The researchers are helped in this by the fact that their membrane material knows two stable states. In other words, it can have two different volume configurations at a given pressure without the need to minimize the larger volume. This is a little like letting the air out of an inflated balloon; it does not shrink back to its original size, but remains significantly larger. Thanks to this bi-stable state, the researchers are able to move air between a more highly inflated chamber and a less inflated one. They do this by applying an electric current to the membrane of the smaller chamber which responds by stretching and sucking air out of the other bubble. When the power supply is switched off the membrane contracts, but not to its original volume; it remains larger, corresponding to its stretched state.
"It is important to find suitable hyperelastic polymers that will enable strong and fast deformation and be durable," points out Metin Sitti. With this in mind, the team has tested different membrane materials and also used models to systematically record the behaviour of the elastomer in the actuator.
Thus far, the elastomers tested by Sitti's team have each had a mix of advantages and disadvantages. Some show strong deformation, but at a slow rate. Others work fast, but their deformation is more limited. "We will combine different materials with a view to combining different properties in a single membrane," says Sitti. This is, however, just one of the next steps he and his team have in mind. They also plan to integrate their actuator in a robot so that it can, for instance, move its legs but still give way if it happens to come across a human. Only then can machine-human interactions be risk-free.Literature
Lindsey Hines, Kirstin Petersen und Metin Sitti
Inflated Soft Actuators with Reversible Stable Deformations
Advanced Materials, 23 March 2016; DOI: 10.1002/adma.201600107Source