Three-dimensional (3D) printing is one of the fastest-growing industrial technologies, but it still lacks versatility. Commercial 3D printers typically use brittle plastics that have limited biocompatibility and cannot print hollow or unsupported structures. Now, a group of researchers from the University of Illinois at Urbana-Champaign has developed a way to 3D-print open structures using an artificial sweetener called isomalt. This paves the way to more biocompatible 3D printed structures that can be used to grow artificial organs and veins.
In order to be used as a scaffold for growing organs and blood vessels, the printed objects need to be easily dissolved after cells have grown on them, while still being strong enough to hold a 3D structure. It is also important that the material, when solid, forms a glass rather than a crystal, so that it can cool into its desired shape without clogging the printer. The class of materials that fits this bill is the sugar alcohols, similar in chemical structure and used as sweeteners, which can form carbohydrate glasses when they solidify.
While several sugar alcohols were tested, the best candidate was isomalt, a sweetener used in sugar-free candy, because it is stable at room temperature and resists crystallization as compared to other sugar alcohols. “We especially focused on using isomalt as it is readily processed and has excellent biocompatibility,” says Rohit Bhargava, who led the research team. “However, its use also needs a careful analysis and validation of the printing process,” he says. As the research group reported in a recent issue of Additive Manufacturing (doi:10.1016/j.addma.2018.04.026), the two principal challenges that had to be overcome were the dehydration of the isomalt and its stability during the extruding process.
Isomalt forms crystals in the presence of water, but remains glassy when dehydrated. Dehydration was achieved by stirring the isomalt in a vacuum to eliminate any dissolved water. Debugging the extrusion process was more challenging. The material needs to quickly solidify as it cools, while maintaining its desired printed shape.
Most 3D printers operate by depositing horizontal layers on top of one another, making structures thick enough to support their own weight. In contrast, the objects the researchers printed were hollow and made entirely of thin rods. Furthermore, the research team faced challenges and structural issues that do not arise during traditional 3D printing, which include thermal contraction as the material cools, the sagging of the rods under gravity, the momentum imparted in the fluid by the moving nozzle, and the inherent viscosity of the rods. Through numerical models the researchers determined that the printed beams are stable if they are thinner than the extrusion nozzle. They then proved their concept by printing structures out of rods that would be impossible with traditional 3D printing methods, including truss bridges and hollow bunnies.
(a) The extruding nozzle in action, printing the scaffolds for a bridge. (b) A hollow 3D printed bunny with pink dye added. Credit: Images adapted from Additive Manufacturing.
The ultimate goal is to be able to use these 3D printed structures as scaffolds for cell culture to grow organs and blood vessels for transplantation. “The first step was to establish the scaffold printing process, which we report in this study,” Bhargava says. “The next step is to deposit and grow cells in a supported manner on these scaffolds. The third major step is to remove scaffold materials such that biological activity is minimally affected and the fabricated tissue structure is structurally stable.” If all goes well with the development of this technology, we can look forward to both lifesaving and delicious printed structures.
