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