1. Department of Biomedical Engineering, University of Texas at Austin, 107 W. Dean Keeton Street, Stop C0800, Austin, TX 78712 e-mail:
2. Department of Aerospace Engineering and Engineering Mechanics, University of Texas at Austin, 2617 Wichita Street, Stop C0600, Austin, TX 78712-1221; Department of Biomedical Engineering, University of Texas at Austin, 107 W. Dean Keeton Street, Stop C0800, Austin, TX 78712 e-mail:
3. Department of Mechanical Engineering, University of Texas at Austin, 204 E. Dean Keeton Street, Stop C2200, Austin, TX 78712-1591; Department of Biomedical Engineering, University of Texas at Austin, 107 W. Dean Keeton Street, Stop C0800, Austin, TX 78712 e-mail: