www.acsami.org Research Article

Three-Dimensional Stretchable Microelectronics by Projection Microstereolithography (PμSL) Yuejiao Wang, Xiang Li, Sufeng Fan, Xiaobin Feng, Ke Cao, Qi Ge,* Libo Gao,* and Yang Lu*

Cite This: https://dx.doi.org/10.1021/acsami.0c20162 Read Online

ACCESS Metrics & More Article Recommendations *sı Supporting Information

ABSTRACT: Stretchable and flexible electronics conformal to human skin or implanted into biological tissues has attracted considerable interest for emerging applications in health monitoring and medical treatment. Although various stretchable materials and structures have been designed and manufactured, most are limited to two-dimensional (2D) layouts for interconnects and active components. Here, by using projection microstereolithography (PμSL)-based three-dimensional (3D) printing, we introduce a versatile microfabrication process to push the manufacturing limit and achieve previously inaccessible 3D geometries at a high resolution of 2 μm. After coating the printed microstructures with thin Au films, the 3D conductive structures offer exceptional stretchability (∼130%), conformability, and stable electrical conductivity (<5% resistance change at 100% tensile strain). This fabrication process can be further applied to directly create complicated 3D interconnect networks of sophisticated active components, as demonstrated with a stretchable capacitive pressure sensor array here. The proposed scheme allows a simple, facile, and scalable manufacturing route for complex, integrated 3D flexible electronic systems. KEYWORDS: stretchable electronics, flexible electronics, 3D microelectronics, 3D printing, advanced manufacturing

■ INTRODUCTION the driven mechanism of compressive buckling leads to a finite Stretchable and flexible electronics holds superior mechanical level of complexities and diversities in the formed 3D characteristics to be bendable, stretchable, and twistable, thus topologies. offering promising opportunities for diverse fields including By contrast, 3D printing selectively builds materials layer by 1−3 4−6 − layer to fabricate required structures. The one-step manufac- biomedical engineering, robotics, human machine ffi interfaces,7,8 and so forth. The two prevailing approaches turing greatly improves production e ciency and lowers costs, and the technology also enables more 3D design freedom and enabling stretchable electronics explore conceptually different 9 material selectivity, thus providing an effective access to principles. One approach utilizes intrinsically stretchable producing electronics. Various 3D printing techniques have materialssuchasconductive nanomaterial-doped soft 10−12 13−16 been reported for creating stretchable and flexible electronics elastomers, conductive polymers, and hydro- 34−38 38−44 17−19 varying from the 2D form to 3D geometries. Here, gels. However, these novel materials exhibit low or

See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. we take the advantage of projection microstereolithography strain-sensitive electrical conductivities, causing unwanted μ Downloaded via SOUTHERN UNIV SCI & TECHLGY CHINA on February 23, 2021 at 07:31:03 (UTC). (P SL), a digital light processing-based 3D printing technol- variations in electrical signals for intended applications. The ogy, that fabricates highly complex 3D geometries with high other approach creates structures to eliminate the strain of resolution (up to 2 μm) through area ultraviolet (UV) traditional high-performance electronic materials including projection which triggers localized photopolymerization.45 metals and semiconductors, such as planar curvy struc- − Using this 3D microfabrication approach, the most stretchable tures20 23 and honeycomb geometries.24,25 The combination 3D geometries according to finite element analyses (FEAs) can of structure design with novel materials has also been applied be easily built up as a polymer template. After sputter coating to additionally increase the stretchability and decrease the thin Au films on the surface of the printed template, the as- resistance change.26 Recent research efforts have been devoted obtained 3D conductive structures demonstrate exceptional to building stretchable 3D architectures by buckling 2D − geometries on a prestrained elastomer support.27 33 A set of 3D layouts of membranes or ribbons from nano- to Received: November 11, 2020 macroscales can be achieved, but the approach requires Accepted: February 5, 2021 complicated programming of initial 2D patterns and a series of sophisticated fabrication procedures including depositing material layers, patterning by photolithography, etching, transferring 2D precursors, and defining bonding sites. Also,

© XXXX American Chemical Society https://dx.doi.org/10.1021/acsami.0c20162 A ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article stretchability (∼130%), conformability, strain-insensitive elec- 3D composite structures can be assembled with functional trical properties (<5% resistance variation under 100% tensile components on substrates and finally turned into practical strain), and stability under cyclic loading. Compared with their applications. 2D counterparts, the 3D microstructures present compact FEA was utilized as shown in Figure 2 to compare different geometries (Figure S3) and can deform freely out of plane to 3D structure designs of the polymer template for accommodat- accommodate much larger stretching strains. Furthermore, the ing large applied deformation, so that the intrinsic strains in the access of PμSL-based 3D printing to intricate free-form coated conductive films on its surface could be minimized. architectures can be exploited to realize integrated circuits by Although the utility of 2D curvy structures as stretchable one-step manufacturing. To provide a demonstration example, interconnects has been established by previous studies, the an array of sophisticated 3D capacitive pressure sensors physical coupling with the substrate leads to a sharp stress interconnected by 3D stretchable microstructures was concentration and limits their performance.31 Obviously, the fabricated. This proposed strategy shows great potential for extension of these structures to 3D construction could further manufacturing complex, integrated 3D electronic systems due improve stretchability since they can deform freely out of plane to its simplicity, high level of design freedom, and scalability. to accommodate the applied in-plane tensile loading. Three representative 3D designs were chosen: a sinusoidal structure ■ RESULTS AND DISCUSSION which corresponds to stretchable out-of-plane wrinkle − The fabrication approach takes advantage of the versatility and structures46 48, a U-shaped structure and a horseshoe-shaped scalability of the PμSL-based 3D printing process to realize any structure, which have been demonstrated to be highly desired 3D circuit layouts even for very complex systems. stretchable in 2D forms.22,49,50 The strain distributions of the Figure 1 illustrates the overall four steps for creating three structures under the same large tensile strain (40%) are given in Figure 2a−c for comparison. Since the maximum strain all occurs at the polymer template surface, it also directly affects the resistance of the coated conductive films. In fact, the metal films deposited in our following experiments are merely tens of nanometers thick, whereas the polymer template has a width (denoted by w in Figure S1a) of around 70 μm, about a thousand time of the metal film thickness. Therefore, the presence of the metal film has a marginal effect on the mechanical properties and deformation process of the polymer structures. It thus can be inferred that the strain distribution of coated metal films approximates the surface strain distribution of the polymer template without coating, if no cracks or debonding happens. In conclusion, the horseshoe-shaped design outperforms the other two by reducing the maximum strain of the sinusoidal design by 53% and U-shaped design by 13%, showing the strongest ability to accommodate large stretching strains. See Supporting Information 1 for the details regarding the geometrical design and deformation distribution of the three structures. The stretchability of the horseshoe- shaped structure with varying geometric parameters was further investigated by an analytical approach (Supporting Information 2). Besides, the tensile load versus strain curves Figure 1. Fabrication process of the stretchable and flexible were also resulted (Figure 2d) by FEA and the horseshoe- electronics based on PμSL 3D printing. (a) 3D geometrical design shaped structure was demonstrated to be the softest design, for mechanical and electrical optimization assisted by FEA. (b) μ which is desirable for developing soft electronics. Schematic illustration of P SL-based 3D printing of the optimal CAD Therefore, the horseshoe-shaped microstructure was chosen models. (c) Schematic illustration of sputter coating of the 3D-printed to be the basic unit of the polymer template for constructing a polymer template with metal films to endow conductivity. (d) 3D stretchable conductor and 3D-printed out. After coating Assembly with LED components and application of the stretchable fi circuits on a human finger. The scale bar of the inset is 3 mm. with thin Au (thickness: 60 nm) lms, the deformation process of the conductive structure was investigated by in situ SEM tensile experiments, as shown in Figure 3. SEM images were stretchable and flexible electronics building on PμSL-based taken from the top view of the structures before deformation 3D printing. First, different configurations of 3D circuits were (Figure 3a) and after uniaxial tensile strain was applied to directly proposed and optimized by the computer-aided design 120% (Figure 3b). No microcracks or debonding of the coated (CAD) and FEA software according to device-working Au films was observed on the structures even under such a requirements. Second, the geometric model of the optimal large overall stretching loading, showing their good adhesion to design was sliced and transferred into a 3D printer to be the polymer template. Moreover, several local regions which photocured layer by layer to form 3D polymer templates. possess high strain distributions according to FEA results were Third, after the printed polymer microstructures were cleared selected for a close-up view. Region i, the upper surface at the to remove uncured photopolymers on their surface, they were crest of 3D structures, is under compressive strains due to coated by metals (e.g., Au here) or other conductive materials bending-dominated deformation (Figure 3d), and thus, small such as graphene and carbon nanotubes (CNTs) (Figure S4). wrinkles of the Au films form, which are reversible according to Once endowed with electrical conductivities, these as-obtained the experimental observation and have little impact on

B https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article

Figure 2. Comparison of max principal strain distribution between (a) sinusoidal structure, (b) U-shaped structure, and (c) horseshoe-shaped structure under 40% stretching strain resulted by FEA. The insets correspond to the magnified views of the lower surface of the top region. (d) Tensile load vs strain curves of these three structures.

Figure 3. In situ SEM tensile test. (a) Undeformed state. (b) Under uniaxial tensile strain of 120%. (c) Schematic illustration of loading and the imaging direction. (d) Schematic illustration of the compressed state of region i under overall tensile deformation. (e) Magnified views of two regions i and ii under the undeformed state and their corresponding deformed state i# and ii#. (f) Magnified views of regions at the left and right side of region ii#. electrical properties. Region ii and regions beside it, the upper Meanwhile, the observed integrity of the Au films is also surface of the bottom line and the nearby part of the horseshoe reasonable because thin metal films with nanoscale thickness shape, suffer the nearly maximum tensile strains of the whole supported by substrates have been widely reported to be able structures but still keep the integrity of Au films. Therefore, to sustain large strains without cracks, although freestanding − these observations convincingly indicate that adopting 3D- metal films rupture at small strains of a few percent.51 55 This printed horseshoe-shaped structures significantly reduces the is because the strain localization of the metal films is retarded maximum strain on the structure as well as coated thin metal by the substrate. Xiang et al. found that Cu films deposited on films compared with the applied overall stretching strains. a polymer substrate could stretch up to an elongation of 10%

C https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article and the majority of the Cu film keeps intact.51 Later, by 50%. The overall maximum tensile strain of the conductor can improving the good adhesion between them, Lu et al. reported reach as large as ∼130% until rupture, with only a ∼6% that polyimide-supported Cu films deform up to a very large resistance increase. The measured large stretchability agrees strain of 40%, while no microcrack was observed.53 Similarly, well with the analytical prediction that adopting the defined 3D Sim et al. demonstrated that a strain of 10% does not generate horseshoe-shaped structure can improve the stretchability of cracks in thin Ag coatings.55 Here, by additionally integrating intrinsic materials (failure strain of 5%, see Figure S2d)by28 the 3D geometrical design to minimize strains in coated Au times (Supporting Information 2). A stable electrical property films in this work, we expect a strain-insensitive electrical is also maintained under bending from a concave state of 90° behavior of the 3D conductor under large deformation. to a convex state of 180°, and the corresponding resistance Further, mechanical and electrical robustness tests of these change is less than 1% (Figure 4b). To investigate the conductive 3D horseshoe-shaped structures under different durability of our stretchable 3D microstructure conductors, we types of deformations were conducted (Figure 4). As shown in also measured the normalized resistance change over 2000 periods of a cyclic tensile loading and unloading of 30% strains (Figure 4c). The normalized resistance degrades slightly less than 2% over so many stretching/releasing cycles, demonstrat- ing that the integrity of the 3D polymer template and coated Au films is still preserved. In addition, these 3D micro- structures can withstand out-of-plane compression deforma- tion up to 20% (Figure 4d), beyond which plasticity appears according to the experimental observation. The extension of the horseshoe-shaped microstructure to the 3D network of stretchable and flexible electronics was constructed, as shown in Figure 5. The 3D-printed large-area array of microstructures is mechanically flexible to be conformably attached on the finger by capillary force after dropping ethanol on it and wrapped on the finger and dried (Figure 5a). An equal biaxial strain as large as 120% can be applied to the whole structure without a fracture (Figure 5e,f). Moreover, the shape of the as-printed 3D microstructures agrees well with the original design (Figure 5b,c). On the other hand, the scalability, high level of design freedom, and customization of the fabrication approach facilitate a straightforward implementation of any desired complex layouts for practical applications (Figure S6). For demonstration, an Figure 4. Mechanical and electrical robustness tests of the conductive example of a hand pattern with the size close to a baby’s hand 3D horseshoe-shaped structures. (a) Under uniaxial stretching ranging from 0 to 128%. (b) Under bending from concave 90° to was easily realized by designing CAD models (Figure 5g). ° These excellent mechanical behaviors of the horseshoe-shaped convex 180 . (c) Cyclic test up to 2000 cycles under stretching fi between 0 and 30%. (d) Under out-of-plane compression. The two design and easy access to the sophisticated con guration of the insets of (a,b,d) show the sample under the start and end points of the process prove the great potentials of PμSL-based 3D printing stretching, bending, and compression test, respectively. in producing wearable electronics. All the above-mentioned microstructures have a small height from 0.9 to 1.1 mm, but it Figure 4a, a negligibly small increase in resistance (<2%) can be further reduced to 0.45 mm by a higher resolution happens as the microstructure conductor is stretched from 0 to PμSL-based 3D printing machine. The smaller version of the

Figure 5. Printed 3D network of the horseshoe-shaped microstructure array for flexible and stretchable electronics. (a) Image of a representative example of the 3D microstructure network wrapped around a finger. (b) Angled image of the 3D network with a close-up view. (c) SEM image of a microstructure array from the front and top views for structural characterization. (d) Smaller version of a 3D microstructure array placed on a fingertip, which is printed by a higher resolution 3D printing machine. (e,f) Undeformed status and biaxially stretched to 120%. (g) Hand pattern of the horseshoe-shaped microstructure array.

D https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article

Figure 6. Stretchable piezocapacitive sensor arrays. (a) Schematic illustration of the structure of the piezocapacitive sensor array. (b) Image showing the flexibility of the sensor array by wrapping it around a finger. (c) Sensor array in its undeformed state. (d) SEM image of a single sensor fi Δ electrode with surface microstructures from an isometric view and (e) magni ed view of a local region. (f) C/C0 response as a function of applied pressure of a single sensor. (g) Real-time capacitance response of a single sensor under increasingly loading pressures. (h) Cyclic test up to 1000 cycles under a pressure of 10.6 kPa, with the inset showing a magnified capacitance curve highlighted in the dark dashed box. (i,j) Pressure distribution mapping of the pressure sensor array under different weights. horseshoe-shaped microstructure network was printed and The sensing properties of a single piezocapacitive sensor placed on a fingertip (Figure 5d). were first evaluated using the as-obtained electrodes. The Δ Δ To further confirm that our approach can build complex sensitivity of the device was given by S =( C/C0)/ P, where Δ integrated microelectronic systems, Figure 6 shows a C, C0, and P represent capacitance change, initial capacitance, demonstration example of a 3D-printed stretchable pressure and corresponding applied pressure, respectively. As shown in sensor array consisting of 4 × 4 piezocapacitive sensors Figure 6f, the sensitivity gradually decreases from 0.073 to −1 − interconnected by 3D horseshoe-shaped microstructures. The 0.030 kPa within the applied pressure range of 0.5 106 kPa. ’ whole electrode arrays were fabricated through one-process The real-time response of device s capacitance under printing, followed by sputter coating of Ag films to endow increasingly loading pressures was also measured, in which electrical conductivity. To improve sensing performance, the increasingly larger steady capacitance values quickly reached under the same applied pressure (Figure 6g). A slight conical microstructures were designed on the surface of each decrease in capacitance before each steady state was observed sensor electrode. Two as-obtained electrode arrays mounted and can be attributed to noncontact sensing behavior. on the PDMS substrate (200 μm) for encapsulation were Moreover, the device shows excellent long-term stability after finally orthogonally laminated onto the dielectric layer of fi μ repeatedly loading/unloading a pressure of 10.6 kPa up to another PDMS lm (100 m) facing with conical micro- 1000 times. Finally, the stretchable pressure sensor array of 4 × patterns on the electrode surface (Figure 6a). The stretchable 4 pixels covering an area of 28 mm × 28 mm was tested for device adopted an island-bridge concept where the sensors are pressure mapping ability. The size of each pixel is 4 mm × 4 the rigid islands of the active components connected by the mm, and the length of each 3D horseshoe-shaped interconnect bridge of 3D stretchable interconnects. The overall stretch- is also 4 mm under the undeformed state. Two small weights ability of the whole device is decided by the 3D horseshoe- were placed on it at two different locations, respectively. The shaped interconnects and can be roughly estimated, as location information was accurately indicated from pressure illustrated in Figure S3b. mapping images, in which the 10 g weight (12 mm diameter)

