1. Department of Prosthodontics; School of Stomatology; Fourth Military Medical University; No. 145 West Changle Road; Xi'an 710032; China
2. State-Key Laboratory for Mechanical Behavior of Materials; Xi'an Jiaotong University; No. 28 West Xianning Road; Xi'an 710049; China
3. Department of Physics and Materials Science; City University of Hong Kong; Tat Chee Avenue; Kowloon; Hong Kong; China