1. KTH Fibre and Polymer Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden, TNO Science and Industry, Bevesierweg (Harssens), P.O. Box 505, 1780 AM Den Helder, The Netherlands, and Swedish Medical Nanoscience Center, Department of Neuroscience, Karolinska Insitutet, Retzius väg 8, S-171 77 Stockholm, Sweden