Laser induced graphanized microfluidic devices

Abstract

With the advent of cyber-physical system-based automation and intelligence, the development of flexible and wearable devices has dramatically enhanced. Evidently, this has led to the thrust to realize standalone and sufficiently-self-powered miniaturized devices for a variety of sensing and monitoring applications. To this end, a range of aspects needs to be carefully and synergistically optimized. These include the choice of material, micro-reservoir to suitably place the analytes, integrable electrodes, detection mechanism, microprocessor/microcontroller architecture, signal-processing, software, etc. In this context, several researchers are working toward developing novel flexible devices having a micro-reservoir, both in flow-through and stationary phases, integrated with graphanized zones created by simple benchtop lasers. Various substrates, like different kinds of cloths, papers, and polymers, have been harnessed to develop laser-ablated graphene regions along with a micro-reservoir to aptly place various analytes to be sensed/monitored. Likewise, similar substrates have been utilized for energy harvesting by fuel cell or solar routes and supercapacitor-based energy storage. Overall, realization of a prototype is envisioned by integrating various sub-systems, including sensory, energy harvesting, energy storage, and IoT sub-systems, on a single mini-platform. In this work, the diversified work toward developing such prototypes will be showcased and current and future commercialization potential will be projected.

I. INTRODUCTION

The advent of various carbon-based nanomaterials like nanofibers, nanotubes, etc., in the recent few decades, has significantly enhanced the production of wearable and flexible microfluidic sensors.¹ Employing carbon nanomaterials in these microfluidic devices has proven to be a boon with respect to the improvement of long-term stability, sensitivity, mechanical, physical, and chemical properties of these devices. Of late, one such material being extensively explored is a 2D allotrope of carbon referred to as graphene. This sp² hybridized, planar structure of hexagonally arranged carbon atoms has captivated considerable attention of researchers in academia and industry. Graphene was first synthesized in 2004, and it was exceedingly rigid and had decent thermal conductivity.² A few of the exceptional properties of graphene are shown in Fig. 1.^3,4

FIG. 1.

Schematic highlighting the general properties of graphene.

These tunable properties have made graphene one of the most likable materials by various sects of industry and academia. There has been continuous growth in commercial applications for graphene like development of energy devices, microelectronic devices, sensors, transponders, antenna development, biomimetic devices, etc. The increase in application leads to an increase in production. The commercial wet synthesis of graphene is conventionally carried out by a bottom-up approach, a top-down approach, mechanical/electrochemical/liquid phase exfoliation, and chemical vapor deposition. Nevertheless, these approaches give scalable yield but have certain limitations like the use of elevated temperatures, polluting reducing agents, pyrolysis, annealing, etc., and often result in the formation of graphene with minimal control over the shape. Cost and economics are also at stake with these approaches.⁵ Therefore, a simple laser ablation method was found to generate graphene over various inexpensive substrates in a single step preparation. This was termed laser-induced graphene (LIG).⁶ Discovery of direct laser engraving and fabrication led to possibilities of preparing LIG. Of late, graphene has been used significantly for several applications like sensing, energy devices, optics, commercial electronic devices, etc. However, the preparation of graphene via chemical process is a multistep–tedious process requiring a chemical synthesis approach. Hence, it requires a large amount of reagents and is non-economic. Traditional methods use a high temperature, strong reducing agents, high pressure, and energy that often gives disruptive morphology, lower efficiency, and are lesser economical when concerned about scaling. LIG is one of the outcomes for finding out the simpler and economic routes of graphene synthesis. In this, a direct laser can be employed to transform carbon substrates into graphene and its derivatives. LIG resembles the properties of graphene, can be prepared in a single step process, cost-effective, uses lesser reagents, and has the capability of scaling, hence, a suitable alternative to conventional graphene. One of the major advantages of LIG is the ability to develop it over flexible and bendable substrates without losing the overall property. Hence, these LIG materials are extensively being used in rapidly prototyped microfluidic devices.

II. RAPIDLY PROTOTYPED MICROFLUIDIC DEVICES

A. Microfluidics: Brief snapshot

Microfluidic systems, which have grown swiftly in popularity in the recent years, have become an effective tool for multiple applications. Vast developments in strategic designs, fabrication approaches, automation, integration, IoT enablement, and applications have taken place. Numerous reports like analytical sensing, biosensing, energy harvesting, and other applications are available.^7–9 Flow rate monitoring, microchannel geometry, and droplet generation are a few of the major aspects of microfluidics essential for efficient device fabrication.¹⁰ Evidently, microfluidics has become a distinctive technological field, but the history of it, going back to a couple of decades, did not have such precise boundaries. There has been a gradual progress through the years via development of strategies and studying various concepts pertaining to fluid flow by leveraging state-of-the-art microfabrication tools. The rise of microfluidic devices is commonly linked to the late 1960s due to crossing over the milestone of gas chromatography and the development of inkjet printer nozzles. However, it was hundreds of years before this in 400 BC., when researchers like Hippocrates performed bedside analysis of urine samples. Though they used flasks for sample analysis and not the sophisticated microdevices but triggered the possibilities of developing diagnosis at point of care.¹¹ Several years later, Jurin and his co-workers studied the behavior of fluids under the influence of pressure while passing through small glass containers and cylindrical pipes, which described the present theory of microfluidic flow.¹² These studies led to significant research in combining the fluid flow with small, miniaturized devices. Toward the end of the 1960s, Finnigan Instrument Corporation, developed the first prototype of gas chromatography controlled using a minicomputer and later, the use of the electrostatic force to print ink drops on paper was explored. These conquered milestones led to the development of laser printer in 1976.¹³

After the establishment of several theories and concepts, a path breaking research article about the fabrication of on chip electrophoresis by Mans, in 1993, using a glass substrate of 1 × 2 cm dimension, with the use of electro-osmotic pressure pumping of fluid came through.¹⁴ Whitesides et al., in 1998, reported fabrication of microfluidic devices using a transparent elastomer, polydimethylsiloxane (PDMS), using an advanced technique, photolithography.¹⁵ Pollack et al., in 2000, developed the concept of digital microfluidics, wherein, droplets of pico- and micro-size could be generated within the devices.¹⁶ Martinez et al., in 2007, described a technique for developing paper based microfluidic devices for bioassays.¹⁷ Kitson et al. in 2012 explored the use of 3D printing technique for fabrication of microfluidic reactors, which is one of the leading methods today.¹⁸ Over the years, there has been a remarkable growth in this field, however, challenges related large scale production, applications, cost-affectivity, integration, and automation are still to be addressed. Emerging advances like integration of microdevices with Internet of things (IoT), smart phones, machine learning, artificial intelligence, etc., could resolve these issues in future.

B. Current status of microfluidic device fabrication

Based upon various types of substrates, applications, and other features like flexibility, bend ability, material compatibility, etc., there are different approaches to fabrication of microfluidic devices. Figure 2 gives a summarized schematic representation of the current state-of-the-art fabrication methods adapted in general.¹⁹ These are the following:

• (1)Photo lithography: Herein, any specified strength optical beam is used for designing patterns as per the requirement over a substrate. Hence, it is also termed optical lithography. Using this approach, microchannels, electrode patterns, designs, etc., can be made over flexible thin film substrates, solid substrates, glass, etc. Patterns as small as 100 nm can be drawn with this approach. Optical beams like ion beam, e-beam, x ray, ultraviolet rays, etc., are employed for this. Figure 2(a) is the schematic representation of the process. A mask is made and placed over the substrate, upon exposure of optical beam, the pattern gets engraved as per the mask.¹⁵
• (2)Soft-lithography: Since early 1998, soft-lithography has been used with a polydimethylsiloxane (PDMS), polyimides, polyurethanes, etc. (liquid materials/polymers) for fabrication of microdevices.²⁰ A master mold, which has desired pattern and structure, is designed by micromachining. A liquefied polymer, which is homogeneously mixed with a curing agent, is poured over the master mold. Further heating to higher temperatures will solidify the polymer, and it acquires the shape of the mold. Figure 2(b) is the schematic representation of the process.²¹
• (3)Embossing: Also termed hot embossing is a simpler approach wherein a mold is made with inert and heat-resistant material like silicon. The mold has desired structure and microchannel patterns. A solid polymer is placed between the mold. High temperature and pressure are applied, which cause the polymer to melt. The melted polymer acquires the shape of the mold grooves. Furthermore, the temperature is reduced to solidify this polymer. Once solidified as a firm structure, the mold is removed to obtain the device. Figure 2(c) is the schematic representation of this approach.¹⁹
• (4)Molding: Herein, a master mold with grooves and patterns is fabricated using photolithography with materials like silicon. Liquefied polymer is injected into this mold, followed by curing and solidifying. Figure 2(d) is the schematic representation of this approach.¹⁹
• (5)3D printing: It is a technique of layer-by-layer material deposition, also termed additive manufacturing. This approach uses different types of 3D printers based upon the material being deposited. Materials like conductive filaments are used. For example, acrylonitrile butadiene styrene (ABS), poly lactic acid (PLA), polyethylene terephthalate (PET), wood fiber (cellulose + PLA), poly vinyl alcohol (PVA), etc., are used. Different commercially available 3D printers are fused deposition molding (FDM), stereolithograhy (SLA), extrusion, lamination, photopolymerization, and multijet molding. Following are the fabrication steps: (1) Designing the device pattern using Computer Aided Design (CAD) software. (2) Conversion of CAD file to Triangle Language (STL) file, i.e., compatible with the 3D printer. This helps in forming a 3D picture into a 2D layer via a G-code file that allows the 3D printer to print through layer by layer deposition. Figure 2(e) is the image of a commercial 3D printer.¹⁹
• (6)Inkjet printing: It is a newer approach, an inkjet commercially available printer is used here. The printer has a nozzle, which is filled with a conductive ink of certain viscosity. Substrates like paper or glass or sheet can be affixed using clamps at the base of the machine. The design pattern is given into the computer as a compatible file. The nozzle sprays the ink as per the pattern. The distance between the nozzle and the substrate is adjusted to get a proper pattern. Once the pattern is drawn completely, it is dried before using. Figure 2(f) shows the real image of the commercial inkjet printer.¹⁹
• (7)Laminating: Stacking of separately cut layers followed by bonding is done using a laminator machine. These devices have three layers in general: bottom layer, intermediate, top layer. Microchannels are designed in the intermediate layer. Bonding of these layers gives a firm device. The number of layers can also be modified as per application. Substrates like poly(methyl methacrylate) (PMMA), glass, polycarbonate, etc., can be used. Adhesive double-sided tapes or thermal bonding is used for bonding. In thermal bonding, heating of the substrate up to the melting point is done. Laser or a knife cutter plotter is used for making channels. However, based on the application and the substrate material, the bonding and channel patterning are done. Figure 2(g) is the schematic of the lamination machine.¹⁹
• (8)Screen-printing: Paper, glass, sheet, or flexible substrates are used here for making microfluidic devices. A wooden frame is employed to fix the substrate over. A pattered mesh or mask made up of nylon or silk is used. The mesh has desired patterns, conducting ink is poured and dried at elevated temperatures. The mesh ensures that the pattern is drawn in the same dimension. Over the printed patterns, PDMA, PMMA, acrylic-, or glass-based microfluidic channels can be placed and bonded to form a device. Figure 2(h) is the schematic of the screen printing method.
• (9)Laser ablation/cut: This approach uses lasers of specified speed and power like UV laser, CO₂ laser, pulsed laser, diode, etc., over substrates like paper, silicon, polymer sheets, plastic, glass, etc., for designing cuts and patterns as microchannels. Furthermore, exposing of the laser to a carbon precursor has reportedly been useful to generate laser-induced graphene (LIG) of various forms. Figure 2(i) is the schematic of the screen printing method.

FIG. 2.

Schematic representation of various types of microfluidic device fabrication techniques.

TABLE I.

Different types of lasers employed for fabricating LIG.^22–33

S. No.	Laser	Carbon substrate	Reference
1	650 nm	Graphene oxide	22
2	450 nm	Graphene oxide	23
3	248 nm (UV)	Graphene oxide	24
4	788 nm (CW)	Graphene oxide	24
5	800 nm (femtosecond)	Graphene oxide	25
6	650 nm	Graphene oxide	25
7	532 nm	Graphene oxide	26
8	450 nm	Polyimide sheet	27
9	10.6 m CO2 pulsed	Polyimide and Polyetherimde	28
10	CO2 pulsed Laser	Polyimide	29
11	514.5 nm Ar–Kr	Sulfonated poly(ether ether ketone)	30
12	10.6 m CO2 pulsed	Paper, coconut, bread, potato, etc.	31
13	9.3 m CO2 pulsed	Teflon	32
14	532 nm	Methane	33
15	10.6 m CO2 pulsed	Wood	34
16	CO2	Single layer graphene	35
17	Ultra short pulsed laser	Graphene oxide	24
18	Nd:YAG/5 ns	Single layer graphene	36
19	CW	Single layer graphene	33

III. LASER-INDUCED GRAPHENE (LIG): WORKING PROCESS

A. Opportunities to synthesize laser-induced graphene

The fabrication of LIG is dependent upon the substrate material and the type of laser being used. Based on this, various types of LIG preparation approaches, like heating, ablation, exfoliation, photochemical and thermal reaction, direct laser writing, etc., can be used. The literature reports suggest that various substrates, like glass, paper, wood, potato skin, bread slice, silicon, plastic, coconut shell, etc., have been used for forming LIG. Several research groups have reported various opportunities to synthesize LIG. Kurra et al. gave a summarized table of such different approaches.

B. Process steps for laser-induced graphene

Figure 3 gives the schematic for the fabrication of LIG. Similar approach can be utilized over various base substrates. Following are the steps in fabrication of LIG. (A) The design of the required pattern is made using a design software, AutoCAD Fusion 360. The design is converted into a .dxf file. This is transferred to a CorelDraw X7 file, which is compatible with the laser. (B) Carbon precursor substrates like paper, polyimide, coconut shells, etc., can be chosen. For materials that are prone to charring, a fire retardant is sprayed before exposing to the laser. (C) The substrate is affixed under the laser beam. The parameters of lasers like speed, power, and height are optimized. (D) The substrate is ablated by adjusting the laser distance. (E) The pattern is obtained on the substrate as per the design in the software.

FIG. 3.

Schematic representation of stepwise process of LIG fabrication.

1. LIG morphological and surface modification

LIG can be used as formed as well as can be further chemically modified for improving the properties and applications. The hybridization, functional groups like oxygen, nitrogen, and other composites as well as chemicals can be modified on the LIG surface. This will enhance mechanical power, conductivity, reactivity, stability, etc. Adjustment of laser parameters is one of the methods for modification. Morphology, wettability, and carbonization can be altered by changing the power, speed, and frequency of the laser.³⁷ The moisture and water contact angle in the LIG surface can be altered by controlling the atmosphere in which LIG is fabricated.³⁸ Exposing the substrate like polyimide multiple times by a laser repeatedly can change the structural morphology of LIG as carbon nanotubes.³⁹

Doping of LIG with various metal catalysts, heteroatoms, etc., can also be achieved. For example, Tour et al. developed boron doped LIG, wherein boronic acid mixed with poly amic acid solution was exposed to CO₂ laser.⁴⁰ In a similar approach, various metal catalyst solutions like cobalt oxide, molybdenum oxide, and iron oxide were mixed with poly amic acid solution and exposed to laser to obtain metal doped LIG. Doping of metal nanoparticles like gold, silver, and platinum and heteroatoms like nitrogen and oxygen is reported.^41–43 Figure 4 is the reprint of the schematic representation of metal doped LIG fabrication.⁴¹

FIG. 4.

Reprint of the schematic representation of fabrication of metal doped LIG.⁴¹ Reproduced with permission from Peng et al., ACS Nano 9, 5868–5875 (2015). Copyright 2015 American Chemical Society.

C. LIG characterization

Post fabrication, the formed LIG has to be physicochemically characterized to study the shape, bonding, hybridization, functional groups, and elemental analysis. Based on the characterization, the LIG shows specific properties and can be used for various applications. Figure 5 is the schematic representation of various common methods of characterization.

FIG. 5.

Schematic representation of various physicochemical characterization techniques.

D. Various forms of graphene

Typically, pure graphene is a single layer of carbon atoms arranged as the thin film. These layers can also be re-deposited over other substrates. Graphene exists in various forms like graphene oxide (GO), reduced graphene oxide (rGO), graphene nanoribbons, graphene nanoplatelets (GNPs), graphene ink, graphene master batches, and graphene quantum dots. Figure 6 gives the schematic representation of various forms of graphene.⁴⁴

• (1)Pristine graphene: The purest form of graphene has a single layer of hexagonally arranged carbon atoms in a honeycomb structure with Sp² hybridization.
• (2)Graphene oxide (GO): Millions of layers of graphene combined together give a 3D structure for graphite. The oxidation of graphite gives oxygen functional groups called graphite oxide, which upon sonication gives graphene oxide as a single or multiple layers.
• (3)Reduced graphene oxide (rGO): The GO can further undergo reduction process. Herein, either chemical or thermal or electrochemical reduction of GO can be done to produce rGO. Each method gives rGO with various properties, surface area, chemical composition, etc.
• (4)Graphene nanoplatelets (GNPs): These are nanoparticles of size 1–15 mm thick. Micromechanical breaking of graphite forms graphene nanoplatelets.
• (5)Graphene quantum dots: These are single, multiple layer quantum dots and can be made from both graphene as well as graphene oxide. The arrangement of the hexagonal structure can be zigzag, armchair, or a mixture of both forms.
• (6)Graphene nanoribbons: These are thin width (2–5 nm), quasi-1D graphene layer.
• (7)Graphene ink: Sol–gel approach based conductive ink of specified viscosity.
• (8)Graphene master batches: These are a combination of polymer and graphene-based materials, which has properties of both.

FIG. 6.

Schematic representation showing structures of various forms of graphene.

IV. LIG BASED MICROFLUIDIC DEVICES

A. Fabrication processes

LIG based microfluidic devices can be fabricated via different approaches depending on the type of application. One of the most commonly used is laser ablation, which was discussed in Sec. III. LIG, being a porous material, can be used for fabrication of microfluidic channels, which can aid fluid flow. Some of the LIG based microfluidic devices reported are discussed here. Tan et al. developed a LIG based microfluidic device from block copolymer (BCP) via laser ablation. A microchannel for fluid transport was formed by etching of BCP via laser and a PDMS top layer.⁴⁵ Griesche et al. developed LIG based electrodes over various substrates using CO₂ laser and transferred LIG electrodes to flexible substrates.⁴⁶ Chen et al. reported microfluidic energy devices and biosensing microdevices with LIG formed using a CO₂ laser.⁴⁷ Likewise, Zhu et al. fabricated LIG fibers based sensors for nonenzymatic detection of glucose in the body.⁴⁸ Gerstl et al. reported fabrication of LIG based immunosensor for the detection of pathogens in a PMMA based microfluidic device fabricated using a CO₂ laser.⁴⁹ Abdul-Aziz et al. reported fabrication of LIG over polyethersulfone polymer (PES) using a CO₂ laser, followed by studying its application as a supercapacitor in wearable devices.⁵⁰

Similarly, Goel et al. have also fabricated several LIG based microfluidic devices. For example, a PDMS-LIG based microfluidic biofuel cell was designed. First, LIG bioelectrodes were formed by CO₂ laser ablation over a polyimide sheet. These were placed over a glass side via adhesives. Over this, a Y shaped, PDMS microfluidic channel, fabricated by soft-lithography was placed. The microchannel has inlets and outlets and was integrated by bioelectrodes. Figure 7(a) is the reprint of their schematic representation of overall device fabrication.³ LIG based microfluidic electrochemiluminescence (ECL) platform was also fabricated by the same group. Herein, a polyimide sheet was taken as a base substrate. The microchannel design was made in CoralDraw software, and the pattern was ablated on the sheet. The laser formed LIG, which was washed and removed giving a microchannel cavity. Furthermore, the sheet was re-ablated with laser, second time to generate LIG electrodes. Figure 7(b) is the reprint of their schematic representation of the device fabrication.⁵¹ An interdigitated microfluidic LIG device for taste sensing application was designed by Goel et al. Polyimide was chosen as a base substrate which was adhered over a glass slide. CO₂ laser was used for engraving the microchannel and interdigitated electrode LIG pattern was drawn using the laser. The microchannel was bonded and sealed with a top acrylic sheet layer. Inlets and outlets were created in the top layer for fluid flow.⁵² Figure 7(c) is the reprint of their schematic representation of the device fabrication. Several other devices fabricated by Goel et al. are discussed further in the application section.

FIG. 7.

Reprints of the schematic representation of LIG-microfluidic device fabrication from the following sources: (a) Reproduced with permission from Rewatkar et al., Trans. Electron Devices 67, 1832–1838 (2020). Copyright 2020 IEEE. (b) Reproduced with permission from Bhaiyya et al., IEEE Trans. Nanobiosci. 20, 79–85 (2021). Copyright 2021 IEEE. (c) Reproduced with permission from Wagh et al., Sens. Actuators A 333, 113301 (2022). Copyright 2021 Elsevier.

B. Device reliability

The testing of device reliability is crucial for commercialization and practical application. The fabricated device initially has to undergo multiple testing for reproducibility and stability. In order to check the reproducibility of the device fabricated, multiple copies of the device are prepared with identical experimental parameters like laser speed, power, and fabrication method. Furthermore, the same concentration of analytes and experimental conditions like pH, temperature, and technique are used to analyze the results. When multiple devices are fabricated from the scratch and tested to give an acceptable error (<5%) or negligible error, the device is reliable for reproducibility. This is crucial in the devices which are disposable or single time usage based. Similarly, a single device prepared is tested multiple times with the same analyte under identical conditions, especially when a device is claimed to be used for multiple times. Such re-usable devices should give acceptable error (<5%) when tested several times. The stability of devices varies from application, device substrate, fabrication method, etc. The maximum limit for re-using the device is established. Another important parameter to be tested is the sensitivity and detection limit of the microdevices. In order to examine the sensitivity, the results obtained with the sensor are validated by conventional clinical procedures to test the reliability of the device. Based on this, microfluidic devices are used in multiple applications.

V. KEY APPLICATIONS

A. Sensing

1. Gas sensors

The distinctive properties of LIG make it applicable for multiplexed sensor applications ranging from optical, electrochemical, gas sensors, and physical sensors. Owing to the flexible nature, the sensors made of LIG can be embedded into systems. For instance, a remarkable LIG gas sensor embedded in a cement substrate has been developed. This was applied for practical application of sensing a mixture of CO₂ and N₂. Herein, thermal conductivity of the gases was measured by applying the potential across LIG channel, which altered the thermal conductivity.⁵³ Similarly, a methane gas sensor was developed using LIG through an interdigited electrode (IDE). IDE was further chemically modified with palladium nanomaterial. An organic solvent electrolyte medium was used and an amperometric response with the change in methane gas concentration was recorded.⁵⁴ Origami based liquid level sensors with various shapes and structures for real time application were developed by Yanan et al.⁵⁵

2. Liquid sensor

Kothuru et al. fabricated a bare LIG based liquid level indication sensor for fluids of various viscosity like oil, water, etc. The sensor indicates the level above or below the sensing point. Furthermore, embedding circuitry was designed in such a way the LED lights glow as an indicator. Figure 8(a) is the reprint of their sensor.⁵⁶ The same author also developed capacitive and resistive touch pads using the bare LIG. Figure 8(b) is the reprint of their sensor.⁵⁶

FIG. 8.

Real images of the LIG based sensors reprinted. (a) and (b) Reproduced with permission from Kothuru et al., IEEE Sens. J. 20, 7392–7399 (2020). Copyright 2020 IEEE. (c) Reproduced with permission from Tian et al., ACS Appl. Mater. Interfaces. 12, 9710–9717 (2020). Copyright 2020 American Chemical Society. (d) Reproduced with permission from Huang et al., Sensors 20, 1–14 (2020). Copyright 2020 Open access (MDPI). (e) Reproduced with permission from Chhetry et al., ACS Appl. Mater. Interfaces. 11, 22531–22542 (2019). Copyright 2019 American Chemical Society. (f) Reproduced with permission from Jeong et al., Sensors 19, 4867 (2019). Copyright 2019 Open access (MDPI).

3. Pressure sensors

LIG based pressure sensor has also been reported. Two layers of bare LIG consisting of sandwiched polystyrene beads layer were developed. This design was bioinspired by a bean pod morphological structure. The sensor could detect about 100 kPa pressure and was applied for real time arterial monitoring in a patient.⁵⁷ Figure 8(c) is the reprint of the schematic and real image of their sensor.

Likewise, Huang et al. developed a wearable strain sensor using LIG–silicone rubber. A polyimide sheet was engraved with LIG and transferred to a substrate of silicone rubber. 3D wavy mold was used to design the pattern. The sensor gave a gauge factor of 37.8. Owing to the wavy nature, the sensor was more stable and gave good repeatability and reversibility.⁵⁸ Figure 8(d) is the reprint of the real image of their device.⁴⁹

In another interesting work, bare LIG doped with MoS₂ was formed by chemically modifying the polyimide sheet and exposing to the laser with specified speed and power. This sensor gave excellent strain sensing piezoresistive sensitivity. Figure 8(e) is the reprint of their device image.⁵⁹ LIG on the PDMS substrate was formed with a pulsed laser ablation with a gauge factor of ∼160. It was sensitive toward the motion of a human finger.⁶⁰ Figure 8(f) is the reprint of their device image.⁶⁰

Xia et al. developed a reduced graphene oxide-cloth-LIG based pressure sensor. LIG gave the linear range of pressure sensing between 20.6 to 30.3 kPa⁻¹. This also enhanced the transfer charge density of a triboelectric nanogenerators up to 270 μC/m², which could be employed in smart electronic skin, wearable medical devices, etc.⁶¹ Jiang et al. developed a flexible LIG-multiwalled carbon nanotubes composite based pressure sensor with a limit of detection of about 1.2 Pa. The sensor was applicable for real time analysis of vocal and breath motion, finger motion, and pulse at the wrist.⁶² A LIG powder was prepared as screen printing conductive ink, which was further used for patterning electrodes over different flexible substrates for pressure sensing. This sensor gave a high pressure sensing up to 1.86 kPa⁻¹ and a low detection limit of 3.4 Pa. This could be applied for examining wrist pulses.⁶³ Fe₂O₃ nanoparticles-LIG based flexible pressure sensor was developed. This sensor gave a linear range of pressure sensing 0–10 kPa.⁶⁴

4. Temperature sensors

Another useful application of LIG is in the development of flexible temperature sensors using LIG substrates. Kun et al. summarized and reported a detailed review on the advances of flexible LIG based temperature sensors.⁶⁵ Han et al. reported a LIG–rGO composite based temperature sensor. An IDE was fabricated which gave temperature sensitivity between the range of 25 and 45 °C. Since it was flexible, this could be used in monitoring of skin temperature wearable device.⁶⁶ Gandla et al. developed a real time wearable temperature sensor. LIG was formed on a Kapton film and was integrated with a flexible printed circuit system. It was assembled as a wearable path that contacts with real human skin and gave a highly stable response toward temperatures.⁶⁷ Similarly, LIG based thermal heating and electro-photo-thermal heating based applications are also reported. For instance, Sharma et al. reported a carbon black adhesive and double-sided tape with LIG based composite tape for electrophoto-thermal applications. This LIG modified composite tape gave remarkable electrothermal behavior with a maximum temperature above 120 °C at 10 V.⁶⁸

5. pH sensor

Apart from temperature monitoring, another physical parameter, pH, can also be monitored using a LIG based sensor. Barber et al. developed a LIG pH sensor with potentiometric measurements, which showed a sub-Nernstian behavior. Furthermore, LIG with redox mediator riboflavin was used for voltammetric pH measurements, which gave a Nernstian behavior.⁶⁹

6. Electrochemical/biosensors

LIG has also been extensively used as base electrodes for electrochemical and biosensor application. Electrochemical sensing of glucose has been extensively reported using LIG. A few of them are discussed here. Tehrani et al. demonstrated a LIG modified with copper nanocubes electrochemical sensor for non-enzymatic detection of glucose. The sensor has a linear range of 0.25 μM to 4 mM and a detection limit of 250 nM. Furthermore, this sensor was tested for selectivity in the presence of various sugars like fructose, lactose, etc.⁷⁰ Lin et al. also reported a LIG–Cu nanoparticles electrode for enzymeless detection of glucose in the range of 1 μM–4.54 mM with LoD of 0.35 μM.⁷¹ Phenolic resin based LIG was fabricated and used for detection of glucose in the range of 0.2–10 mM.⁷² Enzymatic glucose sensing with glucose oxidase enzyme immobilized over LIG fabricated on a glass substrate was demonstrated by Chang et al.⁷³

In addition to glucose, other biochemicals and metabolites of the physiological system have been detected with LIG. For example, detection of urea, a metabolic product was reported. Urease enzyme immobilized over an infrared laser scribed LIG was employed as the sensor. They were able to detect urea with a LoD of 10⁻⁴ M.⁷⁴ Similarly, thrombin, an anti-coagulant enzyme was also detected via LIG electrochemical sensor. 1-pyrenebutyric acid (PBA) modified LIG was used as the sensor. Amino-functionalized bioreceptors (aptamers) were attached to the carboxyl group of PBA. The LoD obtained was 1 and 5 pM in the standard solution and in the real blood serum, respectively. Figure 9(a) is the reprint of their schematic representation of the aptamer based electrode fabrication.⁷⁵

FIG. 9.

Real images of the LIG based sensors reprinted. (a) Reproduced with permission from Fenzl et al., ACS Sens. 2, 616–620 (2017). Copyright 2017 American Chemical Society. (b) Reproduced with permission from Kothuru et al., Trans. Electron Devices 67, 5097–5103 (2020). Copyright 2020 IEEE. (c) Reproduced with permission from Soares et al., ACS Sens. 5, 1900–1911 (2020). Copyright 2020 American Chemical Society.

A multiplexed LIG based miniature electrochemical sensor for detection of ascorbic acid, caffeic acid in various edible samples, and picric acid detection in forensic samples was attempted by Araujo et al.⁷⁶ A flexible sensor over a PDMS substrate with LIG–platinum–gold nanoparticle composite sensor was reported for detection of dopamine. The LoD obtained was 75 nM with no interference from other biochemicals.⁷⁷ Similar way, LIG–platinum modified sensor was reported for multiplexed, non-interference, detection of ascorbic acid, uric acid, and dopamine simultaneously.⁷⁸ Xu et al., reported a LIG–polymer composite electrode wherein poly(3,4-ethylenedioxythiophene) (PEDOT) was used as a redox matrix for detection of dopamine. An appreciable LoD of 0.33 μM was obtained.⁷⁹ Kothuru et al. reported a microfluidic three electrode system designed over a polyimide sheet via laser scribing. Herein, the polyimide sheet mounted over acrylic was exposed to laser to generate LIG in the microchannel, then it was peeled off leaving a cavity as a microchannel. Furthermore, it was ablated again to form three-electrodes of LIG. This device was used for detection of uric acid with a good LoD of 0.61 μM. Figure 9(b) is the reprint of their schematic representation of their device fabrication strategy.⁸⁰

LIG has been used in fabrication of immunosensors, nucleic acid sensors, and enzyme sensors as well. However, modification of LIG with various mediators is needed to immobilize the bioreceptors (antibodies, aptamers, DNA, RNA), etc. A few of the reports in this regard as discussed here. Detection of E. coli was reported wherein, homogenous mixtures of a metallic solution consisting of gold–silver–platinum salts, were spin coated on a polyimide sheet and later exposed to laser to generate metal doped LIG. This was used as a substrate to form the antigen–antibody complex formation.⁸¹ In the same way, detection of Salmonella was carried out over metal catalyst modified LIG. The sensor was tested for real time Salmonella detection in chicken broth with a remarkable LoD of 13 CFU ml⁻¹. Figure 9(c) is the reprint of the schematic representation of the sensor fabrication.⁸²

A notable report was given by Wan et al., wherein micro-RNA (miRNA) detection was carried out using LIG. Nitrogen doped LIG was formed, which was used for detection of miRNA up to 10 fM LoD.⁸³ LIG–ITO electrode which gave TiO₂ modified LIG was used for sensing acetylcholinesterase inhibitor enzyme. Considerable LoD of 5.8 pg ml⁻¹ was achieved by Lei et al.⁸⁴ LIG has also proven to be useful in detection of therapeutic substances like Chao et al. demonstrated detection of trans-resveratrol. This is considered to have anti-cancerous and anti-inflammatory effects. A LoD of 0.16 μmol was obtained.⁸⁵ LIG decorated with copper particles, immobilized with diamineoxidase enzyme was used for detecting histamine in real food samples. A LoD of 11.6 μM was attained.⁸⁶

LIG has also been broadly used in electrochemical detection of environmental pollutants, heavy metals, and other chemicals. For example, detection of hydrazine using an unmodified LIG was reported with a LoD of 70 μM.⁸⁷ Detection of hydrogen peroxide (H₂O₂) was also reported considerably. Eider et al. fabricated LIG–silver nanoparticle modified sensor for detection of H₂O₂ in real time milk samples with a LoD of 7.9 μM.⁸⁸ LIG–platinum nanoparticle composite sensor was developed by Yuhan et al. for H₂O₂ sensing with a LoD of 0.1 μM.⁸⁹ Kothuru et al., from our research group, have also reported detection of H₂O₂ with bare LIG and achieved a LoD of 0.3 μM.⁵⁶ Detection of nitrite up to LoD of 7.45 μM was reported using LIG-multiwalled carbon nanotube (MWCNT)–gold nanoparticle modified electrode.⁹⁰ Garland et al. attempted detection of ammonia and nitrate in soil samples by using a LIG-ion selective membrane. LoD for ammonia was obtained as 28.2 μM and for nitrate as 20.6 μM.⁹¹ LIG-ionic liquid-poly-l-cysteine decorated electrode was prepared for detection of heavy metals in real water samples. LoD obtained was 0.17 μg l⁻¹.⁹²

7. Optical sensors

LIG has showcased promising output when used in electrochemiluminescence (ECL) sensing for development of optical sensors. Our research group has been working comprehensively in various aspects of LIG-ECL, microfluidic, and miniaturized sensors for multiple applications. For example, a simple miniaturized ECL platform was developed using LIG as bipolar electrodes which were further applied for detection of H₂O₂ and glucose via luminol based luminescent reaction. The intensity of the reaction was captured using a smart phone. Figure 10(a) is the reprint of their device.⁵¹ The same author also developed a single electrode based on LIG for multiple analyte detection including H₂O₂, xanthine, dopamine, glucose with luminol chemistry. Each analyte could be detected at specified potentials hence, there was no interference. Furthermore, good LoD as1.25 μM for xanthine, 1.71 μM for H₂O₂, 3.76 μM for glucose, and 3.40 μM for dopamine was achieved. Figure 10(b) is the reprint of their device.⁹³ They also developed a 3D printed device with single and bipolar LIG electrodes for detection of vitamin B12. Android smart phone was used for the analysis as well as the power source for applying voltage hence can be used as a point of care tool. Both the electrodes demonstrated different LoD, single electrode gave 0.094 nM and bipolar gave 0.107 nM. Figure 10(c) is the reprint of their device.⁹⁴ The same group also reported a unique U-shaped bipolar electrode using LIG for detection of lactate, glucose, H₂O₂, and choline. Figure 10(d) is the reprint of their device.⁹⁵

FIG. 10.

Real images of the LIG based sensors. (a) Reproduced with permission from Bhaiyya et al., Trans. Nanobiosci. 20, 79–85 (2021). Copyright 2021 IEEE. (b) Reproduced with permission from Bhaiyya et al., Trans. Instrum. Meas. 70, 1–8 (2021). Copyright 2021 IEEE. (c) Reproduced with permission from Bhaiyya et al., Microfluid. Nanofluidics 25, 1–8 (2021). Copyright 2021 Springer Nature. (d) Reproduced with permission from Bhaiyya et al., Trans. Electron Devices 68, 2447–2454 (2021). Copyright 2021 IEEE.

B. Energy harvesting

The exceptional conductivity and the carbon spatial arrangement of LIG have proven to be useful in energy harvesting application. Consequently, LIG can be employed for fabrication of fuel cells and biofuel cells as base electrodes. The chemical reaction mechanism that generally takes place at the electrodes in a fuel cell is either reduction or oxidation of dissolved oxygen. Hence, bare LIG as well as chemically modified LIG has shown satisfactory results in this regard. It was discovered by Ye et al. that bare LIG modified with metal oxide gives higher oxygen reduction reaction (ORR).⁴¹ Likewise, it was shown by Zhang et al. that the plasma treating the LIG increases the oxidized surface, which enhances the oxygen evolution reaction (OER) mechanism.⁹⁶ Hence, these studies lead to a gateway for application of LIG in fuel cells. Some of the remarkable applications are discussed here.

For instance, a proton exchange membrane-based fuel cell was designed by Tiliakos et al. Herein, they utilized a LIG-Nafion-catalyst as the electrode. The fuel cell gave an efficient current density of about 800 mA cm⁻² and 266 mW cm⁻² peak power.⁹⁷ Of late, our research group has also been significantly working on development of LIG based fuel cells, biofuel cells especially miniaturized/microfluidic device. As a proof of concept, Rewatkar et al. developed an enzymatic biofuel cell. Herein, LIG was laser scribed on polyimide sheet followed by the plasma treatment. Glucose oxidase (GOx) enzyme was immobilized over the treated LIG via linker and its performance for generating current density was studied.⁹⁸ In the extended work, the LIG fabricated was modified with multiwalled carbon nanotube (MWCNT). This gave an increased surface area and led to greater immobilization of enzymes. The bio electrodes were prepared using this modified LIG followed by immobilization of GOx and laccase enzymes on individual electrodes. The electrodes were then integrated into a PDMS based Y shaped microchannel of dimension 25 × 4 × 1 mm to form a device. The device showcased 1.37 times improved result than bare LIG electrode and could generate a power density of 2.2 μW/cm². Figure 11(a) is the reprint of the real image of their device and the fabricated LIG electrode.⁹⁹

FIG. 11.

Real images of the LIG based sensors. (a) Reproduced with permission from Jayapiriya et al., Int. J. Hydrogen Energy 46, 3183–3192 (2021). Copyright 2020 Elsevier. (b) Reproduced with permission from Rao et al., Energy Technol. Assessments 45, 101176 (2021). Copyright 2021 IEEE. (c) Reproduced with permission from Jayapiriya et al., Trans. Nanobiosci. 1241, 1 (2021). Copyright 2022 IEEE. (d) Reproduced with permission from Rewatkar et al. Trans. Electron Devices 67, 1832–1838 (2020). Copyright 2020 IEEE.

Rao et al. fabricated LIG electrodes as anode and cathode and modified with various metallic and carbon nanomaterials like MWCNT, single walled carbon nanotubes (SWCNT) silver nanoparticle ink, MnO₂, TiO₂, etc. Herein, the electrodes were integrated into a PDMS microfluidic channel to form a non-enzymatic fuel cell device. Oxygen was used as an oxidant, a mixture of formic and sulfuric acid as fuel and electrolyte. Among all the catalysts, it was found that silver nanoparticle ink gave better performance with a power density measuring 88.80 μW cm⁻² with a flow rate optimized 24 ml/h. Figure 11(b) is the reprint of the real image of their device.¹⁰⁰

Jayapriya et al. reported a Y shaped-PDMS microfluidic fuel cell device fabricated using LIG as bioelectrodes. Y shaped channel was engraved via laser in the polyimide sheet. LIG electrodes were fabricated with dimension 10 × 1 mm². A uniform spacing of 1 mm was maintained between them. Furthermore, these LIG electrodes were chemically modified with MWCNT followed by immobilization of glucose oxidase and laccase enzymes. The device gave a good performance of 4.7 μ W/cm² power density. Figure 11(c) is the reprint of their real device image.¹⁰¹

Rewatkar et al. also reported LIG bioelectrodes integrated into a PET (Polyethylene Terephthalate) microchannel of dimension 25 × 4 × 0.2 mm³. The overall electrodes and channel were assembled in a PDMS based microdevice. This was studied for enzymatic fuel cell application. LIG-GOx and LIG-laccase enzymes were used as biocathode and bioanode. The device gave a remarkable power density of 13 μ W/cm². Figure 11(d) is the reprint of their real device image.³

In addition to the fuel cell application, LIG has also been used for designing nanogenerators that can convert mechanical energy to electrical energy. For example, a triboelectric nanogenerator has been developed by Jiang et al. LIG-MXene porous film integrated with PDMS was fabricated.¹⁰² Similarly, LIG-polyimide sheet composite wherein, two layers of polyimide were used and one of them was ablated to generate LIG. This was embedded in a PDMS matrix to form a compact triboelectric nanogenerator.⁵³ Hence, these reports depict that LIG has shown great significance in energy harvesting applications.

C. Supercapacitors

The usage of LIG for storage of energy as supercapacitors has been substantially studied over the years. Some of the exceptional reports are discussed here. For example, Chao et al. realized a novel composite fiber of metallic-sulfide and graphene as a nanoribbon and studied it for supercapacitor application. It was a hybrid supercapacitor that showed a specific capacitance as 58.3 mF/cm² at 50 μA/cm², 49.9 μW/cm² at 50 μA/cm² of power density, stability up to 10 000 cycles.¹⁰³ LIG microsupercapacitor was designed by Song et al. Herein, nitrogen doped LIG was fabricated by using a conducting polymer layerpoly(3,4-ethylenedioxythiophene). The LIG obtained was patterned as a pair of interdigitated electrodes of size 8.6  × 0.86 mm², spacing of 0.86 mm and integrated over a silicon rubber substrate. The device displayed a 7.2 mF cm⁻³ volumetric capacitance.¹⁰⁴ A pyrolysis approach was applied by Kyung et al. to generate LIG–nitrogen doped. This was studied for supercapacitor application, and it showed a good specific capacitance of 49.0 mF cm⁻² at 0.2 mA cm⁻².¹⁰⁵A pseudocapacitive microsupercapacitor using LIG modified with various redox mediators, nanoparticle composites: polyaniline (PANI)–manganese dioxide (MnO₂)–ferric oxyhydroxide (FeOOH) was developed which was used in microbatteries.¹⁰⁶ Wang et al. reported a LIG modified with Co₃O₄ for microsupercapacitor application. The studies revealed that the modified LIG gave an enhanced areal specific capacitance and stability up to 10 000 cycles.¹⁰⁷

LIG–LiNi_1/3Mn_1/3Co_1/3O₂ composite electrode was studied for hybrid supercapacitor application. Herein, a remarkable energy density of ∼123.5 Wh/kg, power density of ∼14074.8 W/kg, and capacitance of 141.5 F/g was obtained. The electrode was stable more than 20 000 cycles.¹⁰⁸ Reduced graphene oxide fabricated using ultrashort-pulse laser was examined for microsupercapacitor application. A power density of 83.5 mW cm⁻³ and an energy density of 1.08 mWh cm³ were attained.¹⁰⁹

Graphene oxide films were exposed to laser to get laser derived reduced graphene oxide. These electrodes represented a remarkable area; capacitance 149.7 mF cm⁻² at 3 mV s⁻¹, 7.5 mWh cm⁻³ energy density and stability up to 5000 cycles.¹¹⁰ A 3D LIG was fabricated in presence of KOH and the effect of KOH varying concentration was analyzed. KOH exposure doped nitrogen and oxygen in LIG, hence, an improved performance was obtained. The electrode showed an areal capacitance of 4.27 μWh/cm².¹¹¹ A flexible, stretchable microsupercapacitor was designed using zinc nanosheets modified LIG.¹¹² LIG–poly furfuryl alcohol/graphene oxide composite electrodes were prepared and applied for supercapacitor studies. A specific areal capacitance of 16.0 mF/cm² at 0.05 mA/cm² was attained with stability more than 10 000 cycles. Figure 12(a) is the reprint of their device.¹¹³ LIG–PEDOT [poly (3,4-ethylenedioxythiophene)] composite was tested for supercapacitor application and the device showed stability more than 4000 cycles.¹¹⁴

FIG. 12.

Real images of the LIG based sensors reprinted. (a) Reproduced with permission from Cho et al., Appl. Surf. Sci. 518, 146193 (2020). Copyright 2019 American Chemical Society. (b) Reproduced with permission from Cai et al., J. Mater. Chem. A 4, 1671–1679 (2016). Copyright 2015 American Chemical Society. (c) Reproduced with permission from Awasthi et al., IEEE Sens. J. ■, 1–8 (2022). Copyright 2022 IEEE.

Peng et al. attempted to dope boron atom in LIG. This resulted in an enhanced energy density up to 10 times and improved specific capacitance of 16.5 mF cm⁻² which was three times greater than bare LIG Fig. 12(b) is the reprint of their device.⁴⁰ Cai and his group demonstrated that increasing the wettability of LIG via plasma treatment, the oxygen content was increased resulting in rise of capacitance by 11.6%.¹¹⁵ In a unique approach, Wen et al. demonstrated fabrication of LIG using a graphene oxide–carbon nanotube composite. Polyethylene terephthalate (PET) was affixed over a DVD and the composite prepared was coated followed by laser ablation giving LIG. This was further designed as interdigitated electrodes to study the supercapacitor application. A volumetric capacitance of 3.10 F cm³, power density of 1.0 W cm³, energy density of 0.84 mW h cm³, and current density of 1000 mA cm³ was obtained.¹¹⁶ Goel et al. also recently reported fabrication of LIG over a paper and cloth substrates. Herein, the flexible substrates were laser ablated to generate LIG and further used for microdevice fabrication. Herein, sensing, energy harvesting (fuel cell) and energy storage (supercapacitor) applications were studied. The prepared fuel cell showed 4.5 μW/cm² power density and a capacitance of 15 mF/cm² F. Figure 12(c) is the reprint of their device.¹¹⁷

D. Internet of things (IoT) enablement

IoT enablement is associated with data storage, access, identification, visualization, sensing, and computation with effortless connectivity at any place and time by anyone. Figure 13 highlights some of the features of IoT enablement.¹¹⁸ IoT enablement can be integrated into various areas like health monitoring,¹¹⁹ clinical and biomedical analysis, remote monitoring, retailing, development of smart cities, logistics, etc. However, the application of IoT is still not much explored. Broadly, the technologies that are used to integrate IoT are summarized as various categories like sensing, identification, cloud computing, communication, software and hardware, algorithms, data processing, energy storage, security systems, etc. Figure 14 is the schematic representing various types of wireless technologies for IoT enablement.¹¹⁸

FIG. 13.

Schematic the features highlighted in IoT.

FIG. 14.

Is the schematic representing various types of wireless technologies for IoT enablement.

Of late, several research groups have been working on integration of IoT in various types of sensing technologies and microfluidic devices. For instance, Alahi et al. fabricated interdigitated electrode for sensing of nitrate. A FR4 capacitance sensor was fabricated, which was connected to an impedance analyzer for obtaining data. The impedance analyzer was further connected to an Aurdino Uno Wifi that functions like a microcontroller. This sends the data to IoT cloud server.¹²⁰ Nagabhooshanam et al. made a micro-electrochemical platform for sensing chlorpyrifos. The data collected was transferred with k-stat portable Potentiostat.¹²¹

Recently, Goel et al. reported an IoT enabled microfluidic platform wherein portable thermal management system was used for fabrication of manganese oxide nanoparticles which were electroactive in nature.¹²² An IoT enabled, miniaturized platform with continuous flow microfluidics was also developed. This platform was a portable polymerase chain reaction (PCR) platform for DNA amplification studies.¹²³ An IoT enabled electrochemiluminescence platform for sensing choline and dopamine using a smartphone, photomultiplier tube, and 3D printed electrode system was reported by Goel et al.¹²⁴ Although there is a decent progress in IoT enabling, but there are certain challenges associated with it like connected devices. This leads to traffic in connected devices. Issues related to integration of multiple technologies, data types, devices, networks, data storage, privacy, security from hacking, etc., have to be addressed. IoT integration in microfluidic devices is an added advantage in terms of portability and ease of use by creating the required circuits, with LIG zones, in the same substrates. The microdevices can be fully automated, including the provision of input parameters, results, data storage, sensing, and access to the results obtained can be done remotely. Furthermore, the IoT integration in microdevices can make the testing user-friendly and the entire procedure can be done through simple smartphone based applications.

VI. CHALLENGES AND FUTURE OPPORTUNITIES

A. Challenges

Despite significant advances in usage of LIG in various sensing applications, there are certain challenges associated with fabrication of LIG based devices, especially toward commercialization. Though numerous applications are reported, but only a few have the potential to be used as point-of-care systems. Hence, most of them still work as proof of concept. Another challenging aspect is microdevice fabrication with LIG. Specifically, in terms of choosing the base material, which can be economical produced on a large scale. The most common material adapted for rapid prototyping of the microfluidic device is PDMS. The qualities like gas permeability, transparency, biocompatibility for cell growth, etc., make it one of the most liked materials.¹²⁵ Even though it has these advantages, there are certain challenges associated with use of PDMS. The process of fabrication is time consuming and has clean room requirements mostly. PDMS reacts with organic solvents and swell up. It has uptakes water molecules from aqueous mediums and surface treatment of PDMS is challenging. Hence, choosing of the material that is amenable, cost-effective, and scalable is challenging.^7,126

The other challenge related to portability of LIG microfluidic device is in terms of logistics and portability. It is often observed that microfluidic devices rely on external passive components like pumps, electric supply, computers, etc. Hence, complete automation is difficult leading to increasing the cost of operation. In addition, certain samples need a standardize preparation to avoid the blockage of microfluidic channels and requires vacuum, which make them dependent on bulky hardware. Microfluidic devices are often non-programmable and mostly specific to single application. Hence, the scalability and real time use is challenging. Furthermore, the robustness, reproducibility, and long-term durability of these devices are questionable. All these challenges are to be addressed in the future.¹²⁷

B. Future opportunities

Since, LIG preparation is a single step process and does not require a large reagent volume consumption unlike the conventional wet chemical method, it can be plausibly prepared over various carbon containing substrates like polymers, organic carbon like wood, coconut shell, potato skin, etc. (as reported in the literature),³⁴ and the cost of production will be much lower. Therefore, it can be easily scaled. Recently, some research has shown how LIG can be scalable with roll-to-roll production for several commercial application.^128,129 However, a lot more work needs to be carried out to benchmark LIG to make a fully turnkey and reliable replacement of similar materials in future. In order to overcome certain challenges discussed above, there is a huge scope in future outlook for scalable production of microfluidic devices. There is a constant need of developing novel materials to build the gap which can be used for large scale microfluidic device fabrication, LIG synthesis and application. Furthermore, the functionalization of LIG surface to enhance the bonding, conductivity, and stability can be worked upon in the future. Processes to increase the bonding strength with cost-effective alternatives can also help in increasing the stability of microdevices.¹³⁰ Furthermore, there is a need to explore alternate substrates for LIG as well as microfluidic device fabrications like polystyrene, silicon, glass, olefin copolymer, polycarbonates, etc. In order to minimize the use of external equipment and complete automation, there is a need for studying microdevices with approaches like capillary action, passive fluid pumping, lateral flow, etc. Also, a standard sample preparation using a centrifugal microfluidic chip design can be studied in the future. Advances like plug-play microfluidics devices, programmable devices, IoT integration, full automation, digital chips, biocompatible, wearable designs, etc., are to be studied in detail in the future.

VII. CONCLUSIONS

The present review provides detailed information about the emerging trends in the laser-induced graphene (LIG)-based microfluidic devices. The paper discusses the background of the LIG synthesis and its growth since the time it was first discovered. Detailed description about the properties of graphene and how they can be tuned based on the applications has also been discussed here. A brief snapshot about the design and flow geometry of microfluidics was also discussed, highlighting various types of microchannels. Various approaches for the generation of microdroplets in various forms have been explained in the present paper. The flow geometry and rate controlling mechanisms and the current state-of-the-art microfluidic device fabrication methods were also deliberated thoroughly with suitable schematics explaining the procedures. Approaches for LIG fabrication, physicochemical characterization techniques, and forms of graphene were explained. Furthermore, the LIG-based microfluidic device fabrications with suitable examples from the literature were presented. The examples of key applications like sensing, energy storage, and energy harvesting with possibilities for IoT integration were reviewed in detail. The paper also discusses the present challenges and future aspects of LIG-microfluidic devices for real-time large-scale production and application. Overall, thorough insight into fabrication to application and commercialization was discussed in detail, giving the readers an opportunity to understand the current state of LIG-based microfluidic applications.

AUTHOR DECLARATIONS

Conflict of Interest

The authors have no conflicts to disclose.

Author Contributions

Sanket Goel: Conceptualization (lead); Investigation (lead); Methodology (lead); Resources (lead); Supervision (lead); Writing – original draft (equal); Writing – review & editing (lead). Khairunnisa Amreen: Data curation (equal); Methodology (equal); Writing – original draft (equal).

DATA AVAILABILITY

Data sharing is not applicable to this article as no new data were created or analyzed in this study.