1. Adolphe Merkle Institute and Fribourg Center for Nanomaterials, University of Fribourg, Rte de l'Ancienne Papeterie, CH-1723 Marly, Switzerland
2. Institute of Anatomy, University of Bern, Baltzerstrasse 2, CH-3000, Bern 9, Switzerland
3. Empa, Swiss Federal Laboratories for Materials Science and Technology, Materials-Biology Interactions Laboratory, Lerchenfeldstrasse 5, CH-9014, St. Gallen, Switzerland