1. Department of Physics & Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China, Department of Mechanical Engineering, University of Hong Kong, Pokfulam Road, Hong Kong, China, School of Materials Science and Engineering, Southeast University, Nanjing 210018, China, and Division of Spine Surgery, Department of Orthopaedics and Traumatology, The University of Hong Kong, Pokfulam, Hong Kong, China