1. Department of Mechanical Engineering and Material Science; Duke University; Durham NC 27708 USA
2. Department of Biomedical Engineering; Duke University; Durham NC 27708 USA
3. Department of Mechanical Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA
4. Department of Orthopaedic Surgery; Duke University Medical Center; Durham NC 27710 USA
5. Department of Biomedical Engineering; Columbia University; New York NY 10027 USA
6. Department of Civil and Environmental Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA