1. a Department of Fiber and Polymer Science, Seoul National University, Seoul 151-742, Korea
2. b Department of Fiber and Polymer Science, Seoul National University, Seoul 151-742, Korea
3. c Department of Polymer Science and Engineering, Kumoh National University of Technology, Kyungbuk 730-070, Korea
4. d Department of Fiber and Polymer Science, Seoul National University, Seoul 151-742, Korea
5. e Biomedical Research Center, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea