1. Center for Mechanics and Materials AML Department of Engineering Mechanics Tsinghua University Beijing 100084 P. R. China
2. Department of Materials Science and Engineering Frederick Seitz Materials Research Laboratory University of Illinois at Urbana‐Champaign Urbana IL 61801 USA
3. National Key Laboratory of Science and Technologyon Micro/Nano Fabrication Peking University Beijing 100871 P. R. China
4. Departments of Civil and Environmental Engineering and Mechanical Engineering Center for Engineering and Health Skin Disease Research Center Northwestern University Evanston IL 60208 USA
5. Departments of Materials Science and Engineering, Chemistry, Mechanical Science and Engineering Electrical and Computer Engineering Beckman Institute for Advanced Science and Technology Frederick Seitz Materials Research Laboratory University of Illinois at Urbana‐Champaign Urbana IL 61801 USA