1. WPI-Advanced Institute for Materials Research (AIMR), Tohoku University, Sendai 980-8577, Japan
2. Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan
3. Department of Biomedical Engineering, Texas A&M University, College Station, Texas 77843-3120, USA
4. Materials Science and Engineering, Texas A&M University, College Station, Texas 77843-3003, USA
5. School of Computational Sciences, Korea Institute for Advanced Study, Seoul 130-722, South Korea
6. Department of Physics and Astronomy, Texas A&M University, College Station, Texas 77843-4242, USA