosensors applied in clinical cancer screening and early diagnostic applications. For example, a new 3D microfluidic origami multiplex immunosensing device has been developed for the parallel detection of carcinoembryonic antigen and alpha‐fetoprotein (i.e., cancer biomarkers),^{[ 154 ]} in which a novel nanoporous Ag‐paper electrode and various metal‐ion‐functionalized nanoporous Au–CTS were used as a sensor platform and a tracer, respectively. As a result, the relative standard deviation for the simultaneous detection of 5.0 ng mL⁻¹ biomarker antigens was 2.62% (carcinoembryonic antigen) and 2.25% (alpha‐fetoprotein), indicating good precision and reproducibility. At the same time, a sensitive origami dual‐analyte immunodevice based on PANI–AuNPs electrode and multilabeled 3D graphene nanosheets was also proposed for quantification of both the abovementioned cancer biomarkers^{[ 155 ]} at where aniline electropolymerization was deposited on the AuNPs electrode using as a sensor platform for antibody capture, resulting that this biosensor reached a detection limit of 0.5 pg mL⁻¹ (carcinoembryonic antigen) and 0.8 pg mL⁻¹ (alpha‐fetoprotein). More notably, this 3D electrode has also been further modified from Au–core–shell–Pt NPs to improve their conductivity and surface area, resulting in the highly sensitive determination of carcinoembryonic antigen with a detection limit of 0.4 pg mL⁻¹ and a linear range of 1.0 pg mL⁻¹ to 100 ng mL⁻¹. In other reports, new 3D electrochemical origami immuno‐devices have also been developed through modifications of the cuboid Ag‐paper electrode and nanoporous Ag‐CTS^{[ 156 ]} and nanorod Au‐paper electrode and metal ion coated Au/bovine serum albumin nanospheres,^{[ 157 ]} which were used for sensitive determination of cancer and carcinoembryonic antigens (i.e., cancer antigen 125, carcinoma antigen 199). As a result, the detection limits of cancer antigen 125 and carcinoma antigen 199 were corresponding to 0.08 and 0.10 mU mL⁻¹ using the cuboid Ag‐contained biosensor,^{[ 156 ]} at the same time that the numerous metal ions added to the nanoporous Ag‐CTS significantly amplified the detection signals and the cuboid Ag‐paper electrode with good biocompatibility maintained good stability for the sandwich‐type immunoassay. Meanwhile, the detection limits of the carcinoembryonic antigen and cancer antigen 125 were found, respectively, to be 0.08 pg mL⁻¹ and 0.06 mU mL⁻¹ using the Au‐contained device.^{[ 157 ]} Specifically, the use of the Au‐based electrode in the 3D electrochemical origami immunodevice not only provided a biocompatible platform for the immobilization of antibodies but also amplified the metal ions’ electrochemical signal of metal ions (i.e., Pb²⁺, Cd²⁺). Additionally, another biosensor was also introduced to detect prostate‐protein antigen (i.e., a prostatic cancer biomarker),^{[ 158 ]} in which AuNPs were grown on a freestanding 3D origami device via a screen‐printed technique and manganese oxide (MnO₂) nanowires were electrodeposited on the AuNP‐contained working electrode to catalyze the decomposition of hydrogen peroxide decomposition. This biosensor obtained showed an efficient quantification of prostate‐protein antigen with a detection limit of 0.0012 ng mL⁻¹, which mainly involved enzymatic label redox cycling (i.e., glucose oxidase–an enzyme label, 3,3′,5,5′‐tetramethylbenzidine–a redox terminator, and glucose–an enzyme substrate). Additionally, current approaches of enzyme‐free immunosensors have also developed as an alternative to probably detect cancer biomarkers; for example, a novel enzyme‐free electrochemical immunosensor containing ZnO nanorods on a modified rGO electrode has been designed for sensitive determination of human chorionic gonadotropin, prostate‐protein antigen, and carcinoembryonic antigen,^{[ 159 ]} in which ZnO nanorods was used to offer numerous binding sites for the capture probes. The use of rGO was considered as an enhancement regarding the electron transfer rate. This biosensor could be used as a promising candidate for clinical applications because it exhibited outstanding precision, high sensitivity, and good stability and a wide detection range of 0.001 to 100 ng mL⁻¹ for carcinoembryonic antigen, 0.001 to 110 ng mL⁻¹ for PSA marker and 0.002–120 mIU mL⁻¹ for human chorionic gonadotropin. In other works, AgNPs were combined with porous ZnO spheres to fabricate active nanocomposites labeled on an Au nanorod paper electrode for selective determination of prostate‐protein antigen, using as an enzyme‐free immunosensor,^{[ 160 ]} while rGO could be coupled with PEDOT:PSS to form an active composite for selective determination of carcinoembryonic antigen (i.e., one of the principal markers of lung cancer).^{[ 161 ]} With flexibility in the use of functional materials, these constructed biosensors showed high sensitivity and detection limit performances toward prostate‐protein antigen and carcinoembryonic antigen.

6.2. Alzheimer's Disease

Alzheimer's disease is a neurodegenerative disorder that mainly affects the elderly population and is characterized by an irreversible and progressive loss of selectively vulnerable populations of neurons.^{[ 162} , ^{163 ]} Because there is no certain treatment after the lesion has progressed in the late stage, early‐stage diagnosis of the disease is essential in monitoring the first symptoms of the disease that will be a major discovery from the therapeutic and prevention viewpoints; however, there are still restrictions in early diagnosis of this disease. Thus, bioanalytical electronic devices can represent a promising candidate for biological, clinical, and medical diagnosis techniques, especially low‐cost test approaches that are suitable for diagnosing all stages of Alzheimer's disease. Recently, the diagnostic criteria for Alzheimer's disease can be based on the detection of biomarkers in cerebrospinal fluid (Figure  10 ), which is made up of amyloid‐β (A), tau protein (T), and neurodegeneration (N) biomarkers. Of these, A (i.e., Aβ‐40 and Aβ‐42), and T (i.e., P‐tau 181 and T‐tau) play an important role in the early diagnosis of the disease concerning the initial progress of the disease. Furthermore, the demand for multiplex detection platforms in body fluids for these biomarkers is increasing because the combined analysis of these biomarkers becomes important in the concomitant diagnosis instead of their single analysis.^{[ 164 ]}

Figure 10.

Graphical illustration of the most common biomarkers related to Alzheimer's disease.

For example, electrochemical detection of Aβ protein by delaminated titanium carbide with molecularly imprinted polymer was successfully proposed, using it as a glass carbon electrode;^{[ 165 ]} at the same time, another innovative paper‐based platform was also prepared to carry molecularly imprinted polymer.^{[ 166 ]} Among these biodevices, Aβ‐42 imprinted biosensor with the glass carbon electrode has a much lower detection limit and better sensitivity, while the construction of the paper platform‐based biosensor was unique in its simplicity and low cost. Furthermore, common biorecognition elements can be mentioned for antibodies because biosensor use can strongly connect the affinity of the antibody–antigen reaction with the electrochemical techniques. In recent reports, the Au electrode has been modified by self‐assembled monolayers of mercaptopropionic acid, AuNP, and antibody mAb DE2B4 for the Aβ(1–42) biomarker's determination of the biomarker A (1–42) (i.e., a detection limit of 5.2 pg mL⁻¹),^{[ 167 ]} and a screen‐printed electrode with a dual layer of graphene–rGO has also been designed to immobilize the H31L21 antibody for the Aβ(1–42) biomarker's rapid analysis (i.e., a detection limit of 2.398 × 10⁻¹² mol L⁻¹).^{[ 168 ]} Additionally, biosensors have also been significantly developed to detect the T biomarker, which is based on biorecognition elements of antibodies. For example, an Au electrode has been simply designed and covered with antitau antibodies,^{[ 169 ]} while a more complex ITO electrode coated by PET substrate, in which rGO/AuNP nanocomposite was used to bind antibodies (Figure  11A).^{[ 170 ]} In another report, a four‐electrode system of Au microband electrodes was prepared and covered with a self‐assembled monolayer and protein G, at which the protein G would interact with immobilized antibodies to ensure their most favorable orientation (Figure 11B).^{[ 171 ]} The use of a nanocomposite structure in the biosensor construction indicated a significant increase in the sensitivity and detection limit of the biosensor. In particular, a combination of aptamer and antibodies was also designed and used in the construction of sandwich‐typed assay construction, in which the Au electrode was utilized to act as a cysteamine‐stabilized AuNP carrier coated by biorecognition elements.^{[ 172 ]} The tau protein was determined by an Au electrode coated with a self‐assembled monolayer of 3‐mercaptopropionic acid with immobilized anti‐T‐tau antibodies, at the same time that a screen‐printed carbon electrode with AuNPs–PAMAM dendrimer nanocomposite and antitau capture antibody was designed for the detection of tau proteins.^{[ 173} , ^{174 ]} More details on the biosensors for A and T biomarkers’ detection are listed in Table  5 .

Figure 11.

A) A schematic illustration of immobilization steps for the antitau antibody. Adapted with permission.^{[ 170 ]} Copyright 2020, Elsevier. B) Details of Au microband electrodes and schematic side view of the biosensor. Adapted with permission.^{[ 171 ]} Copyright 2017, Elsevier. C) A schematic illustration of the fabrication process of a biosensing device, including two symmetric dumbbell shapes to contain blood separation zones by using the wax dipping technique. Adapted with permission.^{[ 177 ]} Copyright 2013, Elsevier. D) A schematic representation of i) an electrochemical immunoassay and ii) an electrochemical lateral‐flow immunostrip. Adapted with permission.^{[ 180 ]} Copyright 2014, Royal Society of Chemistry.

Table 5.

Summary of bioanalytical sensors for the detection of amyloid‐β and tau‐protein biomarkers.

Biomarkers	Construction	Detection limit [pm]	Linear range [pm]
Aβ‐42	Fern leaf‐like gold nanostructure with an RNA aptamer[ 175 ]	88.6 × 10−3	0.440–285
	GCE with titanium carbide MXene and MWCNT composite, including MIP[ 165 ]	6.65 × 10−5	2.20 × 10−4 – 2.20 × 10−2
	A dual layer of graphene and rGO with immobilized H31L21 antibody was achieved through Pyr‐NHS[ 168 ]	2.40	11.0–55.0 × 103
	MIP in a paper‐based platform on the carbon ink electrode's surface[ 166 ]	14.8	2.20–2.20 × 104
	Electrode Au with mercaptopropionic acid SAM, AuNPs, and monoclonal antibody mAb DE2B4[ 167 ]	1.15	2.20–2.20 × 102
Aβ‐O	PPy‐3‐COOH was electropolymerized onto gold dendrite with bounded cellular prion protein[ 176 ]	1 × 10−6	10−6–10 × 103
T‐tau	SAM of MPA‐binding anti‐T‐tau antibody on the Au electrode[ 174 ]	–	–
Tau‐441	Antitau antibodies immobilized on an Au electrode[ 169 ]	106–103	–
	ITO‐coated PET electrode with rGO–AuNPs nanocomposite and antitau antibodies[ 170 ]	0.002	2.20 × 10−2–10.9
	Four Au microband electrodes with a layer of SAM, protein G, and antitau antibodies[ 171 ]	0.030	–
Tau‐381	Cysteamine‐stabilized AuNPs with antitau antibody and an aptamer specific to tau‐381[ 172 ]	0.420	0.500–1.00 × 102
Tau	SPCE modified with an AuNPs–PAMAM dendrimer nanocomposite and antitau capture antibody[ 173 ]	0.030	0.110–91.0

*Aβ‐O: Aβ oligomers; PPy‐3‐COOH: poly(pyrrole‐3‐carboxylic acid); SAM: self‐assembled monolayer; GCE: glassy carbon electrode; MWCNTs: multiwalled carbon nanotubes; MIP: molecularly imprinted polymers; rGO: reduced graphene oxide; Pyr‐NHS: 1‐pyrenebutyric acid N‐hydroxysuccinimide ester; MPA: 3‐mercaptopropionic acid; AuNPs: gold nanoparticles; SPCEs: screen‐printed carbon electrode; PAMAM: poly(amidoamine); ITO: indium tin oxide; PET: polyethylene terephthalate.

6.3. Diabetes and Leukemic Diseases

Diabetes, known as diabetes mellitus, is a disorder in blood glucose levels in which the human body is not able to normally respond to insulin, leading to abnormally high glucose levels becoming abnormally high. Conversely, it is concerned with a group of metabolic diseases in which blood glucose levels are abnormally high during a long period. To date, various bioanalytical electronic devices have been exploited and applied efficiently for the determination of glucose levels; for example, a bioanalytical device (Ag/AgCl reference electrode) based on a paper substrate and integrated plasma isolation has been developed for the determination of blood glucose levels, in which this device composed of two symmetric dumbbell shapes to contain blood separation zones (Figure 11C).^{[ 177 ]} The separated glucose in the integrated plasma isolation was detected by immobilizing glucose oxidase in the middle zone of this bioanalytical device (i.e., this detection zone was designed between two symmetric dumbbell shapes, which was known as the middle zone and made using the wax‐dipping method). Then, the reaction of glucose and enzyme occurred to form H₂O₂ and showed a modified Prussian Blue modified screen‐printed electrode. The blood glucose level was in a range of 0–33.1 × 10⁻³ mol L⁻¹; further, this designed device showed efficient flow maintenance, and it was also directly to healthy and diabetes volunteers. Instead of determining glucose levels in blood samples, using bioanalytical devices for urine samples has also been required; in fact, the human body can eliminate glucose through urine when blood glucose levels are too high. For example, a biosensor (transparent ITO electrodes) has been proposed for urine samples and resulted in a color‐related change (from blue to colorless), i.e., an electrochromic reading.^{[ 178 ]} This device acts as an electrolyte for the integrated metal/air battery powers, in which carbon is used as the interface for the cathode and aluminum foil is the anode. The glucose compound in the urine would react with the glucose oxidase‐impregnated electrode, leading to the color‐related signals in the rest of the Prussian Blue‐impregnated electrode, i.e., from blue to colorless. As a result, the detection limit was at 0.1 × 10⁻³ mol L⁻¹ glucose and H₂O₂ in artificial urine samples, revealing potentials in the point‐of‐care detection function with the use of the obtained device.

Another blood‐related disease, leukemia, which usually relates to the uncontrolled growth of abnormal white blood cells in the blood and bone marrow, is considered a cancer of the blood‐forming tissues consisting of the bone marrow and the lymphatic system. Among these cells, K‐562 cells belong to a cancer cell line regarding chronic myelogenous leukemia in humans. To detect these K‐562 cells, a paper substrate has been developed, using it as a simple, low‐cost, portable, and highly sensitive biosensor, in which a 3D hybrid material of AuNPs/graphene was manufactured, used as working electrodes.^{[ 179 ]} For more details, this device with chromatographic paper was established through a solid‐wax printing technique, and the working electrodes were attached to this device by a screen‐printing technique. In the detection step, concanavalin A was used to immobilize this surface and capture K‐562 cells, whereas PdAg NP acted as a catalyst for thionine oxidation with the use of H₂O₂, resulting in a limit of detection reached up to 200 cells mL⁻¹. In particular, H₂O₂ was generated from K‐562 cells through the stimulation of phorbol 12‐myristate‐13‐acetate. As such, the current signal response depended on i) the amount of PdAg NP on the surface of K‐562 cells and ii) the H₂O₂ concentration generated from the K‐562 cells. Therefore, this versatile biosensing platform could be of great clinical application for diagnosis and treatment.

6.4. Myocardial Infarction

Cardiology typically relates to disorders of the heart and circulatory system, in which terms in heart sounds, temperature, blood pressure, pulse, and respiration rate are key hints to prove the importance of monitoring human health with respect to the cardiovascular system. More notably, one of the cardiovascular disorders can be mentioned as a heart attack (i.e., myocardial infarction) that occurs when the blood flow is not adequately provided to the heart muscle and is considered an important dangerous condition. In other words, lack of blood flow can involve a blockage of the heart's arteries, resulting in a heart muscle without blood flow that begins to decrease, or a myocardial infarction can cause lasting heart damage and death if blood flow is not restored in time. Cardiac troponins I and T, creatine kinase‐MB, and cardiac myoglobin are important markers in myocardial infarction diagnosis. Of these, an electrochemical device has been developed to allow one‐step ultrasensitive detection of a cardiac biomarker (i.e., troponin I), serving as an interference‐free electrochemical lateral‐flow immunosensing device (Figure 11D)^{[ 180 ]} in which an asymmetric membrane pad in the lateral‐flow immunoassay allowed the delayed release of the reductant, oxidant, enzyme‐substrate, and antitroponin‐I IgG immobilization in the membrane near the ITO electrode limited complex surface modification of this electrode. As a result, the ultrasensitive detection of troponin I was only within 1 min, while the use of the chronocoulometry technique in the detection of troponin I showed great detectability with a detection limit of 0.1 pg mL⁻¹ and a wide concentration range of 0.1 pg mL⁻¹–100 ng mL⁻¹. Thereby, the biosensor obtained is rapid, simple, and ultrasensitive and is used as a point‐of‐care device.

6.5. Dengue Fever and H1N1 Influenza

Dengue fever is considered a mosquito‐borne, highly infectious disease caused by the dengue virus, meaning that it is a viral infection spread from mosquitoes to humans; its symptoms can include a high fever, skin itching and rash, vomiting, headache, along with muscle, and joint pains. Additionally, this disease can result in low levels of blood platelets or dengue shock syndrome. To date, dengue fever is treated with pain medicine, and there is no specific treatment; however, many potential devices have been successfully constructed to favorably detect and monitor patients’ health status. Particularly, a biosensor connected to a screen‐printed Au electrode has been proposed to detect NS1 protein involving the cause of dengue fever, using as an electrochemical lateral‐flow immunosensor.^{[ 181 ]} This device contained a cellulose glassy fiber paper‐conjugated pad in which the immunoelectroactive nanobeads were added to the device to bind the target markers; at the same time, another specific antibody‐contained pad was marked with ferrocene acetic acid and immobilized on AuNPs. The NS1 protein would react with the specific antibody during the detection process, and the designed Au electrode containing specific capture antibodies formed a sandwich‐like structure. It means that the detection of the NS1 biomarker was converted to an electrochemical signal through a redox reaction that existed on the bead‐immobilized antibody, and an increase of faradic current was also obtained based on the increasing concentration of NS1 protein. The result indicated that the linear range was in a concentration range of 1–25 ng mL⁻¹ with the ultrasensitive detection limit of 0.5 ng mL⁻¹ for both quantitative and qualitative determination of dengue NS1 protein.

Another viral‐infection‐borne disease, i.e., H1N1 influenza, is a contagious viral illness causing high respiratory infections. It belongs to a subtype of influenza A virus, an orthomyxovirus, which has the glycoproteins hemagglutinin (H) and neuraminidase (N). For their specific features, glycoproteins H will induce red blood cells with each other and link with the virus present in infected cells, while glycoproteins N belong to a glycoside hydrolase enzyme that contributes to the virus's movement from the infected cells and promotes their growth from the host cells. Normally, some symptoms of H1N1 influenza can consist of fever, chills, nasal secretions, decreased appetite, and difficulty breathing. In order to monitor the patient's health, a biosensor has been developed to directly apply the H1N1 virus detection,^{[ 182 ]} in which a chromatography paper was modified with hydrophobic silica NPs and stencil‐printed carbon electrodes combined with a hybrid composite of SWCNTs and CTS. Notably, the antibodies would be immobilized by crosslinking with glutaraldehyde. The obtained device reached a detection limit of 113 plaque‐forming unit mL⁻¹ and a linear region of 10–10⁴ plaque‐forming unit mL⁻¹ with a detection time of 30 min. Further, the fabrication process was very easy and used few materials, meaning that it required only a spraying technique and used silica NPs to convert superhydrophobicity onto paper. Thereby, this immunosensor showed potential for the development of point‐of‐care devices.

6.6. Others

Additionally, many other papers have been published on detecting various markers of proteins, DNA, dopamine, cholesterol, uric acid, and ascorbic acid. For the detection of DNA and proteins, a biosensor has been well fabricated for the quantitative detection of oligonucleotides and proteins using a conformational switching assay^{[ 183 ]} so that the detection limits corresponded to 30 nm (DNA) and 16 nm (thrombin). Specifically, the approach in the detection process was based on the principle of target‐induced conformational switching of an aptamer bound to an electrochemical label, meaning that the target analytes (i.e., thrombin or DNA) would link to the redox reporter (i.e., methylene blue) and then led to a conformational change. Similarly, a DNA probe contained both a capture region for target proteins and a redox label to fabricate a highly selective DNA‐based biosensor,^{[ 184 ]} in which a large macromolecule (i.e., protein) linked to a signaling DNA strand generated steric hindrance effects, limiting the hybridizability ability of this DNA to a surface‐attached complementary DNA strand, resulting that a protein detection limit of proteins was <100 nm in blood samples. Additionally, the selection or design of a suitable probe to be immobilized on the electrode surface is often considered to be a main requirement to enhance selectivity, sensitivity, accuracy, and reproducibility of bioanalytical electronic devices for DNA diagnosis and analysis. For example, a new 3D folding biosensor was first introduced to detect DNA markers, which was based on a screen‐printed paper electrode with a modified AuNP/graphene hybrid.^{[ 185 ]} In this modified hybrid material, the AuNP/graphene platform was constructed using a self‐assembly approach based on electrostatic interactions, which could enhance the electronic transmission rate and the development of surface area for the electrochemical DNA biosensors. The ssDNA was immobilized on the surface to capture nanoporous AuNPs bioconjugated with thionine and dsDNA using as signal tags and complementary ssDNA, resulting in outstanding amplification with a detection limit of 2 × 10⁻⁹ mol L⁻¹. In another work, a slip biosensor with magnetic microbeads functionalized with DNA markers and an AgNP‐based electrode has been designed and constructed to detect the hepatitis B virus.^{[ 186 ]} Based on the working principles, Ag⁺ ions would be detected by fast oxidation of the AgNP labels with the existence of KMnO₄ on the paper substrate and reached a detection limit of 8.5 × 10⁻¹¹ mol L⁻¹. Thus, these simple and low‐cost biosensors possessed good precision, high sensitivity, acceptable stability, reproducibility, and excellent performance in human serum assay, which could be favorably used for point‐of‐care testing and public health monitoring.

Additionally, dopamine is known as a neurotransmitter distributed in the hormonal, renal, cardiovascular, and central nervous systems, which can impact the brain process regarding emotional response, movement, pain, and pleasure. More notably, a change in dopamine levels can also affect and cause some neurodegenerative diseases such as memory loss, sleep disorders, and Parkinson's syndrome; therefore, the detection of dopamine is very important for health diagnosis. A new sodium dodecyl sulfate‐modified biosensor has been fabricated to be applied for the selective determination of dopamine in serum samples.^{[ 187 ]} This biosensor system consisted of three layers; furthermore, the top layer used the filter paper containing SU‐8 photoresist, the transparent middle layer consisted of one hole for sample preconcentration and another for the surfactant, and the carbon electrode‐contained bottom layer was used for electrochemical measurements. With respect to the working mechanism, a potential peak would be shifted for the oxidative detection of the dopamine peak toward a more negative potential, which was related to the electrostatic interaction between cationic dopamine and anionic sodium dodecyl sulfate. Therefore, the dopamine peak could be clearly distinguishable and achieved a detection limit of 3.7 × 10⁻⁷ mol L⁻¹, owing to the positive potentials for the oxidation of ascorbic acid and uric acid. Additionally, the detection of ascorbic acid‐containing paracetamol or dopamine was concomitantly reached through the use of dual working electrodes,^{[ 188 ]} which means that ascorbic acid oxidation (i.e., nonreversible agent) would be oxidized at the first electrode, while dopamine and paracetamol (i.e., reversible electroactive species) would be reduced at the second electrode. As a result, the detection limits of dopamine and paracetamol corresponded to 5 × 10⁻⁶ and 6 × 10⁻⁶ mol L⁻¹ without the separation effect. Furthermore, MWCNT and Nafion could be applied well for dopamine detection, using as a working electrode containing a double‐sided conductive carbon tape supported on ITO glass.^{[ 189 ]} Clearly, the combination of Nafion and carbon tape electrodes improved the selective detection of dopamine by preventing ascorbic acid on the electrode surface; at the same time, it also enhanced electrochemical signals of dopamine due to the available adsorption of negatively charged Nafion and cationic dopamine, resulting in a detection limit of 0.1 × 10⁻⁷ mol L⁻¹.

With the appearance of abnormal cholesterol levels, some cholesterol‐related diseases can comprise coronary heart disease, hypertension, brain thrombosis, arteriosclerosis, lipid metabolism dysfunction, and myocardial infarction; therefore, monitoring cholesterol levels is vital for controlling and preventing these cholesterol‐related diseases. Recently, a new mixture of graphene, PVP, and PANI has been used to fabricate a nanocomposite‐based biosensor via an electroporation technique, which could effectively enhance the electrode surface area as well as the sensitivity for cholesterol detection.^{[ 190 ]} Moreover, ascorbic acid has been eliminated using electrostatic repulsion from an anionic sodium dodecyl sulfate coated on the nanocomposite electrode, resulting in the detection limit of the prepared biosensor for cholesterol in a complex biological fluid (i.e., human serum) was found ≈1.0 × 10⁻⁶ mol L⁻¹. Furthermore, AgNPs have been modified with boron‐doped diamond, which was used as a working electrode for cholesterol detection,^{[ 191 ]} resulting in a significant improvement in terms of detection reproducibility and sensitivity by increasing the signal‐to‐noise ratio. In the biosensor working mechanism, the use of AgNPs in the electrode could catalyze the H₂O₂ reduction reaction and favorably enhance the electrochemical signal, in which cholesterol oxidase was dropped into the hydrophilic area of the biosensor, and the H₂O₂ compound was produced and monitored through the enzymatic reaction and reduction correspondingly. These were favorable for eliminating possible interference from other oxidized species in biological samples (i.e., ascorbic acid and uric acid), showing that this new sensing platform could become a potential candidate for cholesterol diagnosis and provide the benefits of low cost, portability, and short analysis time. Additionally, the simultaneous detection of glucose, lactate, and uric acid (as critical health markers in serum) in biological samples using oxidase enzyme reactions (glucose oxidase, lactate oxidase, and uricase) is also suggested by Dungchai et al.^{[ 192 ]} in which a new microfluidic device has been developed using photolithographic and screen‐printing technologies. In this report, to improve selectivity for the amperometric detection of H₂O₂ formed from oxidase reactions, the electrodes have been modified with Prussian Blue functioned as a mediator, which was later modified with suitable enzymes for the determination of small molecule markers in human serum. The detection limits of the biosensor were found correspondingly to 21 × 10⁻⁵, 36 × 10⁻⁵, and 138 × 10⁻⁵ mol L⁻¹ for glucose, lactate, and uric acid. Ascorbic acid (vitamin C) is widely used as a human antioxidant to prevent cells from oxidative damage related to free radicals. To improve the analytical performance of paper‐based biosensors, PANI was used to modify the carbon electrodes using an inkjet‐printing technique for ascorbic acid detection,^{[ 193 ]} which showed high performance, low cost, and disposability. Furthermore, the performance was enhanced by optimizing five printed PANI layers toward the ascorbic acid marker, resulting in a detection limit of 3.0 × 10⁻⁵ mol L⁻¹ of ascorbic acid using this PANI‐modified sensor.

Overall, although bioanalytical electronic devices and smart systems can open potential applications for biological, clinical, and medical diagnostics these developments are still faced with several challenges. These challenges are related to the selection and manufacturing process of functional materials, selectivity, multifunctionality, convenient signal readout circuitry, and concomitant monitoring, in which effective integrations of functional materials on polymeric substrates are considered as a key for enabling the development of bioanalytical electronic devices and smart systems. Of these, the physical, mechanical, and chemical properties of active materials and substrates should be compatible to eliminate possible failures of thermal, electrical, and multilayer integrations. This further leads to challenges arising from numerous aspects regarding active materials and ideal structures that require various manufacturing processes. Biocompatibility requirements in functional materials and substrates are often a main concern for wearable and smart bioelectronics, as well as a smart health monitoring system for bioelectronics.

7. Conclusion, Outlook, and Challenges

In summary, the recent trends and advances in wearable bioelectronics powered by basic algorithms have been discussed above in detail, and these progress were clearly demonstrated in the field of wearable bioelectronics and health monitoring systems using smart bioanalytical devices. Human health monitoring is continuously and widely applied in clinics, the environment, fitness, the fashion industry, and entertainment; nevertheless, several challenges should be suggested to overcome in future research.

For functional materials, the performance and repeatability of active nanomaterials should be further improved. The outstanding advantages of SWCNTs used on large wafers regarding good uniformity, high array density, and low defect density have been well promoted. However, in its future applications, alternative approaches, which utilize graphene as a key component should be further exploited in quality and scope. The recent progress in liquid nanomaterials and their laser‐scribing and R2R printing techniques‐related processing were encouraged for the production of intelligent bioanalytical devices on various flexible substrates cost‐effectively. In wearable bioelectronic systems, comfort and safety are the most important requirements, especially for electronic skin biosensors for not only detecting and analyzing biomarkers in human skin‐excreted biofluids but also continuous monitoring physiological activities, which reveal a promising future in biomedical applications. However, the preparation process of some organic materials and heavy metals in the field of electronic skin biosensors can also be considered to be toxic solutions that may cause a disadvantage to the human body. Meanwhile, the use of superflexible substrates can make it difficult for sweat to evaporate and restrict skin metabolism; of course, there will be unavoidable inflammation or redness during a long‐term wear process. These can directly hinder the potential performance of wearable bioelectronics; consequently, ideas of active material or structural morphology should be suggested to let the gas and water go over. For instance, the nano‐/micromesh‐structured wearable bioelectronics possesses some advantages to solve the problem. For the synthesized nanoparticles, controlling various size and shape of the nanoparticles is still challenging, which is because of possible change in their conformation and the topology for each biosensor. Obviously, the nanoparticles can show a tendency to change their behavior under various environmental conditions.

Several drawbacks faced in the development of bioanalytical sensors can be: i) reaching a low limit of detection; ii) exhibiting on‐site detection of samples; iii) nullifying nonspecific adsorption of interfering species; iv) maintaining repeatability and stability of the sensors quite complicatedly. For the biological analyte, i.e., bacteria, differentiation between living and dead cells is often a big challenge. Moreover, the use of biosensing devices often focuses on single‐target measurements; consequently, noninvasive monitoring approaches of multiple macromolecular biomarkers should be developed further in the future works. It means that this approach can enable a better evaluation of the human physiological monitoring as well as calibrate the sensing signal accurately. Also, the strategy in biosensing detection of functional composites and structures can dramatically contribute to development in wearable multifunctional devices, which can promote more complete human physiology monitoring to probably expand the scope of applicable populations.

Additionally, the interface between the biosensor and the flexible circuit should be optimized to reach high performance and good data readout signals, in which the wearable electronics assisted by machine‐learning‐based algorithms should meet some requirements in further optimization of both software and hardware. On the software side, some key suggestions include i) data readout signals should be outputted consistently with the bioanalytical capacity of electronic devices, which offer a standard for coupled performance of both electronic devices and machine learning algorithms; ii) bioanalytical approaches of physiological signals should be standardized; iii) other potential algorithms need to be exploited further to expand the practical applications of machine learning‐powered by wearable bioelectronics; iv) the combination of chemical and physical signals provide more information about the biological/clinical diagnostics, so the use of synergetic algorithms is absolutely essential to examine the multimodal signals methodically. On the hardware side, the density of computing devices in the bioelectronics system should be enhanced so that the greater‐scale algorithm can work more favorably in situ. The health care process can concomitantly relate to health monitoring and real‐time alerts, likely performed by machine learning‐assisted bioelectronics, which can be used as smart bioanalytical devices. Therefore, in future applications, the smart bioelectronics system can accurately provide health diagnostic capabilities.

Conflict of Interest

The authors declare no conflict of interest.

Author Contributions

T.S.V. and T.H. contributed equally to this work. T.S.V.: conceptualization, design, writing – original draft, writing – review and editing. T.H.: writing – original draft. T.T.B.C.V.: writing – original draft. B.J.: writing – original draft. V.H.N.: writing – original draft. K.K.: conceptualization, writing – review and editing, supervision, project administration, and funding acquisition.

Biographies

Thi Sinh Vo is a postdoctoral researcher as a member of COME Laboratory, Sungkyunkwan University. She achieved her Ph.D. degree in mechanical engineering from Sungkyunkwan University. In 2014, she received the B.S. degree in materials science from Viet Nam National University Ho Chi Minh City–Ho Chi Minh City University of Science (VNUHCM–HCMUS). From 2015 to 2018, she completed the M.S. degree in chemical engineering from Daegu University. Her main background is materials science–polymer and composite materials, in which her current researches relate to functional composites and structures, electrochemical/electromechanical sensors, and water treatment.

Trung Hoang received the M.S. degree in mechanical engineering from Sungkyunkwan University in 2021. In 2018, he received the B.S. degree in petroleum engineering from PetroVietNam University. At present, he is a Ph.D. student from Sungkyunkwan University at Department of Biophysics. His current research focused on biomedical applications of quantum biology and large‐scale manufacturing of macro‐to‐nanoscale biodevices using roll‐to‐roll nanoimprint lithography.

Tran Thi Bich Chau Vo is a Ph.D. student from National Kaohsiung University of Science and Technology at the Department of Industrial Engineering and Management, Taiwan. She graduated an M.S. in industrial system engineering at Viet Nam National University Ho Chi Minh City–Ho Chi Minh City University of Technology (VNUHCM–HCMUT), Vietnam, in 2015. Then, she joined the Faculty of Industrial Management at Can Tho University, Vietnam, as a lecturer. Her research interests include conducting experiments on lean manufacturing, demand forecasting, inventory prediction, business process re‐engineering, and especially intelligent manufacturing.

Byounghyun Jeon earned the B.S. degree in mechanical and automotive engineering from Inje University in 2022. Currently, he is an M.S. student from Sungkyunkwan University at School of Mechanical Engineering, and as a member of COME Laboratory, Sungkyunkwan University. His main research interest is manufacturing of laser‐assisted 2D materials.

Vu Hoang Nguyen earned his M.S. degree in nano science and technology from Sungkyunkwan University in 2020. Prior to this, he obtained his B.S. degree in material engineering from Ho Chi Minh City University of Technology (HCMUT), VNU‐HCM in 2018. Currently, he is pursuing his Ph.D. at Monash University in the Department of Mechanical and Aerospace Engineering, Australia. His research interests include electrochemical energy storage devices, biosensors, and biochips.

Kyunghoon Kim is an associate professor in the School of Mechanical Engineering at Sungkyunkwan University. He received his Ph.D. degree in mechanical engineering from University of California at Berkeley, USA following his completion of both an M.S. degree from Stanford University and a B.S. degree from Sungkyunkwna University. He worked as a senior engineer for Samsung Electronics, where his main research project was strategic analysis for fuel cell business feasibility and design of fuel cell system. Currently, his primary research interests are design, fabrication and various applications of multiscale materials and nanocomposites, bionanoelectronis, and micro/nanofabrication.

Vo T. S., Hoang T., Vo T. T. B. C., Jeon B., Nguyen V. H., Kim K., Recent Trends of Bioanalytical Sensors with Smart Health Monitoring Systems: From Materials to Applications. Adv. Healthcare Mater. 2024, 13, 2303923. 10.1002/adhm.202303923