1. a Department of Clinical Engineering, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK
2. b Department of Surgery, Laval University, and Biomaterials Institute, St. Franpois d'Assise Hospital, Québec G1K 7P4, Canada
3. c Department of Surgery, Laval University, and Biomaterials Institute, St. Franpois d'Assise Hospital, Québec G1K 7P4, Canada
4. d Department of Clinical Engineering, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK