Researchers from the University of Colorado Denver and the Southern University of Science and Technology in China, have created a novel 3D printing material that’s able to imitate the behaviours of biological tissues.
Using the Digital Light Processing (DLP) 3D printing process, the research team developed a honey-like Liquid Crystal Elastomer (LCE) resin. When hit with ultraviolet light, the material cures, and forms new bonds in a succession of thin photopolymer layers, and after being 3D printed into lattice structures, the resin begins to mimic cartilage. The resulting material’s shock-absorbant behaviours open potential new applications in surgical and protective equipment.
“Everyone’s heard of liquid crystals because you stare at them in your phone display,” said mechanical engineering professor Chris Yakacki, PhD. “And you’ve likely heard of liquid crystal polymers because that’s exactly what Kevlar is. Our challenge was to get them into soft polymers, like elastomers, to use them as shock absorbers. That’s when you go down the layers of complexity.”
The difficulty of additive manufacturing biological tissue
LCEs are soft, multifunctional materials that combine anisotropic molecular order of liquid crystals (LCs) with the entropic elasticity of a lightly cross-linked polymer network. While these materials are often utilized to produce soft robotic actuators, they also display the behaviours of biological tissues, such as high energy dissipation and soft elasticity. Using 3D printing, scientists are now able to tailor the geometry of lattices to offer control over their mechanical and dissipative properties, and tailor them for different applications.
Previously produced LCEs were largely limited to thin-film (<150 µm) devices due to their complex synthesis routes, and the need to align the LC groups via surface effects. These materials were produced using the Direct Ink Writing (DIW) 3D printing technique due to recent advances in the technology, which have enabled the fabrication of macroscopic devices.
The researchers opted for a different approach, and developed their new photocurable LC resin using DLP 3D printing instead, because it enabled them to create large-scale soft material devices with high-resolution and complex features. DLP printing is also a high throughput and scalable technology, which makes it an attractive method for the commercial fabrication of architected dissipative lattices. Moreover, the printing method allowed the researchers to specify the device’s overall geometry, and control its mechanical properties to completely optimize a dissipative device.
Using 3D printing to create artificial tissues
To demonstrate the elasticity of their new resin, the research team constructed Bulk LCE test devices with high-resolution details and complex shapes, using bespoke thiol-acrylate LC resin and a custom DLP 3D printer. Utilizing the system’s UV light engine, the researchers projected masked images to photopolymerize the LC resin in a top-down, layer-by-layer process. Once cured, this LC resin formed an elastomer, with highly pronounced dissipative properties at 30°C above its glass transition temperature, a phenomenon that’s not observed in traditional elastomers.
The researchers proceeded to 3D print several structures, including a tiny, detailed lotus flower, and a prototype of a spinal fusion cage, creating the largest spinal LCE device with the most detail. Compressive mechanical testing revealed that the stress-strain responses of the LCE lattices were shown to have 12 times greater rate-dependence, and up to 27 times greater strain-energy dissipation, than those printed from commercially available resins. This greater rate-dependency is caused by the rotation of mesogen and liquid-crystal domains when strained, which adds an additional mechanism of viscous effects to the material.
The researchers’ LCE displayed inherently higher levels of energy dissipation than the TangoBlack resin, as well as neoprene (a common shock absorber), nitrile and silicone, (materials which currently are not DLP-printable). Another advantage of the researchers’ LCE resin is that it can also be printed using commercially available DLP and SLA printers, potentially allowing the rapid development of commercial devices.
Future applications for the novel LCE resin
The high levels of dissipation and rate-dependence of the LCE material, make it well-suited for use in a range of protective applications, such as protective body equipment including helmets, and impact absorbers in industrial equipment and electronics. For instance, small devices placed in small spaces available within mobile phones could reduce the risk of screen cracks when dropped. While the technology has potential shock-absorbing and surgical applications, the researchers have set their sights on the latter for their future research.
“The spine is full of challenges and it’s a hard problem to solve,” said Yakacki. “People have tried making synthetic spinal tissue discs and they haven’t done a good job of it. With 3D printing, and the high resolution we’ve gotten from it, you can match a person’s anatomy exactly. One day, we may be able to grow cells to fix the spine, but for now, we can take a step forward with the next generation of materials. That’s where we’d like to go,” said Yakacki.
In future, the researchers anticipate that the performance of dissipative LCE lattice devices could be further enhanced and tailored, by buckling geometries that are predominantly stretching the members, rather than bending them. For instance, the high level of detail achievable with DLP 3D printing, could allow for gussets to be designed to strengthen structures. Another aspect of DLP printed lattices the researchers did not explore, was how print orientation affects the mechanical responses of the printed structures. Further studies could work to quantify the impact of changing the orientation of a part during printing, in addition to developing further surgical applications of the LCE.
Additive manufacturing and synthetic tissues
University researchers have devised a range of 3D printing methods in recent years to create functional synthetic tissues. In April for instance, researchers from the University of Oxford used 3D printing to enhance the single-droplet resolution 3D bioprinting process, allowing them to create synthetic tissues with greater precision.
3D Bioprinting Solutions, a Russian bio-technical research laboratory, 3D bioprinted bone tissue in zero gravity on the International Space Station (ISS) in December 2019. The research aims to enable the creation of bone implants for astronaut transplantation during long-term interplanetary expeditions.
A team from the University Medical Center (UMC) Utrecht and École polytechnique fédérale de Lausanne (EPFL), Switzerland, developed a volumetric 3D bioprinting process in August 2019. Inspired by visible light projection, the method enabled the creation of free-form tissue structures.
The researchers’ findings are detailed in their paper titled “Liquid‐Crystal‐Elastomer‐Based Dissipative Structures by Digital Light Processing 3D Printing,” published on June 8th, 2020 in the Advanced Materials journal. The research was co-authored by Nicholas A. Traugutt, Devesh Mistry, Chaoqian Luo, Kai Yu, Qi Ge and Christopher M. Yakacki.
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Featured image shows a DLP-printed LCE concept device of a spinal cage with a porous lattice architecture. Image via Advanced Materials.
