3D Printed Liquid Crystals Open up Opportunities in Medical Sensors

 Researchers from Eindhoven University of Technology (TU/e) have developed a groundbreaking liquid crystal ink that can be extruded with direct ink write (DIW) 3D printing. Cholesteric liquid crystals have been used as components in screens for smartphones and high-definition televisions. However, now with additive manufacturing, this material opens up applications in healthcare, energy, and technology.

Cholesteric liquid crystals are a man-made material that have been inspired by iridescent materials found in nature—that is, materials that change color depending on the viewing angle. This includes the plumes of a peacock, the feathers of a pigeon, certain kinds of beetles, butterfly wings, and a pearl in a mollusks’ shell.

Iridescent shader beetle.

Producing cholesteric liquid crystals requires advanced chemistry and are sensitive to heat. On a molecular level, the liquid crystals’ organic molecules have no internal planes of symmetry. The molecules stack on top of each other like cylinders, resulting in the helical structure that causes the material’s unique and interesting optical effects.

In terms of industry, liquid crystals has been used as “smart” materials in light reflectors, switchable windows, and solar energy panels. Theoretically, they are ideal for applications; in healthcare for use in wearable sensors, soft optical sensing mechanisms in robotics, or even decorative lighting. However, the main issue has been that these cholesteric crystals are not viscous enough to produce a solid structure, let alone be extruded through a 3D printer.

Eindhoven University of Technology has solved this problem by developing a custom light reflective liquid crystal ink which can be printed via DIW and programmed to produce complex color gradients. The study shows how the researchers were able to take this liquid crystal elastomer ink and manipulate conditions like writing direction and speed to selectively control the interaction of the material with different light polarizations.

3D printed butterfly using Eindhoven’s breakthrough cholesteric liquid crystal ink. Image courtesy of Eindhoven University of Technology.

Lead author of the study, Jeroen Sol, says, “To successfully print the new ink with DIW, we varied parameters like print speed and temperature. And to get the ink to print properly, we also made an ink containing low-molecular weight liquid crystals. Traditionally, this level of control is only possible with very specialized fabrication devices, so to do this with the new ink and DIW 3D printing is a real breakthrough.”

The ink’s reflective qualities are dependent on the helical arrangement of its molecules, its molecules also self-assemble into mimicking natural iridescent materials, giving way to nature-inspired color changes. Fine-tuning the microscopic elements of a structure printed from this material will change its macroscopic elements creating a programmable iridescent object. In addition to these qualities, the material is also affordable, easy to make, easy to process, and is made with materials already developed by the research group.

Image courtesy of Eindhoven University of Technology.

The study predicts the use of this technology in biomimetic optics for optical sensors, holographic displays, anti-counterfeit labels, intelligent skins, and wearable robotics. Though this material is a first, a similar material was developed last year by the University of Colorado Denver to mimic biological tissue and cartilage, paving the way to a future where it might be possible to grow cells and fix spines, giving people a new lease on life. There’s also this polymer, which is stronger, more lightweight, cost efficient, and recyclable compared to traditional 3D printable composites.