REVIEW Soft Electronics www.advmat.de Materials and Structures toward Soft Electronics Chunfeng Wang, Chonghe Wang, Zhenlong Huang, and Sheng Xu* very well,[10–14] and therefore this article Soft electronics are intensively studied as the integration of electronics with will mainly focus on the stretchable aspect dynamic nonplanar surfaces has become necessary. Here, a discussion of the of soft electronic devices. strategies in materials innovation and structural design to build soft elec- The implications of soft electronics inte- tronic devices and systems is provided. For each strategy, the presentation grating with nonplanar objects are multi- focuses on the fundamental materials science and mechanics, and example fold. First, the intimate contact between the device and the nonplanar object will device applications are highlighted where possible. Finally, perspectives on allow high-quality data to be collected.[15] the key challenges and future directions of this field are presented. With rigid electronics, air gaps at the interface between the device and the object reduce the contact area, and can 1. Introduction potentially introduce noise and artifacts, which compromise signal quality.[5] Second, foldable, low-profile devices can enable Nonplanar surfaces, either static (e.g., complex shaped objects) mobile and distributed sensing, which hold great promise for or dynamic (e.g., biology), are prevalent in nature. Soft elec- Internet-of-Things technology.[16] Finally, in the area of medical tronics that allow interfacing with nonplanar surfaces signifi- devices, which is probably the major driving force of this field at cantly expand the capabilities of conventional rigid electronics present, soft electronics have similar mechanical properties and in sensing, monitoring, diagnosing, and potentially intervening thus cause minimal irritation to the human skin, which can be functions.[1–6] Nonplanar surfaces can be divided into two cate- a key enabling technology for continuous healthcare.[17,18] gories: developable and nondevelopable.[7,8] Developable surfaces Two strategies can be applied to achieve stretchability in elec- can be flattened onto a plane without stretching or compres- tronics: 1) materials innovation, by developing novel materials sion.[7] They have zero Gaussian curvature and are characterized that are stretchable in single or aggregated forms; 2) structural by only bending in one direction at a time. Examples of develop- design, by making nonstretchable materials into specific struc- able surfaces include cylindrical and conical surfaces. Surfaces tures that can absorb the applied strain without fracturing, e.g., that do not satisfy this criterion are nondevelopable surfaces.[8] by inducing mixed modes of mechanical deformations.[9] In this Examples include spherical surfaces and any curvilinear surfaces review, we summarize state-of-the-art advances in both strategies, (such as the human body). By definition, flexible devices can covering a broad range of topics (Figure 1), including hydrogels, only conform to developable surfaces. To integrate seamlessly liquid metals, conductive polymers, and nanomaterials for the with a general nonplanar surface, stretchability is required.[9] A material approach, and waves/wrinkles, “island–bridges,” tex- series of review articles have already covered flexible electronics tiles, origami, kirigami, cracks, and interlocks for the structural approach. We provide outlooks on the challenges in the field and C. F. Wang, C. H. Wang, Dr. Z. L. Huang, Prof. S. Xu possible future research directions at the end of this review. Department of Nanoengineering University of California San Diego La Jolla, CA 92093, USA E-mail: [email protected] 2. Materials for Soft Electronics C. F. Wang The most intuitive approach for soft electronics is to exploit School of Materials Science and Engineering National Engineering Research Center for Advanced Polymer intrinsically soft materials as building blocks. Common building Processing Technology blocks that are necessary for conventional rigid electronic School of Physics and Engineering devices include conductors, dielectrics, and most importantly, Zhengzhou University functional materials, e.g., semiconductors. The intrinsically Zhengzhou, Henan 450001, P. R. China soft counterparts of these building blocks have been identified Dr. Z. L. Huang State Key Laboratory of Electronic Thin Films and Integrated Devices and largely developed recently. Additionally, attractive features University of Electronic Science and Technology of China that are absent in conventional rigid electronics are emerging Chengdu, Sichuan 610054, P. R. China in their soft counterparts, such as toughness, self-healing, and Prof. S. Xu stimuli responsibility, as discussed in the following. Materials Science and Engineering Program University of California San Diego La Jolla, CA 92093, USA 2.1. Hydrogels The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201801368. Hydrogels are hydrophilic polymeric networks with 3D DOI: 10.1002/adma.201801368 microstructures.[17,19] Since their first report in the 1960s,[20] Adv. Mater. 2018, 30, 1801368 1801368 (1 of 49) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advancedsciencenews.com www.advmat.de hydrogels have been widely used as biomaterials in tissue engi- neering,[21] synthetic extracellular matrices,[22] cell culture,[23] Chunfeng Wang received his and drug delivery.[24] Due to their excellent physicochemical B.S. degree in materials sci- properties such as super stretchability, self-healing, biocompati- ence and engineering from bility, stimuli-responsibility, and outstanding conductivity given Zhengzhou University, China, by innovations in polymer chemistry, composite physics, and in 2013. He is currently micro- and nanofabrication, new applications of hydrogels in pursuing a Ph.D. degree soft electronics have emerged (Figure 2).[25–32] in materials processing Traditional hydrogel synthesis methods include chemical engineering at Zhengzhou crosslinking by covalent bonds and ionic interactions, and phys- University. He joined the ical crosslinking by entanglement. The chemically crosslinked group of Prof. Sheng Xu at hydrogels normally exhibit excellent mechanical properties University of California, San including toughness, stiffness, and strength.[33,34] On the other Diego, as a visiting stu- hand, the strong and irreversible covalent bonds usually lead to dent in 2016. His research slow stimuli response,[35] limited stretchability,[36] and poor self- interests include 3D buckled structures and stretchable healing properties.[37] The physical crosslinking method such as electronics. by hydrogen bonds are much easier to break and rebuild than chemical bonds,[38] and thus provides an effective approach for Chonghe Wang obtained the self-healing.[39,40] Physically crosslinked hydrogels typically his B.E. degree in structural have low mechanical properties, which present a challenge for engineering from Harbin their applications in soft electronics.[33] Institute of Technology, Novel strategies including nanocomposites,[41–44] double net- China, in 2016. He is cur- works,[45–48] and sliding crosslinking[49,50] have been developed to rently a graduate research tailor the mechanical properties of the hydrogels. By adding sili- assistant in the Department of cate nanoparticles to the covalently crosslinkable poly(ethylene) Nanoengineering at University network, highly stretchable (1500%) and mechanically robust of California, San Diego, under hydrogels have been demonstrated.[51] In this case, the covalent the supervision of Prof. Sheng crosslinking restricts the polymer chain movements, resulting Xu. His research interests in great elastic properties. At the same time, the physical include soft bioelectronics and crosslinking between the silicate nanoparticles and the polymer translational medical devices. chains forms a viscoelastic network that enhances the hydrogel elongation. In another example, a double network hydrogel that Sheng Xu received his B.S. combines ionic and chemical crosslinking can be stretched to degree in chemistry and over 2000% with a fracture energies of 9000 J m−2.[45] When molecular engineering from this hydrogel is stretched, the ionically crosslinked alginate Peking University, China in network is ruptured to substantially dissipate the strain energy, 2006, and the Ph.D. degree and the covalently crosslinked polyacrylamide network allows in materials science and the hydrogel to return to its original configuration. This double engineering from Georgia network method makes the hydrogel highly resistive to defects. Institute of Technology in The hydrogel maintains a stretchability of 1700% with a notch 2010. He then joined the (Figure 2a). By introducing slide-ring polyrotaxane crosslinkers Frederick Seitz Materials and ionic groups into the polymer network,[49] an extremely Research Laboratory at stretchable and tough hydrogel was demonstrated. In this case, the University of Illinois at the crosslinked α-cyclodextrin molecules can move along the Urbana–Champaign as a postdoctoral researcher from polyethylene glycol chains and the ionic groups help the exten- 2011 to 2015. He is currently an assistant professor of sion of the polyrotaxane crosslinkers in the polymer network, Nanoengineering at the University of California, San Diego. leading to excellent mechanical properties of the hydrogel. His main research interests