1. Joint Graduate Group in Bioengineering, University of California, Berkeley, and University of California, San Francisco, California 94720, Departments of Mechanical Engineering, Materials Science and Engineering, and Bioengineering, University of California, Berkeley, California 94720, Materials Sciences and Physical Biosciences Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Berkeley Nanosciences and Nanoengineering Institute, Berkeley, California 94720