1. Contribution from the Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, Ohio 45433, Department of Engineering Science, Trinity University, San Antonio, Texas 78212, Department of Chemistry and Physics, The Nottingham Trent University, Nottingham NG11 8NS, U.K., Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, and Department of Bioengineering, University of California at Los Angeles, Los Angeles, California 90095