1. Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri 63144
2. Amore-Pacific Co. R&D Center, 314-1, Bora-dong, Giheung-gu, Yongin-si, Gyeonggi-Do 446-729, Korea
3. Department of Material Science and Engineering, Yonsei University, 134 Shinchon-dong, Korea
4. Harvard School of Engineering and Applied Sciences and Department of Physics, Harvard University, Cambridge, Massachusetts 02138