1. a Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea; Interdisciplinary Program in Medical and Biological Engineering Major, Seoul National University, Seoul, South Korea
2. b Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea
3. c Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea
4. d Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea
5. e Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
6. f Department of Biomedical Engineering, Seoul National University, Seoul, South Korea