1. Division of Biological Materials, Northwestern University Medical School, 311 E. Chicago Ave., Chicago, IL 60611-3008; Department of Biomedical Engineering, Robert R. McCormick School of Engineering & Applied Science, Northwestern University, Evanston, IL 60201
2. NESAC/Bio, University of Washington, Departments of Chemical Engineering and Bioengineering, Box 351750, Seattle, WA 98195
3. Department of Biomolecular Engineering, CSIRO, New South Wales, 2113 Australia
4. Division of Biological Materials, Northwestern University Medical School, 311 E. Chicago Ave., Chicago, IL 60611-3008; Department of Biomedical Engineering, Robert R. McCormick Schooi of Engineering & Applied Science, Northwestern University, Evanston, IL 60201