1. Department of Mechanical Engineering & Materials Science, Duke University, Durham, North Carolina 27708, United States
2. A. J. Drexel Nanomaterials Institute, and Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States
3. Department of Electrical & Computer Engineering, Duke University, Durham, North Carolina 27708, United States
4. Laboratory for Soft Machines & Electronics, School of Packaging, Michigan State University, East Lansing, Michigan 48824, United States
5. Departments of Mechanical Engineering, Electrical and Computer Engineering, Michigan State University, East Lansing, Michigan 48824, United States