key: cord-0638561-cu3olgvh
authors: Shirhatti, Vijay; Nuthalapati, Suresh; Kedambaimoole, Vaishakh; Kumar, Saurabh; Nayak, Mangalore Manjunatha; Rajanna, Konandur
title: Laser-patterned multifunctional sensor array with graphene nanosheets as a smart biomonitoring fashion accessory
date: 2021-04-04
journal: nan
DOI: nan
sha: 9d335c8c899c02bce6ee10dcfecc0ab1d51f1e85
doc_id: 638561
cord_uid: cu3olgvh

Biomonitoring wearable sensors based on two-dimensional nanomaterials have lately elicited keen research interest and potential for a new range of flexible nanoelectronic devices. Practical nanomaterial-based devices suited for real-world service, which have first-rate performance while being an attractive accessory, are still distant. We report a multifunctional flexible wearable sensor fabricated using an ultra-thin percolative layer of microwave exfoliated graphene nanosheets on laser-patterned gold circular inter-digitated electrodes for monitoring vital human physiological parameters. This Graphene on Laser-patterned Electrodes (GLE) sensor displays an ultra-high strain resolution of 0.024% and a record gauge factor of 6.3e7 and exceptional stability and repeatability in its operating range. The sensor was subjected to biomonitoring experiments like measurement of heart rate, breathing rate, body temperature, and hydration level, which are vital health parameters, especially considering the current pandemic scenario. The sensor also served in applications such as a pedometer, limb movement tracking, and control switch for human interaction. The innovative laser-etch process used to pattern gold thin-film electrodes and shapes, with the multifunctional incognizable graphene layer, provides a technique for integrating multiple sensors in a wearable fashion accessory. The reported work marks a giant leap from the conventional banal devices to a highly marketable multifunctional sensor array as a biomonitoring fashion accessory.

The human body exhibits ample cues regarding its present state of wellbeing. While the human body possesses the sensory and somatosensory perception of the outside world and its conditions, external devices are required to know the body's status accurately. Up until the turn of this century, a wearable device would just mean a wristwatch or a medical device in the intensive care unit. The swift progression in semiconductor and telecommunication technology has led to a new meaning to the term wearable devices ranging from wrist bands, arms bands, and so forth. Over the last decade, flexible wearable devices have emerged as the primary personal in-house health monitoring devices, which provide clues to the bearer key physiological parameters and help in timely medical intervention. The silicon technology does not cater to these applications principally due to the sensor's rigid structure and inability to comply with the human body shape. Monitoring of human physiological parameters still poses an uphill task and demands sensors with flexibility, stretchability and bio-compatibility, presently in a continual state of improvement.

The most popular sensor employed in commercially available wearable devices is the Photoplethysmography (PPG) sensor used to measure heart rate and respiration rate, 1 which has been researched and developed for decades. The PPG yet encounters certain limitations such as getting affected by optical noise, skin tone dependency, lower sensitivity in low perfusion areas like arms, legs (sensitive on fingers only), longer settling time, delivery of moving average of heart rate, lack of intra-peak details, high power consumption and larger form factor. 2 Sensors dedicated to other physiological measurements such as thermometers and skin-impedance meters used for measurement of body temperature and dehydration, respectively, are still independent devices and have not been integrated into commercially available wearable devices.

Newer sensors termed as e-skins, epidermal electronics, smart textiles are being fervently researched in the field of wearable devices. Sensors based on nanomaterials like graphene, MoS2, MXene have emerged as exciting prospects in accomplishing better detection of physiological parameters. 3 These 2D materials offer considerable improvements in response time, sensitivity, skin conformity, power consumption, and so on, which are reckoning for a new generation of wearable devices. Graphene being the front-runner among 2D nanomaterials, extensive wearable device research has been accomplished based on this material. In its various forms, graphene has been used to develop Flexible wearable devices for motion monitoring and biomedical applications using CVD grown graphene, 4-8 graphene skinconfirming electrodes, 9 capacitive strain sensors, [10] [11] [12] [13] reduced graphene oxide, [14] [15] [16] [17] and nanocomposites based on graphene and metals/semiconductors and polymers. [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] have been extensively reported in the recent past. Similarly, other functional materials like MXene, 36 gold nanoparticles, 37, 38 AgNW, 39 PEDOT:PSS 40 and so forth, have also been employed in a variety of designs as wearable sensors for different biomedical applications. Devices based on nanomaterials have shown immense promise and have effected commendable progress in wearable device technology.

However, certain limitations in the works reported above can be pinned down for further improvement to make the wearable device technology competent and market-ready.

The device sensitivity, an outcome of the sensor thickness, can be further improved in the low strain (<0.1%) regime. 3 Controlling the sensor layer thickness has always been a significant cause of concern since lower thickness leads to film cracking, unreliable electrical contacts, and mechanical instability. A uniform conductive sensing film prescribes higher film thickness (>μm) which compromises the sensor sensitivity and makes it opaque and stodgy. The counterintuitive idea of using 2D nanomaterial in a 3D model can lead to the stacking of graphene sheets and hence squanders the benefits of a 2D material. Sensors that are ultra-thin and transparent such as those using CVD grown monolayer graphene, suffer from lower sensitivity, complex handling process, low scalability, limited customizability, and higher production cost.

The lower sensitivity demands stronger adhesion of the sensor/electrodes with the skin surface, like double-sided adhesive tape, 6 tattoo adhesion. 36 or on-skin printing 41 for maximum strain transfer / electrical interfacing. This arrangement renders the sensor as a single-use device since removal of the strongly bonded sensor without irreparable damage is an improbable task.

The demonstrated sensors have seldom been tested as multifunctional devices, which can greatly enhance their versatility and valuation. The reported sensors, especially foam or sponge-structured, are inherently bulky, opaque, low skin conformal and unappealing. Sensors that have been tested for different physiological and tactile applications have fared satisfactorily for the individual tasks, but consolidating multiple sensors in an ensemble has not been addressed before. Fabricating an ensemble that is practical and fashionable enough to become a marketable accessory still needs to be solicited. The development of a highly sensitive, simple, scalable, skin-conformal, multifunctional, customizable, daily-wear integrated wearable device for biomonitoring and consumer applications is highly demanding and presented in the following work.

In the present work, we have designed, developed and demonstrated a multifunctional sensor using solution-processed graphene nanosheets to monitor human physiological parameters. A unique way to fabricate the sensor electrodes using a laser-etch process has been developed, which forms the essence and the aesthetics of the device. The graphene-ink dropcasted on gold electrodes fabricated using a unique laser-patterning process forms the essence and the aesthetics of the wearable sensor. The 2D graphene nanosheets form a conducting percolative network across a micro-channel created by laser etching of the gold thin film. The sensor's remarkable characteristics and demonstration toward measurements of physiological parameters like heart rate, breathing rate, limb movements, touch input, body temperature and dehydration have been reported. An aesthetically appealing multi-sensor ensemble based on the innovated technique has been presented as a trendy wearable bracelet. 

The physical and chemical properties of the as-synthesized MEGO material were ascertained via different material characterization techniques. The morphology of the MEGO material was examined under the FE-SEM tool, and an exploded form of GO was observed, much like an accordion instrument (figure 2a). This structure is caused due to the intense agitation of the trapped functional groups on the surface of the carbon basal plane, during the graphene oxide synthesis process. The graphene oxide is rendered hydrophilic due to the oxidation process and hence readily adsorbs moisture on the surface. The microwave energy vibrates the functional groups and absorbed water molecules, which eventually split the loosely bonded graphene layers apart and escape. This mechanism leads to the accordion structured MEGO. The high magnification SEM image shows few-layer graphene (<3nm) separated from the stack (figure 2b). To further decipher the graphene layers, Atomic Force Microscopy (AFM) was used. The AFM image reveals the presence of monolayer graphene nanosheets in the dispersion of MEGO (figure 2c). The monolayer graphene nanosheet thickness is ~0.8nm, agreeing to available reports on the thickness of derived graphene nanosheets. 42 The thickness of the sensing layer on the IDE is approximated to ~10nm, extending up to 100 nm for few stacked or un-exfoliated particles (figure S3).

The graphene nanosheets were examined under Tunneling Electron Microscope (TEM) and wrinkly graphene sheets were noticed (figure 2d, 2e). The interlayer spacing was found to be around 0.39nm and the SAED pattern revealed the 6-fold pattern associated to the hexagonal lattice characteristic of the graphene crystalline structure. The vibrational modes of the MEGO were observed in the Raman spectroscopy( figure   2i ). It was deduced that the removal of functional groups from the basal graphene plane due to microwave treatment resulted in the restoration of the defect sites. The ID/IG ratio in MEGO was lower than that in GO, which led to the above conclusion.

The peculiar morphology of MEGO also warranted characterizing the material for its total surface area. The MEGO material was evidently higher in volume than the GO, and extremely light in weight such that the a mild air draft could blow its particles. The surface area of the MEGO was examined via N2 gas absorption and BET analysis. The material was found to have a total surface area of 351.1m 2 /g ( figure S3 ).

The GLE sensor was subjected to known deformation using a Newport Micro-motion system. The sensor was fixed at the ends and deflected at the centre, i.e. at the sensor location, The sensor had an ultra-high strain resolution of 245 µε (~0.024%), indicating immense potential in detecting delicate pressure signals. The sensitivity of a strain sensor is given in terms of Gauge factor (GF), i.e. ratio of change in resistance to applied strain (GF = (ΔR/R)/ε) (figure 3d). A GF of ~15 was observed at the above low strain input. This ultra-low detection limit was found to be better than most sensors reported in the literature so far. High sensitivity sensors, often described as feather touch-sensitive, have reported resolution of not better than 0.1% strain. [44] [45] [46] As the sensor is further strained, the resistance increases exponentially, and at ~1500 µɛ, a 2000-fold increase in the GF is observed. Further increment in the applied strain results in a drastic increase in the sensor resistance reaching the megaohms range, resulting in a GF of 6.38e7 at ~2000 µɛ. This magnitude of GF at the given strain input is unseen and exceeds reported values. 45, 37, 47, 48 . The repeatability of the response was verified by subjecting it to more than 500 cycles of pulse-type mechanical input, as shown in figure 3e, and the response was found steady throughout the experiment. A consistent response was seen for square type cyclic loading experiment (figure 3f). Repeatability of the sensor at higher strain ranges and higher GF has been an area of uncertainty. The conventional judgement indicates that sensor response with such high GF would be unstable, as reported in the literature, 46 where the sensitivity drastically reduces after a few cycles. To examine our sensor for such a behaviour, repeatability tests were carried out to inspect the sensor response at higher strain levels (~1950 µɛ), where a gauge factor of ~6e7 was observed (figure 3g). In our case, the sensor performed consistently for the cyclic loading experiments at different high-load conditions (figure 3g, S6). These experiments adequately substantiate the repeatability and reliability of the sensor. The unprecedented GF and the consistency endorse the use of graphene in the sensor and the merit of the innovative design. The transient response of the GLE sensor was studied by inducing rapid deflection (figure 3h, 3i). The sensor was deflected for 0.2 mm at a speed of 5 mm/s. The sensor's rise time was found to be 54 ms for an input rise time of 40ms, and the fall time of the sensor was found to be 52 ms for a similar input fall time.

The contribution of gold thin-film in the resistance change due to straining was also determined by carrying out experiments on the gold layer exclusively. The gold layer was strained till 5000µɛ, and the corresponding change in resistance was recorded (figure S7). The gauge factor was found to be ~2, which is peculiar of metal thin film type strain sensors. The thin film stayed intact for the magnitude of strain-induced during this experiment. It is, therefore, concluded that the GLE sensor performance is attributed entirely to the graphene nanosheets percolative networks and not to the circular gold IDE.

The GLE sensor fabricated on PET substrate and coated with PDMS layer was subjected to controlled temperature cycling. The temperature response of the sensor is shown in figure 3j. The response was linear for the range of concern, i.e. around the average human body temperature of 37 o C. The sensor displayed negative temperature dependency with a temperature coefficient of resistance ~5x10 -3 Ω/Ω.C -1 . Adopting the same graphene nanosheet material for temperature sensing application offers added advantage in ease-of-fabrication and a measure of temperature compensation for the strain sensors.

Sensing mechanism: The sensing mechanism of the device can be explained based on two principles; change in the contact area of the graphene nanosheets and the tunnelling mechanism between them. The deposition of graphene nanosheets on the sensor surface results in an arrangement where the nanosheets form a percolative network, bridging the IDE structure. The mechanism can be graphically represented, as shown in figure 4b, 4c and mathematically represented as proportionality equation (1).

where R is the resistance of the sensing film, G is the number of graphene nanosheets forming a single conduction path, P is the number of parallel conductive paths established between the two electrodes,

Ac is the cumulative contact area between the graphene nanosheets in the network.

As the strain increases, the nanosheets slide further away, eventually reaching a point of no overlap but remain close enough (<3nm) to maintain tunnel contact. At this juncture, electrons tunnel through the polymer dielectric or void between the nanosheets, and the tunnelling mechanism ensues. A significant increase in the network resistance is witnessed with the rising strain induced in the material. The mechanism can be represented as shown in figure 4d and mathematically explained as per Simmons theory and model explained by Zhang et al. 49, 50 The resistance of the sensing film can be expressed as equation 2, shown below.

where, G and P hold the same notations as above,

h is the Planc's constant, e is the electron charge,

At is the effective area concerning the occurrence of tunnelling,

, where m is the electron mass and φ is the potential barrier between the two conducting particles, d is the distance between the adjacent graphene sheets. A near-linear response of the resistance change followed by exponential rise for higher strain inputs ascribes the sensing mechanism to the above two principles. A similar mechanism has been implemented by several works based on 2D nanomaterials. 51, 52 The discerning factor in the peerless performance of our sensor can be explained as follows. Sensors based on nanosheets or nanomaterials warrant the minimum number of flakes to establish a conductive path between the two probing electrodes. This requirement leads to the obligation of fabricating the sensing layer as thick as necessary, which usually reaches a thickness of few microns. 53, 25, 48 Numerous works have proffered the use of graphene in the form of foam in composition with a binding polymer. 15, 18, [20] [21] [22] [23] [24] [25] 27, 32 This approach is rather antithetical, where a 2D material is redesigned for a 3D sensor model. The natural approach of using graphene would be such that it is indeed used in a 2D model or at least near approaching 2D model. Sensors based on pristine graphene synthesized via the CVD process have also been reported. Fabrication of CVD graphene is a complex, non-scalable and expensive process and not suitable for market commercialization. Fabrication of ultra-thin sensors using 2D graphene nanosheets is deterred by its inherent random arrangement that does not form a conductive path at lower film thickness. Studies showing strain sensors using mechanically exfoliated graphene monolayer have been reported, but mass commercial-scale delivery of such sensor is un-feasible. The appropriate route to achieve ultra-thin sensors adopting graphene nanosheets is demonstrated in our work, where the randomness of the graphene nanosheets has been taken into stride, and an adaptable electrode design has been proposed. The circular IDE allows for a thinner film of graphene nanosheets where sufficient conductive paths are established between the electrodes' digits at ultra-thin film thickness. The influence of this arrangement leads to the outstanding gauge factor as predicted from the equations discussed above. A similar approach was predicted by Hempel et al., 2012, 53 where their simulation work showed that the gauge factor of a 2D material-based sensor would increase with a decrease in the material flake density.

They also showed that a maximum of >10 18 sensitivity factor could be achieved via tunnelling break junction. Practical devices toward this goal have not been witnessed and would seem too ambitious, but our sensor is one of such headed in that direction.

The GLE sensor was tested for a variety of human physiological parameters. The flexibility and compatible nature of the sensor makes it amenable to attach to the human skin and has high skin conformability due to its thin structure.

Heart Rate: Heart rate (HR) is one of the most vital parameters foreshowing the body's present condition. The heart's rhythmic action in pumping blood throughout the body generates pressure waves in the blood volume, particularly in the arteries. The HR can be detected by palpating the arteries that run close to the skin surface by sensing the pressure waves, aka pulses. The pulses can be probed at various locations of the body like the wrist (radial pulse), neck (carotid pulse), groin (femoral pulse) or leg (posterior tibial pulse). Convenient and comfortable measurement of pulse rate can be carried out at the radial artery (wrist) and the posterior tibial artery (ankle), as demonstrated here.

The pulse waveform recorded from the radial artery of a 33-year-old male subject is shown as in figure 5a,5b. The waveform indicates a resting heart rate of 66 beats per minute.

The sensor was able to gather fine details of the pulse by deciphering the various phases of the heartbeat cycle. A pulse wave is generated due to the systolic wave i.e. the upstroke from the heart, and the diastolic wave, which is the pressure reflected from the limbs. The figure 5c shows the main systolic phase (percussion peak), the late systolic phase (tidal peak) and the diastolic wave (dicrotic peak) distinctly. The sensor showed sufficient sensitivity to detect the minute pressure pulses from the radial artery. Available PPG sensors monitor the pulse rate by volumetric measurement of oxygen in the bloodstream. The above method gives a direct quantification of the pressure exerted on the radial artery, which in turn can help in the assessment of other parameters like blood pressure. The pulse waveform shows a deflection (dV/V) of 12.5%, which is impressive and higher than biomonitoring devices reported in the literature. 54 Step counting: The positioning of the sensor below the ankle on the heel also led to the De-hydration detector: Skin impedance is a potent measure of estimating the body dehydration level. The presence of water in the body and essential electrolytes make the skin and underlying epidermis conductive. The percentage of water content in the skin modulates its conductivity and is a good measure of the body hydration level. The Laser-etch process was conveniently used to pattern two electrodes for skin contact. The resistance of the epidermis of a 33-year-old subject was measured half an hour after hydration (drinking half a litre of water).

The skin resistance was measured consequently after each hour for a duration of 6 hours, and the subject did not drink water for the above duration. The measurements reflect an increase in skin resistance due to dehydration ( figure 6j) . The procedure carried out emulates a standard body impedance measurement setup. However, the skin contact impedance of the gold pads could not be minimized since the conductive electrolytic gel was avoided. The use of conductive gel was deemed impractical for a day-to-day wearable device. Further study in the development of regular use wearable dehydration sensor can be explored based on the demonstrated work. The table 1 reveals a superlative performance of the reported work here with respect to the ease of fabrication, sensitivity, sensing layer thickness, multi-functionality and the appearance aspect. The device has been demonstrated with the added feature of wireless communication of the sensor data. The acquired data has been processed runtime in LABVIEW but can be comfortably implemented in smartphones and other smart devices.

The graphene nanosheets on laser-patterned IDE helps in realizing ultra-thin sensing layer via a facile cost-effective approach. The innovative design resulted in ultra-high record sensitivity with repeatable outcomes over different experiments. The sensor had an excellent low-strain resolution, better than 0.02%, effective in detecting minute pressure signals. The GLE sensor was tested for multifunctional roles of strain and temperature sensing.

Biomonitoring applications like measurement of heart rate, breathing rate, body temperature, dehydration, steps and hand movements have been successfully exhibited to establish the immense potential of the sensor. The laser-patterning method has been shown advantageous in fabricating an assemblage of sensors for multifunctional wearable devices. The device is interfaced wirelessly with a data processing system to gather crucial information, an increasing necessity considering the ongoing COVID-19 pandemic. A multifunctional, comfortable, routine-use market-ready smart, fashionable bracelet has been fabricated. The standout feature of the GLE sensor is reported to be ultra-high sensitivity, low-limit strain resolution, thin, skinconformal structure, biocompatible and an aesthetic appeal posing as a genuine marketable fashion accessory.

Material Synthesis: Synthesis of Graphene oxide was carried out via the modified Hummer's method. 63 GO was thermally exfoliated using microwave treatment in an LG Microwave oven at maximum power (800W) for 1 minute. 64 The GO instantly heats up and explodes with bright illumination. A considerable increase in the volume of the MEGO material is observed compared to the precursor GO. The MEGO particles are lighter and darker in appearance. The

Graphene nanosheet ink is prepared by dispersing MEGO in NMP in a pre-defined ratio followed by 30 minutes of ultrasonication. The heavier particles in the solution settle at the bottom while lighter graphene nanosheets form a uniform ink with the solvent. This translucent ink is separated and used to drop-cast on the sensor pattern.

Sensors with different designs and dimensions have been attempted using the Laseretch process. The two types of substrates employed for sensor fabrication are PDMS and PET.

The PDMS polymer was prepared from the combination of base material and curing agent and coated on a glass slide/ PMMA platform. The thickness of the PDMS layer is controlled using a spin-coater to obtain ~60 µm thick layer @1000rpm. The PDMS is heat-treated at 90 o C for 4 hrs allowing it to dry. The PDMS layer is then peeled from the slide and placed back on the slide in order to release its internal stress and convenient to peel the sensor later on. PET substrate of 50 µm thick is used specifically for temperature and dehydration detector. A thinfilm stack of Chromium (10nm) and Gold (125nm) is deposited on the prepared PDMS or PET substrate without breaking the chamber vacuum in a dual target RF/DC Magnetron sputtering system. Laser-trimming of the deposited thin-film is achieved using a fibre laser source (Light mechanics LM-BT-20, 1060 nm wavelength) at optimized power parameters such that the thinfilm is etched, but the substrate remains intact. The Laser-trimming pattern is generated using Autocad software, which provides the flexibility for easy sensor design. A circular IDE pattern has been chosen as the strain sensor electrode design, which allows direction independent strain sensitivity. The IDE arms are ~200um thick with an equal amount of gap between the electrode digits. The surface of the sensor is plasma cleaned for 2 mins to obtain a hydrophilic surface.

The GLE sensor ink is drop-casted on the sensor and heat-treated at 80 o C for 4 hrs to dry. The nanosheets randomly settle on the IDE and form multiple conductive paths. Leads are taken using copper wires and conductive silver epoxy. The sensor is then coated with another layer of PDMS as protection from external factors. The temperature sensor is fabricated using the PET substrate and conventional IDE electrodes. The impedance sensor is a set of two electrodes without the graphene layer. 

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Challenges in Wearable Personal Health Monitoring Systems

Wearable Electronics Based on 2D Materials for Human Physiological Information Detection

A Highly Sensitive Pressure Sensor Using a Double-Layered Graphene Structure for Tactile Sensing

All-Graphene Strain Sensor on Soft Substrate

A Wearable and Highly Sensitive Graphene Strain Sensor for Precise Home-Based Pulse Wave Monitoring

A Piezo-Resistive Graphene Strain Sensor with a Hollow Cylindrical Geometry

Transfer-Medium-Free Nanofiber-Reinforced Graphene Film and Applications in Wearable Transparent Pressure Sensors

Graphene Electronic Tattoo Sensors

Graphene-Based Three-Dimensional Capacitive Touch Sensor for Wearable Electronics

Capacitive Pressure Sensor with High Sensitivity and Fast Response to Dynamic Interaction Based on Graphene and Porous Nylon Networks

High-Range Noise Immune Supersensitive Graphene-Electrolyte Capacitive Strain Sensor for Biomedical Applications

Flexible, Tunable, and Ultrasensitive Capacitive Pressure Sensor with Microconformal Graphene Electrodes

Stretchable Graphene Thin Film Enabled Yarn Sensors with Tunable Piezoresistivity for Human Motion Monitoring

Epidermis Microstructure Inspired Graphene Pressure Sensor with Random Distributed Spinosum for High Sensitivity and Large Linearity

A Laser Ablated Graphene-Based Flexible Self-Powered Pressure Sensor for Human Gestures and Finger Pulse Monitoring

Reduced Graphene Oxide Tattoo as Wearable Proximity Sensor

Piezoresistive Pressure Sensor Based on Synergistical Innerconnect Polyvinyl Alcohol Nanowires/Wrinkled Graphene Film

Space-Confined Graphene Films for Pressure-Sensing Applications

Unprecedented Sensitivity towards Pressure Enabled by Graphene Foam

Multi-Dimensional Flexible Reduced Graphene Oxide/Polymer Sponges for Multiple Forms of Strain Sensors

Multi-Arch-Structured All-Carbon Aerogels with Superelasticity and High Fatigue Resistance as Wearable Sensors

Novel Graphene Foam Composite with Adjustable Sensitivity for Sensor Applications

A Flexible Pressure Sensor Based on RGO/Polyaniline Wrapped Sponge with Tunable Sensitivity for Human Motion Detection

Facilely Prepared Layer-by-Layer Graphene Membrane-Based Pressure Sensor with High Sensitivity and Stability for Smart Wearable Devices

Towards Ultra-Wide Operation Range and High Sensitivity: Graphene Film Based Pressure Sensors for Fingertips

Highly Stretchable Strain Sensors with Reduced Graphene Oxide Sensing Liquids for Wearable Electronics

An Ultra-Sensitive and Rapid Response Speed Graphene Pressure Sensors for Electronic Skin and Health Monitoring

High-Resolution Patterning and Transferring of Graphene-Based Nanomaterials onto Tape toward Roll-to-Roll Production of Tape-Based Wearable Sensors

Fabrication of Low-Cost and Highly Sensitive Graphene-Based Pressure Sensors by Direct Laser Scribing Polydimethylsiloxane

Wearable Multifunctional Printed Graphene Sensors

Bean Pod-Inspired Ultrasensitive and Self-Healing Pressure Sensor Based on Laser-Induced Graphene and Polystyrene Microsphere Sandwiched Structure

A Transparent Bending-Insensitive Pressure Sensor

A Multifunctional Wearable Device with a Graphene/Silver Nanowire Nanocomposite for Highly Sensitive Strain Sensing and Drug Delivery

Highly Sensitive, Scalable Reduced Graphene Oxide with Palladium Nano-Composite as Strain Sensor

Laser-Induced Direct Patterning of Free-Standing Ti3C2-MXene Films for Skin Conformal Tattoo Sensors

High-Performance Flexible Strain Sensor with Bio-Inspired Crack Arrays

Highly Sensitive Strain Sensors Based on Molecules-Gold Nanoparticles Networks for High-Resolution Human Pulse Analysis

Ag Nanowire Reinforced Highly Stretchable Conductive Fibers for Wearable Electronics

Temperature-Pressure Hybrid Sensing All-Organic Stretchable Energy Harvester

Wearable Circuits Sintered at Room Temperature Directly on the Skin Surface for Health Monitoring

Atomic Force and Scanning Tunneling Microscopy Imaging of Graphene Nanosheets Derived from Graphite Oxide

Characterization of X-Ray Irradiated Graphene Oxide Coatings Using X-Ray Diffraction, X-Ray Photoelectron Spectroscopy, and Atomic Force Microscopy

A Stretchable and Highly Sensitive Graphene-Based Fiber for Sensing Tensile Strain, Bending, and Torsion

High-Performance Strain Sensors with Fish-Scale-Like Graphene-Sensing Layers for Full-Range Detection of Human Motions

Ultrasensitive Strain Gauges Enabled by Graphene-Stabilized Silicone Emulsions

An Ultrasensitive Strain Sensor with a Wide Strain Range Based on Graphene Armour Scales

Flexible TPU Strain Sensors with Tunable Sensitivity and Stretchability by Coupling AgNWs with RGO

Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film

Time Dependence of Piezoresistance for the Conductor-Filled Polymer Composites

Tunable Piezoresistivity of Nanographene Films for Strain Sensing

A Novel Class of Strain Gauges Based on Layered Percolative Films of 2D Materials

Sliced Graphene Foam Films for Dual-Functional Wearable Strain Sensors and Switches

3D-Printed Ultra-Robust Surface-Doped Porous Silicone Sensors for Wearable Biomonitoring

Highly Sensitive, Wearable, Durable Strain Sensors and Stretchable Conductors Using Graphene/Silicon Rubber Composites

Strain Sensor with Both a Wide Sensing Range and High Sensitivity Based on Braided Graphene Belts

Multilayer Structured AgNW/WPU-MXene Fiber Strain Sensors with Ultrahigh Sensitivity and a Wide Operating Range for Wearable Monitoring and Healthcare

Skin-Inspired Highly Stretchable and Conformable Matrix Networks for Multifunctional Sensing

Flexible Electrically Resistive-Type Strain Sensors Based on Reduced Graphene Oxide-Decorated Electrospun Polymer Fibrous Mats for Human Motion Monitoring

Flexible and Transparent Strain Sensors with Embedded Multiwalled Carbon Nanotubes Meshes

Ultra-Sensitive Strain Sensors Based on Piezoresistive Nanographene Films

Broad Range Fast Response Vacuum Pressure Sensor Based on Graphene Nanocomposite with Hollow α-Fe2O3 Microspheres

Carbon-Based Supercapacitors Produced by Activation of Graphene

Recent Advances in Smart Wearable Sensing Systems

The authors thank the Micro-nano characterization facility (MNCF) and Advanced Packaging