One of the big challenges in turning nano- or micro-scale objects into useful devices is getting them to switch between states, such as bright and dark for communications or open and shut for releasing a chemical into the body. Hence, the attraction of hydrogels, polymer-rich liquids that quickly swell and shrink depending on the amount of moisture they contain. But such squishy materials are hard to control precisely and can sometimes collapse permanently.
So researchers gave hydrogel more of a backbone. Materials scientist Joanna Aizenberg of Alcatel-Lucent's Bell Laboratories and her colleagues poured a film of gel onto a pincushionlike array of silicon whiskers, each about as tall as a human hair is wide (100 nanometers). In their design, dubbed HAIRS, dry hydrogel clings to the silicon wafer, letting the prongs sag like pesky stubble in shaving cream commercials.
When moisture expands the gel, it fills up the spaces between silicon columns and forces them to stand upright, the group reports in the January 26 Science. Such a property might help create more water-repellant materials, Aizenberg says, because excess water would bead on top of the plumped-up HAIRS surface and roll off.
In other permutations, the silicon slivers tilted like windshield wipers or were joined together to make collapsible pyramid shapes or honeycomblike clusters. Aizenberg says these more complex shapes might lead to small pockets capable of opening and closing to release drug-coated beads or chemicals. For microscopic plumbing in biosensors or research tools, "when these nanostructures bend or tilt they can [in principle] direct the flows on a microscale in different directions," she says.
Aizenberg specializes in creating materials that mimic biological structures. She says her inspiration for HAIRS came from the movements of carnivorous plants and especially from pedicellaria, microscopic three-armed grippers that sea urchins use to grasp for food and defend against parasites and predators.
Other researchers are impressed. According to materials researcher Sergey Minko of Clarkson University, combining hydrogels with a nanoscale skeleton that can take on different patterns is a "very powerful approach" for switching back and forth between states.