Abstract
The spread of point‐of‐care (PoC) diagnostic tests using electrochemical sensors poses a significant environmental challenge, especially in limited‐resource settings due to the lack of waste management infrastructure. This issue is expected to intensify with the emergence of the Internet of Medical Things (IoMT), necessitating eco‐friendly solutions for disposable devices. This review discusses efforts to develop green and sustainable PoC diagnostic devices, clarifying terms like biodegradability and transient electronics. It explores potential transient and biodegradable materials and fabrication technologies, emphasizing sustainable electronics with low‐energy consumption and low‐carbon footprint techniques, particularly favoring printing methods. The review highlights examples of necessary electronic components containing biodegradable materials for electrochemical PoC devices and discusses their role in device sustainability. Finally, it examines the feasibility of integrating these components and technologies into comprehensive biodegradable PoC devices, addressing the imminent need for eco‐friendly solutions in diagnostic testing. This comprehensive discussion serves as a guide for researchers and developers striving to mitigate the environmental impact of PoC testing in the era of IoMT and personalized medicine.
Keywords: Sustainability, Biodegradability, Transient electronics, Point-of-care devices, Electrochemical sensors, Self-powered devices, Electronic circuitry.
The deployment of point‐of‐care diagnosis may lead to sustainability concerns due to the complex management of generated waste. This review examines transient materials and printing technologies to enhance the sustainability of electrochemical point‐of‐care devices, providing examples of components incorporating transient materials for eco‐friendly solutions.
1. Introduction
Over the last decades, electronic technologies have experienced significant and rapid advances. Due to this fast progress and the utilization of device designs that hamper repair, appliances become obsolete or broken after a few years, resulting in short product lifecycles. [1] This results in the amount of Waste Electrical and Electronic Equipment (WEEE), more commonly known as e‐waste, growing every year, which has already become a global challenge.[ 1 , 2 , 3 , 4 ] In 2022, it was estimated that 62 megatonnes of e‐waste were generated worldwide, of which only the 22.3 % was properly collected and recycled, and it is estimated that this value will reach 82 megatonnes in 2030. [5]
A way to tackle this issue would be the introduction of electronic devices into a circular economy model, as it would facilitate the management of generated e‐waste.[ 1 , 3 , 6 ] This can be achieved by designing electronic products in a way that elongates their life span and facilitates dismantling, product and component reuse, and material recycling. [7] Materials used should also present low toxicity to enable safe processing. Figure 1a presents a diagram of the lifecycle of a product integrated into a circular‐economy model and the routes that allow connecting the different stages of the lifecycle. Biodegradation could be a possible circular end‐of‐life process for some electronic devices. These devices would be broken down by microorganisms into simple molecules, which could be used in the production of new materials through biosynthesis processes. [8] A graphical representation of a biodegradation‐based product lifecycle can be observed in Figure 1b.
Figure 1.
a) Diagram showing the different stages of product life cycle in the context of circular economy and the different routes connecting the different stages. b) Diagram showing the life cycle of devices using a biodegradation end‐of‐life route.
Apart from a sustainable end‐of‐life, sustainability should ideally be applied to all the steps of product lifecycle. [9] Fabrication processes with low environmental impacts should be promoted, i. e. the energy requirements, carbon footprint, and pollution associated to processes should be minimized. The environmental impact of material extraction and production processes should also be reduced. When considering biodegradable materials, those produced from biological sources can be obtained from agriculture and forest exploitations. This could lead to competition for food and feed production, land usage, and deforestation, which may result in a negative social and environmental impact. [10] There are also biodegradable materials derived from fossil fuels, which is also unideal, due to the scarcity and the impact of these compounds. A good option would be the usage of waste streams as source for the production of biodegradable materials, as it allows circularity and minimizes the environmental impact.[ 11 , 12 ] Moreover, the principles of green chemistry should be followed during the production of materials, including the utilization of low toxicity solvents, reagents, and by‐products, and the minimization of energy consumption and by‐product generation. [13]
The case of electrochemical sensors is no exception to the problems associated to other kinds of e‐waste. Electrochemical sensors allow rapid monitoring in multiple application fields, such as medical or environmental ones, which has led to an increase in the utilization of this kind of sensors. [14] Specifically, within the biomedical field, electrochemical sensors used as disposable point‐of‐care (PoC) devices may become a serious concern. The use of single‐use PoC diagnostic tests poses significant challenges to both environmental sustainability and human health as identified by Ongaro et al., as these tests are often crafted from unsustainable polymeric materials derived from fossil sources. [15] A PoC test was defined by Schito et al. as “a diagnostic test that is performed near the patient or treatment facility, has a fast turnaround time, and may lead to a change in patient management”. [16] The use of PoC avoids reliance on laboratory facilities and specific equipment for diagnosis and, due to its fast outcome, allows early treatment prescription. [17] PoC devices that integrate electrochemical sensors are desired in specific applications that demand precise and quantitative analyte detection. Employed in healthcare settings for rapid diagnostics, they offer a versatile and efficient solution making them invaluable tools across various fields where detailed and reliable analyte information is paramount.
Nevertheless, waste originated from PoC tests might lead to environmental problems, especially in limited‐resource settings, where there is a lack of complex waste management facilities. [18] In developed countries, on the other hand, this situation will be exacerbated by the rise of the Internet of Medical Things (IoMT). [19] In this field, disposable devices will play a crucial role in the advancement of personalized medicine. Given the high number of sensors that may be required by end‐users themselves, this necessitates the development of eco‐friendly disposable electrochemical PoCs to mitigate the environmental impact associated with the mass utilization of these devices. Therefore, biodegradability would be desirable for PoC tests, especially in the case when electronic components are used, avoiding the toxic materials normally found in non‐biodegradable e‐waste. Facing these near future challenges, this review provides a comprehensive discussion on current efforts made to develop green and sustainable electrochemical PoC diagnostic devices based on biodegradable materials. Essential terms like biodegradability, bioresorbability, and transient electronics, usually misunderstood in this field, will be clarified. Subsequently, this review will explore potential biodegradable materials and pertinent printing and fabrication technologies. Sustainable electronics prioritize materials processed with low‐energy consumption and low‐carbon footprint techniques, favoring printing over traditional clean room technologies. While not exclusively focused, emphasis will be given to materials and components produced through printing methods for enhancing electrochemical PoC devices. Additionally, examples of necessary transient electronic components incorporating biodegradable materials for electrochemical PoC will be provided, showcasing their role in device sustainability. Finally, the feasibility of developing a comprehensive transient PoC device will be examined, integrating these components and technologies into a functional and eco‐friendly diagnostic tool.
2. Biodegradable, Bioresorbable, and Transient Concepts
There is no univocal definition of biodegradability throughout the literature. For instance, the term “biodegradable” is often used instead of other terms that would fit the described situation better, which may lead to confusion among researchers and reinforce the perpetration of misusage of this term. Therefore, in this section biodegradability, bioresorbability, and transient terms are going to be defined and explained, to prevent their confusion and misinterpretation, ensuring clarity and consistency in their usage across scientific discourse.
Biodegradability is the ability of a material to be transformed into simple molecules, such as water, carbon dioxide, methane, and nitrogen, and biomass by the activity of fungi and microorganisms.[ 20 , 21 ] According to the European Standard EN13432, a material is biodegradable when at least 90 % of its mass is able to biodegrade within 6 months in the presence of fungi and microorganisms. [20] Carbon chains of biodegradable organic materials are cleaved into small monomers by enzymes, which are further converted into CO2 through aerobic respiration. When biodegradation takes place in anaerobic conditions, methane is produced instead of carbon dioxide. [21] Biodegradation processes are dependent on the environmental conditions, e. g. temperature, humidity, oxygen concentration and pH. It is important to mention that some materials labeled as biodegradable such as polylactic acid (PLA) degrade in industrial composting conditions, but may remain unaltered in natural environments. [21]
Bioresorbability is the ability of a material to degrade in the presence of physiological fluids, with complete assimilation or elimination of the formed by‐products.[ 21 , 22 ] Bioresorbability is often a desired property for implanted electronic devices. Implanted devices also need to be biocompatible, i. e. they must not elicit an immunological or toxic response in the body. [21] It is worth noticing that in literature the term “biodegradable” is often used instead of the term “bioresorbable”, which can be misleading. [21] In summary, it is recommended to utilize “bioresorbable” for materials in contact with body fluids, while “biodegradable” should be considered specifically for degradation in the environment in the presence of microorganisms.
Transient electronics refers to those electronic components that have the ability of breaking down into small harmless constituents after a desired working time. [20] While in traditional fabrication of electronics the achievement of the optimal performance is one of the main goals, for transient electronics it is important to consider other aspects, such as toxicology and biodegradability/bioresorbability when designing a device. [21] Biodegradable materials are normally abundant, non‐toxic, and inexpensive, favoring the fabrication of safe low‐cost disposable devices. Furthermore, some of them are also flexible and stretchable, desirable for wearable electronic devices. [20]
3. Functional Materials for Transient Electronics
There are both synthetic and nature‐derived materials that can be eligible for the fabrication of transient electronics.[ 20 , 23 ] Some of the advantages of natural materials are that many of them can be extracted from waste, facilitating the integration of its fabrication into circular economy, and that they can be easily degraded by enzymes or compost. However, they may present high variations from batch to batch and may elicit immunologic response when used for intracorporeal applications. [23] In the case of synthetic polymers, their physical properties and their degradation are easily tunable by chemical modification. [23]
Composites are also an option for transient materials. In this case a matrix of a biodegradable polymer is used as structural element and contains particles of a material with a desired physical property, such as high dielectric constant and high electrical conductivity. [23] The matrix material usually degrades faster than the filler material, allowing its dispersion in the degradation medium. In the case of conductive composites, the proportion of filler material has to be higher than the percolation threshold, i. e. the minimum proportion of filler material that allows connectivity between conductive particles.[ 23 , 24 , 25 ] Percolation threshold highly depends on several parameters, including type of filler and synthesis process, shape (aspect ratio) and presence of aggregates, type and properties of the polymer matrix, and dispersion of the filler in the matrix.[ 24 , 25 ]
Another criterion for the selection of materials for sustainable electronics is their abundance. The European Union has developed a list of materials referred to as Critical Raw Materials (CRMs). This list includes those materials that play a crucial role in product fabrication, but their supply during the next ten years is at high risk of shortage. [26] Therefore, CRMs should ideally be avoided in the fabrication of sustainable electronic devices.
This section presents the range of materials, summarized in Figure 2, applicable as substrates, dielectrics, conductors, and semiconductors within the domain of biodegradable and transient electronics.
Figure 2.
Illustration presenting an overview of the commonly sustainable materials utilized in the field of responsible electronics, focusing on key components such as substrates, dielectrics, and the active layers (conducting or semi‐conducting), alongside the conventional 2D and 3D printing methods.
3.1. Biodegradable Substrates and Dielectrics
In electronic devices, the substrate acts as the base material on which all the other layers are fabricated. It is crucial for maintaining electrical insulation, thereby minimizing current losses. Often, substrates comprise the majority of material within the device, both in terms of mass and volume. Consequently, any degradation in the substrate significantly impacts the overall degradation of the device. In traditional electronics, the substrate is a rigid layer normally made out of undoped semiconductors, oxides, or phosphides. Plastics can be used as substrates for flexible devices, although most plastics present poor biodegradability. Therefore, in case of biodegradable and transient devices, other materials need to be used.[ 20 , 23 ] Most of the materials used as substrates can be used as well as dielectric and packaging layers. [20] To be used as dielectrics, the materials must have adequate dielectric constants, low dielectric losses, and high enough dielectric breakdown voltages. [23] Some natural materials that have been used as dielectric materials are DNA nucleobases, saccharides (glucose, lactose, khaya gum…), gelatin, and caffeine.[ 27 , 28 , 29 ] In case of packaging layers, a binder material is normally required as well. [20] Many biodegradable materials that can be used as substrates are based on natural polymers, but there are also some synthetic polymers that present good biodegradability.[ 20 , 23 ] The molecular structures of various natural and synthetic biodegradable polymers suitable for biodegradable electronic devices are depicted in Figure 3.
Figure 3.
Molecular structures of some natural polymers, synthetic biodegradable polymers, conductive organic polymers, and natural dyes and pigments, which could be used in electronics.
3.1.1. Natural Polymers
Most of natural polymers are polysaccharides or proteins that are normally plant‐based or animal‐based, although they can also be obtained from other organisms. Some polymers that can be extracted from plants and algae are cellulose, starch, [20] lignin, [30] agar, carrageenan, [31] and alginate. [32] Alternatively, some examples of animal‐based polymers are silk,[ 33 , 34 , 35 ] chitin/chitosan, [23] collagen, [23] and shellac. [20]
3.1.1.1. Cellulosic Materials
Cellulose is the most abundant biopolymer on earth in terms of mass, as it is the main structural polymer found in plants and can be easily extracted from wood. [36] Cellulose has been used for centuries by humans as building material, energy source, and raw material for the fabrication of paper and textiles. [37] Nonetheless, novel ways of processing cellulose have recently been developed resulting in new possible applications for this material. Cellulose degradation mechanism consists in the hydrolytic cleavage of β‐1,4‐glycosidic bonds present in the molecule by cellulase enzymes found in bacteria and fungi. [38] This causes the release of glucose, which is easily mineralized by living beings. Different cellulosic materials with slightly different properties can be relevant for the fabrication of electronics. Depending on the type of cellulose, dielectric constant may range between 2.8 and 9.0 [39] and electrical conductivities, from 10−14 to 10−8 S/cm.[ 40 , 41 , 42 ]
Paper. It is the result of squeezing a cellulose pulp suspension onto a mesh, which allows dewatering. Further pressing and drying steps are required to obtain high quality paper. [20] Paper is very advantageous to be used as substrate as it has a very low cost and presents flexibility. [43] Moreover, as it has been used for centuries, its fabrication processes are highly consolidated and there is broad knowledge on how to tune its properties. [43] Intrinsic properties are high porosity, high surface roughness, and poor vapor barrier properties, [20] which can result in poor film uniformity when low thickness layers are deposited on its surface. These drawbacks can be alleviated by applying surface treatments or coatings. [36] Nevertheless, porosity can be beneficious for applications that require high specific area such as supercapacitors, apart from helping improve retention and reduce undesired spreading of the deposited material. [44] Many examples of electric and electronic components on paper substrates include supercapacitors, [45] transistor arrays, [44] batteries,[ 46 , 47 , 48 , 49 ] and sensors.[ 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 ] It is also remarkable to mention that the porosity of paper has been exploited in the development of microfluidic devices and flow batteries.[ 46 , 50 , 58 ]
Carboxymethylcellulose (CMC). The properties of cellulose can be easily modified by applying chemical reactions. CMC is one of the most common examples of chemically modified cellulose. CMC is produced by the reaction of cellulose with monochloroacetic acid. [59] It is an anionic polymer, normally combined with sodium counterions, which makes it soluble in water. [60] CMC can be used in many different ways in electronic applications. Huang et al. fabricated a CMC transparent flexible substrate with the ability to dissolve in water within a few minutes. [61] It has also been used in gel form as electrolyte [62] or added to suspensions as a binder or thickener. [63]
Nanocellulosic materials. They are a type of material that contain cellulose fibers or particles with one or more dimensions measuring between 1 and 100 nm. These materials can be obtained through either bottom‐up or top‐down methodologies, such as chemical, enzymatic, and mechanical treatments of natural cellulose fibers.[ 43 , 64 ] There are three main types of nanocellulosic materials: cellulose nanofibers (CNFs), bacterial cellulose (BC), and cellulose nanocrystals (CNCs).[ 37 , 64 ]
CNFs are nanocellulosic materials that have a high aspect ratio, with diameters ranging from 2 to 60 nm and reaching up to a few microns in length. [64] A CNF aqueous gel can be obtained from the delamination of bleached wood pulp by applying several mechanical processes, which can be combined with chemical and enzymatic treatments to decrease the energy requirements. [37] TEMPO‐mediated oxidation, a process that uses (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) as oxidant, is a typical chemical treatment that allows obtaining carboxylated CNFs with enhanced colloidal stability. [65] Films can be obtained from the gel by diluting and filtering the suspension, and then drying the solid material obtained from the filtration. [66] CNF films present high optical transparency, low surface roughness, high tensile strength (224 MPa [66] ), and high Young Modulus (14.5 GPa [66] ),[ 20 , 66 ] and have been used as substrates for electronic applications. [67] CNF has also been used as coating for textile and paper substrates, as it covers the substrate's pores, avoiding ink penetration and improving the conductance of printed patterns.[ 68 , 69 , 70 ]
BC consists of highly crystalline unbranched nanofibers (thinner than 100 nm) produced by bacteria from the species Komagateibacter xylinus. [37] Contrarily to the other types of nanocellulose, BC is obtained by bottom‐up methods, which allows having a higher control on the properties of the nanofibers. The shape and structure of BC gels can be easily tuned by controlling the production conditions, and can be fabricated with low costs and high yields. [37] It is a safe material to use, as it is already being used for alimentary products. BC films have high transparency and good mechanical properties, including flexibility, high tensile strength (274 MPa [71] ), and tensile Young Modulus (12.7 GPa [71] ).
CNCs are crystalline rod‐like structures with widths ranging from 5 to 70 nm. [37] They are produced by degrading the amorphous regions of cellulose from multiple sources, including plants, tunicates, and algae, with an acid hydrolysis process.[ 37 , 64 ] CNCs exhibit outstanding mechanical properties, higher thermal stability than other types of cellulose, and the ability to form chiral nematic liquid crystals, which may be of interest for optical applications.[ 37 , 64 , 72 ] CNCs have been used as a substrate and dielectric in Organic Field Effect Transistors (OFETs) for the fabrication of flexible electronics. This has enabled the development of low‐cost, lightweight, and highly efficient electronic devices.[ 73 , 74 ]
Apart from the mentioned materials, alternative ways of processing cellulose have been used in the fabrication of substrates for electronic devices. Zhu et al. developed a method to fabricate transparent cellulose substrate that allows to save time and energy by simply applying high pressure to bleached wood pieces. [75] Guna et al. used a composite of cellulose, extracted from banana stems, and wheat gluten as a substrate and dielectric material. [76]
3.1.1.2. Silk
Silk is a material consisting of protein fibers and produced by silkworms (Bombyx mori). It contains two different proteins: fibroin, acting as a structural backbone, and sericin, which binds the different fibroin molecules.[ 20 , 33 ] Used for centuries as a textile, it is convenient for electronics due to its good biocompatibility and its easily tunable shelf life. [20] Regarding its biodegradation, it can be hydrolyzed into amino acids by proteolytic enzymes, found in many living beings, due to being based on protein chains. [77] Silk has a dielectric constant of 2.7 and electrical conductivities of the order of 10−9 S/cm at low ambient humidity.[ 78 , 79 ] Both silk textile or just its fibroin component can be used as substrate for electronic applications.[ 33 , 34 , 35 ]
3.1.2. Synthetic Biodegradable Polymers
Apart from natural polymers, some synthetic polymers also present biodegradability, as they can be hydrolyzed into monomers by the enzymes present in microorganisms. These monomers are normally substances present in the organisms, so they can be processed by their metabolism. Some biodegradable polymers are PLA, poly(lactic‐co‐glycolic) acid (PLGA), polyvinyl alcohol (PVA), polycaprolactone (PCL), poly(glycerol sebacate) (PGS), poly(4‐hydroxybutyrate) (P4HB), poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHB/V), polyethylene glycol (PEG), polypropylene carbonate (PPC), and polyanhydrides.[ 20 , 80 , 81 , 82 ] Biodegradable synthetic polymers can be fabricated from both biological sources or fossil fuels. It is interesting to mention that PPC can be synthesized by reducing CO2 with propylene oxide. Therefore, its use as material in electronic circuits could be used to decrease the atmospheric concentration of CO2. [83] Some of these polymers, such as PVA and PEG, present solubility in water.[ 61 , 80 ] Typical values of dielectric constant for synthetic biodegradable polymers range between 2 and 5.[ 84 , 85 ] It has to be noted that dielectric constant can be increased up to several thousand by including the synthetic polymers together with carbon materials in composites. These polymers are clearly insulators as their electrical conductivities are of the order of 10−16–10−13 S/cm. [84] The degradation processes of synthetic polymers highly depend on their chemical structures. As an example, the degradation of some polyesters such as PLA, PLGA, and PCL is a combination of non‐enzymatic hydrolysis and hydrolysis mediated by proteolytic enzymes at temperatures over 40 °C. [86] Another example is the enzymatic degradation of PVA, which consists of two steps: first, the oxidation of alcohol groups to ketones and second, hydrolytic cleavage. [87] PEG biodegradation is a result of successive oxidation steps that lead to the formation of glyoxylate, which is then mineralized. [88]
3.2. Functional Sustainable Conductors and Semiconductors
Typical materials used as conductors and semiconductors in transient electronics include transient metals, carbon‐based materials, organic polymeric semiconductors, and dyes. The molecular structure of some of these materials can be seen in Figure 3. A chart presenting typical conductivity values for some insulators, semiconductors, and metals has been displayed in Figure 4. For instance, semiconductors such as silicon can have conductivity values between 10−4 to 102 S/cm, while metals like copper and aluminum often display high conductivity values, typically in the range of 104 to 106 S/cm.
Figure 4.
Logarithmic‐scale chart presenting the approximate conductivity values of some metals, semiconductors, and insulators. The conductivities of doped organic polymeric semiconductors (PEDOT, PANI, and PPy) have also been included. [94]
3.2.1. Transient Metals
Transient metals are able to oxidize and dissolve when submerged in water or in a mild acidic medium within a few days of exposure. Therefore, they exhibit a bioresorbable behavior, but they cannot be considered to be biodegradable. The most common ones are magnesium (Mg), manganese (Mn), tungsten (W), iron (Fe), zinc (Zn), and molybdenum (Mo). [20] They have been used in the fabrication of transient interconnections, [61] energy harvesters,[ 61 , 80 , 89 , 90 ] sensors,[ 91 , 92 ] and batteries.[ 34 , 47 , 48 , 82 , 93 ] Non‐transient metals, such as gold and silver, are sometimes found as nanoparticles and small quantities in intracorporeal devices, but they are undesirable for mass production of transient devices due to their high cost and non‐reactiveness, which hinders their degradation. [20]
3.2.2. Carbon‐Based Materials
Carbon‐based materials, such as carbon nanotubes (CNTs), graphene, graphite, and carbon black, are another option to be used as electrodes. The abundance of carbon on Earth makes the usage of carbon‐based materials convenient. Moreover, the remarkable properties of CNTs and graphene, including tunable electrical conductivities and excellent mechanical properties, make them suitable for many applications.[ 20 , 95 ] Some biodegradability studies have been performed with carbon‐based materials, which demonstrated that CNTs and graphene can be degraded by specific bacteria and fungi, so they may be biodegraded when left on soil given that these specific microorganisms are present.[ 95 , 96 ] The presence of defects in the carbon structure would facilitate its biodegradation, while highly packed graphene and graphite would biodegrade much slower than single‐layered graphene. [97] Biochar, which can be obtained through pyrolysis of biological waste, is a carbon material that presents no biodegradability, although it is beneficious when left in soil as it can work as carbon sink and soil amendment. [98] However, some carbon‐based materials can present toxicity, which could make them unsuitable for some applications. [96] CNTs have been found to present asbestos‐like toxicity when breathed, being able to cause fibrosis and lung tumors, especially in the case of high‐aspect‐ratio nanotubes. [99] Therefore, precautions must be taken when using CNTs in transient devices.
Graphene. It is a 2D nanomaterial obtained from mechanical or chemical delamination of graphite. It is a zero‐gap semiconductor presenting high electron mobility and electrical conductivity and it also has outstanding mechanical properties, being the material with the highest reported tensile strength. [100] Single graphene sheets have outstanding electrical conductivities of 106 S/cm. [100] However, single‐layered graphene is expensive, so few‐layer graphene powders are habitually used, with conductivities typically ranging from 1 to a few hundreds of S/cm. [101] Graphene and graphite have been used as materials in several applications, including supercapacitors,[ 45 , 102 , 103 , 104 ] sensors,[ 50 , 52 , 53 , 55 , 56 , 57 , 105 , 106 ] and batteries.[ 47 , 49 ] Graphene oxide (GO) is normally used in aqueous inks, due to its superior stability in aqueous media. GO is transferred onto the substrates and later reduced to obtain good electrical properties.
CNTs. They are 1D nanomaterials which consist of single (single‐walled (SWCNT)) or several concentric (multi‐walled (MWCNT)) carbon tubes. Depending on their structure, they can either present semiconductor or metallic properties, with conductivities ranging from 102 to 106 S/cm. Nevertheless, as in the case of graphene, when having a layer compounded of CNTs, apparent conductivity will be lower due to contact resistances between them. This conductivity can be improved by doping mechanisms. [107] Moreover, they present comparable mechanical properties to graphene and remarkable thermal conductivity. [100] CNTs have been used in the fabrication of several electronic components, including electrodes for electrochemical sensors [108] and supercapacitors, [109] conductor material in resistive heaters, [110] and active layer in organic thin‐film transistors (TFTs). [111]
Biosourced carbon. Biomass and natural polymers can be pyrolyzed or carbonized to produce carbon‐based conductive materials, which can be later used in electronics.[ 112 , 113 ] You et al. obtained carbon nanosheets with high conductivity (of the order of 100 S/cm) and dispersibility in multiple solvents by carbonizing 2D‐chitin, obtained from hydrophobization of crab chitin. [113] In another example, activated carbon was obtained by carbonizing banana peel waste and the obtained product was suitable to be used as electrode in lithium ion batteries. [114]
3.2.3. Polymeric Semiconductors
Polymeric semiconductors can be used as materials for the active layer and, if adequately doped, for the electrodes and interconnections. [20] The use of organic semiconductors allows increased flexibility compared to inorganic ones, which makes them more suitable for applications such as wearable electronics. The conductivity of these polymers is determined by the degree of π‐conjugation in the molecule backbone, which can be enhanced by applying oxidation reactions.[ 23 , 115 , 116 ] Conductivity values between 10−2 and 103 S/cm can be achieved with polymeric semiconductors.[ 115 , 116 ] Among the many different synthetic organic semiconductors, polypyrrole (PPy), polyaniline (PANI), and poly(3,4‐ethylenedioxythiophene)‐polystyrene sulfonate (PEDOT:PSS) present good biocompatibility and have already been used in electronic applications. As an example, PEDOT:PSS was used in electrodes for electrochemical sensing[ 33 , 51 ] and PPy and PANI, in electrodes for supercapacitors.[ 117 , 118 ] However, these polymers seem to present poor biodegradability. [116] Owing to the poor biodegradability of these compounds, transience can be achieved by linking oligomers of the polymeric semiconductors with hydrolysable bonds. This would allow breakage of the polymeric structure into small oligomers, but at the expense of some of the potential conductivity. [23]
Many researchers have made efforts to develop transient organic semiconductors. Lei et al. synthesized a polyimine with mobilities that could reach 0.34 cm2 V−1 s−1 and able to degrade in mild acidic media (pH=4.6), although degradation by microorganisms was not tested. [8] In a similar way, Heffernan et al. synthetized a semiconducting polymer (poly(bis((thiophen‐2‐yl)methylene)benzene‐1,4‐diamine)) by using a microwave‐assisted polymerization process. The polymer exhibited an electrical conductivity of 2.08 S/cm, an energy gap of 2.36 eV, and degradation in acidic conditions, although degradation by microorganisms was not studied either. [119]
3.2.4. Dyes and Pigments
As an alternative, natural dyes and pigments such as beta‐carotene, melanin, indigo, indigoidine, and Tyrian purple (6,6’‐dibromoindigo), which are shown in Figure 3, could be used as semiconductors in transistor applications although they present very low mobilities (habitually of the order of 10−4 cm2 V−1 s−1).[ 23 , 27 , 120 , 121 ] Some nature‐inspired dyes, such as indathrene yellow G and indathrene brilliant orange RF, present good biocompatibility, biodegradability, and slightly higher mobilities than natural pigments (of the order of 10−3 cm2 V−1 s−1). [27]
3.2.5. Composites
As mentioned before, a solution to obtain conducting materials with transient properties is to fabricate a composite with a biodegradable matrix and a conductive filler. Many examples of this can be found in literature.[ 23 , 33 , 42 , 65 , 66 , 71 , 112 , 122 , 123 , 124 , 125 ] Lay et al. fabricated several conductive composites using either CNF or BC as matrix and PPy, PEDOT:PSS, and CNTs as fillers.[ 42 , 65 , 66 , 71 ] It is remarkable to mention that for these composites the increase in the filler proportion resulted in a decrease in tensile strength and Young Modulus. The maximum achieved conductivity was 13 mS/cm for a composite consisting of a 45 % of CNF and a 55 % of PPy. [42] A conductive composite consisting of CNFs and PEDOT:PSS was used as metallic layer for metal‐oxide‐semiconductor diodes. [124] López‐Barreiro et al. fabricated conductive fibers using silk fibroin and hydrochar obtained from wood and chitin. [112] Zare et al. fabricated a conductive material based on a composite of dextrin and polypyrrole with electrical conductivities ranging from 0.20 to 1.02 S/cm. [123] Pal et al. used a conductive composite that worked as well as a negative resist for UV‐photolithography consisting of silk sericin and PEDOT:PSS, with which a maximum conductivity of 0.08 S/cm was reached. [33] Another example of a conductive composite is a blend of eumelanin and PEDOT:PSS, which was developed to substitute indium tin oxide as anode of an organic light emitting diode (LED) and presents high electrical conductivity (370 S/cm), adhesion, and water resistance. [125] Jiang et al. fabricated nanoparticles compounded of different semiconducting polymers and a matrix of insulating biodegradable polymers for in vivo imaging purposes. The nanoparticles were degraded by macrophages in rats and cleared out of the body. However, complete mineralization, degradation by microorganisms, and the possible application of these materials to electronic devices were not studied. [122]
4. Printing Techniques
When fabricating sustainable electronics, low energy consumption and low carbon footprint processing techniques, such as printing, may be preferable over other typical fabrication technologies. Over the last years, printed electronics has gained popularity due to the improvement of printing technologies and the advantages they offer, especially for certain types of applications. One of the main advantages is that printing technologies are less costly and energy demanding than traditional clean room techniques.[ 126 , 127 , 128 ] First of all, printing fabrication processes consist of few steps, which not require of extreme conditions, such as high vacuum and temperatures.[ 126 , 129 ] Moreover, the additive nature of printing techniques allows minimizing material waste during fabrication.[ 127 , 130 ] Printing technologies are also easily applicable to industrial processes, allowing automation and large area printing. [129]
Another relevant advantage of printing techniques is that they admit a broad variety of substrates and deposited materials.[ 127 , 128 ] Substrate properties often incompatible with clean room technologies, such as flexibility, foldability, and stretchability, are exploitable with printing techniques and may allow the appearance of novel electronic applications (e. g. electronic skins) and novel industrial processes for electronics (e. g. roll‐to‐roll fabrication).[ 128 , 129 ] Some of the used flexible substrates, such as plastics, paper, and textiles, have low prices, which favors a cost reduction of electronics. In the case of deposited materials, both inorganic and organic materials are easily printable and ink formulations can be tuned according to the required device specifications.[ 64 , 129 ]
One of the main limitations of printing techniques is the achievable resolution of the printed features, much worse than that of traditional clean room technologies. The resolution of a printed pattern is limited by the size of the ink droplets or the features of the printing plates, normally being of the order of the micrometer or even tens of micrometers. Differently, feature resolutions below 10 nm have been achieved using standard microfabrication techniques. However, many emerging applications, including the Internet of Things (IoT) and wearables, do not require of highly miniaturized components and may benefit from printing fabrication. [126]
Considering the advantages of printing technologies, they may be good candidates for sustainable fabrication of devices with feature sizes over 10 μm.[ 131 , 132 ] The reduced energy consumption and lower production of waste material associated to these technologies are related to a reduced carbon footprint. However, printed layers often require of a thermal annealing step, which increases the energy consumption of the whole fabrication process. To mitigate this issue, thermal annealing can be replaced by less energy demanding processes such as photonic curing. Moreover, the broader variety of compatible materials with printing technologies may allow the usage of greener and low‐impact materials.
At present, there exist numerous 2D and 3D printing technologies as illustrated in Figure 2. The most relevant ones are briefly commented in the following subsections. To better understand the significance of printing technologies for biodegradable electronics, it is essential to appreciate the historical evolution of these techniques and their expanding capabilities. As this is beyond the scope of this review article, the reader can find detailed information about printing technology in the reviews of Martins et al. [133] and Tan et al. [134]
4.1. 2D Printing Techniques
2D printing techniques are based on ink transfer to the desired locations of a substrate, according to a designed pattern. It is important that deposited ink spreads in a manner that a continuous and homogeneous film is formed. Therefore, ink rheology and substrate wettability have a big impact on the quality of the printed patterns. Among the 2D printing techniques available, including inkjet printing (IJP), screen printing (SP), gravure, nanoimprinting, spray printing, flexography, and micro‐contact printing,[ 20 , 126 , 128 , 135 ] the most used for the fabrication of electronic devices are IJP and SP.
Screen printing (SP). A highly viscous ink is pushed through a patterned mesh and transferred to a substrate using a squeegee, which is controlled either manually or automatically.[ 128 , 135 ] This was the first printing technique to be used in electronics, so it is highly consolidated and is attractive because of its ease and speed of use. [135] However, SP inks are very viscous and require the addition of binders and thickeners in the ink formulation. Moreover, the control in the thickness is poorer than in other techniques.[ 20 , 126 ] SP has been used in the fabrication of electrodes for batteries and supercapacitors,[ 47 , 48 , 109 , 136 ] sensors,[ 50 , 52 , 136 ] and circuit boards. [61]
Inkjet printing (IJP). Droplets of a low viscous ink are ejected from a nozzle and then reach the substrate, where they spread to form a film. The nozzle moves automatically and sweeps over the whole sample. There are two main types of IJP equipments: continuous and drop‐on‐demand (DoD). [137] In the case of continuous IJP, drops are ejected unceasingly and the extra ejected droplets are deflected and recovered for later use. Differently, in DoD printing droplets are only ejected when required. [137] In this case, droplet ejection is controlled by using piezoelectric materials or thermal pulses.[ 128 , 135 ] The low viscosity of inks makes unnecessary the use of binders and thickeners, but requires making sure that aggregates are not formed, as they could cause nozzle clogging. [135] IJP allows high resolution and non‐contact material deposition. Moreover, the fact of using digital designs and not requiring physic masks facilitates design modification.[ 20 , 126 , 127 ] Nevertheless, DoD IJP is a very slow technique and this could be a deterrent for its usage at industrial level. Continuous inkjet allows faster printing, but at the expense of resolution. Another disadvantage is that material migration during ink evaporation can result in low film uniformity. [126] IJP has been used in the fabrication of sensors[ 51 , 108 , 124 ] and flexible transistors.[ 44 , 138 ]
From the point of view of sustainability, IJP is preferable over SP as it allows printing with almost no waste generation. SP, on the other hand, is always associated to the production of waste. Moreover, SP often requires the usage of organic solvents for mesh cleaning. Nevertheless, the low throughput of IJP and the restrictions of IJP inks to allow proper ejection, which limit the variety of materials that can be used, may favor the usage of SP for industrial applications. [131] Roll‐to‐roll gravure would also be an alternative for industrial production, as it has a high throughput and lower waste generation than SP. Another factor that needs to be taken into consideration for improving printing sustainability is ink formulation. In this regard, the selection of low‐toxicity and low‐impact solvents, binders, and other used chemical reagents needs to be prioritized.[ 131 , 132 , 139 ]
4.2. 3D Printing Techniques
There are many different 3D printing techniques that allow fabricating devices using a big diversity of materials and object geometries. 3D printing is usually compatible with a larger variety of substrates than 2D printing and in some cases can even be used in the absence of a substrate. All these techniques are digitally controlled and are based on layer‐by‐layer deposition, which allows having a precise control over thicknesses. [127] Below, four of the most common 3D printing techniques are explained:
Direct Ink Writing (DIW) is a material extrusion technology, i. e. a non‐Newtonian highly viscous ink is extruded through a nozzle, which automatically moves to the desired deposition locations. [127] DIW was used in the fabrication of supercapacitors,[ 63 , 102 ] triboelectric nanogenerators (TENGs), [81] transistors, [140] and resistive heaters. [110]
Fused Deposition Modeling (FDM) is also a material extrusion technology, but in this case the printed material is provided as a thermoplastic filament that is melted at the nozzle. [127] This differs from DIW, where the printed material is already introduced in a paste form. Materials such as PLA or nylon can be printed using FDM.
Stereolithography (SLA) consists in the solidification of a photocurable liquid resin due to the application of a focused light source, which is normally a laser.[ 127 , 141 ] The surface of the resin is selectively scanned by the laser, which allows building 3D structures due to layer‐by‐layer curing. Highly accurate details in the printed objects can be achieved with SLA.
Selective Laser Sintering (SLS) consists in the application of a laser on a bed of powdered material, typically metal, plastic, or ceramic, to selectively fuse material particles, creating objects layer‐by‐layer.[ 141 , 142 ] This technique allows printing complex geometries.
5. Transient and Sustainable PoC Sensing Devices
When designing a PoC diagnostic test based on an electrochemical sensor, several components must be carefully considered to ensure effectiveness and usability in limited‐resource settings, which are depicted in Figure 5. The electrochemical sensor is the core component responsible for detecting and quantifying the target analyte. It consists of electrodes, typically working (WE), reference (RE), and counter electrodes (CE), immersed in an electrolyte solution. The battery provides the necessary power to drive the electrochemical reactions occurring at the sensor electrodes, ensuring autonomous operation without reliance on external power sources. Some alternatives to batteries as energy‐supplying components include supercapacitors and energy harvesters. The electronic circuitry processes the sensed signal and converts it into readable information. It may also be required to transform the electrical signal generated by the energy‐supplying component to adjust it to the requirements of the sensing device. The readout system presents the results to the user in a clear and understandable format, often through visual displays or simple indicators such an electrochromic display. Finally, the encapsulation ensures the protection and stability of the internal components from environmental factors such as moisture and mechanical shocks. Integrating all these components into a compact and portable device enables equipment‐free operation and ease of use in resource‐limited settings, thereby enhancing accessibility to diagnostic testing.
Figure 5.
Schematic representation of the possible structure of the electrochemical biodegradable PoC diagnostic device highlighting the main components: the electrochemical sensor, the electronic circuitry, the powering part and the read‐out system.
In this section of the review, examples of individual components containing biodegradable materials are presented. Examples of electrochemical sensors, energy‐supplying components, and electronic‐circuitry components are introduced in separate subsections. The usage of printing technologies in the fabrication of components will be highlighted, although examples with other fabrication methods are also presented.
