1. Materials Chemistry Division, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, Department of Neuroscience, Karolinska Institutet, SE-171 77, Stockholm, Sweden, Department of Radiology, Center for Surgical Sciences, Huddinge University Hospital, Karolinska Institutet, SE-141 86 Stockholm, Sweden, and Laboratory for Nanostructured Materials Research, Department of Ceramics, Rutgers University, Piscataway, New Jersey 08854