Engineers have used electrically contracting polymers to demonstrate the potential of artificial muscles in a proof-of-concept study.
The team at Bristol University used its system to mimic the camouflage abilities of certain fish, which use surface muscles to shift around pigment.
To achieve these quick, flexible contractions, the team used a type of material called dielectric elastomers (DEs) — essentially elastic parallel-plate capacitors, which consist of one or more elastomeric dielectric film coated with compliant electrodes.
‘Direct competition might be something like hydraulics, pneumatics or electromagnetics,’ said Dr Jonathan Rossiter of Bristol. ‘All of those are either more complicated or heavier or require a certain scale — it’s very difficult to miniaturise them. With these materials you can make them large with desktop-sized devices or miniaturise them down to a few micrometres.’
Crucially, depending on the arrangement of DE stacks, the material can perform different functions such as radial expansion or diaphragm-type contraction activity.
In the study, two types of artificial chromatophores (muscular pigment cells) were created: the first based on a mechanism adopted by squids and the second based on a rather different mechanism adopted by zebrafish.
In the squid, a central sac containing granules of pigment is surrounded by a series of muscles. When the brain sends a signal to the cell, the contracting muscles make the central sacs expand, dispersing the pigment and generating the optical effect that makes the squid look like it is changing colour.
The Bristol team mimicked this by creating an artificial sac that employed a DE bilayer, applying a voltage-induced transverse compression and planar expansion, which diluted a blue pigment.
By contrast, the cells in the zebrafish contain a small reservoir of black pigmented fluid that, when activated, travels to the skin surface and spreads out, much like the spilling of black ink. The natural dark spots on the surface of the zebrafish therefore appear to get bigger and the desired optical effect is achieved.
This was achieved experimentally by creating a simple hydraulic-type pump with a single DE layer, allowing translocation of a black fluid.
Now that the team has demonstrated these basic functions, it provides a platform for other applications potentially relevant to consumers and industry.
‘You can do lots of cool stuff such as thermal regulation,’ Rossiter said. ‘Imagine taking some of this material and wearing it as a second skin — using some of the mechanisms that we present, you can pump fluid from next to the skin where it’s keeping you warm, then if you get too hot you can pump it to the surface where it radiates and cools down.’
Medical devices and protheses are other obvious applications where the compliant, organic material would make easy-to-integrate pumps. But there are also surpising avenues such as energy generation.
‘For example, where there’s lots of photon activity, you can pump your fluid to that area so you have an active solar cell that moves around to capture as much light as possible,’ Rossiter added.