Flexible, strong hydrogel could replace conventional knee implants
Duke University researchers have created an experimental polymer-based gel that can match the strength and durability of the thin, slippery layer of cartilage naturally present between the bones in the knee.
The layer of cartilage between the bones in the knee is a near-miracle of human evolution: strong enough to withstand a person’s weight, but soft and supple enough to cushion the joint against impact over decades of repeat use.
Unfortunately, many people damage the cartilage in their knees due to injuries, or the cartilage simply gets worn down by age or chronic conditions like osteoarthritis. This limits movement and causes both inflammation and pain. Traditionally, if the cartilage wears down too much, the only treatment has been a surgical knee replacement because that combination of soft-yet-strong cartilage has been difficult to reproduce in the laboratory.
But now, researchers at Duke University, In Durham, N.C., have come up with an experimental gel that they say can match the strength and durability of natural cartilage without wearing out over time.
The material is a hydrogel – materials made of water-absorbing polymers – that consists of two intertwined polymer networks: one made of stretchy spaghetti-like strands and the other more rigid and basketlike, with negative charges along their length. These are reinforced with a third ingredient, a meshwork of cellulose fibres. When the gel is stretched, the cellulose fibres resist pulling and help hold the material together. And when it’s squeezed, the negative charges along the rigid polymer chains repel each other and stick to water, helping it spring back to its original shape.
“Only this combination of all three components is both flexible and stiff and therefore strong,” said Feichen Yang, a member of the Duke research team led by scientists Ben Wiley and Ken Gall.
When the researchers compared the resulting material to other hydrogels, theirs was the only one that was as strong as cartilage under both squishing and stretching. In one experiment, the team subjected it to 100,000 cycles of repeat pulling, and the material held up just as well as porous titanium used for bone implants, “which exceeded our initial expectations,” the team said. They also rubbed the new material against natural cartilage a million times and found that its smooth, slippery self-lubricating surface is as wear-resistant as genuine cartilage and four times more wear-resistant than synthetic cartilage implants currently FDA-approved for use in the big toe.
The next step in the development process is to design an implant that the team can test in sheep, with the ultimate goal of offering new options for people with knee pain.