1. Materials Science and Engineering Program, University of California San Diego, La Jolla, California 92093, USA
2. Department of NanoEngineering, University of California San Diego, La Jolla, California 92093, USA
3. Institute of Materials Research and Engineering, Agency for Science, Technology and Research, Singapore 138634, Singapore
4. Department of Bioengineering, University of California San Diego, La Jolla, California 92093, USA
5. Skaggs School of Pharmacy and Pharmaceutical Sciences, University of California San Diego, La Jolla, California 92093, USA
6. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
7. Department of Radiology, University of California San Diego, La Jolla, California 92093, USA