1. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
2. Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
3. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
4. Microphotonics Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
5. Materials Research Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA