A multidisciplinary team of researchers from the University of Pennsylvania, Harvard University, Duke University, University of California Berkeley and Lawrence Livermore National Laboratory (LLNL) have developed a cutting-edge method to 3D-print cholesteric liquid crystal elastomers (CLCEs), opening the door to dynamic, colour-changing materials that can respond to mechanical stress. This innovative work, combines advanced printing techniques with unique material properties to pave the way for groundbreaking applications in smart sensing, displays, robotics and beyond.
‘The colour changes are caused by the material’s ability to manipulate light, much like a beetle shell reflects light to create a colourful display,’ said Shu Yang, Joseph Bordogna professor and chair of materials science and engineering. ‘These materials have the potential to solve industry problems across medicine, diagnostics, monitoring and can even be used in art. My lab is working with these responsive materials to discover new ways to observe and interact with the world around us.’
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Traditionally, CLCEs have been limited to 2D-planar films, like those in liquid crystal display (LCD) TV screens and monitors. Bringing these light-interactive materials into the third dimension would open doors for more applications. The team’s innovation centres around a novel 3D-printing technique known as coaxial direct ink writing (CDIW), which allows for the precise printing of multi-stable, colour-changing structures.
However, when turning 2D structures into 3D structures, things get a little complicated. In this case, the liquid precursor of the CLCE is so viscous that when pushed through a 3D-printing nozzle, it hinders the formation of the twisted helix structures that are responsible for the colour-changing characteristics of the CLCEs.
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To solve this problem, PhD student Alicia Ng and her colleagues set out to find the perfect CLCE viscosity: thick enough to maintain the structural integrity of the finished product but not too thick to allow the material to easily flow through a printing nozzle. The key ingredient that optimised the process was a completely different material, silicone, which was a game changer in their innovative technique.
‘We developed a transparent silicone shell to serve as a scaffold for the CLCE core,’ said Ng. ‘This unique combination of materials allowed us to preserve the colour-changing properties of the CLCEs while providing the necessary structural strength to support intricate 3D designs.’
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‘We envision using these materials to create devices that give real-time visual feedback, whether it’s in a wearable form or embedded in a larger structure,’ said Yang. ‘This could allow us to observe mechanical stress in a way that was previously impossible with traditional materials.’
Beyond healthcare – for example a smart wrap that indicates when a joint or part of the body swells – the team is exploring how these colour-changing materials can be incorporated into various systems that respond to environmental changes, from robotics to interactive art and environmental sensors.
‘For example, 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 are just a few areas where CLCEs can contribute to new technologies,’ said Katherine Riley, a mechanical engineer at LLNL.
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Looking to the future, the research team is exploring ways to combine CLCEs with temperature- or light-responsive materials to create materials that change colour in response to a variety of stimuli. They also are looking to nature to make these materials more eco-friendly.
‘We may look into incorporating elements from biofilms,’ said Yang. ‘That could allow us to create self-building, regenerative structural colours that are both environmentally friendly and functional.’
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In terms of design, the team has already printed various structures, including lattices, wraps and filaments, and they are eager to explore even more complex designs. With the ability to print directly onto curved surfaces, such as body parts, the team envisions wearable devices that seamlessly conform to a user’s shape.
‘Looking ahead, we are expanding upon this research through an LLNL Laboratory Directed Research and Development Strategic Initiative aimed at combining the design and fabrication of complex architectures with automated materials training to develop “sentient” materials,’ said Elaine Lee, an LLNL engineer.
‘These CLCE materials could play a crucial role in rapidly iterating new designs via remote detection paired with automated on-machine metrology techniques such as digital image correlation,’ added Caitlyn Cook Krikorian, Functional Architected Materials Engineering group leader and deputy director for the Center of Engineered Materials and Manufacturing at LLNL.
The research has been published in Advanced Materials.