1. The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
2. College of Engineering, Peking University, Beijing 100871, P. R. China.
3. Shenzhen Key Laboratory of Nanobiomechanics, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055, P.R. China.
4. Southridge School, Surrey, BC V3S 0B7, Canada.
5. Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, P. R. China.