1. Department of Mechanical Engineering The University of Hong Kong Hong Kong P. R. China
2. Department of Electrical and Electronic Engineering The University of Hong Kong Hong Kong P. R. China
3. School of Electronic Engineering and Computer Science Queen Mary University of London London E1 4NS UK
4. Department of Mechanical and Automation Engineering The Chinese University of Hong Kong Hong Kong P. R. China
5. Department of Micro-Nano Systems Engineering Nagoya University Nagoya 464‐8601 Japan