The thin layer of cartilage that resides between the bones in the knee is an impressive thing. It's strong enough to withstand a person's weight, but soft enough to cushion the joint against impact and decades of repeated use.
Such a combination has been difficult to reproduce in a lab — until now. A group of Duke University researchers took on the task and, in doing so, created an experimental hydrogel that's the first to match knee cartilage's strength and suppleness.
A hydrogel that can withstand heavy loads
Despite the fact that the material created by the researchers is 60 percent water, a single quarter-sized disc can bear the weight of a 100-pound kettlebell without tearing or losing its shape, the researchers say in their paper published in Advanced Functional Materials.
The creators say it is the first hydrogel — a material made of water-absorbing polymers — that is durable and able to withstand heavy loads as well as human cartilage.
The researchers, led by Duke chemistry and materials scientists Ben Wiley and Ken Gall, hope that their work will one day allow for people with knee troubles to get a replacement for damaged cartilage. Such a development would provide an alternative to the 600,000 knee replacement surgeries that are performed in the U.S. alone every year.
While hydrogels have been used for years in cartilage replacements, they have never been durable enough for the load-bearing knee joint.
The Duke team made it their mission to change that: “We set out to make the first hydrogel that has the mechanical properties of cartilage,” Wiley, a chemistry professor at Duke, said in a press release.
A squeeze and squish-resistant material
The new hydrogel is made using two intertwined polymer networks. One is made of stretchy strands while the other is more interwoven and rigid. Both have negative charges along their length. The two polymer networks are then reinforced using a meshwork of cellulose fibers.
The cellulose fibers resist pulling when the gel is stretched and help hold the material together. The negative charges, meanwhile, repel each other and stick to water, helping the hydrogel to retain its original shape when squeezed.
“Only this combination of all three components is both flexible and stiff and therefore strong,” said co-author Feichen Yang, who earned a chemistry Ph.D. in Wiley’s lab.
After comparing their material to other hydrogels — one test saw the researchers subject it to 100,000 cycles of repeat pulling— the team decided that theirs was only one that was comparable to the strength of cartilage under both squishing and stretching.
Taking the material from the lab to the clinic will take another three years at least, Wiley said. While initial tests suggest the material is non-toxic to lab-grown cells, the next step is to design an implant that the team can test in sheep.
Ultimately, the team says their research could constitute an immense breakthrough in health and medicine as it could lead to people with knee pain recovering in a fraction of the time at the same time as doing away with the limited lifetime associated with cartilage repair and knee replacement surgery.