Photo-patternable, large-area solid-state liquid metal lm coated via solution shearing for soft electronics

Liquid metal (LM) is considered one of the most promising conducting materials for soft electronics due to its unique combination of metal-level high conductivity with exceptional deformability and stretchability. However, their practical applicability has thus far been limited due to the challenges of generating chemically and mechanically stable lm over a large-area and the need for non-standard fabrication approaches. Here, we report materials and manufacturing methods that enable multiscale patterning (from microns to centimeters) and multilayer integration of ‘solid-state liquid metal (SSLM)’ with the conventional cleanroom process. In this work, solution shearing of a polyelectrolyte-attached LM particle ink is used to generate SSLM lms. The stabilized LM particles were observed to form a close-packed thin-lm without particle rupture when coated under evaporative regime. This is essential in enabling a subsequent photolithographic lift-off process at wafer-scale to produce high-resolution features (~ 10 µm) of varying thicknesses irrespective of the substrate. Demonstrations of wearable multilayer tactile sensing systems and stretchable skin-interfaced electronics validate the simplicity, versatility, and reliability of this manufacturing strategy, suggesting broad utility in the development of advanced soft electronics. could not be patterned as the SSLM lm was too thick. The triangles represent conditions where patterning was possible, yet with low yield, where 10-30% of the features were removed. The circles represent conditions where uniform and cleanly patterned features were observed. These results summarize the importance of tuning both the thickness of the SSLM lm and the PR in attaining cleanly patterned SSLM lm.


Introduction
With the rising demand for next-generation wearable healthcare devices, soft robotics, and conformable implantable devices, soft electronics are increasingly becoming of pivotal importance. 1,2,3 To enable the aforementioned applications; it is critical that the fabrication method be compatible with the wellestablished standard photolithographic process so that high-resolution, large-area and high-performance devices can be made. 4 The critical component that imparts softness to electronics are the stretchable electrodes and interconnects, and there are generally two main ways to fabricate them. 5 One method is to fabricate serpentine or wavy structures with standard metallic materials, 6, 7 and the other is the use of stretchable materials such as carbon nanotube composite, 8 metal nanowire composite, 9 and conductive polymers. 10 In the former case, high-density integration is di cult due to the extra space needed by the geometric design, and the stretchability of electronics is limited due to relatively small fracture strain of metallic materials. In the latter case, the stretchable materials either lack su cient conductivity and/or are incompatible with conventional photolithographic process.
Gallium-based liquid metal (LM) has recently drawn a great deal of attention as it can potentially overcome the aforementioned limitations, owing to its exceptional deformability and stretchability, and due to its high conductivity comparable to that of metals. 11,12,13 However, its liquid state, ultrahigh surface tension, and immediate formation of oxide layer render it very challenging for patterning and integration. 14  and Zheng et al. utilized stencil printing to pattern LM; 16, 17 however, the patterning dimension was limited to hundreds of micrometers due to the inherent resolution limit of stencil mask.
Herein, we introduce a solution shearing-based deposition of solid-state LM (SSLM) lm using speci cally designed LM particle ink for the fabrication of wafer-scale, multi-layered, and high-resolution, lithographically patterned soft electronics. Supplementary Table 1 presents a comparison between conventional LM-based soft electronics fabrication strategies and our technique. Importantly, our ink formulation with the inclusion of polyelectrolyte greatly stabilizes the dispersion of LM particles (Fig. 1a) and induces self-assembly during deposition by forming bridges between particles, which results in uniform and densely packed LM particle-based thin-lms, while without particle rupturing, during solution shearing (Fig. 1b). Such features enabled effective patterning of the SSLM lm using standard lift-off process (Fig. 1c). LM particle-based inks have previously been reported. 18 However, they have only been utilized by embedding in an elastomer matrix, 11,19,20,21 direct printing, 22,23 and screen printing, 24,25 mainly due to the lack of precise control of LM particles and rupturing during processing, which have limited their applicability towards high-resolution and high-performance soft electronics. As depicted in Fig. 1d (i) and (ii), the thickness of SSLM lm can be precisely controlled, and the lm can be patterned at wafer-scale with micron-level resolution. Furthermore, due to the chemical and mechanical stability of the lm, multilayer patterning is possible (iii). Finally, SSLM can be transferred or directly patterned on a variety of substrates (e.g., PDMS, PET, and gelatin-based biogel 26 ) (iv), rendering our technique a highly versatile process for the fabrication of various soft electronic devices. Our technique facilitates highresolution patterning and large-area fabrication. Furthermore, it is simple, cost-effective, tunable, and environmentally friendly, which is therefore industrially feasible and can be applied to large-scale manufacturing of soft and stretchable electronics.

