1. Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, Division of Chemistry and Chemical Engineering, 210-41, California Institute of Technology, Pasadena, California 91125, and Department of Materials and Institute for Biomedical Engineering, ETH and University of Zürich, Moussonstrasse 18, Zürich, CH-8044 Switzerland