3D printing with a bit of give and take

Materials scientists in the US say they have learned how to make liquid crystal shape-shift.

That may not immediately strike a chord with those who aren’t materials scientists, but it’s the key to a new 3D-printing method the team says could make it easier to manufacture and control the shape of soft robots, artificial muscles and wearable devices.

Shengqiang Cai and colleagues at the University of California San Diego say controlling the printing temperature of the soft, elastic polymers known as liquid crystal elastomers (LCE) makes it possible to control a printed material’s stiffness and ability to contract – its degree of actuation.

What’s more, they write in a paper in the journal Science Advances, they can change the stiffness of different areas in the same material by exposing it to heat.

As a proof of concept, they 3D-printed, in a single print with a single ink, structures whose stiffness and actuation varied by orders of magnitude, from zero to 30%. One area of an LCE structure could contract like muscles, for example, while another was flexible like tendons.

To do this, they first determined that printed LCE filament is made of a shell and a core. While the shell cools quickly after printing, becoming stiffer, the core cools more slowly, remaining more malleable. As a result, they were able to determine how to vary several parameters in the printing process to tune the mechanical properties of LCE.

Preparing the LCE ink does take a few days, they say, but the actual 3D printing can be done in an hour or two, depending on the geometry of the structure being printed.

“Based on the relationship between the properties of LCE filament and printing parameters, it’s easy to construct structures with graded material properties,” says Cai.

When they 3D-printed structures made of two layers of LCE with different properties, they found this gave the material even more degrees of freedom to actuate.

They also printed lattice structures, which they say could be used in medical applications, and a tube that when actuated at high temperatures could adhere much longer to a rigid glass plate, with potential for robotic feet and grippers.

The actuation of the material could be activated not just in hot water but also by infusing LCE with heat-sensitive particles or particles that absorb light and convert it to heat.

The next steps, Cai and colleagues say, include finding a way to tune the material’s properties more precisely and efficiently, and modifying the ink so printed structures can be self-repairable, reprogrammable and recyclable.