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Graphene-based wearable piezoresistive physical sensors Qingbin Zheng 1,2, Jeng-hun Lee 1, Xi Shen 1, Xiaodong Chen 3, Jang-Kyo Kim 1,⇑ 1

Department of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172, China 3 Innovative Centre for Flexible Devices, School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore 2

In the last two decades, wearable piezoresistive physical sensors have attracted tremendous attention due to their broad applications in individual health-monitoring, human–machine interfaces, robotics, sports and therapeutics. Many different nanostructured materials, including nanowires, nanoparticles, nanoribbons, carbon black, carbon nanotubes and graphene, have been explored to construct stretchable piezoresistive sensors on an elastomer substrate. Thanks to its unique two-dimensional geometry, lightweight, flexibility, semi-transparency and outstanding transport and mechanical properties, graphene and its derivatives in particular are considered among the most suitable candidates as wearable sensors. This paper reviews various design strategies established for fabricating flexible, wearable sensors using graphene. The current state-of-the-art developments are discussed of flexible sensors made of 1D fibrous, 2D planar and 3D cellular interconnected graphene architectures for detecting physiological strains, tactile pressures and temperatures. The working mechanisms along with existing applications of flexible sensors are presented. The challenges these sensors are currently facing and potential opportunities for novel applications are revealed to offer new insights into future prospects in this field. Introduction With the development of electronic skins, flexible, stretchable and wearable sensors have drawn much attention because of their vast potential applications in human–machine interfaces (HMIs) [1], soft robotics [2], health monitoring [3], virtual reality [4] and entertainment technology (Fig. 1) [5]. Those capable of multifunctional and easy interactions with human body are considered particularly stimulating [6,7]. Since wearable sensors are designed for mounting mainly on human skin, they should possess not only high flexibility and stretchability, but also excellent sensitivity to detect wide-ranging strains [8]. Therefore, many physico-mechanical requirements, such as the stretchability, sensitivity, simplicity, stability as well as the materials and manufacturing costs need to be taken into account in designing such ⇑ Corresponding author.

sensors suitable for large-scale production [9]. Although significant research has been directed towards developing sensors using a wide range of functional materials on flexible substrates, it is still a challenge to fabricate sensors that possess both high sensitivity and stretchability sufficient to detect a wide range of human body motions from subtle deformations of skin to substantial bodily movements [10]. High sensitivity requires a large structural deformation at a small strain while high stretchability demands structurally interconnected networks even at a large strain [11]. It follows then that the rational design of sensing materials is essential to possessing proper geometric structures and interconnections [12]. Different types of wearable physical sensors have been extensively explored based on various transduction mechanisms, namely piezoresistive [13,14], capacitive [15], iontronic [16], and piezoelectric transmission [17–19]. Among them, piezoresis-

E-mail address: Kim, J.-K. ([email protected]) 1369-7021/Ó 2019 Elsevier Ltd. All rights reserved. https://doi.org/10.1016/j.mattod.2019.12.004

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Emerging applications of graphene-based wearable physical sensors. Images reproduced with permission: sensors on wrist [310] (Copyright 2016, Springer Nature), face [311] (Copyright 2016, Wiley), ear [312] (Copyright 2015, National Academy of Sciences), throat [26] (Copyright 2015, American Chemical Society), chest [313] (Copyright 2014, Springer Nature), knee [30] (Copyright 2011, Springer Nature), foot [314] (Copyright 2011, MDPI), finger [59] (Copyright 2018, Royal Society of Chemistry), and arm [315] (Copyright 2011, Wiley), human–machine interface [315] (Copyright 2015, Wiley), soft robotics [316] (Copyright 2013, American Chemical Society), entertainment and VR [13] (Copyright 2017, Royal Society of Chemistry), and health monitoring [317] (Copyright 2019, Wiley).

tive sensors that transduce the stimuli into resistance changes are widely employed because of their simple read-out mechanism, ease of fabrication, high linearity, low power consumption and potentially-high pixel density [5,20–22]. Other types of sensors usually possess a limited stretching capacity and low resolution, making it difficult to implement them as wearable sensors [5,23,24]. Although traditional piezoresistive sensors based on semiconductors and metal foils are cost-effective, most of them could not serve as wearable sensors due to their low sensitivity or narrow sensing ranges of less than 5% [25]. To develop stretchable piezoresistive sensors having adequate sensitivity, various nanoscale materials, such as metal nanowires [9,26] nanoparticles [27], silicon nanoribbons [28], carbon black [29], carbon nanotubes (CNTs) [30–33] and graphene [2,34–39], have been tested as sensing materials coupled with an elastomer supporting substrate. In particular, graphene has attracted much attention since the first isolation of its single layer by mechanical exfoliation in 2004 [40], along with new techniques developed to synthesize it. Graphene has many characteristics particularly suitable for sensor applications, including tunable 2D assembly structure, flexibility, lightweight, excellent electromechanical properties, extremely high transport properties and remarkable optoelectronic characteristics [41,42]. Based on the surface chemical composition and geometry, graphene can be classified mainly into graphene sheet (GS), graphene ribbon (GR), graphene oxide (GO) and reduced graphene oxide (rGO). GS possesses superior electrical properties [43,44], while patterning GS

into a GR can tune its semiconducting properties [45]. GO is typically nonconductive with an electrical conductivity less than a micro S/m [46,47], while rGO possesses a much improved conductivity thanks to the partially-restored aromatic graphitic structure [48–52]. The graphene-based physical sensors with high flexibility have been employed in many types of skinmountable and wearable sensors which have broad applications in human motion detection, heath-monitoring and HMIs [5]. This review is dedicated to reviewing the current state-of-theart progress in fabrication strategies, selection criteria, working mechanisms and different applications of graphene-based wearable physical sensors. The main strategies employed to create sensing architectures using graphene or GO are summarized, and the working mechanisms of piezoresistive strain, pressure and temperature sensors made of graphene-based flexible structures are elucidated. Several important applications of graphene sensors are highlighted with reference to successful examples. Finally, the challenges that these tactile sensors are facing and potential opportunities in developing new applications for practical devices are discussed.

Synthesis and assemblies of graphene The first challenge to fabricating commercially-viable graphenebased sensors is to synthesize high quality graphene in a scalable and low-cost manner [53]. Two primary methods for the fabrication of graphene, i.e. top-down (TD) and bottom-up (BU)

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approaches, have been developed [54]. The TD approach includes mechanical cleavage, solution exfoliation, chemical method and unzipping of CNTs [53,54]. These techniques are scalable and of low-cost, but often involve breakage of bulk carbon into thinner or smaller graphene sheets, inevitably introducing defects during the exfoliation process [53]. The BU approach includes epitaxial growth, chemical vapor deposition (CVD) growth and total organic synthesis, usually starting with carbon atoms to build or assemble into large size graphene layers or structures. These techniques can produce high-quality graphene with less defects than the TD approach, but are expensive and difficult to scale up for mass production hindering their widespread applications [53]. There are a few unique approaches developed to assemble graphene into different forms of two-dimensional (2D) or threedimensional (3D) macroscopic, freestanding architectures. As shown in Fig. 2, these approaches can be classified into five categories, including the template-directed method, self-assembly, coating method, spinning method and 3D printing. (i) The template-directed methods employ three different types of templates to prepare 3D macroscopic graphene structures. One uses hard templates, such as nickel-based 2D woven fabrics [13,55,56], orthogonally-arranged multi-layer assembly and 3D cellular foams [57–60], CaCO3 particles [61], functional silica nanospheres [62] and metal particles [63]; another is based on soft templates, such as bubbles [64], micelles [65], emulsions [66] and polyurethane (PU) sponges [67]; and the last is icetemplating which is simple and environmentally friendly approach to assemble versatile graphene-based 3D porous structures using ice crystals as templates [68–70]. (ii) Gelation induced by supermolecular interactions, such as p–p stacking [71], hydrogen bonding [72] and electrostatic interactions [73], assisted by freeze-drying is an effective self-assembly approach to produce 3D graphene structures. (iii) A number of coating methods have been utilized to assemble graphene films, and they include dipcoating [53], spin-coating [74], L-B assembly [42,75,76], spray coating [77], “snowing”-coating [78], inkjet printing [79] and transfer printing [80,81]. (iv) The capability to form liquid crystals has been explored to prepare robust, gel-state GO fibers using a custom-built wet-spinning apparatus [82]. Different coagulation baths containing NaOH/methanol [83], amine solution [84], sulfuric acid and chitosan solution [85] were used to achieve continuous GO fibers via direct wet-spinning. Electrospinning is another convenient approach for the generation of continuous nanofibers with large surface areas and high aspect ratios [86]. (v) Various 3D printing techniques including direct ink writing [87], stereolithography [88], fused deposition modeling [89] and selective laser sintering [90] have been utilized to produce graphene-based 3D architectures. The direct ink writing usually involves extrusion and solidification using a high viscosity ink containing graphene, which can retain its shape for a smooth printing [91]. The stereolithography is based on photo curing of photopolymers such as fast-light-responsive composite resin systems with low viscosities to achieve high resolutions [88]. The fused deposition involves simple melting and extrusion of various thermoplastic filaments with different fillers to print 3D architectures [89]. The selective laser sintering technique uses a highly-energized laser to sinter thin layers of graphene-based

composite powders successively [90]. Taking advantages of printing a large range of tunable 3D architectures in a customized manner, the 3D printing technology has become increasingly important to realize complex functional features for wearable sensors [92]. Graphene/flexible matrix composites were also produced via infiltration of flexible polymer into 3D graphene architectures or homogeneous dispersion of graphene sheets in flexible polymer matrices [53,59]. Flexible polymers, such as polydimethylsiloxane (PDMS), rubber, Ecoflex, polyimide (PI) and polyurethane (PU), have been popularly used as the substrates or matrices because of their superior flexibility [55,93]. 3D graphene architectures, including foams, hydrogels, aerogels and sponges, were easily infiltrated with liquid polymers thanks to their unique structural interconnectivities, high porosity and stable mechanical properties [13,94]. Meanwhile, owing to the good solubility and dispersibility in aqueous solution or polymer resins, GO and rGO were also widely used to form graphenebased flexible polymer composites with molecular-level dispersion [95–97]. Depending on the properties of polymer matrices, various mass production techniques including in situ polymerization, solution blending/casting or melt compounding have been utilized to manufacture flexible nanocomposites containing uniformly dispersed graphene flakes [98–100].

Piezoresistive sensing mechanisms and selection criteria The resistance (R) of a conductor under tension can be expressed by: R¼q

L A

ð1Þ

where q, L, and A are the resistivity, length and average cross-sectional area, respectively. When a stress is applied (Fig. 3a), The relative change in resistance, DR/R, is given by two terms representing the geometric and resistivity effects [5,101,102]:

DR Dq ¼ ð1 þ 2mÞ þ R q

ð2Þ

where m is the Poisson ratio of the material and Dq=q is the relative resistivity change. Conventional piezoresistive materials, such as metallic alloys and doped silicon, usually suffer from limited stretchability, nonlinearity and large hysteresis [9,29]. It is also observed that the resistance change of suspended graphene sheets at a strain level below 3%, i.e. before band gap opening, was very limited with a gauge factor (GF) of 1.9 [103], demonstrating a negligible geometric effect.

The second term of Eq. (2) represents the influence of piezoresistivity of sensor materials themselves, i.e., the change of bandgap in inter-atomic spacing [29,104–107]. The valence and conduction bands of monolayer graphene connect in the Brillouin area according to the tight-binding model, as shown in Fig. 3b [108]. Thus, monolayer graphene possesses many exceptional traits, such as ultrahigh mobility, ballistic transport properties and anomalous quantum Hall effects thanks to its unique electronic band structure. Although graphene sheets exhibit the piezoresistive effect under a mechanical strain owing to the change in barrier height, the change in resistivity caused by the deformation is small, resulting in low sensitivity. The stretch of thin films or composite materials causes the overlapped nano3

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Assembly of graphene-based architectures via template-directed method (a), self-assembly (b), coating (c), spinning and printing (d). Images reproduced with permission: Hard template [318] (Copyright 2014, American Chemical Society), soft template [66] (Copyright 2014, Wiley), ice template [188] methods (Copyright 2016, American Chemical Society); Self-assembly by hydrothermal and electrochemical methods [71] (Copyright 2010, American Chemical Society), vacuum filtration [319], (Copyright 2012, American Chemical Society), liquid/liquid and liquid/air interface [320] (Copyright 2009, Wiley), electrophoretic deposition [321] (Copyright 2010, American Chemical Society); Coating methods by dip coating [53] (Copyright 2014, Elsevier), spin coating [74] (Copyright 2008, American Chemical Society), spray coating [322] (Copyright 2016, Royal Society of Chemistry), Langmuir-Blodgett [75] (Copyright 2013, Royal Society of Chemistry); Spinning and printing methods by wet spinning [82] (Copyright 2017, Wiley), electrospinning [323] (Copyright 2010, Wiley), 3D printing [324] (Copyright 2014, Wiley).

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Piezoresistive sensing mechanisms. (a) Geometrical changes. (b) Band gap opening. Adapted with permission [108]. (Copyright 2009, American Physical Society) (c-d) Schematic illustration showing the sensing principles based on contact area change with micropyramid (c) [325] and microdome (d) [31] structures. Adapted with permission [31,325]. (Copyright 2014, Wiley; Copyright 2014, American Chemical Society) (e-f) Schematic illustration (e) [23] of the bio-inspired strain sensor based on microcracks and a tunneling model (f) [326]. Adapted with permission [23,326]. (Copyright 2014, Springer Nature; Copyright 2018, Royal Society of Chemistry) (g) Schematic illustration of the proposed sensing mechanism of pressure sensor based on tunneling effect. Adapted with permission [124] (Copyright 2016, Wiley).

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materials to lose connections through the slippage or stiffness mismatch [109]. Consequently, the electrical resistance increases via the surge in contact resistance at the junctions [9,26,35,36,104,110–113]. Meanwhile, negative resistance coefficients (NRCs) were observed for graphene-based piezoresistive pressure sensors because the increase in contact area under compression led to a fall in electrical resistance (Fig. 3c and d) [104,114–116]. There is another important term constituting DR/R, namely, the tunneling mechanism which is not explicitly articulated in Eq. (2). The tunneling effect describes the crossing or “hopping” of electrons through quantum tunneling junctions [117] and significantly influences the response of sensors with closely spaced conducting nanomaterials [9,118,119]. This mechanism has been extensively utilized by deliberately introducing cracks in conducting materials to improve the sensitivity of sensors (Fig. 3e and f) [23,120]. The reversible opening and closing of micro- or nanocracks with continuously varying crack lengths and densities allowed the sensitivity and stretchability to be accurately tailored [121–123]. Highly sensitive piezoresistive pressure sensors were also designed by utilizing the effective quantum tunneling effect among sea-urchin shaped nanoparticles (Fig. 3g) [124]. The tunneling resistance between two neighboring graphene sheets is related to the shortest distance, d, according to the Simmons's model [5,9,114,125]:

RT ¼

V h2 d 4pd pffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffi expð ¼ 2mkÞ AJ Ae2 2mk h

ð3Þ

where J is the tunneling current density, V is the electrical potential difference, A is the cross-sectional area of the tunnel, e is the quantum of electricity, m is the mass of an electron, h is the Plank's constant and k is the height of the energy barrier.

Apart from the above three basic conduction or resistance mechanisms universal to all graphene-based sensors, there are also other mechanisms specific to each structure, some of which are discussed in the following. It should be noted that the sensitivity and the resistance change of a given piezoresistive sensor arise usually from the combination of several mechanisms. For example, the piezoresistive polymer composites have three major sensing mechanisms, including the band structure changes of fillers, the tunneling resistance between fillers and the variation of percolating pathways [126–130]. For a highly sensitive graphene woven fabric structure, the resistance change originates from several sources, such as flattening of wrinkles, relative sliding of graphene layers and tunneling junctions due to crack initiation and growth [55]. The dominant source of resistance change may gradually shift from one sensing mechanism to another depending on the strain level [59]. Apart from the costs for manufacturing and materials, there are quite a number of parameters that should be taken into account when evaluating the sensing performance and selecting appropriate piezoresistive physical sensors suitable for specific applications. They include the sensitivity, stretchability, durability, linearity, selectivity, detection limit, response time and transparency. The sensitivity is a measure of the ability of sensors to convert external stimuli into electrical signals [131]. For strain sensors, the gauge factor (GF) is popularly used to evaluate the

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sensitivity which is given by the ratio of normalized resistance change, DR=R0 , to applied strain [5,59]: GF ¼

DR=R0 e

ð4Þ

Similarly, the sensitivity of a pressure sensor is defined as:

S¼

DR=R0 DP

ð5Þ

where DP is the pressure variation [132,133]. For thermistors, the sensitivity is usually given by temperature coefficient of resistance, a, which is determined by the ratio of relative resistance change, dRT =RT , to temperature variation, dT:

a¼

1 dRT RT dT

ð6Þ

where RT is the resistance of thermistor at a given temperature T [134].

The stretchability is the maximum tensile strain that the sensor can sustain with stable sensing performance under repeated loading and unloading [135]. The durability is the capability of sensors to maintain stable and reliable electrical functionality and mechanical integrity under long-term continuous loading/ unloading cycles [136]. Many factors, including microstructural changes, oxidation or corrosion of sensing materials and environmental influences, can affect the durability of a sensor [35]. The linearity of resistance response is another important parameter of sensors because nonlinearity makes their calibration complex and difficult [137]. Easy calibration is essential to providing meaningful numerical readings with good monotonicity and resolution. Detection limit is a measure of the smallest quantity with a specified precision or reproducibility of a sensor [138]. For example, the detection limit for carbonized silk-based strain sensor and pressure sensor is around 0.01% strain and 0.8 Pa, respectively [139,140]. The hysteresis of conventional metalbased sensors is often caused by a combination of mechanical and temperature hystereses, whereas the viscoelastic nature of polymers and the interactions between the nanomaterials and polymer substrates are the main source of the hysteresis of stretchable sensors [5,35,141]. The response time is defined as the time needed for a measurable response in the steady state [136,142]. In addition, optically transparent sensors that are “invisible” and “unfelt” are urgently needed for next-generation wearable sensors because such sensors can be worn on the user's skin without affecting daily activities [26,143,144]. These transparent and elastic sensors can be integrated or combined with other components to design skin-like multifunctional electronic devices [21,26].

Graphene-based strain sensor structures Advanced wearable strain sensors with high sensitivity and elasticity are essential components of flexible and soft electronic devices. Conventional metal- and semiconductor-based strain sensors are rigid, fragile and opaque, restricting their applications in wearable electronics. Easy, low-cost fabrication of highly transparent, flexible and sensitive strain sensors is needed for their widespread emerging applications. Figs. 4 and 5 summarizes various graphene structures with different geometries and dimensions designed for strain sensors, while the details of sensor structures, fabrication methods, sensing mechanisms and their relative performances are discussed in the following.

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FIGURE 4

Graphene-based strain sensors. Images reproduced with permission: suspended graphene [103] Copyright 2011, American Chemical Society), graphene on substrate [147] (Copyright 2011, American Institute of Physics), graphene ripples [34] (Copyright 2011, American Chemical Society), graphene fiber [38] (Copyright 2015, Wiley), graphene film [161] (Copyright 2016, Wiley), graphene woven fabric [55] (Copyright 2019, American Chemical Society), graphene tattoo [183] (Copyright 2017, American Chemical Society), graphene foam [59] (Copyright 2018, Royal Society of Chemistry), graphene honeycomb [94] (Copyright 2018, Wiley), graphene/rubber band [36] (Copyright 2014, American Chemical Society), graphene/putty [14] (Copyright 2016, Science), graphene/ CNT [202] (Copyright 2016, Wiley), graphene fluid [208] (Copyright 2018, Royal Society of Chemistry). 7 Please cite this article in press as: Q. Zheng et al., Materials Today, (2020), https://doi.org/10.1016/j.mattod.2019.12.004

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FIGURE 5

Summary of sensing performance of graphene strain sensors in terms of gauge factor vs stretchability.

Single-crystal and polycrystalline graphene An earlier study was conducted to investigate the strain sensing capability of single-crystal graphene. Theoretical calculations reveal that the electrical properties of single-crystal graphene were affected by the applied strain due to the opening of band gap at the Fermi level [145]. For example, the band gap of graphene increased to 0.170 or 0.486 eV at 7.3 or 12.2% strain applied perpendicular or parallel to the C–C bonds, respectively. According to the molecular dynamics simulations, an ideal graphene lattice could sustain up to 40 or 30% strain when loaded perpendicular or parallel to the C–C bonds. Experimental studies using the in situ nanoindentation electrical measurements of suspended mono-layer graphene (Fig. 4a) confirm that the resistance change of defect-free, mechanically-exfoliated graphene almost linearly increased with increasing strain, yielding a gauge factor of 1.9 [103]. However, the maximum stain that a polycrystalline graphene with multiple domains could sustain significantly reduced due to the presence of grain boundaries.[146] The CVD-grown large-size, monolayer graphene with a polycrystalline structure that contained abundant small domains with different orientations exhibited a much higher GF of 151 once the wrinkles were fully flattened at 2.4% strain [147]. When the strain was increased beyond 5%, the graphene structure was damaged through the formation of cracks along the grain boundaries. To increase the working range of multi-layer graphene sheets, a periodically rippled or buckled structure was created by transferring graphene onto a pre-stretched PDMS substrate and releasing the prestrain [34]. The graphene ripples created thereby can accommodate large strains sufficient for flexible electronics applications. Although the reported GF of 0.55 was still low, the stretchability of the buckled graphene structure exhibited a much improved working range over 25% with respectful reversible electrical and mechanical properties in buckling and stretching cycles. However, it should be noted that the single-crystal and polycrystalline graphene produced by the mechanical exfoliation or CVD method are unsuitable for large-scale commercial application because of their low yields and small sizes.

Graphene can be shaped into many different structures, including 1D fibrous form. The fibrous graphene structure has advantages of flexibility, conformability, light-weight and durability, making it an attractive candidate for next-generation wearable electronics conformable to human skin [148]. Graphene fibers are the latest generation carbon-based fibers which can be converted into a broad range of graphene assemblies, such as yarns, fabrics and spacer fabrics, and have the great potential as sensors, supercapacitors and intelligent fibers [149]. Graphene fibers with good mechanical and electrical properties were synthesized using several methods, such as wet-spinning of GO dispersion followed by chemical reduction, scrolling and hydrothermal cross-linking of GO films [83,85,150–152]. In addition, a few approaches were devised to improve the functional properties of fibers, such as introducing nanomaterials into the GO solution [153–156] and generating pores or hollow tubes by freeze-drying, followed by coaxial two-capillary wet-spinning [157,158]. The optimized fibrous graphene structures presented ultrahigh sensitivity for a wide range of tensile strains. For example, Cheng et al. [38] developed a facile method to produce composite fiber strain sensors having a“compression spring”architecture (left of Fig. 4b) which consisted of an elastic PU core and rGO-coated polyester (PE) fibers helically wound around the core. When stretched, the reduced contact area of the PE fiber shell increased the contact resistance between the inner and outer layers. Wang et al. [159] fabricated graphene core/PVA sheath fibers using ultralong CVD-grown graphene bundles. Owing to their excellent electrical conductivities and mechanical properties, the flexible graphene fiber sensors delivered greatly improved strain sensing performance with high sensitivity and cyclic stability. The microcracks generated in tension and bending led to a significant increase in resistance, which were reconnected or restacked upon release of the applied load. A new strain sensor [160] was prepared by layer-by-layer assembly of graphene coating on a PU yarn. The microcracks formed on the graphene layer under stretching were responsible for its high sensitivity through the tunneling effect.

2D networks The sensitivity and stretchability of graphene film sensors were effectively controlled by adjusting the connection channels [161]. While the single layer graphene (SLG) film showed limited stretchability [162], the stretchability was much improved by designing multilayer graphene structures [163,164]. For example, the bilayer graphene (BLG) films applied on polyethylene terephthalate (PET) flexible substrates possessed better electromechanical stability and mechanical flexibility than the SLG counterparts [163]. Ultrathin graphene films (right of Fig. 4b) with a tunable sheet resistance and structural uniformity were fabricated via self-assembly at the liquid/air interface [161]. Shi et al. [164] developed fish-scale-like graphene layers to achieve a combination of high sensitivity and stretchability. The improved sensing performance arose mainly from the reversible slipping between the neighboring overlapped graphene layers, facilitating continuous connections and variable contact resistance. Even higher stretchability has been achieved using a stretchable form of periodic “wavy” silicon consisting of submicrometer

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single-crystal elements [165]. Similar approaches were adopted for graphene films to produce wavy geometries with enhanced stretchability. For example, graphene was transferred to an elastomer substrate biaxially pre-stretched at 12% strain and the strains were released: both the longitudinal and transverse resistances remained stable within 11% tensile strain but increased by an order of magnitude at 25% strain [166]. Self-organized, crumpled structures were also utilized for the same purpose by sequentially releasing the biaxial pre-strains applied to a graphene film/rubber composite [166]. The crumpled structure unfolded when the substrate was stretched in two directions, enabling the crumpled graphene conductor to accommodate an extremely high strain of 450%. More interestingly, the wettability and optical transmittance of large-area graphene were controlled through the reversible crumpling/unfolding process [167]. The optical transmittance was tuned by controlling the crumpling of GO films [168]. The release of pre-buckled strains applied to the GO film which was drop-cast on a silicone rubber substrate resulted in spontaneous buckling and delamination under uniaxial compression [168]. Graphene sheets with wrinkled structures have been fabricated through direct CVD on wrinkled Cu foils [169]. After transferring to an elastomer substrate, the wrinkled graphene presented a transparency of 57% and 40% stretchability without any obvious change in their electrical performance. Textile geometries of graphene have attracted much attention because of the large interfacial resistance between the interlaced ribbons and the microcracks formed in tension. The graphene woven fabrics (GWFs) synthesized via atmospheric CVD on Cu meshes [135,170–176] had several unique structural and functional features, such as high structural integrity, good gas/liquid permeability and an exponential resistive response under strain, making them suitable for strain sensing with high sensitivity [55,170]. Although GWFs with thicker graphene tubes (GTs) showed higher stretchability because of stronger interactions between the upper and lower ribbons, their maximum stretchability was only 10% [174]. Higher stretchability may be achieved by integrating other conductive materials with GWFs. The orientation of GTs within GWFs against the loading direction also significantly affected the sensitivity of GWF/PDMS composites in uniaxial tension [12]. For example, when stretched in the same direction as the orthogonally-interconnected GTs, i.e., at 0° or 90°, their sensitivity was over threefold higher than that loaded at 45° to the GT direction. Inspired by the highly flexible, core–shell spider web architecture, ultrasensitive, wearable sensors were developed using an elastomer-filled graphene woven fabric (E-GWF) (left of Fig. 4c) [55]. The E-GWF/PDMS composite sensors not only presented much improved stretchability up to 30% and excellent linearity compared to those without an elastomer core, they also exhibited a unique reversible switching behavior at a high strain ranging 30–50%. A convenient, low-cost method was recently developed to fabricate graphene mesh fabrics (GMFs) for high-performance strain sensors [177] GMFs were obtained by spinning of GO ribbons on a Teflon substrate followed by drying and reduction. The GMF strain sensors delivered a GF of 20 at strains below 5% along with outstanding stability of up to 500 stretching/releasing cycles. Carbonized silk plain-weave fabric wearable strain sensors

were fabricated by carbonization of pristine silk fabrics [25] The plain-weave structure was maintained even after carbonization. Taking advantage of a highly ordered graphitic structure as well as the unique woven structure, the wearable strain sensors demonstrated combined high sensitivity with GFs ranging 9.6– 37.5 and outstanding stretchability of higher than 500%. Graphene knitted fabrics (GKFs) were fabricated using thermally reduced GO as the colorant on a polyester fabric substrate [178]. The variation of horizontal and vertical interwoven structure of GKFs led to directional sensitivity, allowing all-directional sensing necessary for human motion detection as well as a distinctive negative resistance variation with increasing strain. The strain sensing performance like stretchability and sensitivity have also been improved by creating suitable patterns, such as meshes [179], rosettes [180], lines [181,182], tattoos [183] and matrices [184]. Entangled graphene mesh networks (EGMNs) were produced by selective etching of CVD-grown graphene [179]. The highly stretchable and transparent EGMN delivered a steady monotonic resistance response due to its distinctive structure containing wrinkled, waved and crumpled networks. The strain gauge rosette, which is composed of two or more strain gauges positioned closely, was able to accurately evaluate the strains. A simple line pattern was produced by Meyer-rod coating or pencil-trace drawing. Thanks to the large number of microcracks created on the surface of line patterns when loaded in tension, the sensors featured an impressively high GFs ranging 150.5–804.9 under very low compressive and tensile strains [181,182]. The tattoo-like epidermal sensors possessed a unique advantage of invisibility when worn on human body, such as face. Graphene-based electronic tattoos (right of Fig. 4c) were developed by wet transfer and dry patterning for long-term, high-fidelity biometric sensing [183]. A facile method was proposed to produce a graphene pattern designed by CAD software using a medical tape template [184]. Through disconnection and tunneling effect in the direction perpendicular to the film plane, the graphene pattern showed ideal linearity and reproducible sensitivity with GFs ranging 27.7–164.5 upon application of tensile strains.

3D interconnected structures A variety of graphene-based 3D conductive networks, including foams [59,185,186], sponges [187], and aerogels [94,188] have been designed to prepare highly stretchable strain sensors and conductors. Consisting of an interconnected cellular, flexible network, graphene foams possessed unique electrical and mechanical properties[58,67,189–191]. Because of their stable electrical and mechanical properties even at high strains, however, the 3D graphene foam composites are considered unsuitable for strain sensors, instead more useful as flexible conductors [58]. For example, graphene foam/PDMS composites presented a resistance increase of 30% at 50% tensile strain, equivalent to a very low GF of 0.6 [58]. A thin PET layer was introduced as a substrate to improve the sensitivity [192]. The double-layer graphene foam/PDMS-PET composite exhibited much improved bending sensitivity due to the compression stress introduced by the PET layer. However, the addition of a PET layer greatly limited its stretchability. Fragmenting graphene foam is an effective way to enhance the GF as the fragments 9

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increased the contact area between them [185]. The graphene foam fragment/PDMS composite sensor demonstrated high stretchability of over 70% strain with a GF ranging 15–29. Another facile approach has been adopted to improve the sensitivity of graphene foam structures [59]. Highly stretchable, dualfunctional strain sensors/switches with tunable sensitivities and switching capabilities were obtained by slicing graphene foams into thin layers of varying thicknesses ranging 200–1600 mm (left of Fig. 4d). Their function as sensor or switch was determined by the thickness of graphene foam. 3D bubble-derived graphene foams (BGFs) and 2D bubble-derived graphene porous membranes (BGPMs) were fabricated based on the soft bubbling and hard ice templating methods. Wearable strain sensors were assembled using these graphene structures which can detect both the tensile and compressive strains [186]. Graphene sponge was synthesized via transient CO2 laser heating [187], which possessed a highly porous structure and functioned as multifunctional on-skin sensing electronics with high gas permeability. Porous graphene aerogels (GAs) with long-range ordered porous structures were also used for producing highly stretchable electronics upon them. RGO aerogel was transformed into a flexible architecture with remarkable sensitivity and durability by introducing water-soluble PI followed by freeze casting and thermal annealing [193]. A highly conductive and lightweight graphene honeycomb sandwich was assembled by freeze drying and 3D printing of GA (right of Fig. 4d) [94]. The wall thickness of graphene honeycomb was controlled to serve as either a highly stretchable conductor or a strain sensor. Graphene has also proven to be a promising filler for nanocomposite strain sensors [2,36,114,193]. Highly stretchable 3D macroporous nanopapers composed of crumpled graphene and nanocellulose were developed to fabricate sensors capable of detecting strains of over 100% with a GF of 7.1 [2]. The significantly improved stretchability originated from the continuous changes in the contact area of crumpled graphene during stretching. The solution process made it suitable for mass production at low cost. Boland et al. demonstrated a facile way to produce versatile strain sensors by infiltrating liquid-exfoliated graphene into store-bought elastic bands [193]. The graphene–natural rubber composites (left of Fig. 4e) delivered both high sensitivity with GFs of up to 35 and remarkable stretchability with tensile strains exceeding 800% at a wide vibration frequency up to 160 Hz. G-putty materials were prepared with unprecedented electromechanical properties by homogeneously blending fragmented graphene nanosheets in a lightly crosslinked polysilicone matrix [14] (right of Fig. 4e). The G-putty nanocomposites were extremely sensitive to tensile and compressive strains due to the high dependence of resistance on intersheet separation. In particular, flexible graphene-based composites possess unique characteristics of combining multifunctionalities, such as flexible dielectric properties [194,195], flexible electromagnetic interference (EMI) shielding [196,197] and wearable energy storage [82,198]. For example, flexible graphene/PDMS metacomposites demonstrated tunable negative permittivity because of the inductive character deriving from the conductive networks [194]. Flexible graphene-based composites also showed advanced EMI shielding performance because of their broad

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absorption frequency bandwidths and light weights [196,199,200]. Flexible graphene-based composite fibers were proposed for flexible and wearable batteries [82] and supercapacitors [201].

Hybrid structures Ultra-sensitive and/or ultra-stretchable strain sensors with synergistic “1D and 2D” morphologies, such as graphene/CNT [202] and GO/silver nanowire (Ag NW) [203], or “soft and hard” structures such as GO/few layer graphene [114] hybrid networks have received much attention. A seamless hybrid film of CVD grown graphene and CNTs was produced using an ultrathin CNT film template (left of Fig. 4f) [202]. The CNT embroidered graphene demonstrated a linear, reliable resistance response to applied strains through the enhanced strength and load transfer capability at the joints. GO/Ag NW hybrid structures were assembled by a consecutive co-precipitation, reduction, vacuum filtration and casting process [203]. By adjusting the compositions of Ag NW and GO and the degree of prestretching, the GO/Ag NW hybrid strain sensor exhibited tunable sensitivity, which can be explained using the overlap and crack model. A new method was devised to prepare graphene from graphite by liquid phase exfoliation using a GO dispersant [114]. The large-area, fewlayer graphene sheets were assembled into rGO/graphene/PDMS composites which presented a great potential as transparent flexible electromechanical sensors.

Fluidics Liquid-state flexible sensing devices are an attractive alternative to solid-state sensors for flexible and wearable sensing applications [204]. Liquid metals [205], ionic liquids (ILs) [206] and organic solutions [207] are among popular conductors that have been used to develop wearable sensing devices. Graphene was also recently proposed as sensing element in a liquid for strain sensors with high sensitivity, large stretchability and excellent durability [208]. As shown in the right of Fig. 4f, rGO/deionized water (DI) conductive liquid was filled into Ecoflex rubber using a template method. The microcontact reversible effect enabled the rGO/DI sensing element to detect both tensile and compressive strains with high sensitivity. Although liquid sensors possess the advantages of low cost and simple fabrication processes, they have several critical challenges, such as nonlinear response, liquid leakage and liquid intermixing, which have to be overcome for fully functional sensors and devices [204].

Graphene-based pressure sensors Flexible pressure sensors compliant with arbitrarily curvilinear surfaces have been widely explored for many emerging applications, such as wearable electronic skins and HMIs [209]. Graphene-based piezoresistive pressure sensors are particularly appealing because of their outstanding electrical conductivities and unique nanoscale flexibility [55]. Graphene-based suspended, planar, spongy and double layer microstructures have been developed as highly flexible and sensitive pressure sensors. Figs. 6 and 7 summarize graphene pressure sensors and their sensing performance, which are discussed in detail as follows.

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FIGURE 6

Graphene-based pressure sensors. Images reproduced with permission: graphene on membrane [211] (Copyright 2013, American Institute of Physics), graphene on chamber [210] (Copyright 2016, American Chemical Society), triangular graphene hole [212] (Copyright 2015, American Institute of Physics), single layer graphene [213] (Copyright 2014, Elsevier), graphene with surfactant [214] (Copyright 2016, Wiley), molecule-graphene [215] (Copyright 2019, Wiley), bubble decorated graphene [217] (Copyright 2015, Wiley), rGO paper [218]. (Copyright 2017, American Chemical Society), rGO on nanofibers [219] (Copyright 2016, Elsevier), graphene on fibers [220] (Copyright 2018, American Chemical Society), graphene foam [228] (Copyright 2018, American Chemical Society), rGO aerogel [223] (Copyright 2018, American Chemical Society), foam-like laser-scribed graphene [132]. (Copyright 2015, Springer Nature), GO nanosuspension microfluidic [245] (Copyright 2016, Wiley). 11 Please cite this article in press as: Q. Zheng et al., Materials Today, (2020), https://doi.org/10.1016/j.mattod.2019.12.004

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FIGURE 7

Summary of sensing performance of graphene pressure sensors in terms of sensitivity vs working pressure range.

Suspended graphene The exfoliated and CVD grown graphene sheets not only exhibit strong adhesion with substrates, such as SiO2 and silicon nitride (SiNx), through van der Waals interactions, but are also impermeable to gases, making them suitable for nanoelectromechanical system (NEMS) applications [210]. Lemme et al. [162] designed suspended graphene membrane pressure sensors that could estimate the pressure difference between the two sides of a suspended graphene membrane. Zhu et al. [211] produced graphene meandering patterns located on a square silicon nitride membrane via a standard semiconductor fabrication process. The graphene membrane was able to deflect and deform into a concave shape to different degrees upon application of differential pressures (left of Fig. 6a). The out-of-plane deflection and piezoresistive effect allowed the graphene sensor to estimate the pressures applied. The experiments combined with a theoretical model were used to predict the pressure sensing behavior of the cavity (middle of Fig. 6a) [210]. It is found that the GF of suspended graphene membrane piezoresistive sensor was independent of doping concentration and crystallographic orientation. Triangular holes were also tried to investigate the nanoscale pressure sensing capability of graphene (right of Fig. 6a) [212], which was realized by utilizing the valley Hall effect.

Planar structures Graphene films with pressure-amplifying structures [213,214], a millefeuille-like architecture [215], wavelength-gradient morphologies [216] and bubble-decorated honeycomb-like structures [217] were proposed to fabricate flexible pressure sensors. It is found that the pressure-amplifying structure (left of Fig. 6b) [213] or cavity design (middle of Fig. 6b) [214] was able to enhance the sensor output by inducing additional structural deformation. Various factors, including graphene flake size, out-of-plane resistance, surface treatment and encapsulation, were found to play important roles in pressure sensing performance of percolative graphene film sensors. The molecular graphene hybrid films were synthesized by tethering rGO using molecular pillars (right of Fig. 6b) [215]. The electron tunnelling between the successive rGO sheets drastically enhanced the sen-

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sitivity of piezoresistive pressure sensors, delivering sensitivities as high as 0.82 kPa1 at low pressures ranging 0–0.6 kPa. The wavelength-gradient graphene films were produced to prepare piezoresistive pressure sensors that were capable of detecting the position of equal pressures [216]. Paper-based sensors possessed the advantages of outstanding mechanical flexibility, easy fabrication and low cost [181]. Graphene papers with a porous structure and a high elasticity were also studied for high-performance pressure sensors [218]. The bubble-decorated honeycomb-like graphene film (BHGF) was derived by evaporation of interlayer water during the pyrolysis of oxygen groups (left of Fig. 6c) [217]. The BHGF sensor exhibited an ultrahigh sensitivity of 161.6 kPa1 thanks to the switching effect which depended on the “point-to-point” and “pointto-face” contact modes. Graphene paper pressure sensor (right of Fig. 6c) was produced by mixing tissue paper with GO sheets and reduction in an oven. Owing to the well-organized porous structure, the performance of paper pressure sensors was improved greatly with optimized sensitivity and working range. Graphene textile structures were also proposed as planar piezoresistive tactile sensors [219,220]. For example, Shen et al. [219] fabricated uniform and conductive networks using rGO-coated poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) nanofibers (left of Fig. 6d). The change in conducting pathway under an external pressure led to the change in device resistance. An in situ generated thiolated graphene@polyester (GSH@PET) fabrics were produced via a facile, efficient, inexpensive dip coating process [220]. Taking advantage of the space among the conductive fabric layers (right of Fig. 6d), the GSH@PET pressure sensor exhibited a large working range and excellent sensitivity.

Porous structures Graphene-based 3D porous macrostructures, such as foams [221], sponges [222], aerogels [223] and hydrogels [224], have attracted great interest as flexible pressure sensors with excellent flexibility and stability in compression because the structural scaffolds offered good stability during the repeated loading/unloading cycles [90,225,226]. These 3D graphene porous architectures with desired geometries and densities are usually constructed via a template-directed self-assembly process [221,227]. The graphene foams produced by CVD on a nickel template were able to sense pressures up to 1800 kPa [226]. Using a commercially available PU foam template, MWNT–rGO@PU foam was fabricated via water-based MWNT–rGO ink coating [228]. The formed cracks and increased interactions between the coated PU skeletons under pressure (left of Fig. 6e) facilitated the MWNT– rGO@PU sensors to detect both small-scale and large-scale motions. A similar graphene foam structure was also prepared using a laser-scribed method [132], which exhibited high sensitivity of 0.96 kPa1 and a working range up to 50 kPa1, thanks to the large spacing between graphene sheets and its V-shaped microstructure. Graphene-wrapped sponge composites with a fractured microstructure were developed using PU sponge templates [67,222,229], which had a high resistance change helped by the contact area variation under compression leading to enhanced piezoresistive sensitivity [67]. The pressure sensitivity was tuned by controlling the density of fractured microstructure.

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The graphene sponge composite sensor offered high sensitivity for a broad pressure range due to slipping over micro-scale ridge structures [229]. Similarly, rGO/polyaniline wrapped sponge pressure sensors were developed by coating rGO and polyaniline nanowires on the backbone of melamine sponge [222], which exhibited tunable sensitivity between 0.042 and 0.152 kPa1 for a wide working range of 0–27 kPa. However, it should be noted that the adhesion between graphene sheets and polymer foams or sponge templates was usually of weak van der Waals force, causing easy peeling of graphene coating upon repeated compression and thus limited stability and reliability of the obtained pressure sensors[66]. Other forms of 3D graphene porous structures, such as aerogels [66,193] and hydrogels [224], can make mechanically robust and electrically conductive graphene-based piezoresistive compressive sensors. By introducing water-soluble PI, the rGO aerogel was transformed into superflexible 3D architectures via freeze casting and thermal annealing [193], as introduced previously in the section of 3D interconnected structures. The obtained rGO/PI nanocomposites showed extraordinarily reversible compressibility with high sensitivity of 0.18 kPa1 at 0–1.5 kPa and 0.023 kPa1 at 4–7 kPa, as well as outstanding stability of 2000 loading/unloading cycles. CNT/rGO aerogel pressure sensors with highly ordered hierarchical architectures were synthesized by unidirectional freeze casting, which exhibited both outstanding compressibility and dynamic mechanical stability [209]. RGO sheets were combined with MXene, a new family of 2D materials made of transition metal carbides and/or nitrides [223,230,231], via an ice-template freezing technique [323] to form hybrid MXene/rGO aerogels (right of Fig. 6e). Taking advantage of the large specific surface area of rGO sheets and the MXene’s high electrical conductivity, the hybrid 3D structure exhibited extremely high sensitivity of 22.6 kPa1 and good stability over 10 000 cycles as an excellent pressure sensor. Porous graphene hydrogels are also of particular interest for pressure sensing applications [232]. Qiu et al. [224] developed graphene cellular elastomers made of a graphene hydrogel and stimuli– responsive polymer with an extremely low density and a high electrical conductivity. Thanks to the changes in the contact area between the graphene intersheets upon compression, the graphene-based cellular elastomer exhibited exceptional dynamic piezoresistive response capable of detecting ultralow pressures.

Microstructures Inspired by the interlocked epidermal-dermal layers of human skin, pressure sensors with high sensitivity and a broad detection limit have been developed [31,233,234] based on various interlocked microstructures, including hemispheres [233,235,236], pyramids [237-242], spinosum [243] and prisms[244]. An ultrasensitive pressure sensor with a foam-like graphene structure was prepared using a laser scriber (left of Fig. 6f) [132]. The force-dependent contact between the two laser-scribed graphene (LSG) layers led to a change in electrical path through the crossbar structure, giving rise to an ultrasensitive pressure sensing capability. A bionic spinosum microstructure was further developed using an abrasive paper template to mimic the skin epidermis for high-performance force sensing [243]. Benefiting from

the random distribution spinosum (RDS) graphene microstructure, the pressure sensor showed high sensitivity and a large linear range. Liquid-based microfluidics (right of Fig. 6f) consisting of a GO nanosuspension sandwiched by two layers of soft templates [245] were also developed as highly flexible and stable pressure sensors. First, two distinct layers, i.e., a layer of microchannel in Ecoflex silicone rubber and a layer of PDMS, were prepared separately using standard photolithography technique. Then the two layers were bonded together following the surface treatment. Finally, after the introduction of GO nanosuspension, the fluidic inlet and outlet ports were sealed to form a sensing device. The obtained device was not only highly flexible to withstand various deformations such as tension, compression and bending, but was also capable of recognizing and differentiating distinct motions.

Graphene-based temperature sensors Wearable temperature sensors provide continuous monitoring of temperature gradients between the body and surroundings to reveal the physiological state of a person [246–248]. Several mechanisms, such as thermoresistive effects (thermistors) [249– 251], thermoelectric effects [252] or optical means [253], are utilized to fabricate body temperature sensors, amongst which the thermistor configuration is the most prominent owing to its simple working mechanism of resistance change by temperature [246–248,254]. There are two types of thermistors, namely, positive temperature coefficient (PTC) and negative temperature coefficient (NTC) types, which are defined based on whether the resistance increases with temperature increase (PTC) or temperature decrease (NTC) [10]. The resistance (RT) at temperature T is governed by the material constant (b) of the thermistor based on the following equation [10]:   1 1 ð7Þ  RT ¼ R0 exp b T T0 where R0 is the resistance at reference temperature T0. By taking the natural log of both sides of Eq. (7), a linear relationship is obtained between ln(Rt) and 1/T as follows [250]:



lnRT ¼ lnR0 þ b

1 1  T T0



ð8Þ

where the slope (b) is directly related to the bandgap of the thermistor material. In view of the fact that b varies significantly depending on the thermistor material, both the PTC and NTC is possible for various conductive fillers [255]. Both pristine graphene and rGO were employed as flexible and stretchable temperature sensors thanks to their ultrahigh thermal conductivities and thermal emissivities [256,257]. A few different graphene structures, such as graphene fibers [258], 2D graphene networks [134,250,255] and graphene nanocomposites, [247,248,259] were prepared as wearable temperature sensors with high flexibility and stretchability. Figs. 8 and 9 compare various graphene structures as temperature sensors and their sensing performance, which are discussed in detail as follows.

Fibers Fiber-shaped temperature sensors are flexible, conformable and lightweight, and even more importantly can directly integrate into textiles as wearable sensors to continuously monitor skin temperature [148]. Freestanding single rGO fibers were used to function as highly responsive and sensitive wearable tempera13

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Graphene-based temperature sensors. Images reproduced with permission: fiber[258]. (Copyright 2018, Wiley), micropattern [255] (Copyright 2012, American Chemical Society), serpentine [250] (Copyright 2015, American Chemical Society), textile [134]. (Copyright 2017, American Chemical Society), 3D porous [259] (Copyright 2019, American Chemical Society).

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FIGURE 9

Summary of sensing performance of graphene temperature sensors in terms of sensitivity vs working temperature range.

ture sensors [258]. The rGO fibers were prepared by a simple wet spinning process, which can be directly knitted on fabrics and worn on an arm or finger (left of Fig. 8a). The resistance of these fiber sensors decreased with increasing temperature arising from the enhanced tunneling transport and generated charge carriers. The thermal index was tunable by controlling the conductivity of rGO fibers (right of Fig. 8a).

Films and patterns Graphene [134,260–262] and rGO films [247,248] are found to be sensitive to body temperature changes. As such, graphene nanowalls consisting of vertical graphene nanosheets were combined with a PDMS matrix to furnish a wearable temperature sensor [261]. The device exhibited a PTC of resistivity as high as 0.214 °C1 which is attributed to the high coefficient of thermal expansion (CTE) of PDMS. Bilayer graphene sheets exfoliated from graphite demonstrated a NTC of resistivity about 0.007 K1 [262]. Because the fluffy structure of rGO films exhibited a large volume change upon heating or cooling, they presented a higher NTC than CVD-grown graphene whose resistance change arose mainly from the temperaturedependent electron mobility and electron–phonon scattering [255,260]. For example, the rGO thermistors fabricated by inkjet printing on a flexible PET substrate (left of Fig. 8b) exhibited a rapid reduction in electrical resistance with increasing temperature at a fast response time (right of Fig. 8b) [255]. Graphene channels with a serpentine pattern (left of Fig. 8c) were designed using a lithographic filtration method to achieve higher stretchability while maintaining thermal sensing properties [250]. The 3D crumpled graphene could maintain its temperature sensing functionality (right of Fig. 8c) even at a high tensile strain of 50%. High temperature sensitivity was realized using GWFs prepared by CVD (left of Fig. 8d)[134]. It is found that both the type of substrate and the length-to-width ratio of sensors affected the sensitivity to temperature changes (right of Fig. 8d).

Nanocomposites An ideal wearable thermistor should be strain-independent so that the strain effects can be decoupled from the temperature

effects. However, most reported thermistors showed a strain dependence behavior [250,251], hindering their practical applications requiring accuracy. A possible approach to circumvent the strain dependence is hybridizing a thermistor with a flexible and stretchable matrix. Trung et al. [248] developed dense rGO/ PU nanocomposite temperature sensors via simple spin-coating and lamination. They exhibited stable sensitivity of 0.0134 °C1 even after 10,000 cycles of stretching to 30% strain. Graphite/semicrystalline acrylate polymer composites were produced as flexible and printable thermal sensors [263]. The flexible, active-matrix thermal sensors were able to resolve spatial temperature gradients without interference from the constant tissue motion. Wang et al. [259] developed a cellular graphene/PDMS composite stretchable sensor (left of Fig. 8e) which delivered a stable temperature sensing performance even under different loading conditions (right of Fig. 8e).

Graphene-based multifunctional sensors For practical application of portable electronic devices, the functionality of sensors should not be only limited to a single stimulus acting alone, such as strain, twist, pressure or temperature [246,264–266]. Instead, various functionalities combined with special features like multidimensional [267,268], multimodal [246], self-cleaning [269,270], self-healing [271] and selfpowering [272] may be required simultaneously for sensible differentiation of different stimuli [93,273,274]. Fig. 10 summarizes graphene-based multifunctional sensors with the aforementioned functional features, and the details of integration of these functions to existing sensors and their functional performances are discussed as follows.

Multidimensional sensors Multidimensional strain sensors capable of detecting complex multiaxial strains are needed for wide applications in multipledegrees-of-freedom conditions [275,276]. Many of the reported strain sensors presented strongly coupled, complex responses in multidirections due to the Poisson’s ratio effects [275]. One strategy to design multidimensional sensors capable of detecting multiaxial strains is to create conductive networks with an anisotropic structure [32,275,277–279]. For example, vertically aligned graphene showed polarized and decoupled resistance changes for strains applied in different directions, enabling the detection of both the direction and amplitude of strain vectors [280] (left of Fig. 10a). A flexible e-skin composed of a CNT/GO hybrid 3D structure was fabricated to detect both the normal and tangential forces [281] (right of Fig. 10a). The e-skin exhibited distinctly opposite electrical resistance changes in response to the applied normal and tangential forces, and was able to discriminate the roughnesses of different surfaces.

Multimodal sensors Multimodal sensory integration with sensors is important for medical applications, such as wearable health monitoring, which require simultaneous detection of several physiologically vital signs [59]. Mimicking the human skin capable of simultaneous, multiple sensing, multimodal detection of external stimuli, such as temperature, strain and pressure, was achieved by integrating graphene-based sensing materials into a single pixel using a sim15

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Graphene-based multifunctional sensors. Images reproduced with permission: graphene strain vector sensor [280] (Copyright 2019, American Chemical Society), normal-tangential force sensor [281] (Copyright 2018, Wiley), multimodal E-skin sensor and circuit diagram [246] (Copyright 2016, Wiley), selfcleaning [270] (Copyright 2017, Wiley), self-healing [288] (Copyright 2015, Wiley), self-powered [293] (Copyright 2018, Wiley).

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ple lamination method [246]. As shown in Fig. 10b, the CVDgrown graphene, GO and rGO served as pressure and strain sensors, humidity sensors and temperature sensors, respectively, to monitor a variety of daily life sensations. Each sensor in the integrated sensor matrix was only sensitive to its specific stimulation, avoiding the preparation of several materials separately. Dual functionalities, such as pressure and temperature sensing, were realized using a polyaniline (PANI) hollow nanosphere/CNT composite, which was capable of continuous mapping of pressure and temperature changes [264]. RGO/polyacrylic ester hierarchically wrinkled elastic transparent films were fabricated using a balloon-blowing method [111]. Due to the periodic hierarchical wrinkling morphologies, the rGO/elastomer composite sensors exhibited multi-stimuli responses to a tensile stain and pressure.

Self-cleaning function Self-cleaning offers an additional, desired benefit to wearable sensing electronics with multiple functions [282]. Multifunctional MWCNT/thermoplastic elastomer (TPE) smart coating with a superhydrophobic surface (Fig. 10c) was fabricated by spray-coating [270]. The smart coating not only exhibited a micro-/nanostructured superhydrophobic surface, but also responded to various mechanical deformations, such as stretching, bending and torsion. The superhydrophobic surface offered extreme repellency to water, acid and alkali, extending the lifetime of multifunctional sensors while working under wet and corrosive environments. The perfluorosilane-coated graphene/ PU composite produced via a simple dissolution and resolidification method also demonstrated simultaneously robust superhydrophobicity and excellent electromechanical properties [283]. The unprecedented superhydrophobicity could be maintained even at a high strain up to 400%. Liu et al.[284] further demonstrated a wearable micro-cracked non-woven fabrics strain sensor with both outstanding strain sensing performance and excellent waterproofness, which is achieved by dip-coating in the hydrophobic fumed silica.

Self-healing function Inspired by wound healing ability of human skin, self-healing materials that can heal after being damaged or destroyed have attracted increasing attention [285]. The intrinsic self-healing based on molecular interactions with fast and reversible healing capability, such as hydrogen bonding and p–p stacking, is more suitable for strain sensors than extrinsic self-healing for wearable flexible devices that can be integrated into fully functional applications [286,287]. Self-healing wearable devices have been developed using graphene-based smart materials, such as nanocomposites [288], hydrogels [289] and electronic tattoos [290]. Graphene/PU self-healing composites were prepared by filling furfuryl-contained linear PU in the 3D graphene structure, followed by cross-linking and drying [291]. The flexible strain sensor obtained thereby not only showed excellent stretchability of over 200% strain, but also exhibited effective healing performance activated by heat and microwave. The graphene/PBS composites were prepared by dispersing rGO flakes in polyborosiloxane (PBS), an intrinsic self-healing supramolecular polymer, to detect pressure and flexion [288]. The composite sensor displayed recoverable mechanical and electrical properties

when the cut surfaces after damage were rejoined thanks to its “solid–liquid” behavior and dynamically bonded nature of supramolecular polymer (Fig. 10d). By incorporating graphene into silk fibroin/Ca2+ films, multifunctional electronic tattoos with self-healing and sensing capabilities were fabricated [290]. The electronic tattoos presented high sensitivity to multiple stimuli, such as strains, humidity and temperature, while complete healing was achieved through the reformation of hydrogen and coordination bonds by applying water droplets.

Self-powering In order to reduce the power consumption and broaden their applications, self-powered sensors have become increasingly important for next-generation multifunctional electronic skins (e-skins) [273,292,293]. Piezoelectric, triboelectric, thermoelectric and solar or radio-frequency cells have been utilized to design self-powered multifunctional sensors [292,294–297]. For instance, a multifunctional self-powered e-skin was developed using rGO encapsulated poly(vinylidene fluoridetrifluoroethylene) (PVDF) nanofibers as energy storage materials [273]. The integrated system composed of a microsupercapacitor and a sensor allowed continuous and durable power supply. In another work, a self-powered multifunctional sensing device was fabricated by integrating a highly stretchable graphene/ecoflex nanocomposite and weaved meandering zinc wires [293]. The self-powered strain sensor utilized the selfgenerated current signals from the redox-induced electricity (Fig. 10e).

Integration of sensors in electronic devices Graphene-based piezoresistive physical sensors can be readily integrated into various electronic systems for a large variety of applications, such as artificial electronic skins, human activity recognition, health monitoring, HMIs and entertainment electronics (Fig. 11). Flexible and stretchable, artificial electronic skins that mimic the spatiotemporal sensing and transduction abilities of biological skins have garnered tremendous interest because of their unique sensory capabilities in detecting subtle changes in external stimuli [31,298–300]. A broad range of graphene-based micro-/nanomaterials and structures with diverse detection modes have been designed to fabricate artificial electronic skins [176,301–305]. Artificial throats were developed using laser-induced graphene to realize functional integration of sounds generated and detected by the e-skin with wide bands and frequencies ranging from 100 Hz to 40 kHz [304]. Human activity recognition is a challenging task due to the rigid and brittle nature of conventional electronic devices [30]. Highly sensitive, flexible and stretchable graphene-based strain sensors can detect different signals generated from various human activities, such as stretching and contracting movements of joints involving high strains of well over 50% [12]. Assisted by the advancement of information processing and communication technologies, such as wireless sensor networks, distributed computing and mobile computing, these wearable wireless healthmonitoring systems (WWHMSs) would ideally provide proactive, autonomous and predictive health-care services for elderly people and patients, especially those suffering from dementia, heart 17

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Integration of graphene-based sensors. Images reproduced with permission: human motion monitoring [13] (Copyright 2017, Royal Society of Chemistry), sports tracking [59] (Copyright 2018, Royal Society of Chemistry), human machine interface [315] (Copyright 2015, Wiley), health monitoring [327] (Copyright 2012, Springer Nature), wearable entertainment [55] (Copyright 2019, American Chemical Society), artificial skins [304] (Copyright 2017, Springer Nature).

disease, diabetes or high blood pressure [306–308]. Indeed, a fully-integrated WWHMS has already been realized by seamlessly interconnecting the sensors with a Bluetooth module and smartphone. [59] The HMI, which consists of both hardware and software, is capable of handling the interactions between humans and machine and integrating humans into complex technological systems [9,30,92,309]. Wearable sensory systems can enable a smart HMI by utilizing collected signals from human motion to actuate smart robots [5]. For example, a composite film consisting of graphene platelets and silicon rubber was applied as the HMI controller for a robot [184]. A wireless wearable musical instrument has been demonstrated using a highly sensitive, flexible and skin-mountable GWF/PDMS composite sensor which converted human body motions to play recorded music [13].

Conclusions and perspectives Flexible, foldable and elastic wearable sensors are crucial for wearable electronics which enable a wide range of applications with boosted interactions between humans and smart systems, especially in shape-conforming systems of e-skins, elastic dis-

plays, epidermal sensors, personalized health monitoring and HMIs. Thanks to graphene’s exciting mechanical, physical, optical and transport properties as well as the revolutionary progress of materials processing methods and understanding of new sensing mechanisms, graphene-based flexible piezoresistive physical sensors, such as strain sensors, pressure sensors and temperature sensors, have been rapidly developed. In this paper, we presented a comprehensive review of recent advances in graphene-based piezoresistive wearable sensors. The synthesis strategies of graphene-based sensing structures, including 1D fibers, 2D planar networks and 3D interconnected nanoarchitectures, are summarized. The piezoresistive sensing involves four major mechanisms, including the geometrical effect, band structure change, contact area change and tunneling effect. The structures and performance of graphene-based strain, pressure and temperature sensors are categorized and compared. Potential practical applications are presented of artificial electronic skins, human activity detection, health monitoring, HMIs and wearable entertainment. Although excellent progress has been made in developing graphene-based piezoresistive physical sensors, several

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critical challenges still need to be overcome for practical and emerging future applications. The main challenges and possible solutions are proposed as follows:  It is difficult to combine all performance parameters, such as sensitivity, stretchability, durability and linearity, within one specific sensing material. A trade-off relationship usually exists between the high sensitivity and high stretchability for strain sensors because of the limitations of mechanical and electrical characteristics, hindering their applications in detecting human motions of both subtle and large scales. The durability of device is an important requirement to satisfy long-term use of sensors. To achieve both the wide working range for high sensitivity and the wide linear region for high stetchability is challenging for graphene-based strain and pressure sensors. One possible way to balance the sensitivity and stretchablility is to create hierarchical multiscale sensing structures with optimized strain distributions around the highly localized strains. Apart from designing more stable device structures, soft acquisition systems should be developed to equip these sensors with improved durability. New algebraic functions between the input and output data may solve the problem of multilinear phenomena of most reported sensors for accurate calibration.  Attractive special functional features, such as self-powering, self-sensing, self-healing, self-cleaning, breathability, biocompatibility and biodegradability, are highly desired to broaden the scope of current applications of graphene-based flexible sensors. In addition, because a given sensor has to respond to multiple stimuli simultaneously, it may be difficult to separate the type and the intensity of each stimulus. This means that selective sensing devices with low cross-sensitivity and effective decoupling algorithm are needed to avoid interference between the multiple stimuli for specific end applications. Nature itself can be an excellent inspiration to realize functionalities. For example, well-organized cooperation of multiple biofunctional units of humans may inspire the design of function-oriented sensing devices via a rational combination of microstructures. Human skin is not only self-healable, but also capable of detecting and distinguishing multiple stimuli, such as pressure, strains and temperature. It is possible to develop skin-like sensors with multiple sensations to complex deformation and self-healing capability by mimicking the existing skin structure and known functions.  Integration of graphene-based sensors with other flexible electronics and their functions, such as interconnects, power supply, signal transduction, data transmission and analysis, actuation and delivery of feedback, may be an exciting approach. Key challenges to seamless integration include the development of reliable stretchable interconnects, power management, wireless communication and information processing. Due to the mismatch in mechanical, thermal and electrical characteristics between the multiple components of sensors, multilevel interconnections such as stretchable conductors are required to preserve the mechanical and structural integrity while delivering power and data in a stable manner, both of which are essential to maintaining highperformance functionality. Sustainable power supply may be

realized via wireless powering, self-powering or using solar cells accompanied with reliable energy storage devices. Although wireless communication has been realized using the Bluetooth technology, its high power consumption significantly reduces the operational capability. Advanced technologies, such as miniaturized low-power Bluetooth, nearfield communications (NFCs), radio frequency (RF) communications, and high-frequency (HF) and ultrahigh-frequency (UHF) technologies, may offer controllable wireless power transfer and efficient communications. Further investigation of information processing interfaced with graphene sensors is an emerging research area that may lead to many frontier applications, such as artificial intelligence, intelligent robotics, medical monitoring and healthcare measurement.  To produce highly functional, economical and fashionable devices for large-scale commercial success, emerging approaches are needed to achieve low-cost construction of large area graphene-based sensors with high performance. New physical and chemical eco-friendly assembly strategies may help large-scale production of graphene-based macroscopic structures with tunable electrical properties. New, high functional elastomers are also needed to resolve various challenges, such as limited elasticity, poor durability, low stability and obvious hysteresis. The 3D printing technology offers new exciting possibility of synthesizing graphene-based sensing materials with desired structures and morphologies on a large scale and low cost. New processing platforms and fundamental understanding of printable materials are required to realize fully printable electronics. With the development of new strategies and technologies as well as a deeper understanding of wearable sensor systems, we believe that graphene-based flexible electronics will play an increasingly more important role in driving the development of a wide range of emerging applications in biomedicine, healthcare, robotics, artificial intelligence and entertainment technologies.

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