1. Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
2. Center for Mechanics and Materials, Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, P.R. China.
3. National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Peking University, Beijing 100871, P.R. China.
4. Departments of Civil and Environmental Engineering, Mechanical Engineering and Materials Science and Engineering, Center for Engineering and Health, and Skin Disease Research Center, Northwestern University, Evanston, IL 60208, USA.
5. Department of Automotive Engineering, Tsinghua University, Beijing 100084, P.R. China.
6. School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai 200092, P.R. China.
7. Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.