1. Mechanical Engineering, University of Michigan, Ann Arbor, MI 48104, USA.
2. Theoretical and Applied Mechanics, Cornell University, Ithaca, NY 14853, USA.
3. Brain and Cognitive Sciences and Center for Bits and Atoms, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
4. Mechanical Engineering, Delft University of Technology, NL-2628 CD Delft, Netherlands.