1. State Key Laboratory of Oral & Maxillofacial Reconstruction and Regeneration National Clinical Research Center for Oral Diseases Shaanxi Key Laboratory of Stomatology Department of Prosthodontics School of Stomatology The Fourth Military Medical University Xi'an Shaanxi 710032 P. R. China
2. State Key Laboratory of Oral & Maxillofacial Reconstruction and Regeneration National Clinical Research Center for Oral Diseases Shaanxi Engineering Research Center for Dental Materials and Advanced Manufacture Department of Periodontology School of Stomatology The Fourth Military Medical University Xi'an Shaanxi 710032 P. R. China
3. State Key Laboratory of Oral & Maxillofacial Reconstruction and Regeneration National Clinical Research Center for Oral Diseases Department of Dental Materials School of Stomatology The Fourth Military Medical University Xi'an Shaanxi 710032 P. R. China
4. Department of Physics The Chinese University of Hong Kong Shatin, N.T. Hong Kong SAR 999077 P. R. China