1. Department of Electrical Engineering and Computer Sciences; University of California; Berkeley CA 94720 USA
2. Berkeley Sensor and Actuator Center; University of California; Berkeley CA 94720 USA
3. Materials Sciences Division; Lawrence Berkeley National Laboratory; Berkeley CA 94720 USA
4. School of Mechanical Engineering; Tianjin University; Tianjin 300072 China
5. Key Laboratory of Mechanism Theory and Equipment Design of Ministry of Education; Tianjin University; Tianjin 300072 China
6. National Center for Computer Animation; Bournemouth University; Bournemouth BH12 5BB UK