1. Department
of Mechanical Engineering, Northwestern University, Evanston, Illinois 60208, United States
2. Theoretical
and Applied Mechanics Program, Northwestern University, Northwestern University, Evanston, Illinois 60208-3111, United States
3. Department
of Mechanical Engineering, Stanford University, Stanford, California 94305-4040, United States
4. Max Planck Institute for Intelligent Systems, Heisenberg Street, Stuttgart 70569, Germany