1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering, Harvard University, Cambridge, MA 02138, USA.
3. Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong.
4. Department of Mechanical Engineering, University of Washington, Seattle, WA 98195, USA.
5. Mechanical, Materials, and Aerospace Engineering Department, Illinois Institute of Technology, Chicago, IL 60616, USA.
6. SRI International, Menlo Park, CA 94025, USA.