Researchers publish comprehensive guide on 4D printing with liquid crystals

Researchers from the Eindhoven University of Technology (TU Eindhoven) have published a new, comprehensive review of the liquid crystal additive manufacturing landscape.

Liquid crystals are a class of material that exhibit properties of both conventional liquids and solid crystals. As well as being a key component of liquid crystal displays (LCDs), liquid crystals have advanced applications as smart materials in everything from light reflectors and switchable windows to solar panels.

When used in conjunction with 3D printing, liquid crystals allow for programmable, reversible, anisotropic actuation in both dry and wet environments, making them a very powerful ‘stimuli-response material’ with 4D capabilities. The list of potential additive manufacturing use cases includes energy generation, sensing, and even soft robots.

Unlike conventional academic reviews, the TU Eindhoven paper goes beyond just listing noteworthy research projects in the area, and is instead designed to serve as a handbook for non-specialists.

Jeroen Sol, a fourth year Ph.D. student and co-author of the work, writes, “Reading from front to back gives the reader a comprehensive understanding of the options and challenges in the field, while researchers already experienced in either liquid crystals or additive manufacturing are encouraged to scan through the text to see how they can incorporate additive manufacturing or liquid crystals into their own work.”

4D printed liquid crystal soft robots can be actuated with heat, UV light, and other mechanisms. Image via TU Eindhoven.
4D printed liquid crystal soft robots can be actuated with heat, UV light, and other mechanisms. Image via TU Eindhoven.

The importance of liquid crystal alignment

According to the paper, the molecular order of a liquid crystal is a primary factor in the quality of its anisotropy and has a major effect on how it responds to stimuli. As such, controlling the molecular alignment during the 3D printing process is crucial (especially for 4D applications), but there are several common ways of doing this.

One of the most widely used methods of aligning the mesogens in a liquid crystal is by using a chemical surface treatment. This method commonly utilizes a glass cell with alignment layers on the top and bottom surfaces. Once the cell cools to the liquid crystal phase, the molecules can be made to undergo phase transitions and self-organize in a predetermined manner.

Liquid crystals can also be aligned by applying electric or magnetic fields during the 3D printing process, or by leveraging mechanical force fields such as shear and elongation.

Applications of 4D printed liquid crystals

The paper also goes on to discuss some of the more advanced applications of 4D printed liquid crystal structures, the first being soft robotics. In soft robots, contracting strips of liquid crystal materials are used as artificial muscles. These artificial muscles are usually actuated via thermal energy, light exposure, or humidity exposure, and can be 3D printed on their own or woven together to embed additional smart properties.

Additively manufactured liquid crystals also have potential applications in dynamic biomedical implants. While they haven’t yet been approved for use in human implants, the paper states that prior research has suggested the materials may offer a degree of biocompatibility. With a process like direct ink writing, for example, it’s possible to print a urological implant that has a programmable thermal contraction when blasted with infrared light (which travels through skin quite well).

Finally, liquid crystals can also be 3D printed for photonic devices, owing to their widespread conventional use in optical devices. In fact, in a separate experiment conducted by the TU Eindhoven team, a novel color-changing liquid crystal ink was developed for use with microextrusion. Inspired by iridescent materials such as the exterior of jewel beetles, the ink is expected to have implications for decorative lighting and even augmented reality optics.

Butterfly 3D printed using the new liquid crystal ink. Photo via Eindhoven University of Technology.
Butterfly 3D printed using the iridescent liquid crystal ink. Photo via Eindhoven University of Technology.

Further topics such as 3D printing methods, print parameter refinement, and liquid crystal actuation methods can be found in the paper titled ‘4D Printing of Liquid Crystals: What’s Right for Me?’.

The 3D printing of optical devices is a cutting-edge sector within additive manufacturing. Earlier this year, researchers from the King Abdullah University of Science and Technology (KAUST) developed a new method of 3D printing photonic crystal fibers – a special type of optical fiber. The team constructed a purpose-built SLA-based 3D printer for the project, one that enabled the scientists to customize their optical fibers with previously impossible internal geometries.

Elsewhere, scientists from the University of Freiburg and 3D printer manufacturer Nanoscribe recently utilized a two-photon-polymerization technology to fabricate glass silica microstructures with a sub-micrometer resolution. In the future, the team believes that their unique printing, debinding, and sintering process could be deployed to produce next-generation micro-optics with potential biomedical applications. 

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Featured image shows the TU Eindhoven campus. Photo via TU Eindhoven.