1. Institut für Technische Chemie, Abt. TC Makromolekularer Stoffe, Technische Universität Braunschweig, Hans-Sommer-Strasse 10, 38106 Braunschweig, Germany, National ESCA and Surface Analysis Center for Biomedical Problems, Departments of Bioengineering and Chemical Engineering, Box 351750, University of Washington, Seattle, Washington 98195, and Gesellschaft für Biotechnologische Forschung, 38124 Braunschweig, Germany