Dual-core capacitive microfiber sensor for smart textile applications

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Applications of Polymer, Composite, and Coating Materials

Dual-core capacitive microfiber sensor for smart textile applications Longteng Yu, Yuqin Feng, Dinesh S/O M Tamil Selven, Liangsong Yao, Ren Hao Soon, Joo Chuan Yeo, and Chwee Teck Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10937 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 21, 2019

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ACS Applied Materials & Interfaces

Dual-core capacitive microfiber sensor for smart textile applications Longteng Yu1, Yuqin Feng1,2, Dinesh S/O M Tamil Selven1, Liangsong Yao3, Ren Hao Soon1, Joo Chuan Yeo4, Chwee Teck Lim1,2,4,5* 1Department

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

Singapore. 2NUS

Graduate School for Integrative Sciences and Engineering, National University of

Singapore, Singapore 119077, Singapore. 3Department

of Materials Science and Engineering, National University of Singapore, Singapore

117575, Singapore. 4Institute

for Health Innovation and Technology, National University of Singapore, Singapore

117599, Singapore. 5Mechanobiology

Institute, National University of Singapore, Singapore 117411, Singapore.

KEYWORDS: stretchable microtube, conductive microfiber, motion sensing, electronic textile, wearable microfluidics

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ABSTRACT: Wearable sensors have garnered tremendous interest in recent years with an enormous impact on smart textile applications such as human machine interface and digital health monitoring. Here, we report a soft capacitive microfiber sensor that can be woven seamlessly into textiles for strain measurement. Comprising a dual-lumen elastomeric microtube and liquid metallic alloy, the microfiber sensor enables continual strain perception even after complete cuts. In addition, our microfiber sensor is highly stretchable and flexible, and exhibits tuneable sensitivity, excellent linearity, fast response, and negligible hysteresis. More importantly, the microfiber sensor is minimally affected by strain rate and compression during strain sensing. Even under drastic environmental changes, the microfiber sensor presents good electrical stability. By integrating the microfiber sensor imperceptibly with textiles, we devise smart textile wearables to interpret hand gestures, detect limb motion and monitor respiration rate. We believe this sensor presents an enormous potential in unobtrusive continuous health monitoring.

INTRODUCTION Flexible electronics has a profound impact on robotics, prosthetics, digital health and humanmachine interface1–5. Among these, wearable sensors are especially essential in digitizing human body motion which is critical for a myriad of applications in healthcare monitoring4,6–8, soft robotics9, prosthetics10 and human-machine interface11. To this end, many researchers have developed wearable strain sensors by incorporating conductive elements in stretchable and flexible matrices8,12–18. However, most of the sensors exist in a patch form. In contrast, fiber sensors are more favorable as they allow for better conformity and breathability for wearable and smart textiles applications19–21. To produce a fiber with mechanical sensing capabilities, researchers created a single conductive pathway in the fiber, either through filling conductive liquid into a microtube22, 2 ACS Paragon Plus Environment

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co-extruding conductive core and sheath23,24, coating conductive materials outside a fiber20,25, or mixing conductive fillers in a rubber matrix19. These fiber sensors work on resistive principle such that the resistance of the conductive pathway changes with external loading.

Alternatively, capacitive sensors are receiving more attention recently due to their higher linearity and lower hysteresis compared to resistive sensors13,17,26,27. However, the fabrication of a capacitive fiber sensor is not trivial since two conductive pathways are required in the fiber. Using a multi-layer printing technique, Lewis group fabricated a wearable capacitive fiber sensor that can detect strain, where the two conductive layers were concentrically separated by an annular dielectric layer28. Similarly, by sandwiching a rubber layer with conductive films, Baughman and co-workers demonstrated a twistable capacitive fiber that can measure strain and torsion29. More recently, Dickey group proposed a simple approach to build a capacitive strain and torsion sensor by intertwining two strands of soft conductive fibers30. These works highlight the huge potential of capacitive fiber sensors for wearable strain measurement. In view of these exciting milestones, it is desirable for us to enrich the utility of capacitive fiber sensors in smart textiles applications. To this end, the overall dimensions of the sensor should be reduced, as prior works are usually larger than 1 mm which can be incompatible with the microscale dimensions of the common fibers used in the textiles today31. As such, we aim to create a capacitive fiber with micron thickness and implement it in various smart textile applications. Furthermore, soft fiber strain sensors suffer from two key disadvantages in general, including both resistive and capacitive sensors. Firstly, in many applications, tension is usually coupled with compression. Few strain sensors have reported an accurate strain without undue influence from compression. Secondly, fiber sensors are rather fragile and prone to damage by cuts and abrasions. Even with self-healing capability, the fiber may 3 ACS Paragon Plus Environment


take hours to recover from damage, not to mention that special chemistry and synthesis processes are usually required to prepare the materials32. As such, a microfiber sensor that can measure strain with high accuracy and improved robustness is highly sought after, especially for smart textile applications.

Here, we report a dual-core wearable microfiber sensor that can be woven into textiles for capacitive strain sensing. Comprising a dual-lumen elastomeric microtube and liquid metallic alloy, the microfiber sensor is soft, compliant and highly conductive, which enables efficient transmission of electrical signals, and superior strain sensing with minimal influence from compression. Most importantly, our microfiber sensor remains functional even after destructive cuts, something which has yet to be demonstrated in prior art. The sensor exhibits tuneable sensitivities, exceptional linearity, rapid response, negligible hysteresis, and excellent stability across various environmental conditions. Furthermore, owing to its excellent stretchability, flexibility and tiny footprint, the microfiber sensor conforms readily to complex 3D curvilinear contours. This allows us to seamlessly integrate it with a variety of fabrics to build smart textiles. Following this strategy, we demonstrate hand gesture recognition, limb motion detection and respiration monitoring by wearing the smart textiles. Table S1 in supporting information compares recent works on capacitive fiber sensors and highlights the novelty and contribution of this work. With these attractive features and fascinating applications, we believe our microfiber sensor can open up new avenues for wearable electronics, robotics, prosthetics, digital health, and humanmachine interface.


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RESULTS AND DISCUSSION Structure of the microfiber sensor We constructed a microfiber sensor by filling conductive elements into a dual-lumen elastomeric microtube (Figure 1a and b). The conductive filler creates the electrical pathway to enable sensing while the microtube protects the inner cores. As will be discussed later, a liquid metallic alloy (eutectic Gallium-Indium, eGaIn) was determined to be the most suitable conductive element as compared to others (Figure 2). At the interfaces between the sheath and cores of the microfiber sensor, we anticipate the existence of a thin solid oxide skin33, which is reported to improve wetting of the liquid metal34. Due to the oxide skin and low viscosity, the liquid metal can flow easily and freely in the microtube35, enabling an instantaneous response of the microfiber sensor to mechanical stimuli. Wires were attached to one end of the microtube for connection to an external circuit (Figure 1a). The ends were then sealed with epoxy resin. The microfiber sensor is able to perceive tensile strain based on capacitive variation, which is detailed in the Supporting Information. To achieve optimal integration in textiles, we controlled the footprint of our microfiber sensor to be below 600 µm in diameter (Figure 1b). Furthermore, owing to the excellent elasticity of the microtube and shape reconfiguration of the conductive filler, the microfiber sensor was highly stretchable and flexible. We demonstrated that it can be easily knotted and threaded through a needle hole, thus allowing us to weave it onto a variety of fabric garments to perform wearable sensing.

Cutting test The ability to survive physical damage is especially important in wearable applications and our dual-core capacitive microfiber sensor provides a means to attain such functionality. To 5 ACS Paragon Plus Environment


demonstrate this, we separately prepared a single-core microfiber sensor which can respond to tensile strain based on a resistive sensing mechanism22, in addition to a dual-core microfiber sensor. We then attached the microfiber sensors on separate rubber bands (Figure 1c) and stretched the rubber bands to impose uniform tensile loading on the sensors. Both microfiber sensors showed swift responses to repeated stretch and release cycles (Figure 1d and e) before it was severed by a scalpel (Movie S1, Supporting Information). After cutting, the capacitive microfiber sensor remained functional and was able to respond well to an additional three cycles of stretching, although the baseline of its capacitance readout became lower as the length of the conductive cores was decreased (Figure 1d). Besides, eGaIn will not leak out from the opening of the severed sensor unless a critical surface stress (~0.5 N/m) is reached35. Based on Young–Laplace equation35, around 5 kPa pressure will be required to reach this critical surface stress, given the diameter of the opening is 400 µm, which is about the largest inner diameter of our microfiber sensors. When the opening is smaller, a higher pressure is needed to squeeze out eGaIn. However, even some eGaIn is squeezed out from the opening, the remaining eGaIn in the sensor will still complete the capacitor structure of the sensor, and thereby the sensor will still be functional. In comparison, the resistive microfiber sensor completely lost its sensing capability after cutting since the electrical conduit was destroyed (Figure 1e). In this regard, this ability to survive complete cuts enables our capacitive microfiber sensor to outperform its resistive counterpart. We believe the ability to survive severe damage is particularly sought after for various wearable electronics, robotics, and prosthetics applications, where functional devices will be exposed to frequent collision and abrasion.


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Figure 1. Overview of the dual-core capacitive microfiber sensor. (a) The assembly of this sensor includes a dual-lumen soft microtube filled with liquid metal, hook-up wires inserted into the cores, and epoxy resin applied to the open end and wire-tube joints. (b) Photos demonstrating the tiny size and flexibility of the sensor. Inset figures show the close-up view of the sensor and a Scanning Electron Microscope (SEM) image of its cross-section. (c) Photo showing a capacitive-type and resistive-type microfiber sensor adhered to a red and blue rubber band respectively. The sensors are uniformly stretched as the rubber bands are stretched. (d) Real-time capacitance signal shows the capacitive microfiber sensor responding well to three stretch and release cycles before and after it was severed. After cutting, the baseline of the capacitance signal decreased. (e) Real-time resistance signal shows a complete loss in function of the resistive microfiber sensor to conduct strain sensing after it was severed.

Choice of materials To achieve an optimal sensing performance, we explored different materials for both the sheath and cores of our microfiber sensor. For the sheath, two elastomers, polydimethylsiloxane (PDMS, SylgardTM 184, Dow Corning) and EcoflexTM 00-30 (Smooth-On) were investigated. These elastomers are skin-friendly, popular and commonly used in many flexible electronics applications5,36. For the fabrication of dual-lumen microtubes, we improvised the customized 7 ACS Paragon Plus Environment


coating technique developed in our previous work37 (Figure S1a, Supporting Information). Briefly, two nylon wires were coated with the elastomer by drawing through a polymer pool and then heated up in a hot chamber. Subsequently, the nylon wires were gently extracted by hand from the elastomer coating to produce a dual-lumen microtube (Figure S1b, Supporting Information). Importantly, various dimensions were achieved and the smallest dual-lumen microtube measured 350 μm in outer diameter. To compare, we performed tensile tests on the microfiber sensors made from PDMS and Ecoflex (Figure S2, Supporting Information). Their sensitivities to strain were found to be the same, showing a gauge factor of 0.19. However, the PDMS microfiber sensor snapped at 60% strain while the Ecoflex microfiber sensor was able to withstand a significantly higher strain at 350%. Therefore, we selected Ecoflex as the material for the sheath of our microfiber sensors in this study.

For the selection of the sensing element of the microfiber sensor, we shortlisted five conductive elements and tested their stability in mass and conductance over 50 days (Figure 2). These elements included 2M NaCl aqueous solution, 1-Ethyl-3-methylimidazolium dicyanamide (EMIM DCA), eutectic Gallium–Indium (eGaIn), agarose and polyacrylamide ionic hydrogels (2M NaCl added). As presented in Figure 2a and b, when exposed to the atmosphere, only eGaIn was able to keep its mass over 50 days while the rest exhibited significant mass variations in the initial 12 hours. We then filled these conductive elements into Ecoflex microtubes but could not ameliorate the mass changes (Figure 2c and d). The loss in mass of NaCl solution and ionic hydrogels was ascribed to severe dehydration38 while the increasing mass of EMIM DCA was attributed to its hygroscopic property39. Inevitably, the mass fluctuations adversely affected the conductance of the microfiber sensors (Figure 2e and f). A loss of water resulted in a breakage of electrical connection while 8 ACS Paragon Plus Environment

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moisture absorption accelerated the rate of corrosion of wires (Figure S3, Supporting Information). The corrosion of wires quickly resulted in a significant loss of conductance of the EMIM DCA filled microtube (Figure 2f and Figure S4, Supporting Information). Over 50 days, eGaIn filled microtube demonstrated perfect mass conservation (Figure 2d), consistent conductance (Figure 2f) and stable capacitance (Figure S5, Supporting Information). Therefore, eGaIn is more suitable than other conductive elements for our microfiber sensors.

Figure 2. Investigation on conductive elements for the microfiber sensor, including 2M NaCl solution, EMIM DCA, eGaIn, ionic hydrogels using agarose and polyacrylamide (PAAm). In open air, loss of mass (a) during the first 12 hours and (b) for over 50 days. Only eGaIn (green line) was stable while the rest significantly gained or lost mass. When filled into Ecoflex microtubes, loss of mass (c) during the first 12 hours and (d) for over 50 days, and change in electrical conductance (e) during the first 12 hours and (f) for over 50 days. Only eGaIn (green line) filled microtube was stable in both mass and conductance while the rest showed significant fluctuations for both parameters. In all figures, a value of -1.0 indicates a complete loss of mass or conductance.



Characteristics of the microfiber sensor Having determined the materials for the sensor, we investigated the electrical responses of the microfiber sensor to mechanical stimuli (Figure 3) and environmental fluctuations. To clearly show the variation, the capacitance signals were converted into normalized capacitive change ΔC/C0, where ΔC is the change in capacitance and C0 is the original value. We first carried out tensile tests on microfiber sensors with different dimensions (Figure 3a), in order to obtain the sensitivities and sensing ranges, as well as explore how the dimensional parameters govern the sensing capability. Results revealed an extraordinary linearity (R2=0.999) between the capacitance readout and the tensile strain across all samples. Importantly, we observed that the sensitivity was dependent on the value of λ, which is calculated by C0 and the dimensions of the microtube including the length L0, inner diameter a0 of the cores and distance d0 between them (see Supporting Information). Samples with lower λ showed higher sensitivities with the highest recorded sensitivity at 0.32 per unit of strain. In general, the sensing range was wider for samples with lower sensitivities with the largest achievable strain at 500%.

A fast response time is critical for effective real-time sensing. To examine the responsiveness of our microfiber sensors, we synchronized mechanical loading with the electrical output (Figure 3b) and computed the time lag between them (Figure S6, Supporting Information). Even at high strain rates (0.36, 0.48 and 0.60 s-1), the microfiber sensor responded almost instantly, showing a ramp response time of around 75 ms. Furthermore, we explored the effect of strain rate on the sensing performance of the microfiber sensor (Figure 3c and Movie S2, Supporting Information). In this regard, the microfiber sensor was stretched to strain of 180% at strain rates ranging from 0.006 s-


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1

to 0.6 s-1. We observed the capacitance changing linearly between 0.0 and 0.35 regardless of

different strain rates, thereby implying that strain sensing was independent of strain rate.

Next, we conducted cyclic testing to verify the durability and fidelity of the microfiber sensor. Basically, we stretched the microfiber sensor to 200% strain for 1000 cycles. Figure 3d shows no drift in the capacitance readout, and the strain sensing was equally accurate for the first ten cycles and last ten cycles. From this result, we inferred that the sensing capability did not degenerate after repeated use. Additionally, we looked into the hysteresis of the microfiber sensor during cyclic loading. Figure S7 in Supporting Information depicts the capacitive response vs. tensile strain during selected loops of the cyclic loading. We noticed that initially the capacitance output was slightly lower (0.126 vs. 0.121 at ε = 1.0) during the unloading phase than the loading phase (Figure S7a, Supporting Information). This small mismatch can be attributed to the viscoelasticity of Ecoflex40. After four cycles, the slight hysteresis vanished (Figure S7b, Supporting Information) and did not occur again (Figure S7c, d, e, Supporting Information).

In real applications, the sensor may experience both compression and tension simultaneously. Many strain sensors are unfortunately sensitive to pressure, which is unfavourable for an accurate measurement of strain. To evaluate the response to compression, we pinched a microfiber sensor using a rigid indenter (Figure S8, Supporting Information). As depicted in Figure S9 in Supporting Information, even though the pressure applied on it soared to 2400 mmHg (equivalent to 323 kPa), which is around 24 times of blood pressure, there was little changes in the capacitance of the microfiber sensor (< 4% increase). Further tests on microfibers with larger dimensions also



indicated that they were not sensitive to compression. In comparison, resistive microfiber sensors with the same dimensions exhibited significantly pronounced signals when subjected to the same forces (Figure S10, Supporting Information). Moreover, when the whole sensor sustained the same compressive force, the capacitance signal remained at a low level which is around 0.06 to 0.20 (Figure S11 and S12, Supporting Information). Hence, we concluded that the microfiber sensor can provide reliable strain measurements even when subjected to compression.

Subsequently, we considered the influence of environmental changes such as humidity and temperature on the performance of the sensor. As shown in Figure S13a in Supporting Information , the capacitance of microfiber sensor remained stable at room temperature (23 oC) even though we alternated the humidity between high (up to 75%) and low levels (down to 48%) multiple times. As for temperature, the microfiber sensor provided stable readings within the range of -20 oC to 20 oC, with a minor elevation in the capacitance at higher temperature (Figure S13b, Supporting Information). The average increase in their capacitances was calculated to be 0.23%/oC from 20 oC

to 100 oC. This slight fluctuation can be a consequence of the temperature dependent change in

the permittivity41,42 of the polymer (Ecoflex) used in the microfiber sensors. Moreover, we found that even when the environmental temperature was only -5 oC, which is much lower than the melting point (~15.5 oC)35 of eGaIn, our sensors still maintained strain sensing capability. As depicted in Figure S14, Supporting Information, after being frozen for 6 hours, the sensors showed clear and reproducible signals when they were cyclically stretched by hand in the refrigerator.


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Figure 3. Characteristics of the microfiber sensor for strain sensing. (a) Normalized capacitive change ΔC/C0 increased linearly with increasing tensile strain. Red line: a0 = 215 µm, d0 = 270 µm, L0 = 47 mm, C0 = 18 pF. Blue line: a0 = 385 µm, d0 = 580 µm, L0 = 39 mm, C0 = 16 pF. Yellow line: a0 = 125 µm, d0 = 170 µm, L0 = 27 mm, C0 = 16 pF. Green line: a0 = 185 µm, d0 = 290 µm, L0 = 28 mm, C0 = 14 pF. Grey line: a0 = 295 µm, d0 = 625 µm, L0 = 30 mm, C0 = 13 pF. (b) Dynamic capacitive responses matched well with mechanical stimuli using strain rate of 0.36, 13 ACS Paragon Plus Environment


0.48 and 0.60 s-1. (c) Dynamic capacitive response when strain of 1.8 was applied using 11 different strain rates from 0.006 to 0.6 s-1. (d) Dynamic capacitive response of 1000 cycles of tensile loading with strain of 2.0. Insets show the detailed signal during the first and last 10 cycles.

Smart textiles using the microfiber sensor We embedded the microfiber sensor into textiles to monitor body movements in real-time (Figure 4). In doing so, we demonstrated the ease of incorporating our sensors into existing textile garments. Additionally, the garments serve to accommodate any supplementary electronics for wireless transmission. As a start, we present the detection of finger motion and recognition of hand gesture, which is useful in evaluating finger mobility for rehabilitation43 and human-machine interface11. Specifically, we devised a smart glove by sewing microfiber sensors onto the index, middle and ring fingers (Figure 4a). On each finger, one microfiber sensor was positioned over the middle and proximal phalanges (Inset of Figure 4a) such that they were stretched as the fingers were bent. As shown in Figure 4b, during the ramp-hold-release motion of the index finger, the capacitance of the microfiber sensor increased and maintained, before returning to its original value as the finger was straightened (Movie S3, Supporting Information). Following that, we detected the movement of multiple fingers. Wearing the smart glove, we flexed the index finger twice, and repeated the same action for the middle finger and the ring finger successively (Figure 4c and Movie S3, Supporting Information). It is clear that the sequential action could be tracked very precisely, as the capacitance signals went up and down twice almost instantaneously as each finger was moved.

Next, we wove a microfiber sensor into a bandage (Figure 4d) and wore it as a smart textile on the body (Figure 4e). The microfiber sensor was aligned with the longitudinal direction of the bandage 14 ACS Paragon Plus Environment

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to fully capture the stretch. Compared to the glove, the smart textile can cover larger skin area and monitor a larger scale of movement. In Movie S4, Supporting Information, we wrapped the smart textile conformally around the abdomen to monitor the respiration rate and intensity. During inhalation, the strain of the smart textile was released as a result of the contraction of the abdomen, leading to a drop in the capacitance readings. Likewise, exhalation induced an increase in the capacitance. Notably, we distinguished breath with high and low frequencies using the smart bandage (Figure 4f), highlighting its potential in the evaluation of respiratory disorders such as asthma44. Furthermore, we applied the smart textile around the elbow to detect the flexion and extension of the upper limb (Figure 4g and h, Movie S4, Supporting Information). The microfiber sensor managed to responsively record the stepwise movements of the elbow (Figure 4i).

Similarly, the motion of the lower limb can be captured using the microfiber sensor. Instead of a bandage, we used a kinesiology tape which is essentially a fabric strip with adhesive on the back. We then made a smart kinesiology tape by adhering a microfiber sensor on the kinesiology tape, and subsequently attached the smart tape on the knee for motion detection (Figure 4j). As shown in Movie S5 in Supporting Information, the smart tape conformed perfectly to the knee during exercise, enabling a responsive motion detection through the microfiber sensor. Figure 4k illustrates the capacitance signal recorded in the test, including three cycles of fast squatting and three cycles of slow squatting. Notably, the signal magnitudes were almost the same for the fast and slow motions whereas the fast motion produced a higher signal frequency than the slow motion. This indicates the accuracy of this motion detection with our sensor. The slight drop in the baseline of the signal was caused by the displacement of the dangling wires (see Movie S5, Supporting Information), which can be avoided by employing wireless transmission. The real-time monitoring 15 ACS Paragon Plus Environment


of limb motion, including elbow flexion and knee bending, would be especially useful for virtual reality gaming45 and physical activity monitoring46.

Figure 4. Strain measurement using the smart textiles equipped with microfiber sensors. (a) A smart glove with three microfiber sensors woven on the index finger, middle finger and ring finger. Inset shows closer view of the microfiber sensors in the glove. Dynamic capacitive responses of the smart glove showing (b) the flexion, holding and extension of the index finger alone, and (c) the successive bending (twice) of index finger, middle finger and ring finger simultaneously. (d) A smart bandage with a microfiber sensor sewn on it. Inset shows closer view of the microfiber sensor in the bandage. (e) The smart bandage was wrapped around the abdomen for real-time respiration monitoring. Red arrow indicates the location of the sensor (Same below). (f) Dynamic capacitive responses showing fast and slow cycles of respiration detected by the smart bandage. (g) The smart bandage was wrapped around the elbow to detect the motion of the upper limb including being fully extended and (h) partially flexed. (i) Dynamic capacitive response showing the step-wise flexion and extension of the upper limb monitored through the smart bandage. (j) A 16 ACS Paragon Plus Environment

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smart kinesiology tape with a microfiber sensor adhered on it. The smart tape was attached on the knee for motion detection of the lower limb. (k) Dynamic capacitive responses showing fast and slow cycles of squat detected by the smart tape.

Conclusions We reported a dual-core capacitive microfiber sensor for smart textiles that can unobtrusively monitor tensile strain based on capacitive principle. The microfiber sensor possesses enhanced robustness such that it can remain functional even after severe damage. We examined different materials and chose liquid metallic alloy as the sensing element and Ecoflex rubber as the matrix material for microfiber. The microfiber sensor features valuable mechanical and electrical characteristics including high stretchability, high linearity, tuneable sensitivity, fast response, negligible hysteresis, and good stability in changing environments. During strain measurement, the microfiber sensor was shown to be independent of strain rate and relatively free from the influence of compression. By integrating the microfiber sensors into textile garments, we demonstrated the recognition of hand gestures and monitoring of respiration and limb motion. Taken together, our microfiber sensor possesses enormous potential for wearable electronics, robotic, prosthetics, digital health, and human-machine interface.

EXPERIMENTAL SECTION Fabrication of the dual-lumen microtube We designed and built a customized fabrication platform (Figure S1, Supporting Information) to produce dual-lumen microtubes. The platform comprises a cylindrical container for the polymer, a metal tube as heater, and a step motor to draw wires through the container and heater. Two nylon 17 ACS Paragon Plus Environment


wires were positioned in the center of the container and the heater, and connected to the motor. At the bottom of the container, each wire was passed through a small metal tube to avoid touch with the other wire. On the top of the container, a spacer was inserted between the two nylon wires to control their distance. Nylon wires were tightened and aligned vertically before fabrication started. In the final design of the microfiber sensor, we used a skin-like commercial elastomer EcoflexTM 00-30 (Smooth-On, USA) to make the dual-lumen microtube. To prepare the prepolymer, a surfactant Slide STD (Smooth-On, USA) was firstly added into Ecoflex Part B with 1.5% weight ratio, followed by mixing Ecoflex Part A and Part B with 1:1 weight ratio. The mixture (~16g) was degassed for 10 minutes to remove air bubbles and then poured into the container of the fabrication platform. Ecoflex prepolymer was coated on the nylon wires when they were drawn up by the motor. The elastomer coating were cured when the wires passed through the hot chamber (100 oC) of the heater. Finally, we pulled the nylon wires out from the Ecoflex coating to produce the dual-lumen microtube by hand. For microtubes made from Polydimethylsiloxane (PDMS, Dow Corning, USA), we mixed the base and agent with weight ratio of 10:1 and degassed for 30 minutes to prepare the prepolymer. Then we waited for the prepolymer to be viscous enough (~9 hours after degassing) before coating it on the wires using our platform. The PDMS coated wires were soaked into acetone and sonicated for 2 hours to facilitate the removal of the wires. Preparation of the conductive elements Sodium Chloride (NaCl), 1-Ethyl-3-methylimidazolium dicyanamide (EMIM DCA), eutectic Gallium-Indium (eGaIn), agarose, acrylamide (AAm), potassium persulfate (KPS), N,N’Methylenebis(acrylamide) (MbAAm), N,N,N’,N’-Tetramethylethylenediamine (TEMED) were purchased from Sigma-Aldrich and used as received. Firstly, we prepared 2M NaCl solution as one of the conductive elements, and also the solvent for ionic hydrogels. Agarose ionic hydrogel


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was made by dissolving agarose powder into near-boiling 2M NaCl solution. The weight ratio of agarose to water was 1%. To synthetize polyacrylamide (PAAm) ionic hydrogel, the monomer solution of was firstly prepared by dissolving 2M AAm in 2M NaCl solution. Then KPS (2%), MbAAm (0.5%), and TEMED (0.25%) were added into the monomer solution to trigger polymerization. The percentages represents their weight ratio to the weight of AAm in the hydrogel. Testing on the conductive elements In the first test, we exposed the conductive elements (50 μL each) to ambient air and recorded their masses using a weighing balance (EL204-IC, Mettler Toledo). In the second test, we filled each conductive element into a dual-lumen Ecoflex microtube (Inner diameter, ID, 215 µm, outer diameter, OD, 800 µm), and recorded the changes in mass using a weighing balance. In the third test, we filled each conductive element into a single-core Ecoflex microtube (ID 215 µm, OD 800 µm), and connected with wires to measure their electrical resistance, using a True RMS Industrial Multimeter (EX530, Extech Instruments). In the last test, we prepared a microfiber sensor using a dual-lumen Ecoflex microtube (ID 215 µm, OD 800 µm) and eGaIn, and measured its capacitance using a capacitance meter (U1701B, Keysight Technologies). All above samples were kept in a dry cabinet (45% humidity) unless they were taken out for measurements. The experiment was conducted under room temperature 23 oC. We measured them hourly for the first 12 hours and then once per day for 50 days. When measuring the conductance and capacitance, wires were inserted into the microtubes and in contact with the conductive elements all the time. Fabrication of the microfiber sensor We injected the liquid metallic alloy (eGaIn) into the two channels of the microtube using a syringe. The injection into each channel was done in one attempt from one end to ensure complete perfusion of the eGaIn in the microtube. Next, hook-up wires were inserted into each channel from the same 19 ACS Paragon Plus Environment


end of the microtube. The outer diameter (OD) of the syringe tip, and the diameter of the hook-up wires were chosen to be slightly larger than the inner diameter (ID) of the microtube. Finally, the open end of the microtube, and the joint of the wires and microtube were sealed with epoxy resin. Cutting of the microfiber sensor We prepared a capacitive microfiber sensor as described above and a resistive capacitive microfiber sensor using a single-core microtube, using the materials used and dimensions (ID, OD and length). We adhered each of them on a long latex balloon using Dow Corning 732 MultiPurpose Sealant. In the test, we stretched them by hand, cut them using a scalpel, and monitored their electrical signals using a digital multimeter (PXIe-4082, National Instruments). Characterization of the microfiber sensor The response of the microfiber sensor to tension and compression was characterized using an Instron Universal Mechanical Tester. In tensile tests, we glued the two ends of the microfiber sensor on two pieces of plastic using epoxy resin and clamped on the plastic sheets to pull the sample. In compressive tests, we placed the microfiber sensor on a rigid plate and pressed it using an indenter with a cylindrical tip (radius 2.5 mm, Figure S8, Supporting Information). While the mechanical tester recorded displacement and force, we used a digital multimeter (PXIe-4082, National Instruments) to record the capacitance or resistance of the sample. For the humidity test, we alternated three same microfiber sensors (ID 215 µm, OD 800 µm) in a dry cabinet and lab environment every hour. Before each shift, we measured their capacitances using a capacitance meter (U1701B, Keysight Technologies) and the cabinet/lab humidity using a digital hygrothermometer (445703, Extech Instruments). In the temperature test, we prepared another three identical microfiber sensors and measured their capacitances in lab refrigerators at -20 oC (UGL2320V Thermo Fisher Scientific) and then 4 oC (MPR-311D(H) SANYO Electric). Next, we 20 ACS Paragon Plus Environment

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put them in a temperature controllable chamber (Thermosel System, AMETEK Brookfield) and measured their capacitances from 20 oC to 100 oC. We kept the microfiber sensors in each temperature for one hour to ensure that their temperatures reached a steady state. Testing on the smart textiles We wove microfiber sensors (ID 140 µm, OD 500 µm) in a fabric glove and a stretchable bandage (Profore, Smith&Nephew) using a sewing needle. We adhered a microfiber sensor (ID 215 µm, OD 800 µm) on the cotton side of a fabric kinesiology tape (KT Tape®) using epoxy resin. Written informed consent was obtained from all participants prior to the experiments. For the glove, on index, middle and ring fingers, the microfiber sensor was positioned along the longitudinal direction and covered middle and proximal phalanges. Wires of the microfiber sensors were attached on the inside back of the glove and interconnects were exposed from the end of the glove (Figure S15, Supporting Information). Wearing the smart glove, the subject was asked to make different hand gestures. On the smart bandage, the microfiber sensor was aligned with the longitudinal (stretchable) direction so that it experienced the same extension of the bandage. To monitor respiration, we wrapped the smart bandage around the abdomen without stretch and asked the subject to breath freely. To monitor elbow flexion, we gently wrapped the smart bandage around the elbow and asked the subject to perform flexion and extension. On the smart kinesiology tape, the microfiber sensor was aligned with the longitudinal direction of the tape. To monitor knee bending, we aligned the smart kinesiology tape with the leg and attached it on the knee, and then allowed the subject to squat. In all the experiments, the real-time capacitance signals were monitored and continuously recorded using a digital multimeter (PXIe-4082, National Instruments).



ASSOCIATED CONTENT Supporting Information Working principle of the microfiber sensor, Table S1 and Figure S1-S15 (PDF) Supporting Movie 1: Microfiber Sensors Before and After Cutting (MP4) Supporting Movie 2: Tensile Test of the Microfiber Sensor (MP4) Supporting Movie 3: Microfiber Sensors Woven into a Glove for Strain Sensing (MP4) Supporting Movie 4: Microfiber Sensor Woven into a Bandage for Strain Sensing (MP4) Supporting Movie 5: Microfiber Sensor Attached on a Kinesiology Tape for Strain Sensing (MP4)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Longteng Yu: 0000-0001-7449-0303 Ren Hao Soon: 0000-0003-4188-540X Joo Chuan Yeo: 0000-0001-8759-016X Chwee Teck Lim: 0000-0003-4019-9782 Author Contributions L.Yu and C.T.L. conceived the research; L.Yu, Y.F., D.S., L.Yao, and R.H.S. performed the experiments and collected data; L.Yu analyzed data and plotted the figures; L.Yu, R.H.S., J.C.Y.,


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and C.T.L. prepared the manuscript; C.T.L. supervised the research. All authors have given approval to the final version of the manuscript. Funding Sources This research was supported by the Academic Research Fund Tier 1 Grant (R-397-000-247-112) and the Hybrid-Integrated Flexible (Stretchable) Electronic Systems Program (R-397-000-277731) at the National University of Singapore. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Research Centre of Excellence, Mechanobiology Institute, Ministry of Education Academic Research Fund Tier 1 Grant, the Hybrid-Integrated Flexible Electronic Systems (HiFES) Initiative and the MechanoBioEngineering Laboratory at the Department of Biomedical Engineering of the National University of Singapore. L.Yu thanks Dr. Chen Shi from the same laboratory for the valuable discussion and help during the early stage of this work.

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Table of Contents Graphic


Dual-core capacitive microfiber sensor for smart textile applications

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