1. Department of Mechanics Tianjin University Tianjin 300350 China
2. John A. Paulson School of Engineering and Applied Sciences Harvard University Cambridge MA 02138 USA
3. Department of Electronics, Information and Bioengineering Politecnico di Milano Milan 20133 Italy
4. Department of Materials ETH Zürich Zürich 8093 Switzerland
5. Wyss Institute for Biologically Inspired Engineering Harvard University Cambridge MA 02138 USA
6. Kavli Institute Harvard University Cambridge MA 02138 USA