In Fig. 1, the two middle bars of each panel were inadvertently transposed. This version of Fig. 1 has been replaced with the correct version.
The corrected online version of Fig. 1 is at http://stm.sciencemag.org/content/5/181/181ra51.full and the corrected PDF version of the article is at http://stm.sciencemag.org/content/5/181/181ra51.full.pdf.
- Copyright © 2013, American Association for the Advancement of Science
