1. Galician Institute of Ceramics, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain, Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, 226-8503 Yokohama, Japan, Department of Biomaterials, Max Planck Institute of Colloids and Interfaces, D-14424 Potsdam, Germany, Present address: International Iberian Nanotechnology Laboratory, 4710-299 Braga, Portugal