1. Program in Biophysics, Harvard University , Cambridge MA 02138, USA
2. Wyss Institute for Biologically Inspired Engineering, Harvard University , Cambridge MA 02115, USA
3. John A. Paulson School of Engineering and Applied Sciences, Harvard University , Cambridge MA 02138, USA
4. Harvard–MIT Division in Health Sciences and Technology, Massachusetts Institute of Technology , Cambridge MA 02142, USA
5. Department of Bioengineering, Imperial College London , London SW7 2BP, UK