1. a Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 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 Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, South Korea
4. d Biomaterials Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 136-650, South Korea;, Email: soohkim@kist.re.kr