1. Center for Self-Powered Artificial Muscle and Department of Biomedical Engineering; Hanyang University; Seoul 04763 Korea
2. Intelligent Polymer Research Institute; ARC Centre of Excellence for Electromaterials Science; University of Wollongong; Wollongong NSW 2522 Australia
3. The Alan G. MacDiarmid Nano Tech Institute; University of Texas at Dallas; Richardson TX 75083 USA