Xiaofeng Jia1, Mario I Romero-Ortega2, Yang D Teng3. 1. Department of Neurosurgery, University of Maryland School of Medicine, 10 South Pine Street, Medical School Teaching Facility 559 A, Baltimore, MD 21201, USA ; Department of Biomedical Engineering, Physical Medicine & Rehabilitation, Anesthesiology & Critical Care Medicine, Johns Hopkins University, Baltimore, MD 21205, USA. 2. Department of Bioengineering, The University of Texas at Dallas, Richardson, TX, USA ; Surgery Department, University of Texas Southwestern Medical Center, TX, USA. 3. Departments of Neurosurgery and Physical Medicine & Rehabilitation, Harvard Medical School, Brigham and Women's Hospital, Spaulding Rehabilitation Hospital and VA Boston Healthcare System, Boston, MA, USA.