1. Shaanxi Key Laboratory of Degradable Biomedical Materials; School of Chemical Engineering; Northwest University; Taibai North Road 229 Xi'an Shaanxi 710069 China
2. Department of Materials Science & Engineering; Faculty of Engineering; National University of Singapore; 7 Engineering Drive 1 117574 Singapore
3. Department of Anatomy; Yong Loo Lin School of Medicine; National University of Singapore; 4 Medical Drive, MD10 117597 Singapore
4. Key Laboratory of Advanced Materials; School of Material Science & Engineering; Tsinghua University; Beijing 100084 China
5. Beijing National Laboratory for Condensed Matter Physics; Institute of Physics; Chinese Academy of Sciences; Beijing 100190 China