Human knees come with a pair of built-in shock absorbers – the menisci – hunks of cartilage that cushion every step we take. Daily wear or a single wrong step can permanently damage these key supports, leading to pain and an increased risk of developing arthritis.

The adult meniscus has limited ability to heal on its own and attempts to repair torn or damaged tissue often lead to it being partially or completely removed. Available implants either do not match the strength and elasticity of the original cartilage or are not biocompatible – they do not support the growth of cells to encourage healing around the site.

Hydrogels have been gaining traction as a replacement for lost cartilage. Biocompatible with a similar molecular structure to cartilage, researchers have struggled to create recipes for synthetic hydrogels.

“The current gels that are available are really not as strong as human tissues, and generally, when they come out of a printer nozzle they don’t stay put—they will run all over the place, because they are mostly water,” says Benjamin Wiley, an associate professor of chemistry at Duke University in Durham, North Carolina, and co-author of the paper on printable hydrogels, which appears in ACS Biomaterials Science and Engineering.

Feichen Yang, a graduate student in Wiley’s lab and co-author on the paper, experimented with mixing together two different types of hydrogels – one stiffer and stronger, and the other softer and stretchier – to create a double-network hydrogel.

“The two networks are woven into each other,” Yang says. “And that makes the whole material extremely strong.”

By changing the relative amounts of the two hydrogels, Yang can adjust the strength and elasticity of the mixture to create a formula that best matches that of human cartilage.

He also mixes in a nanoparticle clay to make the mock-cartilage 3D-printable. Adding clay causes the hydrogel to flow like water when placed under shear stress, such as when being squeezed through a small needle, but as soon as the stress is gone, the hydrogel immediately hardens into its printed shape.

“We’ve made it very easy now for anyone to print something that is pretty close in its mechanical properties to cartilage, in a relatively simple and inexpensive process,” Wiley says.

The hydrogel-based material is the first to match human cartilage in strength and elasticity while also remaining 3D-printable and stable inside the body. To demonstrate, researchers used a $300 3D printer to create custom menisci for a plastic model of a knee.

“A meniscus is not a homogenous material,” Yang says. “The middle is stiffer, And the outside is a bit softer. Multi-material 3D printers let you print different materials in different layers, but with a traditional mold you can only use one material.”

Meniscus implants could also benefit from 3D printing’s ability to create customized and complex shapes, the researchers say.

In a simple demonstration, Yang took a computed tomography (CT) scan of a plastic model of a knee and used the information from the scan to 3D print new menisci using his double network hydrogel. The process, from scan to finished meniscus, took only about a day.

“This is really a young field, just starting out,” Wiley says. “I hope that demonstrating the ease with which this can be done will help get a lot of other people interested in making more realistic printable hydrogels with mechanical properties that are even closer to human tissue.”

Duke University