1. Joint Graduate Program in Bioengineering, University of California San Francisco/University of California Berkeley, California 94720-1762, Department of Bioengineering and Center for Tissue Bioengineering, University of California Berkeley, California 94720, Department of Chemistry, Stony Brook University, Stony Brook, New York 11794-3400, and Department of Medicine, University of California San Francisco, San Francisco, California 94143