1. Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province and Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, P. R. China
2. Soft Matter Research Center (SMRC), Zhejiang University, Hangzhou 310027, P. R. China
3. Department of Mechanical and Materials Engineering, University of Nebraska-Lincoln, Lincoln, NE 68588-0526, USA