1. a Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea; School of Life Science and Biotechnology, Korea University, Seoul 136-701, South Korea
2. b Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea
3. c Department of Materials Science & Engineering, Gwangju Institute of Science and Technology, 1 Oryong-dong, Buk-ku, Gwangju 500-712, South Korea
4. d School of Life Science and Biotechnology, Korea University, Seoul 136-701, South Korea
5. e Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea