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