1. Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore
2. Department of Aerospace Engineering, Texas A&M University, 701 H.R. Bright Building, College Station, TX 77843, USA
3. Department of Materials Science and Engineering, Texas A&M University, 207 Reed McDonald Building, College Station, TX 77843, USA
4. School of Biochemistry, Biomedical Sciences Building, University of Bristol, University Walk, Bristol, BS8 1TD, UK