5.1. Electrochemical Sensors Containing Biodegradable Materials
Electrochemical sensors allow determining the concentration of a redox‐active substance in the sensing medium. Many electrochemical sensors use a standard three‐electrode amperometric configuration (WE, CE, RE) or a two‐electrode potentiometric configuration (WE, RE). [143]
The WE is where the chemical reaction takes place. The properties required for a good WE include high electrical conductivity, high surface area, and good stability in the electrolyte solution. Common materials used for WEs include metals such as gold, platinum, and silver, as well as polymeric semiconductors and carbon‐based materials. The CE is used to complete the electrical circuit and maintain electrical neutrality in the solution during the electrochemical reaction. The properties required for a good CE include high electrical conductivity, low impedance, and good stability in the electrolyte solution. Common materials used for CEs include platinum, gold, and graphite. The RE is used as a reference point for the measurement of the electrical potential of the WE. The properties required for a good RE include stable and reproducible potential, low impedance, and good stability in the electrolyte solution. Common materials used for REs include silver/silver chloride and saturated calomel. In summary, the materials used in electrochemical sensors as WE, CE, and RE should possess high electrical conductivity, low impedance, good stability in the electrolyte solution, and reproducible potential. The specific materials used for each type of electrode will depend on the specific application and requirements of the sensor.
In this section, examples of electrochemical sensors that could be used in PoC tests and that contain biodegradable materials are presented. The selected sensors have been chosen according to the relevance of their analytes for PoC diagnosis, including glucose, vitamins (ascorbic acid), and hormones and neurotransmitters (dopamine, melanin, and serotonin).
Nie et al. fabricated microfluidic electrochemical sensors on paper substrates. [50] Carbon and Ag/AgCl electrodes were screen printed on a cellulose paper and microfluidic channels were later patterned using either photolithography or wax printing. Detection of heavy metals using square‐wave anodic stripping voltammetry and glucose using chronoamperometry after glucose oxidase (GOx) functionalization were demonstrated. [50]
Valentine et al. fabricated glucose electrochemical sensors by drop‐casting a CNT suspension onto paper substrates with different porosities. [54] Methods such as laser cutting, drop casting, and sputtering of gold electrodes were used during fabrication, and transportation of the analyte to the electrodes was achieved with origami (Figure 6a). They observed that the porosity of the paper affected the limit of detection (LoD) of the fabricated sensor, so the choice of a paper as substrate had a great influence on the device performance.
Figure 6.
Electrochemical sensors containing biodegradable materials. a) Schematic of the fabrication process of a paper‐based electrochemical glucose sensor. Reproduced from ref. [54], Copyright (2020), with permission from American Chemical Society. b) Inkjet printed breathalyzer on a paper substrate: i) schema of the working principle of the device and ii) image of the printed sensor. Reproduced from ref. [51], Copyright (2016), article distributed under CC‐BY license c) Silk‐based dopamine and ascorbic acid sensor: i) schema showing the fabrication process of the device and ii) image of the finished sensors. Reproduced from ref. [33], Copyright (2016), with permission from Elsevier.
Ramadoss et al. fabricated a cellulose composite consisting of hydroxyethyl cellulose, sodium CMC, citric acid, and glycerol. [144] This composite could be used as a WE for electrochemically quantification of glucose in artificial sweat without the need of GOx. The material degraded within 15 days when buried in soil. The same material was also used for ethanol quantification in artificial sweat with a LoD of 0.34 mM. [145]
Another electrochemical sensor for glucose detection in artificial sweat was fabricated by Franco et al. [55] In this case, graphene electrodes acting as WE were hand printed on a cellulose paper substrate and modified by drop‐casting with Cu2O nanoclusters, while the RE was hand printed with a commercial Ag/AgCl paste. Similarly, a paper‐based sensor for detection of sweat glucose was presented by Bushra et al. [146] This sensor was based on a transistor with a reduced GO channel modified with Ag‐Cu2O nanoparticles, which catalyze glucose oxidation. In this device, the drain‐source current of the transistor depends on the glucose concentration.
Da Costa et al. fabricated a fully‐printed dopamine sensor in water on a paper substrate in the linear range from 10 to 100 μM. [108] A set of WE, CE, and RE were inkjet printed using a MWCNT aqueous ink. A mineral oil ink was also inkjet printed to define a passivation layer.
Vilouras et al. fabricated a serotonin sensor by depositing platinum electrodes on a 2.5 μm thick film made out of GO and chitosan. [106] This sensor was flexible and could be rolled up to a curvature radius of 500 μm allowing its integration into a syringe tip. Being able to detect serotonin from 0.2 μM to 2 mM, it could be used for the diagnosis of carcinoid syndrome.
Camargo et al. fabricated an electrochemical sensor on a waterproof paper substrate for the quantification of melatonin, whose concentration was related to sleep disorders and other diseases, with a linear range from 0.08 to 100 μM and paracetamol. [57] An ink consisting of graphite and nail polish was used for the WE and CE, and a silver ink was used as pseudo‐RE. Electrodes were directly painted using an adhesive mask to pattern the substrate. The same research group developed a similar sensor, which consisted of shellac and graphite electrodes painted on a waterproof paper, for the detection of the antibiotic sulfomethoxazole. [56]
Bihar et al. fabricated a disposable breathalyzer, i. e. a sensor for detecting ethanol concentration in human breath, on a paper substrate. [51] An Organic Electrochemical transistor (OECT) consisting of two PEDOT:PSS stripes was inkjet printed on the paper. A collagen‐based gel containing alcohol dehydrogenase (ADH) and Nicotine Adenine Dinucleotide (NAD+) was drop‐casted between the source‐drain and the gate (Figure 6b). Except for PEDOT:PSS, all the materials used would be able to biodegrade.
Pal et al. fabricated an electrochemical chronoamperometric sensor able to detect dopamine and ascorbic acid non‐specifically (LoD of 15 μM), and glucose when functionalized with GOx (LoD of 1.2 mM). [33] This sensor was based on silk proteins and consists of a cross‐linked fibroin substrate and of an active surface that was a composite of sericin and PEDOT:PSS. This composite acts as well as a negative resist for UV‐lithography, so it can be easily patterned using this technique (Figure 6c.i).
De Lima et al. developed a thermoplastic conductive composite consisting of two biodegradable polymers, PHB/V and poly(butylene adipate‐co‐terephthalate), and graphite microparticles. [147] This composite, modified with polyethyleneimine and creatine phosphate, was used for the specific detection of creatine kinase myocardial band (CK‐MB), a biomarker for acute myocardial infarction diagnosis, in saliva and urine samples using electrochemical impedance spectroscopy as sensing technique. With a LoD as low as 0.26 ng/mL, the sensor was suitable for CK‐MB detection in medical samples, as clinical concentrations range from 5 to 100 ng/mL. Photodegradation, thermal degradation, and biodegradation tests were performed with the composite. [147]
Sahraei et al. developed an electrochemical sensor on chromatographic paper substrates for the detection of cancer‐derived exosomes. [148] Carbon electrodes and an Ag/AgCl RE were hand printed on the paper. A composite of mesoporous carbon, CNTs and gold nanoparticles was deposited on the WE and later modified with antibodies specific for the recognition of a protein overexpressed in exosomes secreted by cancer cells. Sensing was performed in a medium containing a ferrocyanide/ferricyanide solution. The presence of exosomes in the medium caused a decrease in the current obtained from differential pulse voltammetry measurements. The sensor exhibited a linearity range from 102 to 107 exosomes/μL. [148]
An environmentally low‐impact biodegradable sensor for the detection of omeprazole was developed by De Moraes et al. [149] The used substrate contained PLA and a biodegradable photosensitive resist and was 3D‐printed using FDM and SLA. A set of graphite WE, CE, and RE was pencil drawn on the substrates. The fabricated sensors were water‐resistant and presented good repeatability, reproducibility, and selectivity.
A biodegradable testing device was demonstrated by Rengaraj et al. [52] They fabricated an impedimetric sensor for detection of bacterial contamination in water. In this case, a carbon‐based WE was screen‐printed on hydrophobic paper. The electrode was functionalized with lectin, which interacts with the mannose molecules found on the cell surface of some bacteria species resulting in an increase in the charge‐transfer resistance of the electrodes when reducing redox species are added to the medium, ferrocene in this case. Therefore, measured resistance could be related to the bacterial concentration. The LoD was 1.9 colonial forming units per mL (CFU/mL) and the sensing dynamic range was maintained up to 106 CFU/mL.
5.2. Powering Components Containing Biodegradable Materials
5.2.1. Batteries
Batteries are circuit elements which are able to convert chemical energy stored in certain substances into electricity, thus being able to power the circuit. The working principle of a battery is the electrochemical reaction between two redox‐active materials, which are spatially separated and connected electrically. [150] Batteries consist of an anode (electrode where the oxidation happens), a cathode (electrode where the reduction happens), an electrolyte (to allow ion conduction between the electrodes), and a separator (to avoid the possibility of short‐circuit between the electrodes).[ 150 , 151 , 152 ] A current collector is also required when the redox‐active materials are not electrically conductive. A solid electrolyte can act as a separator by itself, simplifying the design of the battery.
Primary batteries are those which cannot be recharged, so reuse is not possible after the materials have reacted. [150] In the case of a PoC diagnostic devices with the intended end‐of‐life route being biodegradation, reuse would be unnecessary, so primary batteries are the most interesting ones. Traditional primary batteries use highly toxic substances, but in the case of transient and biodegradable batteries, these substances must be avoided.[ 6 , 153 ] Some of the reported transient batteries are intended for powering medical implants, as their transience avoids the need of surgery for removal. In this case it is important to ensure the biocompatibility of the used material and complete bioresorption. [21] Apart from primary batteries, some examples of secondary batteries are also presented.
Esquivel et al. fabricated a fully biodegradable metal‐free flow battery that was activated when adding water. [58] Water dissolved the solid material containing both the electrolytes and the redox‐active substances, which could reach the porous carbon current collectors and react. The battery contained a cellulose‐based absorbent pad, which ensured ion conduction between the two electrochemical cells by capillary flow (Figure 7a.ii). Quinones were used as redox‐active substances. Cellulose and bee wax were used in the packaging. The open circuit voltage (OCV) and maximum power density for a single cell were 0.75 V and 6.8 mW/cm2, respectively, although voltage could be scaled up to 3.0 V by cell stacking. The batteries performed similarly to pure cellulose in anaerobic biodegradation tests, proving their biodegradability (Figure 7a.iii). [58] Another quinone‐based biodegradable flow battery, intended for precision agriculture applications, was developed by the same group. [154] Two big reservoirs for the dissolved redox species ensured that the battery was able to work at 0.2 mW/cm2 for more than three days. The reactant consumption and generated power depended on the evaporation rate of the solution, which was controlled by the area of paper pad exposed to air. Aerobic biodegradation and germination assays demonstrated the biodegradability and non‐toxicity of the fabricated battery. [154]
Figure 7.
Examples of powering components containing biodegradable materials. a) Biodegradable metal‐free flow battery: i) Image of the battery, ii) schema showing the material layers compounding the battery and its working principles, and iii) results of a biodegradability assay of the battery, the same battery without reactants, and cellulose acetate. Reproduced from ref. [58], Copyright (2017), with permission from John Wiley & Sons. b) Screen printed secondary batteries on paper: i) image showing the printed batteries and their flexibility and ii) SEM cross‐section showing the layers compounding the printed battery. Reproduced from ref. [48], Copyright (2021), article distributed under CC‐BY license (John Wiley & Sons). c) Schema showing the material layers compounding the food‐based edible supercapacitors. Reproduced from ref. [160], Copyright (2016), with permission from John Wiley & Sons. d) Schema showing the fabrication process and the materials compounding a BC‐based TENG. Reproduced from ref. [89], Copyright (2021), with permission from Elsevier.
Another example of water‐activated primary battery was developed by Poulin et al. [155] A zinc anode and graphite carbon collectors and cathode, which worked by reducing air oxygen, were stencil printed on a paper substrate containing sodium chloride. When water was added, the sodium chloride acted as electrolyte activating the battery, which presented an OCV of 1.2 V per cell. [155]
Rahmanudin et al. presented a stretchable redox‐diffusion battery. [156] This battery would use PGS as encapsulation, a composite of nanographite and cellulose fibers as current collectors, and an aerogel of CNF and PEDOT:PSS as electrodes. The ion‐selective membrane consisted of a composite of biomass‐derived polyurethane, CNF, and PSS, which was soaked in a H2SO4/PVA hydrogel electrolyte containing plant‐based redox‐active species. Due to the choice of materials, the battery should be able to biodegrade.
Yin et al. used Mg as anode and Mo, W, or Fe as cathodes for a bioresorbable battery. [82] The metal in the cathode would catalyze oxygen reduction and water reduction to hydrogen in aqueous media. An OCV of 1.05 V was achieved when using Mo as cathode. Phosphate buffer saline (PBS) solution was used as electrolyte and polyanhydride as packaging. A four‐battery stack setup would fully dissolve within 19 days in PBS at 85 °C. [82]
Huang et al. proposed a similar battery to the one presented by Yin et al., [82] but with some modifications. [93] They used Mg as anode and Mo as cathode current collector as well, but MoO3 nanoparticles were added as cathode active material. They used a hydrogel based on sodium alginate as electrolyte, instead of using a liquid electrolyte. PLGA was used as binder for the MoO3 nanoparticles and polyanhydride as packaging. The fabricated battery had a high OCV of 1.6 V and a maximum power density of 0.27 mW/cm2 and was able to operate for 13 days in dry conditions at 25 μA/cm2. Full dissolution was observed both in PBS and in vivo, i. e. as an implant, in less than one month. [93]
Jia et al. fabricated a battery based on silk fibroin, as this protein was used as packaging and as matrix for electrolyte and the cathode current collector. [34] A solid electrolyte was obtained by introducing choline nitrate ionic liquid in a fibroin matrix. Mg was used as anode and gold nanoparticles catalyzed the reduction of oxygen and water in the cathode. An OCV of approximately 1.5 V was achieved. Battery dissolution was observed when using proteases, although gold nanoparticles remained unaltered. [34]
Karami‐Mosammam et al. fabricated a stretchable and bioresorbable battery. [157] The battery consisted of Mg foil anode, MoO3 paste cathode, Mo foil current collector, a calcium alginate hydrogel separator, and PGS hydrogel encapsulation. The foils were patterned utilizing a laser to make them stretchable. The achieved OCP and maximum power density were 1.6 V and 196 μW/cm2, respectively. Over 70 % of the battery dissolved when submerging it in PBS at 37 °C for 10 weeks. The battery was used attached to the skin to power a wearable sodium sensor. [157]
Yang et al. developed a screen printed secondary battery using a hydrogel‐embedded cellulose paper as both substrate and separator. [48] The used hydrogel promoted liquid uptake by the cellulose paper, accelerating its biodegradation when buried in soil and facilitating the absorption of liquid electrolyte. The electrodes (a Zn/ZnO anode and a MnO2 or a Ni(OH)2 cathode) were screen printed in opposite sides of the paper (Figure 7b). The fabricated batteries showed good flexibility and cyclability and had higher energy and capacity densities than other paper‐based energy storing devices. [48]
Another example of a flexible paper‐based secondary battery was developed by Pandey et al. [49] The cathode consisted of a carbon paper sheet coated with graphite flakes and modified with electrochemically deposited polyanthraquinone. Aluminum foil was used as anode and origami paper soaked with NaCl electrolyte, as separator. The battery presented good cyclability and no alteration after the application of 1200 bending cycles. [49]
An edible bioresorbable secondary battery was developed by Chen et al. [158] The battery consisted of composite electrodes of active carbon and gelatin embedded in a gelatin separator. During the charging process, water splitting happened at the electrodes with generated gases being adsorbed in the pores of active carbon. The adsorbed gases would be transformed into water during the discharging process. The battery was dissolved in 20 minutes in a simulated gastric solution. [158] Another edible secondary battery was presented by Galli et al. [159] This coplanar battery was fabricated using food ingredients and food additives. The battery had an OCV of 0.65 V and presented some biodegradation in seawater biodegradation tests.
5.2.2. Supercapacitors
Supercapacitors are alternative energy devices to batteries characterized by their higher power density and charging cyclability. [150] In supercapacitors, energy is mainly stored in the electrodes in the form of an electric double layer, although sometimes a small quantity of a redox‐active species is also added. [150]
An edible and digestible supercapacitor was fabricated by only using food, food supplements, or food additives. [160] Gelatin was used as packaging, cheese as impermeable segregation layer, charcoal as electrodes, roasted seaweed as cell separator, Gatorade drink as electrolyte, egg albumin as binder, and gold as current collector (Figure 7c). The supercapacitor showed good cyclability (an 8 % of specific capacity loss after 1000 cycles) and was suitable for applications such as bacterial disinfection and powering of a commercial USB camera. [160]
Gouda et al. developed an electrode material for a pseudocapacitive supercapacitor. [161] The electrodes were based on a composite of sepia melanin and carbon quantum dots deposited on a carbon paper current collector. Filter paper was used as separator and an aqueous Na2SO4 solution, as electrolyte. The supercapacitor showed good cyclability. [161]
Chen et al. fabricated a wood‐based sandwich supercapacitor as carbonized wood blocks were used as electrodes and a wood membrane, as separator. [162] MnO2 was grown on the carbonized wood cathode and a PVA‐LiCl gel was used as electrolyte. The fabricated supercapacitor showed very high energy and power densities and high areal capacitance. [162] Another plant‐based supercapacitor was developed by Gao et al. [163] In this case, the materials for the substrate, carbon electrodes, and aerogel separators were obtained from an aloe leave. The supercapacitors were then implanted in an aloe leave using its fluids as electrolyte and was characterized using a golden wire for external connection.
A self‐healable flour‐based supercapacitor was presented by Hu et al. [164] The electrodes were made out of flour, water, sodium chloride, phosphoric acid, and activated carbon, while the separator was made out of flour, water, and sodium chloride. The supercapacitor was able to self‐heal after breakage with negligible performance loss and could also be stretched up to 50 % after healing. These devices were able to break down in both simulated gastric juice and soil. [164] Another example of a self‐healable supercapacitor was presented by Hsu et al. [103] This all‐in‐one supercapacitor, i. e. it did not have a layered structure, contained a hydrogel consisting of gelatin methacrylate, tannic acid, and CNCs, which was then coated with reduced GO and PANI. In this case, the biodegradability of the device was not tested, although many biodegradable materials were used in its fabrication. [103]
Another supercapacitor containing multiple biodegradable materials was fabricated by Figueroa‐Gonzalez et al. [104] It was based on graphene‐coated coconut‐fiber electrodes, one of them being modified with MgTiO3 nanoparticles. Rice paper soaked with electrolyte was used as separator, copper was used for the current collectors, and a mix of gelatin, water, and pectin was used as encapsulation. [104] An electrolyte film consisting of PCL and guar gum and doped with LiClO4 was developed by Sumana et al. [165] This solid electrolyte showed biodegradation in soil and was used in a supercapacitor using active carbon electrodes.
5.2.3. Triboelectric Nanogenerators
The inclusion of an energy harvester, i. e. a component that transforms small amounts of energy present in the environment into electricity, could be an alternative way of fabricating a self‐sustaining PoC test. However, it is possibly more challenging to integrate an energy harvester into a diagnosis test than a battery, due to the requirement of an external stimulus in order to power the device. One example of energy harvesters are TENGs, which convert deformations and vibrations into electricity. When two different materials are brought into contact and later separated, charge accumulates due to contact electrification, which can be used to power electronic circuits. [166]
Chen et al. fabricated flexible TENGs based on a composite of PGS and CNTs using DIW. [81] When using lipases, PGS easily degraded while CNTs remained unaltered, allowing their reuse. Zhang et al. presented a cellulose‐based TENG, containing a BC membrane sandwiched between two BC‐CNT‐PPy electrodes (Figure 7d). [89] The used CNTs‐PPy could be reused after degrading the device with cellulase. Zheng et al. also fabricated a transient TENG, which had Mg electrodes, a PLGA and PVA encapsulation, and triboelectric active layers made out of different combinations of biodegradable polymers (PLGA, PVA, PCL, and PHB/V). [80] The output voltages and currents depended on the used polymers, but the combination of PLGA and PCL was the one achieving the best performance. The devices were fully dissolvable in PBS and in vivo. Jiang et al. developed a similar TENG, but using natural biodegradable polymer films as friction layers and encapsulation. [90] The tried materials were rice paper, cellulose, chitin, silk fibroin, and egg white, showing excellent biocompatibility and bioresorbability when implanted in rats. A TENG developed more recently consists of a poly(trimethylene carbonate) encapsulation, Mg electrodes, a composite of PEG and polypropylene glycol as tribopositive layer, and a composite of PCL and ethylcellulose as tribonegative layer. [167] The device showed complete dissolution in PBS and in vivo. A conductive textile consisting of polypyrrole polymerized on Lycra fiber and modified with chitosan and phytic acid was proposed as a single‐electrode TENG. [168] Due to the flame‐retardant properties of the textile, it was proposed to be integrated in firefighters’ clothes and use human skin as tribopositive material. This material disappeared when buried in soil proving the biodegradability of the materials, although it has to be said that non‐biodegradable materials were used in small quantities, such as spandex found in Lycra textile.
5.3. Transistors Containing Biodegradable Materials
Active circuit elements are those capable of controlling, amplifying, or generating an electric signal.[ 169 , 170 ] Active elements include electricity generators (which have already been explained), diodes, and all the types of transistors, among others. [170] In the case of an autonomous sensing device, in order to control and process the electric signals of the sensor, the incorporation of an electronic circuitry with many transistors is required.
Several transistors fabricated on biodegradable substrates can be found in the literature. Mitra et al. demonstrated the fabrication of fully inkjet printed TFT arrays on a paper substrate with acceptable electrical performance. [44] Granelli et al. [140] presented fully‐printed OECTs on a cellulose acetate substrate using a DIW system. The OECTs consisted of a syringe barrel and an electroactuated piston, containing silver contacts, carbon source/drain electrodes, a PEDOT:PSS channel, and an electrolyte of PSS:Na+NaCl. These OECTs were successfully integrated in unipolar inverter circuits and also used as ion detecting sensors. [140] Similarly, Stucchi et al. also fabricated transistors on biodegradable substrates, using both IJP and chemical vapor deposition. [138] The reported device was an all‐polymer field effect transistor (FET) on Mater‐Bi substrate, a biodegradable plastic extracted from supermarket bags. Although the performance of these devices was inferior compared to other devices on plastic substrates processed at higher temperatures, they exhibited good reproducibility, mechanical robustness, and excellent flexibility (Figure 8a). The substrate with the printed FETs exhibited similar biodegradation by microorganisms in seawater than the bare substrate. [138]
Figure 8.
a) Extreme flexibility of OFETs fabricated on Mater‐bi substrate i) when rolled and ii) crumpled into a ball. Reproduced from ref. [138], Copyright (2021), article distributed under CC‐BY license. b) Pseudo‐MOS logic circuits fabricated on ultrathin cellulose acetate substrate with excellent flexibility i) when placed on a human hair and ii) when adapted to an avocado rough surface. Reproduced from ref. [8], Copyright (2017), with permission from PNAS. c) Degradation of dextran‐based OFETs by the action of Penicillium sp. on an orange surface. Reproduced from ref. [171], Copyright (2022), article distributed under CC‐BY license (John Wiley & Sons). d) Electrical circuit diagram and image of the screen‐printed self‐power OECT‐based halide sensor on paper substrate. Reproduced from ref. [136], Copyright (2020), with permission from IOP Publishing.
In addition to the use of biodegradable substrates, other researchers have focused on developing transient or biodegradable semiconductors. Lei et al. fabricated a transient pseudo‐complementary metal‐oxide‐semiconductor (CMOS) logic circuit that was fully dissolvable in an acidic cellulase solution. [8] The circuit was built on an ultrathin cellulose substrate, and used thermal evaporated Fe as electrodes (a version with gold electrodes was also fabricated), and atomic layer deposited alumina as dielectric. These devices demonstrated hole mobilities of 0.12 cm2 V−1 s−1, ON/OFF ratios of 104, and excellent flexibility (Figure 8b). The used semiconductor was a polyimide synthesized by the authors from dye molecules (diketopyrrolopyrrole and p‐phenylenediamine) and claimed to be biodegradable. Although the decrease in absorbance when keeping the polyimide in acidic conditions (pH=4.6) would indicate degradation of the monomers, degradation tests with microorganisms were not conducted. [8] Organic semiconductors such as natural Indigoidine and Tyrian purple biodegradable dyes, were used in ambipolar transistors fabricated on silicon using clean‐room technologies.[ 120 , 121 ] Nevertheless, low mobilities were achieved for Indigoidine, being of the order of 10−7 and 10−8 cm2 V−1 s−1 for electrons and holes, respectively. [121] For Tyrian purple, higher mobilities were achieved, being 3.1 ⋅ 10−2 and 5.8 ⋅ 10−4 cm2 V−1 s−1 for electrons and holes, respectively. [120]
Finally, several researchers have explored the use of biodegradable materials for the gate dielectric layer in transistors. Tall et al. used spin‐coated khaya gum, a biodegradable polysaccharide extracted from Khaya senegalensis trees, as capacitor and gate dielectric in OFETs. [28] These devices showed low operation voltages (VGS<−3 V and Vth=−1.3 V) and field‐effect mobilities of 0.3 cm2 V−1 s−1, which outperformed other transistors using natural materials as gate dielectric. Another proposed material as biodegradable gate dielectric was a bilayered dielectric consisting of toluene diisocyanate‐terminated PCL (TPCL) cross‐linked with a composite of PVA and CNCs. [111] This material was used in the fabrication of bottom gate top contact organic TFTs that used SWCNTs as semiconductor. The addition of the TPCL layer allowed increasing hole mobilities and ON/OFF ratios, while reducing hysteresis and humidity sensitivity. Although the materials are individually biodegradable, the biodegradability of the whole structure was not tested. Yang et al. proposed glutaraldehyde‐crosslinked dextran, a plant‐based material, as dielectric material for OFETs. [171] The crosslinking of dextran allowed this polysaccharide to be impermeable to water and resistant to high relative humidity. The devices exhibited a high mobility of 4.22 cm2 V−1 s−1, operation in relative humidities up to 80 %, and outstanding flexibility, being able to operate with bending radii of 0.0125 mm. The devices showed transient behavior in the presence of Penicillium sp. and Aspergillus flavus fungi (Figure 8c). Gelatin, a biodegradable material based on collagen, was used as gate dielectric in flexible OFETs, which achieved low operational gate voltages of −3 V and average mobilities of 1.6 cm2 V−1 s−1. [29] Pectin is another biodegradable material that have been proposed for low‐voltage transistors. [172] Li et al. developed an ink for IJP containing CNF and PVA and used it as dielectric material for the fabrication of synaptic transistors. [173] A cellulose‐based dielectric (cyanoethyl cellulose) was also used for OFETs fabricated on PVA‐coated paper substrates. [174] The fabricated transistors had relatively low operation voltages, between −5 V and 5 V. These devices were subjected to biodegradation assays in soil and combustion tests, showing that most of the OFET mass decomposed, although metallic layers remained unaltered. [174] Four different types of pine resins were also explored as dielectric materials for OFETs. [175] Sun et al. used a polymer electrolyte consisting of chitosan, dextran, and LiClO4 as gate dielectric to achieve solid‐state OECTs. [176] The transistors were fabricated on PET substrates by using screen‐printing and drop‐casting. PEDOT:PSS was used as semiconductor material. Finally, Rullyani et al. used PPC, a polymer that can be synthesized by using CO2 as raw material, as substrate and gate dielectric in the fabrication of transistors. [83] This polymer easily degraded when submerged in a lipase solution.
5.4. Integration of Individual Components in a Self‐Powered Sensing Device Containing Biodegradable Materials
The works presented above deal with the fabrication of individual electronic components using biodegradable materials found in literature. However, there has been minimal research published on the integration of multiple electronic components to create biodegradable autonomous PoC devices, particularly in the case of electrochemical sensors. This section provides a review of sensors that use biodegradable materials and are integrated with a powering device, including both those that are applicable and not applicable to PoC testing.
Nair developed a transient electrochemical PoC testing device, which serves as an example of how such a device could be constructed. [136] It consisted in a halide sensor based on an OECT, which was screen printed on paper and powered by printed batteries. The transistor channel was made of PEDOT:PSS and silver contacts were used, with the paper substrate serving as the gate dielectric. The reaction between silver and halide ions caused variations in the source‐drain current, which enabled the device to detect chloride ions in sweat samples. To facilitate read‐out, a paper well was built to contain the sample and a current‐to‐voltage converter was printed. The batteries used were based on the Zn‐MnO2‐ZnCl2 chemistry. However, the device was not fully printed, as commercial microcontrollers, regulators, and a LED for read‐out had to be glued to the paper substrate to process the signal (Figure 8d). Despite these limitations, this device integrated both sensing and powering on the same biodegradable paper platform, setting an important milestone in the development of biodegradable electrochemical PoC diagnostic tests.
Sardana et al. developed another example of a self‐powered sensor using a different approach. [177] They fabricated an ammonia gas sensor using an MXene/TiO2/CNF heterojunction that was powered by a TENG consisting of MXene and cellulose acetate nanofibers as triboelectric layers. MXene are 2D inorganic materials which present metallic conductivity. The sensor and TENG were attached to a human foot to use stepping motion as powering energy source. The researchers used a chip containing electronic circuitry to process the signals and LEDs as indicators. While some biodegradable materials were used, the researchers did not aim to create a fully transient device, and the components were not integrated into the same platform. Nonetheless, this work presents a promising approach to self‐powered sensing and sets the foundation for further development of biodegradable electrochemical PoC diagnostic tests.
Recent research has explored the incorporation of biodegradable materials in sensors including the use of antennae and inductor coils, which enable both powering and read‐out by using radiofrequency waves. The devices developed were capable of sensing humidity [178] and pressure, [179] and their wireless capability allowed for placement in locations that are difficult to reach, such as underground and inside a living organism. Nevertheless, these systems were dependent on an external device for powering and read‐out, which may not be ideal for PoC devices that require portability and ease of use.
Alternatively, TENGs have been used as self‐powered motion sensors, where their responsivity to movement was exploited. In one case, the TENG was connected to a LED that would illuminate upon motion detection. [81] In the other case, multiple TENG‐based sensors were used as an interactive virtual keyboard and connected wirelessly to a computer. [89] However, motion detection would hardly be of interest to be used in PoC devices, and the proposed self‐powering strategies would probably not be useful for these applications. Additionally, other self‐powered sensors that could be used for medical applications, based on energy harvesters, and fabricated using biodegradable materials can be found in literature. These include a sensor for detecting E. coli bacteria, [180] temperature, [181] pressure (it is proposed to be used for tooth health monitoring), [182] and motion caused due to cardiorespiratory activity. [183] Nevertheless, in most of these cases external instrumentation was required for the read‐out, so these proposals are still far from a fully integrated system.
6. Conclusions and Critical Discussion
The development of affordable PoC devices is crucial to ensure early diagnosis, which is particularly important in resource‐limited settings. Nevertheless, this may lead to the production of e‐waste, which would be problematic due to the lack of advanced waste management facilities in these areas. Therefore, the development of biodegradable PoC tests can facilitate proper waste management and contribute to sustainable healthcare.
From the materials point of view, there are plenty of biodegradable insulators, both natural and synthetic materials, which can be used as substrates, dielectrics, encapsulation, or packaging in electronic devices. Materials such as bioresorbable metals, carbon materials, and polymeric semiconductors are used as conductors and semiconductors. Nevertheless, due to the limited biodegradability of some of these materials, they are often incorporated in composites with a biodegradable polymeric matrix to enable a transient behavior. Apart from material biodegradability, the environmental impact of fabrication and material extraction and production processes also needs to be considered. For properly optimizing sustainability and minimizing carbon footprint, lifecycle analysis (LCA) should be performed when designing an electronic component or selecting a specific material or fabrication technique. [184] Unfortunately, LCA is not commonly addressed in most of the studies reporting electronic components fabricated with biodegradable materials. Nevertheless, certain general strategies can be implemented to reduce environmental impact. These include sourcing materials from waste streams to promote circularity, and following the principles of green chemistry. Furthermore, low carbon footprint processes, such as printing technologies, should be employed for device fabrication.
Despite the extensive efforts that have been done for the incorporation of these biodegradable materials in the development of novel devices, they are usually incorporated in limited areas of the sensors, powering components, and active electronic components. This is stated because prototypes of fully biodegradable PoC diagnostic devices with self‐powering capabilities have not yet been reported. The halide sensor developed by Nair [136] may be one of the closest devices to this ideal, but it still relies on commercial microprocessors and non‐biodegradable materials, and biodegradation assays were not performed. Therefore, integrating printed processors into the device is one of the next necessary steps, despite significantly increasing its complexity.
It is highly remarkable that most of the presented devices still contain non‐degradable materials, such as gold and silver, which is not ideal for transient applications due to the loss of valuable materials and the potential introduction of these materials into natural environments. Therefore, it is crucial to avoid these materials during the design steps of transient devices. In the event that these materials are difficult to replace with biodegradable alternatives, implementing PoC device recycling systems could be a viable solution to recover these valuable materials, thus adhering to the principles of circular economy. This would involve developing advanced recycling technologies that allow for the efficient separation and recovery of valuable materials, thereby avoiding resource loss and reducing the environmental impact associated with the extraction and production of new materials.
From the point of view of the use of biodegradability concept in this review, several examples of electrochemical sensors, powering components, and transistors containing biodegradable materials have been presented. On the one hand, it is important to note that the term “biodegradable” is often misleadingly used in the literature. In many cases, “transient” or “bioresorbable” would be more appropriate terms for the reported devices. Moreover, a plethora of methodologies can be found in literature to claim biodegradability, including disappearance in vivo, dissolution in aqueous media with certain pH and temperature conditions, enzymatic degradation, and disappearance in soil or compost. However, only the capacity of materials to decompose due to the action of microorganisms should be considered as true biodegradability. On the other hand, some of the tests used to claim biodegradability are insufficient, and there is no control over the degradation by‐products formed during enzymatic degradation or dissolution in aqueous media. [21] One of the observed issues is that there are numerous standardized tests for evaluating biodegradability, making it difficult to compare the “claims” of biodegradability between materials. Moreover, the assays performed to demonstrate biodegradability of fabricated devices are often inaccurate. Various organizations have standardized methods for assessing this property, among which the tests by the Organization for Economic Cooperation and Development (OECD) stand out for their widespread international use. We want to emphasize the importance of rationalization of the number of standardized tests and optimizing the existing ones to make them more accessible to potential users. This would facilitate the comparison of biodegradability across different materials and ensure more reliable and consistent results. Additionally, simplifying and optimizing these tests could accelerate the development and adoption of biodegradable materials in various applications, contributing to environmental sustainability and reducing the impact of non‐degradable waste.
As the future of the IoT rapidly approaches, the number of sensor devices or individual components being manufactured in a sustainable way is still relatively low to cover the expansive scope that communication technologies envisions. Very few works have focused on the development of self‐powered sensors using biodegradable materials. While the concept holds promise for revolutionizing various sectors, including healthcare through the IoMT, there is a pressing concern regarding the environmental repercussions, particularly in limited‐resource settings. As disposable devices become integral to personalized medicine advancements, there is an inevitable surge in their utilization. However, the absence of sophisticated waste management facilities in these settings exacerbates environmental problems stemming from PoC tests. With non‐biodegradable electronic waste posing significant toxicity risks, the need for eco‐friendly electrochemical PoCs based on disposable systems becomes increasingly urgent. Therefore, the integration of biodegradability into PoC tests emerges as a crucial consideration, offering a viable solution to mitigate the environmental impact and ensure a sustainable transition towards the interconnected future envisioned by communication technologies.
Abbreviations
- ADH
Alcohol dehydrogenase
- BC
Bacterial cellulose
- CE
Counter electrode
- CFU
Colonial forming units
- CK‐MB
Creatine kinase myocardial band
- CMC
Carboxymethylcellulose
- CMOS
Complementary metal‐oxide‐semiconductor
- CNCs
Cellulose nanocrystals
- CNFs
Cellulose nanofibers
- CNTs
Carbon nanotubes
- CRMs
Critical Raw Materials
- DIW
Direct ink writing
- DNA
Deoxyribonucleic acid
- DoD
Drop‐on‐demand
- FDM
Fused deposition modelling
- FET
Field effect transistor
- GO
Graphene oxide
- GOx
Glucose oxidase
- IJP
Inkjet printing
- IoMT
Internet of the Things
- IoMT
Internet of the Medical Things
- LCA
Lifecycle analysis
- LED
Light emitting diode
- LoD
Limit of detection
- MWCNTs
Multi‐walled carbon nanotubes
- NAD+
Nicotine Adenine Dinucleotide
- OCV
Open circuit voltage
- OECD
Organization for Economic Cooperation and Development
- OECT
Organic electrochemical transistor
- OFETs
Organic field effect transistors
- P4HB
Poly(4‐hydroxy‐butyrate)
- PANI
Polyaniline
- PBS
Phosphate buffer saline
- PCL
Polycaprolactone
- PEDOT:PSS
Poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate
- PEG
Polyethylene glycol
- PGS
Poly(glycerol sebacate)
- PHB/V
Poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)
- PLA
Polylactic acid
- PLGA
Poly(lactic‐co‐glycolic) acid
- PoC
Point‐of‐care
- PPC
Polypropylene carbonate
- PPy
Polypyrrole
- PVA
Polyvinyl alcohol
- RE
Reference electrode
- SLA
Stereolithography
- SLS
Selective laser sintering
- SWCNTs
Single‐walled carbon nanotubes
- TEMPO
(2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl
- TENG
Triboelectric nanogenerator
- TFT
Thin‐film transistor
- TPCL
Toluene diisocyanate‐terminate polycaprolactone
- WE
Working electrode
- WEEE, e‐waste
Waste Electrical and Electronic Equipment
Conflict of Interests
The authors declare no conflict of interest.
7.
Biographical Information
David Batet is a predoctoral researcher at the IMB‐CNM (CSIC). He has a Bachelor in Nanoscience and Nanotechnology from the Autonomous University of Barcelona and a Master in Nanomaterials and Nanophysics from Aalborg University. His current research interests include printed electronics, biodegradable materials, and self‐powered sensors.
Biographical Information
Gemma Gabriel is Tenured Scientist at the IMB‐CNM (CSIC) and also forms part of CIBER‐BBN. She has a PhD in Materials Science from the Autonomous Univ. of Barcelona. Her work is transversal, spanning the entire fabrication process from materials development, sensor design to microelectronic manufacturing and characterization for specific applications in the biomedical field. She has authored more than 50 scientific contributions and is inventor in 5 patents, 2 of them licensed.
Acknowledgments
The authors thank the help provided by J.P. Esquivel. The authors acknowledge financial support provided by the project CEL‐SENS (TED2021–132164B‐C22) with funds from MCIN/AEI/10.13039/501100011033 and the European Union ′NextGenerationEU′/PRTR and the project ECOTRONIC (RTI2018–102070‐B‐C21, MCIN) with funds from the Ministerio de Ciencia e Innovación of the Spanish Government. D B also acknowledges the support provided by FPI PhD fellowships (PRE2019–090103). The authors also thank the support of the SU‐8 Unit of the CIBER in Bioengineering, Biomaterials, and Nanomedicine (CIBER‐BBN) at the IMB‐CNM (CSIC) of ICTS ′NANBIOSIS.′ This work also made use of the Spanish ICTS Network MICRONANOFABS partially supported by MEINCOM.
Batet D., Gabriel G., ChemSusChem 2025, 18, e202401101. 10.1002/cssc.202401101
Data Availability Statement
Data sharing is not applicable to this article as no new data were created or analyzed in this study.
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Data Availability Statement
Data sharing is not applicable to this article as no new data were created or analyzed in this study.