1. Cox Laboratory for Biomedical Engineering, Institute of Biosciences and Bioengineering and Department of Chemical Engineering, Rice University, P.O. Box 1892, Houston, Texas 77251-1892, Department of Orthopaedic Surgery, Mayo Clinic, 200 First Street SW, Rochester, Minnesota 55905, and Department of Chemistry, Rice University, P.O. Box 1892, Houston, Texas 77251-1892