1. Design of Microsystems, Department of Microsystems Engineering (IMTEK), Albert-Ludwigs-University Freiburg, Freiburg D-79110, Germany e-mail:
2. Design of Microsystems, Department of Microsystems Engineering (IMTEK), Albert-Ludwigs-University Freiburg, Freiburg D-79110, Germany
3. Department of Orthopaedics and Trauma Surgery, University Medical Center, Albert-Ludwigs-University Freiburg, Freiburg D-79095, Germany