1. Research Institute for Biomimetic and Soft Matter; Provincial Key Lab for Soft Functional Research; Xiamen University; Xiamen 361005 P. R. China
2. Department of Physics; National University of Singapore; 2 Science Drive 3 Singapore 117551 Singapore
3. School of Physics; Chongqing University; Chongqing 401331 P. R. China
4. NUS Graduate School for Integrative Sciences and Engineering; National University of Singapore; 28 Medical Drive Singapore 117456 Singapore
5. Department of Biological Sciences; National University of Singapore; 14 Science Drive 4 Singapore 117543 Singapore
6. Institute of Materials Research and Engineering; 2 Fusionopolis Way Singapore 138634 Singapore