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Researchers Develop Light-Responsive Material for Soft Robots

A red toy robot
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Researchers have designed a new, 3D-printable, light-responsive material that can be used to make machines that move without any electronics.

These “soft robots” or “soft machines” are made of a novel material, called a liquid crystal elastomer, embedded with gold nanorods. So far, the researchers have successfully used their material to print cylinders that can roll, “crawlers” that move forward and lattice structures that oscillate when exposed to laser light.

Robots made from materials like this could one day be used to explore caves or other environments where it is unsafe for humans to go, the researchers say. The new material is described in a new study published in Matter.

A light-responsive printable ink

Liquid crystals are a strange state of matter. They are made up of slightly cross-linked molecules that flow like liquids normally, but when allowed to settle, they stack periodically into a crystal arrangement like a solid.

Liquid crystal elastomer (LCE) materials operate in a similar fashion, using liquid crystal structures incorporated into a stretchy elastomer network. This unusual structure can give the material some unexpected properties.

Previously, LCE materials have been used to make textile fiber that reacts to heat. As the fiber heats up, the liquid crystal molecules begin to fall out of alignment and pull the elastomer network closer together, causing woven textiles to get tighter. This could be useful for compression garments or other adaptable clothing items.

Now, researchers are using LCE materials embedded with gold nanorods to make 3D-printable “inks” that move in response to laser light. In this case, the movement of the LCE material is due to a process called photothermal actuation.

“When you have this composite material — in this case, these gold nanorods in these liquid-crystal elastomers — it has a photothermal effect,” explained principal investigator Caitlyn Krikorian (Cook), a polymer engineer and group leader within the Materials Engineering Division at the Lawrence Livermore National Laboratory.

“With [infrared] light, it creates a heating effect, which causes the aligned molecules to become misaligned. During that misalignment process, if there's uniform heating, you’ll have a global shape change. But in this case, we can have localized heat change, which is how you can get those localized regions of shape morphing to do things like locomotion.”

New materials for soft robotics

Using their new LCE composite material, in combination with a laser and a computer vision system with cameras and tracking software, the researchers were able to print and control the movement of a 3D cylinder.

The computer tracking system continuously monitored the position of the cylinder, adjusting the laser beam as needed to get the cylinder rolling and keep it progressing in a controlled manner. 

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The researchers were also able to print some more sophisticated soft machines, including a latticed sheet that could oscillate in response to the laser and some asymmetric “crawlers” that could shuffle forward.

The team demonstrated the usefulness of these machines in fulfilling basic tasks, such as the moving cylinder transporting a length of wire as it moved.

These materials could be useful in the field of “soft machines” or “soft robots,” the researchers say. This field of robotics concerns itself with robots made from materials that are more flexible and compliant than traditional metal or plastic parts. Soft robots could be useful in the medical field in implantable devices that flex with the body’s natural motion or to create advanced prosthetic limbs, for example.

“Rigid robots maybe wouldn’t be ideal for humans to interact with, so we need systems and materials that are more compliant,” said the paper’s first author Michael Ford, a postdoctoral researcher in the High Performace Materials group at the Lawrence Livermore National Laboratory. “You start with components that make up our robots, and one of those components is an actuator. That’s where these materials come in; they could potentially be an actuator.”

“It reduces computational complexity; you're making a material that gets rid of onboard electronics and replacing them with a single material that can do all those things,” he continued. “That will allow you to put more computational complexity into another component or drive power to other sensors that you wouldn't have been able to do with traditional rigid materials.”

Towards autonomous, sentient materials

There are still some notable challenges that need to be overcome before this new LCE material can be used for broader robotics applications. The researchers note that some of the structures they created had a tendency to flip over or otherwise move unpredictably, which complicated things when they tried to execute very specific instructions. Building better computer simulations of how these materials move could help to negate this aspect, they say.

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The team is hopeful that new control systems and computer algorithms could allow these soft machines to interact with their environment in a more intelligent way.

“We’re all thinking about ways to make materials more autonomous; sentient materials that can sense, respond, be programmed, learn, decide and communicate,” Cook said. “These liquid crystal elastomers are responsive materials — they’re able to sense a stimuli and respond, and will respond repeatedly every time — but it doesn't have a sense of memory or a way to learn the repeated stimuli and respond accordingly. It doesn't have a means to communicate yet, other than potentially being able to pair it with some type of mechanical computing. These are really the materials that we're striving towards, and this might be a five- to 10-year timespan of effort.”

Next, Cook’s team is looking to develop LCE materials that can respond to other types of stimuli beyond light and heat, such as humidity changes or energy absorption. They also want to look at the sort of conditions that such a material might experience in space and other harsh environments.

Reference: Ford MJ, Porcincula DH, Telles R, et al. Movement with light: Photoresponsive shape morphing of printed liquid crystal elastomers. Matter. doi:10.1016/j.matt.2024.01.006

This article is a rework of a press release issued by the Lawrence Livermore National Laboratory. Material has been edited for length and content.