1. State Key Laboratory of Military Stomatology; Department of Operative Dentistry and Endodontics, School of Stomatology; The Fourth Military Medical University; Xi'an 710032 P. R. China
2. Department of Biologic and Materials Sciences; University of Michigan; Ann Arbor MI 48109 USA
3. Department of Biomedical Engineering; University of Michigan; Ann Arbor MI 48109 USA
4. Macromolecular Science and Engineering Center; University of Michigan; Ann Arbor MI 48109 USA
5. Department of Materials Science and Engineering; University of Michigan; Ann Arbor MI 48109 USA