Lawrence Livermore National Laboratory researchers and their collaborators have created a new responsive material called a liquid crystal elastomer, made by incorporating liquid crystals into the molecular structure of a stretchable material. Adding gold nanorods to the material, they created photo-responsive inks and 3D printed structures that could be made to bend, crawl, and move when exposed to a laser light. (Image: courtesy of Michael Ford)

Researchers at Lawrence Livermore National Laboratory have furthered a new type of soft material that can change shape in response to light, a discovery that could advance “soft machines” for a variety of fields, from robotics to medicine.

The novel material, called a liquid crystal elastomer (LCE), is made by incorporating liquid crystals into the molecular structure of a stretchable material. Adding gold nanorods to the LCE material, scientists and engineers created photo-responsive inks and 3D printed structures that could be made to bend, crawl, and move when exposed to a laser that causes localized heating in the material. The results were recently published online by Matter.

As described in the paper, the LLNL team, along with their collaborators from Harvard University, North Carolina State University, and the University of Pennsylvania, used a direct ink writing printing technique to build a variety of light-responsive objects, including cylinders that could roll, asymmetric “crawlers” that could go forward, and lattice structures that oscillated. By combining shape morphing with photoresponsivity, researchers said the new type of material could change the way people think about machines and materials.

“At LLNL, we’ve focused on developing static materials and architectures for some time,” said principal investigator Caitlyn Krikorian (Cook). “We’ve made complex types of structures like hierarchical lattices, and we’ve even started exploring more responsive materials, like shape memory polymers, that have a one-time shape memory response. But the Lab hadn't previously delved into creating architectures that can go through a 3D-to-3D shape change.

The researchers said the new material could be used to create a “soft machine” — a type of machine made from these flexible LCE composite materials — capable of responding to external stimuli and even mimicking the movements and behaviors of living organisms. Soft robots made of this shape-morphing material could crawl, swim, or fly, and explore environments that are too difficult or dangerous for humans to access, like caves or outer space. Soft machines could also be used in medical applications, such as implantable devices that can adapt to the body's movements, or prosthetic limbs that move like natural limbs, as well as other applications that aren’t possible with machines made from rigid materials, like metal or plastic.

“Rigid robots wouldn’t be ideal for humans to interact with, so we need systems and materials that are more compliant,” said the paper’s lead author Michael Ford, who began working on responsive materials while a postdoc at Carnegie Mellon University. “You start with components that make up our robots, one of which is an actuator. That’s where these materials come in: they could potentially be actuators. It reduces computational complexity if you're making a single material that replaces some onboard electronics. 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.”

Researchers said the movement of the LCE material is driven primarily by a process known as photothermal actuation, which involves converting light energy into thermal energy resulting in a mechanical response from the material. Driven by the interaction between light, gold nanorods, and the LCE matrix, the process enables the printed structures to exhibit dynamic and reversible movements in response to external stimuli.

“This composite material has a photothermal effect,” Cook explained. “Infrared light creates heating, which causes the aligned molecules to become misaligned. With uniform heating, the misalignment will cause a global shape change. But in this case, we can have localized the heat change, which is how you can get localized regions of shape morphing to do things like locomotion.”

In the study, researchers used a computer vision system, involving cameras and tracking software, to control the movement of a printed cylinder. The tracking system monitored the position of the rolling cylinder and continuously adjusted the position of the laser to raster the edge of the cylinder. This continuous tracking and adjustment allowed for the cylinder to maintain its rolling motion in a controlled manner.

By leveraging computer vision with the photothermal actuation of the cylinder, the researchers achieved a sophisticated level of manipulation of the soft machine's movement, showcasing the potential for advanced control systems in the field of soft robotics and soft machines. The team also showed that responsivity could be controlled so the soft machines could perform useful tasks, such as a moving cylinder carrying a wire.

“Doctor Ford did some awesome work in using computer vision to control the locomotion of the printed cylinder, using a rastering laser to force it to move,” said co-author Elaine Lee. “But once you start to get into much more complex motion — like using various rastering speeds and light intensities on a printed lattice, causing it to move in various different modes — those were outside of what our high-performance computing (HPC) simulations were able to predict, because those codes are based on uniform heating on the lattice. So, using computer vision and machine learning to learn the actuation speeds, and what doses of light can cause locomotion from that printed architecture, will push us a lot further in understanding how our materials will respond.”

The researchers said there are still some challenges that need to be overcome before the material can be used in practical applications. The team found that the structures they created could flip over or exhibit other unpredictable motions, thereby making it difficult to design specific motions. They said they will continue to work on models that can describe the complex motion to better design future machines and develop new materials and manufacturing techniques to create soft machines that are more durable, reliable, and efficient for a variety of applications.

New control systems and computer algorithms also could enable soft machines to move and interact with their environment in a more intelligent and autonomous way, they said. Cook said the team is looking at incorporating responses to different types of stimuli, beyond thermal and light stimuli, into areas like humidity and energy absorption, and conditions that the material might experience in space. She added that they are looking at starting a new strategic initiative at the Lab to focus on autonomous materials and “move the needle” towards sentient materials.

“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 stimuli and respond, and respond repeatedly, every time — but they don’t have a sense of memory or a way to learn the repeated stimuli and respond accordingly. This might require a five- to 10-year timespan of effort to perfect.”