1. a Department of Mechanical Engineering , National University of Singapore, 9 Engineering Drive 1 , Singapore , 117576 , Singapore
2. b Faculty of Engineering , Center for Nanofibers and Nanotechnology, Nanoscience and Nanotechnology Initiative, National University of Singapore, 2 Engineering Drive 3 , Singapore , 117576 , Singapore
3. c NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore , Singapore , 117576 , Singapore