1. Department of Engineering Mechanics, Soft Matter Research Center, and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310027, China
2. Department of Mechanical Engineering, University of Houston, Houston, TX 77204;
3. Department of Electrical and Computer Engineering, University of Houston, Houston, TX 77204;
4. Department of Biomedical Engineering, and Materials Science and Engineering Program, University of Houston, Houston, TX 77204
5. Department of Engineering Mechanics, Soft Matter Research Center, and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310027, China e-mail: