Unveiling a Revolution: 300 Times More Flexibility in 3D Printing Design Using Architected Auxetics

November 30, 2023

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by Jerry Grillo,

Georgia Institute of Technology

There are young children celebrating the holidays this year with their families, thanks to the 3D-printed medical devices created in the lab of Georgia Tech researcher Scott Hollister. For more than 10 years, Hollister and his collaborators have developed lifesaving, patient-specific airway splints for babies with rare birth defects.

Personalized Airway Support Devices are manufactured from a biocompatible polyester known as polycaprolactone (PCL). The Food and Drug Administration has approved this material, which is formed into a solid structure using selective laser sintering—a method of heating the powdered polyester. Devices constructed from PCL have consistently demonstrated their safety when implanted into patients.

Nevertheless, PCL’s comparatively stiff and linear mechanical attributes present a hurdle, inhibiting its functional application in certain vital biomedical demands, for instance, soft tissue engineering. The question, therefore, is transforming a rigid thermoplastic into a flexible material capable of adapting and growing with the patient. Hollister’s lab may have discovered a solution to this issue.

Jeong Hun Park, a research scientist in Hollister’s lab who spearheaded the team’s recent study showcasing successful 3D printing of PCL for soft tissue engineering, brings attention to “3D auxetic design.” Unlike traditional, standard elastics, auxetic materials bear a negative Poisson’s ratio. In other words, longitudinal stretching of an auxetic material results in lateral expansion as well. In contrast, other materials become laterally thinner due to their positive Poisson’s ratio.

An auxetic structure’s ability to expand biaxially is beneficial when considering biomedical applications for humans. The human body, composed of varied textures and densities, often changes in size and shape over time. The team at Hollister set out an endeavor to introduce new auxetic properties to the usually firm PCL.

“Although the mechanical properties and behavior of the 3D structure depend on the inherent properties of the base material—in this case, PCL—it can also be significantly tuned through internal architecture design,” explained Park.

Park guided the design of 3D-printed structures made up of tiny struts, arranged at right angles—imagine the bones of very tiny skyscrapers. The team began by creating cube-shaped structures first, to test the auxetic design’s flexibility, strength, and permeability.

The work is published in the journal Advanced Functional Materials.

Basically, an auxetic material is a network structure designed by assembling unit cells. These unit cells consist of struts and their intersecting joints, which are an important aspect of an auxetic device’s behavior. The rotation of those intersecting joints within the network, under compression or extension, causes negative Poisson’s behavior. It also enables advanced performance for a printed device, including impact energy absorption, indentation resistance, and high flexibility.

“When you look at the numbers, based on Jeong Hun’s work, the new structure is about 300 times more flexible than the typical solid structure we make out of PCL in our lab,” said Hollister, professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University, where he also holds the Patsy and Alan Dorris Chair in Pediatric Technology and serves as the department’s associate chair for translational research.

The combination of flexibility and strength in a device is particularly important here, Park said, because the ultimate goal of the research is to “apply this structure to develop a breast reconstruction implant that has comparable biomechanical properties to native breast tissue. Currently, we don’t have a biodegradable breast implantation option in the clinical setting.”

He explained that these biodegradable breast reconstruction implants serve as a kind of scaffold. The idea is, the biocompatible material (PCL) eventually degrades and is absorbed into the body, while maintaining similar mechanical properties to native breast tissue.

“We expect that native tissue will be first infiltrated into the pores of the biodegradable implant,” Park said. “Tissue volume will then increase within the implant as it degrades and eventually the device itself is replaced with the tissue after complete degradation of the implant.”

Basically, the purpose of the 3D-printed breast implant is to both offer reconstructive aid and, additionally, promote the development of new tissue.

The tiny struts provided by this larger device create a vital space, leading to a flexibility and softness which wouldn’t have been possible otherwise. These spaces over time could be filled with a hydrogel designed to assist in promoting cell and tissue growth.

The team’s engineered auxetics further incorporate the creation of internal voids and areas within the struts, providing a type of microporosity that allows for the transfer of oxygen, nutrients, and metabolic products required for the extension and growth of a cellular network.

Currently, Park is coordinating with Emory surgeon Angela Cheng to obtain a grant for more extensive research and trials relating to the breast implant. The team has already begun adapting this technology for other uses, with one of their research partners, Mike Davis, focusing his lab at Emory on cardiac regeneration.

“Because of the great flexibility, they’re using it to reconstruct infarcted or necrotic myocardial tissue,” Hollister said.

And Park has developed an auxetic version of the pediatric tracheal splint. “The advantage there is, with this design, it can expand in two directions,” he said. “So, as young patients grow, the new device will grow with them.”

More information: Jeong Hun Park et al, 3D Printing of Poly‐ε‐Caprolactone (PCL) Auxetic Implants with Advanced Performance for Large Volume Soft Tissue Engineering, Advanced Functional Materials (2023). DOI: 10.1002/adfm.202215220

Journal information:Advanced Functional Materials

Provided by Georgia Institute of Technology

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