1. Department of Dental Materials and Biomaterials Research; RWTH Aachen University Hospital, Pauwelsstrasse 30; Aachen 52074 Germany
2. Department of Molecular and Cellular Anatomy; RWTH Aachen University Hospital, Wendlingweg 2; 52057 Aachen Germany
3. B. Braun Aesculap AG, Am Aesculap-Platz; 78532 Tuttlingen Germany