Advances in Functionalized Applications of Graphene‐Based Wearable Sensors in Healthcare

Graphene has earned significant attention in the present world due to its light weight and extremely good conductive properties, which are used in different functional materials and smart devices. With skyrocketing demand, wearable sensors are evolving with many essential functionalities and flexibility in use. Moreover, wearable sensors can show some marvelous activities easily when they are incorporated with different nanomaterials and two‐dimensional (2D) materials. Therefore, after the immense effort and diligence of scientists over the years, wearable sensors can successfully exhibit numerous potential applications, such as motion detection, artificial intelligence, prosthetic skin, intelligent robotics, and human‐machine interface and interaction. With the rapid development of flexible, perceptible electrical devices, graphene‐based wearable sensors play an eminent role in healthcare. In this work, a comprehensive overview of recent research on wearable sensors and integrated systems for various sections of healthcare is demonstrated. Along with discussing the basic properties of graphene and the fabrication methods for graphene‐based wearable sensors, this work can help the scientists address them and set a projection for future studies. Wearable graphene‐based sensors have great potential to make healthcare facilities more accessible and enhance the quality of sensing activities, which has enormous implications for the future of healthcare.


Introduction
Wearable technology has been a booming industry, with an anticipated global market worth USD 165 billion by 2030, up from DOI: 10.1002/adsr.202300120 30 billion back in 2022, at a compound annual growth rate of 19.1% for medical science and healthcare. [1]hese wearables include different types, such as smart watches, glasses, jewelry, bandages, electronic textiles, wrist/chest bands, smart clothing, rings, a realtime glucose regulator, smart tattoos, face masks, and more. [2]In general, wearable devices consist of different blocks, including a substrate and electrode, a power unit, a sensing unit, and a decision-making unit. [3]Wearable sensors play a crucial role in wearable technologies by facilitating the detection of biometrics, collecting biological data from the wearer, and then adapting to the body's requirements.The first wearable sensor in the healthcare sector was invented several decades ago, in 1960, named the Holter monitor, which is still very useful in monitoring heart activity. [4]ertainly, wearable sensors are the analytical tools that integrate with wearables by combining and collecting the responses of point-of-care systems automatically with built-in units. [5]Such sensors can efficiently detect a very small change in physiological response and excellently monitor the biometrics of the human body in a regular manner. [6]Wearable sensors have gained significant popularity in the fields of disease detection and monitoring, as well as in many healthcare and fitness applications.At present, wearable sensors are not only confined to human body applications but also expand their reach to the healthcare of livestock and pets. [7]Besides, wearables have made healthcare facilities more affordable, which increases their popularity and acceptability among the masses. [8]Also, wearable sensors are more user-friendly and easily accessible than the conventional clinical diagnosis system. [9]n this current era, various types of wearable sensors are being intensively utilized and developed in the medical field for the well-being of humans and animals.Textile-based, paper-based, hydrogel-based, etc., are the common sorts of wearable sensors in the health sector. [10]Besides, nanoscale materials like metallic nanowires, [11] carbon-black elements, [12] nanoparticles, [13] carbon nanotubes, [14] nanorods, [14c,15] and also graphene, [16] can be utilized in developing stretchable and flexible wearable sensors by coupling with elastomers for adequate sensitivity.Since the very first invention of single-layer graphite through mechanical exfoliation back in 2004, graphene has garnered a significant amount of attention. [17]Graphene, a two-dimensional material, [18] has many desirable properties, such as high electron mobility through the material (350 000 cm 2 (Vs) −1 ), [19] thermal conductivity (5300 Wm −1 K −1 ), [20] Young's modulus (1 TPa), [21] superior specific surface area (2600 m 2 g −1 ), and reduced thickness (0.34 nm). [22]Among all these amazing attributes, graphene's thermal conductivity is one of the most effective in comparison to other common materials (i.e., even higher than metals and carbon nanotubes). [23]Thus, graphene and its derivatives can play a significant role in developing wearable sensors. [24]hey can easily make the bridge between the sensors and the device itself.These materials could be integrated into a multilayer stack to create heterostructure-based, multifunctional devices with exceptional characteristics and function control.Researchers and individuals are taking an active role in using the unique features of graphene to produce sensors that can efficiently and rapidly detect and convert minute variations in an open system.Additionally, efforts are being made to optimize its utilization in the healthcare industry.
Wearable sensors in the healthcare sector essentially must maintain optimal mechanical flexibility as well as outstanding sensing ability for comfort and proper integrity with the human body. [25]In this case, the graphene-based materials, including graphene, graphene oxide (GO), and reduced graphene oxide (rGO), are performing excellently as healthcare wearable sensors due to their outstanding biophysical and biochemical sensing abilities along with ultrahigh flexibility. [26]Graphenebased materials are an appropriate carrier that can be transformed into a chemical sensor to detect fluids and gases because of their extremely high specific surface area.Highly flexible graphene-based physical sensors have been used in a wide range of skin-mountable and wearable sensor types, with applications ranging from human motion detection to health monitoring to human-machine interfaces. [27]Figure 1 demonstrates a summary of graphene-based sensors for various activities, such as monitoring some important biophysical, biochemical, and en-vironmental signals, as well as effective devices for the nervous system, cardiovascular sections, digestive systems, and locomotor areas.However, persistent endeavors are currently underway to adapt and augment the functioning of materials based on graphene for specific sensing applications through different innovative approaches. [26]For example, Qing et al. [28] integrated silk nanofiber to increase the conductivity and biocompatibility of the rGO scaffold for tissue engineering.Graphenebased wearable sensors are utilized for different purposes in the healthcare sector, such as health monitoring (i.e., pressure sensing [29] ), human motion monitoring, [30] electrophysiological signal monitoring, [31] electronic skin and multifunctional sensing, [32] temperature sensing, [33] and more.Noticeably, graphene-based wearable sensors can also make a revolutionary contribution to pandemics like COVID-19 by sensing and monitoring viral infections. [34]In the future, the potential of wearable electronics with graphene sensors for telemedicine is significant, owing to the utilization of advanced technologies involving graphene-based materials.
In this article, the development of graphene-based wearables and recent trends in human health monitoring have been comprehensively reviewed.This review illustrates how fast the latest advancements have happened in graphene-based wearable devices, with a particular emphasis on fabrication, features, and significant wearable applications.Different types of fabrication techniques for graphene-based wearable sensors in healthcare applications are discussed.The wide applications of wearable graphene-based sensors in the medical sector are thoroughly described, as is the sensing mechanism for monitoring health issues.Recent developments in graphene devices have been discussed, along with the challenges they present and possible approaches to resolving those challenges for future researchers.Many research articles are found that discuss graphene-based wearable sensors and their application in different industries.This review particularly focuses on recent significant research and demonstrates an overall view regarding the utilization of graphene-based wearable sensors in healthcare.The emphasis on crucial applications of the sensors along with the fabrication parts makes this review more unique and novel than the other published papers to date.Furthermore, the paper concludes with an illustration of potential future research scopes on graphene-based wearable sensors in healthcare that will help researchers advance wearable devices for the well-being of humankind and the animal world.

Wearable Sensors in Healthcare Sector
The current healthcare sector in the world is blessed with different types of wearable sensors. [35]The constituent components of a wearable sensor determine its functionality, mechanical, and physical properties. [36]Nanotechnologies, conductive polymers, and 3D-structured materials are mostly preferred to manufacture wearable sensors. [37]Textiles have gained significant prominence as a substrate material in the fabrication of wearable sensors because of their notable attributes of being lightweight and flexible.These qualities are deemed crucial to meeting the primary needs of wearable sensors. [38]Based on the fabrication material, there are many wearable sensors that are used for multifunctional applications.The most researched wearable sensors in healthcare are discussed in this section of this review.Table 1 demonstrates and summarizes different wearable sensors with their properties and applications.

Textile-Based Wearable Sensors
Textiles have highly flexible properties, such as being stretchable, compressible, twistable, and deformable.In addition to these features, textiles also have other unique properties that make them a great carrier and substrate for wearable electronics, including breathability, lightweight, and high integration with other materials. [39]Electronic textiles are one of the most researched areas that enable wearable sensors to achieve different properties like easy integration with clothing, superior flexibility, and good abrasion resistance. [10]Also, the lightness and deformation resistance properties of the textile-based sensor made it more suitable as a wearable sensor. [40]Especially textile-based pressure sensors are performing very well in different sectors, such as medical diagnostics, health monitoring, motion detection, and home care. [41]Along with the sensibility and detectability properties of the sensor, the flexibility, comfortability, and durability of the sensor should also be considered. [42]Textile-based sensors can exhibit and maintain these properties effectively.Promphet et al. [43] designed and fabricated a modified cotton thread colorimetric sensor that can effectively and simultaneously detect urea and glucose in the sweat of the human body.A mix of chitosan graphene oxide and cellulose nanofiber was used to chemically treat the cotton thread.This made it better at immobilizing enzymes (Figure 2a), which means it could be picked up by sensors more easily.Due to the flexibility of the cotton thread, this sensor can easily integrate with the apparel and accessories, which enables it to monitor and regulate glucose and urea levels in the sweat of the wearer's body.As a result, diabetes and kidney disease can easily be monitored by this sensor.

Paper-Based Wearable Sensor
Manufacturing cost is one of the most crucial factors in making wearable sensors.In addition to production costs, factors such as flexibility, substrate degradation, and process complexity also exert influence.Paper-based sensors are getting more attention from researchers due to their multiple unique properties.Primarily, the cost of making them is comparatively low for paper-based wearable sensors.Besides, paper-based sensors have flexibility, are lightweight, and are available in raw materials.One notable feature of these sensors is their recyclability and biodegradability, making them increasingly favored in the context of sustainability and the circular economy.However, there are some drawbacks to these sensors.For instance, paper is inherently insulative; thus, it is needed to make it conductive through different complex processes. [44]In addition, wrinkles are another problem with paper that can result in reduced service life as well as different sensing errors.Also, a wet environment like underwater is not suitable for paper-based sensors, as paper easily degrades with the touch of water. [10]However, Liu et al. [45] fabricated a superhydrophobic paper-based strain sensor that successfully overcame the wetting property of the paper-based sensor.Scorpion and lotus leaf were utilized to make this water-repellent and highly sensitive paper-based strain sensor (Figure 2b).The coupling bionics strategy was used to manufacture this sensor, which enabled it to monitor real-time human motion as a regular wearable electronic in all environmental conditions, even underwater.In general, paper-based sensors may encounter many limitations that researchers are actively attempting to address.Nevertheless, their cost-effective manufacturing process makes them a viable option for large-scale production, [10] and their biodegradable properties enhance their superiority over other wearable sensors.

Hydrogel-Based Wearable Sensor
Hydrogel is a three-dimensional porous networked structure made of different hydrophilic polymers, such as polyvinyl alcohol, polyacrylamide, etc. [46] One of the unique properties of the hydrogel is that it can retain water up to 99%, which is more than human tissue (≈70%). [47]In addition, hydrogels are biocompatible and electric conductive, as well as having distinct mechanical properties like less elastic modulus, excellent stretchability, and outstanding tuneability that make them an appropriate alternative for the traditional sensor fabrication material like an elastomer.Besides, hydrogel, with its excellent mechanical features, can properly imitate human skin properties like self-healing; thus, it is an appropriate material for manufacturing long-term wearable sensors with intact functionality.Wang et al. [48] developed a wearable strain sensor capable of sensing real-time sweat quantity by using the remarkable water retention capability of hydrogel materials.A super absorbent hydrogel, consisting of polyacrylic acid and polyvinyl alcohol, was utilized in this study.This hydrogel has the ability to swell upon contact with sweat, hence activating the strain-sensing fabrics.These fabrics were created by covering them with conductive carbon ink (Figure 3a). Figure 3b-d shows the schematic diagram of the superabsorbent hydrogel and the workflow of the sweat volume monitoring method.This sensor can perfectly measure Gas sensor rGO and nanomaterials Machine learning-enabled and IoT-based graphene gas sensing Calorimetric analysis [95]   the volume of sweat by changing the resistivity of the strainsensing fabrics, which was further displayed on the monitor. [48]owever, hydrogel is still facing some drawbacks that need to be addressed to be a suitable constituent material for wearable sensors.For example, the weakening of hydrogel adherence to the electrode during electrochemical performance owing to swelling and shrinking is a serious barrier to long-term application with good functionality. [10]

Graphene-Based Wearable Sensor
Graphene is one of the most suitable fabrication materials for wearable sensors due to its multiple excellent characteristics.
Classic sensors have a lot of problems, like being expensive, not being biocompatible, having rigid substrate materials, being hard to make, and not being able to sense strain.Graphene presents a potential remedy for the aforementioned limitations owing to its cost-effectiveness, heightened sensitivity, and expansive operational scope, among other attributes. [49]Graphene has excellent electron mobility, thermal conductivity, and flexibility; as a result, various mechanical sensors can easily be obtained from it, such as breath, sound, pulse, human motion, etc.The excellent performance of graphene as a chemical sensor is a result of its large specific surface area and ultralight properties. [22]Overall, graphene is an excellent candidate for the fabrication of wearable sensors due to its excellent sensing, mechanical, optical, thermal,  [43] Copyright 2023, Elsevier Limited.b) The schematic manufacturing and design concept of a strain sensor based on superhydrophobic paper and SEM analysis of the surface morphology of a scorpion's slit unit and a papilla (lotus leaf).Adapted with permission. [45]Copyright 2021, American Chemical Society.Adapted with permission. [48]opyright 2023, Elsevier Limited.
and electrical properties. [10,50]Graphene-based wearable sensors can be of various types based on their functions and properties, such as mechanical sensors [49a,51] (i.e., human motion, pulse, and breath), fluid sensors, [52] acoustic sensors, [53] electrophysiological sensors, [54] and gas sensors. [55]49a,56] Yang et al. [57] prepared a polyester fabric-based graphene mechanical sensor that can detect both large and small human body motions.Very small signals like diastolic waves, percussion waves, and tidal waves can be effectively detected by this excellent wearable sensor.This sensor can detect not only the subtle signals of the human body but also significant movements such as the bending of the knee, wrist, elbow, and finger.Figure 4a,b shows the attachment of this sensor to different parts of the body for sensing small and large motions.Due to its excellent thermoacoustic and piezoresistive effects, graphene is also being extensively researched for sound sensing [58] and for sound emission. [59]Graphene-based wearable acoustics sensors are superior in many ways, such as graphene sensors have lower fluctuations and a wider range of frequency. [60]Additionally, all kinds of graphene, including multilayer or single-layer, can be utilized in acoustic applications. [22]On the other hand, for measuring bioelectricity, the traditional method used wet electrodes placed on the human body. [61]However, it has been found that wet electrodes are not good for human skin as their constituents can maintain good attachment with the human body even during exercise and home care.In the case of dry electrodes, graphene performs wonderfully due to its super flexibility and good conductivity. [22]62c] Graphene-based wearable sensors also work well in body fluid sensing like glucose, sweat, etc., and different gas sensing.The large specific surface area of graphene helps excellently in gas and fluid sensing. [22]Li et al. [54] prepared a wearable sweat capture sensor by utilizing patterned graphene arrays.The patterned graphene arrays were super hydrophilic and showed strong bonding to capture droplets of sweat even in modest physical exercise.Figure 4c shows the sweat capture band that was made with patterned graphene arrays on a pristine polyimide substrate.The strong attachment between the water droplets (sweat) and the patterned graphene arrays in the band's positions of upward, downward, and sideward is demonstrated in Figure 4d-f. [54]

Fabrication Methods
Graphene should have been structured into different patterns to fabricate functional devices.As a result, numerous strategies have been proposed. [26]The method used to synthesize graphene has a significant influence on the patterning method utilized.Moreover, resolution, cost, and substrate are all important factors to consider when selecting a patterning method. [22]This section briefly describes various patterning approaches used in the fabrication of graphene devices.Figure 5 summarizes all the fabrication methods, including different printing techniques for sensor development

Photolithography and Plasma Etching
Photolithography is a widely employed form of lithography projection that involves the selective exposure of a photosensitive polymer to light through a mask.The objective is to generate a desired image in a nano-polymer, thereby creating a patterned entrance to the elementary polymer. [96]Figure 5a depicts the photolithography and plasma etching processes.Plasma etching is the process of removing unwanted material from a treated surface by using plasma with overlay accuracy.Photolithography is expected to be an effective process for graphene due Adapted with permission. [57]opyright 2018, American Chemical Society.Digital image of a wristband wearable sweat capture sensor; c) water captured by the wearable sweat capture sensor when the sensor is d) upward, e) sideward, f) downward.Adapted with permission. [54]Copyright 2019, American Chemical Society.
They used 200 ppm of NO2 gas with a strain of 0.5%.Moreover, after forming two electrodes with Ag pastes, they used a poly (methyl methacrylate) layer for transferring the graphene fabricated in the chemical vapor deposition process.For better evaluation, they evaluated their current-voltage dynamic sensing performance in both strained and relaxed situations.Most importantly, they demonstrated photolithography with a higher vacuum process for fabricating graphene sensors with higher performance.Next, Wang et al. [99] developed micro-patterned sensors over skins for monitoring human physiological signals by photolithography process for strain sensing.Their fabricated wearable membrane-based sensors could separately manage physiological parameters and different physical activities, like athletic movements.Various etching methods are used for graphene, but O 2 plasma etching is considered to be the best option for patterning. [97]In 2011, the use of nanosphere lithography with low-power O 2 plasma proved to be a good choice for locating various branches, chains, and connected circular rings. [100]Another procedure for thinning graphene in a layer-by-layer way was established using mild N 2 plasma along with post-annealing in Ar/O 2 . [101]The O 2 plasma approach was favored despite being more effective due to the formation of defects on the surface of the graphene lattice. [101]Graphene hydrogenation by ion-etching plasma was developed in 2011 and could reverse the hydrogena-tion of graphene to its initial phase. [102]After a few years, in 2017, it was found that cyclic etching (two-step plasma etching process) can prevent the graphene surface from being destroyed. [103]he benefits of using the plasma etching process include improved surface treatment, removal of metal-coated substrates despite their strong adhesion to the surface, [104] precise control of multilayer graphene, thickness uniformity without causing surface changes, and suitability for nano-electronic devices based on two-dimensional (2D) materials. [103]The high price of the permanent mask and the inability to treat honeycomb graphene's extremely rough surface are two of the process's drawbacks. [97]

Laser Scribed Graphene
Laser-scribed graphene (LIG) is one of those fabrication processes that use a programmed procedure for patterning rather than a mask (Figure 5b). [97]A mobile and disposable nonenzymatic glucose sensor strip, initially based on gold nanoparticles, was developed after modifying laser-scribed graphene, and it was a cost-effective way for applications such as glucose level self-monitoring and nutrition management. [105]Wearable energy storage electronics seek to be mechanically conformable for optimal comfort, adaptability, and thermal stability.Therefore, there is a lot of research going into creating supercapacitors (SCs) as a replacement for microbatteries in microelectronics.Using a laser-induced graphene-based flexible interdigital elec- Various patterning methods for graphene-based wearable sensor development a) Photolithography and plasma etching process of graphene, [126] b) Illustration of the laser-scribed graphene process, [127] c) Transfer printing process, [127] d) Illustration of inkjet printing technique, [127] and e) Graphene hard mask patterning technique. [128]ode, Awasthi et al. [106] created a tiny supercapacitor that is stable throughout a wide temperature range.A flexible laser-induced graphene (LIG) supercapacitor electrode fabrication method using a CO 2 laser was introduced in the study.EDX spectroscopy and scanning electron microscopy both validated the morphology and development of LIGs.Charge-discharge cycles, capacitive performance, power density, and cyclic life were all improved with interdigital LIG electrodes designed specifically for supercapacitors on a flexible substrate.The 3D porous LIG supercapacitor, made possible by an optimized PVA-KOH gel electrolyte, has an areal capacitance of 83.33 mF cm −2 at 0.5 mA cm −2 current density, 4.62 Wh cm −2 at the same current density, and a power density of 0.2 mW cm −2 at the same current density.One possible benefit of this method is that it can do both growth and pattern formation at the same time, thanks to a programmable process that gets rid of the need for a mask.Additionally, this method offers a time-saving advantage and allows for the possibility of grooving the pattern on a carbon source subsequent to its capture by a laser platform. [97]This method has the limitation of producing a sloppy pattern with poor resolution.In terms of commercialization, laser scribing technology allows for rapid, large-scale, and low-cost production of the LSG, which is a significant advantage in the development of pressure sensors. [107]

Transfer Printing
Transfer printing is an excellent choice for applications such as transistors and energy storage where patterns with extremely high, clear, and precise resolution are required. [97]It can be a positive or negative pattern transfer depending on the extraction or repositioning of materials between layers of substrate and mask, as shown in Figure 5c.As a transfer stamp, the gold film was used for transferring the pattern of graphene, and this technique was suitable for applying to any substrate over large areas, which paved the way for its use in electrical applications. [108]ater on, by using the soft transfer printing method, ultra-large graphene oxide sheets (up to 100 micrometers) were transferred onto PET substrates, which was proven to be a cheap, melodious, efficient reduction process and also a promising technique to produce flexible TCFs as well. [109]Oren et al. [110] reported a facile way to pattern graphene sensors by transferring the printing process with drop-casting of graphene film from a prepatterned polydimethylsiloxane (PDMS) membrane to the target tape.The micrometer-sized graphene was transferred on the final tape, and this process was quite easy to implement and did not require any heavy equipment for the patterning process.Transfer of chemical vapor deposition graphene to an assigned 3D molecular surface was possible by applying the water transfer printing method, which had the advantages of using uncomplicated tools, operating at room temperature, and ingesting low amounts of reagents. [111]The transfer printing method can also be combined with other methods such as holographic lithography, plasma etching, [112] laser scribing method. [113]This method has a number of benefits: it is easy to set up, it works well for large-scale industrial applications, [114] it has a technique that can be used by other people, it has a high and accurate pattern resolution, [97,115] and it allows for separate assembly and device component fabrication. [116]The disadvantages include the high costs brought on by using fixed masks and the high interface requirements. [97]The most common screen-printing mask is a stainless-steel sheet scribed with a laser.Secor et al. combined screen printing and transfer printing to create the gravure printing of graphene. [117]The development of appropriate inks and printing parameters allows for the fabrication of patterns with resolutions as low as 30 μm.

Inkjet Printing
Inkjet printing, which uses a programmable controller instead of a mask to realize patterning, has been one of the most demanding fabrication techniques over the years. [97]As the material passes through the print head, nozzles in the head spray ink onto the material (Figure 5d).Graphene inks are typically prepared using liquid-phase exfoliation. [118]Silver, gold, graphene, and carbon nanomaterials are the primary components of inkjetprinted conducting materials. [119]A water-based inkjet printable ink was created using electrochemically exfoliated graphene, and it was found to have greater sheet resistance than any other ink produced by LPE. [118]Another fabrication technique was used on photographic paper for the application of a fully inkjet-printed gas sensor matrix, which demonstrated excellent stability and flexibility under various bending and long-term dynamic sensing tests. [120]Other amazing works on inkjet printing include the combination of inkjet printing for healthcare and wearable devices, [121] the development of stretchable graphene conductors for wearable technology, [122] and so on.Wearable sweat sensors have the potential to advance precision medicine by collecting health-related molecular data in a non-invasive manner.Unfortunately, it is still difficult to continually detect several critical biomarkers with existing wearables.The benefits of inkjet printing include low-cost changes in printed design due to the lack of a mask, ease of development, [119] non-contact manufacturing techniques, and essential sensing capabilities.Disadvantages include the need for more time for ink preparation in order to achieve high concentration, [118] and low resolution. [97]

Hard Mask
The hard mask technique, which uses a hollow pattern for the materials being passed through and has numerous applications, has been used for several years.After placing a hard mask on the surface, graphene ink is dropped, and any excess ink is discarded into hollow parts.After the ink has dried, the mask's hollow shapes are turned into graphene patterns on the substrate.There are two methods for the hard mask of graphene: etching and lifting off, which are indifferent to photolithography. [97]The only difference is that the photolithography process has a transfer pattern, and the hard mask process uses a hollow pattern (Figure 5e). [97]The process can be combined with other methods, such as lithography, for defining graphene patterns. [123]As a flexible power source, the printed finger structure can be applied to a micro-supercapacitor array and integrated with a flexible strain sensor.Xiong et al. [124] created a patterned GO/polyaniline (PANI) gel by combining spraying and the hard mask.The advantages of this method include a confrontation with proximity effects and the ability to control lift-off reliability with the hard mask method.Additionally, underexposure to the honeycomb pattern results in an increase in the process window and improved resolution. [125]The disadvantages include increased expenses due to the need for masks and the requirement for more time.

3D Printing
The fabrication of graphene-based wearables via 3D printing is a fascinating convergence of modern materials and manufacturing technology, giving a variety of new possibilities for developing useful, flexible, and high-performance wearable devices.Graphene, a one-atom-thick sheet of carbon, is well known for its superior electrical, mechanical, and thermal properties, making it an excellent option for usage in wearables. [129]3D-printed graphene wearables have been used for a variety of reasons in healthcare.They can, for example, be used to continuously monitor vital signs such as heart rate, body temperature, and respiration rate.Furthermore, graphene-based sensors can be programmed to detect specific biomarkers in human fluids, enabling early illness detection and tailored health monitoring. [22]he initial stage in making graphene-based wearable sensors for healthcare applications is to make graphene-based ink.Graphene flakes or nanoplatelets are dispersed in a suitable solvent to create this ink.To attain the correct ink viscosity for compatibility with 3D printing procedures, the solvent and graphene content in the ink must be carefully determined. [130]After the graphene ink and substrate have been created, the 3D printer must be set up for printing.To ensure compatibility with the graphene ink, printer settings must be adjusted, including nozzle size, temperature, and layer thickness, among other characteristics.To obtain the desired precision in printing, proper alignment and calibration are required. [130]The 3D printer must be adjusted for printing after the graphene ink and substrate have been created.It is critical to adapt the printer settings to ensure compatibility with the graphene ink, which includes considerations for nozzle size, temperature, and layer thickness, among other characteristics. [129]Proper alignment and calibration are required to obtain the requisite precision in printing. [22]Following the printing procedure, the graphene ink on the substrate must dry and cure.This is usually accomplished through a combination of thermal and chemical treatments.The curing procedure is critical for maintaining proper adhesion between the graphene and the substrate as well as improving the printed sensor's electrical conductivity. [130]D bioprinting with different functional 2D materials has added a different dimension as a present-day technology.It is also considered a part of additive manufacturing, which shows ground-breaking potential in modeling and remodeling approaches in healthcare applications.It can generate living tissues from the tissues of the mother cell, and it can provide different and highly accurate biological signals with biosensors made with materials like graphene or other 2D materials.Because of this, graphene can be combined with the 3D bioprinting method and the ideas behind biomimetic acts. [131]Figure 6 demonstrates different aspects of incorporating graphene in 3D printing and its applications.Bio-fabrication processes in human organs or tissues may happen in three ways: pre-bioprinting, 3D bioprinting, and post-bioprinting.The entire process includes selecting the materials and appropriate processing techniques, evaluating the material flow, and finally getting through to the application stage. [132]Additionally, mature constructs are cultured in the post-printing period in a substantial environment that is almost close to the native environment of the cell. [132,133]eople all over the world have suffered a lot and faced a devasting state in healthcare due to COVID-19 during the 2020 pandemic. [134]The situation was quite out of hand due to the spread and growth of infectious diseases.3D printing technologies with graphene have the ability to fight against infectious diseases, and they perform so efficiently as vaccines, therapeutic agents, and in vitro modeling. [135]Graphene-based inks are also used in 3D-printing constructs for different biosensors and wearable sensors, with diverse scopes for applications.To fabricate a 3D structure through printing with an appropriate rheological property, the friction efficiency in the system should be maintained as low as possible; hence, the obtained construct will have an ensured cursive deposition with very few defects.At the beginning of the world of 3D printing, Das et al. [136] considered mechanical flexibility as a core issue in the use of graphene in electronics and wearables.They considered the fast fabrication of ink-jet-printed graphene circuits at a cheaper cost and in a sustainable way.Therefore, this non-toxic agent can process ink that can be used in conductive stripes on different flexible wearables.Using a powdered bed with graphene with injection molding and binder in the feed is not a bad idea at all.Azhari et al. [137] introduced the process of fabricating quasi-ink, and they could develop some regular slices of the sensing materials.Moreover, this process can easily be incorporated into different sensing applications like immunodetection and therapeutics.When the problem was about the cost and processing time of sensor fabrication with 3D printed sensors, Davoodi et al. [138] used dip coating with porous polymers with graphene to fabricate conductive layers integrated into the silicone sensors that are ordered and interconnected to each other.The 3D-printed mold was dip-coated into the silicone rubber surface, and with a stable coating, the sensors had stable electric conductivity for ≈ 12 months.This sensor can easily take signals from human movement, small deformations like a human pulse, and many more.
Though graphene inks are quite capable of having some abstract properties to endow superior functionalities, there are some parts to consider in processing them for 3D printing, especially in healthcare applications. [129]For example, the ink and its fabrication need to be in accordance with user and eco-friendly procedures and environmental factors, and the variations must be evaluated strictly.The best-fitting printing technique for the Better surface treatment, removal of metal-coated substrates, [ 104] precise control, thickness uniformity, suitability for nano-electronic devices based on 2D materials [ 103] High cost, presence of an unchangeable mask, and not being able to treat honeycombed graphene with an extremely bristle surface [ 97] Hard mask Medium No Confrontation with proximity effects and lift-off reliability can be controlled.Underexposure to the honeycomb pattern, increment of the process window, and better resolution [ 125] More cost, use of masks, and requirement of more time.
Transfer Printing High No Simple equipment, beneficial for industrial-scale applications, [ 114] has transferrable technique, high and precise resolution of pattern, [ 97,115] and enables distinct operation of assembly and device component fabrication [ 116] High cost, presence of an unchangeable mask, demand of high interface [ 97] Inkjet Printing Low Yes Changes in printed design with low cost, no use of masks, ease of development, [ 119] non-contact manufacturing techniques, essential sensing capabilities Requirement of more time for ink preparation in order to achieve high concentration, [ 118] low resolution [ 97] 3D printing High Yes It can make prototypes rapidly, and it can be modified faster.It can generate complex 3D structures It requires highly skilled manpower and a slower fabrication process, and there has been a scarcity of materials Laser Scribed Graphene Low Yes Growth and pattern in one step, programmable procedure, no use of mask, less time-consuming, possibility of pattern being grooved on a carbon source after taking output pattern on a laser platform. [ 97]w-resolution pattern best precursor should be fixed beforehand to get the best output from the process. [132]Researchers are working more on developing more tunable and geometrically controllable sensing materials with graphene in the wearable sector. [22]able 2 summarizes all the features and attributes of different fabrication techniques for graphene-based wearable sensors.Furthermore, the table summarizes the advantages and disadvantages of different fabrication techniques and gives a relative idea of the cost of production.Correspondingly, different sensing applications with graphene-based wearables in healthcare have been demonstrated in the following section of this review.

Graphene in Wearable Sensors For Healthcare Applications
Graphene's exceptional properties of high thermal conductivity, superior electron mobility, and a large surface area with limited thickness have propelled it to a point where a wide range of applications are now readily available.All of this is possible because the properties are fortunately concentrated in a single material, graphene. [97]There are several applications of graphene, and this section discusses the application of graphene-based wearable sensors.Figure 7 illustrates different healthcare applications of graphene-based wearables.

Human Movement, Gesture, And Health Monitoring
The 3D graphene sensors are pretty amenable to application in terms of human health detection as well as monitoring from time to time. [139]After numerous efforts by researchers to develop a non-destructive human monitoring device, Chen et al. [140] were finally successful in developing a specific approach to a pressure sensor with 3D microstructures in 2019, using the extremely useful properties of graphene.The path was not that easy for them, as the pulse and the blood vessels are just beneath the skin, and thus the very delicate and tiny signals are tough to detect.An interfacial self-assembled graphene film, or ISG, has been developed in ultra-large form via self-assembly and an environmentally friendly process, and this structure produces some brilliant outputs for ready-made 3D sensors, particularly with excellent sensitivity in the detection of the pulse rate as well as the motion since the wrist was bent.In addition to that, this innovation has aggrandized the research in graphene a lot as it provides a novel, superior, and tunable sensitivity within the range of 1.04 to 1875.5 kPa −1 since the range of detection has ranged from 1 to 40 kPa. [141]valuating and monitoring the human body's locomotion has been an important part of quantifying human health conditions and performance, as well as providing patients with care when they are not in the hospital. [29]Furthermore, graphene-based wearable sensors have enormous potential for biomedical applications and human motion monitoring.Musculoskeletal functions and human gait are important kinetic parameters in games and sports in terms of quantitative evaluation, and they also have a great impact on the detection and running diagnosis of various diseases prior to cardiopathies, aging, dementia, and some other neurodegenerative disorders like Parkinson's and other sclerosis. [142]xtensive research has been conducted on textile sensors designed for application in wearable monitoring.The sensor systems require a large sensing area, adaptability, and a manufacturing process that can be scaled up.Choudhry et al. [143]  fabricated textile-based flexible pressure sensors using a single layer of material for use in smart wearable electronics.Using conductive threads and fabrics covered with metallic and graphene nanoplatelets, single-layer piezoresistive sensors were created using a machine stitching process.Because the area of contact between the conductive thread and fabric altered in response to pressure, these sensors worked.The single-layer construction ensured flexibility, reducing physical drift caused by human activity and boosting both comfort and efficiency.Scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize coated fabrics.Wearability and sensing capability were validated via extensive physical and electromechanical testing, with the sensors demonstrating a 100 kPa working range, extraordinary sensitivity, and resistance to mechanical deformations.The successful integration of sensors into garments for real-time monitoring and posture correction has proved their potential as durable, adaptable, and highly effective pressure sensors suitable for smart wearable applications.
The sensors can monitor the high-risk patient's actions over time using wearable sensors since there is an assistive tool established in the body to obtain information from asymptomatic individuals and patients.The most often employed and accessible sensors for gait analysis include inertial sensors, optical sensors, and angular sensors.However, the inertial sensors include multiple sensor combinations among the accelerometers, gyroscopes, and magnetometers.The inertial sensors function mostly based on Newton's third law, and they are set to the inertial measurement unit of the body (IMU).Moreover, multiple sensors are integrated into it, and thus the generated signals drive the IMU to measure angular velocity, angle of flexion, 3D linear acceleration, and coordination with the reference frame.The below and above joints are the best places to fix the rigid small IMU, i.e., the knee, hip, elbow, toe, shoulder, neck, and ankle are the places where it can monitor the walking speed, condition, joint movement, and running conditions.However, the IMU is the most perfect method for clinical gait analysis.The integration of graphene can make it easier and more convenient. [144]Zhang et al. [145] fabricated a graphene-coated fiber sensor with superior sensitivity and high reproducibility in terms of performance.They attached the sensors to several parts of the body of the athletes, including the shoulder, elbow, wrist, knee, and ankle, to capture the motion of the human body.As a result of analyzing the raw data, they were able to obtain precise monitoring of the motion.
There is a growing interest in graphene-based aerogel/spacer fabric composites for use in healthcare, notably in sensing.These composites, based on the porous structure of aerogels and the amazing characteristics of graphene, provide a viable platform for healthcare sensors.They are perfect for building sensors for medical monitoring, diagnostics, and individualized healthcare due to their lightweight, extremely porous structure and graphene's superior electrical and mechanical characteristics.For this better sensing prospect and potential with protection, Wang et al. [146] created wearable multifunctional graphene-based aerogel/spacer fabric composites.A flexible composite material was created by combining the mechanical qualities of the spacer fabric with the electrical features of graphene-based aerogels.A 1.48 strain sensitivity, a temperature of −0.004 °C in the sensitivity, and a 45 percent protection factor were all displayed.This ultra-thin, waterrepellent, thermally insulating material worked well throughout a wide temperature range, from ≈ 180 °C down to much lower temperatures.Previous studies indicated that the multifunctional composite could be useful not only as a building material but also as an intelligent cushion in the medical field.
Interactive wearable sensors have become so prevalent that the need for flexible electronics and devices has been skyrocketing in healthcare.Larimi et al. [147] fabricated an inexpensive and stretchable strain sensor for monitoring the motion of the human body and organs using bio-signals (Figure 8a).They introduced the piezo-resistive sensor for measuring a wide range of human motion.This highly stretchable graphene-based sensor can keep the strain withstand able to a significant extent (up to 350%).It sustains itself even after 10000 cycles of strain.Moreover, this Figure 8. Graphene-based wearable sensors in human motion monitoring a) preparation of GNF-pad by infusing the graphene nanopowder into a clear adhesive pad.Adapted with permission. [147]Copyright 2023, Elsevier Limited.b) a biocompatible strain sensor with conductive polymers.Adapted under the term of Creative Commons Attribution License 4.0 (CC BY). [148]Copyright 2021, The Authors, published by Licensee MDPI.
sensor also works fine in robotic fingers with real-time monitoring of different human interactions.Jiong et al. [148] developed a biocompatible strain sensor on elastomeric conductive polymer composites to develop human monitoring devices (Figure 8b).They used an e-skin substrate with polydimethylsiloxane (PDMS) and a photosensitive polyimide (PSPI) precursor, followed by the formation of LIG/PDMS/PSPI composites with the laser direct writing (LDW) process.A combination of minimalistic oscillation and physiological signals was generated for intelligent sound sensing.Moreover, it showed an excellent sensing range (120%), a higher gauge factor of ≈ 380, and a very short response time of 90 ms.
When tiny and strong pressure sensors are available with a wide range of monitoring abilities, a 3D graphene-based wearable pressure sensor can easily evaluate human motion.Joint bending is comparatively a robust movement with a value of >20 kPa, and the movements generated from speaking and pulsating cause barely <1 kPa.Therefore, it has been a challenge to distinguish the movements and monitor them.Researchers were trying to find a way to get out of this limitation. [149]Finally, Dong et al. [149] came up with a solution.The group has fabricated a 3D rGO/polyaniline sponge (RGPS) for detecting human motion by constructing multi-level microstructures with a feasible method.The 3D graphene sponge has a delicate microstructure, and its positively charged PANI and negatively charged rGO make it a hybrid sponge.The synergistic impact of it provides a flexible and ultra-light sensor with high sensitivity to monitor swallowing, opening of the mouth, speaking, blowing, shouting, crying, breathing, and squatting.Its sensitivity was adjustable, ranging from 0.042 to 0.152 kPa −1 , and it could detect both large and small movements.Its response time (96 ms), functioning range (0-27 kPa), and durability (> 9000 cycles) steal the limelight easily. [141,150]Islam et al. [151] fabricated a fully printed and conductive energy-storing wearable sensor to monitor human movements with different bio-signals (Figure 9a).Highly scalable printing for sensor development with graphene ink could detect different activities of the human body.Moreover, it showed superior aerial capacitance with excellent stability.Additionally, it can monitor brain activities for different electrophysiological signals.Bai et al. [152] developed an inexpensive technique to fabricate graphene-based yarn sensors (GYS) in combination with graphene oxide (GO) coatings with polyester (PE) and wound spandex yarns (Figure 9b).It had the ability to tune the sensitivity as well as the gauge factor from 0 to 50%.This direct wound yarn sensor can essentially monitor different motions in the human body with muscles in relaxing and contracting conditions.
The strain sensors let the device get information from the human body and keep the device and the human body in touch.The thin, hydrophobic, flexible, human-skin-colored sensor has been directly set to the human body, a handicapped prosthetic limb, or a functioning robot.PDMS was used as a main part of the strain sensors because it is hydrophobic, easy to shape, has low interfacial free energy, and is stable at high temperatures and chemicals.As a carbon-based material, graphene has attracted  [151] Copyright 2022, Elsevier Limited.b) Stretchable graphene-based thin film-coated yarn sensors for human motion monitoring.Adapted under the term of Creative Commons Attribution License 4.0 (CC BY). [152]Copyright 2019, The Authors, published by Springer Nature.
great attention for its higher thermal conductivity and optical transparency. [153]3D porous graphene structures (3-D PGS) can be implanted in so many sensors that they have a great impact on fabricating a device for human motion monitoring.Laserinduced graphene can release properties like graphene.So many researchers have already worked on it. [154]Although ample effort has been made in the development of wearable sensors, it still requires a long way of study to make it more pragmatic for precise, long-term, durable, and continuous human monitoring activities.The efficient form factor, high flexibility, robustness, and efficacy of the final device with respect to energy consumption are places where it still needs to be improved.It is still challenging to integrate piezoresistive materials into an apparel product with a superior gauge factor and better stretchability up to a satisfactory level.144b] Flexible sensors play a crucial role in the development of intelligent electrical gadgets.Strain sensing is one of their most important uses and underpins several other technologies.To usher in the next generation of smart electronics, it is crucial that high-performance flexible strain sensors be developed.For autonomous oral health monitoring, Li et al. [155] developed 3D extruded graphene thermoelectric threads.Using graphene-based thermoelectric composite threads, researchers created a selfpowered, ultrasensitive strain sensor.Extreme stresses of over 800% were tolerated by the optimized thermoelectric composite threads.Excellent thermoelectric stability was maintained even after 1000 bending cycles.Using the thermoelectric effect, this technique can measure strain and temperature with extreme precision.These thermoelectric threads allowed for the autonomous monitoring of physiological signals in wearable devices, such as the degree of mouth opening, occlusal frequency, and tooth force when eating.This breakthrough yielded useful information that can be used to promote better dental health and healthier diets.
As the demand for polymer-based composite strain sensors rose by that time, Sankar et al. [156] developed flexible hydrogenexfoliated graphene (HEG)-based strain sensors with piezoresistive action with waterproof poly(vinylidene difluoride) (PVDF) to get higher conductivity in the sensors used in different wearables and human health monitoring devices (Figure 10a).This PVDF/HEG-based strain sensor exhibited 0.3 S cm −1 of electrical conductivity and a maximum gauge factor (GF) of 10, which made the sensor exclusive to the other available devices.Some of the cardiovascular difficulties happened in the absence of portable and flexible monitoring of real-time health.To reduce the mortality rate, Chen et al. [157] fabricated a device to detect physiological signals with a LIG-based E-skin that can monitor the health condition and respond in alarm as an output (Figure 10b).Moreover, the sensor showed excellent acoustic and mechanical performance with higher sensitivity (316.3 units) and could detect bio-signals related to pulse, respiration activities, and many more.Shathi et al. [158] fabricated a smart textile sports bra Figure 10.Graphene-based wearable sensors in healthcare a) PVDF/HEG-based strain sensor for human health monitoring.Adapted with permission. [156]Copyright 2020, American Chemical Society.b) Laser-induced graphene (LIG)-utilized electronic skin (E-skin)-based sensor.Adapted with permission. [157]Copyright 2023, John Wiley & Sons.c) From cutting the fabric electrodes and setting them in a sports bra: the notion indicates left straps (L), right straps (R), and bottom rib electrodes (H), and then the performance of the smart textiles was evaluated.Adapted with permission. [158]opyright 2023, John Wiley & Sons.
with a coating of graphene that can be used in human health monitoring (Figure 10c).They used the pad-dry-cure method to fabricate the electrode, and the electrical performance was significantly improved.Then, they incorporated rGO into the layer-bylayer (L-b-L) method.With a highly sensitive response rate, this smart wearable can easily monitor different parts of the human body.
COVID has taught us a great lesson in the prevention of the spreading of the virus; hence, facemasks play a very important role in resisting virus-containing fluids or air from entering our bodies.The virus directly attacks the lungs, which eventually creates problems with breathing.In this circumstance, it is important to monitor the health status of the patient from time to time.However, collecting respiration information from the human body and real-time tracking of the signals are not easy jobs to maintain.Moreover, fast responses and biocompatibility have been issues in wearables for a long time.As a response to the above-mentioned issues, Bidsorki et al. [159] developed a wearable and smart face mask with graphene materials that could easily be used in healthcare for real-time monitoring of human respiration.The smart face masks were fabricated with graphene nanoplatelets and polycaprolactone (PCL) over normal surgical masks in a certain polymeric matrix.With excellent durability and >1000 cycles of stability, the face masks exhibited a very fast response time of nearly 42 ms during the tests.Moreover, it can easily distinguish normal and abnormal breathing in the patient with very nuanced information, and the ultimate evaluation is visible in the mobile application.Hence, it has made diagnosis and monitoring very accessible with a facile fabrication process of simple coating with functional graphene materials.
Despite the presence of some necessary cracks, graphene has a significant piezoresistive impact in terms of its flexible mechanical performance in sensors.49a,56] To provide a solution to that, Yang et al. [57] fabricated a textile sensor with graphene to detect precise and largescale human motion.Aside from accurately capturing pulse signals, it could also detect changes in relative resistance during mouth opening and closing, as well as waves such as the tidal wave (T-wave), percussion wave (P-wave), and diastolic wave (Dwave).The electronic skin for the epidermal part, with multilayered and laser-scribing graphene, developed by Qiao et al, [160] could be able to capture the respiratory signals and could identify the levels and conditions of graphene when it has been set to the throat or mask.A graphene-paper sensor, on the other hand, can detect subtle pressures such as pulse and gas pressure.A strain sensor on woven fabrics, fabricated by Wang et al. [161] by the CVD method, was quite amenable to defining the physiological signals and human-generated micromotions when it was fixed to the mouth, and it could detect the change in facial expression, the resistive changes, as well as the blinks, and the respires, respectively.Graphene-based wearable sensors can be used in invasive and non-invasive applications.Moreover, the invasive applications include the nervous system, cardiovascular system, digestive system, and locomotor system.On the other hand, non-invasive applications are associated with biophysical, environmental, and chemical stimulation, and they are mostly hard to monitor.

Electronic Skin and Multifunctional Sensing
Natural skin is a combination of various senses and activities, such as flexibility, intelligence, high sensitivity, quick response, and a built-in healing process.The urge to create a skin-like sensor that can function as an intelligent sensor with multifunctional capabilities drove the researchers to draw inspiration from natural skin.Furthermore, because of its self-healing properties, the skin part can easily recover the mechanically injured part on its own. [162]The PC/rGO/PVA hydrogel with PC/rGO composite and PVA-borax hydrogel has been a bionic tactile component, fabricated by Liu et al., and this solution propagates the way to prepare the electronic skin from PC/rGO/PVA hydrogel, necessarily assembled electrodes and adhesives.PVA-borax can activate selfhealing effects, and it also improves biocompatibility and compensates for the poor elasticity.162c,163] With the idea of the human fingertip, a micro-structured ferroelectric skin has been fabricated by Park et al. [164] with the composition of rGO and PVDF composite film, and they comprise a layer to retrieve the sensing information with the piezoelectric and piezoresistive properties accordingly.The microstructured skins, which can easily separate them according to different surfaces, could have captured dynamic or static tactile stimuli like pressure, temperature, and acoustic waves.However, the E-skin has gained great interest for the nature of the sensor as well as its spatiotemporal sensing and transduction abilities. [164]Octadecane-implemented titanium dioxide nano capsule (OTNs)-graphene/PU film has been utilized with flexible wearable electronics by Chen et al. [165] this material provides properties like thermal insulation, self-healing, and ultraviolet protective properties.The ductility and sensing properties have been amazing both before and after healing because PU has the self-healing property as well as the H-bonds and disulfide bonds.The peak remained intact even after being cooled down and heated 20 times, which set up the notion that the device had great thermal insulation.Most surprisingly, the coated fabric showed great resistance to UV radiation due to the presence of TiO 2 , which is quite amenable to absorbing UV light.The soft robots coated with the electronic skin on the prosthetic finger can perform a lot of activities through these important skin attributes.This invention aligns with artificial intelligence, and when it combines with the composition of hydrogel and PC/rGO, properties like strain sensitivity (higher sensitivity up to GF = 14.14), stretchability (>5000%), compliance (≈1 mm), and self-healing push it over the mark in terms of its efficacy.The implementation of 3D graphene drives it even beyond the abovementioned applications. [141]hun et al. [166] developed a water-resistant e-skin for wearable fabric sensors using graphene (Figure 10a).Getting inspired by the structure and the micro-suckers of an octopus, they delivered an inexpensive and ultra-sensitive skin adhesive graphenecoated fabric (GCF) sensor that is sensitive to pressure and strain.Through this sensor, it can monitor a wide range of activities of the human body efficiently, including wrist pulse, movement, heart rate, and many other physiological signals.Moreover, it can also detect speech vibration and wrist bending, even in wet surroundings.This skin-adherent sensor has huge potential in the future (Figure 11a).Ye et al. [33] fabricated a sensor that has specific sensing with directive selectivity with a femtosecond laser fabrication process (Figure 11b).They could determine different kinds of faults and defects in graphene in the laser patterning system.Some specific patterns (i.e., basically four fundamental patterns: square, arrays, triangle, circle, and hexagon with right, none, obtuse, and acute angles) of imperfect graphene were introduced, and they could be controlled and minimized by the laser patterning process.They evaluated the sensing capability of graphene with different patterns, and correspondingly, a circle pattern was used for strain fluctuation, a triangle shape for temperature monitoring, and a hexagon for having information related to gas sensing.This sensor can essentially be integrated with e-skin and can perform various necessary sensing activities like temperature detection, pulse, heart rate, and detecting detrimental gases adjacent to the human body or clothing.Gelatin nanofiber films have a much higher failure tensile stress (35 MPa) and mostly possess better mechanical properties than many plastics like polyethylene and polysulfones.As a result, Liu et al. [167] fabricated GO film-based sensors to incorporate as eskin for cardiovascular monitoring, and they used highly tough and robust gelatin nanofiber to ensure its durability (Figure 11c).The sensor can perform activities like apexcardiogram recording, sound recognition, and pulse spectrum evaluation and monitoring.Apexcardiogram recording is quite an essential element for hemodynamic and cardiac health recognition.It can monitor ventricular contraction, diastole, systole, blood ejection, atrioventricular valve direction, semilunar valve open/close, and general valve open/close.With the input unit connected as an e-skin, the device can be used for cardiac health monitoring even with just a mobile phone.
Human skin's tactile sensing is quite complicated to imitate, and thus, fabricating a flexible and multifunctional E-skin with higher tactile sensitivity has been a tough task in terms of mimicking the sensing attributes of human skin.However, sensitivity has been a crucial factor for the smart sensing E-skin as it has to maintain the properties to sense vibration, ambiance, environment, temperature, and different shapes and materials. [164,168]roper flexibility, sensitivity, and quick responses to commands in a sensor can even make it act like a humanoid robot.Moreover, these robots can modify objects and get the essence of the living beings around them.Researchers assumed that someday this robot would mimic human nature and function even better than the somatosensory system present in the human body.Among all the efforts made on developing a multifunctional E-skin with specific sensors, a self-powered E-skin with triboelectric  [166] Copyright 2019, American Chemical Society; b) graphene-based wearable sensors with pattern directive sensing fabricated by femtosecond laser fabrication.Adapted with permission. [33]Copyright 2023, John Wiley & Sons c) GO films form gelatin nanofibers as e-skin for cardiovascular monitoring, Adapted with permission. [167]Copyright 2023, John Wiley & Sons.
nanogenerators' integration has been developed by Wang et al. [168a] and it could perform simultaneous pressure sensing and monitoring, temperature sensing, and recognizing the materials. [169]

Electrophysiological Signal Monitoring
Graphene-based wearable sensor is basically another feather in the cap of medical science in the modern era of smart transducers, electronic materials, and smart wireless systems.That is why it has attracted such interest in the community of researchers.58b,161,170] As a response to the demand for low-cost, reliable, and easy disease detection and monitoring, miniature instruments in medical science such as portable electrocardiography, glucometers, electronic sphygmomanometers, and paper-based diagnostics have already been invented.Thus, these instruments can alleviate the hospital's burden by detecting chronic disease early and precisely, as well as keeping a record of the long-term physiological condition to ensure continuous health monitoring.Long-term electrophysiological signal monitoring via wearable sensors with physical and cognitive functions has become popular for health monitoring and diagnosis.Therefore, these devices are more stable compared to mechanical signaling devices. [171]Electrophysiological signals are necessary for the treatment and prevention of patients.
Wearable sensors have come a long way with the crucial development of multi-sensing applications regarding real-time monitoring of biosignals and many other non-invasive signals.However, many researchers raised the issue of the comfort of the wearables, as the device has to be carried along with the wearer.Ali et al. [172] fabricated wearable fabric-based electrodes with graphene materials that can exhibit enhanced, real-time, and stable biosignal detection ability.This study revealed a comfortable and flexible method to fabricate electrodes to monitor and evaluate ECG signal detection, and they just developed the sensor by depositing a graphene-based nanocomposite over the fabric substrate.By getting out of the very traditional methods of fabricating electrodes, they reduced the importance of gel in the fabrication of sensors.Moreover, it can stay directly in contact with the skin.Most of the results of the study come from values of a higher signal-to-noise ratio (SNR) with lower skin-electrode impedance.Hence, the ECG signals could maintain a superior quality and effectively distinguish all crucial ECG signals according to their important features.For example, 40 dB was the range of the ECG signal-to-noise ratio (SNR) according to the record in the monitoring devices.In addition, it is worth noting that the ECG signal intervals during each cardiac cycle exhibit little temporal fluctuation.The electrodes can be utilized multiple times following washing due to the presence of the superhydrophobic characteristic.In conclusion, the PVDF/GNP electrodes that have been produced exhibit several desirable characteristics, including reusability, biocompatibility, effective skin-electrode interaction, and the absence of skin irritation.This research holds promise for more comfortable and effective ECG signal detection, with potential applications in healthcare and biomedical monitoring.The non-intrusive nature of wearable electronic textiles (e-textiles) made of graphene bodes well for the next generation of personalized healthcare applications.However, their broad use is restrained by inferior performance, decreased comfort, and increased material costs.Wearable e-textile sensors based on graphene have been developed by Tan et al. [173] for bio-signal detection; they are highly scalable, sensitive, and ultra-flexible.An optimized two-layer sensor was further assessed as a breathing sensor to gauge the practical bio-signal sensing potential of the piezoresistive sensors.In the supplementary material, the plux system was employed as a reference measurement, positioning the dual-layer sensors beneath the plux elastic band at the chest's center.The sensor was also incorporated for recording resistance shifts during breath analysis.These two-layer piezoresistive sensors exhibit the capacity to function as wearable sensors for both human movement and bio-signal detection, characterized by their rapid response and recovery times.A novel and facile technique has been described in this study for making electro-conductive yarn out of graphene, which could then be embroidered into piezoresistive sensors.The multilayer sensors were more sensitive, responded quickly, and recovered from failure quickly.
There are a lot of electrical phenomena that have been produced from the human body as well as the animal tissues prior to the active cells, and they are mostly known as the "bioelectrical signal."Due to the transmembrane flow of the ions, the resting potential (RP) and the action potential (AP) follow the mechanism of bioelectrical signals.The potential difference between the inside and outside of the cell membrane produces the static potential, whereas the active potential comes from the cell's stimulation when it is excited from the outside.As the initial membrane potential rises, there are a series of transient changes followed by a certain recovery but a slow approach to the resting potential. [174]Additionally, electrodes are mainly the sensing component in the device for capturing the signal from the human body.With the help of silver/silver chloride (Ag/AgCl) electrodes, the wet electrodes were fixed on the human body to evaluate the bioelectricity from several physical activities, in the conventional methods.As the wet electrodes possess a high risk of skin irritation and allergic reactions to the gel and adhesive materials, the wet electrodes are not that amenable in the long-term.Moreover, it has been reported the irritation may last even longer than 12 hours in the body of the patient and it requires a replacement of the gels once they get dry. [175]as et al. [176] fabricated a wearable sensor to monitor human motion and electrophysiological signals (Figure 12a).They adopted a vacuum filtration process with GO and nylon membranes, and they used no harmful elements or chemicals in the entire process.The docile process can fabricate an epidermal sensor with high sensitivity and a sheet resistance of 40 Ω sq −1 .as the impedance in skin contact was ≈20 kΩ at a very low frequency.With an amenable tensile strain, the sensor showed an excellent fast response time and relaxation time of 0.5 s, which made the device more flexible in bending-stretching response.Most importantly, the sensor can easily generate and monitor electroencephalography (EEG), electrocardiography (ECG), and electromyography (EMG), showing better biocompatibility than was tested with a cytotoxicity test.Zhang et al. [177] fabricated a portable, flexible, and multimodal on-skin graphenebased sensor for electrophysiological signal monitoring with low impedance (Figure 12b).The on-skin electrode sensors in a wireless communication module can essentially exhibit the three-inone attributes of monitoring ECG, EMG, and EEG.Additionally, it makes the device more comprehensive for medical applications and the field of wearable sensors.This low-cost ($25) and very light-weight (22 g) solution can provide real-time electrophysiological signal monitoring, and the human-machine interface with a mobile app can provide the services of fitness tracking, diagnosis, and very easy health check-ups from time to time.Qiu et al. [178] fabricated graphene-based wearable sensors inspired by the structure of the avian nest (Figure 12c).They produced a skin electrode to detect electrophysiological signals from a semiembedded and highly graphitized electro-spun fiber/monolayer graphene (GFG)-converting soft elastomer for making the conductive film.The sensor showed a higher conductivity of 150 Ω m −1 and stable electrical performance with biometric signals.
Therefore, as an alternative, the dry electrodes are more compatible and easy-going for the patients.as they can maintain good contact with the human body during movement and even during exercise.After a lot of attempts, various dry sensors have been fabricated, like conductive elastomers, conductive fabrics, and metal-type electrodes.Their rigid structures and the greater contact impedances create discomfort for the wearers, and sometimes they may cause damage to the skin after a long time.Therefore, in search of flexible materials, the researchers found dry sensors made from CNTs, PDMS, and rGO that were easy to implement.As most of these are quite expensive to fabricate, they found thermally reduced graphene oxide to be one of the best candidates for a biocompatible and cheap sensor material.175a,179] Surprisingly, Das et al. [176] developed an excellent high-performance and paperbased epidermal sensor that is placed over the biocompatible nylon membrane (NM), and the most important thing is that they did not use any detrimental elements in the process.The epidermal sensors under the conventional tattoo have been fabricated with metal films and silicon membranes by Ameri et al., [62c] and they came up with a method called "wet transfer, dry patterning," which was cost-and time-effective and known as graphene electronic tattoo (GET).The sensor, with a thickness of 463 ± 30 nm, higher sensitivity (>40%), and excellent adhesion to the skin, could capture various bioelectrical signals, including the ECG, EEG, and EMG.Moving forward, low-cost, high-performance bioelectrodes have been developed by Yun et al. [180] through the solution-based method with rGO and porous polydimethylsiloxane (PDMS).Moreover, they were highly stretchable (strain> 149%), durable (up to 5000 cycles), and low sheet-resistant (1500 Ω −2 ) which denoted their potential for the world of wearable sensors at a very low cost.A multifunctional on-skin electrode with laser-patterned porous graphene had been fabricated by Sun et al. and could exhibit a superior water-vapor permeability of 18 mg  [176] Copyright 2023, Elsevier Limited.(b) Portable wearable sensors with low-impedance graphene electrode sensors for electrophysiological monitoring.Adapted with permission. [177]Copyright 2022, Wiley VCH-GmbH.(c) A transparent and bioinspired graphene electrode sensor for monitoring electrophysiological signals.Adapted with permission. [178]Copyright 2020, American Chemical Society.cm −2 .h−1 and a water-wicking rate of 1 cm/30 s.Because this sensor can also monitor air permeability and reduce the risk of inflammation, it can also address the issue of comfort and is better for long-term monitoring. [181]174a]

Therapeutics and Temperature Sensing
Temperature has been a crucial parameter for the physiological monitoring of the human body and monitoring healthrelated problems, as having precision and accuracy in body temperature is quite important to monitor continuously.Although there was an abundance of material with the ability to provide good resolution and accurate temperature measurement, they are mostly contact-based and rigidly shaped, which makes them not so suitable for the body's long-term usage.Therefore, real-time temperature monitoring became a great challenge to execute. [23,182]Localized temperature sensing has been quite problematic because the body surface is not flat.Therefore, measuring and monitoring the temperature from the curved surface as well as defining the signals from the skin's surface considering the consumer's mobility have yet to be in-vented.As the IR devices and thermal imaging cameras were able to provide the temperature without any contact with the surface, this idea helped to come up with the idea of developing wearable temperature sensors with the ability to monitor the temperature in dynamic environments and places with spatial variations. [23]Although thermocouples, [182,183] thermosresistance, [184] and thermal-responding field-effect transistorbased temperature sensors [185] have come a long way, they fail to provide a wide range of thermal properties.However, delicate electronic circuits are necessary so that the device can provide accurate detection.To form efficient, flexible thermal sensors, graphene is one of the most superior elements because of its exceptional electrical, physical, thermal, and chemical properties.The exceptional thermal properties make graphene different from any other material in terms of fabricating a temperature sensor.Moreover, it has an excellent thermal-responsive ability to provide a great temperature-sensing application. [186]or optical transparency, conductivity, carrier mobility, and flexibility, graphene materials are selected for different microelectronics and sensors.In terms of biosensing, active electrochemical defects due to low-density graphene deposition have been a limitation in the development of graphene-based sensors for thermal and therapeutic applications.Lee et al. [52] fabricated the wearable sensor with gold-doped graphene in a gold mesh with electrochemical activity that can create a patch so that it can Figure 13.Graphene-based wearable sensors in thermal and therapeutic applications a) reduced graphene oxide-based strain sensor for monitoring temperature sensing in healthcare.Adapted with permission. [187]Copyright 2023, Elsevier Limited.b) Incorporation of graphene flakes to produce graphene-based wearable sensors for thermal sensing.Adapted under the term of Creative Commons Attribution License 4.0 (CC BY). [189]Copyright 2019, The Authors, published by American Chemical Society.
evaluate and remark on the diabetes condition of the patients.Moreover, it can also generate precise feedback for the wearer.A heater with different sensors (i.e., temperature, pH, humidity, and glucose) and a polymeric and thermally activated microneedle for drug delivery are the main components of this wearable sensor.This sensor can smoothly deliver metformin to reduce the diabetes level in the bodies of diabetic mice.Nanofiber composites with conductive polymers for wearable sensing devices are now quite popular due to their flexibility and high precision in different biomedical attributes.However, this innovation still faced some challenges in maintaining proper wetting properties, durability, and multifunctional sensing operations.Xiao et al. [187] fabricated a multifunctional graphene-based strain sensor with rGO and a polymer nanofiber core that can essentially be used as a temperature sensor for high-performance functional uses and biomedical applications (Figure 13a).Moreover, it can also be used for body motion monitoring, and it showed high sensitivity with great durability for >1000 cycles of use.Thermal imaging is an essential part of human condition evaluation, and it helps to provide a general idea regarding the human body and hints for different diseases.Moreover, heat therapy and thermography are used to cure many diseases and injuries in the skin tissue.Kang et al. [188] developed a graphene-based patch for thermal monitoring used in thermal distribution and evaluating thermography.The thermal patch can keep track of continuous skin temperatures for sensing and generate thermography for self-care treatment facilities.The capacitive sensor in the graphene-based ther-mal pad with a communication module can monitor temperature continuously, and it has been proven by different trials on animals.Most smart wearables are generally integrated with sensors in the body, textiles, or materials.However, manufacturing smart textiles by using conductive yarn was quite expensive before Afroj et al. [189] introduced the docile, ultrafast, washable, and highly scalable graphene-based textile sensor, where graphene flakes were dispersed to coat the textile yarn with graphene ink (Figure 13b).In its knitted structure, the graphene-based wearable can essentially send information regarding health continuously.Most importantly, it can sometimes work in self-powered mode for a low-powered Bluetooth in an RFID system.It showed superior thermal sensitivity and conductivity.It projected a highscale production rate of 1000 kg/h for fabricating electroconductive yarns.
For flexible temperature sensors to be useful in real-time wearable health care, they must have high sensitivity, great linearity, and wireless monitoring.Particularly demanding sensor performance is the multichannel body temperature monitoring system, which must constantly keep tabs on a massive volume of data.Chen et al. [190] designed a system for wirelessly monitoring body temperature that makes use of an in-situ temperature sensor array with high sensitivity and excellent linearity.The porous graphene/polydimethylsiloxane sensing layer allowed them to create a flexible temperature sensor.In the region of 30-70 °C, this sensor showed both high sensitivity (5.203% °C−1 ) and excellent linearity (R 2 = 0.996).These high-performance temperature sensors have found use in a variety of contexts, including the monitoring of human body temperature and breath in both normal and abnormal breathing rates.To further facilitate distant and multichannel body temperature monitoring, the researchers set up a high-throughput wireless body temperature monitoring system consisting of wireless sensor modules, a cloud server, and a portable electronic device.Thermal imaging is an effective method for evaluating a patient's health and aiding in the diagnosis of a wide range of medical disorders.Heat therapy, often known as thermotherapy, is also effective in treating damage to the skin and other superficial tissues.In this research, we introduce a thermal patch that may be worn to simultaneously give thermotherapy and track the user's skin temperature in realtime.This cutting-edge tool provides a highly efficient self-care option.For the purposes of thermography and temperature distribution mapping, Kang et al. [191] developed a wireless graphenebased thermal patch.The system consisted of a capacitive sensor based on graphene, a thermal pad made of graphene, and a flexible readout board that was paired with a wireless communication module.The wearable sensor exhibited continuous monitoring of temperature fluctuations across a significant surface area of the skin, measuring 3 × 3 cm 2 , with remarkable precision and sensitivity.In addition, thermotherapy was implemented by incorporating a heater based on graphene, which was positioned at the bottom of the device.The diagnostic capability of the system for many diseases has been verified through animal tests.
A flexible temperature sensor has been fabricated by Rogers et al. and implemented over the serpentine gold and PIN diodes with Si nanoribbons on elastomers. [192]184a] With the help of a bimodal sensor array, Park et al. [193] came up with the idea of continuous and simultaneous sensing of pressure and temperature, and those sensors can monitor the precise and regular thermal state of the human skin.Unfortunately, they provided an inferior thermal response.As a thermally sensitive element in conventional micronanofabrication techniques, monolayer or bilayer graphene was deployed on silicon materials.However, it fell behind as there was a lack of flexibility.As a solution to it, Trung et al. [186b,c] suggested a flexible temperature sensor with the ability to detect slight changes in temperature.That experiment was implemented by polymer substrates with the reduced graphene oxide thermal-responsive field impacting the transistor.Then again, the stretchability and the temperature responsibility were quite inferior and limited.Finally, the graphene nanowalls (GNWs) from the graphene nanosheets vertically over the substrate provided a floor for the gas sensor and the biosensors.That used to have great stretchability to a greater extent for the vertically standing graphene nano walls that are perpetually interlaced.Therefore, it enabled the material to be flexible, stable, highly sensitive, and a simple temperature-responsive material. [194]There are still several fields in graphene-based temperature sensors requiring more research and their performance and efficacy have not been up to the mark yet.
All these applications are quite eminent in healthcare in the modern world.Since healthcare is too expensive and inaccessible in so many countries, these technologies can bring solutions at a very cheap price to the people of those countries.Table 3 demon-strates the applications of graphene-based wearable sensors.The sensors can also help in combating pandemics, and some of the brilliant research regarding pandemics and the ways to fight future pandemics has been demonstrated in the following section of this review.

Graphene Sensors in Tackling Future Pandemics
The side effects of COVID-19 have still been around in the postpandemic environment.Due to its different routes of transmission, the extremely contagious spread of the viral infection was almost impossible to stop.Therefore, it is important to become more aware of and prepared for pandemics.Graphene-based wearable sensors can play a vital role in defending against the foreshadowed pandemics in the future, as we never know when they may happen.Furthermore, due to overuse and overexploitation of resources, the implementation of sustainable products with properties such as ease of fabrication, resource and cost efficiency, environmental friendliness, and lower energy requirements is now unavoidable. [225]The laser-induced graphene materials from the single-step laser scribing can fight against the coronaviruses HCoV-OC43 and HCoV-229E and destroy the virus within 15 minutes of photothermal action.By adding metal and metal oxides, the nanofibrous textured edges could enhance their antibacterial effects.The highly conductive property improved its joule heating properties as well as its self-sterilization into air and water.The oxygen overpotential enabled the great electrochemical properties.Aside from that, the LIG can destroy microbes using hydrogen peroxide, putting the microbes under oxidative stress.Electroconductive filters made of LIG can be used in high flux and have been coated with PVA and GO.The membranes can kill microorganisms with their very high flow rate and in flowthrough mode.A lot of researchers are interested in using LIG in wearable sensors because it has the best texture, conductivity, and durability of the membrane surface.Additionally, LIG's remarkable self-sterilization properties and the fabrication process have been recognized for their immense potential as a control and prevention technology. [226]hen graphene was incorporated into conventional LFP (lithium iron phosphate) batteries, they were charged way faster than lithium-ion batteries.Furthermore, those have the advantages of being lighter and having a larger capacity. Figure 14 illustrates the use of graphene in detecting COVID-19 and preventing its spread through protective measures.Therefore, it can be advantageous to the sensors operating with graphene-based materials too.A graphene-based sensor with the ability to capture the real-time detection of tiny molecules has been fabricated by Cardea (presented by San Diego Nanomedical Diagnostics). [227] resistive biosensor made of graphene has been deployed to detect the COVID-19 virus with a graphene-modified electrode.The hepatitis B virus was detected with a composite of reduced graphene oxide and gold nanoparticles, and it happened within a very short detection limit.A rGO nanocomposite with gold nanoparticles was used to detect diarrhea.Single-layer graphene is also applicable in biosensing applications for its robust thermal conductivity, good elasticity, high mechanical strength, and greater surface area. [228]Graphene was driven to detect the SARS-CoV-2 virus in the laboratory of the University of Illinois at Chicago by their researchers. [229]The single-atom-thick carbon  Capturing body motion with the negative temperature coefficient effect (NTCE) [187]   material could make resonant vibrations.Furthermore, the vibrations could be precisely counted, which became known as phonons.The resonant vibration plays a very specific role as it dismantles the structures of SARS-CoV-2 molecules as soon as it interacts with graphene.The research to develop graphene-based sensors at the atomic level to detect COVID and cancer. [230]raphene has a strong antibacterial impact due to its high inhibitory capability.The composite antivirals are made up of sulfate, heparin, and graphene.The antiviral activity against the African virus was discovered in rGO sulfate compounds created for thermal power.DPG (dendritic polyglycerol) with sulfation, incorporated with rGO could detect various orthopoxviral strains by working as an antivirus.A carbon blanket restricts the micro-organisms from growing, and the GO flakes are used to do the main job at that place.SARS-CoV-2′s protein receptor agent is used to interact with heparin and modify its own conformation. [230,232]raphene has been known as an outstanding antiviral substrate to fight against the spread of viruses, diseases, and infections.As a result, graphene-based wearables may be able to com-bat pandemics in the future.According to the WHO, frontline workers are suggested to wear graphene-based facemasks that might minimize the transmission of the virus.The research focusing on wearables, sensors, and functional textiles must be stepped forward so that the technology can be useful in terms of controlling and protecting from the spread of viruses.Graphenebased wearable sensors play a vital role in the pandemic.Moreover, graphene facemasks are recyclable by the photocatalysis method or the thermal setting process.34b,233] Moreover, the sensors are always there to detect and provide the necessary information.Research must go on about this topic, and the study on graphene-based wearable sensors will surely bring out some outstanding technology for any future outbreaks or pandemics.

