1. Andrew and Peggy Cherng Department of Medical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
2. Department of Electrical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
3. Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA.
4. Department of Mechanical and Civil Engineering, California Institute of Technology, Pasadena, CA 91125, USA.