1. Department of Bioengineering School of Engineering and Applied Sciences University of Pennsylvania Philadelphia PA 19104 USA
2. BioFrontiers Institute and Department of Chemical and Biological Engineering University of Colorado Boulder CO 80309 USA
3. Translational Musculoskeletal Research Center Corporal Michael J. Crescenz VA Medical Center Philadelphia PA 19104 USA
4. Department of Orthopaedic Surgery McKay Orthopaedic Research Laboratory University of Pennsylvania Philadelphia PA 19104 USA