Large-Scale Fabrication of Highly Elastic Conductors on a Broad

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Functional Inorganic Materials and Devices

Large-scale fabrication of highly elastic conductors on a broad range of surfaces Lixue Tang, Lei Mou, Wei Zhang, and Xingyu Jiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20460 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Materials & Interfaces

Title: Large-scale Fabrication of Highly Elastic Conductors on a Broad Range of Surfaces Lixue Tang1, 2, Lei Mou1, 2, Wei Zhang1, 2, and Xingyu Jiang1, 2, 3 * 1. Beijing Engineering Research Center for BioNanotechnology and CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, CAS Center for Excellence in Nanoscience, National Center for NanoScience and Technology, No. 11 Zhongguancun Beiyitiao, Beijing 100190, P. R. China; 2. University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing, 100049, P. R. China; 3. Department of Biomedical Engineering, Southern University of Science and Technology, No. 1088 Xueyuan Rd, Nanshan District, Shenzhen, Guangdong 518055, P. R. China * Corresponding author, E-mail: [email protected] KEYWORDS: Stretchable conductors, liquid metal, printable electronics, wearable devices, large-scale fabrication, low-cost. ABSTRACT Recently, a great stretchability progress has been witnessed in elastic electronics. However, such electronics are either costly, toxic, or cannot pattern on a broad range of substrates which limit their large-scale fabrications and applications. Here, to overcome those limitations, an ink comprising liquid metal particles and desirable polymer solutions is developed. The polymer solutions in our ink can be adjusted to print on different surfaces and avoid toxic organic solvents in most cases. The ink can be sintered by small strain (~10%) in room temperature. Using our ink, conductors with high stretchability (380,000 S/m at a strain of 1000%) can be printed in low consumption (liquid metal consumption 3.27 mg/cm2), in large area (bestrew entire surface of a T-shirt) and in high throughputs (~105 cm2 per hour). The ink can be printed on a T-shirt to achieve a smart wearable platform that integrates electronics for strain/electrophysiology/electrochemistry detection, and temperature monitoring/controlling.

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1. INTRODUCTION Printable, stretchable, and non-toxic interconnects are the key points to achieving the soft wearable/implantable devices compatible with human tissues 1, 2. Currently, there are two main strategies to obtain stretchable interconnects. One way is to design special structures such as serpentine or wavy structure that can offset the deformations 3, 4. However, such strategies have limited stretchability (~150%), low productivity, limited area (typically no larger than the area of a silicon wafer), and high cost. Another way is to print conductive materials such as carbon nanomaterials, micro-structured silvers, and liquid metal that can bear large deformations 5-12. Carbon nanomaterials have poor performances in conductivity and stretchability, they are most used in stretchable semi-conductors and sensors 11, 12. Elastic silver flake inks have both excellent conductivity and stretchability 8, 9. But such inks contain toxic substances, such as silver and organic solvent, thus making wearable/implantable electronics harmful to health. The sintering of such silver inks usually requires high temperature (>100 oC) which may damage the substrates with low melting point. In addition, the noble metal silver is more expensive than most other metal used in electronics. For example, the cost of silver is four times that of the liquid metal 13. Another most reported class of materials that can bear large deformations are liquid gallium alloys 13-22. The liquid state in room temperature endow gallium alloys both excellent stretchability and conductivity. At the same time, the gallium alloy is low in toxicity which is capable for bio-applications 13-15. However the huge surface tension of liquid metal confines its patterning only to hollow channels 15, 18 or substrates that can be wetted by liquid metal 16, 17.

To avoid the surface tension-caused problems, liquid metal can be fabricated into

nano/micro-particles which are composed by conductive core and non-conductive oxide layer 13, 19-22.

To form conductive path, oxide layer on the particles should be broken. Researchers

usually use a marker/tip to draw desirable patterns on a thin layer of deposited particles, and 2 ACS Paragon Plus Environment

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conductive paths will form on the contact region due to the break of the oxide layer of particles 19-22. However, Conductive patterns obtained by such methods are not able to bear large deformations, because stress in applications will also break the rest of particles, causing undesirable parts conductive. In addition, such methods have low liquid metal utilization, most particles of metals are wasted. Here, we report the scalable metal-polymer conductors (SMPC) that can be large-scale printed by an ink comprising liquid metal particles (gallium alloys) and desirable polymers solutions. We make liquid metal into a true sense of printable ink that can be patterned on freeform surfaces. The SMPC have both excellent stretchability and repeatability. We carried out mechanical and microscopic characterization to the SMPC, we found that appropriate strains will break most of the metal particles in SMPC, forming stretchable conductive paths (strain sintered). We demonstrate that we can use different polymers to fabricate our ink, because polymers in our ink serve to fix particles on the stretchable substrates to transmit stress from the substrate to particles, thus the oxide layer of the particles can be broken when stretching the substrates.We used different polymers to fabricate the SMPC ink to meet the printability of different substrates and avoid toxic organic solvent in most cases. To demonstrate the potential of our ink for printing elastic electronics in large-scale, we printed the SMPC on a T-shirt to achieve a smart wearables platform for strain, electrophysiological, and electrochemical detection as well as temperature monitoring and controlling.

2. RESULTS AND DISCUSSION 2.1 The Fabrication of the SMPC. We sonicated liquid metal alloys in desirable polymer solutions for several minutes to fabricate the ink of SMPC for laboratory fabrication (Figure S1a). We chose the gallium alloys as the conductive part of the ink of SMPC. Because gallium alloys such as gallium-indium alloy (Melting point: 15 oC) and gallium (Melting point: 29.9 oC, the gallium particles are liquid at room temperature because of the supercool 3 ACS Paragon Plus Environment


effect) have lower price, at about one third of silver. To fabricate the ink of SMPC in a large quantity (>20 mL) at low cost, we can also use a blender to disperse liquid metal in polymer solutions (Figure S1a).

Figure 1. Conductive mechanism of the SMPC. (a) Printing SMPC on substrates in large area and large scale. (b) A schematic shows the sintering of liquid metal particles by strain. (c) SMPC can be activated by small strains. Scale bar, 15 mm. (d) The conductance of SMPC with different diameters versus the strain. (e) SEM characterizations of the SMPC before and during 50% strains. Scale bar, 10 m. (f) Size and volume distribution of the unbroken liquid metal particles in patterns before and during 50% strains. We used a cost-competitive strategy, screen printing, to directly print such ink on substrates in large area and large scale (Figure 1a). After the evaporation of the solvent in room temperature, we found that a slight strain can break the particles on the entire surface of the substrate and make the SMPC conductive (Figure 1b and Figure 1c). As a result, we obtained the highly stretchable SMPC in room temperature with high efficiency. 4 ACS Paragon Plus Environment

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2.2 The Characterization of the SMPC. We applied strains on the SMPC and characterized them during strains. We used our inks containing liquid metal particles and polyvinyl pyrrolidone (PVP) solution (PVP in n-Decyl alcohol) to print 2 mm *20 mm strips on the silicone film (Rogers Corporation, HT-6240, US) as samples for scanning electron microscope (SEM) and mechanical characterizations. We measured the electric conductance of the SMPC when applying strains from 0% to 120%, then releasing to 0% using dynamic mechanical analysis (Figure 1d). We printed the SMPC ink with diameters of 2.31 m, 1.15 m, 0.86 m, and 0.39 m for the mechanical analysis. The diameters of the liquid metal particles in inks depend on the sonication time (Figure S1b). We analysed the strain-conductance curve of the SMPC strips (2 mm *20 mm) with average diameter of 2.31 m. We found that the SMPC strips are non-conductive before the strain reaches 10%, the SEM characterizations show that liquid metal particles still keep a round and smooth form, oxide layer unbroken (Figure 1e, Figure S2a, and Figure S2d). When the strain reaches about 12%, at which we call it the activation strain, the conductance of the SMPC strips increases sharply, because most of the particles in the SMPC broke and release the conductive core, forming conductive paths (Figure 1b and Figure 1e). However, there still remains some unbroken particles. Figure 1f shows the size distribution of particles at strain of 0% (without pre-stretch) and 50%, respectively. Before stretching, particles are unbroken and isolated by the oxide layer, and the liquid metal particles with larger diameter takes most of the volumes of liquid metal. After the activation strain, almost all large particles (>2 m) broke, the released liquid metal merge into conductive paths (Figure 1e and Figure S2b). When we continued to increase the strain to 120%, the larger strain will gradually break some particles with smaller particles. SEM characterization indicates that beyond this point, very few particles continue to break to result in increase of conductance (Figure S2e, and 5 ACS Paragon Plus Environment


Figure S2f). We noted that the conductance of SMPC strips increases slightly first and then decrease slightly. We hypothesized that this conductance change of the SMPC with the increase of strain contains two parts: the breaking of the particles and the fusion of liquid metal will increase the conductance, while the elongation of the pattern and decreased cross section of the conductive parts of the SMPC will decrease the conductance due to the Ohm’s law. At large stain (>80%), the increase of the conductance as a result of more broken liquid metal particles cannot match the decrease of the conductance as a result of decreased cross section, causing a drop of the conductance. To test if the relaxed pattern can still be conductive, we gradually released the strain from 120% to 0%. We defined the conductance of patterns at strain of 0% after stretching as the released conductance/resistance. We removed the strain on the pattern, but the pattern still keeps high conductivity and can still be conductive after repetitively bearing large deformations. The SEM characterization shows broken, wrinkled, and interconnected particles (Figure S2c and Figure S3a bottom). Thus, we obtained conductive patterns with ultra-high stretchability. To test if smaller liquid metal particles can be fabricated into the MPC ink, we found that the SMPC with smaller diameters have lower conductance on large deformations (Figure 1d). The SMPC lost its conductivity when the diameter is as small as 0.39 m. The SEM characterizations of the SMPC with small particles show that cracks (PVP-based ink) or wrinkles (TPU-based ink) will form on the surface of the SMPC after stretching, and the magnified SEM images show those small particles are not break (Figure S4). We believe this can be explained by the formula in our previous work.13 According to our previous work which is a model simplified as a gallium particle embedded in elastomer matrix, the Tresca’s equivalent stress on the oxide layer of the particle σe =kREm/t (k, Em, , R, and t is the constant term, Young’s modulus of the polymer, strain of the substrates, the radius of particles, and the thickness of gallium oxide, respectively) 13. In 6 ACS Paragon Plus Environment

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order to break the oxide layer of the particles, we can tune the radius of particles and the strain of the substrate to make the σe larger than the yield stress of gallium oxide σy=200 MPa. For liquid metal particles with large diameter, we need to apply a small strain on the substrate to break such particles. While for liquid metal particles with small diameter, we need to apply a large strain on the substrate to break such particles. However, the liquid metal particles in this study are not embedded in the substrates, they are fixed on the substrates by polymers. Forces can still be transmitted to particles via polymers when the substrate get stretched. But the polymers in our ink cannot firmly bond the liquid metal particles on the substrates, when the liquid metal particles are smaller enough, with the increasing of the strain, the polymers will detached from the substrate before the breaking of the oxide layer of liquid metal particles (Figure S4a), cracks or wrinkles will form on the surface of SMPC. Thus, stress cannot be transmitted to particles via polymers, causing liquid metal particles with small diameters unbroken under strain. This result also suggests that we do not have to spend time and energy to fabricate liquid metal particles with very small diameter. The released conductance of the pattern depends on the maximum strain applied to it. We used PVP-based SMPC strips to carried out another strain-conductance test by measuring the conductance as a function of applied stains (0%-15%-0%-15%-0%-30%-0%-30%-0%50%-0%-50%) (Figure 2a). We found that when the SMPC initially reaches a certain strain, for example, when the first strain increases from 0% to 15%, we can see a sharp conductance increase caused by breaking particles. If we repeat such process, for example when the strain increases from 0% to 15% after the initial strain and relaxation, there shows a conductance increase caused by the thickness increase of the conductive paths (Figure 2d inset). We fabricated a batch of unstretched patterns and apply 0%, 15%, 30%, 50%, 80%, 100%, and 120% strains on them and released them to 0% strain, respectively. Regardless of the difference in the strain, the released resistance decrease at first and finally tends to be a constant (Figure S5), which we call the post-stretch conductance/resistance. It suggests that 7 ACS Paragon Plus Environment


there is no more particles can break further to contribute to the conductivity when the strain is beyond a certain threshold (50%).

Figure 2. Characterization of the SMPC. (a) The change in conductance of the SMPC as a function of applied stains (0%-15%-0%-15%-0%-30%-0%-30%-0%-50%-0%-50%). (b) Poststretch conductivity dependence on the mass fraction of polymers (PVP and thermoplastic polyurethane (TPU)) in the ink of SMPC. (c) Post-stretch conductivity and thickness of the SMPC versus the liquid metal volume fraction in ink. (d) Conductivity of the SMPC fabricate by different polymers dependence on tensile strain. Inset, the change in thickness of the SMPC with different strains. (e) The resistance change of SMPC fabricated by different polymers depending on a strain up to 1000%. (f) The resistance change of SMPC printed by inks contain different volume of liquid metal depending on a strain up to 1000%. Inset, the resistance change within 150% strains. (g) Changes in resistance of the SMPC by stretching the patterns from 30% to 80% for 1,000 cycles. Error bars in this paper represent standard error (from five parallel samples). The addition of polymers in the ink of SMPC is slight (1.25%-2.5%) but inevitable. Patterns without polymer are not conductive no matter how much strain is applied. The SEM characterization shows that when and after strains (30%) are applied to the substrates, cracks 8 ACS Paragon Plus Environment

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which are perpendicular to the direction of stretch will form on the patterns (Figure S3b). Particles which compose the patterns keep a round and smooth shape, which shows no difference with that before stretching. By contrast, no cracks form on patterns with polymers, there shows merged liquid metals released from particles to form the conductive paths. According to such observations, we believe that the force that can break the liquid metal origins from the force we applied on the substrates, when there is no polymer, the affinity (mainly van der Waals force) between the substrates and liquid metal particles are not enough to synchronize the strain, forces cannot transmit to particles when substrates get stretched. By contrast, polymers can greatly enhancing the affinity between the substrates and liquid metal particles by increasing the contact area (decrease the surface tension of the liquid metal) or bonding liquid metals onto the surface of the substrates. Thus the polymer can fix particles on the stretchable substrates to conduct the strain. The particles and the substrates will have synchronous strains during stretching. When the strain of the substrates reaches about 10%, the inelastic oxide layer on particles will break and the released liquid metals will merge into stretchable conductive paths. As a result, these stretchable conductive paths endow the SMPC excellent stretchability. We demonstrated that most of the polymers solutions can be used to fabricate the ink of SMPC and quantified the amount of added polymers. We used non-elastic polymer PVP and elastic polymer thermoplastic polyurethane (TPU) with different concentrations as the additive, respectively. Figure 2b shows that both polymers can lead to a conductive and stretchable patterns, which further demonstrates that it is not the polymer that contribute to the stretchability of the pattern (polymers may break during the stretch, but merged liquid metal particles as conductive paths have formed within the SMPC to ensure the stretchability). The best addition range is about 1.25%-2.5%. The over-addition (>2.5%) of polymers will cause the formation of continuous polymer film and the isolation of the particles by polymers, which will make the SMPC non-conductive (Figure S6). Different from the reported elastic 9 ACS Paragon Plus Environment


ink whose stretchability partially derives from the elastic polymer, we can use desirable polymers, not limited to elastic polymer, to fabricate the elastic inks, because the stretchability of the SMPC derives from the merged liquid metals. We explored the relationship between the conductivity, thickness of patterns and the volume fraction of the liquid metal in the ink of SMPC. We fixed the concentration of polymer PVP at 1.25%. We found that before the volume fraction of liquid metal reaches about 25%, the post-stretch conductivity of SMPC increase with the liquid metal volume fraction. After the volume fraction reaches 25%, the post-stretch conductivity shows no significant enhancement (Figure 2c). We used a profiler to measure the thickness of the SMPC printed by inks with different volume fraction of liquid metal. Patterns show a rough surface (Figure S7) and the thickness of the pattern almost linearly increase with the volume fraction of liquid metal (Figure 2c and Figure S7). We found that the best volume fraction of liquid metal is 25%, at which patterns can keep good electric performances and have economical liquid metal consumption (About 3.27 mg/cm2). To demonstrate that the SMPC have excellent stretchability and repeatability, and SMPC fabricated by different polymers have similar performances. We choose three different polymers PVP, TPU, and ethyl cellulose to fabricate the SMPC ink and print them on 3M VHB tapes and polyurethane (PU) films to test their stretchability. We fixed the addition of different polymers at 1.25% (mass fraction).We found that SMPC fabricated by different polymers have high conductivity and stretchability (Figure 2d). For example, the TPU-based SMPC have high conductivity (post-stretch conductivity) (420,000 S/m) at the strain of 0%. The conductivity shows an increase until the strain reaches about 300%, keeping a conductivity of 1,110,000 S/m. We supposed that this observation is caused by the increasing thickness of the conductive paths between particles (H2>H1 in Figure 2d). But the conductive paths begin to thin after the strain of 300%, which will decrease the conductivity of patterns (H3150 oC) for sintering at which PLC membrane will melt. The casting and peeling strategy in our previous work 13 cannot achieve such patterns because the porous structure of the ES membrane will be infiltrated by cast polymers. We also tried to directly print the SMPC ink on the surface of textile (warp-knitted structure). The SMPC particles tend to gather between fibbers due to the capillary force. The SMPC on the textile have high conductivity at unstretched length after stretch. However, the SMPC will lost the conductivity when stretched perpendicular to the warp (about 30%) because of the inhomogeneous deformations of the textile which will cause huge gaps (about 100 m) between the bundled fibbers (Figure S10). To make stretchable wearable devices on textile wearables, we usually print a layer of polyurethane on the textile to smooth the surface which we will discuss in the following. To avoid the toxic organic solvents in our ink, we further developed the water-based SMPC. We used the water-based SMPC ink comprising Polyethylene oxide (PEO) solution (PEO in water) to print interconnects on the surface of gloves on which electronic devices can be integrated. Large deformations will not break such conductive interconnects (Figure 3d). Combining with the strategy of screen printing, we can print the SMPC on curved surface. We printed SMPC ink (Ethyl cellulose in ethanol) on the surface of latex balloons. To demonstrate the excellent stretchability of patterns on balloon, we inflated the balloon to make its diameter to ~ 20 cm. The LED linked with the SMPC on the balloon can indicate the conduction of the circuitry. The LED keeps lit during the inflation process (Figure 3e, inset), 13 ACS Paragon Plus Environment


which demonstrate the excellent conductivity of the SMPC under stretch on balloons (Figure 3e and Movie S1). Electronics on balloon will have great potential in medical applications as catheters for in-vivo detection and treatments such as electrophysiological detection and ablation therapy 23. We greatly simplified the fabrication process of electronics on balloons. We printed the SMPC on sports T-shirt as interconnects to fabricate highly integrated smart wearables (Figure 4a, and Figure 4b). Our smart wearables contain electrophysiology sensors for electrocardiogram (ECG) detection, electrochemical sensor for potassium ion detection, strain sensors for motion detection, and heating modules for keeping a constant temperature for human body. The printing process of our SMPC is the same as the T-shirt screen printing process in industry. To make washable SMPC interconnects on wearables, we sandwiched the SMPC within the insulating layer printed by commercial polyurethane inks (Figure 4a). The whole printing process is as highly efficient as the traditional T-shirts screen print techniques.


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Figure 4. The integrated wearable T-shirt. (a) The photograph of the SMPC printed T-shirt for smart wearables. Inset, the structure of the encapsulated SMPC interconnects. (b) A schematic shows the highly integrated T-shirt. (c) The strain sensor printed on the silicone substrates using SMPC. (d) Electrocardiogram signals using commercial 3M electrodes (top), and printed electrodes on the T-shirt (bottom). (e) The SMPC serpentine patterns printed on the Tshirt as a heater. (f) The resistance response of the strain sensor in chest expansions. The peak represent expanding chest for high speed. The plateau exhibits keeping chest expansion for a period. (g) The open circuit potential responses of the potassium sensors treated by KCl solutions with different concentrations and strains. We conducted the wash test for the printed T-shirt to demonstrate the T-shirt is washable. The washing test of the SMPC printed T-shirt was conducted by washing-machine as a 45minutes procedure. The encapsulated SMPC keeps its conductivity after the test which demonstrate that neither the water nor the violent environment in the washing-machine can break the SMPC interconnects on the T-shirt. We integrated ECG electrodes on the corresponding sites of the T-shirt for on-body ECG detection (Figure 4b). We stuck the commercial hydrogel pads to the SMPC interconnects as 15 ACS Paragon Plus Environment


the electrodes, such electrodes can firmly attach to the skin. To demonstrate the good stretchability of our wearables, the T-shirt was stretched (30% strain) for 10 cycles before the on-body ECG recording. Figure 4d shows a comparison of single-lead ECG recordings from our wearables and the commercial ECG electrodes. On-body ECG signals at the same leads have similar morphologies. To demonstrate that an arbitrary number of ECG electrodes can be integrated on the T-shirt, we carried out an 8-leads (6 electrodes) ECG on-body test (Figure S11 and Movie S2), ECG signals with typical features can be collected elegantly. To keep a constant temperature for human body in low temperature, we developed a constant temperature system for a T-shirt. Such a system is formed by a serpentine heater printed by our SMPC and temperature sensors. It is a challenge for temperature sensor to accurately monitor the temperature on wearable devices, because the resistance change of the sensors and interconnects caused by external stimuli will bring measurement deviations 24, 25. In our study, we used rigid temperature sensors in the wearable T-shirt and SMPC as interconnects. In deformations, the resistance change of the SMPC interconnects are the main reason for influencing the temperature sensors. The strain of the wearables are usually no more than 50%, according to figure 2f, the resistance change (R/R) of PVP-based ink on PU substrate is about 12.4%. So the deformations of the wearables will bring measurement deviation of resistance up to 7 Ω, causing a measurement deviation of temperature less than 1.7 oC. Such measurement deviations make the accurate sensing of temperature difficult, but it is enough for the constant temperature system to keep an appropriate temperature for human body (Figure 4e). We integrated strain sensor on a T-shirt for motion detections. The strain sensors which present a serpentine structure with line width of 200 m are directly printed by SMPC on the silicone membrane (Figure 4c). The strain sensor was applied a pre-strain of 30% and fixed on a T-shirt to detect the chest movements when expanding chest (Figure 4f). These peaks in 16 ACS Paragon Plus Environment

