Far more than a simple hinge, the human knee is a complex, intricate mechanism, and a knee injury is a painful and debilitating of condition that's difficult and expensive to repair. Duke University is developing a cartilage-like material based on hydrogel that may make the task of repairing knees easier. The 3D-printable hydrogel allows bioengineers to create bespoke artificial replacement parts for injured knees that are tailored to match the old part both in shape and mechanical properties.

3D printing has been a boon to surgeons. By using virtual models of a patient's body parts from computer tomography or magnetic resonance imaging scans, surgeons can provide new hips, cranial sections, and spinal vertebrae that are close matches to the original. They're even used to produce detailed models of the human heart for cardiac surgeons to plan complicated operations or to fashion mechanical implants that fit exactly.

Such a technology should be a godsend for repairing damaged knees, but knee anatomy is very tricky. One of the key components are the menisci, and it is this component that the researchers used to demonstrate the potential of the new material. Menisci are ear-shaped pieces of cartilage that sit inside the knee between the femur and tibia. They act as shock absorbers and lubricating bearings that allow the joint to bend and move smoothly in a surprisingly subtle and dynamic manner, and they can withstand decades of use that would destroy a man-made device.

However, hard use or even an awkward stumble can rip the menisci, resulting in a painful chronic injury and the increased risk of arthritis because, in adults, the menisci cannot heal quickly or completely. This means that injured knees often require surgery, removal of the damaged meniscus, or replacing it with plastic implants. But these implants are a poor match for the original in strength or elasticity. They are also often non-biocompatible, so the surrounding tissues cannot heal properly.

3D printing should be an improvement, but the meniscus must fit exactly or it could slip out or cause great pain. In addition, the material itself must be tailored for the job.

"A meniscus is not a homogenous material," says graduate student Feichen Yang. "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."

So far, that looks like an endorsement for 3D printing, but what to print the meniscus out of? A favored candidate is hydrogel, which is stable inside the body, biocompatible, and has a similar molecular structure to cartilage with long-chain molecules holding in water. The catch is finding a hydrogel that has the same properties as cartilage and can be printed.

The current hydrogels lack the strength and ooze away when put through a 3D printer due to their high water content, so the Duke researchers are looking at a new hydrogel-based material that they claim is the first to match human cartilage in strength and elasticity, yet is 3D-printable. It was created by Yang, who combined a stiff strong hydrogel with one that is soft and stretchy. When mixed, the polymer chains wove together to create a new hydrogel that is strong and elastic. More importantly, by altering the proportions, these properties could be controlled across different parts of the artificial meniscus.

Yang then added a nanoparticle clay to the hydrogel to make it printable. When subjected to shear stress, the clay particles collapse into a smooth-flowing liquid, but once in place after oozing through a printing needle, they set hard to hold up the hydrogel structure.

Using a US$300 printer, Yang was able to print a replacement meniscus with the new hydrogel in only a day – showing that what was once a daunting manufacturing process could soon be simple and inexpensive.

"This is really a young field, just starting out," says Benjamin Wiley, an associate professor of chemistry. "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."

The research was published in ACS Biomaterials Science and Engineering.