Results
Preparation and characterization of ink with functionalized LM particles.
LM particle-embedded ink was prepared as presented in Fig. 2a. Eutectic GaIn-based LM and polystyrene sulfonate (PSS, molecular weight (MW): 70,000 g/mol) were dispersed in deionized water (DI) containing 5 vol% acetic acid (AA) using ultrasonication. The ultrasonication provides mechanical and thermal energy that induces the formation of LM particles covered with gallium oxide layer. 11,27 Fig. 2b are photographs and zeta potential values of LM dispersed in DI (left), DI/AA (middle), and DI/AA/PSS (right). The DI by itself does not effectively disperse the LM, as evident by the near transparency of the solution and the sedimentation of LM at the bottom of the container. However, with the addition of AA, the solution became turbid with a gray color, con rming the dispersion of LM in the solution. As previously been reported, this can be attributed to the carboxylic acid group in the AA. 28, 29 Furthermore, the addition of AA increased the zeta potential of LM particles from +42.5 mV to +76.8 mV (Fig. 2b, Supplementary Fig. 1), due to the increased acidity of the solution, and such an increase may enhance the electrostatic coupling of PSS to LM particles. With the inclusion of PSS in the solution, the zeta potential was a negative value at -5.4 mV, suggesting that the PSS (known as negatively charged polyelectrolyte) is surrounding the LM particles. 30,31,32 We have concluded that such an interaction also induces the bridging of LM particles via PSS, as schematically depicted in Fig. 2a. 33 The ink without and with PSS behaved differently upon the addition of HCl. Since HCl dissolves the gallium oxide layer, in the case of ink without PSS, all of the LM particles merged into one large LM droplet in the solution. 27 For the ink with PSS, such behavior was not observed, where the LM remained dispersed in the solution ( Supplementary Fig. 2). This further corroborates our assumption of PSS surrounding the LM particles, thereby enhancing their stability.  Supplementary Fig. 3). To study the rheological behavior of the LM ink (DI/AA/PSS), apparent viscosity was measured as a function of shear rate (Fig. 2d). With increasing PSS concentration, the apparent viscosity decreased. Previous studies indicate that with higher variability in particle size, the viscosity decreases (inset of Fig. 2d, Supplementary Fig. 4). 34 SEM images con rm that at high PSS concentration, LM particles in some regions aggregated and showed less spherical geometry ( Supplementary Fig. 5). This suggests that the aggregated particles behave as large particles, thus increasing the effective particle size variability of the dispersion.
To verify the compatibility of our ink for solution shearing process, wettability on polyimide substrate (hydrophobic surface) was measured by observing contact angles of different inks (Fig. 2e). 35,36 Firstly, the contact angle of 50 μl droplet was measured; subsequently, 25 μl was withdrawn from the droplet and the contact angle was measured again. The ink without PSS exhibited contact angle of 74.61° and 50.34°b efore and after ink withdrawal, respectively; whereas, that of the ink with PSS exhibited contact angle of 63.99° and 16.47°, respectively. The droplet with PSS did not decrease in diameter after ink withdrawal (i.e., the droplet was pinned), which was the reason for the larger decrease in contact angle. On the contrary, for the droplet without PSS, the decrease in contact angle was not as large due to the decrease in diameter upon ink withdrawal. This result suggests that PSS acts as a surfactant that decreases the interfacial energy between the ink and the substrate, which enhances the wettability of ink ( Supplementary Fig. 6, and Supplementary Movie 1, 2). Such a forced wetting property of PSSincorporated LM ink enables the formation of uniform liquid layer and thin-lm during solution shearing, as described below. due to poor dispersion of LM particles in ink. DI/AA exhibited improved coverage and uniformity; however, the coverage was still incomplete. This can be attributed to the lack of forced wetting ability and selfassembly of LM particles in the absence of bridging polymers. Furthermore, without PSS, rupturing and reduction of LM particles was frequently observed ( Supplementary Fig. 7), which rendered it di cult to conduct the lift-off process, as we will describe below. DI/AA/PSS generated a completely covered and uniform SSLM lm (Fig. 2g), which can be ascribed to well-dispersed LM particles and the bridging between them that aids their self-assembly during thin-lm formation. We have also con rmed the uniform distribution of components through energy-dispersive X-ray spectroscopy ( Supplementary Fig. 8) Fig. 2h and 2i are representative SEM images and particle size distribution as a function of sonication time, respectively, for the SSLM lms generated with two different MW of PSS: 70,000 and 1,000,000 (the two LM solutions contained the same molarities of PSS). In the case of larger MW PSS lm, nonspherical LM particles were commonly observed, where the interfacial area between the particles was relatively large. On the other hand, for low MW PSS lm, the LM particles were dominantly spherical. Such a difference in lm morphology can be attributed to reduced surface energy and stronger inter-particle attraction of the LM particles with the longer polyelectrolyte chains. 37 Moreover, as evident in Fig. 2i, the average LM particle size can be tuned via sonication time ( Supplementary Fig. 9). The inset of Fig. 2i con rms that with reduced average LM particle size, smaller features can be attained. We have determined that to attain a 10 μm line width, an average particle size of ~3.69 μm 2 is required. Large-area coating of SSLM lm through solution shearing. Fig. 3a is a schematic representation of SSLM thin-lm formation using solution shearing. Solution shearing is a technique analogous to blade coating, where the solution is sandwiched between a heated substrate and a moving blade. A meniscus (curved liquid-air interface) naturally forms between the blade and the substrate, and as the blade moves, thin-lm is deposited across the substrate via liquid-to-solid transition occurring near the edge of the meniscus, i.e., contact line (substrate-solution-air interface). 35 Solution shearing enables accurate control of uid dynamics and localizes the solvent evaporation at the meniscus, enabling uniform coating and precise tuning of thin-lm properties such as packing density and lm thickness. 35, 36, 38, 39 We have observed that during solution shearing, the LM particles continuously migrated towards the meniscus, which was critical for uniform thin-lm formation (Supplementary Movie 3). This can be attributed to effects such as capillary, Marangoni, pressure-driven, and boundary-driven ow that drive the transport of solute towards the contact line. 35 Supplementary Table 2 summarizes the bene ts of solution shearing in generating SSLM thin-lm compared to that of coating techniques.
As a means to observe the SSLM lm formation process in real-time, high speed (1000 frames per second) in-situ microscopy was used. Fig. 3b, 3c, and 3d, 3e are side and top view schematic and video images of the meniscus near the contact line in the presence and absence of pre-existing SSLM thin-lm acting as seed particles (i.e., seed lm), respectively. In the former case, observation of top and side view real-time videos (Supplementary Movie 4, 5) con rm that LM particles swiftly move towards the contact line and self-assemble into a close-packed lm to continue the growth of the seed lm. This is on the contrary to the latter case, where the majority of LM particles clump-up on the blade at its edge (Supplementary Movie 6). During solution shearing, the solvent passes through underneath the clumps, ultimately resulting in a non-uniform lm with a signi cant amount of voided regions (Supplementary Movie 7). Our results suggest that for SSLM lm formation, apart from the aforementioned effects, the inter-particle attraction is also an important factor in effectively transporting LM particles towards the contact line and self-assembling them into a thin-lm.
To achieve lift-off-based photo-patterning of SSLM lm, it is critical that the lm is coated in the evaporative regime (See Supplementary Fig. 10 for further explanation), where lm thickness increases with the decrease of shearing speed (Fig. 3f), and the lm grows at the contact line simultaneously with the moving blade (< 2 mm/s). Here, self-assembled packing of LM particles was observed ( Fig. 3b, 3c), and such particle-packed morphology (i.e., dry lm) maintained after thin-lm formation, as shown in Fig.   3g. 35 At lower shearing speeds, PSS with long-chain yielded higher lm thickness, which can be attributed to the stronger inter-particle attraction. At high coating speeds (> 10 mm/s) (Supplementary Movie 8), known as the Landau-Levich regime, the thickness increases with shearing speed (Supplementary Fig.   11). 35 In this regime, the lm exhibited bulk-like continuous morphology (Fig. 3h), indicating breakage and merging of LM particles during lm formation (see Supplementary Fig. 10 for further explanation). As will be discussed below, this 'wet lm' was not compatible with the lift-off process. In the wet lm, indium oxide peaks were detected in the XPS spectrum; whereas, for dry lm coated in the evaporative regime, such peaks were not present ( Fig. 3i and Supplementary Fig. 12). The indium oxide likely formed due to the oxidation of EGaIn, as the LM particles were being ruptured and annealed. 30 Fig. 3j is the surface roughness pro le of SSLM lm formed via solution shearing, where the lm was coated uniformly over a large-area with an RMS roughness of 710.6 nm. On the contrary, lm formed via blade coating could not be deposited uniformly on the substrate (see further discussion in Supplementary  Fig. 13 and Fig. 14). Fig. 3k is the pixel color distribution of optical images (25 × 25 mm) of spin-coated and solution sheared lms. Spin-coating generated relatively non-uniform lm, where the LM particle packing density decreased moving radially away from the center of the substrate (inset of Fig. 3k). These results con rm that solution shearing is, by comparison, a highly feasible SSLM lm deposition technique. Furthermore, due to the ink's excellent wetting ability, the SSLM lm can be solution sheared on a variety of surfaces, as depicted in Fig. 3l, making it highly versatile for the fabrication of various devices.
Lift-off-based photo-patterning and multilayer electronics demonstration.
Previous lithographic technique utilized the lift-off process of LM in its liquid state, which required prepatterned gold to form an alloy with the LM. 15 Furthermore, because LM in its liquid state is mechanically and chemically unstable, multiscale/multilayer fabrication and direct use as metal contacts is infeasible. Since our technique, on the contrary, utilizes LM as a solid-state lm, the aforementioned features are achievable, as standard cleanroom processes (i.e., lithographical patterning, spin coating, and reactive ion etching) can be conducted. Fig. 4a is a photograph of SSLM lm-based interdigitated electrodes transferred onto a soft elastomer substrate, 6,40 which was then placed on a nger (Supplementary Fig. 15 and Fig. 16). Fig. 4b are SEM image and depth pro le data of patterned SSLM lm with 100 μm line width and spacing, demonstrating high uniformity and cleanly lifted off edges. Bath sonication during lift-off (solvent: acetone and isopropanol) was essential in attaining such edges. Fig. 4c are thin (4 μm) and thick (15 μm In attaining high-density and high-resolution (< 50 μm) features, stronger ultrasonication was required to attained cleanly patterned SSLM lm. However, under these conditions (in the case of using 70,000 MW PSS), peeling-off (blue-bordered area) and rupturing (red-bordered area) of the SSLM lm occurred, as depicted in Fig. 4f. Fig. 4g is the peel-off ratio (i.e., ratio of peeled off area to total area) for 40 μm and 100 μm resolution serpentine structure, patterned using SSLM lm with 70,000 and 1,000,000 MW PSS.
The latter case exhibited a lower peel-off ratio, indicating that high MW PSS is necessary for highdensity/resolution patterning. This can be attributed to the stronger inter-particle attraction, as mentioned above. Fig. 4h is a wafer-scale pattern of SSLM lm with high-resolution (The smallest line width: 20 μm), obtained using 1,000,000 MW PSS. From this point onward, SSLM lm with 1,000,000 MW PSS was used.
Lift-off was conducted after coating a wet lm of SSLM (left) and with ink without PSS (right) (Fig. 4i). For both lms, continuous bulk-like morphology was attained (Fig. 3h, Supplementary Fig. 7). Since this results in highly attractive interaction within the bulk of the lm, both of the lms could not be patterned via lift-off process. 30 This emphasizes the importance of forming particle-packed morphology for lift-offbased patterning.
In order to fabricate highly-integrated, large-scale electronics, SSLM lm should be compatible with conventional cleanroom-based fabrication processes such as multilayer deposition, spin-coating, and reactive ion etching (RIE). Fig. 4j are cross-sectional SEM images of multilayer SSLM lms with (right) and without (left) polyimide lm in between, con rming that vertically insulating or conductive architectures can be fabricated. In the former case, polyimide needed to be spin-coated (4000 rpm) on top of the SSLM lm, and the SSLM lm remained rmly attached to the substrate during spinning, owing to the strong adhesion of SSLM lm to the substrate surface and the mechanical stability of the lm itself ( Supplementary Fig. 17). For multilayer lithography, the use of RIE with oxygen plasma is inevitable to remove polymer layers. Unfortunately, RIE can also damage the metal layers, which can deteriorate the electrical performance or even destroy the entire device. The SSLM, on the other hand, maintained the same chemical composition and morphology after RIE. Fig. 4k and Supplementary Fig. 18 are XPS spectra of elements in SSLM before and after RIE. There was no change in XPS spectra, further con rming the stability of SSLM lm under RIE. To the best of our knowledge, this is the rst demonstration of utilizing standard cleanroom processes to pattern and incorporate LM, opening up a wide variety of possible soft electronic applications that require multilayer, high-resolution, and highdensity features over a large-area.
One critical aspect of LM-based electronics is the ease in which it can be rendered conductive (i.e., activated). 18 Our SSLM lm becomes activated during the transfer process or during the substrate peeloff process to a resistivity of 8.67x10 -7 Ωm. The resistivity can be further reduced by applying strain. Fig.  4l is resistance as a function of time under strain at 30%. When strain is applied initially, the resistance suddenly decreases from 1.445 to 1.034 Ω. Thereafter, the resistance continues to decrease under strain but eventually saturate to 1.032 Ω. This value is maintained and only a negligible (0.001 Ω) resistance change was observed under strain release and reapplication. Hence, our SSLM lm-based soft electronics can be activated under normal device operation without the need of additional (e.g., mechanical rubbing) activation steps. 11,41 Characterization and demonstration of soft electronic with SSLM lm.
As mentioned in Fig. 3l, SSLM lm can be directly coated on a variety of substrates. Therefore, SSLM lm-based electronics can be directly fabricated on soft substrates such as PDMS (see Supplementary  Fig. 19 and Methods section for detailed fabrication steps). This direct coating and patterning on soft substrate improve its yield by skipping the cumbersome transfer-printing step. Also, conventional electronic components (μ-LED in this case) can be integrated with SSLM-based soft electronics (Fig. 5a).
The inset shows that LEDs can be lit using SSLM as electrodes. Interestingly, as presented in Fig. 5b, our SSLM lm demonstrated consistent electrical impedance under various strains and AC frequencies.
Typically due to skin effect, metal electrodes, including conventional LM, suffer from an increase in impedance with AC frequency. 42 The absence of such an effect in our lm can be attributed to the presence of ions, as previously been reported. 43 Such consistency in electrical impedance allows stable operation of electrical circuits under various conditions for different applications. I-V curve of an LED connected to an SSLM lm-based stretchable interconnector was measured with and without strain to corroborate stable operation (Fig. 5c). Only a negligible change in current was observed under the application of 50% strain.
The variation in resistance with strain can be controlled by tuning the thickness of the lm, as depicted in Fig. 5d. In the case of thick lm (15 μm), there is only a negligible change in resistance under the application of strain, making it an ideal material for electrodes or interconnects. On the other hand, the resistance of the thin lm (4 μm) increases with strain, which therefore can be used as strain sensors, 44 or deformable heaters. 45 The deviation from theoretical gauge factors for both lms indicates that the resistivity is decreasing with strain. The dotted line represents theoretical gauge factor (2), assuming resistivity remains constant. For the thicker lm, a higher amount of electrical paths are likely to be generating under strain, which largely compensates for the increase in resistance due to geometrical change. Further investigation is needed to elucidate the exact underlying mechanism, and is the subject of our future work.
Utilizing all of the unique capabilities of our SSLM lm, (e.g., multilayer, large-area, high-resolution fabrication, lm thickness control, and direct patterning on soft substrates), soft arti cial nger that can decouple pressure and strain was demonstrated (Fig. 5e). The arti cial nger consisted of two SSLM layers electrically connected through a via hole (the multilayer fabrication process is presented in Supplementary Fig. 20). The rst layer consisted of thick SSLM lm-based interdigitated electrodes. On the interdigitated electrodes, pyramid structured PDMS coated with a conductive polymer (polypyrrole) was laminated, which together functioned as piezoresistive pressure sensor ( Supplementary Fig. 21). 46,47 The second layer consisted of a thin SSLM lm, functioning as a strain sensor. Fig. 5f is photograph of the arti cial nger placed on a human nger, and Fig. 5g are real-time measurements of pressure and strain. The resistance between terminals 1' and 3' (both belonging to the rst layer) decreased with pressure, and the resistance between terminal 2" (belonging to the second layer) and terminal 1' increased with strain (see Supplementary Fig. 22 for description of device architecture and operation).
When pressure and strain are independently applied (blue and red regions respectively), only the corresponding signal undergoes change. Moreover, when both stimuli are applied simultaneously (yellow region), the corresponding signals change concurrently without cross-interference, verifying that pressure and strain can be effectively decoupled. Furthermore, unlike conventional LM, one key feature of SSLM lm is their high stability under repeated contact with another surface. Fig. 5h is resistance variation of the pressure sensor under repeated application of 30 kPa over 10,000 cycles. Inset in Fig. 5h is SEM image of the pyramid surface after cycling test. Despite the repeated contact with the pyramid surface, the SSLM lm did not rub off of the substrate. We also demonstrated closed-loop antenna with SSLM lm, which can be applied to soft-wireless devices (Supplementary Fig. 23). 46, 48 Such a demonstration was achievable owing to the aforementioned attributes of our SSLM lm.
Demonstration of skin-interfaced electronics with SSLM lm.
Softness, deformability, biocompatibility, and excellent conductivity of SSLM lm make it a compelling option for the construction of bio-integrated electronics. 3,14,25,49 To demonstrate its potential, we built skin-interfaced wearable electronics for electrophysiological monitoring. More speci cally, we fabricated fully-rubbery surface electromyography (sEMG) sensor, integrating SSLM lm as skin-interfacing electrodes and as interconnects, as presented in Fig. 6a. Here, by using a mask aligner, SSLM electrode patterns were connected to a conventional wireless system via anisotropic conductive lm. The EMG sensor consists of three soft electrodes, which are used for ground, reference, and measurement. The intrinsically exible, stretchable nature of the sensor allows not only intimate integration with the curvilinear surface of the skin but also dynamic adaptation to skin deformation (Fig. 6b), thereby allowing stable measurement of sEMG signals. Real-time monitoring of sEMG signals on a forearm with repeated wrist bending motions veri es capability of the SSLM skin sensor for high-quality electrophysiological measurements (Fig. 6c). For measuring electrophysiological signals or for delivering electrical stimuli to the skin, direct contact of electrodes with bio-surface is inevitable. In this regard, the stability of the electrode should be examined. The SSLM lm does not leave any residue on the skin after intimate contact with the skin for several hours, as seen in Fig. 6d. Furthermore, several studies have shown biocompatibility of GaIn-based LM, suggesting bio-safety of SSLM for use as a skin-interfaced material. 25,48 These demonstrations together further con rm the potential applicability of SSLM as biointegrated electronics, which often requires high-resolution features, metallic electrical properties, while maintaining soft, biocompatible, and stable properties. 50 Conclusion Soft electronics are expected to play a critical role in the forthcoming electronic applications, where devices will make intimate and conformal contact with the soft tissues of the body. Herein, liquid metal is the most appropriate material for interconnects and electrodes, and their applicability relies on their compatibility with well-established cleanroom based photolithographic patterning, as large-area, multilayer, multiscale, and high-resolution features are needed for integration with conventional electronic systems. These features have not yet been realized due to the di culty of making mechanically and chemically stable LM lm uniformly over a wafer-scale that can also be lithographically patterned. Our new ink formulation stabilizes the LM particles and enables the formation of highly uniform and stable solid-state lm using solution shearing. We also con rmed that the particle-packed morphology of SSLM lm achieved by coating under the evaporative regime allows photo-patterning with a conventional lift-off process. The lm can be coated and patterned on a variety of substrates, and its thickness control enables tuning of conductance. These attributes were utilized to fabricate SSLM multilayer interconnects, tactile sensors, and skin-interfacing electrodes for sEMG measurements. We anticipate that our technique will provide the pathway for signi cant advancement of soft electronics, thereby bringing forth new opportunities in soft robotics, and wearable and implantable devices in the near future.

Method
Materials. All chemicals were used without further puri cation and acquired from Sigma-Aldrich unless otherwise mentioned. To prepare and characterize the LM ink, eutectic gallium indium alloy (EGaIn, Rich-Metals, China), poly(styrene sulfonate) (PSS, with an average molecular weight of 70,000 and 1,000,000), acetic acid (99%), ethanol (SAMCHUN, 99.5%), acetone (SAMCHUN, 99.5%), isopropyl alcohol (IPA, SAMCHUN, 99.5%), dimethyl sulfoxide (DMSO, 99.9%), hydrochloric acid (HCl, 37%), and Span 80 were used. For lithographical patterning and device fabrication, we used trichloro (1H,1H,2H SSLM lm coating via solution shearing. The LM ink was coated on the various substrate (glass, PET lm, Si wafer, PI lm, PR lm, PDMS, Au) by using a customized shearing machine. Before solution shearing, samples were treated with oxygen plasma (CUTE, Femto Science) at 100 W for 1 min to clean and activate the surface. The substrates were heated to 70 ℃ during lm coating. Here, two types of glass slides (25 mm × 75 mm/ 50 mm × 75 mm) were utilized as the coating blade depending on the width of the lm needed. We xed the angle (5°) and the gap between blade and substrate (200 μm) for all the lms coated. 100 μl of LM ink was injected between the coating blade and substrate right before solution shearing. After the shearing process, SSLM lm-coated substrates were placed on a 70 ℃ hot plate to completely evaporate away any remaining solvent. For blade coating, the shearing angle was modi ed to 90° (blade vertically standing), and for the wet lm coating, the speed of moving substrate increased to > 10 mm/s. SSLM lm patterning with lift-off process. For patterning of the SSLM lm, NR9-3000py, and 8000py (liftoff PR) were spin-coated and selectively exposed to UV light using a mask aligner (MJB4). Spin-coating and UV exposure conditions were tuned according to the thickness of the PR. The exposed PR was baked at 110 ℃ (150 sec), and developed in MIF 300 developer for 50 sec. Finally, the substrate was exposed to oxygen plasma treatment for surface activation. Then, SSLM lm was coated on the PR patterned substrate via solution shearing as described above. SSLM lm coated sample was immersed in acetone/IPA (4:1) and sonicated in bath sonication for lift-off.
Fabrication of soft electronics through transfer printing of patterned SSLM lm. Schematic illustration in Supplementary Fig. 15 depicts the overall patterning and transfer printing process of SSLM lm. For this process, PMMA working as a sacri cial layer, PI working as a temporary substrate, and PR layers for patterning are coated on the substrate (glass, silicon wafer) sequentially. PMMA solution (4 wt% dissolved in anisole) was spin-coated (3000 rpm, 30 sec) on oxygen plasma-treated substrate. Subsequently, PMMA was then annealed at 180 ℃ for 3 min, and the edges were removed with acetone.
PI solution was spin-coated onto the PMMA-coated glass at 4000 rpm for 60 sec and cured at 250 ℃ for 60 min. Thereafter, PR patterning, SSLM lm coating, and lift-off-based patterning were conducted, as described above. Once SSLM lm was patterned, PMMA sacri cial layer was removed by putting the sample in the acetone overnight. Then, PI lm with patterned SSLM lm was delaminated from the original substrate and transferred to a soft substrate by stamping or using conventional thermal releasing tape. After transferring, oxygen plasma etching was conducted (100 sccm, 200 W, 1 hour) to remove PI lm.
Fabrication of soft electronics through direct patterning of SSLM lm. Schematic illustration in Supplementary Fig. 19 depicts the overall direct fabrication process of soft electronics with SSLM lm. Before coating PDMS lm on rigid glass substrate, the glass substrate was modi ed as a hydrophobic surface with PFOCTS by chemical vapor deposition to facilitate the delamination of the lm. Here, all four edges of the glass were sealed with PI tape prior to hydrophobic coating process to maintain hydrophilicity so that delamination can be prevented during solution-based processing (e.g., development, lift-off). After chemical vapor deposition, PDMS solution (base and curing agent were mixed in a weight ratio of 10:1) was poured on a glass slide and spin-coated (1000 rpm, 30 sec), and cured at 70 °C to form a PDMS lm. Subsequently, PI solution was spin-coated on the PDMS-coated glass at 4000 rpm for 60 sec and cured at 250 ℃ for 60 min. PR was patterned with the aforementioned methods. Afterwards, oxygen plasma etching was conducted (50 sccm, 200 W, 30 min) to remove PI lm in the regions absent of PR. Subsequently, SSLM was coated and patterned directly on the PDMS lm by following the abovementioned methods. Remaining PI was removed by oxygen plasma etching (100 sccm, 200 W, 1 hour). For the multilayer structure, we repeated the same sequence from the PI coating on the SSLM patterned PDMS. Finally, the sample was cut along the hydrophobic/hydrophilic border, and the PDMS was lifted off of the glass substrate.
Fabrication of pressure sensor. Schematic illustration in Supplementary Fig. 21 depicts the overall fabrication process of the conductive polymer-coated pyramid pressure sensor. For the structuring of the pyramid pattern, the silicon substrate is chemically etched as previously reported 47 and subsequently coated with PFOCTS to facilitate the delamination of PDMS lm. PDMS solution (base and curing agent were mixed in a weight ratio of 10:1) was then poured on the mold and spin-coated at 800 rpm for 30 sec.      Electrical characterization of SSLM lm and various soft electronic demonstrations. a, Photograph of large-scale soft electronics with SSLM lm-based interconnect connected to a μ-LED. Inset is the μ-LED being turned on. Scale bar, 3 cm. b, Impedance variation of SSLM lm and bare LM lm as a function of strain and frequency. c, I-V curve of the LED connected to a stretchable SSLM lm-based interconnector with (red) and without (black) 50% strain. d, Relative resistance change versus strain of a thick (~15 μm) and a thin (~4 μm) SSLM lm. Inset presents the initial resistances of thick and thin SSLM lms. e, Illustration of multilayer SSLM lm-based arti cial nger that can measure and decouple pressure and strain. Pyramid-patterned PDMS coated with polypyrrole was laminated on the interdigitated electrode for pressure sensing. Inset is a cross-sectional SEM image of a stacked structure. f, Photograph of SSLMbased arti cial nger placed on a glove. g, Real-time monitoring of pressure and strain with SSLM-based arti cial nger. Pressure and strain are successfully decoupled. h, Resistance change of the pressure sensor upon repeated application of 30 kPa of pressure for 10,000 cycles. Inset is an SEM image of a pyramid structured pressure sensor after the cycling test. Figure 6