Spirally Structured Conductive Composites for Highly Stretchable

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Spirally Structured Conductive Composites for Highly Stretchable, Robust Conductors and Sensors Xiaodong Wu, Yangyang Han, Xinxing Zhang, and Canhui Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06256 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 28, 2017

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ACS Applied Materials & Interfaces

Spirally Structured Conductive Composites for Highly Stretchable, Robust Conductors and Sensors Xiaodong Wu, Yangyang Han, Xinxing Zhang,* and Canhui Lu* State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, Chengdu 610065, China. Keywords: conductive composite, spiral structure, stretchable conductor, strain sensor, functional thread

Abstract:

Flexible and stretchable electronics are highly desirable for next generation devices. However, stretchability and conductivity are fundamentally difficult to combine for conventional conductive composites, which restricts their widespread applications especially as stretchable electronics. Here, we innovatively develop a new class of highly stretchable and robust conductive composites via a simple and scalable structural approach. Briefly, carbon nanotubes are spray-coated onto a self-adhesive rubber film, followed by rolling up the film completely to create a spirally layered structure within the composites. This unique spirally layered structure breaks the typical trade-off between stretchability and conductivity of traditional conductive composites and, more importantly, restrains the generation and propagation of mechanical micro-cracks in the conductive layer under strain. Benefiting from such structure-induced

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advantages, the spirally layered composites exhibit high stretchability and flexibility, good conductive stability and excellent robustness, enabling the composites to serve as highly stretchable conductors (up to 300% strain), versatile sensors for monitoring both subtle and large human activities, and functional threads for wearable electronics. This novel and efficient methodology provides a new design philosophy for manufacturing not only stretchable conductors and sensors but also other stretchable electronics, such as transistors, generators, artificial muscles, etc.

1. Introduction Research in flexible and stretchable electronics is fully fuelled by diverse technological needs for next-generation devices, such as electrodes, transistors, sensors, generators, batteries, supercapacitors, antennas, and so on.1-4 One of the oldest and most investigated strategies to prepare flexible and stretchable electronics is to incorporate conductive fillers into elastomers. This strategy allows the fabrication of numerous flexible and stretchable devices, but also presents a fatal disadvantage: the trade-off between conductivity and stretchability.5 Specifically, increasing the conductive filler loading for higher conductivity intrinsically increases the material stiffness and thus diminishes the flexibility and stretchability. This dilemma makes it difficult to simultaneously attain good electrical conductivity and high stretchability for fillerfilled conductive elastomers. Another important strategy is to engineer stretchable structures from established materials. Examples include buckled geometry,6-8 helical structure,9 wavy structure,10-11 network,12 porous structure,13 cross-stacked structure,14 etc. However, for stretchable electronics based on the aforementioned strategies, the conductive nanomaterials are generally deposited on the substrate surface and could exfoliate when exposed to abrasion and repeated deformation. Alternatively,


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stretchable electronics with serpentine structures have also been fabricated successfully,15-17 which, however, require relatively expensive and time-consuming photolithography technique. Their mechanical strength is also limited. More recently, integration of novel nanomaterials and stretchable structures,18 dynamic self-organization of fillers under stress,5 and utilization of nanoconfinement effect19 have been proven to be promising ways to construct stretchable electronics. Despite these pioneering achievements, nevertheless, complicated chemical synthesis or sophisticated assembling processes are usually involved. Therefore, developing simple, scalable, cost-efficient and eco-friendly manufacturing techniques for stretchable electronics using commercial raw materials still remains a great challenge. Nature has always been a source of inspiration for understanding the structure-function relationships in living organisms, which enables to design and fabricate new types of novel materials. Snail is a class of Gastropoda that have a spirally coiled shell for them to retract into (Figure S1a). Generally, the shell has two main functions. Firstly, the shell acts as a coiled skeleton for muscle attachment and spatially segregates their internal organs. More importantly, the shell serves as a layer for protection from mechanical injury and predators. The unique spirally coiled shell has enabled snail to survive on earth for hundreds of millions of years.20 Here, an interesting idea arises: can we design and fabricate anisotropic materials or devices with spirally layered structure by mimicking snail, as illustrated in Figure S1b? This, however, has rarely been explored and reported in literature.21 In this work, inspired by snail, we develop a new class of highly stretchable, robust and costefficient conductive composites with a spirally layered structure via a simple, scalable and ecofriendly approach. Specifically, carbon nanotubes (CNT) are uniformly spray-coated onto selfadhesive epoxidized natural rubber (ENR) films, followed by rolling the CNT/ENR films into


