1. Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA
2. Department of Biology, Duke University, Durham, NC 27708, USA
3. Departments of Botany and Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA
4. Departments of Chemical and Biological Engineering and Mathematics, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z2