1. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut, USA
2. Current affiliation: NanoEngineering Corporation, Branford, Connecticut, USA
3. Department of Cellular and Molecular Biology, Centro Nacional de Biotecnología (CNB), CSIC, Madrid, Spain
4. Department of Macromolecular Structures, Centro Nacional de Biotecnología (CNB), CSIC, Madrid, Spain