1. a Department of Periodontology and Biomaterials, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
2. b Department of Periodontology and Biomaterials, Radboud University Nijmegen Medical Center, Nijmegen, The Netherlands
3. c Department of Bioengineering, Rice University, MS-142, P.O. Box 1892, Houston, TX 77251, USA