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the figure represents the fast expansion of the chest while the flat hump represents keeping the chest expansion for several seconds. We integrated electrochemical sensors on the smart wearables to monitor electrochemical signals in real time. We chose potassium ion as our target, because the concentration of the potassium ions in sweat is related to exercise-associated hypokalemia. K+ sensors are printed by carbon inks (working electrode) and Ag/AgCl ink (reference electrode) on PET film (Figure S12) and directly connected with interconnects on our T-shirt. Figure 4g shows the open circuit potential of K+ sensors in the KCl solutions with 1 mM to 100 mM which covers the physiological potassium concentrations. We apply 30% strain on the K+ detection process, there shows negligible influence to the results (Figure 4g). This demonstrates that deformations of our wearables during vigorous exercise will not influence the electrochemical detection results. 3. CONCLUSIONS We, for the first time, fabricate liquid metal into a true sense of printable ink that can be sintered by strains and patterned on freeform surfaces, we demonstrate that SMPC ink fabricated by different polymers have excellent performance in conductivity and stretchability. We make a comparison with exist elastic conductive ink, our SMPC ink have better performances in conductivity, stretcahbility and cost (table S1). Compared with the reported liquid metal patterning strategies which are confined to hollow channels or specific substrates that can be wetted by liquid metals, our SMPC can be printed on most substrates and surfaces no matter they are smooth or rough, flat or curved, small or large, since the printability can be tuned by choosing desirable polymer solutions to meet different substrates. The most reported strategies to fabricate conductors using liquid metal particles usually deposit a layer of liquid metal particles on the substrates and use a marker/tip to sinter liquid metal particles one by one to obtain desirable patterns, most particles wasted. Besides, stress 17 ACS Paragon Plus Environment


in application may sinter undesirable particles and make these parts conductive, causing short circuit. By contrast, our ink is ready-to-use after printing, we apply about 10% strains on substrates to make the SMPC on entire surface conductive immediately, and no other strategies can sinter the liquid metal particles with such straightforwardness. We make full use of the liquid metal particles and avoid the undesirable sintering. Compared with our previous work, we greatly improve the practicability and productivity, we can directly print SMPC on desirable substrates and special surfaces. We avoid the time consuming cast-andpeel procedure. Elastic silver flake inks also have excellent performance in conductivity and stretchability. However, nano/micro structured silver are highly corrosive in biological environments because of the high oxidation tendency of Ag, which limit their applications in bioelectronics26. By contrast, the gallium alloy is non-toxic since it has been adopted for dental restorative material, tumor scanning, and drug delivery. Liquid metals already have some applications on microfluidics 27, 28 and soft robotics 29, 30, we believe our SMPC ink can further promote the development of microfluidics and soft robotics. The SMPC can provide the microfluidic chips with different circuits for driving droplets or for electrochemical detections. With our ink, we can print the SMPC on the surface of electroactive polymer or shape memory polymer to fabricate the artificial mussel for the soft robotics. Also, SMPC can be printed on gloves as the electro-skin for soft robotics to detect pressure or movements. We printed SMPC on a T-shirt to fabricate a smart wearable platform on which various electronics and sensors can be integrated. Except for these devices mentioned above, more function modules can be integrated to achieve complicated functions. Our strain sintered and wettability tuneable ink for large-scale fabrication of highly elastic conductors will greatly decrease the cost for elastic electronics and promote the development of wearable/implantable devices for widespread applications. 18 ACS Paragon Plus Environment

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4. METHODS Preparation of the ink of SMPC. SMPC for screen printing was prepared by sonicating 3 g gallium-indium alloy (Ga: In =4:1, Hawk, HK3284, China) or gallium (Hawk, China) in desirable polymer solutions. PVP solutions were made by dissolving 0.5 g polyvinyl pyrrolidone powders (Aladdin, Mn=1,300,000, China) in 10 mL n-Decyl alcohol (98%, MACKLIN, China) solution and stirring for 24 hours. TPU solutions were made by dissolving 0.5 g TPU (BASF, 9390, German.) in 10 mL Tetrahydrofuran and stirring for 24 hours. Ethyl cellulose solutions were made by dissolving 0.5 g Ethyl cellulose (Bioruler, Mw=288.38, China) in 10 mL ethanol and stirring for 24 hours. The sonication of liquid metal alloy was achieved by a sonicator (Scientz, Scientz-IID, China), the sonication time is 1 min with the power of 300 W. The preparation of the water-based ink of SMPC is slightly different. To avoid the slow chemical reaction between the gallium and water, we fabricated and stored the water-based SMPC in ethanol. Water-based ink of SMPC was prepared by sonicating 3 g liquid metal alloy in 5 mL ethanol for 1 minute. After the precipitation of the particles by centrifuge at 1500 r/min for 2 min, we discarded the ethanol solution and replaced by 1 mL PEO aqueous solution and stirred evenly to obtain the SMPC ink. The PEO aqueous solution was obtained by adding 0.6 g PEO (Aladdin, Mn=1,000,000, China) and 200 L surfactant (Capstone FS-30, Dupont, US) to 20 mL deionized water. The large scale fabrication of the ink of SMPC is achieved by adding 100 g gallium-indium alloy in 30 mL 5% PVP solution (PVP in n-Decyl alcohol) and stirring by a blender (IKA, T18 ULTRA TURRAX, German) with a speed of 15,000 rpm for 20 min.



Printing of the SMPC. We used the screen printing strategy to print the SMPC. We chose the screen printing plates with 250 mesh to print the SMPC on different substrates including silicone films (Rogers Corporation, HT-6240, USA), the polyurethane films, the PDMS films (Sylgard 184, Dow Corning, USA), the examination glove (Medicom, SafeTouch, US), and the PLC (Poly(L-lactide-co-ε-caprolactone) 70:30, Evonik, Resomer LC 703S, Germany) electrospinning membrane by using a simple screen printing equipment (Taobao, China). We obtained the PDMS films by mixing base and curing agent with a ratio of 10:1 and casting on a glass plate, curing at 80 oC for 20 min. We obtained the polyurethane films by casting waterborne polyurethane (Wanhua, Archsol 8560, China) on a glass plate. We obtained the PLC ES membranes on the aluminum foil by electrospinning PLC solution (10% wt%; voltage 15 kV). After the printing, patterns were placed in room temperature for 20 min or in an oven at 80 oC for 2 min to make the solvent fully evaporate. After the evaporation of the solvent, we applied strains on the PDMS films to make patterns conductive. Characterization of the SMPC. Samples for SEM characterizations should be deposited by a thin layer of gold for 50 s using ion sputter (Hitachi, E-1010, Japan). The stretched samples were fixed by conductive tapes. We use Scanning electron microscopy (SEM, Hitachi, S8220, Japan) to obtain the SEM images. The thickness of the patterns were obtain by stylus profiler (Bruker, DektakXT, USA). Strain test of the SMPC. We used the ink of SMPC to print 2 *20 mm strips on the PDMS substrates strain test. We used Dynamic mechanical analysis (DMA Q800, TA Instruments, USA) to apply stains at a rate of 20% /min. We measured the electric conductance by using the electrochemical workstation (1040C, CH Instruments, USA) (amperometric i-t curve at potential 0.001 V) of the patterns when applying strains. Fabrication of the smart T-shirts. Sports T-shirts (RIGORER, 84% polyester fiber, 16% spandex, China) (Sbart, 82% polyamide, 18% spandax, China) were fixed at Screen printing 20 ACS Paragon Plus Environment

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table, we used screen printing plate with 100 mesh to print waterborne polyurethane (PU) inks (Zhongyi, Cornera WG-NY102, China) on the T-shirts. After the curing of the inks in room temperature for 10 hours, we printed (100 mesh) water-based ink of SMPC on the PU layer. After the evaporation of the solvents, we printed (100 mesh) another PU layer to encapsulate the liquid metal layer. We applied 30% strains on the T-shirts to make patterns conductive. We fabricated an 8-leads ECG detection on the T-shirts which contains I, II, III, avr, avl, avf, V2, and V5. Hydrogel electrode pads (Top-Touch, Japan) were fixed on the T-shirts in a way they would correspond to the locations of electrodes of I, II, III, avr, avl, avf, V2, and V5. The recording of the ECG signals were obtained by wireless electrocardiograph (GOOD FRIEND, EL-194, China). Informed signed consent has obtained from the human subjects. The fabrication of printed electrochemical K+ sensors are fabricated by screen printing and modified by K+ selective membrane. Briefly, we used screen printing strategy to print (250 mesh) the carbon ink (Gwent, C20319P4, UK) as the working electrodes, and the Ag/AgCl inks (ALS, 011464, Japan) as the reference electrodes on PET films. The K+ selective mixture for drop-casting the working electrode was fabricated by adding 2 mg valinomycin (Sigma-Aldrich, US), 1 mg sodium tetraphenylborate (NaTPB) (SigmaAldrich, US), 33 mg polyvinyl chloride (PVC) (Sigma-Aldrich, US) and 65 mg bis (2ethylehexyl) sebacate (DOS) (Sigma-Aldrich, US) into 350 μl of cyclohexanone and stirring for 24 h. The polyvinyl butyral (PVB) (Sigma-Aldrich, US) solution for drop-casting the reference electrode was prepared by adding 158 mg PVB， 100 mg of NaCl, 4 mg of F-127 (SigmaAldrich, US), and 0.4 mg of multiwall carbon nanotubes (Sigma-Aldrich, US) into 2 ml methanol (Sigma-Aldrich, US). We used 5 L K+ selective mixture and PVB solution to drop-cast the working electrodes and reference electrodes, respectively, and allowed them to dry overnight. 21 ACS Paragon Plus Environment


The heaters were screen printed (100 mesh) by water based ink of SMPC as a serpentine shape on the T-shirt. A temperature sensor PT 1000 (HAYASHI DENKO, CRZ2005, Japan) is adopted to monitor the temperature of the heater and keep the temperature in a constant value. The Infrared Radiation photos of the heater are obtained by heat gun (FLIR, E40, USA). The strain sensors are screen printed (250 mesh) by SMPC on the PDMS substrates as a serpentine structure. We applied 100% strains on these strain sensors to activate them. The strain sensor was applied a pre-strain of 30% and fixed on the T-shirt.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/. Additional details include schematic illustration of fabrication of the SMPC ink, SEM image of the SMPC printed by ink with different polymers under different strains, schematic and image of stretched SMPC with small particles, the releasing resistance of the SMPC dependence on the maximum strains applied, the thickness of the SMPC printed by different inks, the SEM image of the SMPC on different substrates, the electrophysiological and electrochemical signals collected by T-shirt, a comparison of the conductivity, stretchability, and cost of the presented conductive ink. AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] Author Contributions X. J. conceived and supervised this project. L.T. designed, fabricated and characterized materials and devices. L.M. carried out the electrochemical experiment. L.T., L.M., W.Z., and X.J. wrote the manuscript. All authors discussed the results and commented on the manuscript. Notes The authors declare no competing financial interest ACKNOWLEDGMENTS We thank Shuaijian Yang for experimental assistance. We thank the National Key R&D Program of China (2017YFA0205901), the National Natural Science Foundation of China (21535001, 81730051, 21761142006) the Chinese Academy of Sciences (QYZDJ-SSWSLH039, 121D11KYSB20170026, XDA16020902) for financial support. REFERENCES 1. Kim, D. H.; Lu, N.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S.; Wu, J.; Won, S. M.; Tao, H.; Islam, A. Yu, K. J.; Kim, T.; Chowdhury, R.; Ying, Ming; Xu, L.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.; Huang, Y.; Coleman, T.; Rogers, J. A. Epidermal Electronics. Science 2011, 333, 838–843. 22 ACS Paragon Plus Environment

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The table of contents entry Liquid metals are made into a true sense of ink that can be sintered by small strains in room temperature. The solvents of the ink can be tuned to be printable on different substrates or environmentally friendly. Conductors printed by our ink exhibit an extraordinary stretchability that is comparable with the most stretchable, reported conductors. Keywords: Stretchable conductors, printable electronics, liquid metal, wearable devices. Lixue Tang, Lei Mou, Wei Zhang, and Xingyu Jiang1, 2 * Title: Large-scale Fabrication of Highly Elastic Conductors on a Broad Range of Surfaces


Large-Scale Fabrication of Highly Elastic Conductors on a Broad

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