1. Department of Materials Science and Technology, Science University of Tokyo, Yamazaki 2641, Noda, Chiba 278, Japan, International Center for Biomaterials Science, Research Institute for Biosciences, Science University of Tokyo, Yamazaki 2669, Noda, Chiba 278, Japan, and Institute of Biomedical Engineering, Tokyo Women's Medical College, 8-1 Kawada, Shinjuku, Tokyo 162, Japan