By Daniel Cressey
A sticky species of beetle has inspired researchers to develop a device that uses switchable 'liquid bridges' to attach to a variety of surfaces.
Bioengineers Michael Vogel and Paul Steen, of Cornell University in Ithaca, New York, based their device on a technique used by the leaf beetle, which sticks to leaves using the combined surface tension of many drops working in concert to generate an adhesion force of more than 100 times its own body weight.
The device consists of a plate punctured with hundreds of tiny holes, through which liquid is pumped. Pushing droplets through these holes creates 'liquid bridges' when the plate comes close enough to another surface, generating an adhesive force through surface tension, they report in Proceedings of the National Academy of Sciences USA.
"It's an example of things to come," says Steen. "We might have something [commercial] in five years using these types of controllable capillary surfaces."
Researchers have previously attempted to mimic the mechanism that geckos use to run up vertical surfaces. Gecko feet are thought to rely on dry adhesion, with nanofibres creating the stickiness through Van der Waals forces. This differs from the leaf beetle's tactic of wet adhesion, in which the combined force of the surface tension of multiple droplets can be hugely powerful.
Vogel and Steen learned of the leaf beetle Hemisphaerota cyanea from the work of insect researcher Thomas Eisner2. In 2000, Eisner and his colleague Daniel Aneshansley, both also at Cornell, reported that this beetle could withstand pulling forces of around 60 times its own body weight for 2 minutes, and could even withstand far stronger forces for brief periods.
There are no solid moving parts in Vogel and Steen's device. Instead it consists of three plates: a bottom plate acts as a reservoir for the fluid used to create the liquid bridges; below that sits a porous plate dotted with lots of small holes through which the droplets protrude, sandwiched by a third plate. The team initially used water in the reservoir, but they found that if the holes in the device were very small, water evaporated too quickly and oil had to be used instead.
The researchers pushed liquid through the plate in just a fraction of a second by applying a low-voltage pulse that drives a process known as electro-osmotic flow.
As the liquid squeezes through the holes in the porous plate, the droplets form bridges to the third plate, which is held at a set distance from the reservoir using spacers. When turned upside down, the device can hold the third plate and small payloads attached to it. Each individual droplet sticks to the surface with a tiny force, but the combined strength of hundreds of droplets allows the device to stick.
Pumping the water back unsticks the device again. Both of the states are stable, so the applied voltage is used only to switch between them.
Vogel and Steen show that the strength with which their device adheres to surfaces is inversely related to the size of the liquid droplets. "The real engineering challenge is to scale the holes down," says Steen. The pair think that there is "no reason" why the holes can't be scaled down to around 0.1 micrometres (m).
In its current form, the holes in the device are around 150 m in diameter and it can hold a weight of about 10 grams per centimetre square. Reducing the holes to 0.1 m across should enable up to 13 kilograms to be suspended from a device measuring 1 centimetre square.
At very small hole sizes, the adhesive strength of the device approaches that of commercial adhesives, but unlike the device, these lack the ability to quickly switch between stuck and unstuck states.
"I am very impressed," says Jon Barnes, a biologist who works on bio-adhesion technologies at the University of Glasgow, UK. "Whereas animal adhesion has lots of features that should be really exciting and developable, to date the results have been a little disappointing." But Vogel and Steen's device is "particularly promising", he says.