1. Department of Mechanical Engineering, Biomanufacturing Center, Tsinghua University, Beijing 100084, China
2. Biomanufacturing and Rapid Forming Technology Key Laboratory of Beijing, Beijing 100084, China
3. Key Laboratory for Advanced Materials
Processing Technology, Ministry of Education, Beijing 100084, China
4. Department of Mechanical Engineering, The College of New Jersey, Ewing, NJ 08628
5. Department of Mechanical Engineering, Drexel University, Philadelphia, PA 19104 e-mails: ;