1. Department of Electronic Engineering The Chinese University of Hong Kong New Territories Hong Kong SAR 999077 China
2. Department of Mechanical and Automation Engineering The Chinese University of Hong Kong New Territories Hong Kong SAR 999077 China
3. Department of Biomedical Engineering The Chinese University of Hong Kong New Territories Hong Kong SAR 999077 China
4. Department of Mechanical Engineering Northwestern University Evanston IL 60208‐3109 USA