Recent Advancements in Flexible and Stretchable Electrodes for

Mar 10, 2017 - conductive network; desirable feature; flexible and stretchable electrodes; sensing .... Materials and Wearable Devices for Autonomous ...
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Recent Advancements in Flexible and Stretchable Electrodes for Electromechanical Sensors: Strategies, Materials, and Features Songfang Zhao,*,† Jinhui Li,‡ Duxia Cao,† Guoping Zhang,*,‡ Jia Li,† Kui Li,† Yang Yang,† Wei Wang,§ Yufeng Jin,∥ Rong Sun,*,‡ and Ching-Ping Wong⊥ †

School of Material Science and Engineering, University of Jinan, Jinan 250022, Shandong, China Guangdong Provincial Key Laboratory of Materials for High Density Electronic Packaging, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, China § National Key Laboratory of Science and Technology on Micro/Nano Fabrication and Institute of Microelectronics, Peking University, Beijing 100871, China ∥ Shenzhen Graduate School, Peking University, Shenzhen 518055, China ⊥ School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, Georgia 30332, United States ‡

ABSTRACT: Stretchable and flexible sensors attached onto the surface of the human body can perceive external stimuli, thus attracting extensive attention due to their lightweight, low modulus, low cost, high flexibility, and stretchability. Recently, a myriad of efforts have been devoted to improving the performance and functionality of wearable sensors. Herein, this review focuses on recent remarkable advancements in the development of flexible and stretchable sensors. Multifunction of these wearable sensors is realized by incorporating some desired features (e.g., self-healing, self-powering, linearity, and printing). Next, focusing on the characteristics of carbon nanomaterials, nanostructured metal, conductive polymer, or their hybrid composites, two major strategies (e.g., materials that stretch and structures that stretch) and diverse design approaches have been developed to achieve highly flexible and stretchable electrodes. Strain sensing performances of recently reported sensors indicate that the appropriate choice of geometric engineering as well as intrinsically stretchable materials is essential for high-performance strain sensing. Finally, some important directions and challenges of a fully sensor-integrated wearable platform are proposed to realize their potential applications for human motion monitoring and human−machine interfaces. KEYWORDS: flexible and stretchable electrodes, wearable sensor, conductive network, sensing mechanism, desirable feature oxide films, possess an intrinsic brittle and rigid nature, which limits their applications requiring large deformation (bending, twisting, and stretching, etc.) or intimate integration with curvilinear surfaces.4−9 Recently, numerous efforts have been devoted to developing novel stretchable electrodes, which could be capable of sustaining a large level of strain (≫1%) with the preservation of conductive pathways.10−12 As Rogers et al. suggested,13,14 there are two major strategies to address the challenges in stretchable and flexible electrodes, namely, “structures that stretch” and “materials that stretch”. Common stretchable structures are fabricated by designing geometries of conventional materials, including wavy geometry,15,16 percolating

1. INTRODUCTION Aligned with the fast expansion of the personal information platform and Internet of Things, flexible and stretchable electronic devices are being seamlessly integrated in modern electronics and related multidisciplinary fields at an exponentially increasing rate. Stretchable electronics, which builds electronic circuits on top of a stretchable substrate or embeds them in a stretchable matrix, enables devices to be deformed into arbitrary shapes while maintaining the performance and reliability of the devices. In particular, wearable sensors can be attached onto the textile or even directly mounted on the human skin by adhesive tapes or straps, thus monitoring physical signals for disease diagnosis and healthy monitoring.1−4 As one of the most fundamental components of wearable sensors, conductive electrode has become a dominant building block of electronic devices. Conventional electrodes, comprising single-crystal silicon, polycrystalline metals, or metal © 2017 American Chemical Society

Received: October 28, 2016 Accepted: March 10, 2017 Published: March 10, 2017 12147

DOI: 10.1021/acsami.6b13800 ACS Appl. Mater. Interfaces 2017, 9, 12147−12164

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ACS Applied Materials & Interfaces network,17,18 helical structure,19 and serpentine20 and mesh shapes.21 Additionally, intrinsic stretchability is achieved by incorporating novel conductive materials such as carbon black (CB),22 graphene,23,24 carbon nanotubes (CNTs),25 and metal nanowires (NWs)12 or metal particles,26 into the stretchable matrix. Stretchable conductors can be used for interconnection or sensor applications depending on the electrical properties of the stretchable conductors under mechanical strains. Normally, stretchable conductors with huge changes of electrical properties under deformations are employed as strain sensors in stretchable devices. To provide high sensitivity, micro-/ nanostructured designs have been employed to improve the relative change of the electrical signal upon deformation.27 Also to provide a sustainable power supply, integration of current sensors with stretchable nanogenerators is another important feature for wearable sensors.28 Apart from high sensitivity, stretchability, and self-powering capability, wearable strain sensors should have a range of other features including selfhealing capability, self-cleaning capability, and printability, etc.2,3,5 So far, numerous work has been conducted on fabrication and integration of wearable sensors with multifunctional features, which stimulates the advance of stretchable electronics. More recently, several reviews have been reported of latest achievements in flexible and stretchable electronics.2−4,26,29,30 However, these review papers mainly focused on the progress in printable and flexible sensors,3 flexible sensors based on nanoparticles,26 nanomaterial-enabled stretchable conductors,4 or thin-film electrodes with high deformability.10 A detailed overview of emerging desirable features driving the advance of multifunctional electronics, remarkable advances and technological issues of diverse approaches to achieve intrinsic stretchability, emerging geometric engineering, and combination of three-dimensional (3D) geometric engineering and elastomeric substrates, has not been reported. Thus, this review focuses on the latest development of flexible and stretchable electrodes for electromechanical sensors with additional desirable features (Figure 1). In section 2, we address some important features (e.g., linearity, self-powering, and selfcleaning) integrated in electromechanical sensors, which drives the advance of stretchable electronics. In section 3, we highlight the characteristics of several conductive building blocks employed in stretchable sensors, such as metallic NWs, graphene, conductive polymers, or their hybrid components. In section 4, we review the latest successful approaches to achieve stretchability by developing intrinsically stretchable materials and geometrical design of conventional materials, along with a discussion of the associated technological innovations and issues. Finally, some technological challenges and outlook in this field are also discussed.

Figure 1. Brief introduction of wearable sensor platform: strategy achieving stretchability, features, and applications. Strategy achieving stretchability: Multicore−shell fiber. Reprinted in part with permission from ref 31. Copyright 2015 John Wiley and Sons. ZnO NW@ polyurethane (PU) fiber. Reprinted in part with permission from ref 32. Copyright 2016 John Wiley and Sons. Isotropic buckled CNT film. Reprinted in part with permission from ref 33. Copyright 2016 American Chemical Society. Elastomeric carbon adhesive. Reprinted in part with permission from ref 34. Copyright 2016 American Chemical Society. Fish-scale-like graphene. Reprinted in part with permission from ref 35. Copyright 2016 American Chemical Society. Ag NW elastic conductor. Reprinted in part with permission from ref 36. Copyright 2014 American Chemical Society. Knitted CNT/Spandex (SPX) structure. Reprinted in part with permission from ref 37. Copyright 2016 American Chemical Society. Elastomeric conductive polyaniline (PANI). Reprinted in part with permission from ref 38. Copyright 2016 American Chemical Society. Features: Self-powering. Reprinted in part with permission from ref 39. Copyright 2015 American Chemical Society. Reprinted in part with permission from ref 40. Copyright 2016 American Chemical Society. Biocompatibility. Reprinted in part with permission from ref 41. Copyright 2012 Nature Publishing Group. Reprinted in part with permission from ref 42. Copyright 2015 American Chemical Society. Printability. Reprinted in part with permission from ref 43. Copyright 2016 Royal Society of Chemistry. Reprinted in part with permission from ref 44. Copyright 2014 John Wiley and Sons. Self-cleaning. Reprinted in part with permission from ref 45. Copyright 2015 American Chemical Society. Transparency. Reprinted in part with permission from ref 46. Copyright 2015 American Chemical Society. Self-healing. Reprinted in part with permission from ref 47. Copyright 2015 John Wiley and Sons. Application in detecting: UV. Reprinted in part with permission from ref 32. Copyright 2016 John Wiley and Sons. Drug-delivery. Reprinted in part with permission from ref 48. Copyright 2014 John Wiley and Sons. Wound-healing. Reprinted in part with permission from ref 49. Copyright 2014 John Wiley and Sons. Hydration. Reprinted in part with permission from ref 50. Copyright 2014 John Wiley and Sons. Temperature. Reprinted in part with permission from ref 32. Copyright 2016 John Wiley and Sons. Personal health. Reprinted in part with permission from ref 51. Copyright 2014 John Wiley and Sons. Reprinted in part with permission from ref 35. Copyright 2016 American Chemical Society. Human motion. Reprinted in part with permission from ref 31. Copyright 2015 John Wiley and Sons. Reprinted in part with permission from ref 52. Copyright 2014 John Wiley and Sons.

2. DESIRABLE FEATURES FOR ELECTROMECHANICAL SENSOR To realize the effective and large-scale applications of electromechanical sensors in human motion monitoring and human−machine interfaces, some important features (e.g., sensitivity, self-powering, self-healing, and transparency) should be considered to be incorporated into wearable sensors. 2.1. Sensitivity or Gauge Factor. Normally, stretchable conductors with high conductivity under mechanical deformation are employed as electrodes or interconnections. Stretchable conductors with high piezoresistivity are promising candidates for electromechanical sensors which can be easily

attached onto the human body. The slope of the relative change of the electrical signal (e.g., resistance and capacitance) vs applied strain or stress reflects the sensitivity or gauge factor (GF) of strain sensors. For resistive-type strain sensors, the GFs mainly rely on mechanisms such as disconnection between sensing elements, crack propagation in thin film, tunneling effect, and micro-/nanostructures of strain sensors. For instance, interlocked microdome arrays and fractured microstructure designs could enable the conductive networks to 12148

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Figure 2. (A) Schematic evolutions of the conductive pathways in CB@PU sponge during continuous compressive deformation. (B) SEM images of the microcrack junctions on compressed CB@PU sponge. Reprinted in part with permission from ref 55. Copyright 2016 John Wiley and Sons. (C) Electrical resistance change of Au NWs/latex strain sensor under various strains. GF could be derived by linear fitting. Reprinted in part with permission from ref 56. Copyright 2015 John Wiley and Sons. (D) Relative resistance change vs strain of GA/polydimethylsiloxane (PDMS) nanocomposites. Reprinted in part with permission from ref 57. Copyright 2016 American Chemical Society.

Figure 3. (A) Self-healing electronic sensor. The SHP channel loading [OMIm][PF6] functions as (a) self-healing circuit and (b) self-healing electronic sensor. Reprinted in part with permission from ref 47. Copyright 2015 John Wiley and Sons. (B) Self-powered sensitive sensor. (a) Schematics of TES. (b) Changes of the voltage with time collected from the TES stuck onto shoulder during sleep monitoring. Reprinted in part with permission from ref 64. Copyright 2016 American Chemical Society. (C) Self-powered triboelectric microfluidic nanosensor (TMN). (a) 3D schematic of TMN integrated with microchannel. (b) SEM image of Kapton nanostructure. (c) CA of water on the Kapton surface. Reprinted in part with permission from ref 45. Copyright 2015 American Chemical Society.

2.2. Linearity. Apart from sensitivity, linearity is another important parameter because it endows the signal detection with feasibility and accuracy. Linearity refers to the relationship between the relative change of the electrical signal and applied strain, which can be graphically represented as a straight line. Most of the resistive-type strain sensors exhibit linearity at low strains and nonlinearity at large strains, which brings some blocks in the information processing. Generally, the nonlinearity of strain sensors is mainly resulted from the occurrence of nonhomogenous morphology upon stretching. For example, homogeneous microcrack generation in conductive networks made of Au NWs or graphene aerogel (GA) enabled the

achieve giant tunneling piezoresistance and high pressure sensitivity.27,53 Inspired from the crack-shaped slit organs of spiders, the opening and closure of microcracks by elongation/ relaxation cycles enabled the sensor with high stretchability and sensitivity to detect multiple human motion.54 Generally, diverse mechanism and structure design are combined to enhance the sensitivity. The combination of the microcrack junction sensing mechanism and the compressive contact of CB@PU conductive backbones endowed the sponges with high sensitivity and capabilities of monitoring subtle and large motions (Figure 2A,B).55 12149

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Figure 4. (A) SWCNT/PU-PEDOT:PSS-based strain sensors. (a) Sensors attached to the human to sense skin strains. (b) Schematics of the crosssection of the sensor. (c) Transmittance spectra of the three-layer stacked sensor. Reprinted in part with permission from ref 46. Copyright 2015 American Chemical Society. (B) e-3DP of strain sensor. (a) Illustration of the embedded 3D printing process. (b) Photographs of a glove with embedded e-3DP strain sensors. (c) Resistance change under different positions. Reprinted in part with permission from ref 44. Copyright 2014 John Wiley and Sons. (C) Temperature and strain sensor. (a) Schematic structure of the integrated sensor platform. (b) Infrared thermograms of the neck. Reprinted in part with permission from ref 70. Copyright 2016 John Wiley and Sons.

to be repaired not only mechanically but also electrically, which is attractive for functional restoration. Thus, a myriad of work has been carried out to integrate self-healing ability into the current sensors.47,60−62 By incorporating thermally sensitive ionic liquids into self-healing polymer (SHP), the as-prepared sensors exhibited repeatable and identical thermal sensitivity after breaking and self-healing (Figure 3A).47 Subsequently, diverse conductive sensing channels based on single-walled CNT (SWCNT), graphene, and Ag NWs were incorporated into self-healing hydrogels, which enabled the sensors to possess fast electrical healing speed (within 3.2 s) and high selfhealing efficiency (98 ± 0.8%), and to sustain high elastic deformation (up to 1000%) with GF of 1.51.60 Although this work is quite promising toward the development of self-healing electronic sensors, additional work should be carried out to improve their stability and sensitivity. 2.4. Self-Powering. Providing long-lasting power is another desired feature for the applications of wearable sensors in human motion monitoring and personal healthcare. A number of promising technologies have recently been developed for incorporating power generation and storage devices into wearable sensor-integrated platforms.63,64 A selfpowered patchable strain sensor platform was developed based on the multifunctional Ag NW/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/PU nano-

sensors to exhibit an initial linear dependency upon the applied strain (Figure 2C,D).56,57 Interestingly, for CNT−polymer composites, linearity and sensitivity could be improved by their hybrid structures. Ni-coated CNT-epoxy nanocomposites (GF ∼ 155) and CNTs-graphite flake hybrid thin film coated on poly(ethylene terephthalate) (PET) (GF ∼ 7.8) exhibited high linearity and sensitivity, and low stretchability (ε ≤ 1.5%).2,58 Capacitive-type sensors showed excellent linearity and low sensitivity. It should be noted that the linearity is limited to a certain amount of strains, due to the variation of Poisson’s ratio at large strains. Transparent and stretchable strain sensors were fabricated, whose relative capacitance change was linearly dependent on strain (0−50% strain).59 Overall, a trade-off relationship between “high sensitivity” and “high linearity and stretchability” exists in a majority of strain sensors. In addition, highly stretchable sensors require intact morphology under large stretching, and highly sensitive sensors require considerable structure changes upon stretching, while highly linear sensors require homogeneous morphology changes upon stretching. These conflicting requirements enable the development of strain sensors with high stretchability, sensitivity, and linearity simultaneously to be a challenge. 2.3. Self-Healing. All electronic devices are subjected to damage, and thereby they would malfunction during operating. The impartation of self-healing function enables the electronics 12150

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simultaneously monitor subtle change in skin temperature and strain during human activity (Figure 4C).70 Recently, a multifunctional wearable wrist band was employed as both heater for thermotherapy and sensor for personal health and motion monitoring.71

composite, which was also used for electrodes in an all-in-one triboelectric nanogenerator (TENG) and supercapacitor (SC). More importantly, integrated sensor systems could be easily extended for integration in other sensors, such as physical, chemical, or biological sensors, and act as a new class of patchable systems.28 Very recently, a flexible and low-cost TENG based on a patterned aluminum−plastic film and an entrapped cantilever spring leaf was developed.64 The high power, sensitivity, and stability enabled TENG to be used as a self-powered sensitive triboelectric sensor (TES), which could be fixed on a shoulder of human body (Figure 3B). However, the current power generation and power capacity should be further improved to satisfy the practical applications. 2.5. Self-Cleaning. A self-cleaning function could guarantee that the electronic devices remain clean despite their surroundings. A number of works have proposed integration of a self-cleaning function into wearable sensors, guaranteeing their stable working. Carbon NPs could endow the functional devices’ surface with superhydrophobicity. A contact angle (CA; >150°) and a sliding angle ( 40%) and possessed a tunable GF (2−14) via controlling the density of Ag NWs. Thanks to thermal heating (200 °C for 20 min), the surfactant was removed and Ag NWs were fused, resulting in the decrease of electrical resistance of the Ag NWs networks. Moreover, a series of novel high-strain sensors based on crumpled graphene and graphene foams were developed.104−106 Note that graphene networks with unique crumpled morphologies or porous structures were essential to be embedded in elastomers. To further improve the sensitivity and stretchability of the strain sensors, Ha et al. 106 demonstrated a highly stretchable and sensitive strain sensor based on fragmentized graphene foam (FGF) and PDMS. Three-dimensional percolation networks were formed via contact of FGFs with 3D structure. The fabricated sensors demonstrated high stretchability over 70% and tunable GF (15−29) (Figure 6B). Compared to the partial breaking and cracking of the graphene foam when stretched, the contact area between adjacent FGFs was apparently larger, resulting in high sensitivity. As already discussed, the elastomer-embedding approach is capable of achieving a myriad of promising characteristics simultaneously, such as high conductivity, stretchability, sensitivity, and transparency by tuning the volume fraction of

building blocks. Since the approach involves the transfer printing process, the removal of the composites from the original substrates is still a challenge. Moreover, the constructed networks may hinder the infiltration of polymer, resulting in the incompletion of elastomer. Thus, the rheological behaviors of polymer solution, such as viscosity, surface tension, volatility, and wettability, should be optimized to improve the infiltration processes. 4.1.3. Design Elastomers-Constructing Conductive Network. The low compatibility between building blocks and polymers would result in fillers aggregation at low volume fractions before the formation of the percolation threshold. An alternative approach is to in situ introduce conductive blocks on top of or within elastomers. It begins by designing a stretchable matrix, and then the conductive blocks are deposited on top of or absorbed in the elastomers, or the absorbed precursors are reduced to form conductive blocks. The approach could introduce a high payload of conductive blocks without phase separation, incompatibility, and aggregation, endowing the composites with excellent mechanical and electrical behaviors. A hybrid 3D CNT/reduced GO (rGO) network was assembled on a porous PDMS (p-PDMS) elastomer (pPCG).107 The collaborative structure not only effectively prevented the aggregation of carbon nanomaterials but also simultaneously improved the conductivity of pPCG under large strains. Consequently, the electrical conductivity of the pPCG reached 27 S m−1 with only 2 wt % CNT/rGO, and the value remained constant after 5000 bending cycles and 100 stretching−releasing cycles of 5% strain (Figure 7A). In other 12155

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conductivity of 360 S m−1) by reducing the diameter of the rubber core from 2 mm to 150 μm while maintaining constant sheath thickness. In addition to single-axial prestrain, multiaxial prestrain is also employed to develop a novel scheme to detect random strain.59,111,112 Ko et al.111 intersected two prestrained Ag NWs percolation network layers to fabricate stretchable and sensitive multidimensional strain sensor, detecting principal and perpendicular directional strain. Very recently, Zhu et al.112 fabricated a hierarchically wrinkled elastic transparent conductor (HWETC) via a “balloon-blowing” method. Unlike the previous wrinkles formed under uniaxial and biaxial tension, the rGO film exhibited a hierarchical (short- and long-period) wrinkle pattern (Figure 8B). The advantage of the periodic hierarchical wrinkles endowed the HWETC structure with high conductivity (100−457 Ω □−1) and transmittance (67−85%) under stretching (>400%) and bending deformation (180°) conditions. Moreover, the HWETC-based sensors could detect human motion and spatial distribution of external force. Alternative approaches are being developed to avoid the prestrain procedure, which is still a grand challenge for largescale manufacturing processes. One approach is that the surface tension effect results in the shrinkage of the CNTs film due to the rapid evaporation of alcohol.113 It should be noted that the adhesion between the CNTs and PDMS is strong enough to hold the buckled structures, which can be reversibly stretched and compressed. Wrinkled Au NP layers are also self-assembled on shrink-memory polymer substrate via thermal heating, and the porosity, topography, and morphology of Au film could be well-tuned through the electroless deposition parameter manipulation and shrink-induced wrinkling.114,115 Moreover, conductive Ag film wrinkles are formed on an elastomer by combining polymer swelling with electroless deposition, the amplitude and wavelength of wrinkles could be tuned via the cross-linking ratio of polymer substrate.116 4.2.2. Open-Mesh Structure. High stretchability could also be achieved via introducing open-mesh geometries. When the film with open-mesh geometries is stretched, open holes deform to allow stretching, while the strips act as bending units. In detail, in-plane motion enables the open mesh to accommodate strain via its reconfiguration, while the mesh undergoes instability and defects out of plane at larger strain, enabling it to stretch greatly. Inspired from these, graphene woven fabric (GWF) assembled by intersecting micrometersized graphene ribbons possesses excellent mechanical and electrical performance. Zhu et al.117 assembled a wearable strain sensor by adhering the GWFs on PDMS and medical tape composite. The GWFs-based sensors possessed an extremely high GF of 35 under small strain of 0.2%. The GWF strain sensors could detect subtle signals. To improve the working range and sensitivity of the sensors, they118 assembled a novel GWF-based sensing system via the crisscross interlacing of graphene microribbons in an over-and-under fashion (Figure 8C). The novelty of GWF configuration lay in their unique structural characteristics, namely, network geometry, crisscross interlaced ribbons, and expected local flaws, which had a synergistic effect on improving sensitivity. Meanwhile, the stretchability was also tailored via adjusting graphene growth parameters and adopting oblique angle direction stretching. These favored engineering and process optimizations enabled the sensors to detect human motion. Additionally, conformal lamination of devices based on serpentine mesh was achieved,

work, transparent CNT films were deposited on a PDMS substrate, and a spring-like structure made of CNTs was generated upon stretching and releasing.59 Some additional techniques such as forced impregnation107 or surface treatment59 are required to ensure the penetration of blocks to the matrix. Except the conductive blocks deposited on the surface of the elastomers, they could also be infused inside the elastomers. Lee et al.108 coated poly(styrene-block-butadiene-styrene) (SBS) polymer on the surface of poly(p-phenyleneterephthalamide) (Kevlar) fiber, and the SBS-coated fiber was immersed in a 15 wt % AgCF3COO solution for 30 min to absorb Ag precursors. The absorbed Ag precursors were chemically reduced by N2H4·4H2O, generating Ag NPs inside the SBS layer. A capacitive sensor was fabricated by coating PDMS on the surface of the conductive fiber and stacking the PDMScoated fibers perpendicularly to each other, which possessed a high sensitivity of 0.21 kPa−1 in the low-pressure region and fast relaxation time of less than 10 ms (Figure 7B). Additionally, liquid-exfoliated graphene was also infused into natural rubber to create electrical conductive composites.109 To facilitate the diffusion of conductive blocks, the elastic matrix should swell in solution, resulting in the extension of molecular chains. In general, different approaches are combined to fabricate the stretchable and flexible electrodes. Highly conductive, sensitive, and stretchable composites comprising CNTs, Ag NPs, and hydroxyl-SBS (OH-SBS) polymers were fabricated.43 To this end, CNT-dispersed OH-SBS suspension was evaporated directly under mild heating conditions, followed via an iterative process of silver precursor absorption and reduction, generating large amounts of Ag NPs on both the surface and inner regions of the CNT-embedded composites. The obtained CNT-Ag NP embedded composites possessed a superior electrical conductivity of 1228 S cm−1 and a high GF of 26500 (Figure 7C). It should be noted that the compatibility between the conductive blocks and elastomer matrix, the microstructure of the elastomer, and the evaporation rate of the solvent played important roles in the above approaches. 4.2. Design of Structures That Stretch. Generally, the aforementioned conductive fillers/polymer composites are stretchable with reasonable conductivity, but the conductivity decreases with the applied strain due to the reduced contact area among the nanomaterials under larger strain. Only by further introducing stretchable structures, such as mesh-shaped or buckled structures, could the stretchable electrodes be enabled with high conductivity. 4.2.1. Buckled Structure. Buckled structure is a promising design to enable the electrodes to be stretchable. Thin conductive networks are transferred to a prestrained matrix, and buckled films are formed upon relaxation of the matrix. Uniaxial prestrain causes the linear buckles, while biaxial prestrain results in herringbone buckled patterns. Note that the adhesion between the conductive network and the substrate should be relatively strong, otherwise, resulting in poor durability. Using the prestrain−release−buckling strategy, CNT sheets (NTS) were wrapped oriented in the fiber direction on stretched rubber fiber cores.110 After releasing, the fibers exhibited distinct short- and long-period sheath buckling that occurred reversibly out of phase in the axial and belt directions (Figure 8A). Taking advantage of the hierarchically buckled structure, the fibers possessed a resistance change of less than 5% for a 1000% stretch. Moreover, the conductivity of a fiber stretched to 870% increased by a factor of 13 (realizing a 12156

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Figure 9. (A) Spring-like CNT ropes. (a) Illustration of the spinning process. (b) SEM image of CNT loops. Reprinted in part with permission from ref 120. Copyright 2012 John Wiley and Sons. (B) Graphene-based fiber with compression spring architecture. (a) Schematics of the fabrication process. (b) Current signal of the fiber sensor to a quasi-transient step strain of 0.5%. (c) Resistance variation for multiple-cycle tests at 0−30% (sensor 1) and 0−50% strain (sensor 2). (d) Response signal of wearable sensor in monitoring finger bending. Reprinted in part with permission from ref 121. Copyright 2015 John Wiley and Sons.

lies in the change of winding angle and the generated gaps, which accommodates the external strain. 4.2.4. Sponge Structure. Sponges possess 3D-interconnected porous scaffolds, which can deform to accommodate the strain under stretching, enabling them to be compressible and stretchable. Inspired from this, Yu et al.123 demonstrated a binary-network-structured PU sponge-Ag NW-PDMS (PUS-Ag NW-PDMS) stretchable conductor with high performance. For this, PU sponges with 3D-interconnected microfiber networks were dipped into the Ag NWs solution, and the microfibers were uniformly wrapped by the Ag NWs after evaporation of ethanol. The resulting PUS-Ag NWs-PDMS stretchable conductors possessed excellent electromechanical stability under high tensile strain (50%) and a small bend radius (1 mm) (Figure 10A). The superiority of such sponge structure lies in strut rotation and bending, which mitigates the accumulation of the strain in the individual struts. Similarly, a novel conductive composite was developed based on PU sponge and solution-processed thin metal coating.124 Mechanical simulation proved that the 3D sponge structure could effectively release the strain on the metallic layer by transferring the strain deformation to rotation movements, preventing the occurrence of cracks. Moreover, a series of electrodes with sponge structure have been developed via layer-by-layer (LBL) and templating approaches.125,126 4.2.5. Other Stretchable Structures. Apart from the aforementioned stretchable structures, others have been developed to achieve stretchability, such as percolation network,127 serpentine,20,128 and island bridge.129,130 Onedimensional nanomaterials tend to form a percolation network via infiltration, spray-coating, or other techniques. One-

which possessed a large resistance change under the uniaxial or biaxial strains.119 4.2.3. Coiled Structure. In general, the electrodes with 3D coiled structure are more stretchable than that with 2D serpentine structure, because the coiled structure could suppress the local stress formed in the conductive layer during the stretch. Moreover, the nonplanar motion of the coil could mitigate the local maximum strain of the 3D coiled structure effectively. Cao et al.120 reported a yarn-derived spring-like CNT rope with uniform, neat loops with perfect arrangement over a long distance. The advantage of loops opening and straightening during elongation endowed the spring-like ropes with significant elongations and tensile strains (285%), resulting in high toughness (28.7 J g−1) for fracture (Figure 9A). In addition, no degradation in the conductivity of the rope was observed after repeated strain cycles within a moderate strain of 20%. Recently, Sun et al.121 demonstrated graphene-based composite fiber with “compression spring” architectures, which possessed an ultrahigh sensitivity to tensile strain (detection limitation of 0.2% strain), a wide maximal sensing range (up to 100% strain), a fast signal response (