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The first gel-based cartilage substitute that is stronger and more durable than the real thing could transform knee operations, a Duke University team has said.
Knee pain generally comes from the progressive wear and tear of cartilage known as osteoarthritis, which affects nearly one in six adults – 867 million people – worldwide.
Mechanical testing reveals that the newly developed hydrogel – a material made of water-absorbing polymers – can be pressed and pulled with more force than natural cartilage, and is three times more resistant to wear and tear.
Implants made of the material are currently being developed by Sparta Biomedical and tested in sheep before clinical trials in humans next year.
The researchers took thin sheets of cellulose fibres and infused them with a polymer called polyvinyl alcohol – a viscous goo consisting of stringy chains of repeating molecules—to form a gel.
Image credit: Benjamin Wiley, Duke University
The cellulose fibres act like the collagen fibres in natural cartilage and they give the gel strength when stretched. The polyvinyl alcohol helps it return to its original shape. The result is a jelly-like material, 60 per cent water, which is supple yet surprisingly strong.
The hydrogel is about 26 per cent stronger than natural cartilage in tension, something like suspending seven grand pianos from a key ring, and 66 per cent stronger in compression – which would be like parking a car on a postage stamp.
“It’s really off the charts in terms of hydrogel strength,” said Duke chemistry professor Benjamin Wiley, who led the research along with mechanical engineering and materials science professor Ken Gall.
The team had already created a gel in 2020 that was strong enough for knees. But putting the gel to practical use as a cartilage replacement presented additional design challenges.
One was achieving the upper limits of cartilage’s strength. Activities like hopping, lunging, or climbing stairs put some 10 megapascals of pressure on the cartilage in the knee. But the tissue can take up to four times that before it breaks.
“We knew there was room for improvement,” Wiley said.
In the past, researchers attempting to create stronger hydrogels used a freeze-thaw process to produce crystals within the gel, which drive out water and help hold the polymer chains together. In the new study, instead of freezing and thawing the hydrogel, the researchers used a heat treatment called annealing to coax even more crystals to form within the polymer network.
By increasing the crystal content, the researchers were able to produce a gel that can withstand five times as much stress from pulling and nearly twice as much squeezing relative to freeze-thaw methods.
The improved strength of the annealed gel also helped solve a second design challenge: securing it to the joint and getting it to stay put.
Cartilage forms a thin layer that covers the ends of bones so they don’t grind against one another. Previous studies haven’t been able to attach hydrogels directly to bone or cartilage with sufficient strength to keep them from breaking loose or sliding off. So the Duke team came up with a different approach.
Their method of attachment involves cementing and clamping the hydrogel to a titanium base. This is then pressed and anchored into a hole where the damaged cartilage used to be. Tests show the design stays fastened 68 per cent more firmly than natural cartilage on bone.
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