By Jerry Grillo

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. 

These personalized Airway Support Devices are made of a biocompatible polyester called polycaprolactone (PCL), which has the advantage of being approved in medical devices by the Food and Drug Administration. Researchers use selective laser sintering to heat the powdered polyester, which binds together as a solid structure. Devices made of PCL have a great safety record when implanted into patients.

Unfortunately, PCL has the disadvantage of having relatively stiff and linear mechanical properties, which means this promising biomaterial has yet to be applied functionally to some other critical biomedical needs, such as soft tissue engineering. How do you make a firm thermoplastic into something flexible, and possibly capable of growing with the patient? Hollister’s lab has figured out how.

“3D auxetic design,” said Jeong Hun Park, a research scientist in Hollister’s lab who led the team’s recent study demonstrating the successful 3D printing of PCL for soft tissue engineering. An auxetic material, unlike typical common elastics, has a negative Poisson’s ratio. That means if you stretch an auxetic material longitudinally it will also expand in the lateral direction, whereas most materials will get thinner laterally (because they have a positive Poisson’s ratio).

So, an auxetic structure can expand in both directions, which is useful when considering biomedical applications for humans, whose bodies and parts can change in size and shape over time and comprise many different textures and densities. Hollister’s team set out to give usually firm PCL some new auxetic properties.

“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 developed 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.

Flexible Behavior

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.”