Scientists have developed an artificial cartilage from Kevlar – a material used for making bulletproof vests – that can withstand forces within the body without getting damaged. Cartilage in the human body, which is about 80 per cent water, can withstand some of the toughest forces in our bodies.
“We know that we consist mostly of water – all life does – and yet our bodies have a lot of structural stability,” said Nicholas Kotov, professor at University of Michigan in the US, who led the study. “Understanding cartilage is understanding how life forms can combine properties that are sometimes unthinkable together,” said Kotov.
Many people with joint injuries would benefit from a good replacement for cartilage, such as the 850,000 patients in the US who undergo surgeries removing or replacing cartilage in the knee. While other varieties of synthetic cartilage are already undergoing clinical trials, these materials fall into two camps that choose between cartilage attributes, unable to achieve that unlikely combination of strength and water content.
The other synthetic materials that mimic the physical properties of cartilage do not contain enough water to
transport the nutrients that cells need to thrive, Kotov said. Meanwhile, hydrogels – which incorporate water into a network of long, flexible molecules – can be designed with enough water to support the growth of the chondrocytes cells that build up natural cartilage.
Yet those hydrogels are not especially strong. They tear under strains a fraction of what cartilage can handle. The new Kevlar-based hydrogel mimics real cartilage by combining a network of tough nanofibres from Kevlar – the “aramid” fibres best known for making bulletproof vests – with a material commonly used in hydrogel cartilage replacements, called polyvinyl alcohol (PVA).
In natural cartilage, the network of proteins and other biomolecules gets its strength by resisting the flow of water among its chambers. The pressure from the water reconfigures the network, enabling it to deform without breaking. Water is released in the process, and the network recovers by absorbing water later.
This mechanism enables high impact joints, such as knees, to stand up to punishing forces. Running repeatedly
pounds the cartilage between the bones, forcing water out and making the cartilage more pliable as a result. Then, when the runner rests, the cartilage absorbs water so that it provides strong resistance to compression again.
The synthetic cartilage boasts the same mechanism, releasing water under stress and later recovering by absorbing
water like a sponge. The aramid nanofibers build the framework of the material, while the PVA traps water inside the network when the material is exposed to stretching or compression.
Even versions of the material that were 92 per cent water were comparable in strength to cartilage, with the 70-per cent version achieving the resilience of rubber. As the aramid nanofibers and PVA do not harm adjacent cells, Kotov anticipated that the synthetic cartilage may be a suitable implant for some situations, such as the deeper parts of the knee.
The potential applications of the material are not limited to cartilage. Similar networks, with different proportions of aramid nanofibers, PVA and water, may be able to stand in for other soft tissues, researchers said.