Congmin Xu1,2, Huaiqiu Zhu1, Peng Qiu3. 1. Department of Biomedical Engineering, College of Engineering, Peking University, Beijing, 100871, China. 2. Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, 30332, USA. 3. Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, 30332, USA. peng.qiu@bme.gatech.edu.