1. The BioRobotics Institute, Scuola Superiore Sant'Anna, P.za Martiri della Libertà, 33, Pisa 56127, Italy
2. Center for International Research on Integrative Biomedical Systems, Institute of Industrial Science, University of Tokyo, Tokyo 153-8505, Japan
3. Department of Mechanical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA