1. Department of Materials Science and Engineering and Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
2. Department of Mechanical Engineering and Department of Civil and Environmental Engineering, Center for Engineering and Health, and Skin Disease Research Center, Northwestern University, Evanston, IL 60208, USA.
3. Center for Mechanics and Materials, Tsinghua University, Beijing, 100084, P.R. China.
4. Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
5. Department of Materials Science and Engineering, Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea.
6. Department of Civil Engineering and Architecture, Zhejiang University, Hangzhou 310058, P.R. China.