Semiliquid Metal Enabled Highly Conductive Wearable Electronics for

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Functional Inorganic Materials and Devices

Semi-liquid Metal enabled Highly Conductive Wearable Electronics for Smart Fabrics Rui Guo, Huimin Wang, Xuyang Sun, Siyuan Yao, Hao Chang, Hongzhang Wang, Jing Liu, and Yingying Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08067 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 28, 2019

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ACS Applied Materials & Interfaces

Semi-liquid Metal enabled Highly Conductive Wearable Electronics for Smart Fabrics Rui Guo, Huimin Wang, Xuyang Sun, Siyuan Yao, Hao Chang, Hongzhang Wang, Jing Liu*, and Yingying Zhang* R. Guo, H. Wang and Corresponding Author Prof. Dr. J. Liu Department of Biomedical Engineering, School of Medicine, Tsinghua University Beijing, 100084, China *E-mail: [email protected] H. Wang, Prof. Y. Y. Zhang Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China *E-mail: [email protected] S.Yao, H. Chang, Dr. X. Sun, Prof. J. Liu Beijing Key Lab of CryoBiomedical Engineering and Key Lab of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. *E-mail: [email protected]; Prof. J. Liu School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049, China

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Abstract Wearable electronics incorporating electronic components into commonly used fabrics can serve as new generation personalized healthcare systems for applications ranging from healthcare monitor to disease treatment. Conventional rigid materials including gold, silver and copper generally require to experience a complicated fabrication process to be sewed into clothes. At the same time, other high stretchable nonmetal materials such as conductive polymers generally have limitations of low electroconductivity, restricting their further applications. Recently, gallium-based liquid metals exhibit superior advantage in flexible electronics and present valuable potential in creative printing technologies. Here, we proposed a novel wearable electronics prepared through roller printing technology based on the adhesion difference of semi-liquid metal (Cu-EGaIn, eutectic gallium‐indium mixed with copper microparticles) on cotton fabrics and polyvinyl acetate (PVAC) glue. Results have showed that the surface topography and chemical interaction of fabrics and PVAC glue determines the adhesion effect with CuEGaIn mixture. The electric experiments have demonstrated the electromechanical stability of the fabricated lines on fabrics. Further, a series of smart fabrics were developed including interactive circuits, stretchable light emitting diode (LED) array and thermal management device with advantages of easy operation, low cost and large-area fabrication to show practical applications in the method. This strategy may play an important role in the design and fabrication of smart fabrics, contributing to the development of customized healthcare systems.

Keywords: Cu-EGaIn, semi-liquid metal, selective adhesion, wearable electronics, smart fabric


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Introduction With excellent compliance, wearable electronics present broad potential impact to produce a new generation smart fabric, such as wearable heating devices,1,2 large-area electronic systems,3,4 human motion detection,5,6 wearable communication devices7,8 and human-intimated system.9,10 Different from traditional rigid devices, wearable electronics require to have similar mechanical properties with human skin tissues, and usually are applied to skin or clothing. As a type of well-known cheap, good air permeability and bio-compatibility material, fabrics have attracted broad attention to produce large-area and low cost wearable electronics. Due to their high conductivity, conventional metal materials, such as gold, silver and copper, have been sewed in clothes and connected with rigid components, in the past decades.11,12 While, the huge differences in mechanical performance between rigid metal materials and fabrics disable its applications in flexible and wearable devices.13-15 At present, extensive studies have been carried out on the mechanical design of rigid metal to improve its compliance.16,17 However, this implementation strategy increases the manufacturing costs and is very complicated. Recently, numerous attempts have been made to develop new non-metal materials showing high stretchability, such as metal nanoparticle composites,18-20 carbon-based conductive materials,21-23 molybdenum disulfide (MoS2) 24 and organic conductive materials.25,26 Unfortunately, the conductivities of those non-metallic materials (carbon nanotubes: 5.03×103 S m-1, Carbon: 1.8×103S m-1, Poly(3,43 -1 ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT: PSS): 8.25×10 S m ) are usually far lower than metal materials (Ag: 6.301×107S m-1, Au: 4.52×107S m-1, Cu: 5.9×107S m-1).27 Moreover, the complicated and costly producing processes limit their scalable applications. Therefore, new materials with high conductivity, compliance, low-cost and fast fabrication present a wide range of possibilities for the further development of wearable electronics for smart fabrics. Recently, with excellent electrical conductivity (EGaIn24.5: 3.4×106 Sm-1),28 favorable flowability,29 low vapor pressure and low toxicity compared to mercury,30 gallium based liquid metals (LMs, such as gallium, gallium-indium eutectic alloys and gallium-indium-tin alloys) have attracted considerable interest in various applications, including soft robotics,31 electronics,32 biomaterials33 and tumor therapies.34 Especially, EGaIn24.5 with a melting point of 15.5 oC shows superior advantages in flexible electronics, such as stretchable sensors,35 injectable electrodes,36 reconfigurable antennas37 and self-healing circuits.38 Through combined with other metals, nanoparticles or organic polymers, liquid metal were endowed with new-emerging properties and multifunctional applications.39 Till now, due to the high surface tension and low adhesion on many surfaces, much effort has been paid on the fabrication method of patterning liquid metals on flexible substrates. For example, fabricating techniques including direct writing,40 microcontact printing,41 roller pen printing,42 masked deposition,43 atomized spraying deposition44 and laser printing45 have been developed. Besides, various materials such as copper,46 nickel47 and iron particles48 were incorporated into liquid metals to enhance the electrical conductivity as well as the wettability and adhesion behavior on various surfaces such as polyvinyl chloride (PVC), polyethylene terephthalate (PET) and paper. On the other hand, novel printing methods based on the adhesion improvement were developed via involving ethyl-2cyanoacrylate, polyurethane (PU)49 and polymethacrylates (PMA) glue50 to a variety of substrates. With the advantages of high conductivity and flexibility, the liquid metal would serve as an ideal material for



flexible electronics. In this article, we proposed and demonstrated a new wearable electronics for smart fabrics fabricated by a roller printing method based on the selective adhesion of liquid metal mixed with Cu particles (CuEGaIn) on cotton fabrics and polyvinyl acetate (PVAC) glue. Cu-EGaIn, which has a high conductivity (6×106 S m−1) and a suitable viscosity, was first printed on cotton fabrics to prepare wearable electronics. We observed the significant adhesion differences of Cu-EGaIn on cotton fabrics and PVAC glue. The printed Cu-EGaIn conductive layer showed high compliance (strain to 75%) and excellent stability (1000 cycles), due to its semi-liquid state (liquidity and viscosity). With the advantages of convenient preparation and high conductivity, a series of applications, such as interactive circuits, stretchable LED array, and thermal management device were presented, showing its great potential in health monitoring, human-machine interfaces and thermal management.

Materials and Methods Materials. The Gallium (99.999% purity) and indium (99.999% purity) were purchased from Anhui Minor New Materials Co. Ltd. The copper micro-particles (99.5% purity, mean diameter of 12um) were purchased from Beijing DK Nano Technology Co., Ltd. Gallium-indium eutectic alloys (EGaIn, 74.5% Ga and 24.5% In by weight) was prepared by heating the metals at 200 °C for 2 h. The melting point of EGaIn was 15.5 °C and its electrical conductivity was 3.4 × 106 S/m. The polyvinyl acetate (PVAC) glue was purchased from Hong Kong Yihui Co., Ltd. The Ecoflex 00-30 (Smooth-On, PA, USA) substrate was prepared by mixing its two precursors with a volume ratio of 1:1. and heated at 60 °C for 3 min. Fabrication Process of the Semi-liquid Metal (Cu-EGaIn). There was a particle internalization process between Cu particles and EGaIn performed in the NaOH solution.46 At first, the EGaIn (weight of 100g) was placed in a beaker. Later, different weight of Cu particles (weight of 5g, 10g, 15g and 20g) were scattered on the surface of EGaIn. Then, the NaOH aqueous solution (100mL, 1.0 mol L−1) was poured into the beaker, and agitated constantly by a glass bar for 3 minutes. Finally, the NaOH aqueous solution was evaporated at 100 °C for 30 minutes, and the EGaIn mixed with Cu particles was prepared, which was called Cu- EGaIn. Here, the packing ratio was defined as ϕ = mCu/mEGaln, where mCu and mEGaln represent the mass of Cu particles and EGaIn, respectively. The fabrication process of CuEGaIn was showed in Figure S1. Characterization and Electrical Signal Measurement. The standard four-point measuring method was used to measure the electrical conductivity of Cu-EGaIn with different mixing ratios. All the samples were filled into a groove (length of 750mm and cross section of 5.1 × 4.1 mm). The X-ray Photoelectron Spectroscopy (PHI Quantera II) was used to definite the composition of metal oxides on the surface of Cu-EGaIn. The Environmental Scanning Electron Microscope (Quanta 200, operating at 5 KV) was used to characterize the morphologies of the fabric, PVAC glue and Cu-EGaIn. The resistance and strain-resistance curve of Cu-EGaIn based wearable electronics on fabric were measured with a universal testing machine (Shimadzu, Model AGS-X) and a digital source meter (Keithley, Model 2400). Adhesion Characterization. We used three tests to show the remarkable selective adhesion of CuEGaIn on fabric and PVAC glue. Firstly, the contact angle meter (XG-CAMC) was used to measure the


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contact angle of the Cu-EGaIn droplets on fabric and PVAC glue. Secondly, the adhesion force of fabric and PVAC glue with Cu-EGaIn was measured by the dynamic contact angle measurement system (DCAT 11, Data Physics Instruments GmbH). Here, four rounded fabrics (woven cotton fabric, knitted cotton fabric, woven cotton fabric covered PVAC glue, and knitted cotton fabric covered PVAC glue, diameter of 6mm) were attached to a stress probe. Then, the four rounded fabrics were pushed into the Cu-EGaIn, which was placed in a container, and then pulled out. The inserting depth was 1mm, and the moving speed was 0.05 mm/s. The interaction force between fabric and Cu-EGaIn was recorded by the dynamometer and the maximal force was selected as the adhesion force. Finally, the Cu-EGaIn droplets were put on four tilted slopes covered different kinds of fabrics (woven cotton fabric, knitted cotton fabric, woven cotton fabric covered PVAC glue, and knitted cotton fabric covered PVAC glue). The photos of Cu-EGaIn droplets rolling off from the tilted slopes were recorded by a camera (Canon EOS 800D). Interactive Circuit. Here, the touch switch chip (TTP223-BA6) and two capacitors (C1=10Pf, C2=100nF) were purchased from Shenzhen Jointwel Technology Co. Ltd. Characterization of the Wearable thermal management circuit. An infrared thermal imager (Thermovision A40, FLIR, USA) was used to track the temperature evolution of the DC-powered and wireless-powered wearable thermal management circuits. A kind of thermochromic coating was printed on the woven cotton fabric to show the temperature distribution.

Results and Discussion Preparation of Cu-EGaIn based Wearable Electronics. Figure 1a shows the photos of Cu-EGaIn of different packing ratio ϕ (ϕ = mCu/mEGaln), which presented gradually enhanced shaping ability from suspensions to pastes. Meanwhile, the electrical conductivity of Cu-EGaIn gradual increased as ϕ increased, as shown in Figure S2. The increases in conductivity had been explained in previous research,46 which showed that the Cu particles were more conductive than EGaIn and enhanced its conductivity. With good shaping ability, this kind of Cu-EGaIn (ϕ=20%) was not easy to flow away when it was printed on substrates using a roller. In this research, we selected the Cu-EGaIn of packing ratio ϕ=20% (conductivity of 6 × 106 S m−1) to prepare the wearable electronics for smart fabric. X-ray photoelectronspectroscopy (XPS) was used to characterize the Cu-EGaIn oxidation state, as shown in Figure 1b. The coexistence of Ga2O and Ga2O3 in Cu-EGaIn surface was evidenced on the peaks at 19.2±0.1 and 20.2±0.1 eV. The higher binding energy peak at 18.0± 0.1 eV corresponded to Ga in the metallic state, and the peak at 16.5 eV was attributed to In in the metallic state. This result suggested that sufficient amount of oxidation of gallium covered on the surface of Cu-EGaIn, which increased its adhesion on various substrates.49,50 However, there were some agglomerates of Cu particles in EGaIn (the SEM image in Figure 1c). The uneven distribution of agglomerates resulted in the rugged surface of Cu-EGaIn. The polyvinyl acetate (PVAC) glue was a viscous water-based glue and curing into a film at 25°C for 10 minutes, as shown in Figure 1d. Figure 1e illustrates the fabrication of semi-liquid metal based wearable electronics for smart fabrics. Two kinds of weaving cotton fabrics, woven and knitted cotton fabrics, were selected as the printing



substrates. Firstly, a screen printing mask was placed on the cotton fabric. Then, PVAC glue was poured on the mask and squeezed onto the fabric by a scraper. Later, the Cu-EGaIn (ϕ=20%) was rolling printed on the cotton fabrics using a roller. Due to the significant adhesion difference of semi-liquid metal on cotton fabric and PVAC glue, the Cu-EGaIn could be only printed on PVAC glue, while not the cotton fabric. Finally, the Cu-EGaIn was encapsulated into an elastic matrix (Ecoflex). Especially, some rigid components, such as LEDs, could be connected to the Cu-EGaIn lines using the Cu tapes and then encapsulated into Ecoflex, as shown in Figure S3, 4. Different from traditional conductive materials, the Cu-EGaIn could be reshaped under external force and the good adhesion of Cu-EGaIn on PVAC glue, which made the Cu-EGaIn elongated during stretching, as shown in Figure 1f. Using this preparation method, we designed a series of wearable electronics on woven and knitted cotton fabrics, such as thermal management circuit, wireless powered LED array and interactive circuit (Figure 1g).

Figure 1. Preparation process of Cu-EGaIn based wearable electronics for smart fabric. (a) The photos of Cu-EGaIn of different packing ratio ϕ. (b) Ga 2p fitted XPS spectra of the Cu-EGaIn. (c) The surface SEM image and internal structure schematic diagram of Cu-GaIn. (d) The photo of PVAC glue and its molecular formula. (e) Fabrication of semi-liquid metal based wearable electronics for smart fabrics. (f)


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Schematic diagram of Cu-EGaIn on fabric and PVAC glue during stretching. (g) Illustration showing large area wearable electronics for smart fabric. Adhesion of Cu-EGaIn on Fabrics and PVAC Glue. We observed the remarkable selective adhesion of Cu-EGaIn on fabrics and PVAC glue. We first showed the adhesion of Cu-EGaIn on four substrates (woven cotton fabric, knitted cotton fabric, woven cotton fabric covered PVAC glue and knitted cotton fabric covered PVAC glue) using three tests (contact angle test, push-and-pull test and tilted slopes test). Figure 2a showed the contact angle images of Cu-EGaIn droplets on four substrates. The contact angles of Cu-EGaIn droplets on woven and knitted cotton fabrics were 150° and 131°, while these on two kinds of fabrics covered PVAC glue were 53° and 65°, as shown in Figure 2b. According to the previous research,51 the higher contact angle between liquid metal droplets and substrate mean low wettability. It was obvious that the wettability of Cu-EGaIn droplets on fabrics were significantly higher than that on PVAC glue. Furthermore, a push-and-pull test was adopted to characterize the adhesion forces of Cu-EGaIn on four substrates. These curves in Figure 2c showed the force on the probe during the push-and-pull process. Adhesion force increased sharply as the probe began to pull-out the sample, demonstrating there was some Cu-EGaIn adhered to the probe. It could be seen that during this process the adhesion force on the probe covered PVAC glue increased faster than woven and knitted cotton fabrics, which mean that more Cu-EGaIn was adhered to the PVAC glue than these fabrics, as shown in the inset photos of Figure 2c. The peak of woven and knitted cotton fabrics covered PVAC glue (1.67mN and 1.81mN) further reached about 7 times higher than that of woven and knitted cotton fabrics (0.23mN and 0.26mN). Finally, we found that the Cu-EGaIn droplets could not remain on woven and knitted cotton fabrics stably and would roll away at incline angle of 46° and 32° (Figure 2d). In contrast, the Cu-EGaIn droplets would be adhered to the PVAC glue and not roll away even at the vertical plane, as shown in Figure S5. In order to reveal the principle of the selective adhesion of Cu-EGaIn on fabrics and PVAC glue, the surface morphology of various surface was characterized, as shown in Figure 2e. SEM micrographs showed that the woven and knitted cotton fabrics had a very rough surface morphology, which decreased the effective contact area between Cu-EGaIn oxide layer and substrates, further prevented Cu-EGaIn adhere to surface. On the contrary, the PVAC glue printed on fabrics was filled into the fiber gaps and formed smooth film, which provided enough contact area for Cu-EGaIn oxide layer. The SEM images of Cu-EGaIn lines also exhibited that the Cu-EGaIn only adhered to smooth PVAC glue, while not rough cotton fabrics. Besides, the previous research had demonstrated that there was hydrogen bond interaction between Cu-EGaIn oxide layer and polymer glue.50



Figure 2. Adhesion characterization of Cu-EGaIn on fabrics and PVAC glue. (a) Photos of Cu-EGaIn droplets on four substrates. (b) The contact angle images of Cu-EGaIn droplets on four substrates. (c) The adhesion force curve of Cu-EGaIn on four substrates and the photos of probes (woven cotton fabric with & without PVAC glue) pulled out from Cu-EGaIn. (d) Incline angles of four substrates when the Cu-EGaIn droplets rolled away. (e) SEM micrographs of woven and knitted cotton fabrics, printed PVAC glue and printed Cu-EGaIn (scale bar = 500 um). Electrical Performance of the Cu-EGaIn Electronics on Fabrics The Cu-EGaIn electronics on woven and knitted cotton fabrics showed excellent electrical stability. As shown in Figure 3a, the resistance of Cu-EGaIn lines (line length of 8cm) increased gradually with the decrease of the lines width (n=4). However, the resistance of Cu-EGaIn lines with different width was not proportional, which was mainly due to the uneven cross-sectional morphology of Cu-EGaIn (Figure 3b). Considering the difference in tensile properties, the twisting and bending experiments were carried out using the Cu-EGaIn lines on woven cotton fabrics (width of 1mm, length of 3cm), and series of stretching tests were carried out using the Cu-EGaIn lines on knitted cotton fabrics (width of 1mm, length of 2cm). In the twisting experiments, the Cu-EGaIn line was subjected with twisting angles from 0° to 180° (Figure 3c). It was obvious that twisting the line did not significant affect its conductivity, and there was only slight variation (R/R0 = 1.03). In addition, the Cu-EGaIn line was bent with bending radii ranging from 1 to 10mm. Figure 3d showed the photograph of the bent Cu-EGaIn and the


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resistance curve under different bending radii. The curve showed that the resistance did not change substantially, and there was slight increase (R/R0 = 1.02) under the bending radii of 1mm. Figure 3e depicted the resistance curve of the Cu-EGaIn line under various strains (large strains up to 75%), exhibiting excellent linear relationship between the strains and the resistance. Furthermore, the resistance variation of the Cu-EGaIn line indicated no obviously frequency dependence under different frequency (20% strain, 0.25Hz, 0.5Hz and 1Hz), which exhibited frequency stability for practical applications, as shown in Figure 3f. Figure 3g showed the response of the Cu-EGaIn line to 1000 times cyclic loading of 20% strain at a frequency of 1 Hz. In the stretching process to 20% strain, there was only slight resistance change (R/R0 = 1.37), indicating its remarkable stability. Moreover, a LED light was connected with the Cu-EGaIn line (width of 1 mm) by the Cu wires, as shown in the schematic diagram of the Figure 3h. The I–V curves of the Cu-EGaIn lines with LED under various strains (0% to 75%) showed that the Cu-EGaIn lines had remarkable stability. Although the LED light was stretched to 75% strain, the whole electric circuit maintained normal work, as shown in Figure 3i.

Figure 3. Electrical performance of Cu-EGaIn electronics on fabrics. (a) Resistance of different width of Cu-EGaIn lines (n = 4). (b) Cross-sectional profile of Cu-EGaIn line with 500 μm width and its SEM image. (c) Resistance of Cu-EGaIn line with twisting angles from 0° to 180°. (d) Resistance of CuEGaIn line with bending radii from 1 mm to 10 mm. (e) Resistance of Cu-EGaIn line under various strains. (f) Resistance variation of the Cu-EGaIn line under different frequency (20% strain, 0.25Hz, 0.5Hz and 1Hz). (g) Resistance of Cu-EGaIn line with a strain rate of 20% for 1000 cycles. (h) The



structure of Cu-EGaIn lines connected with LED light. (i) The I–V curves of the Cu-EGaIn lines with LED under various strains. Applications of the Cu-EGaIn based Wearable Electronics With excellent flexibility and electrical stability, the Cu-EGaIn based conductors were able to fabricate wearable electronics for smart fabrics. For example, an interactive circuit was printed on a woven cotton T-shirt using the roller printing method, as shown in Figure 4a, b. The LED lights would illuminate when the finger approached the sensing area (Video S1). Figure S6 showed the circuit diagram of the interactive circuit, and there was a chip (TTP223-BA6) detecting changes in capacitance of the sensing area to control the LED lights. Besides, a stretchable LED array on knitted cotton fabric was folded into multiple shapes and still maintain normal working status (Figure 4c). Figure 4d showed another kind of LED array on knitted cotton fabric, which could be stretched to 75% strain (Video S2). Here, the LED array was printed on knitted cotton fabric covering on a cylinder (diameter of 7cm). Then, this LED array was re-sleeved on another cylinder (diameter of 10cm), as shown in Figure 4e. It was obvious that the LED lights could maintain stable state during these two stretching tests. In particular, the LED array with packaging Ecoflex was water-repellent, as shown in Figure S7 (Video S3). Finally, we designed a wirelessly powered LED array, which could be sewed on clothes and keep circuit function under 10% strain (Figure 4f). Above all, Cu-EGaIn based conductors had great potential as large area and multifunctional wearable electronics.


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Figure 4. Interactive circuit and stretchable LED arrays. (a) Details of the interactive circuit. (b) The interactive circuit printed on a woven cotton T-shirt. (c) A type of stretchable LED array printed on knitted cotton fabric. (d) Another kind of LED array printed on knitted cotton fabric was stretched to 75% strain. (e) LED array was printed on the knitted cotton fabric covering on cylinders of different diameters. (f) A wirelessly powered LED array. Particularly, we developed a thermal management device on the woven cotton fabric using the serpentine Cu-EGaIn conductors (line width of 2mm, length of 78cm). Figure 5a showed the temperature curve of the serpentine Cu-EGaIn conductors during heating/cooling process under input current of 0.5A (Video S4). As could be seen from Figure 5b, the temperature gradually increased with the current intensity, and thermal management device could provide the highest heating temperature (58°C) under input current of 0.5A at 120s. These pictures in Figure 5c showed the maximum temperature under different input currents (from 0.1 A to 0.5 A) and the corresponding temperature distribution at 120 s recorded by an Infrared (IR) camera. Here, the thermal management device was sewn on a T-shirt, and heating the human body under input current of 0.5A, as shown in Figure 5d. Besides, a piece of woven cotton fabric with color changing pigment was covered on the thermal



management device (Figure 5e). The thermal management device could induce the color change of the thermochromic materials on the top layer, and the color distribution of the woven cotton fabric could visually display the distribution of temperature, as shown in Figure 5f. In addition, we designed multiple Cu-EGaIn patterns on the woven cotton fabric, which could be heated by the electromagnetic heating coil (Figure 5g) using the electromagnetic induction to generate electricity inside the Cu-EGaIn, relying on the energy of these eddy currents for heating purposes. Compared with other flexible conductive materials with high resistances, such as carbon nanomaterials and metal nanowires, the Cu-EGaIn based thermal management device just required a low driving voltage to reach the targeted temperature, which was safe for human body and had wide application prospect as ski suits and electrical-heat diving suits.

