1.
Introduction
Strain or pressure sensors, detecting the electrical deviations under dynamic destruction, have drawn considerable attention for applications in health-care monitoring[1]. Particularly, flexible pressure sensors with ultrahigh sensitivity within ultralow strain are highly desirable for development of human-machine interface and precise detection of human activity[2]. Interest in realizing this function in an artificial skin is motivated by the promise of creating advanced humanoid robots, biomedical prostheses, surgical gloves, and wearable health-monitoring devices. Traditionally, conventional silicon complementary metal–oxide semiconductor (CMOS) based sensors have excellent properties, such as high sensitivity, but the rigid and fragile nature dramatically hamper their applications in portable and implantable devices. On the contrary, the rapid-progressing flexible sensors possess innumerable merits, including skin-mountable assembly, portable and foldable characteristics, feasible preparation feature, easy-to-deal signal collection and output mechanism. Soft, sensitive and inexpensive flexible sensors are in an urgent and imperative demand for the next generation of wearable and portable electronic product markets. Flexible and implantable physical sensors embedded into the epicuticle to generate e-skins[3, 4] can transform various and multiple stimuluses into detectable signals to indicate the type and amplitude of the ambient spurs applied on human body. Specifically, highly stretchable piezoelectric devices based on PVDF nanofibers, self-powered sensors based on electrohydrodynamically printed nano/microfibers, stretchable wireless LC strain sensors have been prepared[5–8].
Well-evolved manufacturing process of novel functional materials and innovative device structure design are both driving forces for the further advance of stretchable physical sensors. The unique and attractive properties of flexible physical sensors enable them underlying application in electric skin (e-skin)[3, 4–9], human-machine interfaces[2, 10, 11], human-activity detection, and individual-centered healthcare[12] utilizations. In addition to high stretchability and flexibility, ultra-light weight, large-area fabrication, easy feasibility of extensive utilization in veritable life, long-term durability, cost-effective manufacturing process[6], ultra-low hysteresis, extremely-low power consumption and noninvasiveness property are also demonstrated to be basic qualifications and key components for a successful stretchable sensor to fulfill its functions. Recently, the developments of flexible sensors have been reported and summarized. However, these reports either focus on the computational analysis or device performances[13–16]. Other reports focus on the classification of stretchable sensors[17] and illustrate the mechanism on the physics aspects. In addition to sensing mechanism, materials choice and property optimization are of significant importance. This review includes the integral configurations of flexible strain sensors, covering from transduction mechanism, materials consideration, property optimization and human-related applications. Main applications of flexible strain sensors are illustrated (Fig. 1).
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Figure1.
(Color online) Characteristic features of recent advanced flexible strain sensors to detect human movement and activity under external deformation: Tiny capacitance changes detection on placing and removing a bluebottle fly (20 mg)[27]. Hydrogel based sensors dramatically accommodating the finger movements[28]. Smart glove attached to the composite fiber on each finger[29]. E-skin devices under rolling condition[30]. A piezoelectric sensor in conformal contact with the top of a human wrist[31]. A sensor unit mounted on the joint of a finger[32].
In this paper, recent developments towards the direction of flexible and stretchable sensors are reviewed, followed by sensing mechanisms and essential components of strain-responsive sensors. In addition, further optimization to enhance the overall performance and perspectives on the dramatically expended area are brought into consideration. Favorable applications (including human-integrated and skin-mountable applications) and current challenges are highlighted. At last, current challenges and further remarks are provided.
2.
Transduction mechanisms of flexible sensors
Traditionally, flexible and stretchable sensors can be classified into strain sensor, piezoresistive sensor[18], capacitance sensor[19], triboelectricity sensor[20], piezoelectric sensor[21, 22], and so on. Fig. 2 shows the transduction mechanisms of flexible sensors. However, it is the sophisticated measuring instrument, relative low-resolution behavior, poor sensitivity and stability under dynamic condition that make a multitude of the as-mentioned sensors still challenging and therefore narrows their practical utilization. Furthermore, the desirable performance and easy-to-fabricate processing of other type sensors are difficult to get good balance, except for flexible strain sensors. On basis of the above-mentioned analysis, herein, we focus our current main interests only on piezoresistive sensors due to its simple sensing mechanism, easy-to-assemble characteristic, as well as ultra-low energy consumption compared to its counterparts.
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Figure2.
(Color online) Three claimed modes to explain the piezoresistivity of flexible sensors: (a) piezoresistivity, (b) capacitance, and (c) piezoelectricity[37].
Sensitivity (S) is a primary parameter of flexible sensors to evaluates its piezoresistive performance, it can be termed as S = δ(ΔX/X0) / δP. Where, X denotes electronic signal output, P denotes applied pressure. And, X can be different under various test conditions. In compressing process, X always means current signal output, however, X can be resistance in tensile process. Specifically, ΔX is referred to current or resistance variation and can be interpreted as ΔX = X – X0 (X is current value under external deformation, X0 is initial parameter value). When an ambient stimulus is exerted on the stretchable sensor, the pressure or tensile force can cause resistance changes which include: (1) geometric structure of functional materials. (2) contact resistance variation among sensing elements. (3) disconnection mechanism between active materials. In addition to the proposed mechanisms, other transduction mechanisms, such as crack propagation mechanism[12, 29, 30] and tunneling mechanism[31–33], are also brought out to prove the pressure-sensing behaviors of flexible sensors.
2.1
Geometric structure variation
As known to all, R = ρL/A, where ρ refers to resistivity, L is the length of the specific conductor, and A is the cross-sectional area of the conductor. When a strain sensor is subject to pressure or strain, the length or area of the conductor changes respectively or they simultaneously change with the external deformation. Subjected to a stretching or pressing force, the capacitance-type flexible sensors experience the relative change of the areas and the length show a continual vibration upon the external deformation, which contributes to the change of the capacitance. The as-mentioned situation also suits the pressure-responsive stretchable sensors. However, in most cases, the resistance vibration shows a unique phenomenon: the resistance change demonstrates a linear function versus the exerted stretching forces within the low-pressure ranges; and a nonlinear dependence upon the applied force in the large scopes.
A new type of transparent and flexible sensor based on Ag NW/PEDOT:PSS/PU nanocomposites[25] was developed with an excellent performance. Upon cyclic tensile strain, the contact areas and pathways of electrodes were altered, thus changing resistance of the fabricated device. Furthermore, different tensile strains and operating time both caried various substantial weight to resistance change of the strain-type sensors. Yu et al.[34] fabricated a flexible and highly pressure-sensitive sensor by employing graphene as the functional element and polyurethane as the substrate, which can detect an ultra-low pressure as low as 9 Pa and the GF (gauge factor) is 0.26 kPa?1 in the range of 0–2 kPa. The pressure-sensing mechanism can be explained by resistance vibration with the increase of pressure imposed on the artificial skin. When suffered with ever-growing deformation, the rGO (reduced graphene oxide) wrapped PU (polyurethane) microfibers closely contact with each other, resulting in a large increase in the contact area, which corresponds to the decrease of resistance.
The geometry-induced resistance vibration could also be presented in other groundbreaking works. The bubble-like honeycomb-like graphene film[35] shows three different sensing regions in the dynamic loading/unloading process, because the shape changes clearly demonstrate diverse influences on the resistance-responsive behavior of the sensor. Moreover, under the compressing condition, the resistance change from a lower value to a higher one confirmed the mechanism that geometric variations played an important role in explaining the transduce mechanism. Compared to the slight resistance change of Ag NWs film, the Ag NWs-penetrated microstructured PDMS[36] showed greater resistance variation due to the connection and disconnection of Ag NWs during the stretching process, which caused the rearrangement of Ag NWs in the substrate.
2.2
Piezoresistive effect
Piezoresistive phenomenon, which can be termed as resistance variation of materials caused by the external mechanical deformations, has been discovered both in inorganic and organic based devices, such as BaTiO3[38–41], PZT[42] and PVDF[43, 44]. Lee’s group[45] fabricated strain sensors by embedding crumpled graphene and nanocellulose in flexible elastomer matrix. Through this method, the stretchable nanopaper[45] could be stretched to 100% without evident deformation, a resistance change of 710% and a high GF exceeding 7 could also be obtained. And this sensing platform is preferable to CNT[46] and graphene[45] based flexible sensors. The outstanding property of this sensor could be attributed to piezoresistive effect, where contact areas and interspacings can be accommodated upon high strains. Piezoresistive effect of 1D nanomaterials (such as CNT[47, 48] and Ag NWs[36, 49, 50] as sensing components) can be termed as structure-dependent percolative behaviors, where external strain can be oriented upon sliding and reconfiguring of sensing elements with loading and unloading of the applied destruction. In graphene-sensing based flexible sensors, the piezoresistive effect is derived from graphene lattice turbulence, resulting in a modified electronic band structure and contact resistance variation.
In graphene-based stretchable sensors, the resistance derives mainly from graphene itself, graphene interconnections and graphene lamellas. However, the contribution of piezoresistivity may be weakened due to the undesirable mismatch[16] because of diverse components in the flexible sensors. A hollow-sphere microstructure based on polypyrrole was employed by Bao’s group[51]. The structure-derived elasticity coupled with conductive features endow the sensing materials excellent electrical and mechanical behavior. And these properties can be tuned by a controllable engineering design and the toughness or spherical asperities of the surface. Significantly, the relationship between log R and log P demonstrated inverse linearity and was accordant with the as-known equation about contact resistance:
$${R_{ m{c}}} = {left( {{ ho ^2}eta varPi H/4F} ight)^{1/2}}.$$  |
In which formula, Rc denotes contact resistance, ρ is the reflection of electric resistivity and η is termed as an empirical coefficient, H is the reflection of hardness of the material and F depicts the outward load. It is a popular belief that the contact resistance variation is determined by conductive paths and contact areas, which is mainly decided by the roughness or jaggedness of the material interface. Moreover, the sensitivity and overall property of the fabricated flexible physical sensor can be further enhanced by employing the microstructure engineering strategy. The improved sensitivity further demonstrated the piezoresistivity and the transduction mechanism for flexible and stretchable sensors.
2.3
Disconnection mechanism
Among the functional nanostructured materials of active matrix, interconnected conductive material networks can form tightly overlapped structure to entitle the elements excellent electrical property. Imposed by stretching force or pressure on the surface of intricate array, the contact areas of the fabricated sensors show great variation, thus giving rise to electrical resistance in the stretching cycles. On the contrary, the recovering process endows the nanomaterials to reconnect former structure again, including layer-by-layer structure (2D materials, such as graphene), rod-to-rod structure (1D materials, such as Ag NWs, polypyrrole nanorods, and polyaniline nanofiber). It is inevitable to disconnect from adjacent conductive elements, because the weak interfacial binding and stiffness mismatch between nanomaterials and flexible substrate upon the imposed deformation (Fig. 3). Upon stretching cycles, the buckling nature and fracture morphology of Ag NWs on the PDMS layer surface of simple-fabricated flexible sensor generate a mass of loss in contact areas. Segregating of Ag NWs from the PDMS substrate also occurs during the stretching process, which decreases the number of conductive pathways and therefore the resistance of nanowire-networks increases irreversibly.
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Figure3.
(Color online) SEM illustrations depict Ag NWs/PDMS surface during the (a) stretching and releasing process at 20° and (b) cross-sectional directions[36]. Shape deformation at the contact point of the sensor unit under press, stretch, and flexion is showed in (c) and (d) picture[52].
A metal nanoparticle based flexible sensor was reported[53], in which device Ag NPs thin film patterned PDMS was employed as the sensing material to fabricate a stretchable sensor. During the loading/unloading process, the sliver nanoparticle films with original cracks show stress concentration, resulting in more resistance change and higher sensitivity upon the applied strain (Fig. 4). A gecko-inspired[54] sensor which utilized “biomechanics” and “mechanotransduction” mechanism was fabricated. The lower half of the device was composed of laminated microhair structures. The microhair lower sheets had various contact areas with the PEN/Cr/Au/PVA/pyramidal-shaped PDMS layer during the touch process, enabling it to detect the deep-lying JVPs (jugular venous pulses), 12 times enhancement in SNR (signal-to-noise ratio) through effective contact between the devices and the epidermis surface. Flexion-sensitive e-skins, which utilized the composite Al/PMMA/AFWCNT/ PET to fabricate flexible sensors was also reported[55].
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Figure4.
(Color online) Working principle of the metal nanoparticle thin film based strain sensors: (a) The crack variation during the elongation/relaxation process. (b) The current variation during the stretching and releasing course. (c) The corresponding resistance variation during the dynamic course[53].
The newly fabricated PMMA/AFWCNT-based flexible sensor can demonstrate high accuracy, high linearity, ultra-low energy consumption and excellent stability. The as-mentioned sensor can tolerate a loading-unloading procedure beyond 15 000 cycles. Upon the bending-unbending cycles, the sheet resistance between AFWCNTs and Au or Al electrodes is increased by the introduction of PMMA wrapping. For the composite-based e-skins, the current passes through the device from bundles to bundles directly, but the current path or the resistance variation can show a significant change during the bending process, because of displacement or mismatching of different components in the dynamic loading, thus resulting in a separation of the tightly connected AFWCNTs.
Compared to the stacked sheets with π–π interaction, the 3D graphene foam[56] based highly sensitive flexible sensors show much more cracks during the stretching process, leading to more bypasses than the initial state, which gives rise to more obvious resistance variation. According to the 3D percolation theory, when the concentration of Ag NPs embedded in Ag NWs wet-spinning poly (styrene-block-butadienstyrene) (SBS) matrix[25] reached the percolation threshold, the conductive pathways were effectively formed due to the continuous connection of Ag NPs between each other, resulting in long-range connectivity and electric conductivity. While under high stretching condition, the sensing materials undergo disconnection and separation, and even the breaking of Ag NPs/Ag NWs inner the SBS substrate occurs, contributing to a more obvious variation in conductive pathways and resistance. When the 3D nanostructured CPS[57] is compressed by external forces, the individual CNT become squeezed with one another, enhancing the contact areas and contact sites, and changing the transduction routes. When the strain or compression is exerted on the framework of conductive and stretchable sponge, the connections between neighboring CNTs is increased so that the CPS has more conductive pathways. This combination of conductive property and flexible characteristic show a great potential in flexible sensors and mountable skins.
2.4
Other mechanism
Fig. 5 demonstrates the traditional working mechanism of flexible physical sensors. In addition to the as-mentioned several sensing mechanisms, other transduction mechanisms were also introduced in recent years (Figs. 6 and 7). Zhu et al.[55] fabricated a versatile flexible e-skin based on AFWCNT/polymer composite. The magnificent property of the e-skins derives from the anisotropic conductivity of AFWCNT array embedded in PMMA polymer, and the “tube-to-tube” response upon bending and stretching also carries substantial weight on the distinct sheet resistance vibration. A simple method to fabricate stretchable physical sensors based on cotton was prepared[61]. The commercially available cotton was dip-coated in Ag NWs solution for several times and the obtained sensor behaved ultra-high flexible, cost-effective and environmentally friendly features. According to the conductive theory, the following equation can explain the junction resistance of composite conductive materials:
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Figure5.
(Color online) Finite element analysis (a) showing von Mises stress distributions of the PEDOT:PSS/PUD coated micropyramid array based pressure sensors and the enhanced pressure sensitivity (b) of pyramid-structured sensor relative to unstructured films[58]. (c) The transduction mechanism of MC-Gel based force-sensitive flexible sensors and the corresponding resistive variation response (d) of the hydrogel based sensors under external deformation[59]. (e) The sensing mechanism of ultrathin gold nanowire-impregnated tissue paper based sensors[60].
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Figure6.
(Color online) (a) SEM images of the RGO-PU sponge, hydrothermally treated RGO-PU sponge (RGO-PU-HT), and the compressed treated RGO-PU-HT sponge (RGO-PU-HT-P) during the pressing and releasing process. (b) Pressure-response curves for RGO-PU sponges and RGO-PU-HT-P sponges, respectively. (c) Multiple-cycles tests of repeated loading and unloading pressure with different values. The minimum value of detectable pressure is as low as 9 Pa[34].
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Figure7.
(Color online) (a) The structure elasticity of the hollow-sphere-structured PPy based sensors. (b) SEM and TEM images of PPy revealing its interconnected hollow-sphere structure[51]. (c) SEM illustrations showing the morphology of a CNT fiber and pre-strained CNT fiber of the CNT-based flexible sensors in the stretching course. (d) Schematics demonstrating the sliding-disconnecting mechanism of the fabricated sensors[63].
$${R_{{ m{total}}}} = {R_{{ m{buck}}}} + {R_{{ m{in}}}}.$$  |
In the as-mentioned formula, Rbulk is bulk resistance of the materials, and Rin refers to the interface resistance between the two composite conductive materials. The resistance vibration can be marked as follow:
$$Delta R = {R_{{ m{in}}}} - R_{{ m{in}}}^prime ,$$  |
where Rin is the interface resistance of the flexible sensors, and R'in is the interface resistance upon the external dynamic forces.
All the analysis above demonstrates that the interface resistance plays a significant role in the sensing property of the stretchable sensors to enhance the sensitivity.
G, short for gauge factor, is the parameter to depict how the resistance variation changes upon the external dynamic forces and the ambient deformation, including compressions, bending, flexion, torsion, and strain ε. And the relationship of G and strain can be interpreted as follow: ΔR/R0 = Gε. The conductivity of average materials demonstrates independence upon stretching, and it can be shown upon dimensional changes:
$$Delta R/{R_0} = varepsilon (2 + varepsilon ).$$  |
For the traditional rigid sensors based on inorganic semiconductors, the gauge factor is ~2[62] at a low strain. For the flexible nanostructure-based composites, the conductivity of the nanomaterials is strain-dependent, that is, the conductivity will also vary because of the shape change suffered from dynamic forces. Therefore, at a low strain, we can partially attribute conductivity to the changes induced by tensile process:
$$R/{R_0} = (1 + 2nu )varepsilon + Delta sigma (varepsilon )/{sigma _0},$$  |
in which equation, the first segment demonstrates the effects of dimension or shape variation, while the second segment demonstrates the dependence of conductivity upon strain (ν is Poisson's ratio). When it comes to the nanocomposite-based flexible sensors, external strain exerted on the stretchable conductor can alter the spacing between conductive filling nanoparticles, nanowires, or nanofibers, leading to tremendous conductivity change. It turns out that the GF value can be as high as 1000 in theory. However, most of reported sensors exhibit a GF less than 50.
3.
Materials consideration for flexible sensors
Flexible and stretchable strain sensors can transduce physical stimulus into electrical signals (mostly referring to variation of resistance or current), in which process the integrity and combined action of diverse parts have great influence on the monolithic properties and measured parameters. High GF, large detective ranges, distinguished flexibility and good linearity are priorities in designing a flexible strain sensor. In pursuit of these properties, materials wise selection is crucial, and technology advances in stretchable sensors have been possible thanks to the progress of new materials and innovative processing approaches. Traditionally, the materials utilized in stretchable electronics include substrates, sensing elements, electrodes and dielectric layers. Nanomaterials often offer an advantageous incorporation of mechanical compliance and compatibility with excellent conductive performance, as well as large-area processing fabrication.
3.1
Substrates
The widespread availability and well-studied properties of PDMS (Polydimethylsiloxane)[64] have empowered it overwhelming advantages over other flexible building blocks. Its advantages include excellent transparency to fabricate top-performance devices, inertness to various chemical reagents, stability over a broad scope of temperature changes, high mechanical properties (including stretchability, compressibility, bendability, twisting ability et al.), and the capability to adhere to other elements.
Generally, for the sake of highly sensitive and low detective ranges, microstructured flexible substrates with bionic hierarchical structures[65–67] are utilized for pressure sensors fabrication. However, the easy-to-contaminate trait of PDMS in dusty air condition imposes restrictions on its pervasive practical application. Hence, a great magnitude of innovative substrate materials are employed for the rapid development of stretchable strain sensors, including PU (polyureth ane)[55, 68–70] sponges, PET (polyethylene terephthalate)[71], PI (polyimide) substrate[72], tissue paper[73], textiles[74], elastomeric fibers[32] and cotton[75]. A conductive fiber-based flexible sensor was put forward[76]. For fabricating the sensor, the Kelvar fiber was utilized as substrate, followed by pouring SBS polymer on the surface. Then Ag+ precursor solution was dip-coated on the substrate surface and further reduction of the procure was conducted to generate conductive Ag NPs. The Kevlar substrate showed excellent dynamic stability (3000 bending tests) and superb durability with negligible hysteresis performance.
The core-shell structured PU/cotton/CNT was reported[77] and the native cotton yarns were entwined tightly around elastic PU fiber. Both the components acted as substrate with stretchability as high as 300% and the composite substrate behaved excellent stability exceeding 300 000 cycles under dynamic deformation. A stretchable conductive fabric based on intertwined composite fibers was prepa- red[28], in which device the rubber element did not perform as substrate but as piezoresistive materials. Owing to the high flexibility of the fabric composite substrate, the flexible sensor could maintain up to 50% of its initial sensitivity without break under harsh testing condition. To augment the real-time detection of flexible pressure sensors, micro- or nanostructured substrates were developed by using molding technique. Silk-molded flexible sensor was presented by Wang et al.[74], and the patterned PDMS was fabricated by splashing PDMS precursor and curing agent on the textile fabric surface. The sensors with unique nanostructure demonstrate fast response time, high stability upon dynamic force and superiority sensitivity as high as 1.81 kPa?1. A toluene treated elastic band[62] can be an excellent substrate with highly outstanding properties, such as wide working range exceeding 800%, 10?4 fold of resistance variation along with an ultra-high GF as high as 35. Furthermore, the N-Methyl pyrrolidone (NMP)-assisted liquid exfoliated graphene possesses less defected flakes with lateral size of 500–1500 nm with the thickness of several layers, which empowers the elastic band/graphene based flexible sensors supreme sensitivity in the dynamic measuring process. The wet-spinned SBS matrix[29] can act as a perfect candidate for the fabrication of flexible sensors because of ultra-high stretchability with 900% elongation, demonstrating a large conductivity variation of 4.4% at 100% strain.
A poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride] (PMETAC) modified substrate was fabricated[78] and treated with oxygen plasma to generate oxygen containing functional groups. After immerging in toluene solution, the substrate was treated by METAC with KPS as the initiator to trigger polymerization. The innovative approach endowed the manufacturing sensing platform remarkable advantages, including sponge-like structure, durable performance exceeding 5000 cycles, reliable behavior under 50% strain. Furthermore, high conductivity was also achieved to lighten a LED bulb.
3.2
Active materials
Flexible strain sensors can detect both subtle pressure and ultra-high pressure in a wide range of scopes. To achieve the accurate detection of tiny strain (pulse vibration[80] and the gravity of a corn kernels[81]), active materials play an important role to ensure the precise and sensitive strain-responsive behaviors of the flexible devices. Upon this consideration, sensing elements with ultra-high sensing ability stand at the core of this emerging devices. Nanomaterials are demonstrated to be promising building blocks for new-fashioned flexible strain sensors, and devices based on several typical nanostructures, such as Ag NWs[44, 82], graphene[83, 84], CNT (carbon nanotube)[9, 55, 70, 85], conductive polymers (PVDF-TrFE)[43], PbTiO3/graphene composite materials[86] and carbon black[87, 88] have been reported.
3.2.1
Organic sensing elements
As one-dimensional material, CNT shows fantastic properties, such as light weight and perfect hexagon-connected structure, and have attracted far-reaching concerns all over the world. The high specific surface area, high L/D ratio, and superior mechanical properties of CNT endow it impressive electrical conductivity and incomparable mechanical strength. Therefore, it has been proved to be an excellent candidate to perform as sensing material in flexible sensors. Excellent tensile property combined with electrical conductive characteristics renders CNT widespread accessibility to be wielded in device-fabricating process. CNT ribbons[85] directly drawn from CNT forests were performed as stretchable electrodes. The CNT ribbons obtained from well-aligned CNT were straight-arrayed and parallel to the adjacent ones. The CNT ribbons behaved high aspect ratio, superior conductivity, and mechanical toughness propertiesas well. Additionally, CNT/ICPs (intrinsically conducting polymers) hybrid based composite materials have been demonstrated to possess outstanding properties superior to individual components, which can be termed as synergistic effect.
Graphene oxide glue[84] were used to enhance the adhesion between CNT and substrate. The vertical CNT column was bonded on the substrate by rGO (reduced graphene oxide), which acted as the electrode. Elastically flexible strain sensor was fabricated by stretching the highly oriented CNT fibers grown on a flexible substrate (Ecoflex)[63], in which device the sensor can be stretched 900% owing to the wise combination of pre-strained structure of Ecoflex and highly aligned CNT fibers. With a high GF of 64, the mechanism underlying the dramatic resistance vibration can be termed as the sliding and disconnecting phase of highly-aligned CNT fibers during the stretching process.
Graphene, a two-dimensional and hexagonal conductive material with infinite virtues, is drawing numerous consideration around the scientific circles[89–93]. Under the tensile loading process, the structure of the honeycomb-like structure partially destructed, which results in variation of the electronic band and conductance characteristics. Thus the resistance change demonstrates significant variation. On considering the intrinsically high degree of crystallization and the facility to be fabricated into thin films, a novel piezoelectric flexible sensor based on PVDF-TrFE nanofiber[43] with high performance and ultrahigh stability was assembled by utilizing the piezoresistive PVDF-TrFE nanofiber. Other innovative materials are also assembled into flexible strain sensors through the engineering process, such as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS)[58], graphene oxide[94], PVDF-TrFE, polyaniline (PANI)[95, 96], polypyrrole (PPy)[97] or combination of them. Given the cost-effective feature and large-area compatibility of carbonization technique, the carbonized silk nanofabric[98] was demonstrated to be sensing elements to introduce N-doped sp2-hybridized graphitic structures by easy calcining process, free of intricate chemical modification or complicated equipment.
In addition to tunable reactive property, light-weight and cost-effective features, nanofibrous membranes also possess controllable porosity distribution, ultra-high porosity and low-density characteristics, which renders it another perfect candidate for fabricating the flexible sensors through a simple and scalable method. With the trait of supreme electrical conductivity, facile to synthesis and cost-effective property, conductive polymers, which enjoyed tremendous attention due to their potential applications in supercapacitor and flexible sensors in the past several years, are another significantly booming materials for electrode or sensing element.
Conductive polymers, traditionally classified into ICP (whose polymer chains include ultra-long conjugated bonds to enhance highly conductive performance) and CPC, are mushrooming due to excellent properties and easy-to-prepare feature. Among the as-prepared conductive polymers, the most utilized categories include PANI[99], PPy[100], PEDOT:PSS[58] and their derivatives. A new method was introduced to fabricate stretchable and flexible conductive physical sensors based on patterned PANI/PVDF composite membrane[95]. The fabrication procedure included electrospinning and in situ polymerization of aniline. Because of the patterned structure, the as-prepared flexible sensors could be stretched beyond 110% strain (2.6 times than the commonly fabricated nonpatterned PANI/PVDF flexible sensors) and the conductivity demonstrated a tremendous variation in the tensile process. Moreover, the flexible sensing platform could endure 10000 loading-unloading reversible cycles.
Apart from PANI, another conductive polymer PPy can also be utilized to fabricate stretchable and conductive conductor. Transparent conducting composite film, based on CNT and hierarchically nanostructured PANI nanorods hybrid coplanar network[101] can be used to fabricate flexible sensors for gas sensing. Chemical oxidation polymerization of aniline accompanying with in situ growth in a functional WCNT solution was realized to form composite matrix membrane. The sensing material was simultaneously precipitated onto PET substrate and the fabricated device exhibited splendid transparency of 85%. A PPy/Ag NWs hybrid aero-sponge was reported[102], where INCG (Incipient Network Conformal Growth) technique was utilized to prepare aerogel via wet chemical and supercritical drying procedure on a large scale. Through wise optimization of the proportion of PPy to Ag, the resistance variation of the prepared hybrid aero-sponges-based flexible sensors demonstrated independence from temperature effect, negligible time hysteresis and tiny detection as low as 4.93 Pa. A hollow-sphere microstructure hydrogel based on PPy film[51] was prepared by two-phase reaction, in which approach the sensing materials showed magnificent elasticity and low elastic modulus. Furthermore, the elastic micro-structured thin film based pressure sensor showed sensitive detection of ultra-low pressures less than 1 Pa, negligible response time, credible durability in the loading-unloading cycles and excellent dynamic stability.
3.2.2
Inorganic sensing elements
Inorganic sensing elements are rigid and hardly implantable, which dramatically demonstrate disadvantageous aspect for human-movement surveillance and person-focused healthcare therapy, but they do have excellent superiorities, including high sensitivity, effective costing, and excellent processing procedure. In addition to flexible and portable sensors, inorganic materials have been also widely utilized in catalyst for water splitting and oxygen evolution reaction[103, 104]. Opposite charge-stabilized CB (carbon black) are utilized to coat polyurethane sponge to fabricate a stain sensor with versatility. Carbon black with microcracked-structure[81] renders the conductive backbones outstanding parameters, including an ultra-high detection limit as low as 91 Pa and 0.2% strain, a large motion monitoring of 16.4 kPa, a fast response time less than 20 ms.
ZnO NWs based strain sensors have a higher sensitivity and therefore are another perfect candidate for improving sensing property of the devices. Wang’s group[105] fabricated a novel strain sensor using ZnO NWs-polystyrene nanofibers hybrid materials on PDMS substrate. The number of conductive pathways between the ZnO NWs and PSNFs (polystyrene nanofibers) would be reduced when a large strain was exerted on the devices. Zhao et al. exploited a facile method where Ag NPs are covalently bonded with N-doped CNT to form Ag NPs hybrid sponges as sensing elements[106]. Given the high sensitivity of Au NWs, a sandwich-structure based stretchable pressure sensor[60] was fabricated by utilizing ultrathin Au NWs dip-coated on the surface of tissue paper substrates. Fast response time less than 17 ms and high stability of 50 000 loading-unloading dynamic cycles made it a perfect candidate for the generation of flexible and portable devices. Except for the as-mentioned carbonized active materials, the pyrolysis cotton[35], prepared by simply heating at different temperatures, was assembled into sensors, which possessed many advantages, including high porosity, good flexibility, high sensitivity (8.4 kPa?1) and wide detective range up to 700 kPa.
CB (carbon black), another perfect candidate for the sensing elements with attractive properties, has been utilized as active material resulting from its naturally low density, high environmental durability, excellent chemical stability and cost-effective fabrication process. CB nanoparticle embedded silicon elastomer composite materials based stretchable strain sensors was reported[88]. The increasing addition of porous CB particle can decrease the resistance variation of the flexible sensors by 7 times of magnitude when the CB concentration reaches a certain regime. However, the response behavior changes sluggishly when the porous CB loading exceeded the as-mentioned concentration, which can be termed as percolation threshold. The resistance-sensing regime can be classified into insulating, percolation and post-percolation ranges, which corresponds to 0.5 wt% and 2.5 wt% based on the analysis of the CB loading.
On basis of mere connection-disconnection or crack mechanism, most fabricated flexible sensors exhibit a limited detection regime due to the prone destruction of conductive sites and pathways, impeding their widespread application in large deformation monitoring. And this also renders the nature-bionic inspired stretchable sensors impractical by far. To settle the presented question, a microcrack-structured CB@PU sponge versatile pressure-type flexible sensor was reported[81], which behaved both simple and economic fabrication process with highly impressive and versatile properties. Assembled by LBL (layer-by-layer) method, the CB coated 3D porous PU sponge based sensors which was inspired by spider-bionics, could detect both tiny pressure as low as 91 Pa at 0.2% strain. And large human-activity motion of 16.4 kPa was also moonitored.
It is well known that to maintain ultra-high electrical conductivity and highly robust mechanical stretchability simultaneously is brimming with difficulties since these two irreconcilable parameters are always exclusive. To solve this puzzling question, highly stretchable conductive fibers based flexible sensors were introduced[25]. The fabricated process includes two steps: a wet spin-coating procedure and formation of Ag NPs. In the second stage, after absorption and reduction of Ag precursor, Ag NPs were embedded in wet spin-coated SBS elastomeric substrate to form a sensing hybrid matrix. The implanted Ag NWs could perform as “conductive bridge” among each other during the tensile process, which prevented the disconnection or separation of conductive pathways. The Ag NWs-SBS-Ag NPs hybrid conductive fibers exhibited reliable stability of electrical property under high strain, which could be attributed to the fact that the Ag NWs were arrayed along uniaxial direction and therefore can perform as a joint between the separated networks of Ag NPs in the stretching course. The conductivity of the Ag NWs implanted SBS matrix increases with the length of conductive fibers, resulting from the lower percolation threshold when 20 μm length fiber was applied.
3.3
Electrodes
Electrode fabrication must be taken into thorough consideration when it comes to make a flexible strain sensor. In certain cases, active materials can act as electrodes for flexible sensors as well. In manipulating the mountable devices, the transduction mechanism can be regarded as the resistance variation between two ends of the electrodes. In addition, the contact area deformation between the electrodes and active materials under stretching condition is another factor, thus inducing the resistance variation and electrical signal output.
CNT and graphene can be outstanding candidates for stretchable electrodes, because of their superior conductive property and mechanical flexibility derived from high aspect ratio structure (1D CNT) and honeycomb-like cellular structure (2D graphene), respectively. The almost perfect crystal structure suits them for high conductive conductor for flexible sensor applications.
CNT ribbon[85] directly drawn from high-aligned MWCNT (multi-walled carbon nanotube) forests was obtained. The aligned CNT ribbons can perform as electrodes for flexible sensors because of excellent conductivity and advantageous durability upon elongation up to 100%. Carbonized cotton fabric[79] was skillfully utilized as stretchable electrode for a flexible sensor, which exhibited attractive performances, including large loading strain range (> 140%), ultra-high GF (25 in strain of 0–80% and 64 when stretched by 80%–140%), and negligible value deviation. Furthermore, prolonged stability, cost-effective fabrication, simplicity in device manufacturing were also obtained.
Au nanowires coated tissue paper assembled by PDMS was reported[60]. The fabricated flexible devices can be pressed, bended, and twisted with fantastic properties, including ultra-low power consumption, fast response time and reliable durability[81]. A CNT and Ag NPs hybrid nanostructure based stretchable electrode was made[106] in which process the “dipping and coating” procedure was repeatedly conducted to wrap the active elements on polyurethane substrate to fabricate a piezoresistive sensor. High flexibility and good conductivity were achieved because of the wise combination of 1D material and 2D material.
Transparent electrodes are essential components for a mass of electric devices, such as OLED, solar cells, and energy storage devices. Recent years have seen the rapid-emerging progress of flexible and wearable electronics, which created an urgent demand for fundamentally innovative mode for those devices. And encoding physical flexibility and bendability into transparent electrodes is a desirable research direction. Generally, indium tin oxide (ITO) is the most commonly utilized materials for fabricating transparent electrodes, but its brittle and fragile characteristic makes it a poor candidate for applications in transparent flexible and wearable electronics. Therefore, alternatives are devised in the past several years for better realizing the transparency property. Those materials should include conducting polymers (CPs)[107], CNT[108], graphene[109, 110] and nanopatterned metal lamella[111].
Traditionally, electrospinning is widely utilized in most cases to fabricate such free-standing, successive nanofiber networks of polymers to act as a template and form the nanotrough. The fabrication process includes depositing metals onto the rigid template and subsequently dissolving the template selectively. After electrospinning PVP (polyvinyl pyrrolidone) on the surface of aluminum frame to form free-standing fibers, the transparent electrodes were fabricated by employing photolithography technique to pattern the nanotrough and graphene to form metal nanotroughs-graphene hybrid electrodes[112]. The presence of CNT can act as templates to enhance dispersibility of PANI nanoparticles and simultaneously enhance the transparency of nanostructured composite membrane compared to the mere existence of PANI nanoparticles.
4.
Property optimization of flexible sensors
To explore the promotion of the high-performance (high sensitivity, fast response time with negligible hysteresis and ultra-low-pressure detective limitation) of the flexible and strain sensors, considerable efforts have been devoted by many research groups around the world. Among the numerous parameters, high sensitivity and outstanding durability with good stability are the foremost factors for better human movement detection and more precisely anticipating human health-related signals. Traditionally, the sensitivity of capacitance-responsive flexible sensors is higher than resistance-responsive stretchable sensors because capacitance-type sensors possess easy-to-read characteristic with a gigantic amplitude. Specially designed structure can be introduced to improve the sensitivity such as interlocking structure[80], sponge-like matrix with folds or cracks structure[64, 113], hollow sphere structure[51] and sheath-core structure[114] (Figs. 8 and 9). Endeavors to optimize the overall properties of flexible sensors are high on the agenda, including newly-applied nanomaterials and fantastically designed structures.
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Figure8.
(a) SEM and AFM images showing the morphology of a patterned PDMS film molded from an E. aureum leave confirming the hierarchical structures. The enhanced sensitivity, reliability, and stability of ACNT/G pressure sensors. (c) The high stability performance of the fabricated sensors under external dynamic compression of 150 Pa at 2.3 Hz for 35 000 cycles[79].
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Figure9.
(Color online) (a) Schematic illustration showing the fabricated process of micro-structured PDMS films and the corresponding pressure sensors. (b) The excellent sensitivity of the bio-inspired pressure sensor under different applied pressures. (c) Current changes and the desirable piezoelectric mechanism of mimosa-inspired flexible pressure sensors in response to pressure and its reversibility[67].
4.1
Structure design optimization
In most reported cases, focuses have been located on the improvement of the gauge factor of the flexible sensors. The strategy either concentrates on its detectable pressure range or on the linear relationship between the output electrical signal and the applied external pressure. Currently, it is possible for pressure-sensitive sensors to obtain a high sensitivity in theory. But most fabricated pressure sensor at present cannot maintain their excellent sensitivity when it is imposed as-mentioned pressure ranges. Moreover, it is primary for the newly fabricated stretchable sensors to discriminate feeble variations in the relatively ultra-high compression range. A good sensitivity under external pressure within 10 kPa which corresponds to a gentle touch is difficult to get. If the manufactured physical sensors cannot implement exactly in a linear mold with high sensitivity, then it is impossible for customers to acquire accurate message from the output signals, which would be devastating for the further advance of the totally new field.
On basis of above comprehensive analysis, flexible devices with innovative geometry structures may be the favorable answer to surmount such restricted linear relationship. To overcome the adverse facts, a high-performance piezoresistivity-type flexible sensor with a linearity between electrical output and external pressure was fabricated. The sensor possesses a high sensitivity of 8.5 kPa?1 in the wide workable regime (0 to 12 kPa). The hierarchically bio-inspired structure based flexible sensor is made up of PDMS covered with graphene monolayer[65], which demonstrates a superior robustness upon 10 000 cycles. Figs. 10–12 demonstrate the enhanced overall properties induced by subtle structure design. Another hierarchically bionic structure based pressure sensor[66] was fabricated with high sensitivity. As for the manufacturing process, the micro-structure of a common leaf is transferred on the surface of PDMS substrate, which entitles the sensor easy-to-prepare feature and superior performance compared to other sophisticated microstructure designs.
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Figure10.
(Color online) (a) SEM image of hierarchically structured graphene/PDMS array. Inset is an amplifying image of an individual structure. (b) An illustration depicting the transduction mechanism of the fabricated hierarchical structure based flexible pressure sensors under normal compressive pressure. The contact areas increase with the applied pressure. (c) Superb transparent property of the pressure sensor attached on the wrist and the corresponding electric signal variation showing the health condition of human being[65].
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Figure12.
(Color?online)?(a) Schematic diagram illustrating the overall structure design of the fabricated ultra-sensitive flexible sensor. (b) Relative current variation of the two e-skin sensors in the low detective ranges[123]. (c) The nanoarchitecture of graphene-CNT binary percolation networks based all-carbon conductive platform. (d) Frequency response of the fabricated epidermal sensor with an input frequency of 1 Hz. (e) Relative resistance variation of the networked platform versus time under different strain (with an external frequency of 0.1 Hz). (f) AWP (artery wrist pulse) output signal of a healthy tester before (blue line) and after (red line) sports[10].
Ag NPs coated electro-spun SBS fibers were performed as a sensing platform[115]. The Ag NPs were obtained by reduction reaction of as-prepared procure and the fabricated strain senor demonstrated to behave high conductivity up to about 2200 S/cm under 100% strain. A sandwich-like silver nanowire based strain sensor was fabricated. First, the Ag NWs solution was poured onto the PI (polyimide) tape mold to form a conductive film. Second, another half-cured PDMS was coated on the surface of Ag NWs film.
A novel approach to fabricate a mimosa-inspired flexible pressure sensor was reported[67]. In this fabrication approach, the PDMS precursor was poured onto the mimosa leaf, so that the irregular micro-domains mode (with an average height of 16.1 ± 3.7 μm) could be transformed to the PDMS sheet thus endowing the flexible sensors excellent properties, such as a higher sensitivity as high as 50.17 kPa?1 in low detecting region, a lower response time less than 20 ms, stable loading-unloading dynamic cycles over 10000 times. Likewise, reduced graphene oxide film coated plasma-treated PDMS was performed as an ultra-sensitive strain sensor[116]. The continuous film possessed a wrinkled surface with obvious roughness and overlapped morphology, resulting in a modified sensing mechanism.
An innovative method to fabricate a flexible sensor was skillfully introduced[70], in which process the CNT was deposited on PS (polystyrene) substrate by spray gun. After that, the modified building block was heated to generate wrinkled structure. Subsequently CNT was transferred onto the PDMS sheet by dissolving PS substrate in toluene solution, which could be defined as sacrificial template method. The resulting wrinkled CNT based sensor showed high flexibility up to 750% strain, high piezoresistivity behavior, and a higher GF.
The LBL assembly and vacuum-assisted precipitation were traditional procedures to prepare membrane, but the sophisticated trait posed great challenges. A versatile SWCNT (single-walled carbon nanotube)/Ag NPs hybrid paper based flexible sensor was fabricated with a facile and scalable method[52, 70], in which process the functional CNT was mixed with Ag NPs solution, and then freeze-drying and compressing technique were conducted to form a 3D porous aerogel-like hybrid film based stretchable conductors. The unique aerogel-like building blocks of as-prepared miscellaneous SWCNT hybrid paper endowed the flexible sensors folded, compartmental structures, sufficient void space for liquid penetration and stress release. Moreover, the innovatively scalable and universal manufacturing method could demonstrate great universality for fabricating next generation of flexible and portable devices. Notably, the controllable construction, long-term durability and one-step fabrication without pondering on phase separation are superior to other flexible devices.
Apart from flexible conductive polymers and intrinsically stretchable organic/inorganic sensing nanomaterials, metal can still be excellent candidate for pressure-responsive sensor fabrication due to its high-performance in good conductivity, mechanical robustness and fantastic stability. Significantly, when prepared into out-of-planar wrinkles or in-plane waves nanostructured thin films, metal derivatives can be foldable, stretchable and flexible sensing elements or active materials. MACP[78], a technique short for matrix-assisted catalytic printing, is compatible with inkjet printing, screen printing et al. This technique can be utilized for extensive fabrication of metal patterning, including Cu, Ag, Ni and Au whose characteristic sizes gaps range from nanometer to several micrometers. Moreover, in addition to flexible, fordable and stretchable traits, MACP technique can be easily realized to manufacture nanostructured metal conductors at room temperature with cost-effective feature, highly conductive property (2.0 × 108 S/m).
An all-carbon collaborative nanoarchitectures based epidermal sensor was demonstrated by Dong[11]. The fabricated binary conductive percolation networks prepared by nanostructural engineering strategy possessed superior advantages derived from 3D graphene sponginess and 1D nanoscaled CNTs. The manufactured conductive networks behaved high stretchability, excellent sensing performance, and ultra-high SNRs (signal-to-noise ratio). In detail, the strain platform could endure various deformation, including tension, bending, and torsion. Under dynamic loading-unloading of 5000 cycles, the conductive building blocks showed negligible changes at 2 Hz, rendering it reliable durability in practical application. Furthermore, the introduced networked flexible device could meet human-activity detection demand, and AWP (artery wrist pulse), JVP (jugular venous pressure) were monitored accurately. Superb stretchability (≈ 85%), high sensitivity (GF = 35) and fast response time were integrated in a portable sensor for the first time, broadening the potential utilization of human-machine interface. Currently, the graphene-CNT binary pressure-responsive skeleton opened a totally new door for E-skin, smart robots, human motion real-time detection and individual-centered health monitoring.
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Figure11.
(Color?online)?(a) Conductive GNN patterns on line, tetragonal-dots and cylindrical pillars and the corresponding SEM images (b) without depositing metal on the samples. (c) Illustration depicting the sensor to detect amplified bio-signals with the presence of micro-lines patterns and the corresponding amplitude obtained by mapping of the capacitive signals[117].
4.2
Materials optimization
Since the utilized materials have an apparent influence on the overall property of the flexible and stretchable sensors, it is in a great demand to put more booming materials into practical device-fabrication process. Apart from high sensitivity and good stretchability, broad pressure-detective range, good linear response property under ambient deformation, easy accessibility of manufacturing and cost-effective encapsulating process throw light on the developing directions. Meanwhile, low hysteresis behavior during the dynamic loading and reliable stability also deserves painstaking considerations.
Flexible carbon tissue paper was tactfully assembled into a highly stretchable sensor[73]. The carbonized paper was prepared by direct pyrolysis at 800 °C in nitrogen ambience for several times, and the manufacturing approach showed great advantages in cost-effective trait. The newly-designed CP (carbon paper) based sensor (with GF of 25.3) showed steady response within low frequency. A sheath-core structured strain sensor[114] was developed utilizing graphite sheet as sensing material. The outstanding loading stability made it a perfect candidate for application as wearable devices. Enlightened by 3D porous and laminar structures (mushrooms and spongia for examples) commonly seen in nature, an ultra-high subtle capacitance-type flexible sensor based on cellular PDMS thin film has been reported to possess excellent property. When measured at external dynamic deformation, the bioinspired porous structure based devices demonstrated high-performance including high sensitivity (0.63 kPa?1), credible endurance at over 10000 cycles, negligible hysteresis behaviors and low detective pressure range as low as 2.42 Pa.
A valid and inexpensive strategy was reported to assemble a flexible sensor by dipping hairs into GO solution[118] and GO was then reduced to rGO. The measured current signals showed prominent vibrations with the bending radians change. Water-soluble PI was selected to double the elasticity and flexibility of GO aerogels[119]. A light-weight and super-elastic strain sensor was achieved due to the synergistic effect of rGO and PI, which rendered the nanocomposite based sensing platform desirable conductivity and compression sensitivity during pressing, bending and torsion tests. A multifunctional mechanical sensor was fabricated by Yu et al.[28] in which device intertwined electronic fiber and piezoresistive elastomer were employed as active components, and the Ag nanowire was utilized as core electrodes. The unique synergistic effect of Ag NWs coated elastic thread and the conductive rubber made the sensor maintain 50% GF value even when it was stretched up to 100%.
Metal-organic frameworks (MOFs)[120] were employed to fabricate ultrasensitive flexible sensors which allowed a detection limitation as low as 0.73 Pa and the working voltage could be as low as 1 V. The stretchable devices demonstrated ultra-high sensitivity (6.25 kPa?1), negligible hysteresis (< 10 ms), excellent energy saving property (< 0.1 mW), and amazing repeatability (> 10 000 times). It was noteworthy that the Cu TCNQ nanowire arrays, obtained by reaction of Cu and TCNQ solution, were utilized to secure a highly sensitive and stretchable device. The MOF based sensor possessed excellent properties, such as facile fabrication, cost-effective solution-processing method and fantastic human-motion signals detection ability. The as-introduced sensing platform could detect human radial artery pressure in real time, making the fabricated sensing platform a perfect candidate for the widespread availability to monitor heart-related diseases.
3D graphene foam composite[56] based flexible sensors demonstrated highly stretchable and sensitive properties were prepared. The addition of ammonium sulfide solution and ammonia solution to the GO solution to induce reduction reaction was performed. Freeze-drying treatment was subsequently carried out to obtain the graphene foam. Encapsulated by PDMS, the prepared sensor demonstrated marvelous dynamic stability during the tensile process. The ultra-high sensitivity (98.66 upon 5% strain), good conductivity property and the mechanically robust tensile strength rendered the prepared 3D graphene foam/PDMS composite materials based electric devices exhibit great potential utilization in the fields of human health-interactive applications.
Apart from the as-mentioned 3D graphene foam, CNT based porous and laminar aerogel was also prepared to fabricate flexible sensors[121]. However, two main questions still restricted its broad application. The first problem resulted from large inter-blocks disconnection happening during the tensile or compression process due to the weak van der Waals interactions between individual building blocks compared to the CNT membrane. Another issue was that it was difficult to obtain stable and ultra-high resistance variation signals during the cyclic loading course without impairing the initiative hierarchical and porous CNT morphology. A versatile method was reported where N-doped CNT/Ag NPs HPG (hyperbranched polyglycerol) based flexible sensors were prepared[106]. The composite sensing materials acted as bridges to form covalent bond with acidified N-CNTs, while Ag NPs could not only perform as interlocked nanodomes to enhance compression stability, but also generate larger resistance variation via increasing contact areas and conductive pathways. Another type of MWCNT aerogel based 3D sponge[122] with ultra-low density and marvelous compressible ability was also fabricated. The manufactured flexible sensor showed high conductivity up to 3.2 × 10?2 S/cm, and the value could be enhanced to be 0.67 S/cm upon external dynamic deformation, which made it a grand candidate for gas sensing devices.
In the past several years, self-healing materials[124–126] have attracted considerable focuses due to its excellent mechanic properties and conductive performances. Particularly it exhibited great potentials in flexible sensor areas. Apart from hydrated ICP (intrinsically conducting polymers), conductive hydrogel based composite materials were initially utilized in bionic interface field, implantable bioelectrodes[75] and biosensors. And this was due to the mechanical analogy to human skins, biocompatibility and excellent conductive performance. New species of extremely flexible self-healing piezoresistive-type[127] strain sensors were introduced. And the utilized sensing elements should include CNT[128–130], graphene as well as other materials including PEDOT:PSS[131]. When strained to 1000%, the as-prepared conductive cross-linked hydrogels demonstrated an ultra-high reversible recovery of its initial state. The resistance could recover 98 ± 0.8% within 3.2 s, a negligible hysteresis time, which exhibited supreme self-healing property. Immune to external stimuluses such as thermal heat, pH and light, the self-healing hydrogel based flexible sensors were liable to distinguish different movements, including strain, flexion and torsion without obvious breakage under large deformation and dynamic forces. Fascinatingly, when cut into two furcate parts, the open circuit resistance reached an infinite value. However, the detached two segments became contact in short time as quick as 3.2 s. After recovery process, a lower resistance was obtained compared to the incipient value and this could result from free ions transferring in the 3D cross-linked hydrogel. Reduced hydrogel based on rGO also behaved identical procedure. Interestingly, without addition of sensing element MWCNT, the mere hydrogel without electrical conductor also manifested a large resistance variation of 533% during the stretching process, which attributed to the ions transfer of Na+, H+ and water molecule. Currently, hydrogels have attracted enormous attention due to its fascinating behaviors, and great endeavors were put to assemble elastically conductive hydrogel based flexible sensors[125, 126, 132–134].
Apart from the as-prepared 3D CNT and 3D graphene conductive foams, another marvelous 3D porous foam based on conductive metal-coated PDMS sponge was fabricated[64], which demonstrated to be 3D flexible and stretchable electrical conductors. The 3D sponge could be prepared as follows: sugar-templated method for preparing the PDMS sponge, silanization of the sponge, in-situ free radical polymerization of vinyltrimethoxysilane (VTMS) and PMETAC (poly[2-(methacryloyloxy)ethyl-trimethylammoniumchloride]). Finally, various metals were coated on the chemically modified 3D continuous sponges. The sugar cubic-templated technique proved to be cost-effective and widespread available with the attribute of 3D sponge-like structures, and the intrinsically compressible sponge could be compressed to a large extent. Furthermore, the conductive 3D PDMS sponge demonstrated to be stable at 40% elongation and only 20% deviation at 50% elongation after 5000 loading-unloading cycles in the stretching process. It also possessed excellent characteristics under stretchable, compressible, and bendable treatment, which proved a facile method for next generation of flexible sensors.
Similarly, a flexible, stretchable, hydrophobic and low-density CNT-PDMS 3D sponge (CPS) is presented[57]. The commercially obtained sugar cubic was utilized as a template for preparation of 3D PDMS sponge. After dissolving the template in hot DI water, the sponge with cellular and porous structure was prepared. This manufacturing approach was economic and cost-effective. Finally, oxygen plasma treated 3D PDMS sponge absorbed functionalized CNT to assembly a flexible sensor. The as-prepared CPS sensing platform showed a variable Young’s modulus in the 22 to 200 kPa range. This was due to the applied tensile elongation and the tremendous porous structure. The CNTs hardly affected the porosity of the composite materials, and the porosity of both PS and CPS tested by volume absorbance were maintained about 64%. The intrinsically porous feature of CNT forest with a porosity of approximately 99% could account for the negligible effect of the role of the CNTs.
5
Human-motion monitoring
Now, flexible and stretchable sensors have been employed in considerable applications. For examples, low-strain measurement was realized by highly sensitivity strain sensors to achieve accurate detections of health-care monitoring[136, 137], mass variation detection[138] and pressure responsive performances[53]. Besides the tiny stimulus change, large-scale motions could be real-time monitored by the flexible strain sensors. The smart textiles, intelligent gloves, percolation sponges, highly stretchable and transparent electrodes were utilized to reflect human-related motions. The stretchable and wearable sensors have enjoyed the widespread accessibility and far-reaching development due to the rapid evolution of the Internet of things. The applications of strain sensors are widely, however, human-motions detection was the main application to achieve human-machine interactions.
3D graphene foam based strain sensor[56] was fabricated and gauged human activities. In addition to excellent piezoresistive behaviors, the conductive foam could also respond to wrist bending and finger bending, which is important for healthcare application. Highly stretchable E-skin[139], incorporated porous poly (dimethylsiloxane) (pPDMS) and rGO, was a perfect candidate to realize EMG detection and knee movement monitoring. Notably, the current variation during the high wrist bending degree (~80°) could be recorded by the self-healing and ultrasensitive hydrogel based strain sensor[140]. Suo and his coworkers prepared a fantastic ionic skin[141] by utilizing PAAm (polyacrylamide) as hydrogel building block. The capacitance sensor could accurately reflect the capacitance change under external pressure. Furthermore, the responsive curve also presented an identical trend with pressure application. Large-scale smart textile based pressure sensor[142] could not only respond to finger bending and pressing, but also realize real-time acoustic vibrations, words distinction, various hands motions and pulse waveforms detection. Hollow-architecture microcapsules based pressure sensor[143] could monitor the loading of a hand and distinguish different heart rates under static or running conditions. A cost-effective and easy-prepared graphene-paper pressure sensor was manufactured by Ren. Wrist pulse of ~70 min–1 and another pulse of ~68 min-1 after exercise were precisely detected by this sensor. Besides, body motions, such as jumping, pushing, squatting and walking were clearly monitored. Thread-based pressure sensor[28], stretchable and conductive PDMS[144] and self-healable mineral hydrogel[24] could define different finger configurations, wrist bending, throat motion and blood pressure. Specifically, a wireless flexible sensor[145] was utilized to control a gripper robot to realize different gesture identification. Smart glove systems[12, 45, 146] were beneficial for developing surgical procedures which were out of reach or dangerous. Human-environment interaction could be detected by sensory artificial skin[147, 148], which allowed the soft robots to obtain stimulus from the surroundings.
6.
Current challenges of stretchable sensors
Under ambient loading deformation, the stretchable piezoresistive sensors are employed to output current or resistance variations. Driven by recent advances, the sensing conductors have been verified to possess profound applications in human-motion and health-care fields.
Though the flexible and stretchable sensors have received enormous evolution during the past years, and several devices concerning health-centered application have been brought into practice (Fig. 13), great challenges still exist. Firstly, to further develop the innovative devices, the transduction mechanism must be understood to enhance the overall property thus obtaining a higher GF. Recently, various mechanisms[149–152], including geometry effect, piezoresistive effect, connection-disconnection mechanism, and tunneling effect, were declared to account for the current or resistance variation during the stretchable or compressed process. However, the uniform transduction mechanism is far from accessibility to explain the operating principle of flexible sensors in all cases. Apart from a better comprehension of the underlying sensing mechanism, we should well know that further exploration of a structure design is also worthy of prudent consideration. More significantly, simultaneous acquisition of high stretchability with negligible brittleness and delicate sensitivity still remains tremendously challenging[29, 69].
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Figure13.
(Color?online)?(a) The desirable illustration of nanopile interlocking to achieve both high-adhesion and high flexibility. (b) Finite Element Modeling simulating the strain distribution among the fabricated sensor with and without nanopiles under external 10% strain. (c) SEM image showing the cross-section view of the stretchable Au film with nanopiles penetrating into PDMS. Insets show the front and reverse sides. The reverse illustration black for sake of absorption of nanopiles[135].
Particularly, the current situation may be even worse for individual-focused health and medical applications where highly ingenious sensors with robustly mechanical property are in great demand. In addition to the as-mentioned challenges, excellent linear relationship between output electrical signals and external deformation is difficult to obtain. Flexible strain sensors which allow for detecting multi-directions movements and destructions with energy storage property[63, 153, 154] are rarely reported at present. Therefore, totally new materials and nanostructure engineering[23, 35, 80, 155] are perfect blueprints for the further advance of stretchable physical sensors.
For the matter of measurement aspects, stable and distinct signal output with easy data-read mechanism at harsh condition is also of vital importance for the practical applications. The utilization of robustly conductive wire to act as electrodes in sensor devices realms for human-activity signal attaining is incompatible and implantable, resulting in impairing the flexibility[156, 157], that is, the as-fabricated strain sensors are not totally soft devices. Significantly, most of the dedicated endeavors are focused more in the sensing property enhancement rather than sensor assembly procedures, which is disastrous for industrialization[158]. On basis of the incisive analysis, substantial and prudent attention must be concentrated on the perfecting the sensitivity and enlarging the practical industrialization for better feasibility in real life. And the solution can be obtained by employing newly developed active elements[159, 160] and well-designed geometry structures[158, 161] of the manufactured building blocks. However, the trouble to enhance all the property parameters, such as linearity relationship between electric signal output and external pressure, reliable durability in extremely harsh condition, should not be overlooked when it comes to practical application. For human-machine applications, cost-effective stretchable strain sensors[81] with ultra-low energy depletion[23] are especially in urgent need. Currently, microfluidic assembly of the miniature sized chip-scale elements are presented to show immense practical potential for human-activity detection. Moreover, nonintrusive and compatible wearable sensors, which utilize biocompatible substance to perform as flexible substrate and sensing materials are also in crying demand and have caught huge interest around the world[162].
Another hardship for the improvement of flexible and wearable sensors focuses on fabricating versatile sensor matrixes rather than merely single functional devices, which can achieve multi-monitoring output of various physical signals using one device. All these as-mentioned claims can not only be questions for the further advancement but also developing direction, resulting the emerging field a vibrant and lively area[124, 163–165].
Despite the seemingly promising future, the fabrication process of such systems is sophisticated, time-consuming[166]. Precise human-oriented detection and stable current signals output are difficult to achieve balance. The conformal adhesion of flexible and stretchable sensors on the surface of human epicuticle[130] should also be taken into consideration to detect the tiny human motions which reflects the health condition. Therefore, weak adhesion may lead to sensors falling from skin, aggravating signal noise and hysteresis in electrical response of strain sensors, thus misinterpreting external stimulus[167].
7.
Conclusion
During the past two decades, the accelerating development of flexible devices has expedited dramatically because of the facile material preparation and the delicate structure design. Thanks to the progress of this unrivalled trend, the area of flexible strain sensors is becoming an impressive hit[168]. More importantly, motivated by the prosperous advancement of organic electronic domain, flexible pressure-responsive devices have been evolving in an incomparable pattern. Ultra-sensitive flexible physical sensors[29, 169, 170] based on optimal active materials choice and nanostructure design are now attracting great attentions to perform as perfect candidates for wearable and portable devices. The enormous success in device fabrication and implantable assembly into human epicuticle also exhibit their promising applications in wearable electric areas, which can be an indispensable field in future. Simultaneously, despite the progress[58, 171, 172] depicted above, a good deal of challenges still act as severe obstruction before wearable physical sensors are put into real application. Other auxiliary attribute such as transparence, touchable panels are widely considered as a crucial driver for the future advances of the wearable and flexible sensors. Inorganic metal oxides including ITO, organic sensing materials including CNTs and graphene, well-familiar transparent active materials, even self-healing materials, have been demonstrated promising in the practical application. Nowadays, flexible and stretchable sensors with transparency more than 80% have been prepared. Reduplicate exploration of such stretchable sensors could afford new chances for wearable devices and data-security utilization. Merging of innovative and emerging technology into flexible and wearable sensor can be conducive to the newly booming wearable device markets. Pressure sensors with self-healing performances[173–175] are recently drawing a great deal of attention. Self-rehabilitation behaviors derived from new materials and optimal structure design can secure the devices to recover in short time and maintain original properties. The conductive pathways show negligible change under ambient deformation, thus exhibiting stability and durability.
In summary, gigantic accomplishment has been achieved in the direction of sensitivity enhancement and property optimization. Stretchable and portable sensing platforms with trait of good flexibility, superior transparency and easy-to-obtain signal output behaviors will render wearable and portable devices a fresh and novel realm. Despite the challenges may pose great difficulties at present, the desirable perspectives is certain to appear soon.
