1. AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China;
2. Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China
3. Center for Mechanics and Materials, Tsinghua University, Beijing 100084, China e-mail:
4. Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL 60208;
5. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208;
6. Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208;
7. Skin Disease Research Center, Northwestern University, Evanston, IL 60208