1. Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Hong Kong
2. John A. Paulson School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, USA
3. Wyss Institute for Biologically Inspired Engineering, Harvard University, Boston, MA 02115, USA
4. Department of Mechanical and Civil Engineering, California Institute of Technology, Pasadena, CA 91125, USA