Recent Advancements in Flexible and Stretchable Electrodes for

Subscriber access provided by University of Newcastle, Australia

Review

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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13800 • Publication Date (Web): 10 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

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,‡,* 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, Beijing100871, China ⊥

Peking University Shenzhen Graduate School, 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 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 improve the performance and functionality of wearable sensors. Herein, this reviews 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 appropriate choice of geometric engineering as well as intrinsically stretchable materials is essential for the high-performance strain sensing. Finally, some important directions and challenges of fully sensor-integrated wearable platform are proposed to realize their potential applications for human motion monitoring and human-machine interfaces.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords:

flexible and stretchable electrodes, wearable sensor, conductive network, sensing

mechanism, desirable feature

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 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 oxide films, possess intrinsic brittle and rigid nature, which limits their applications requiring large deformation (bending, twisting, stretching, etc.) or intimate integration with curvilinear surfaces.4-9 Recently, numerous efforts have been devoted to develop 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 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 network,17, 18 helical structure,19 serpentine20 and mesh shapes21. 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/nano-structured designs have been employed to


Page 2 of 44

Page 3 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 self-healing capability, self-cleaning capability, printability and 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 reviews focuses on 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 self-cleaning) 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.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1 A brief introduction of wearable sensor platform: strategy achieving stretchability, features, and applications. Strategy achieving stretchability: Multi-core-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.


Page 4 of 44

Page 5 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. While stretchable conductors with high piezoresistivity are promising candidates for electromechanical sensors, which can be easily attached onto human body. Slope of the relative change of the electrical signal (e.g., resistance and capacitance) versus 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 achieve giant tunneling



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

piezoresistance and high pressure-sensitivity.27,53 Inspired from the crack-shaped slit organs of spiders, the opening and closure of micro-cracks 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 and B).55 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 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 non-homogenous morphology upon stretching. For example, homogeneous microcrack generation in conductive networks made of Au NWs or graphene aerogel (GA) enabled the sensors to exhibit an initial linear dependency upon the applied strain (Figure 2C and 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 In overall, a trade-off relationship between “high sensitivity” and “high linearity and stretchability” exists in 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.


Page 6 of 44

Page 7 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. 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 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 thermal-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 3.2s) and high self-healing efficiency (98±0.8%), and to sustain high elastic deformation (up to 1000%) with GF of 1.51.60 Although these work is quite promising towards the development of self-healing electronic sensors,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 self-powered patchable strain sensor platform was developed based on the multifunctional Ag NW/ poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS)/PU nanocomposite, which was also used for electrodes in triboelectric nanogenerator (TENG) and supercapacitor (SC) in all-in-one. 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.

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


Page 8 of 44

Page 9 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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. 2.5 Self-cleaning Self-cleaning function could guarantee that the electronic devices remain clean despite their surroundings. A number of work has been proposed to integrate self-cleaning function into wearable sensors, guaranteeing their stable working. Carbon NPs could endow the functional devices’ surface with superhydrophobicity. A contact angle (CA) (>150o) and a sliding angle (40%), and possessed a tunable GF (2-14) via controlling the density of Ag NWs. Thanks to thermal heating (200 oC 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. 3D 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.

Figure 6 A) Sandwich-structured PDMS/Ag NW/PDMS strain sensor. a) Schematics for assembling strain sensor. b) Motion detection of index and middle fingers. Reprinted in part with permission from ref. 103. Copyright 2014 American Chemical Society. B) FGF/PDMS strain sensor. a) Schematics for fabricating strain sensor. b) Relative resistance change vs. strain curves


Page 18 of 44

Page 19 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the sensor with different content of FGF/ isopropyl alcohol (IPA). Reprinted in part with permission from ref. 106. Copyright 2015 John Wiley and Sons. 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 percolation threshold. An alternative approach is to in situ introduce conductive blocks on top of or within elastomers. It begins by designing 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 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 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 PDMS-coated 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.

Figure 7 A) Stretchable pPCG conductor. a) Schematics of the preparation. b) Normalized electro-conductivity of the pPCG vs. the cycles. Reprinted in part with permission from ref. 107. Copyright 2014 John Wiley and Sons. B) SBS/Ag NP composite-coated Kevlar fiber. a) Schematics for fabricating conductive fiber, and surficial SEM image of the conductive fiber. b)


Page 20 of 44

Page 21 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pixelated pressure sensor to measure the spatial pressure distribution. Reprinted in part with permission from ref. 108. Copyright 2015 John Wiley and Sons. C) Stretchable and conductive CNT-Ag NP embedded composites. a) Schematics of fabrication process, and cross-sectional SEM images and EDS mapping images. b) Resistance change ratio vs. tensile strain. c) Initial conductivity vs. cycles of absorption and reduction of Ag ions. Reprinted in part with permission from ref. 43. Copyright 2016 Royal Society of Chemistry. 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, 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 above-mentioned 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 enable the stretchable electrodes 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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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 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 (180o) 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 large-scale 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 well tuned through the electroless deposition parameter manipulation and shrink-induce 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 crosslinking 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


Page 22 of 44

Page 23 of 44

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 micrometer-sized 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 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 lied 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 optimization enabled the sensors to detect human motion. Additionally, conformal lamination of devices based on serpentine-mesh was achieved, which possessed large resistance change under the uniaxial or biaxial strains.119



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8 A) Hierarchically buckled sheath-core fibers. a) Steps in fabricating an NTSm@fiber. b) Illustration of the structure of an NTSm sheath. c) SEM images of an NTS180@fiber at 100% strain. Reprinted in part with permission from ref. 110. Copyright 2015 The American Association for the Advancement of Science. B) Hierarchically wrinkled elastic transparent conductor. a) SEM images and illustrations of typical wrinkles in the rGO layer. b) Homemade system containing an electrochemical workstation and an electronic universal testing machine. c) Relative resistance change vs. time. Reprinted in part with permission from ref. 112. Copyright 2016 John Wiley and Sons. C) GWF-on-polymer sensor. a) Schematic of a GWF-based strain sensor, and top-view optical image of GWF on a Si/SiO2 substrate. b) Measured relative resistance of pulse measured in real time. Reprinted in part with permission from ref. 118. Copyright 2015 American Chemical Society. 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 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), wide maximal sensing range (up to 100% strain), fast signal response (

Recent Advancements in Flexible and Stretchable Electrodes for

Recommend Documents