Figure 5. Thermal management device printed on woven cotton fabrics. (a) Temperature curve of the serpentine Cu-EGaIn conductors during heating/cooling process under input current of 0.5A. (b) Temperature curve of the serpentine Cu-EGaIn conductors during heating process under various input currents (from 0.1 A to 0.5 A). (c) Picture of the thermal management device and its infrared temperature distribution images at 120 s under various input current (from 0.1 A to 0.5 A). (d) Infrared temperature distribution image of the thermal management device sewn on a T-shirt. (e) Structure diagram of the woven cotton fabric with color changing pigment. (f) Thermochromic display of the serpentine CuEGaIn conductors. (g) Infrared temperature distribution image of multiple Cu-EGaIn patterns heated by the electromagnetic heating coil.


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In fact, the Cu-EGaIn will change to solid status when the temperature is lower than 15.5 °C. Here, a LED array printed on the knitted cotton fabric was placed on ice, where the temperature was close to zero 0 °C. The pictures in Figure S8 shows that the LED array could maintain normal working condition when the fabric was bent. However, when the fabric was stretched, the Cu-EGaIn lines would be pulled off, which resulted in circuit failure. The broken Cu-EGaIn lines would reconnect if the fabric returned to the original length. Besides, the micrographs of Cu-EGaIn lines also show the connection status (Figure S8d).

Conclusion In summary, we develop highly conductive wearable electronics for smart fabrics based on the semi-liquid metal (Cu-EGaIn). In addition to high conductivity (6×106 S m−1), Cu-EGaIn shows significant adhesion differences on cotton fabrics and PVAC glue, which can be attributed to the differences in surface topography and chemical interaction. A fast fabrication of Cu-EGaIn based wearable electronics using screen and roller printing method is developed. This method makes it possible to fabricate large-area and low cost wearable electronics on fabrics. Besides, a series of electrical experiments have been fabricated, showing the excellent electromechanical stability of Cu-EGaIn lines on woven and knitted cotton fabrics. Furthermore, we design interactive circuits, stretchable LED arrays and thermal management devices to demonstratethe practical applications for large-area and low cost wearable electronics. However, there are still some obstacles, which limit the large-scale production of the Cu-EGaIn based wearable electronics. For example, the connection between rigid components and Cu-EGaIn lines is not strong enough due to their mechanical performance difference. In the future, we will design some adapter plates to strengthen the connection. With the advantages of high conductivity, we believe that the Cu-EGaIn based wearable electronics will contribute to the practical applications of next generation smart fabrics.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: (). The fabrication process of Cu-EGaIn, electrical conductivity of Cu-EGaIn with different contents of Cu particles, fabrication and effect of Cu-EGaIn printed on woven cotton fabrics, fabrication and effect of Cu-EGaIn printed on knitted cotton fabrics, tilted slopes test of CuEGaIn on four substrates, circuit diagram of the interactive circuit, waterproof test of LED array, and freezing tests of Cu-EGaIn based smart fabric (PDF) Video S1. The interactive circuit. (MP4) Video S2. The stretchable LED array. (MP4) Video S3. The washable LED array. (MP4) Video S4. The wearable thermal management circuit. (MP4)

Acknowledgements This work is partially supported by the NSFC Key Project under Grant No.91748206, Dean’s Research Funding and the Frontier Project of the Chinese Academy of Science.



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Semiliquid Metal Enabled Highly Conductive Wearable Electronics for

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