1. Center for Cell and Macromolecular Therapy, Department of Materials Science and Engineering, Gwangju Institute of Science and Technology, 1 Oryong-dong Puk-gu Gwangju 500-712, Korea and Biomedical Research Center, Korea Institute of Science and Technology, 39-1 Haweolgog-dong, Sungbook-gu, Seoul 136-791, Korea