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fibrous composites. In the resultant CNT/ENR composites, highly dense CNT layers are segregated by spiral ENR layers, forming an alternately and spirally layered structure. This spirally layered structure realizes the directional locating of CNT in ENR matrix, which breaks the mutual restriction between mechanical and electrical characteristics and attains simultaneous enhancement of stretchability and conductivity of the CNT/ENR composites. On the other hand, such spirally layered structure not only protects the CNT layers from abrasion under diverse harsh conditions, but also restrains the generation and propagation of mechanical micro-cracks in CNT layers under strain, endowing the CNT/ENR composites with excellent conductive stability. Hence, the as-prepared CNT/ENR composites possess desirable conductivity, high stretchability, good conductive stability, tunable strain sensitivity and excellent robustness. These merits enable the composites to serve as highly stretchable conductors (up to 300% strain), versatile sensors for monitoring both tiny and large human activities, and functional threads for wearable electronics. This work demonstrates that highly stretchable and versatile electronics could be fabricated based on such cheap raw materials and simple method, opening up new opportunities for the widespread fabrication and application of stretchable electronic devices.

2. Results and Discussion The overall preparation process of the spirally layered CNT/ENR composites is illustrated in Figure 1 and recorded in Figure S2. It is worth pointing out that the elastomer used in this work should have self-adhesive behavior, which is indispensable for successfully rolling up the CNT/elastomer film into fibrous composites. Theoretically, any elastomer with self-adhesive behavior could be used to fabricate similar spirally structured composites. In this work, we chose ENR as the elastomer because of its desirable self-adhesive property, commercial availability,


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and excellent suspension stability of ENR latex which could greatly facilitate the spray-coating process. Initially, ENR latex was sprayed to form a self-adhesive and elastic rubber layer, onto which a conductive CNT layer was spray-coated uniformly. The electrical conductance of the CNT layer could be regulated by modulating the amount of sprayed CNT suspension (Figure S3). Then, the CNT/ENR film was rolled up continuously from one side until the film was scrolled completely into fibrous CNT/ENR composites. More details can be found in experimental section. Notably, no toxic or dangerous chemicals were involved during the whole fabrication process, revealing the good safety and environmental friendliness of this method. The diameter of the fibrous CNT/ENR composites could be easily modulated via varying the thickness of ENR layer (Figure 2a, Figure S4). In the obtained CNT/ENR composites, the conductive CNT layer was segregated between spiral ENR layers, yielding an alternately and spirally layered structure.

Figure 1. Schematic diagram for preparation of the spirally layered CNT/ENR composites.

As given in Figure 2b, a spirally layered structure can be seen very clearly in the sliced CNT/ENR composites under optical microscope. In SEM images, a spiral CNT layer (bright


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line) with high regularity was distinctly segregated by spiral ENR layers (dark region) over the whole cross-section of the composites (Figure 2c), forming an alternately segregated and spirally layered structure. In the central area, an Archimedes spiral pattern could be clearly recognized (Figure 2d). Under higher resolution, alternating CNT layers (≈1µm in thickness) and ENR layers could be observed more obviously (Figure 2e) in the CNT/ENR composites. It is worth pointing out that such beautiful spirally layered structure has been rarely observed and reported in other literature. Moreover, it is noticed that the CNT was directionally located between ENR layers, forming a highly dense CNT conductive layer (Figure 2f). In this conductive layer, CNT overlapped and even entangled with each other, generating a robust conductive network as confirmed by TEM observation (Figure S5). Compared with conductive composites with randomly dispersed fillers, such alternately segregated and spirally layered structure is expected to achieve high conductivity using small amount of CNT, thus to break the trade-off between conductivity and stretchability as well as to signally reduce the manufacturing cost of the conductive composites.


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Figure 2. (a) Photograph of the as-prepared CNT/ENR composites with different diameters. (b) Optical micrograph of the spirally layered CNT/ENR composites, scale bar: 200 µm. (c-f) SEM images of the spirally layered CNT/ENR composites, scale bars: 1 mm, 200 µm, 100 µm and 5 µm for c, d, e, and f, respectively.


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For traditional conductive composites prepared via directly blending (Figure 3a), they generally exhibit a trade-off between conductivity and stretchability due to the mechanical enhancement effect of the rigid fillers. As verified in Figure 3d, CNT-ENR composites (via directly blending) with higher CNT content showed higher conductivity but lower stretchability and flexibility, exhibiting the typical trade-off between mechanical and electrical characteristics, as depicted in the yellow elliptical region. In contrast, this dilemma can be easily broken via the proposed spirally layered structure design (Figure 3c). Specifically, when a small quantity of CNT (0.87 wt%) was used, the spirally layered CNT/ENR composites reached a desirable conductivity of 5.3 S/m, exhibiting a significant enhancement of conductivity (~9 orders of magnitude) compared with that of CNT-ENR composites. Moreover, the spirally layered CNT/ENR composites showed much higher stretchability (356%) and much lower Young’s modulus (1.8 MPa) compared to that of CNT-ENR composites with similar conductivity. These superior properties of the spirally layered CNT/ENR composites could be ascribed to the fact that CNT was directionally located between spiral ENR layers in the anisotropic CNT/ENR composites, forming an alternately segregated and spirally layered structure (Figure 2c-f). This unique structure significantly enhanced the stretchability and flexibility of the CNT/ENR composites without sacrificing their conductivity. More importantly, this spirally layered structure could also improve the conductive stability of the composites under deformation. As given in Figure 3e, the resistance of CNT-ENR composites with 10 wt% CNT soared dramatically under strain. With higher CNT loading, CNTENR composites exhibited slower change in electrical resistance under strain (i.e. better conductive stability). This is because that higher CNT loading resulted in denser CNT network with more junctions or entanglements between CNT, which was more resistive to deformation as


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illustrated in Figure S6. For the spirally layered CNT/ENR composites, CNT was directionally located between ENR layers, forming an extremely dense CNT layer as verified in Figure S5 and Figure 2f. Such highly dense CNT layer was more endurable and robust compared with random and sparse CNT networks in CNT-ENR composites. Hence, the spirally layered CNT/ENR composites possess much higher conductive stability. Along with the desirable conductivity, high stretchability and good flexibility, the spirally layered CNT/ENR composites reveal great potential to serve as stretchable electronics. For conductive composites based on surface coating strategy (Figure 3b), the conductive nanomaterials on elastic substrates are usually susceptible to mechanical and chemical stimuli, which may result in signal deterioration in conductivity. As a contrast experiment, we prepared CNT@ENR composites via surface coating using the same ingredients (Table S1). As depicted in Figure 3f, ENR@CNT composites showed rapid change in electrical resistance under increasing strain due to the different rigidity between conductive layer and elastic substrate. The mismatching in rigidity led to generation and propagation of mechanical micro-cracks in the conductive layer under strain, as confirmed in Figure 3g. Similar micro-cracks have also been observed by different researchers in other composite systems.22-24 Such micro-cracks could significantly deteriorate the conductance of the ENR@CNT composites (Figure 3f), hindering their application as stretchable electronics. In contrast, for the spirally layered CNT/ENR composites, the conductive CNT layer was embedded in and protected by ENR matrix (Figure S5, Figure 2f), which could restrict the generation and propagation of micro-cracks in the conductive layer under deformation.25 As demonstrated in Figure 3h, under successively increasing strain, the conductive CNT layer was stretched uniformly, and no micro-cracks were observed even under very high strain (300%), benefiting from the protective effect of the ENR


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layers. Moreover, the spirally layered structure could also endow the CNT/ENR composites with outstanding tolerance to other harsh conditions, e.g. washing, ultrasonic treatment, peeling off, 180o bending and repeated stretching (Figure S7). All these results demonstrate the excellent robustness of the spirally layered CNT/ENR composites. In addition, compared with bulk (3D) conductive composites prepared by directly blending and planar (2D) conductive composites based on surface coating, these spirally layered CNT/ENR conductive composites are in fibrous forms (quasi 1D), which means they take less space and thus allow to manufacture more compact, sophisticated and smaller electronic devices.

Figure 3. Comparison of conductive composites prepared via different methods. (a-c) Schematics for 3D conductive composites prepared via directly blending (a), 2D conductive


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composites based on surface coating (b), and quasi 1D conductive composites via roll-up approach (c). (d-e) Electrical conductivity, stretchability, and Young’s modulus (d) as well as relative resistance change (e) of CNT-ENR composites via directly blending and spirally layered CNT/ENR composites. (f) Conductive stability of CNT@ENR composites via surface coating and spirally layered CNT/ENR composites under increasing strain. (g-h) Optical microscope images showing the morphology evolution of the conductive layer in CNT@ENR composites by surface coating (g) and simulated spirally layered CNT/ENR composites (h) under increasing strain.

The spirally layered CNT/ENR composites are electrically conductive, mechanically strong and highly stretchable with the maximum tensile strength of 4.6 MPa and tolerable strain up to 356% (Figure S8a), making them good candidates as stretchable and robust electronics. Firstly, we evaluated the conductive stability of the composites under deformations. Figure 4a shows typical plots of relative resistance change (∆R/R0) and gauge factor (GF) versus strain for CNT/ENR composites with different CNT contents. It is noticed that CNT/ENR composites with higher CNT loading showed higher conductivity (Figure S9) and better conductive stability. Specifically, CNT5/ENR composites prepared with 5 mL CNT suspension exhibited good conductive stability under strain, with relative resistance change less than six folds (not exceed one order of magnitude) at high strain up to 300%, enabling them to act as stretchable conductors that require good conductive stability under deformations. In contrast, CNT1/ENR composites prepared with 1 mL CNT suspension showed relatively rapid variation in resistance under strain, making them suitable to serve as stretchable sensors that require desired sensitivity to


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deformations. Hereinafter, CNT5/ENR composites were used as stretchable conductors and CNT1/ENR composites were used as stretchable sensors unless otherwise specified. To investigate the reliability of the stretchable conductors, repeated stretching-releasing and bending-releasing cycles were applied. Figure 4b gives a plot of ∆R/R0 versus strain during the first three stretching-releasing cycles. It can be seen that the resistance increased smoothly under increasing tensile strain. In the first releasing cycle from 300% to 0% strain, the conductance of the conductor was partially recovered due to some residue strain (Figure S8b) and rearrangement of conductive fillers, which is very common for stretchable electronics.8,19,26 Subsequently, during another two bending-releasing cycles (up to strain of 300%), the resistance showed no obvious change after releasing in each cycle (except for the first cycle). This implies that a stable conductance could be achieved by pre-stretching-releasing process. Actually, during repeated stretching-releasing cycles, the resistance of the conductor exhibited a slight decrease instead of increase (Figure 4c), which is favorable for stretchable conductors. This might be caused by the reconstruction of more joints between CNT during the repeated deformation process. To intuitively present the good conductive stability of the conductors, we constructed a basic LED circuit connected by a CNT/ENR conductor during stretching. As shown in Figure 4e, when the strain increased gradually up to 300%, no obvious reduction in brightness of the LED light was observed (see Movie S1 for a real-time demonstration), revealing the satisfactory conductive stability of the CNT/ENR conductors. In practice, desirable stretchable conductors need to withstand not only tensile strain but also bending deformation. Hence, we tested the conductive stability of the CNT/ENR conductors during repeated bending. As shown in Figure 4c inset, during the first bending cycle, the resistance of the conductor had a slight increase (15%) at the bending radius of 1.0 mm and then


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partly recovered after unbending. As the bending test continued after the first cycle, the resistance became very stable during 2000 bending cycles (Figure 4c), demonstrating the good conductive stability of the CNT/ENR conductors. More intuitively, Figure 4f-i show that the illumination of the LED light remained unchanged when the conductor was bent, twined, knotted, and even partly cut, confirming the outstanding flexibility and robustness of the conductors. The results above demonstrate that the spirally layered conductors are highly stretchable with good robustness and can be stretched, folded, twisted, knotted, and even partly cut without signal deterioration in conductance, exhibiting superior stretchability and conductive stability compared with many recently reported stretchable conductors based on cross-stacked structure, CNT ribbon, buckling structure, networks, composites, and so on (Figure 4d, detailed data are listed in Table S2). Furthermore, looking more carefully, one may find that most of the reported stretchable conductors are naked. Thus, short-circuit possibly happens when they are utilized as conductors or flexible electronics. Compared with these naked conductors, an additional attractive feature of our conductors is that they are internally conducting but superficially insulating (Figure S10), resembling ordinary electric wires, which enables the CNT/ENR stretchable conductors to be utilized in a facile and safe way.


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Figure 4. Conductive stability of the CNT/ENR composites. (a) Variation in resistance and GF (inset) of CNT/ENR composites with different CNT contents under increasing strain. (b) Relative resistance change (∆R/R0) of CNT/ENR composites during the first three stretchingreleasing cycles. (c) Conductive stability of CNT/ENR composites during 2000 stretchingreleasing cycles (100% strain) and 2000 bending-releasing cycles (bending radius: 2 mm). The inset in (c) gives change in ∆R/R0 during the first three bending-releasing cycles. (d) Comparison of CNT/ENR conductors with some recently reported stretchable conductors (supporting reference 4-17). (e) Photographs showing a lighted LED light via a CNT/ENR conductor under increasing strain from 0% to 300%. (f-i) Pictures giving the stable illumination of the LED light when the CNT/ENR conductor was bent (f), twined (g), knotted (h), and partly cut (i).


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Considering the desirable conductivity and good stretchability, the CNT/ENR composites can also serve as flexible and stretchable sensors. Here, CNT1/ENR composites were employed to fabricate strain sensors due to their higher sensitivity to tensile strain. Figure 5a shows a typical plot of ∆R/R0 and GF versus strain of the CNT1/ENT sensors. It is noticed that the resistance of the sensors increased correspondingly under increasing strain, which was due to the reduced junctions between CNT under deformation. Besides, GF, defined as the ratio of ∆R/R0 versus strain, also showed an increase with strain, implying that higher strain caused faster change in conductance. The strain-dependent variation in conductance provides the prerequisite for the sensors to detect deformations. Moreover, under repeated strain loading-unloading cycles, the sensors exhibited continuous and stable current signals to strain variation in the range of 1%300% (Figure 5b), exhibiting excellent reproducibility and a wider working range than that of some recently reported strain sensors24-25,27-33 benefiting from the high stretchability. Due to the desirable strain sensing behavior, the CNT/ENR based sensors are expected to detect and monitor different human activities. As a proof-of-concept, firstly, we evaluated the capability of the CNT/ENR sensors in monitoring small-scale human physiological activities. As given in Figure 5c, a CNT/ENR sensor was attached to the neck of a tester to noninvasively detect the muscle motions near the throat. When the tester pronounced different words (e.g. tea, coffee, cocktail), the sensor generated different characteristic current patterns associated with the words, as shown in Figure 5d and recorded in Movie S2. This is because that each word caused particular movement form of the vocal muscle, thus giving distinguishing current signal. The significant difference among these current signals reveals good potential for the CNT/ENR sensors to serve as voice recognition devices.34 Furthermore, when the tester conducted other laryngeal motions by opening mouth, coughing, swallowing and shaking head, relevant


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characteristic signals could also be obtained (Figure 5e-h). It is noticed that when one motion was repeated, nearly invariable current patterns were recorded, indicating the good reproducibility and reliability of the CNT/ENR sensors. Next, the sensor was attached to the cheek near the eye to detect the eye movements (Figure 5i). As given in Figure 5j-k, looking upwards and closing eyes also caused distinguishing and repeatable current peaks, implying the feasibility of the sensor to monitor specific eye movements. So, based on the capability in monitoring small-scale physiological activities, we expect that the CNT/ENR sensors can be used to manufacture diverse health care equipment and handy control systems for physically challenged patients. Apart from small-scale physiological activity detection, the CNT/ENR based sensors are also qualified for large-scale movement monitoring due to their good stretchability, such as finger touching, wrist motions (Movie S3), elbow bending and knee bending. These results are detailedly discussed in Figure S11. Moreover, after subjected to 10,000 stretching cycles, the sensor exhibited similar sensing behavior to that of pristine sensor (Figure S12), revealing good reproducibility and durability of the sensors.


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Figure 5. Sensor application of the CNT/ENR composites. (a) Change in relative resistance and gauge factor (GF) versus increasing strain. (b) Responsive current curves under different applied strain/force in cyclic test. (c-h) Photograph showing a CNT/ENR sensor attached to the throat (c) and the recorded current signals when the tester pronounced different words (d) and conducted


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diverse physiological activities (e-h). (i-k) Picture of a sensor attached to the cheek (i) and the recorded current patterns when the tester conducted different eye movements (j-k).

Compared with bulk (3D) and planar (2D) stretchable conductive composites, our quasi 1D CNT/ENR composites could be prepared into very thin and long fibers (shown in Figure 6a) and woven into various textiles to produce electronic fabrics. Figure 6b-c shows that a thin CNT/ENR thread (≈300 µm) with spirally layered structure passed through a needle hole, which enables the threads to be sewed or woven into textiles to prepare wearable electronics. As a proof-of-concept, we sewed CNT/ENR threads into every finger part of a glove, as shown in Figure 6d. Then, the tester conducted three different finger bending-release motions, which are labeled as state I, II and III (Figure 6e, upper). It was observed that the CNT/ENR threads gave different responsive signals to different motions (Figure 6e, bottom), i.e., larger bending-release motions caused higher intensity of the current signal. Besides, we fabricated a grid of CNT/ENR threads to create a simple device that has spatial resolution (Figure 6f). When stretching the upper margin (Figure 6g-h), the estimated corresponding color contrast map based on resistance variation (Figure 6i) was consistent with the strain profile over a 2D area. These results reveal the potential application of the CNT/ENR threads in manufacturing wearable electronic fabrics.


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Figure 6. Spirally layered CNT5/ENR threads and their potential in fabricating smart textiles and wearable electronics. (a) Photograph of a long CNT/ENR thread wrapped on a glass rod. (b-c) SEM images showing a CNT/ENR thread passing through a needle hole, scale bar: 1 mm for b and 200 µm for c. (d) Photograph of the CNT/ENR threads sewed into every finger part of a glove. (e) Pictures and representative current signal of CNT/ENR threads during different bending-releasing motions. (f-i) Photographs showing CNT/ENR thread-integrated textiles before (f) and after stretching (g), along with the stretching model (h) and corresponding map of the estimated strain profile over a 2D area (i). The signal processing algorithm of the strain profile is based on the following equation: Npixel=(R/R0)row×(R/R0)column, where Npixel is the


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numerical value of each pixel, and (R/R0)row and (R/R0)column are the relative resistance on the corresponding row and column.

3. Conclusions In summary, enabled by a unique spiral structure design, we successfully developed a new class of highly stretchable, robust and cost-efficient conductive composites. The structureperformance relationship was well investigated. We also demonstrated the potential application of this strategy in manufacturing highly stretchable conductors (up to 300% strain), versatile sensors (capable of monitoring both subtle and large human activities), and wearable electronic fabrics. It is worth pointing out that only commercial raw materials were used and no toxic or dangerous chemicals were involved during the whole fabrication process, revealing attractive large-area compliance and good environmental friendliness. Besides, this simple but efficient strategy presents

a

general

methodology

for

fabrication

of

not

only stretchable

conductors/sensors but also other novel electronic devices (e.g. transistors, generators, supercapacitors, artificial muscles, etc.) using diverse nanomaterials, such as carbon black, metal nanowires or nanoparticles, graphene, single-walled CNT, and so on.

4. Experimental Section 4.1 Preparation of ENR and CNT sprayable systems A sprayable ENR aqueous system (ENR content: 15wt%) was prepared by mixing ENR latex (ENR-25, provided by Agricultural Products Processing Research Institute, Chinese Academy of Tropical Agricultural Science) and an aqueous suspension containing commercial cross-linking agents (see Table S1 for detailed formula). CNT suspension (0.1 wt%) was prepared by


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sonication of multiwall CNT (with a mean diameter of 10-20 nm and a length of 10-30 µm, obtained from Chengdu Organic Chemicals Co., Ltd Chinese Academy of Sciences) in ethanol. More specifically, the desired amount of CNT powder was directly added into alcohol to form a CNT/alcohol mixture by magnetic stirring. Then, the mixture was sonicated using an ultrasonic cell crusher (JY99-IIDN, Ningbo Xinzhi Biotechnology Co., LTD, China) at 1000 W for 20 min, yielding CNT suspension (0.1 wt%). 4.2 Preparation of spirally layered CNT/ENR composites A desired amount of ENR aqueous system was spray-coated onto a polytetrafluoroethylene (PTFE) substrate (50mm×50mm) to form an ENR film using a commercial airbrush at a distance of ~10 cm, which was accelerated by hot-air drying. Then, a desired amount of CNT suspension (0.1 wt%) was spray-coated onto the ENR film under conditions mentioned above. It should be noticed that the thickness of ENR film and the amount of CNT coated on ENR film could be well controlled by regulating the volume of sprayable systems. Sample prepared based on x g ENR aqueous system and y mL CNT suspension was labeled as CNTy/ENRx correspondingly. After dried, the CNT/ENR film was rolled up manually to create fibers with a spirally layered structure, followed by complete drying and vulcanization at 160 oC for 20 min to form the eventual spirally layered conductive composites. For stretchable conductors and sensors fabrication, two pieces of aluminum foil (20 µm in thickness), which act as two electrodes, were attached onto two ends of the CNT film and rolled into spirally layered composites, as shown in Figure S2. 4.3 Preparation of CNT-ENR, CNT@ENR, CNT@PDMS and CNT@PU composites For comparative purposes, CNT-ENR composites with different CNT loadings were prepared by direct two-roll mixing of CNT, ENR and other additives. The obtained solid mixture was cut


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into pieces and molded using compression molding and vulcanized at a temperature of 160 oC and a pressure of 10 MPa for 10 min. x%CNT-ENR and x%CNT/ENR means that CNT-ENR and CNT/ENR composites were prepared using a formula containing x wt% CNT. CNT@ENR composites were prepared by direct spray-coating CNT onto the ENR film and then vulcanized using the same conditions as CNT/ENR composites. Besides, CNT@PDMS and CNT@PU composites were prepared using a similar spray-coating method as mentioned above except using PDMS and PU as the substrates, which are two of the most used elastomers in fabrication of stretchable electronics in literature. 4.4 Characterization Scanning electron microscopy (SEM) was performed on a microscope (JEOL JSM-5600, Japan) at the operating voltage of 20 kV. The CNT/ENR composites were cut with a sharp blade and the cross-section was sputter-coated with a thin layer of Au for SEM observation. Optical microscopy was performed using a microscope (UB 200i, Chongqing UOP Photoelectric Technology Co., Ltd., China) with a digital camera. To evaluate the morphology evolution of CNT layers in CNT/ENR composites under deformation, a dense CNT layer embedded between two ENR layers, mimicking the segregated and layered structure of CNT/ENR composites, was observed under strain with an optical microscope. For transmission electron microscopy (TEM) observation, the composites were cryo-microtomed using Leica EM UC6 equipment at about -30 o

C to get ultrathin cryo-sections of 70-80 nm thickness, which were collected and directly

supported on a copper grid for observation using a transmission electron microscope (JEOL JEM-100CX, Japan). Mechanical property tests were conducted on a versatile testing machine (Instron-5560, USA) at room temperature at the tensile speed 50 mm/min. For sensor performance evaluation, the tensile speed was adjusted in different strain ranges to obtain stable


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electrical signals. The electrical conductivity was conducted by a two-point measurement with a resistance meter (UT61, Uni-Trend, China) for R2×108 Ω. The current signals of the sensors were measured in real-time by a two-point measurement with a Keithley 2601B source meter and copper clamps. For detection of diverse human activities, CNT1/ENR1 composites were employed due to their better flexibility and higher sensitivity to deformations.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Discussion of snail-inspired materials or devices with a spirally layered structure; preparation process of CNT/ENR composites; spray-coating of CNT onto ENR film; SEM images of crosssections of CNT/ENR composites; TEM images of CNT/ENR composites; evolution of percolated conductive networks fabricated with different CNT density; comparison in robustness during various harsh conditions; mechanical properties and electrical conductivity of CNT/ENR composites; anisotropic conducting behavior of CNT/ENR composites; capability of CNT/ENR sensors in detecting large-scale human motions; durability test of the spirally layered sensors; experimental vulcanization formula of CNT/ENR composites; summary of performance of some recently reported stretchable conductors (PDF) Video showing the excellent conductive stability of a CNT/ENR conductor in a LED circuit (Movie S1); movies showing the capability of CNT/ENR sensors in detecting tiny (Movie S2) and large (Movie S3) human activities (AVI)


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AUTHOR INFORMATION Corresponding author: Xinxing Zhang and Canhui Lu *E-mail address: [email protected], [email protected] Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors thank the National Natural Science Foundation of China (51673121 and 51473100) and Outstanding Young Scholars Fund of Sichuan University (2016SCU04A16) for financial support. References 1. Trung, T. Q.; Lee, N. E. Recent Progress on Stretchable Electronic Devices with Intrinsically Stretchable Components. Adv. Mater. 2017, 29, 1603167. 2. Huang, Y.; Kershaw, S. V.; Wang, Z.; Pei, Z.; Liu, J.; Huang, Y.; Li, H.; Zhu, M.; Rogach, A. L.; Zhi, C. Highly Integrated Supercapacitor-Sensor Systems via Material and Geometry Design. Small 2016, 12, 3393-3399. 3. Wang, Z.; Huang, Y.; Sun, J.; Huang, Y.; Hu, H.; Jiang, R.; Gai, W.; Li, G.; Zhi, C. Polyurethane/Cotton/Carbon Nanotubes Core-Spun Yarn as High Reliability Stretchable Strain Sensor for Human Motion Detection. ACS Appl. Mater. Interfaces 2016, 8, 24837-24843.


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Schematic diagram for preparation of the spirally layered CNT/ENR composites. 77x33mm (300 x 300 DPI)


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(a) Photograph of the as-prepared CNT/ENR composites with different diameters. (b) Optical micrograph of the spirally layered CNT/ENR composites, scale bar: 200 µm. (c-f) SEM images of the spirally layered CNT/ENR composites, scale bars: 1 mm, 200 µm, 100 µm and 5 µm for c, d, e, and f, respectively. 127x163mm (300 x 300 DPI)



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Comparison of conductive composites prepared via different methods. (a-c) Schematics for 3D conductive composites prepared via directly blending (a), 2D conductive composites based on surface coating (b), and quasi 1D conductive composites via roll-up approach (c). (d-e) Electrical conductivity, stretchability, and Young’s modulus (d) as well as relative resistance change (e) of CNT-ENR composites via directly blending and spirally layered CNT/ENR composites. (f) Conductive stability of CNT@ENR composites via surface coating and spirally layered CNT/ENR composites under increasing strain. (g-h) Optical microscope images showing the morphology evolution of the conductive layer in CNT@ENR composites by surface coating (g) and simulated spirally layered CNT/ENR composites (h) under increasing strain. 140x110mm (300 x 300 DPI)


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Conductive stability of the CNT/ENR composites. (a) Variation in resistance and GF (inset) of CNT/ENR composites with different CNT contents under increasing strain. (b) Relative resistance change (∆R/R0) of CNT/ENR composites during the first three stretching-releasing cycles. (c) Conductive stability of CNT/ENR composites during 2000 stretching-releasing cycles (100% strain) and 2000 bending-releasing cycles (bending radius: 2 mm). The inset in (c) gives change in ∆R/R0 during the first three bending-releasing cycles. (d) Comparison of CNT/ENR conductors with some recently reported stretchable conductors (supporting reference 4-17). (e) Photographs showing a lighted LED light via a CNT/ENR conductor under increasing strain from 0% to 300%. (f-i) Pictures giving the stable illumination of the LED light when the CNT/ENR conductor was bent (f), twined (g), knotted (h), and partly cut (i). 130x96mm (300 x 300 DPI)



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Sensor application of the CNT/ENR composites. (a) Change in relative resistance and gauge factor (GF) versus increasing strain. (b) Responsive current curves under different applied strain/force in cyclic test. (ch) Photograph showing a CNT/ENR sensor attached to the throat (c) and the recorded current signals when the tester pronounced different words (d) and conducted diverse physiological activities (e-h). (i-k) Picture of a sensor attached to the cheek (i) and the recorded current patterns when the tester conducted different eye movements (j-k). 187x196mm (300 x 300 DPI)


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Spirally layered CNT5/ENR threads and their potential in fabricating smart textiles and wearable electronics. (a) Photograph of a long CNT/ENR thread wrapped on a glass rod. (b-c) SEM images showing a CNT/ENR thread passing through a needle hole, scale bar: 1 mm for b and 200 µm for c. (d) Photograph of the CNT/ENR threads sewed into every finger part of a glove. (e) Pictures and representative current signal of CNT/ENR threads during different bending-releasing motions. (f-i) Photographs showing CNT/ENR threadintegrated textiles before (f) and after stretching (g), along with the stretching model (h) and corresponding map of the estimated strain profile over a 2D area (i). The signal processing algorithm of the strain profile is based on the following equation: Npixel=(R/R0)row×(R/R0)column, where Npixel is the numerical value of each pixel, and (R/R0)row and (R/R0)column are the relative resistance on the corresponding row and column. 177x146mm (300 x 300 DPI)



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Spirally Structured Conductive Composites for Highly Stretchable

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