1. Istituto per i Processi Chimico-Fisici del CNR, via G. Moruzzi 1, I-56124 Pisa, Italy
2. Department of Polymer Chemistry and Biomaterials, Faculty of Science and Technology, MIRA Institute for Biomedical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands