1. Division of Chemistry and Chemical Engineering, 210-41 California Institute of Technology, Pasadena, California 91125, Institute for Biomedical Engineering and Department of Materials, ETH-Zurich and University of Zurich, 8044 Zurich, Switzerland, Max Planck Institute fur Polymerforschung, Postfach 3148, 55021 Mainz, Germany, and Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, California 90089