Perspectives, Challenges, and Conclusion
Graphene is now being used as a sensor and signal transducer in various wearable and mobile health device prototypes.More Figure 14.Application of graphene-based wearable sensors in tackling COVID-19 and future pandemics. [231]nterestingly, graphene has paved the way for the use of other 2D materials in sensors and mobile health-monitoring devices.The various types of sensors discussed here revealed that for highperformance sensors, the right combination of doping materials, device architectures, and synergistically contributing interfacial effects is critical.Recent research promotes the utilization of graphene and its derivatives, along with the functionalization of graphene with other substances, as a sensing material in a wide array of wearable sensors.Due to the multifunctional properties of graphene-based materials, this review paper focuses on newly developed graphene-based wearable sensors for detecting temperature, gas, strain, piezoresistive pressure, various chemical detections, and electrophysiological signals.In addition, this review shows that graphene can detect a wide range of signals.This allows researchers to explore how graphene can be used in wearable health monitoring devices that work in real-time.These devices are getting a lot of attention from researchers across many fields because they can track important body functions and find diseases early.Graphene-based wearable sensors show promising advances as an emerging technology that will have a positive impact on healthcare.Nevertheless, it is still suffering from multiple challenges and limitations that need to be addressed.
The first and foremost drawback of graphene application is the difficult and complex synthesis process.Different types of production processes have already been developed, but unfortunately, all of them have some individual benefits and shortcomings.For instance, the chemical vapor deposition process is one of the most promising methods to synthesize graphene on an industrial scale due to its large and top-quality graphene production with controllable layers, but the transfer process of this method is not efficient.Again, the exfoliation method is cheap and simple, but the size and layers of graphene cannot be controlled properly.The researchers are still facing challenges in producing high-quality graphene on a mass scale at a cheap cost.In the current scenario, the processing cost of graphene is much higher compared to similar carbon materials like activated carbon, which limits its application on a broad scale.To overcome this issue, more research is required to develop and upgrade the synthesis methods for graphene. [234]Besides, another matter of concern is the biocompatibility of graphene-based composites.It has been found that some of them are biocompatible, and again, some are toxic. [235]The toxicity of graphene has been explored in fish, plants, fungi, bacteria, and animals.The impact of graphene on these organisms is mostly negative and varies from one to another.The possible ways of graphene leaching are graphene production, graphene-based wearable manufacturing, graphene-based wearable use, reuse, and recycling, and unplanned disposal of graphene-based wearables. [236]To decrease the exposure of graphene to life, the production of graphene needs to be secured, standards need to be followed for the care and laundry of graphene-based wearables, and the disposal of graphene-based wearables has to be done more safely.The human health and environmental impact of graphene also need to be considered for further study to make it more sustainable and human-friendly, especially the exfoliation-based synthesis techniques.Reproducibility and recycling of graphene materials have been an underlying demand in the current world.The success of biomedical devices depends significantly on their production and processing methods.Also, an intensive study of the molecular structure and the mechanism of action of graphene is necessary to derive efficient properties from it.Therefore, more research on controlling the factors with minimal environmental impact can build a sustainable future by ensuring large-scale production of them.
In the case of manufacturing graphene wearable sensors, there are two primary units: 1) conductive mesh, which functions as a signal-capturing device; and 2) flexible substrate, which provides protection to the conductive mesh and also functions as a base.Therefore, the substrate materials need to be chosen carefully to impart optimum flexibility and protection.Though the graphene electrodes are flexible, stress corrosion cracking has been noticed, especially during ECG, due to enhanced electronic impudence that resulted in low sensitivity. [237]Some novel techniques are currently being studied to enhance flexibility and durability.For instance, bridging materials like silver nanowires or carbon tubes can be incorporated to improve mechanical stability.Again, lubricant can be used to reduce the stress on the strain sensors.Also, a brick-and-mortar structure can be developed between the substrate and nanomaterials by forming ionic bonds and − bonds that will increase the overall stability of the sensor. [238]After all, to overcome this issue, the selection of processing method, substrate, and compatibility should be considered carefully.Further study is needed to detect the reasons for the poor sensitivity and durability of the graphene wearable healthcare sensors.It is also noticeable that graphene-based wearable sensors show a narrow sensing range to detect signals, which can be inappropriate for real-world applications.Thus, highly sensitive sensors with a wide range need to be developed through further study.Furthermore, comfort is a mandatory property for wearable sensors.It has been found that wearable sensors use conventional tape or bandages to attach the sensors to the body or skin, which could be annoying for the users.Hence, selfadhesive wearable sensors need to be developed, which is possible by using gecko-inspired microfibers, ultrathin packaging, microneedle arrays with swellable tips, or chemical adhesives (e.g., polydopamine) [239] In addition, sweating is a usual bodily function that needs to be vaporized properly.Wearable clothes should not be an obstacle to sweat vaporization from the skin.Thus, the wearables must be breathable, which can be obtained by using textiles or a 3D sponge to fabricate the sensors, which will also increase their flexibility.Besides, graphene-based wearable sensors need to show responses only based on the human body without considering environmental factors.However, as graphene is a very sensible material, it may show different responses due to changes in temperature, moisture, or pH values. [238]As a result, graphene-based wearable sensors need further study to protect their responses from outside factors.Hydrophobic coatings and thermal insulator material can be utilized to make them waterproof and thermally stable.For the long-term use of wearable sensors, sustainable power sources can be an issue that can be mitigated by incorporating a self-powered generator.For instance, a triboelectric nanogenerator can harvest energy from the environment and human movement [240] which can be incorporated with graphene-based wearable sensors for self-powered capability.Future research can be focused on developing graphene-based wearable sensors that are lightweight, flexible, comfortable, sustainable, cheap, reusable, battery-free, and multifunctional with a proper response to stimuli.
Flexible electronics based on graphene will play a bigger role in the development of many promising fields in biomedicine, healthcare, robotics, artificial intelligence, and entertainment technologies.Because of the interface's adaptability and userfriendliness, real-time monitoring is possible, which will increase the amount of data.Graphene-based wearable sensors can be used in clinical or medical settings when combined with machine learning and big data techniques.Wearable graphene sensors may improve people's lives in the future.Perhaps diagnosis and treatment will become more common in the future.Wearable sensors based on graphene are receiving a lot of attention in this highly competitive field of research.Additional novel approaches based on graphene and its various forms may be developed in the near future.
Mahmuda Akter has been working as an associate professor at the Bangladesh University of Textiles.She has completed her Ph.D. from Erciyes University, Turkiye, under the prestigious CoHE (Council of Higher Education) Ph.D. Fellowship.Prior to that, Dr. Mahmuda obtained her B.Sc. and M.Sc. in Textile Engineering from the Bangladesh University of Textiles (BUTEX).Before becoming a faculty member of BUTEX, she had professional and industrial experience in textile factories.Her major fields of study and research interests are wearable technology, graphene and 2D materials, textile materials, fiber-reinforced polymer composites, textile structural composites, nanocomposites, biocomposites, technical textiles, and sustainable materials.
Habibur Rahman Anik is an inquisitive graduate student, who is embarking on an exciting journey in material science and polymer engineering.With a BS in Textile Engineering from Bangladesh University of Textiles, Anik has been devoted to the world of materials science.His research interests span polymer chemistry, composites, fiber science, hydrogel, bio-based materials, and nanoparticles, exploring intricate properties.Anik excels at identifying complex problems and crafting ingenious solutions.He aspires to become a formidable scholar in the field of smart materials, garnishing his achievements with international acclaim through captivating presentations, groundbreaking research, and inventive problem-solving.

Figure 1 .
Figure 1.An outline of graphene-based sensors for diverse applications in healthcare.

Figure 2 .
Figure 2. a) Schematic fabrication process of a modified cotton thread colorimetric sensor developed by Promphet et al.Adapted with permission.[43]Copyright 2023, Elsevier Limited.b) The schematic manufacturing and design concept of a strain sensor based on superhydrophobic paper and SEM analysis of the surface morphology of a scorpion's slit unit and a papilla (lotus leaf).Adapted with permission.[45]Copyright 2021, American Chemical Society.

Figure 3 .
Figure 3. a) Image of the strain-sensing fabric b) sweat-absorbing hydrogel; dry hydrogel absorbs sweat and swelling till final equilibrium (left to right); c) cross-section view of hydrogel-attached sweat gland; d) hydrogel's response to sweat along with real-time signal monitoring.Adapted with permission.[48]Copyright 2023, Elsevier Limited.

Figure 4 .
Figure 4. Schematic of the sensor attachment for detecting a) subtle motion and b) large movement of the human body.Adapted with permission.[57]Copyright 2018, American Chemical Society.Digital image of a wristband wearable sweat capture sensor; c) water captured by the wearable sweat capture sensor when the sensor is d) upward, e) sideward, f) downward.Adapted with permission.[54]Copyright 2019, American Chemical Society.

Figure 9 .
Figure9.a) A fully printed graphene-based multifunctional sensor for monitoring human movement.Adapted with permission.[151]Copyright 2022, Elsevier Limited.b) Stretchable graphene-based thin film-coated yarn sensors for human motion monitoring.Adapted under the term of Creative Commons Attribution License 4.0 (CC BY).[152]Copyright 2019, The Authors, published by Springer Nature.

Figure 11 .
Figure11.E-skin in different healthcare monitoring applications a) graphene-based skin-adhesive fabric sensor for e-skin.Adapted with permission.[166]Copyright 2019, American Chemical Society; b) graphene-based wearable sensors with pattern directive sensing fabricated by femtosecond laser fabrication.Adapted with permission.[33]Copyright 2023, John Wiley & Sons c) GO films form gelatin nanofibers as e-skin for cardiovascular monitoring, Adapted with permission.[167]Copyright 2023, John Wiley & Sons.

Shariful
Islam Tushar is currently pursuing his MSc from Oklahoma State University (OSU), majoring in Apparel Design and Production.He is working as a Graduate Teaching Assistant at the Department of Design and Merchandising of OSU.His research interest belongs to soft materials including fiber, polymer, and textiles, and their inclusion with the Internet of Things (IoT).He graduated with a bachelor's degree in Textile Engineering from the Bangladesh University of Textiles, Dhaka, Bangladesh in 2021.Imana Shahrin Tania is an Associate Professor in the Department of Wet Process Engineering under the Faculty of Textile Chemical Engineering.She is a fellow of the National Science and Technology ('NST'), Ministry of Science and Technology, Bangladesh.Dr. Tania has an outstanding academic background.She completed her Ph.D. from the Bangladesh University of Engineering and Technology (BUET).She completed her graduation and post-graduation in textile engineering with a specialization in wet process engineering from Bangladesh University of Textiles.Besides teaching, her prior interest lies in effective research work.Her research interests focus on advanced functional finishing, nanotechnology, surface modification, characterization of textile fibers, eco-friendly dyeing and finishing technology, and technical textiles.

Table 1 .
Different wearable sensors with their properties.

Table 2 .
A comparison of various graphene patterning techniques.