IEEE TRANSACTIONS ON COMPONENTS, PACKAGING AND MANUFACTURING TECHNOLOGY, VOL. 5, NO. 9, SEPTEMBER 2015 1201 Mechanical Designs for Inorganic Stretchable Circuits in Soft Electronics Shuodao Wang, Yonggang Huang, and John A. Rogers Abstract— Mechanical concepts and designs in inorganic circuits for different levels of stretchability are reviewed in this paper, through discussions of the underlying mechanics and material theories, fabrication procedures for the constituent microscale/nanoscale devices, and experimental characterization. All of the designs reported here adopt heterogeneous structures of rigid and brittle inorganic materials on soft and elastic elastomeric substrates, with mechanical design layouts that isolate large deformations to the elastomer, thereby avoiding potentially destructive plastic strains in the brittle materials. The overall stiffnesses of the electronics, their stretchability, and curvilinear shapes can be designed to match the mechanical properties of biological tissues. The result is a class of soft stretchable electronic systems that are compatible with traditional high-performance inorganic semiconductor technologies. These systems afford promising options for applications in portable biomedical and health-monitoring devices. Mechanics theories and modeling play a key role in understanding the underlining physics and optimization of these systems. Index Terms— Biomimicking electronics, buckling, inorganic semiconductor, stretchable electronics. I. I NTRODUCTION R ECENT developments in stretchable inorganic electronic systems have attracted increasing interest, partly due to their ability to support electrical performance that Manuscript received January 8, 2015; revised March 13, 2015; accepted March 26, 2015. Date of publication May 6, 2015; date of current version September 18, 2015. The work of S. Wang was supported by the National Science Foundation of China (NSFC) under Grant 11272260, Grant 11172022, Grant 51075327, and Grant 11302038. The work of Y. Huang was supported in part by the Division of Civil, Mechanical and Manufacturing Innovation within NSF under Grant CMMI-1400169 through the Division of Materials Research under Grant 1121262, and in part by the National Institutes of Health under Grant R01EB019337. The work of J. A. Rogers was supported in part by the U.S. Department of Energy, Washington, DC, USA, in part by the Office of Science, and in part by the Basic Energy Sciences, under Award DE-FG02-07ER46741. Recommended for publication by Associate Editor H. Jiang upon evaluation of reviewers’ comments. (Corresponding author: Shuodao Wang and John A. Rogers.) S. Wang is with the School of Mechanical and Aerospace Engi- neering, Oklahoma State University, Stillwater, OK 74078 USA (e-mail: shuodao.wang@okstate.edu). Y. Huang is with the Department of Civil and Environmental Engineer- ing, and Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208 USA (e-mail: y-huang@northwestern.edu). J. A. Rogers is with the Frederick Seitz Materials Research Laboratory, Department of Materials Science and Engineering, Department of Electrical and Computer Engineering, and the Department of Chemistry, Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Champaign, IL 61820 USA (e-mail: jrogers@ illinois.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCPMT.2015.2417801 can approach that of established rigid, brittle, and flat semiconductor technologies, with the ability to offer flexible stretchable formats that are compatible with soft, elastic biological systems. Realization of large mechanical stretcha- bility in these systems enables many innovative bioinspired and biointegrated electronic systems, with potentially significant impacts in important medical and healthcare applications. Examples include electronic eye cameras [1], [2], conformable skin sensors [3], [4], smart surgical gloves [5], structural health monitoring devices [6], [7], transient healthcare electronics [8], and wearable soft powering components [9]. Success of stretchable electronics depends on mechanical designs in inorganic electronic materials and structures that can be highly bent, stretched, compressed, and twisted [10], [11] in both one- time stretching and cyclic conditions [12]. Extensive amount of important work [10], [13]–[21] has been done on the development of mechanical concepts and ideas that overcome the mechanical incompatibility between rigid, brittle, and flat inorganic semiconductor materials and soft, elastic, and curvi- linear elastomers/biotissues. Due to the limitation on length, this paper provides a review on related work conducted by the authors. The fundamental aspects of the designs are discussed through mechanics modeling, prediction, optimization, and their quantitative comparison with experiments. Section II presents the stretchability of buckled/wrinkled interconnects of various geometric arrangements, and a prestrain strategy that significantly increases the stretchability of interconnects of many different shapes, as well as different designs for biomedical and healthcare applications. Compatibility of these strategies, and in particular, the classes of materials, the device designs, and the circuit layouts with large-scale manufacturing and packaging of conventional electronics represents a critically important aspect of the types of stretchable soft electronics summarized here. Some concluding remarks appear in Section III. II. MECHANICAL DESIGNS OF STRETCHABLE CIRCUITS The main challenge is to design structures that are made of inorganic materials, e.g., silicon, for stretchability, such that they can conform to elastomers or biotissues. The difficulty is that all known inorganic semiconductor materials are brittle and fracture at strains of the order of 1%. Several types of strategies were developed during the past 10 years with gradually improving levels of stretchability (from ∼30% to as high as ∼300%). Several of the most promising schemes are summarized in the following. 2156-3950 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.