1. Department of Mechanical Engineering, University of California, Berkeley, CA 94720;
2. Department of Mathematics, University of California, Berkeley, CA 94720
3. Department of Chemical and Biomolecular Engineering, Institute for Medicine and Engineering, University of Pennsylvania, Philadelphia, PA 19104
4. Department of Radiology, Perelman School of Medicine of the University of Pennsylvania, Philadelphia, PA 19104
5. Department of Mechanical Engineering, University of California, Berkeley, CA 94720 e-mail: