1. Laboratory for Biomolecular Materials Research, Departments of Surgery and Bioengineering, Emory University School of Medicine, Atlanta, Georgia 30322, and School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30320, and Laboratoire de Chimie des Polymères Organiques, UMR ENSCPB-Université Bordeaux I-CNRS, Avenue Pey-Berland-BP 108, 33402 Talence Cedex, France