1. Spine & Spinal Cord Institute, Department of Neurosurgery, College of Medicine, Yonsei University, Seoul 03722, Republic of Korea
2. School of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea
3. Graduate Program in Bioindustrial Engineering, Yonsei University, Seoul 03722, Republic of Korea