1. Department of Materials Science & Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, United States
2. Space Biosciences Division, NASA-Ames Research Center, Moffett Field, California 94035, United States
3. Department of Biological Engineering, Massachusetts Institute of Technology, 500 Technology Square, Cambridge, Massachusetts 02138, United States
4. Department of Engineering, Cambridge University, 11 JJ Thomson Ave., Cambridge CB3 0FF, U.K.
5. Department of Electricity and Electronics, University of the Basque Country UPV/EHU, Sarriena s/n, Bilbao 48940, Spain