1. NSF Research Triangle Materials Research Science and Engineering Center, Department of Biomedical Engineering; Duke University; Durham NC 27708 USA
2. NSF Research Triangle Materials Research Science and Engineering Center, Department of Chemical and Biomolecular Engineering; North Carolina State University; Raleigh NC 27607 USA
3. Soft Active Materials Laboratory, Department of Mechanical Engineering; Massachusetts Institute of Technology; Cambridge MA 02139 USA
4. NSF Research Triangle Materials Research Science and Engineering Center, Department of Mechanical Engineering and Materials Science; Duke University; Durham NC 27708 USA