1. Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
2. Innovative Micro Technology, Santa Barbara, California 93117
3. Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
4. Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany
5. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139