1. Department of Mechanical and Aerospace Engineering, Cornell University, Ithaca, NY 14853, USA.
2. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA.
3. Center for Micro-BioRobotics@SSSA, Istituto Italiano di Tecnologia, Viale Rinaldo Piaggio 34, I-56025 Pontedera, Italy.