Anomalous Stretchable Conductivity Using an Engineered Tricot

A variety of wearable devices that perform diverse functions are already conceptualized or commercially available targeting different spots in the hum...
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Yong-Hee Lee,† Yoonseob Kim,‡ Tae-Ik Lee, Inhwa Lee, Jaeho Shin,§ Hyun Soo Lee,^ Taek-Soo Kim, and Jang Wook Choi*,†,#

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Anomalous Stretchable Conductivity Using an Engineered Tricot Weave Graduate School of Energy Environment Water and Sustainability (EEWS), Department of Mechanical Engineering, §Department of Chemical and Biomolecular Engineering, and #Kolon Lifestyle Innovation (LSI) Center, Korea Advanced Institute of Science and Technology (KAIST), Yuseong-gu, Daejeon 305-701, Republic of Korea, ‡Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States, and ^Kolon Corporation, Kolon-ro 11, Gwacheon, Kyeonggi 427-709, Republic of Korea

ABSTRACT Robust electric conduction under stretching motions

is a key element in upcoming wearable electronic devices but is fundamentally very difficult to achieve because percolation pathways in conductive media are subject to collapse upon stretching. Here, we report that this fundamental challenge can be overcome by using a parameter uniquely available in textiles, namely a weaving structure. A textile structure alternately interwoven with inelastic and elastic yarns, achieved via a tricot weave, possesses excellent elasticity (strain up to 200%) in diagonal directions. When this textile is coated with conductive nanomaterials, proper textile engineering allows the textile to obtain an unprecedented 7-fold conductivity increase, with conductivity reaching 33,000 S cm 1, even at 130% strain, due to enhanced interyarn contacts. The observed stretching conductivity can be described well using a modified 3D percolation theory that reflects the weaving effect and is also utilized for stretchable electronic interconnects and supercapacitors with high performance. KEYWORDS: stretchable conductivity . wearable electronics . textile engineering . modified 3D percolation theory . tricot weave

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he human body is a main prospect of upcoming wearable IT electronics. A variety of wearable devices that perform diverse functions are already conceptualized or commercially available targeting different spots in the human body in the form of glasses, clothes, shoes, and skin patches.1 4 In adapting new IT devices onto the body, constructing devices on textile platforms is one viable direction because physical properties of textiles that support bending and stretching motions as well as coherent body integration are well aligned with the wearable capability of the devices. However, key components5 9 (field-effect transistors, displays, power sources, actuators, etc.) and their integration need to be developed based on distinctive designs and strategies as compared to the conventional counterparts based on rigid substrates. Among all key components, stretchable electronic conductors are the very first ones to establish the wearable integrated devices.10 12 The stretchable conduction is, LEE ET AL.

however, very challenging since conductivity and stretchability are mutually exclusive parameters.13,14 Available approaches so far to surpass this contradiction are (1) employing unconventional device geometries (wavy, serpentine, etc.)15 17 and (2) using intrinsically stretchable materials in the form of film matrix11,13,18,19 or yarn.20 22 The first approach has an advantage of being compatible with conventional microfabrication processes; however, its strain limit is relatively low (∼50%).3,23,24 In the case of the second approach, despite recent findings that self-organization of conductive nanomaterials can provide high tolerance for large stretching motions,19 the approach generally suffers from a significant conductivity drop in conductivity upon stretching and inferior sustainability over repeated stretch recovery cycles.25,26 In similar efforts, textiles-based electrodes have been introduced27 36 because the weaving structures of the textiles can provide certain stretchability. However, most of the woven VOL. XXX

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* Address correspondence to [email protected]. Received for review August 31, 2015 and accepted October 22, 2015. Published online 10.1021/acsnano.5b05465 C XXXX American Chemical Society

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ARTICLE Figure 1. Basic structure of the interwoven textile electrode designed using tricot method. (a) A schematic illustration of the tricot-weaving structure consisting of two kinds of yarn (green: polyester yarn, orange: Spandex yarn). (b) A focused view of the weaving structure made of both yarns. (c) A further zoom-in view of the weaving point to illustrate that each yarn consists of a bundle of fibers (the thickness of each fiber is ∼8 μm). Optical microscope images of the actual textile electrode from (d) the front and (e) the rear sides after Ag NP coating, along with a rear view schematic.

textiles reported thus far bear only inelastic yarns so their stretchability and stretchable conductivity are limited.27 29,31 (See our control samples in Figures S1g and S1h.) Although a few papers reported woven textiles comprised of both elastic and inelastic yarns,33,34 they also suffer from conductivity drop upon stretching, as observed with our control case where elastic core fibers are covered by inelastic shells (Figure S1i). Overall, the existing approaches all remain within the known boundary restricted by a trade-off relation between conductivity and stretchability. In an effort to overcome this fundamental limit, herein, we pay attention to a parameter uniquely available in textile-based devices, namely the weaving method. The textile of our interest consists of inelastic (polyester) frame yarns interwoven with elastic (Spandex) yarns, using a tricot weave. In particular, an elasticity analysis reveals that diagonally oriented textile possesses robust elasticity even under a strain up to 200%. Furthermore, additional textile engineering allows the textile coated with conductive nanomaterials to hold an unprecedented 7-fold electric conductivity increment, with the conductivity reaching 33,000 S cm 1, even at 130% strain via enhanced interyarn contacts. Given that there exists a vast LEE ET AL.

number of textiles with different weaves and yarns, this study focuses on a commercially available tricot weave to emphasize the possibly of similar opportunities with many commercial textiles based on appropriate engineering and structural understanding upon stretching. The observed stretchable conductivity can be consistently described by a modified 3D percolation theory and experimental results. The usefulness of the stretchable conductivity was demonstrated in advancing supercapacitors and light-emitting diode (LED) circuits toward stretchable forms. RESULTS AND DISCUSSION Figure 1a presents a schematic illustration of the weaving structure of the tricot-weave textile. While polyester yarns (inelastic, green line) comprise the basic framework of the textile, Spandex yarns (elastic, orange line) are woven together by being wound around the polyester loops, constituting a repeating zigzag pattern (Figures 1b and S2a). Thus, when the textile is stretched, both kinds of yarn mutually affect each other, and the given textile deforms in a collective fashion. Although a single polyester yarn is nonstretchable, the yarns in the tricot weave can be stretchable to some extent by three geometric changes from original VOL. XXX

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ARTICLE Figure 2. Stretch recovery behavior of the tricot-weave textile. (a) Schematic illustrations of initial state of the textile. (b) Comparative stress strain curves in three different directions. The green dots indicate the respective ends of the linearly increasing regimes. (c) Elastic recovery ratios at different strains. At each strain, stretch recovery was repeated 500 times. (d) SEM images of the textile after Ag NP coating, at low and high magnifications (inset). (e) Nonstretching conductivity of the textile electrode with varying Ag precursor concentrations. The blue dots represent the experimental data, and the green line is the simulated conductivity based on 3D percolation theory. (f) Dependence of electric conductivity on the strain direction.

state (Figure S2b). Due to their inherent elastic properties, the Spandex yarns enable the textile to stretch beyond the range achievable with the weaving structure. Also, each type of yarn consists of a bundle of fibers (diameter: ∼8 μm) (Figure 1c), so that the textile has enhanced adhesion with coated conductive nanomaterials.30,37,38 When viewed from the front (Figure 1d) and rear (Figure 1e) sides, the actual weaving structure turned out to match well with our schematic drawings. However, for a clear structural understanding, both yarns in the schematic graphics were drawn thinner than they actually are to allow a visualization of the underlying yarns (Figure S3). As a stretchable electrode, the textile consisting of the two kinds of yarn with a zigzag pattern (Figure 2a) has unique stretching properties because of the different characteristics of each yarn. Since Spandex has much higher elasticity (details in Figure S4), the recovery behavior of the entire textile is governed mainly by the elastic property of the Spandex yarns. However, the maximum strain of the textile depends on the weaving structure of the polyester yarns due to their nonstretchability. From the geometric consideration, the stretching mechanics can be followed along three different stretching directions: vertical, horizontal, and diagonal (Figure 2a,b). Notably, each sample showed a linear increase (Figure 2b, inset) in the stress up to a certain strain level (horizontal ε = 125%, diagonal ε = 150%, vertical ε = 250%), after which the stress rose sharply. The linear increment in the stress can be explained by a recovery process dominated by the elasticity of the Spandex yarns; this linear process was LEE ET AL.

indeed consistently simulated in the stress analyses (see Figure S5, and the 'Unit elasticity measurements of Spandex yarns' section in Supporting Information). The sharp stress increase in each direction arises from a tightening of the weaving structure and therefore depends on the orientation of the polyester yarns (details in Figure S6a d). Cyclic stretch recovery tests were also performed to examine the elasticity under various strains (Figure 2c). For each strain, the textiles underwent 500 stretch recovery cycles and exhibited relatively high recovery (>85% at ε = 200%) in all directions. In the case of diagonal elongation, in particular, elastic deformation was detected up to ε = 200% (Figure S7). Figure 2d presents scanning electron microscopy (SEM) images of the textile coated with silver nanoparticles (Ag NPs, average diameter ∼30 nm). In this investigation, we have chosen the chemical reduction method for uniform coating of Ag NPs on the 3D porous framework.11 Various Ag precursor (AgCF3COO) concentrations were tested without stretching in correlation with conductivity (Figure 2e), and 16 wt % was chosen as the main concentration for all the samples in this study because the coating, at concentrations above this level, starts to peel off. The adhesion of Ag NPs is based on van der Waals interaction and is strong enough to keep the conductivity as evidenced by an in situ conductivity test upon peeling (Movies S1 and S2). The uniform coating of Ag NPs along the surface of the yarns was confirmed by a cross-section analysis on the single yarn scale (Figure S8). The conductivity behavior was mathematically described based on the 3D percolation power law, VOL. XXX

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ARTICLE Figure 3. Prestrain approach for robust conductivity upon diagonal elongation. (a) Domain formation and chemical structure of Spandex. (b) Orientation and thickness changes of the polyester yarns during prestrain treatment. (c) The changes in resistance as a function of strain for the different degrees of prestrain. (d) Changes in conductivity with increasing strain, together with representative data from previous reports. Blue square: this work; black triangle: SBS composite/Ag NPs;11 green rhombus: polyurethane/self-organized Au NPs;19 and red circle: PVDF/composite of CNT and Ag NPs.10

with good fitting outcomes (see the “3D percolation theory”39 in Supporting Information). The conductivity values we report hereafter are obtained from these fitting outcomes. After coating of the Ag NPs, the conductivity change of the textile was monitored for the different tensile directions (Figure 2f). Both the horizontal and the diagonal elongations showed increments in resistance until certain points (ε = 75% and 50%, respectively), followed by gradual decreases upon further strain rise. In contrast, the vertical elongation exhibited a steady increase throughout the entire strain range. This result stems from the distinct interyarn contact areas, which are again governed by the tricot weave. A detailed interpretation is provided later, in Figures 4 and S9. In order for this type of material to serve as an electrode for stretchable electronics, stable electric conductivity during repeated stretch recovery motions LEE ET AL.

is highly desirable. In this regard, the diagonal elongation of the given tricot-weave textile is most promising due to its superior elasticity, but still suffers from increased resistance at low strains (Figure 2f). We resolve this limitation by concentrating on the weaving method and textile engineering. To this end, it is useful to grasp the chemical structure of Spandex. Spandex is ureaurethane copolymer that contains two types of domain, soft and hard (Figure 3a).40 Its chemical identity was verified by the Fourier transform infrared (FT-IR) spectrum (Figure S10a). The phase separation into the two kinds of domain enables high elasticity against stretching motion (Figure S10b).41 In the recovery process, in particular, the intradomain interactions in the hard domains, such as π π and hydrogen-bonding interactions, work as a driving force to recover the original dimension. This structural understanding has led us to a textile engineering strategy in which the textile is VOL. XXX

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ARTICLE Figure 4. Multiregime behavior of the prestrained (50%) textile upon diagonal elongation. (a) Stress strain curve. (b) Resistance (blue circle) and conductivity (red square, obtained from fitting the experimental values based on 3D percolation theory) at different strains. (c,d) Optical microscope images of the prestrained (50%) electrode at 0% and 80% strains, along with schematic illustrations of the corresponding polyester yarn orientations. (e) Cross-sectional thickness change as a function of strain. (f) Resistance changes (R/R0) after various stretch recovery cycles with the strain varied from ε = 20% to 80%.

heat-treated (100 C, 2 h) under prestrain conditions (Figures S10d,e) because this treatment can decrease the number of intradomain interactions in the hard domains and thus leaves the textile in a permanently stretched state.42 Importantly, this prestrained textile can avoid the initial conductivity loss due to readily available interyarn interactions, as will be discussed in the next paragraph. On a single yarn scale, stretching of a yarn inevitably impairs interparticle connections of coated conductive nanomaterials. This weakened percolation consequently increases the resistance of the yarn. Nevertheless, this intrinsic conductivity stretching contradiction can be overcome using interyarn interactions controlled by the weaving structure. As shown in Figure S2b (Supporting Information, motions 1 3), the tricot weave LEE ET AL.

allows enhanced interyarn interactions upon stretching (Figure 3b). In particular, the prestrained states after heat-treatment can activate these interyarn interactions even at the very beginning of stretching (as indicated by the polyester angles shown in Figure 3b: 55 @ε = 0% and 45 @ε = 50%), leading to a situation in which damaged percolation on the single-yarn scale can be readily compensated for. As can be seen in Figure 3c, the onset strain for interyarn interaction moves to smaller strain values as the degree of the prestrain rises. Interestingly, the conductivity of the 50% prestrained textile increased immediately from the beginning of the stretching, in contrast with other reported data10,11,19 overlaid in Figure 3d. The 50% prestrained textile presents interesting percolation behaviors because the overall stretching VOL. XXX

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ARTICLE Figure 5. 50% prestrained textile as a stretchable electrode. (a) Current voltage (I V) curves of the circuit (inset) that connects LEDs and the textile conductor in parallel when measured at different strains. (b) Magnified graph from the dotted box in (a). (c) A photograph of the testing instrument. Two linear stages are set up in a perpendicular position (initial area 5  5 cm2) for biaxial stretching. (d) Changes in resistance as a function of strain when equal strains are applied in x- and y-directions. Consistent brightness of LEDs upon (e) uniaxial (200% strain) and (f) biaxial (50% strain) stretching.

phenomena can be separated into three different regimes in terms of the stress (Figure 4a) and the resistance (Figure 4b, pristine conductivity σ = 4100 S cm 1). In regime 1 (0% e ε e 80%), the weaving structure tightens on the same plane mainly through basic stretching motions (motions 1 and 2 in Figure S2b). The basic stretching motions are reflected in the decreased angle of the V-shape polyester pattern from 45 at ε = 0% to 25 at ε = 80% (Figure 4c,d). This structural change was consistently observed in SEM images at low and high magnifications (Figure S11). The increase in the stress and the conductivity via interyarn interaction in this regime is relatively weak. In regime 2 (80% < ε e 130%), the tightening in the weaving structure can no longer be absorbed by the same plane rearrangements that occurred in regime 1; the tightening starts to lead to overplane rearrangement, as indicated by the thickness change (645 μm @ ε = 80% vs 509 μm @ ε = 130%) LEE ET AL.

(Figures 4e and S12a). This overplane tightening elevates the stress sharply (Figure 4a) and further increases the conductivity via amplified interyarn interactions (Figure 4b). In regime 3 (ε > 130%), the strain increment is no longer accommodated by tightening of the weaving structure, and so-called “over stretching”, in which polyester yarns are untied and broken, begins to take place (Figure S12b), leading to damage of the percolation pathways and a rebound of the conductivity (Figure 4b). The conductivity behavior in these three regimes is well described by a modified power law, and its excellent fitting to the experimental data validates the given approach (see details in 'Modified power law for the 50% prestrained textile' and Figures S13 17 in Supporting Information). Importantly, with a realistic strain (0% e ε e 80%, Regime 1), diagonal elongation of the Ag NP-coated electrode turned out to be quite robust, as the resistance changes (R/R0) after 500 stretch recovery VOL. XXX

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ARTICLE Figure 6. Applications of stretchable conductive electrode. (a) Charge discharge profiles and (b) gravimetric capacitances at various strains when MWCNT-coated tricot-weave textile was tested for supercapacitor electrodes. (c) Specific capacitances at various strains and current densities. (d) CV curves under with and without dynamic strain (ε = 0 50%) at a scan rate of 80 mV s 1. (e) The cycling stability under both modes. The current density is 1.0 A g 1. (f) The cycling performance under biaxially stretched (ε = 50%, red period) and recovered (ε = 0%, blue period) states.

cycles were below 1.04 at different values of strain (ε = 20, 40, 60, and 80%) (Figure 4f and inset). The stretchable conductivity of the Ag NPs coated textile (50% prestrained electrode) was further confirmed by various experiments. For evaluating stretchable conductivity, the textile electrode was set up as an interconnect bridge between two LEDs connected in parallel (Figure 5a inset), its current voltage (I V) curves were monitored, while the strain of the textile was varied (Figure 5a,b). The LEDs were turned on at V = 2.58 V, which is determined by the energy bandgap of the LEDs. The I V profiles were fairly persistent over the strain range, reconfirming the robust stretchable conduction of the prestrained textile. Indeed, highmagnification profiles (Figure 5b) showed behavior consistent with the observation in Figure 4b; the current increases up to ε = 120%, and the current LEE ET AL.

rebounds thereafter. The consistent operation observed over the great range of strain is unprecedented in previous reports10,11,43 45 that usually suffer from the conductivity drop with the strain. To simulate more realistic use, the textile was also stretched in biaxial directions (Figure 5c,d). For quantitative application of strain along the x- and y-directions, two linear stages were set up in a perpendicular position (Figure 5c). When stretched with equal strains in both directions, the stretchability of the textile was not as large as when stretched in one direction. However, the overall trend of resistance upon stretching is somewhat similar to that in the uniaxial stretching case (Figure 5d). Figure 5e,f visualizes the consistent brightness of the LEDs when the textiles were stretched in uniaxial and biaxial directions, supporting the feasibility of the textile to be used as actual conductive electrodes. VOL. XXX

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EXPERIMENTAL SECTION Preparation of Ag NP- and MWCNT-Coated Electrodes. For the Ag NP-coated electrode, the pristine textile (Tricot textile, HYUNJINKNIT, South Korea) was first dipped into a precursor solution containing 16 wt % silver trifluoroacetate (AgCF3COO, SigmaAldrich) in ethanol, followed by a drying step at room temperature for 20 min. Next, the Ag precursor was chemically reduced using hydrazine hydrate (50%) in ethanol/DI mixture (v:v = 1:1). The dipping in the hydrazine hydrate solution was controlled to be short (1 3 s) because a long exposure to hydrazine could impair ester functional groups and degrade the strength of the textile.46 The residual reducing agent was then washed several times with deionized water, and this process was followed by a drying step at 100 C for 2 h. According to thermal gravimetric analyses (TGA), the amount of the coated Ag in our main sample is 9.9 wt % (Figure S18). The MWCNT-coated electrode was prepared through a similar simple dipping process. Ten mg mL 1 sodium dodecylbenzenesulfonate (SDBS, Sigma-Aldrich) surfactant was dissolved in deionized water; 3.0 mg mL 1 MWCNTs (Hanhwa Chemical, South Korea) was then dispersed using probe-sonication at 70 W for 30 min. Next, the pristine textile was dipped into the MWCNT ink and then washed in deionized water several times, followed by a drying step at 100 C for 2 h. The mass loading of MWCNT was ∼0.8 mg cm 2. Characterization. The morphology of the conductive textile was characterized using SEM (HITACHI, S-4800) and an optical microscope (NIKON, ECLIPSE LV100ND). The chemical identity

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one stretching or recovery process. The textile SC was also tested under biaxial stretching mode (Figure 6f) by using the same equipment setup used for the experiment in Figure 5c. When stretched at ε = 50% in both directions, the capacitance was well retained, which is comparable to that at recovered state, suggesting that stretchable energy storage remains functional in biaxial strain conditions.

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To see the practical applicability of the 50% prestrained textile, the textile coated with multiwall carbon nanotubes (MWCNTs) was tested as stretchable electrodes for supercapacitor (SCs). In evaluating a SC using galvanostatic measurements with a wide strain range of 0 200%, charge discharge profiles followed almost the same track (Figure 6a), and the specific capacitance was well preserved (Figure 6b), indicating decent tolerance of the textile electrodes against large strains during its electrochemical charge discharge process. When the strain returned to 0% after the testing at 200%, the original capacitance was recovered. Furthermore, the consistent capacitance over different strains was retained when current density was varied (Figure 6c). The stable stretching operation was further supported by real-time stretch recovery tests that also showed persistent capacitance at the same strain range (Movie S3). Figure 6d shows cyclic voltammetry (CV) profiles under both with and without dynamic strain operation. In the dynamic strain mode, each charge or discharge consists of one round of stretching (ε = 50%) and recovery process at 10% strain s 1. Both profiles under with and without strains fully overlapped each other, implying that ion storage is not interrupted at all during real-time stretching motions. The textile SC maintains the initial capacitance well for 1500 stretch recovery cycles, verifying the robust nature of the textile electrode (Figure 6e). The observed capacitance fluctuation (Figure 6e, inset) is attributed to the increased conductivity during stretching; faster charge discharge was engaged compared to that in Figure 6d such that each cycle corresponds to

CONCLUSIONS In summary, many wearable electronic and energy storage devices can be expanded to unimaginable dimensions when properly integrated with textiles. The first step in realizing such a great challenge is to develop stable electric conductors that perform well even during repetitive stretching. Even though stable conduction and stretching motion are contradictory, the present investigation suggests a new direction to overcome this limitation, namely a weaving method and textile engineering. Stretchable SC was demonstrated as an immediate application that would benefit from the stretchable conductors. Moreover, the textile investigated in the present study is commercially available, and thus the introduced approach can be readily applied. Although the present tricot weave turned out to be better at achieving stretchable conduction than were other weaves and those reported previously, textiles made with various weaving methods and components (i.e., elastic/inelastic yarn ratio, etc.) can be further explored to tune conductivity and stretchability simultaneously for use in wearable electronic and energy storage devices that require different levels of stretchable conductivity.

of the functional groups for Spandex was detected by FT-IR spectroscopy (IRTracer-100, SHIMADZU). The sheet resistance of the electrode with an area of 4  4 cm2 was measured using a 4-point probe system (FPP-2400, DASOL ENG). For the resistance strain measurements, the specimens with dimensions of 5  35 mm2 were stretched at a constant displacement rate of 30 μm s 1, while the resistance was measured using a 4-point probe system (Keithley 2000 multimeter). For the cyclic resistance strain tests, the samples were stretched and recovered 500 times for each strain at a displacement rate of 1 mm s 1. Strain stress tests were conducted using a highprecision micromechanical test system (DTS Company, USA). Supercapacitor (SC) Testing. Stretchable symmetric SCs were prepared by assembling the MWCNT-coated electrodes for both sides of the electrodes. The pristine textile was also used as a separator, and 1 M sodium sulfate (Na2SO4) aqueous solution was used as the electrolyte. To support stretchable operations, nitrile rubber was used as an outer case (bluecolored in Figure 6b inset). The electrochemical properties of the SCs were galvanostatically measured in the two-electrode potential range of 0 0.8 V using a battery cycler (Bio-Logic VSP). To see the dependence of the strain on the electrochemical performance, the same galvanostatic measurements were carried out, while the SC was held in the lab-built sliding instrument, and the strain was varied. By using the same setup, CV and galvanostatic measurements were carried out under in situ stretching recovery mode.

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Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b05465. More illustrations of diverse weaving structures, details on modified 3D percolation theory, additional analyses using optical microscope, SEM. FT-IR, TGA, and mechanical tests(PDF) In situ conductivity test during peeling by scotch tapes (Ag electrode) (AVI) In situ conductivity test during peeling by scotch tapes (MWCNT electrode) (AVI) In situ stretch recovery tests while measuring electrochemical performances (AVI) Acknowledgment. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2012-R1A2A1A01011970), the KAIST End-Run Program funded by the Ministry of Science, ICT & Future Planning (N01150080), the Climate Change Research Hub Project of the KAIST EEWS Research Center (N01150039), NST (National Research Council of Science & Technology, Grant 13-2-KIST), Office of Naval Research Global (N62909-14-1-N271), the R&D program of MSIP/COMPA (2015K000214), and the KOLON Corporation, Korea, through the KOLON-KAIST Lifestyle Innovation Center Project (MI, Korea) (MI-2013- 10044519).

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