The Institute for Soldier Nanotechnologies' Team 04 has already reported a breakthrough in switchable surfaces. The team, led by MIT bioengineer Robert Langer, has collaborated with researchers at the University of California at Santa Barbara and at Berkeley, and with the Lawrence Berkeley Laboratory, to create a molecular surface that can switch between wettable and water-repellent states in response to an electric field-the first time surface wettability has been controlled by a nonchemical process.

The new system is based on self-assembled monolayer (SAM) chemistry. A basic insight into how to remove chemical reactions from the process was the realization that molecules in SAMs are too closely packed, preventing any mechanical movement of the molecules. Team 04's strategy was to find a way to create more widely spaced SAMs in which the con-stituent molecules could move in response to an electric field. The researchers attached a large molecular group to the end of their SAM molecules so that when the structures self-assembled, the large end groups would force a wider spacing than usual. The end groups are then removed, leaving freestanding molecules with enough space in between to bend into different configurations without interacting chemically.

The molecules had a structure in which a water-repelling stem segment terminated with a hydrophilic (wettable) end group. Standing freely on a gold substrate, the molecules present hydrophilic caps with a negative electrical charge to the environment, creating a wettable surface. When a positive electrical potential is applied, the hydrophobic stems are presented, mutating the SAM into a water-repellent surface: an electromechanical system organized on the nanoscale that can switch between two chemically distinct states.