Pulsing gels could power tiny devices -

A gel that pulses regularly when doused with certain chemicals has been modelled in detail for the first time. The scientists behind the modelling say it may one day be used to power miniature robots or other devices.
So-called Belousov-Zhabotinsky (BZ) gels were first discovered in 1996 by researchers at the National Institute of Materials and Chemical Research in Japan. They consist of long polymer molecules containing a metal catalyst made from ruthenium – a rare metal similar in structure to platinum.
When the right nitrogen compound solution is added to the gel, a cyclical reaction starts. The ruthenium catalyst alternately gains and loses electrons, causing the polymer strands within the gel to oscillate in length.
"A sample of this gel beats autonomously by itself like a little heart in a Petri dish," says Anna Balazs, a chemist at the University of Pittsburgh, US. "A piece a few millimetres across will go for hours until the fuel reagent runs out."
Robot muscles
Such materials have the potential to power small mechanical devices, Balazs says, but until now only crude models describing the gel's transformative properties have existed. Balazs worked with colleague Victor Yashin to create a model that describes these changes in two dimensions for the first time.
"If you want to use the gels as muscles for tiny robots, you have to know how they work," explains Balazs. "This model should help people to design gels that perform in a particular way."
Their model describes the gel as a lattice of tiny springs connected at specific points. The gel's pulsing movements are defined through thermodynamic equations that calculate the energy liberated by chemical reactions. "From that energy, we can consider the force on every point on the lattice," explains Balazs, "and that can be used to calculate how the points move."
"This really is a prediction out there for people to test," says Balazs. So far, there is little detailed research on the shape changes undergone by BZ gels, but preliminary tests suggest that the model rings true, she adds.
Working devices
Jonathan Howse, who works on polymer-based artificial muscles at the University of Sheffield, UK, agrees that the model should help experimentalists. "Having a theoretical model puts you in a much better position to design a working device," he told New Scientist.
Also, he notes that conventional polymer muscles must be treated chemically each time a new physical change is required. "BZ gels could allow a much more intelligent structure because they don't need those changes," he says.
But Howse believes the usefulness of BZ gels could be limited by a lack of structural uniformity. "The cross links between the polymer molecules are distributed randomly," he explains. "This means they are more likely to fail because they can pull in different directions."
By contrast, conventional polymer materials can be made with more regular molecular arrangements, making them much less vulnerable to stress fatigue.
Journal reference: Science (vol 314, p798)