An easier way to produce and control the shape of artificial muscles, soft robots, and wearable devices using a new 3D-printing technique has been developed by a team of researchers from the University of California, San Diego (UC San Diego).
Video Credit: University of California San Diego.
The team demonstrated that by regulating the printing temperature of liquid crystal elastomer (LCE), they can regulate the material’s level of stiffness and capacity to contract—also referred to as the degree of actuation. Furthermore, the researchers showed that the stiffness of various areas in the same material could also be altered by exposing it to heat.
As a proof of concept, the team 3D-printed in a single print, with a solo ink, structures whose actuation and stiffness differ by orders of magnitude, from 0% to 30%. For instance, one area of the LCE structure could be made to contract similar to muscles, while another area could be made flexible similar to tendons.
This innovation was possible because the researchers closely examined the LCE to properly comprehend its material properties.
The researchers, guided by Shengqiang Cai, a professor in the Department of Mechanical and Aerospace Engineering at the UC San Diego Jacobs School of Engineering, have explained their work in the Science Advances journal in the September 25th, 2020 issue.
The team was motivated to develop this material with varying degrees of actuation by taking examples in nature and biology. Besides the combination of muscle and tendon, the researchers took cues from the beak of the squid, which is very stiff at the tip but relatively softer and malleable where it is linked to the mouth of the squid.
3D-printing is a great tool to make so many different things—and it’s even better now that we can print structures that can contract and stiffen as desired under a certain stimuli, in this case, heat.
Zijun Wang, Study First Author and PhD Student, Jacobs School of Engineering, University of California, San Diego
Wang is a Ph.D. student in Cai’s research team.
Understanding Material Properties
To figure out how to tweak the material characteristics of LCE, the team initially examined the material in detail. They subsequently established that the printed LCE filament is composed of a core and a shell. The shell cools off rapidly after printing, becoming stiffer, while the core cools off more gradually, staying more malleable.
Therefore, the scientists were able to establish how to differ a number of parameters in the printing process, particularly temperature, to tweak the mechanical characteristics of the LCE. In brief, the greater the printing temperature, the more malleable and flexible is the material.
Although the preparation of the LCE ink requires a few days, the actual 3D print can be accomplished in merely 1 to 2 hours, based on the geometry of the structure to be printed.
Based on the relationship between the properties of LCE filament and printing parameters, it’s easy to construct structures with graded material properties.
Shengqiang Cai, Professor, Department of Mechanical and Aerospace Engineering, Jacobs School of Engineering, University of California, San Diego
Varying Temperature to 3D-Printing Structures
For instance, an LCE disk was printed at 40 °C (104 °F) and heated up to 90°C (194°F) in hot water. The disk changed into a conical shape. However, an LCE disk made up of areas that are printed at varying temperatures (for example, 40 °C, then 80 °C, and then 120 °C), changed into a totally different shape upon heating.
The team also 3D-printed structures composed of two layers of LCE that have different characteristics and demonstrated that this imparted the material even more degrees of freedom to actuate. They also printed lattice structures using the material, which could be utilized in medical applications.
Ultimately, as a proof of concept, the researchers 3D printed an LCE tube that they had tweaked during 3D printing and demonstrated that it could stick to a rigid glass plate much longer when actuated at elevated temperatures, approximately 94 °C (201 °F), than a standard LCE tube with homogenous characteristics. This could result in the production of improved robotic feet and grippers.
The material's actuation could be triggered not just in hot water but also by filling LCE with heat-sensitive particles, or particles that absorb light and turn it to heat—any material from graphene to black ink powder. Another procedure would be to 3D print the structures using electric wires that produce heat embedded in LCE.
The subsequent steps would involve discovering a method to tweak the material’s properties more efficiently and precisely. The team is also working to alter the ink so that the printed structures can be reprogrammable, self-repairable, and recyclable.
Wang, Z., et al. 2020 Three-dimensional printing of functionally graded liquid crystal elastomer. Science Advances. doi.org/10.1126/sciadv.abc0034.