1. Applied Mechanics Laboratory, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, People's Republic of China
2. Center for Flexible Electronics Technology and Center for Mechanics and Materials, Tsinghua University, Beijing 100084, People's Republic of China
3. State Key Laboratory for Manufacturing and Systems Engineering, School of Mechanical Engineering, Xi'an Jiaotong University, Xi'an 710049, People's Republic of 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