1. Department of Materials Science and Engineering, University of Pennsylvania, 211 LRSM, 3231 Walnut Street, Philadelphia, PA 19104 e-mail:
2. Department of Bioengineering, University of Pennsylvania, 240 Skirkanich Hall, 210 S. 33rd Street, Philadelphia, PA 19104 e-mail:
3. Department of Mechanical Engineering, University of Minnesota—Twin Cities, 7-105 Nils Hasselmo Hall, 312 Church Street SE, Minneapolis, MN 55455 e-mail:
4. Department of Biomedical Engineering, University of Minnesota—Twin Cities, 7-105 Nils Hasselmo Hall, 312 Church Street SE, Minneapolis, MN 55455 e-mail:
5. Departments of Bioengineering and Neurosurgery, University of Pennsylvania, 240 Skirkanich Hall, 210 South 33rd Street, Philadelphia, PA 19104 e-mail:
6. Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, 2048 Jonsson Engineering Center, 110 8th Street, Troy, NY 12180 e-mail: