In a leap forward for materials science, a multi-institutional team of researchers has developed a pioneering method of 3D printing cholesteric liquid crystal elastomers (CLCEs), enabling complex, color-changing responsive materials and paving the way for novel applications like smart textiles and advanced robotics.
Using a cutting-edge method known as Coaxial Direct Ink Writing (DIW), the team of engineers and scientists from the University of Pennsylvania (Penn), Harvard University and Lawrence Livermore National Laboratory (LLNL) invented 3D-printed, multi-stable structures capable of changing colors in response to stress, with a goal of combining the unique materials and techniques to help redefine smart materials. The research was published in the journal Advanced Materials.
Key to this research are the CLCEs, a unique class of materials known for their vivid colors and elasticity. To put it simply, CLCEs are soft, rubbery substances that can change color depending on their shape and the stress applied to them. This remarkable property is due to their ability to manipulate light, while the molecular structure of CLCEs allows for the creation of intricate color displays, like the material used in mood rings, aquarium thermometers and Boogie boards.
The inspiration for this inventive approach stems from the collaborative vision of Penn researchers Alicia Ng (also an LLNL summer student intern) and Shu Yang, with contributions from LLNL engineers Elaine Lee, Katherine Riley and Caitlyn Cook Krikorian. The breakthrough came when they partnered with Harvard researchers Jennifer Lewis and Rodrigo Telles, coming up with the idea to print a transparent silicone shell to serve as a scaffold for the CLCE core. The journey was not without challenges, researchers said.
"We wanted to bring the work [into the Lab] to have a mechanically or strain-stimulated material, where the color change can be used for remote detection," Cook explained. "Once we found a viable material, we spent considerable time fine-tuning the DIW conditions, particularly shear rates, to maintain vibrant colors while also ensuring print fidelity to construct these bistable dome structures. In this paper we demonstrated we could print these structures, but this is just the tip of the iceberg as far as achievable print architectures."
The potential applications of these innovative materials are vast. LLNL's Riley highlighted that multi-stability has been used to create "robotic grippers that can snap open and closed, metamaterial sheets that can passively sense and record mechanical loads, and mechanical logic systems that can compute without conventional electronics."
By using CLCEs, the team could 3D print flexible structures with a special ink that changes color when stressed. This color change acts as a visual signal, showing how the structure is being used and whether it's under pressure or strain.
"When arranged into 2D or 3D metamaterial arrays, this architecture could be used for passive displays, and for sensors that display different colors to indicate different levels of mechanical loading have been applied," Riley said.
The researchers foresee these materials serving as remote strain detection sensors that can be directly printed into specific shapes, providing the capability to experimentally map and verify strain and stress in complex architectures. Looking ahead, the team is eager to expand their research through a Laboratory Directed Research and Development Strategic Initiative aimed at developing a new class of 3D-printed sentient materials with even more complex liquid crystal elastomer-like synthetic muscle and sensing properties. Cook and Lee, who both lead the effort, would like to enhance the complexity of the printed architectures and compare the results to stresses observed in computational simulations.
Researchers are investigating how the materials can alter and lock their stiffness in real time based on different loading conditions. These CLCE materials could play a crucial role in rapidly iterating new designs via LLNL's automated materials lab and machine learning reinforcement strategies, Cook said.
As the team continues to refine their printing capabilities, they envision a practical integration of these vibrant, responsive materials, with innovative fabrication techniques that could also advance industries reliant on smart materials, robotics and beyond.
For more, visit Advanced Materials and Manufacturing at LLNL.
