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Room-temperature method transforms liquid crystal 'fabric' into soft robotic actuators
(Nanowerk Spotlight) Soft robotics has emerged as a promising field, with the potential to revolutionize areas such as prosthetics, wearable devices, and human-machine interaction. Unlike traditional rigid robots, soft robots are made from highly compliant materials that can safely interact with delicate objects and adapt to complex environments.
However, the fabrication of soft robotic actuators - the components that enable movement and force generation - has remained a significant challenge, often requiring specialized equipment, harsh processing conditions, or intricate manufacturing techniques. These challenges have limited the widespread adoption of soft robotics and largely confined it to research labs.
Now, a team of scientists from China has developed a new approach that could make soft actuators much easier to fabricate and customize, potentially opening the door to more accessible and scalable soft robotic technologies.
The researchers, led by Professor Yan Ji from Tsinghua University, have devised what they call a "cloth-to-clothes-like" method for creating soft actuators. The key innovation is the separation of the core material production from the process of shaping it into a functional actuator. This two-step approach significantly simplifies fabrication compared to traditional methods and allows greater flexibility in customizing the actuators for specific applications.
The team reported their findings in Advanced Materials ("“Cloth-to-Clothes-Like” Fabrication of Soft Actuators"),
At the heart of this method is a type of liquid crystal elastomer (LCE). LCEs are rubbery polymers containing rod-like "mesogen" molecules that can align in response to stimuli like heat, causing the material to change shape. While LCEs have attracted significant interest for soft robotics due to their large, reversible deformations, getting them into the right initial shape to serve as an actuator has been challenging.
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Scheme of the “cloth-to-clothes-like” preparation strategy of soft actuators. a) Comparison of fabricating clothes and soft actuators. b) Advantages of the “cloth-to-clothes-like” strategy. c) Flexible designs of soft actuators via this strategy. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Professor Ji's team discovered that by carefully controlling the chemistry and processing of their LCEs, they could imbue them with a unique "hysteretic behavior." This means that after being stretched and released, the material retains some of that deformation for an extended period, almost like a "memory" of the stretched state. The researchers liken this to how clothing retains the shape of the body even after being taken off. Just as a t-shirt can be stretched out and holds that shape, their LCE material can be deformed into a desired actuator shape and will maintain it at room temperature without requiring any extra equipment or harsh conditions.
The LCEs used in this study are synthesized via a thiol-acrylate Michael addition reaction using commercially available monomers. A key component is the incorporation of a transesterification catalyst, which enables the rearrangement of the polymer network that locks in the programmed shape. After the pre-stretched LCE is left at room temperature for a few days, the catalyst triggers a gradual reorganization of the network, solidifying the mesogen alignment. The result is an actuator that returns to the pre-programmed shape when heated and reverts to a flat sheet when cooled.
This catalyst, a neutralized form of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), plays a critical role in the room-temperature "fixing" of the actuator shape. It promotes transesterification reactions between ester groups in the LCE network, allowing a reshuffling of the network topology without changing the overall chemical composition. This dynamic bond exchange is what enables the LCE to transition from a temporary stretched state to a permanent actuator configuration.
Using this approach, the team created actuators in a variety of shapes, including coils, bends, and twists. By selectively stretching different regions of a single LCE sheet, they could program complex deformations, like an artificial flower that opens and closes with temperature. Importantly, the "remembered" shape is not fixed forever - the dynamic bonds in the LCE allow the actuator to be reshaped and reprogrammed later if needed.
The "cloth-to-clothes" method's simplicity and versatility could significantly lower barriers to creating custom soft actuators. The LCE material could be mass-produced in large sheets, then shipped out for others to fashion into actuators as needed. The programming process is straightforward enough that even non-experts could potentially do it, opening possibilities for personalized soft robotic devices.
However, there are still some limitations to overcome. Locking in the programmed actuator shape currently takes a few days, and repeated cycles to high temperatures can gradually degrade performance. The researchers are now working on new LCE formulations that could speed up programming and improve long-term stability. Accelerating the shape-setting process and enhancing thermal stability will be important for realizing the full potential of this approach.
Despite these challenges, this work represents a significant step toward more accessible and scalable soft robotics. By decoupling the production of the "smart material" from the process of shaping it into an actuator, the "cloth-to-clothes" approach could enable a much wider range of people to participate in creating soft robotic devices. In the future, the researchers envision pre-stretched LCE sheets becoming widely available, allowing anyone to cut them to size and craft custom soft actuators.
Just as fabric can be sewn into endless varieties of garments, these "programmable" LCE sheets could be fashioned into all manner of soft machines, from biomimetic grippers to morphing airplane wings to responsive prosthetic limbs. By making the process as simple as possible, this new approach to fabrication could help unlock the full potential of soft robotics, paving the way for more accessible, customizable, and widely applicable soft robotic technologies.
As soft robotics continues to advance, innovations in materials and manufacturing, like the "cloth-to-clothes" method developed here, will be crucial for translating research breakthroughs into practical, real-world devices. With simpler and more scalable fabrication techniques, soft robots may soon become much more commonplace, ushering in a new era of adaptable, safe, and personalized robotic systems that could transform fields from healthcare to manufacturing to space exploration.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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