1. Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, IL 60208 e-mail:
2. Theoretical and Applied Mechanics, Northwestern University, Evanston, IL 60208 e-mail:
3. Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708 e-mail:
4. Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208 e-mail: