Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber

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Highly stretchable, weavable and washable piezoresistive microfiber sensor Longteng Yu, Joo Chuan Yeo, Ren Hao Soon, Trifanny Yeo, Hong Hui Lee, and Chwee Teck Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19823 • Publication Date (Web): 27 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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ACS Applied Materials & Interfaces

Highly stretchable, weavable and washable piezoresistive microfiber sensor Longteng Yu#, †, Joo Chuan Yeo#, †, ‡, Ren Hao Soon†, Trifanny Yeo†, Hong Hui Lee†, Chwee Teck Lim*, †, ‡, ǁ †

Department of Biomedical Engineering, National University of Singapore, 117583, Singapore

‡

Mechanobiology Institute, National University of Singapore, 117411, Singapore

ǁ

Biomedical Institute for Global Health Research & Technology, National University of

Singapore, 117599, Singapore

KEYWORDS: stretchable microtube, conductive microfiber, pulse monitoring, electronic textile, wearable microfluidics

ABSTRACT: A key challenge in electronic textiles is to develop an intrinsically conductive thread of sufficient robustness and sensitivity. Here, we demonstrate an elastomeric functionalized microfiber sensor suitable for smart textile and wearable electronics. Unlike conventional conductive threads, our microfiber is highly flexible and stretchable up to 120% strain, and possesses excellent piezoresistive characteristics. The microfiber is functionalized by

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enclosing a conductive liquid metallic alloy within the elastomeric microtube. This embodiment allows shape reconfigurability and robustness, while maintaining an excellent electrical conductivity of 3.27 ± 0.08 MS/m. By producing microfibers the size of cotton threads (160 µm in diameter), a plurality of Stretchable Tubular Elastic Piezoresistive (STEP) microfibers may be woven seamlessly into a fabric to determine force location and directionality. As proof of concept, the conductive microfibers woven into a fabric glove were used to obtain physiological measurements from the wrist, elbow pit, and less accessible body parts, such as the neck and foot instep. Importantly, the elastomeric layer protects the sensing element from degradation. Experiments showed that our microfibers suffered minimal electrical drift even after repeated stretching and machine washing. These advantages highlight the unique propositions of our wearable electronics for flexible display, electronic textile, soft robotics, and consumer healthcare applications.

INTRODUCTION Significant attention has been devoted to wearable and flexible electronics technologies in the past decade.1–3 Among which, recent research in electronic textiles has revealed many smart clothing applications for electromagnetic shielding,4 wearable antennas,5 energy harvesting,6 and healthcare monitoring.7–9 Underpinning its success lies electrically conductive layers that are flexible, stretchable and lightweight. These properties enable the seamless integration of wearable electronics into woven and non-woven fabrics. Two strategies are typically employed. The first involves the forming of wearable electronics by printing metallic inks directly on the non-woven fabric.9,10 However, this requires special ink formulations for high stretchability


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without delamination.11 Furthermore, additional coating formulations are necessary to protect the ink from external environment, increasing manufacturing costs. Another strategy utilizes conductive threads woven directly into fabrics. In the past few years, many researchers have sought to improve the conductivity of the conductive threads, while maintaining its mechanical flexibility. As a result, fibers coated with either carbon-based materials (for example, graphene,12,13 carbon nanotube14–16) or various inorganic nanostructured compounds (silver,17,18 copper,19 etc.) have been constructed. While these have shown considerable success, several challenges still exist in satisfying the wearability requirements owing to the mismatch in material properties.12,13,16 Moreover, commercially available electrically conductive yarns possess limited functional sensing capabilities, making integration cumbersome and ineffective.

Increasingly, the embodiment of such smart fabrics or prosthetic skin responsive to human interface and environment stimuli has been of interest with its wide applicability towards sports, healthcare, consumer electronics and robotics.20,21 However, it requires excellent electrical conductivity, sensitivity, stability, robustness, durability, flexibility, and stretchability.10,17 While established manufacturing methods for conductive threads and printed fabrics have been reported, these electrically conductive layers often lack the mechanical flexibility and stretchability to withstand daily use and frequent washing. Besides, multiplexed sensing requires the embodiment of functional sensor packages which are extremely difficult to solder on and may be damaged during washing as well. Several research groups have reported washable conductive yarns recently,6,22–25 indicating the surge in interest for such requirements. For example, Zhao et al reported a woven electronic fabric comprising copper coated polyethylene terephthalate yarns suitable for triboelectric nanogenerators to monitor respiration rate.6


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However, the resistance of the conductive yarns increased several folds after washing which may compromise its performance. Even more recently, Le Floch et al produced a hydrogelelastomeric hybrid fiber that is both stretchable and washable, but possesses high resistivity.22

Besides solid-state sensing elements, liquid metallic alloys have been proven to be stable and ideal for sensing applications, owing to their excellent electrical conductivity.26,27 Furthermore, due to its shape reconfigurability, liquid metal within flexible elastomers enables robustness over a wide range of pressure.28,29 When forces are applied on the elastomers, the liquid metal is displaced from the microchannels, altering its electrical properties. These flexible elastomers could be created in planar geometries or even in dome structures.30 Importantly, the versatility allows for sensing in multiple dimensions for different applications. More recently, stretchable electronic fibers have been developed using a liquid metal alloy core.31 However, these fibers measure more than 500 µm in diameter, making them incompatible to cotton yarns or polyester threads.

To overcome this, we present a Stretchable Tubular Elastomeric Piezoresistive (STEP) microfiber that is soft, thin, flexible, stretchable, and washable. Electrical functionality is achieved by depositing a liquid metallic alloy, eutectic Gallium Indium (eGaIn), into an elastomeric microtubular structure. The STEP-microfibers may be woven into a fabric to produce a fully functional wearable electronics to sense force, position, and directionality. To demonstrate its robustness and durability, we subjected the functionalized fabric to typical laundering cycles in a washing machine. Due to its elasticity, the STEP-microfibers remain


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highly conductive and functional even after repeated washing. These superior properties highlight the high potential utility of our STEP-microfibers for wearable electronics into smart clothing applications. As proof-of-concept, we embedded the STEP-microfiber into a fabric glove, and applied it to various arterial sites to obtain physiological pulse waveforms. Other healthcare applications requiring pressure monitoring have also been demonstrated.

RESULTS AND DISCUSSION Fabrication and Features of STEP-microfiber. To produce STEP-microfiber, we propose a soft, flexible, and stretchable microtube made from silicone elastomer, polydimethylsiloxane (PDMS). This soft microtube serves as the insulating and deformable envelope of the STEP-microfiber (Figure S1, Supporting Information). The fabrication process follows a customized dip coating technique.32 Briefly, a thin metal wire was dipped into uncured elastomeric solution and extruded immediately to create a uniform uncured elastomeric layer around a microscale metal wire. The wire was then removed to form the elastomeric microtubular structure. EGaIn was injected into the tubular structure to form the conductive pathway. To enclose the microfibers, metal pins were inserted into the outlets and sealed with uncured elastomer. The metal pins can be easily connected to flexible PCB interconnects for electronics integration. In this work, we report a highly conductive STEPmicrofiber of 3.27 ± 0.08 MS/m, which is at least four orders of magnitude better than those previously reported.33–35 We also demonstrate the ability to produce STEP-microfibers of various lumen diameters. This versatility enables the selection of different STEP-microfibers for various fabrics. The use of the liquid metallic alloy accounts for its high conductivity and deformability


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beyond the conventional limits attributed to mechanical fracture. Figure 1 depicts the STEPmicrofiber woven in a fabric glove. The size of the microfiber (Ø160 µm) is clearly comparable to the cotton fabric yarns. Figure 1a further shows the position of the STEP-microfiber on the fabric glove. The STEP-microfiber can only be seen under a 4X optical magnification, highlighting its imperceptibility. Furthermore, the one-dimensional format of our STEPmicrofibers enables high compliance even to a highly curved three-dimensional surface of the cotton glove. In fact, the diameter of the STEP-microfiber is smaller than the size of an ant (Figure 1b). The STEP-microfiber shows high stretchability up to 120% strain (Figure 1c) and high flexibility (Movie S1, Supporting Information). Furthermore, we demonstrate excellent durability of the STEP-microfiber in repeated loading cycles. In particular, we stretched the microfiber over 100 cycles and did not observe any electrical drift or fatigue (Figure S2, Supporting Information). Importantly, its excellent characteristics in flexibility, stretchability, and durability allow the STEP-microfiber to be woven into a fabric using standard sewing and stitching processes.


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Figure 1. Highly stretchable elastomeric piezoresistive (STEP) microfiber woven in a glove. (a) Photo highlighting the STEP-microfiber woven into a fabric glove. Inset shows the magnified view with the red arrow pointing at the STEP-microfiber. Scale bar represents 15 mm. (b) Magnified image showing an ant on top of the STEP-microfiber. Scale bar represents 2 mm. (c) The STEP-microfiber can be stretched up to 120% strain. Scale bar represents 10 mm.

The normalized resistance of the STEP-microfiber under tension can be calculated as

(1)

where L represents the length of the microfiber, the subscript 0 denotes original resistance value and ∆R is the change of resistance. Notably, when the fiber is stretched, the resistance increases significantly due to the square law, implying a highly sensitive sensing element. We stretched our STEP-microtubes of various diameters, and found that the experimental results are in agreement with the theoretical model (Figure S3, Supporting Information).