E https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article causes a larger capacitance change in a larger area, whereas the demonstration of 3D-printed polymer microstructures to be 5 g weight (8 mm diameter) generates a more focused assembled with other functional materials including CNTs, graphene, capacitance change. and Ag nanowires (Figure S4), the printed sample was dipped into their highly concentrated solutions and then pulled out for drying. This process was repeated many times until the measured resistance ■ CONCLUSIONS of the sample reaches a constant value. We have demonstrated the application of PμSL 3D printing in Fabrication of the Capacitive Pressure Sensor Array. The 3D fabricating stretchable and flexible 3D microelectronics. The printing setup of the pressure sensor electrode was kept the same as advantage of 3D printing to fabricate free-form structures can stated above for printing horseshoe-shaped microstructures using Nano Arch S140. Then, the as-obtained structures were put into the be exploited to utilize the geometrical design to achieve the sputter machine for coating Ag films (70 nm) from both the bottom most stretchable 3D microstructures and by coating functional ff and top sides. Adhesive silver paste was used to connect sensor materials on their surface presents an e ective way for electrodes to narrow conductive copper foil as current collectors. stretchable electronics. The 3D design enables a larger Electromechanical Measurements and In Situ SEM Tensile stretchability (∼130%) and a more compact configuration Tests. The electrical measurement of resistance was performed by the compared to traditional 2D layouts. After sputter coating of Digital Multimeter (F15B+, FLUKE) and capacitance by the LCR thin Au films, the structure demonstrates a superior ability to Meter (TH2827A, Tonghui). The monotonic tensile test was applied minimize the mechanical strains in the conductive films under by a customized manual stretcher assisted with a ruler (Figure S5a), deformation, thus leading to stretchable conductors with whereas the cyclic tensile test was carried out by a miniature electric slide table. The compression test was conducted by Microtest strain-insensitive electrical performances. For 3D-printed (Gatan). Three different mechanical testing systems were chosen due structures with less exposed areas to explore in the future, to the limitation of the maximal/minimal loading speed, displacement other coating methods can be combined such as electroless range, and ease of operation. For the in situ tensile test, SEM (Quanta deposition, dip coating, and vapor deposition evaporation. FEG 450, FEI) equipped with Microtest (Figure S5b) was used to Other functional materials can also be integrated with the 3D- observe the deformation process of horseshoe-shaped microstructures printed polymer template such as dip coating of Ag nanowires, and surface Au films. graphene, and CNTs (Figure S4).56 Moreover, the fabricating scheme based on PμSL 3D printing has relevance not only in ■ ASSOCIATED CONTENT fl fabrication exible and stretchable interconnects but also in the *sı Supporting Information direct implementation of complex layouts for integrated 3D The Supporting Information is available free of charge at electronic systems due to its high design freedom, scalability, https://pubs.acs.org/doi/10.1021/acsami.0c20162. and customization. Interconnected arrays of 3D capacitive pressure sensors with surface microstructures were exemplified FEAs of three different 3D structure designs; stretch- by one-step printing, followed by coating Ag films. Further, by ability analyses of the horseshoe-shaped structure; utilizing PμSL-based 3D printing to create corresponding 3D measured stress−strain curve of the 3D-printed polymer mask for locally depositing different functional materials, more materials; comparison of the occupied area between 3D sophisticated 3D microsystems can also be explored in the and 2D horseshoe-shaped structures; overall stretch- future, affording powerful opportunities in many emerging ability of the island-bridge design of capacitive pressure areas from biointegrated electronics to soft robots. sensor arrays; resistance of the 3D horseshoe-shaped microstructure conductor coated by Ag/Au films; optical ■ MATERIALS AND METHODS image of 3D horseshoe-shaped microstructures coated by CNTs; mechanical testing setups; realization of Finite Element Analyses. Three-dimensional FEA techniques different circuit layouts; optical images of a single sensor were carried out by commercial software ABAQUS 6.14. The elastomeric substrate and three design models were simulated using electrode and a sensor electrode array; SEM image of 3D deformable solid parts, and conventional static analysis was surface microstructures of a single sensor electrode; and adopted. For comparison, the three structures were built up with a resistance of 3D horseshoe-shaped microstructures same height of 900 μm and a wavelength of 762 μm. The short coated with different materials (PDF) bottom line segments for connecting unit cells and attaching them on the substrate were fixed at 56 μm. The 3D-printed polymer materials and substrate materials were assumed to be linear elastic, but ■ AUTHOR INFORMATION geometry nonlinearities were considered. Young’s modulus and Corresponding Authors ’ Poisson s ratio are Epolymer = 2 Gpa and vpolymer = 0.34 for the 3D- Qi Ge − Department of Mechanical and Energy Engineering, printed polymer and Esubstrate = 6 Mpa and vsubstrate = 0.34 for the elastomeric substrate, respectively. Southern University of Science and Technology, Shenzhen Fabrication of the Polymer/Conductive Microstructure. The 518055, China; orcid.org/0000-0002-8666-8532; horseshoe-shaped microstructures of different layouts were printed Email: [email protected] out from a photosensitive resin via a PμSL-based 3D printing machine Libo Gao − School of Mechano-Electronic Engineering, Xidian (Nano Arch S140, BMF Material Technology Inc.). The CAD models University, Xian 710071, China; orcid.org/0000-0002- were sliced and photocured layer by layer by the 3D printer with a 5964-2337; Email: [email protected] thickness of 10 μm. The smaller horseshoe-shaped microstructures for Yang Lu − Department of Mechanical Engineering, City demonstration (Figure 3d) were fabricated by another higher University of Hong Kong, Kowloon 999077, Hong Kong resolution 3D printing machine (Nano Arch P130, BMF Material SAR, China; Nanomanufacturing Laboratory (NML), City Technology Inc.), which enables a thinner minimum layer thickness of 5 μm. Once the polymer structure was successfully printed, it was University of Hong Kong Shenzhen Research Institute, washed with ethanol and heated under 70 °C for 15 min after drying. Shenzhen 518057, China; CityU-Xidian Joint Laboratory of Finally, to endow conductivity, the printed polymer structures were Micro/Nano-Manufacturing, Shenzhen 518057, China; put into the sputter machine (ISC 150T ion sputter coater, SuPro orcid.org/0000-0002-9280-2718; Email: yanglu@ Instruments LTD) for coating a thin metal film. For the cityu.edu.hk

F https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article

Authors (5) Terryn, S.; Brancart, J.; Lefeber, D.; Van Assche, G.; Yuejiao Wang − Department of Mechanical Engineering, City Vanderborght, B. Self-Healing Soft Pneumatic Robots. Sci Rob. University of Hong Kong, Kowloon 999077, Hong Kong 2017, 2, No. eaan4268. SAR, China; Nanomanufacturing Laboratory (NML), City (6) Bartlett, N. W.; Tolley, M. T.; Overvelde, J. T. B.; Weaver, J. C.; University of Hong Kong Shenzhen Research Institute, Mosadegh, B.; Bertoldi, K.; Whitesides, G. M.; Wood, R. J. A 3d- Printed, Functionally Graded Soft Robot Powered by Combustion. Shenzhen 518057, China; orcid.org/0000-0003-0512- Science 2015, 349, 161−165. 454X − (7) Gao, L.; Wang, Y.; Hu, X.; Zhou, W.; Cao, K.; Wang, Y.; Wang, Xiang Li Nanomanufacturing Laboratory (NML), City W.; Lu, Y. Cellular Carbon-Film-Based Flexible Sensor and University of Hong Kong Shenzhen Research Institute, Waterproof Supercapacitors. ACS Appl. Mater. Interfaces 2019, 11, Shenzhen 518057, China; CityU-Xidian Joint Laboratory of 26288−26297. Micro/Nano-Manufacturing, Shenzhen 518057, China (8) Xu, H.; Gao, L.; Wang, Y.; Cao, K.; Hu, X.; Wang, L.; Mu, M.; Sufeng Fan − Department of Mechanical Engineering, City Liu, M.; Zhang, H.; Wang, W. Flexible Waterproof Piezoresistive University of Hong Kong, Kowloon 999077, Hong Kong Pressure Sensors with Wide Linear Working Range Based on SAR, China; Nanomanufacturing Laboratory (NML), City Conductive Fabrics. Nano-Micro Lett. 2020, 12, 159. University of Hong Kong Shenzhen Research Institute, (9) Rogers, J. A.; Someya, T.; Huang, Y. Materials and Mechanics for − Shenzhen 518057, China Stretchable Electronics. Science 2010, 327, 1603 1607. Xiaobin Feng − Department of Mechanical Engineering, City (10) Chen, M.; Tao, T.; Zhang, L.; Gao, W.; Li, C. Highly University of Hong Kong, Kowloon 999077, Hong Kong Conductive and Stretchable Polymer Composites Based on Graphene/Mwcnt Network. Chem. Commun. 2013, 49, 1612−1614. SAR, China; Nanomanufacturing Laboratory (NML), City (11) Cheng, T.; Zhang, Y.-Z.; Lai, W.-Y.; Chen, Y.; Zeng, W.-J.; University of Hong Kong Shenzhen Research Institute, Huang, W. High-Performance Stretchable Transparent Electrodes Shenzhen 518057, China − Based on Silver Nanowires Synthesized Via an Eco-Friendly Halogen- Ke Cao School of Mechano-Electronic Engineering, Xidian Free Method. J. Mater. Chem. C 2014, 2, 10369−10376. University, Xian 710071, China; orcid.org/0000-0001- (12) Yang, H.; Gong, L. H.; Zheng, Z.; Yao, X. F. Highly Stretchable 7857-4467 and Sensitive Conductive Rubber Composites with Tunable Piezoresistivity for Motion Detection and Flexible Electrodes. Carbon Complete contact information is available at: − https://pubs.acs.org/10.1021/acsami.0c20162 2020, 158, 893 903. (13) Wang, Y.; Zhu, C.; Pfattner, R.; Yan, H.; Jin, L.; Chen, S.; Molina-Lopez, F.; Lissel, F.; Liu, J.; Rabiah, N. I.; Chen, Z.; Chung, J. Author Contributions W.; Linder, C.; Toney, M. F.; Murmann, B.; Bao, Z. A Highly Y.W. led the structure designs, FEA, and experimental work Stretchable, Transparent, and Conductive Polymer. Sci. Adv. 2017, 3, and wrote the main part of the manuscript. X.L. conducted a No. e1602076. part of finite element modeling. S.F. took SEM images. X.F. (14) White, M. S.; Kaltenbrunner, M.; Głowacki,E.D.; and K.C. helped to prepare the samples. Q.G., L.G., and Y.L. Gutnichenko, K.; Kettlgruber, G.; Graz, I.; Aazou, S.; Ulbricht, C.; supervised the research. All authors contributed to the final Egbe, D. A. M.; Miron, M. C.; Major, Z.; Scharber, M. C.; Sekitani, manuscript. T.; Someya, T.; Bauer, S.; Sariciftci, N. S. Ultrathin, Highly Flexible and Stretchable Pleds. Nat. Photonics 2013, 7, 811−816. Notes (15) Oh, J. Y.; Kim, S.; Baik, H.-K.; Jeong, U. Conducting Polymer The authors declare no competing financial interest. Dough for Deformable Electronics. Adv. Mater. 2016, 28, 4455−4461. (16) Savagatrup, S.; Chan, E.; Renteria-Garcia, S. M.; Printz, A. D.; ■ ACKNOWLEDGMENTS Zaretski, A. V.; O’Connor, T. F.; Rodriquez, D.; Valle, E.; Lipomi, D. J. Plasticization of Pedot:Pss by Common Additives for Mechanically These authors would like to acknowledge the financial support Robust Organic Solar Cells and Wearable Sensors. Adv. Funct. Mater. from Shenzhen Science and Technology Innovation Commit- 2015, 25, 427−436. tee under the grant JCYJ20170818103206501. A part of this (17) Guo, J.; Liu, X.; Jiang, N.; Yetisen, A. K.; Yuk, H.; Yang, C.; project was supported by the National Natural Science Khademhosseini, A.; Zhao, X.; Yun, S.-H. Highly Stretchable, Strain Foundation of China (no. 61904141), Natural Science Sensing Hydrogel Optical Fibers. Adv. Mater. 2016, 28, 10244− Foundation of Shaanxi Province (no. 2020JQ-295), Key 10249. Research and Development Program of Shaanxi (Program (18) Zhao, Z.; Zhang, K.; Liu, Y.; Zhou, J.; Liu, M. Highly no. 2020GY-252), and City University of Hong Kong (Project Stretchable, Shape Memory Organohydrogels Using Phase-Transition nos. 7005070, 9667164, and 9667194). Microinclusions. Adv. Mater. 2017, 29, 1701695. (19) Tian, K.; Bae, J.; Bakarich, S. E.; Yang, C.; Gately, R. D.; Spinks, G. M.; in het Panhuis, M.; Suo, Z.; Vlassak, J. J. 3d Printing of ■ REFERENCES Transparent and Conductive Heterogeneous Hydrogel-Elastomer (1) Choi, S.; Lee, H.; Ghaffari, R.; Hyeon, T.; Kim, D.-H. Recent Systems. Adv. Mater. 2017, 29, 1604827. Advances in Flexible and Stretchable Bio-Electronic Devices (20)Gray,D.S.;Tien,J.;Chen,C.S.High-Conductivity − Integrated with Nanomaterials. Adv. Mater. 2016, 28, 4203−4218. Elastomeric Electronics. Adv. Mater. 2004, 16, 393 397. (2) Gao, L.; Cao, K.; Hu, X.; Xiao, R.; Gan, B.; Wang, W.; Lu, Y. (21) Li, T.; Suo, Z.; Lacour, S. P.; Wagner, S. Compliant Thin Film Nano Electromechanical Approach for Flexible Piezoresistive Sensor. Patterns of Stiff Materials as Platforms for Stretchable Electronics. J. Appl. Mater. Today 2020, 18, 100475. Mater. Res. 2005, 20, 3274−3277. (3) Feiner, R.; Dvir, T. Tissue-Electronics Interfaces: From (22) Zhang, Y.; Xu, S.; Fu, H.; Lee, J.; Su, J.; Hwang, K.-C.; Rogers, Implantable Devices to Engineered Tissues. Nat. Rev. Mater. 2018, J. A.; Huang, Y. Buckling in Serpentine Microstructures and 3, 17076. Applications in Elastomer-Supported Ultra-Stretchable Electronics (4) Wehner, M.; Truby, R. L.; Fitzgerald, D. J.; Mosadegh, B.; with High Areal Coverage. Soft Matter 2013, 9, 8062−8070. Whitesides, G. M.; Lewis, J. A.; Wood, R. J. An Integrated Design and (23) Huang, Z.; Hao, Y.; Li, Y.; Hu, H.; Wang, C.; Nomoto, A.; Pan, Fabrication Strategy for Entirely Soft, Autonomous Robots. Nature T.; Gu, Y.; Chen, Y.; Zhang, T.; Li, W.; Lei, Y.; Kim, N.; Wang, C.; 2016, 536, 451−455. Zhang, L.; Ward, J. W.; Maralani, A.; Li, X.; Durstock, M. F.; Pisano,

G https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX ACS Applied Materials & Interfaces www.acsami.org Research Article

A.;Lin,Y.;Xu,S.Three-DimensionalIntegratedStretchable Activatable Liquid-Metal-Silicone Inks. Adv. Funct. Mater. 2020, 30, Electronics. Nat. Electron. 2018, 1, 473−480. 1906683. (24) Lee, W.; Kobayashi, S.; Nagase, M.; Jimbo, Y.; Saito, I.; Inoue, (39) Zarek, M.; Layani, M.; Cooperstein, I.; Sachyani, E.; Cohn, D.; Y.; Yambe, T.; Sekino, M.; Malliaras, G. G.; Yokota, T.; Tanaka, M.; Magdassi, S. 3d Printing of Shape Memory Polymers for Flexible Someya, T. Nonthrombogenic, Stretchable, Active Multielectrode Electronic Devices. Adv. Mater. 2016, 28, 4449−4454. Array for Electroanatomical Mapping. Sci. Adv. 2018, 4, No. eaau2426. (40) Liu, S.; Shi, X.; Li, X.; Sun, Y.; Zhu, J.; Pei, Q.; Liang, J.; Chen, (25) Kumaresan, Y.; Kim, H.; Pak, Y.; Poola, P. K.; Lee, R.; Lim, N.; Y. A General Gelation Strategy for 1d Nanowires: Dynamically Stable Ko, H. C.; Jung, G. Y.; Dahiya, R. Omnidirectional Stretchable Functional Gels for 3d Printing Flexible Electronics. Nanoscale 2018, Inorganic-Material-Based Electronics with Enhanced Performance. 10, 20096−20107. Adv. Electron. Mater. 2020, 6, 2000058. (41) Shin, S. R.; Farzad, R.; Tamayol, A.; Manoharan, V.; Mostafalu, (26) Kim, J.; Park, H.; Jeong, S.-H. A Kirigami Concept for P.; Zhang, Y. S.; Akbari, M.; Jung, S. M.; Kim, D.; Comotto, M.; Transparent and Stretchable Nanofiber Networks-Based Conductors Annabi, N.; Al-Hazmi, F. E.; Dokmeci, M. R.; Khademhosseini, A. A and Uv Photodetectors. J. Ind. Eng. Chem. 2020, 82, 144−152. Bioactive Carbon Nanotube-Based Ink for Printing 2d and 3d Flexible (27) Zhang, Y.; Yan, Z.; Nan, K.; Xiao, D.; Liu, Y.; Luan, H.; Fu, H.; Electronics. Adv. Mater. 2016, 28, 3280−3289. Wang, X.; Yang, Q.; Wang, J.; Ren, W.; Si, H.; Liu, F.; Yang, L.; Li, H.; (42) Wang, Z.; Guan, X.; Huang, H.; Wang, H.; Lin, W.; Peng, Z. Wang, J.; Guo, X.; Luo, H.; Wang, L.; Huang, Y.; Rogers, J. A. A Full 3d Printing of Stretchable Piezoresistive Sensor with Hierarchical Mechanically Driven Form of Kirigami as a Route to 3d Porosity and Multimodulus Architecture. Adv. Funct. Mater. 2019, 29, Mesostructures in Micro/Nanomembranes. Proc. Natl. Acad. Sci. 1807569. U.S.A. 2015, 112, 11757−11764. (43) Yan, D. J.; Chang, J. H.; Zhang, H.; Liu, J. X.; Song, H. L.; Xue, (28) Xu, S.; Yan, Z.; Jang, K.-I.; Huang, W.; Fu, H.; Kim, J.; Wei, Z.; Z. G.; Zhang, F.; Zhang, Y. H. Soft Three-Dimensional Network Flavin, M.; McCracken, J.; Wang, R.; Badea, A.; Liu, Y.; Xiao, D.; Materials with Rational Bio-Mimetic Designs. Nat. Commun. 2020, Zhou, G.; Lee, J.; Chung, H. U.; Cheng, H.; Ren, W.; Banks, A.; Li, 11, 1180. X.; Paik, U.; Nuzzo, R. G.; Huang, Y.; Zhang, Y.; Rogers, J. A. (44) Hensleigh, R.; Cui, H.; Xu, Z.; Massman, J.; Yao, D.; Berrigan, Assembly of Micro/Nanomaterials into Complex, Three-Dimensional J.; Zheng, X. Charge-Programmed Three-Dimensional Printing for − Architectures by Compressive Buckling. Science 2015, 347, 154−159. Multi-Material Electronic Devices. Nat. Electron. 2020, 3, 216 224. (29) Yan, Z.; Zhang, F.; Wang, J.; Liu, F.; Guo, X.; Nan, K.; Lin, Q.; (45) Ge, Q.; Li, Z.; Wang, Z.; Kowsari, K.; Zhang, W.; He, X.; Zhou, Gao, M.; Xiao, D.; Shi, Y.; Qiu, Y.; Luan, H.; Kim, J. H.; Wang, Y.; J.; Fang, N. X. Projection Micro Stereolithography Based 3d Printing Luo, H.; Han, M.; Huang, Y.; Zhang, Y.; Rogers, J. A. Controlled and Its Applications. Int. J. Extreme Manuf. 2020, 2, 022004. Mechanical Buckling for Origami-Inspired Construction of 3d (46) Kim, D.-H.; Song, J.; Choi, W. M.; Kim, H.-S.; Kim, R.-H.; Liu, Microstructures in Advanced Materials. Adv. Funct. Mater. 2016, 26, Z.; Huang, Y. Y.; Hwang, K.-C.; Zhang, Y.-w.; Rogers, J. A. Materials 2629−2639. and Noncoplanar Mesh Designs for Integrated Circuits with Linear (30) Nan, K.; Luan, H.; Yan, Z.; Ning, X.; Wang, Y.; Wang, A.; Elastic Responses to Extreme Mechanical Deformations. Proc. Natl. − Wang, J.; Han, M.; Chang, M.; Li, K.; Zhang, Y.; Huang, W.; Xue, Y.; Acad. Sci. U.S.A. 2008, 105, 18675 18680. Huang, Y.; Zhang, Y.; Rogers, J. A. Engineered Elastomer Substrates (47) Wang, C.; Wang, C.; Huang, Z.; Xu, S. Materials and Structures for Guided Assembly of Complex 3d Mesostructures by Spatially toward Soft Electronics. Adv. Mater. 2018, 30, 1801368. Nonuniform Compressive Buckling. Adv. Funct. Mater. 2017, 27, (48) Ma, Y.; Jang, K.-I.; Wang, L.; Jung, H. N.; Kwak, J. W.; Xue, Y.; 1604281. Chen, H.; Yang, Y.; Shi, D.; Feng, X.; Rogers, J. A.; Huang, Y. Design of Strain-Limiting Substrate Materials for Stretchable and Flexible (31) Jang, K. I.; Li, K.; Chung, H. U.; Xu, S.; Jung, H. N.; Yang, Y. − Y.; Kwak, J. W.; Jung, H. H.; Song, J.; Yang, C.; Wang, A.; Liu, Z. J.; Electronics. Adv. Funct. Mater. 2016, 26, 5345 5351. Lee, J. Y.; Kim, B. H.; Kim, J. H.; Lee, J.; Yu, Y.; Kim, B. J.; Jang, H.; (49) Kim, D.; Shin, G.; Kang, Y. J.; Kim, W.; Ha, J. S. Fabrication of a Stretchable Solid-State Micro-Supercapacitor Array. ACS Nano Yu, K. J.; Kim, J.; Lee, J. W.; Jeong, J. W.; Song, Y. M.; Huang, Y. G.; − Zhang, Y. H.; Rogers, J. A. Self-Assembled Three Dimensional 2013, 7, 7975 7982. Network Designs for Soft Electronics. Nat. Commun. 2017, 8,1−10. (50) Xu, S.; Zhang, Y. H.; Cho, J.; Lee, J.; Huang, X.; Jia, L.; Fan, J. (32) Kim, B. H.; Liu, F.; Yu, Y.; Jang, H.; Xie, Z.; Li, K.; Lee, J.; A.; Su, Y. W.; Su, J.; Zhang, H. G.; Cheng, H. Y.; Lu, B. W.; Yu, C. J.; Jeong, J. Y.; Ryu, A.; Lee, Y.; Kim, D. H.; Wang, X.; Lee, K.; Lee, J. Y.; Chuang, C.; Kim, T. I.; Song, T.; Shigeta, K.; Kang, S.; Dagdeviren, Won, S. M.; Oh, N.; Kim, J.; Kim, J. Y.; Jeong, S. J.; Jang, K. I.; Lee, S.; C.; Petrov, I.; Braun, P. V.; Huang, Y. G.; Paik, U.; Rogers, J. A. Huang, Y.; Zhang, Y.; Rogers, J. A. Mechanically Guided Post- Stretchable Batteries with Self-Similar Serpentine Interconnects and Assembly of 3D Electronic Systems. Adv. Funct. Mater. 2018, 28, Integrated Wireless Recharging Systems. Nat. Commun. 2013, 4, 1803149. 1543. (33) Nan, K.; Kang, S. D.; Li, K.; Yu, K. J.; Zhu, F.; Wang, J.; Dunn, (51) Xiang, Y.; Li, T.; Suo, Z.; Vlassak, J. J. High Ductility of a Metal Film Adherent on a Polymer Substrate. Appl. Phys. Lett. 2005, 87, A. C.; Zhou, C.; Xie, Z.; Agne, M. T.; Wang, H.; Luan, H.; Zhang, Y.; 161910. Huang, Y.; Snyder, G. J.; Rogers, J. A. Compliant and Stretchable (52) Lacour, S. P.; Wagner, S.; Suo, Z. Stretchable Conductors: Thin Thermoelectric Coils for Energy Harvesting in Miniature Flexible Gold Films on Silicone Elastomer. Mater. Res. Soc. Symp. Proc. 2004, Devices. Sci. Adv. 2018, 4, No. eaau5849. 795, 193−198. (34) Muth, J. T.; Vogt, D. M.; Truby, R. L.; Mengüc,̧ Y.; Kolesky, D. (53) Lu, N. S.; Wang, X.; Suo, Z. G.; Vlassak, J. Metal Films on B.; Wood, R. J.; Lewis, J. A. Embedded 3d Printing of Strain Sensors Polymer Substrates Stretched Beyond 50%. Appl. Phys. Lett. 2007, 91, within Highly Stretchable Elastomers. Adv. Mater. 2014, 26, 6307− 221909. 6312. (54) Li, T.; Huang, Z.; Suo, Z.; Lacour, S. P.; Wagner, S. (35) Sun, G.; An, J.; Chua, C. K.; Pang, H.; Zhang, J.; Chen, P. Stretchability of Thin Metal Films on Elastomer Substrates. Appl. Layer-by-Layer Printing of Laminated Graphene-Based Interdigitated Phys. Lett. 2004, 85, 3435−3437. Microelectrodes for Flexible Planar Micro-Supercapacitors. Electro- (55) Sim, G.-D.; Hwangbo, Y.; Kim, H.-H.; Lee, S.-B.; Vlassak, J. J. 2015 − chem. Commun. , 51,33 36. Fatigue of Polymer-Supported Ag Thin Films. Scripta Mater. 2012, 66, (36) Mohammed, M. G.; Kramer, R. All-Printed Flexible and 915−918. Stretchable Electronics. Adv. Mater. 2017, 29, 1604965. (56) Gao, L.; Song, J.; Surjadi, J. U.; Cao, K.; Han, Y.; Sun, D.; Tao, (37) Guo, F.; Jiang, Y. Q.; Xu, Z.; Xiao, Y. H.; Fang, B.; Liu, Y. J.; X.; Lu, Y. Graphene-Bridged Multifunctional Flexible Fiber Super- Gao, W. W.; Zhao, P.; Wang, H. T.; Gao, C. Highly Stretchable − capacitor with High Energy Density. ACS Appl. Mater. Interfaces 2018, Carbon Aerogels. Nat. Commun. 2018, 9,1 9. 10, 28597−28607. (38) Zhou, L. Y.; Fu, J. Z.; Gao, Q.; Zhao, P.; He, Y. All-Printed Flexible and Stretchable Electronics with Pressing or Freezing

H https://dx.doi.org/10.1021/acsami.0c20162 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX