1. Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230027, China e-mail:
2. Department of Obstetrics and Gynecology, Second Affiliated Hospital of Chongqing Medical University, Chongqing 400010, China e-mail:
3. Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230027, China; Department of Biomedical Engineering, The Ohio State University, Columbus, OH 43210 e-mail: