1. Biomaterials Center, National Institute for Materials Science, Ibaraki 305-0047, Japan
2. Department of Materials Science and Technology, Nagaoka University of Technology, Niigata 940-2188, Japan
3. Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo 152-8550, Japan
4. Advanced Nano-Characterization Center, National Institute for Materials Science, Ibaraki 305-0003, Japan