Lab on a Chip View Article Online CRITICAL REVIEW View Journal | View Issue Design and application of ‘J-shaped’ stress–strain Cite this: Lab Chip,2017,17,1689 behavior in stretchable electronics: a review Yinji Ma,ab Xue Feng,a John A. Rogers,c Yonggang Huangb and Yihui Zhang *a A variety of natural biological tissues (e.g., skin, ligaments, spider silk, blood vessel) exhibit ‘J-shaped’ stress–strain behavior, thereby combining soft, compliant mechanics and large levels of stretchability, with anatural‘strain-limiting’ mechanism to prevent damage from excessive strain. Synthetic materials with similar stress–strain behaviors have potential utility in many promising applications, such as tissue engineer- ing (to reproduce the nonlinear mechanical properties of real biological tissues) and biomedical devices (to enable natural, comfortable integration of stretchable electronics with biological tissues/organs). Recent advances in this field encompass developments of novel material/structure concepts, fabrication ap- proaches, and unique device applications. This review highlights five representative strategies, including de- signs that involve open network, wavy and wrinkled morphologies, helical layouts, kirigami and origami constructs, and textile formats. Discussions focus on the underlying ideas, the fabrication/assembly routes, Received 18th March 2017, and the microstructure–property relationships that are essential for optimization of the desired ‘J-shaped’ Accepted 21st April 2017 stress–strain responses. Demonstration applications provide examples of the use of these designs in de- formable electronics and biomedical devices that offer soft, compliant mechanics but with inherent robust- DOI: 10.1039/c7lc00289k ness against damage from excessive deformation. We conclude with some perspectives on challenges and rsc.li/loc opportunities for future research. 1. Introduction a Significant progress in the development of stretchable Department of Engineering Mechanics, Center for Mechanics and Materials, 1–8 AML, Tsinghua University, Beijing, 100084, China. electronics based on inorganic materials enables systems that E-mail: [email protected] combine extremely deformable mechanics and high-performance b Published on 24 April 2017. Downloaded by Northwestern University 15/06/2017 15:00:58. Departments of Civil and Environmental Engineering, Mechanical Engineering, electrical properties. These unconventional technologies have sig- and Materials Science and Engineering, Northwestern University, Evanston, IL nificant commercial potential in biomedicine, either to replace 60208, USA c the function of biological tissues/organs (e.g. skin-like Departments of Materials Science and Engineering, Biomedical Engineering, 9–13 14–17 Chemistry, Mechanical Engineering, Electrical Engineering and Computer Science, prosthesis, biomimetic robots, etc.) or to integrate with 18–24 Center for Bio-Integrated Electronics, Simpson Querrey Institute for Nano/Biotech- biological tissues/organs (such as epidermal electronics, flexi- 25–28 nology, Northwestern University, Evanston, Illinois 60208, USA ble implantable medical instruments, etc.). Yinji Ma is a postdoctoral fellow Xue Feng is a Professor of Engi- at the Northwestern University neering Mechanics at Tsinghua of USA. He received his PhD de- University. His research interests gree from Tsinghua University of include flexible electronics, solid China. His research interests in- mechanics and experimental me- clude experimental mechanics, chanics in extreme environ- solidmechanicsandflexible ments. He is an associate editor electronics. of “Journal of Applied Mechan- ics” (ASME Transactions), and serves on the editorial board of “npj Flexible Electronics” and “Flexible and Printed ” Yinji Ma Xue Feng Electronics . This journal is © The Royal Society of Chemistry 2017 Lab Chip,2017,17,1689–1704 | 1689 View Article Online Critical review Lab on a Chip Many biological tissues, such as skin,29,30 ligaments,31 spi- the development of engineering materials as substrates or der silk,32,33 blood vessels,34,35 etc., exhibit ‘J-shaped’ stress– superstrates in stretchable electronics focuses on the develop- strain behaviors36 as depicted in Fig. 1a,29 as a result of their ment of designs that can precisely reproduce the ‘J-shaped’ curved and chained microstructures (e.g., collagen triple helix, stress–strain behavior of real biological tissues. collagen fibril, collagen fiber in Fig. 1a). This type of stress– To enable natural, comfortable integration of stretchable strain response is typically characterized by three different electronics with biological tissues/organs, an important de- stages. In stage I (Fig. 1a), wavy and crimped collagen fibers sign consideration is to reduce the stresses induced on the begin to unfurl by bending and twisting, thereby resulting in skin by the presence of the devices to within thresholds for negligible stiffness and compliant mechanics (linear). As the somatosensory perception. Specifically, the electronics must applied strain increases (stage II), the collagen fibers uncoil, be sufficiently compliant to accommodate deformations of leading to an increase of the tangent modulus. After the colla- soft biological tissues,37,38 typically within several to tens of gen fibers straighten (stage III), the stress–strain behavior is percent. Materials with ‘J-shaped’ stress–strain behavior are dominated by stretching of the fibers, thereby offering a rela- well-matched to this requirement due to their low elastic tively linear response and a high tangent modulus. A goal in modulus at small strains.39 In the aforementioned applications, another challenge is in the development of materials/structures and strate- John A. Rogers is the Louis gies to minimize the possibility of mechanically induced – Simpson and Kimberly Querrey device failure.40 42 Several design concepts allow stretch- Professor of Materials Science able, integrated systems of hard, functional, inorganic – and Engineering, Biomedical En- components and soft, elastomeric substrates.43 48 Here, gineering, Mechanical Engineer- an emerging design strategy involves the use of a layer ing, Electrical Engineering and with ‘J-shaped’ stress–strain behavior placed between the Computer Science, Chemistry electronics and the biological tissue. This embedded and Neurological Surgery at layer has a high elastic modulus at large strain to shield Northwestern University where the electronics from the potential for large deforma- he also serves as Director of the tions,39,49 thereby providing a so-called “strain-limiting” Center on Bio-Integrated function. Electronics. Rogers’ research fo- Fig. 1b–f summarize five structural designs that can of- ‘ ’ – 49 John A. Rogers cuses on unusual electronic and fer J-shaped stress strain behavior: (i) network designs photonic devices, with an em- that adopt wavy, horseshoe microstructures patterned in phasis on bio-integrated and bio-inspired systems. He has pub- periodic lattices (e.g., triangular, right-top; honeycomb, lished nearly 600 papers and is a member of the National Acad- left-bottom; Kagome, right-bottom; Fig. 1b); (ii) wavy and – emy of Engineering, the National Academy of Sciences and the wrinkled designs50 53 induced using a pre-strain [unidirec- American Academy of Arts and Sciences. tional (top) or bidirectional (bottom) in Fig. 1c] strategy; Published on 24 April 2017. Downloaded by Northwestern University 15/06/2017 15:00:58. Yonggang Huang is the Walter Yihui Zhang is an Associate Pro- P. Murphy Professor of Mechani- fessor of Engineering Mechanics cal Engineering, Civil and Envi- at Tsinghua University. Before ronmental Engineering, and Ma- joining Tsinghua, he was a Re- terials Science and Engineering search Assistant Professor at at Northwestern University. He Northwestern University. His re- is interested in mechanics of search interests include mechani- stretchable and flexible electron- cally guided 3D assembly, bio- ics, and mechanically guided de- inspired soft materials, and terministic 3D assembly. He is a stretchable electronics. His re- member of the US National cent honors and awards include Academy of Engineering. His re- the ASME Journal of Applied Me- Yonggang Huang cent research awards (since Yihui Zhang chanics Award (2017), MIT Tech- 2013) include the Drucker Medal nology Review's 35 Innovators in 2013, the Nadai Medal in 2016 from the American Society of Under 35 (2016), and Qiu Shi Outstanding Young Scholar Award Mechanical Engineers, and the Prager Medal in 2017 from the So- (2016). He is an associate editor of Journal of Applied Mechanics ciety of Engineering Sciences; and he is listed as a Highly Cited (ASME Transactions), and serves on the editorial board of Pro- Researcher. ceedings of the Royal Society A, npj Flexible Electronics and Acta Mechanica Solida Sinica. 1690 | Lab Chip,2017,17,1689–1704 This journal is © The Royal Society of Chemistry 2017 View Article Online Lab on a Chip Critical review Published on 24 April 2017. Downloaded by Northwestern University 15/06/2017 15:00:58. Fig. 1 Microstructures of biological tissues and five representative strategies to enable ‘J-shaped’ stress–strain behavior. (a) Hierarchical structure of a representative biological tissue spanning the nanoscale dimensions of collagen triple helices to microscale networks of collagen and elastin fi- bers, and its representative ‘J-shaped’ stress–strain behavior. (b) Network design: optical image (left-top) of a 2D network [triangular (right-top),