1. Department of Biomechanical Engineering University of Twente Drienerlolaan 5 Enschede 7522 NB The Netherlands
2. Department of Chemical and Biomolecular Engineering Johns Hopkins University Baltimore MD 21218 USA
3. Department of Material Science and Engineering Johns Hopkins University Baltimore MD 21218 USA
4. Department of Biomedical Engineering University Medical Centre Groningen University of Groningen Groningen 9713 AV The Netherlands