1 The John Scales Centre for Biomedical Engineering, Institute of Orthopaedics and Musculoskeletal Science, Division of Surgery and Interventional Science, University College London, The Royal National Orthopaedic Hospital Trust, Brockley Hill, Stanmore, Middlesex, HA7 4LP, UK; Department of Materials and IRC in Biomedical Materials, School of Engineering and Materials, Queen Mary University of London, Mile End Road, London E1 4NS, UK.