1. Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, USA
2. Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
3. Department of Physics, Utah State University, Logan, UT 84322, USA
4. Department of Biochemistry and Biophysics, University of Pennsylvania, Philadelphia, PA, 19104, USA