1. Department of Engineering Mechanics, CNMM and AML Tsinghua University Beijing 100084 P. R. China
2. School of Mechanical and Aerospace Engineering Nanyang Technological University Singapore 639798 Republic of Singapore
3. Mechanical Engineering and Materials Science Washington University St. Louis MO 63130 USA
4. NSF Science and Technology Center for Engineering Mechanobiology St. Louis MO 63130 USA