On the other hand, when the STEP-microfiber is compressed, the fluid movement is different. The relationship between normalized resistance and normalized pressure could be expressed as

(2)

where α represents the ratio between the length of the constricted portion and total length, λ is the correction factor based on the ratio of the outer diameter and the inner diameter of the microtubular envelope (refer to Supporting Information). For enhanced sensitivity, the ratio of


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the outer diameter and inner diameter has to be near to unity (Figure S4, Supporting Information).

Characteristics of the STEP-microfiber upon different loads. The PDMS-based STEP-microfiber enables high stretchability without damage. A tunable diameter of STEP-microfiber (between 100 µm and 1500 µm) may be achieved, realizing different stress-strain characteristics. This tunability allows the choice of different microfibers for various applications. To compare the durability of the STEP-microfiber to the textile, we wove the STEP-microfiber into the fabric and stretched the smart textile using a universal loading machine (5848 MicroTester, Instron, Norwood, MA). Figure 2a shows the experimental setup before and after stretching. The normalized electrical resistance, defined by the resistance change over original resistance (∆R/R0), of the STEP-microfiber was recorded while the fabric was stretched (Figure 2b). Here, we observed that when the electronic textile is stretched to 35% strain, the normalized electrical resistance of the electronic textile is much lower than the STEPmicrofiber alone. This suggests that the stretching is mostly loaded on the fabric. This further indicates that our STEP microfiber is highly stretchable. In fact, the small kinks in the normalized resistance represents textile yarns breaking at various strains (inset of Figure 2b), yet the STEP-microfiber remains functional beyond these strain values. This strongly demonstrates the robustness of the STEP-microfiber under extreme deformations.

Moreover, a major requirement for wearables is dependent on the functional elements to withstand laundering procedures. Many conventional sensors suffer catastrophic failure due to


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the chemical reactions of the washing detergent coupled with heavy mechanical actions. However, in our study, the conductive and sensing element eGaIn is confined within the silicone elastomeric microfiber, protecting it from external environment. Besides, the conductive liquid maintains in its liquid-state which allows shape reconfigurability even under extreme mechanical loadings and washing. Furthermore, eGaIn reacts spontaneously to form a thin surface oxide. This oxide layer is reported to increase its overall yield stress, surface tension, and improve wetting.36,37 Importantly, the surface characteristics ensure its mechanical durability and protect the inner conductive core from chemical reaction (Figure S5, Supporting Information). To simulate washing, we wove four STEP-microfibers into a red dyed common woven fabric and subjected this textile to immersion and agitation in a beaker of 600 mL deionized water of temperatures above 32 ℃ (inset of Figure 2c and Movie S2, Supporting Information). The red dye was quickly cleansed from the fabric, showing high agitation and stirring within the beaker. The conductivity of the STEP-microfiber was measured before and after each experiment. Here, we showed that the conductivity of the STEP-microfiber remained unchanged despite continuous washing for 3 hours (Figure 2c). To further demonstrate its washability, we placed the textile in a commercial washing machine together with 2 kg of ballast and liquid detergent, and subjected it to washing steps according to the ISO 6330 standards (Movie S2, Supporting Information). The laundering steps involved repeated washing, rinsing, and spinning cycles, lasting for 35 minutes. Notably, even after six cycles of wash, we found no change in functional integrity of the STEPmicrofiber between each washing (Figure 2d). At the seventh cycle, we observed electrical discontinuity at the interconnects, where the connectors were dislodged out of position (Figure S6, Supporting Information). However, SEM images of the microfibers before and after washing showed no visible damage (Figure S6, Supporting Information). This shows that the failure was


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attributed to the connectors, and could be better improved with additional protection on the interconnects. Taken together, it proves the high robustness and durability of STEP-microfibers even under repeated washing.

Figure 2. Characteristics of the STEP-microfiber upon different loads. (a) Experimental setup of the electronic textile before stretching and after 40% strain. (b) Relationship of the normalized electrical resistance (∆R/R0) of the STEP-microfiber with strain. Inset shows a magnified view of the ∆R/R0 up to 40% strain. (c) Electrical resistance of the STEP-microfiber following washing in a beaker using magnetic stirring bar and temperature > 32 °C. Inset shows the photograph of the experimental set up. (d) Electrical resistance of the STEP-microfiber after washing cycles in a front load washing machine.


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Pressure sensing and reliability performance of the STEP-microfiber. Owing to the high conductivity of the liquid metallic alloy within the deformable elastomeric tubular envelope, the STEP-microfiber possesses force sensing capabilities as applied forces cause the liquid metallic alloy to be displaced within the microfiber. We measured the force sensing characteristics of the STEP-microfiber with different diameters (Figure 3). The force sensitivity is dependent on the ratio of the inner diameter (ID) and the outer diameter (OD) of the tubular envelope (Figure 3a). Again, this provides the choice of STEP-microfibers for different force sensing applications. Importantly, the sensitivity and detection limit of our STEPmicrofiber supersedes most of the sensors reported in the literature (Figure 3b), further highlighting the utility of our STEP-microfiber.

Furthermore, due to the small size, STEP-microfibers may be woven together in a fabric, forming a cross-stitched network (3 cm × 3 cm), as illustrated in Figure 3c. Using a plurality of STEP-microfibers, both the magnitude and the location of the force applied may be determined by the spikes in the normalized electrical resistance (∆R/R0) of corresponding STEP-microfibers. Figure 3d illustrates the electrical signals when the corresponding positions on the fabric were pressed. Here, the signal peaks denote the force magnitude. By comparing the spatiotemporal electrical signals, the position of the force applied may be established. A localized heat map may also be produced based on the peak electrical intensities, enabling position and force recognition (Figure S7, Supporting Information). Force directionality may also be computed by observing time lag of the signal peaks between STEP-microfibers (Figure 3e). For example, when the user swipes from left to right, its corresponding time delay (red bands in Figure 3e) of the electrical


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resistance between STEP-microfibers R1 and R3 denotes the force direction and its corresponding velocity. Multiple forces sensing is also possible with the STEP-microfibers woven fabric (Figure S8, Supporting Information). The results strongly illustrate the potential of our STEP-microfibers for soft robotic and wearable human-computer interface applications.

Figure 3. Pressure sensing and reliability performance of the STEP-microfiber. (a) Force sensing characteristics of the STEP-microfiber. (b) Comparison of sensitivity and pressure values of our STEP-microfiber with existing sensors reported in literature.38–45 (c) Photograph of the microfibers in a cross-stitched network. Intersection points are labelled in yellow. Microfibers are labelled with R1, R2, R3, and R4, respectively. (d) Electrical signals of the respective microfibers when the following positions on the fabric were pressed on, points A, B, C, and D, respectively. (e) Electrical signals of the respective microfibers with different swiping actions.


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Pulse monitoring using the STEP-microfiber system. Real-time pulse monitoring has been of profound importance, especially for healthcare monitoring and disease diagnosis.46,47 To demonstrate the utility of the STEP-microfiber, we wove the conductive microfiber on a fully functional fabric, such as on a finger of the fabric glove. As proof-of-concept, the user wearing the glove positioned the finger on various parts of the body, i.e., the wrist, the elbow pit, and even less accessible positions, such as the neck and the foot instep. These locations indicate the arterial blood flow to the various parts of the body. Here, arterial palpations were obtained from radial, brachial, carotid, and dorsalis pedis arteries respectively, signifying our capability to perform real-time pulse recording and heart rate monitoring (Figure 4 and Movie S3, Supporting Information). Notably, similar pulse rates were measured across all the locations on the body, indicating its high sensitivity, responsiveness, and repeatability. Furthermore, by observing the subtle pulse differences based on the reference locations on the body, we can potentially establish cardiac abnormalities,40 such as arterial stiffness, atherosclerosis, or high blood pressure. Importantly, the subtle forces from the physiological flows captured by our STEP-microfiber were recorded and displayed continuously in real-time, enabling versatility and robustness towards tele-rehabilitation applications and clinical diagnosis.


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Figure 4. Pulse monitoring using the STEP-microfiber system. (a) Photograph showing a user measuring the carotid pulse near the neck. Normalized electrical resistance (∆R/R0) indicating the pulse waveforms at the corresponding location. (b) Photograph showing a user measuring the brachial pulse near the elbow pit. ∆R/R0 indicating the pulse waveforms at the corresponding location. (c) Photograph showing a user measuring the radial pulse near the wrist. ∆R/R0 indicating the pulse waveform at the corresponding location. (d) Photograph showing a user measuring the dorsalis pedis pulse near the foot instep. ∆R/R0 indicating the pulse waveform at the corresponding location.

Other applications of the STEP-microfiber system. To further prove the versatility of the STEP-microfiber, we wove the microfiber into several fabric products. For example, we sewed the STEP-microfiber onto an elastic bandage (Profore, Smith & Nephew, UK). This conductive microfiber serves as a strain gauge on the bandage. Figure 5a shows the micrograph of the STEP-microfiber on the bandage. By measuring its


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electrical signal, the extent of the stretch on the bandage may be obtained (Figure 5b). Different strain levels on the bandage may be achieved and quantified using the STEP-microfiber. Furthermore, the signals obtained were highly responsive (

Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber

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