1. Harvard John A. Paulson School of Engineering and Applied Sciences Harvard University Cambridge MA 02138 USA
2. State Key Laboratory for Strength and Vibration of Mechanical Structures and School of Aerospace Xi'an Jiaotong University Xi'an 710049 P. R. China
3. Department of Materials ETH Zürich 8093 Zürich Switzerland
4. Kavli Institute Harvard University Cambridge MA 02138 USA
5. Wyss Institute for Biologically Inspired Engineering Harvard University Cambridge MA 02138 USA