A SMART surface that can either attract or repel water at just the flick of a switch is being hailed as a breakthrough, as it is the first designed to instantly change its physical properties.
The surface is made from a substance called 16-mercapto-hexadecanoic acid (MHA). Molecules of the acid are almost tree-shaped – a long water-repelling chain forms the trunk, on top of which sits a negatively charged carboxylate group that attracts water.
The key to the smart surface is how these molecules are arranged. The designers allowed millions of molecules to self assemble into a monolayer sitting atop a layer of gold. When the gold is made negatively charged, it repels the carboxylate group, forcing the molecules to stand up straight. In this configuration, the carboxylate treetops form a surface that attracts water.
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Making the gold surface positively charged has the opposite effect: it pulls the carboxylate groups to the ground, bending the trees over to expose their trunks. In this configuration, the trunks form a surface that repels water.
One stumbling block was that the monolayer is usually so dense that the molecules can’t bend over because of their neighbours. The designers overcame this problem by attaching oversized heads to the carboxylate groups to prevent the molecules packing together too closely. Removing the heads let the molecular trees bend in two (Science, vol 299, p 371).
Instantly changing the surface’s physical properties allows it to be switched like an electromagnet, says team member Samir Mitragotri at the University of California, Santa Barbara. Cells or proteins attracted to the surface could be pulled out of a biological mixture as other components are washed away. They could then be released simply by flicking a switch. In the same way, the researchers speculate that dividing the surface into pixels that attract or repel ink would allow it to be used as a programmable printing press.