1. Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA
2. Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
3. Department of Mechanical and Aerospace Engineering, Materials Science and Engineering Program, University of California San Diego, La Jolla, California 92093, USA
4. Laboratory for Integrated Micro-Mechatronic Systems, CNRS-IIS (IRL2820), Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan