1. Materials Science and Engineering Program; University of California at San Diego; La Jolla California 92093-0418 USA
2. Department of Bioengineering; University of California at San Diego; La Jolla California 92093-0412 USA
3. Department of Biology; Lamar University; Beaumont 77710 Texas USA
4. Department of Electrical and Computer Engineering; University of California at San Diego; La Jolla 92093-0407 California USA