Shape memory alloy makes moves for micro-robots

Shape memory alloys have the ability to “remember” their shape, meaning they can be deformed and then returned to their previous form when heated.

However, existing shape memory alloys are typically only able to change from one shape to another, or in response to one particular temperature change.

By developing functionally graded shape memory alloys, a team of UK researchers are hoping to modify these properties at different points in the material. In this way the material could change its shape in response to different temperatures at various points on the device, for example.

This would allow much more complex and controllable micro-robots to be built, according to Professor Duncan Hand at Heriot-Watt University, who is developing the alloys in an EPSRC-funded project alongside his colleague Professor Bob Reuben.

To produce the tailored materials, the researchers are developing a new technique known as functionally graded Laser Induced Forward Transfer (FG-LIFT).

Thin films of metal, such as nickel, titanium or copper, layered on a transparent polymer, are deposited onto a substrate using a pulse laser, said Hand.

“So you have a ribbon of transparent polymer with these thin metal films in parallel on it, and then like an old-fashioned typewriter ribbon you transfer bits of this ribbon over on to the [receiver] substrate,” he said. “So you might put down three layers of titanium, four layers of nickel and a layer of copper.”

In this way the process can build up three-dimensional elements, which the team describe as voxels, consisting of layers of different metals.

The material can then be heat treated to carefully control a process of diffusion between the metal layers in each voxel, allowing the researchers to precisely control the composition of the material.

By adding voxels together, the researchers can then build up 3D microstructures with properties that differ at particular points across the material.

“So some parts of the device might be a shape memory alloy, some parts will not have that effect, and some will have a shape memory alloy that operates at a different temperature,” said Hand.

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