1. Materials Department, University of California, Santa Barbara, Santa Barbara, California 93106; Laboratoire Physico-Chemie Curie, CNRS-UMR 168, Institut Curie-Section de Recherche, 11 rue P et M Curie, 75231 Paris Cedex 05, France; and Bioengineering Department, University of California, Los Angeles, Los Angeles, California 90049