Biomaterials
“In our case, the silver is alloyed, more or less in the surface layer, so it shouldn’t be released. And the surface should keep this antibacterial property even when it’s implanted for a longer time in the body,” Bertrand says. This is also likely why their surface doesn’t interfere with osseointegration as much as other silver-modified implants, she adds.
Bioactive coatings
Metals are widely used for implants due to their mechanical properties, yet often they are inert. To get around this, you could add bioactive coatings to their surface to change the environment that the bone cells are sensing. This can help to drive osseointegration, says biomedical engineer and senior lecturer at the University of Technology, Sydney, Jiao Jiao Li.
For instance, you can add certain trace elements already present within our bones, like zinc and magnesium, onto the surface. “Those have the inherent ability to cause bone formation,” says Li. One method that holds promise is micro arc oxidation (MAO), where rough and porous oxide films are formed on the implant surface. MAO has been found to increase the rate of osteoblast formation while these coatings can be enhanced when other elements are added. For instance, incorporating hydroxyapatite (HA), a mineral present in our bones, can promote bone growth by controlling macrophage activity – cells that control inflammation when an injury heals. In the early stages of healing, macrophages release signals that can kickstart the bone formation process. Though if they stay activated they’ll cause chronic inflammation, which can interfere with bone growth. This HA-modified coating switches the macrophages on after the implant is put in but deactivates them once a good amount of bone has formed by responding to an enzyme released by the bone cells. You can also add coatings that directly trigger molecular pathways that bone cells use, Li explains. Here, there’s a broad category called layer-by-layer self-assembly, where layers of oppositely charged materials adhere to each other on top of a charged substrate. “You have thin layers of biological coating and they may be different materials,” she says. “It’s combinations of different materials to convey different effects.”
Collagen is of particular interest here as it’s naturally found in bone, and so can mimic the natural interface of the tissue. This can encourage osteoblasts to adhere to the implant surface and improve osseointegration. However, it can be tricky to ensure that coatings like these will have a reliable effect. “Biological
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materials are variable – even one batch of collagen is going to differ from the next,” says Li. “There’s a preservation period as well, because biological materials can’t just sit on the shelf forever.”
Building better implants
In the US alone, demand for hip replacement revision is estimated to rise by 137% from 2005- 2030; for knee replacements, that figure is 601%. By 2060 in the UK, demand for hip and knee implants is set to grow by almost 40% since 2022. Creating implants that have a better chance of survival could make a difference for thousands while alleviating the burden on health systems. But ideally, one day we won’t need to use artificial implants at all, says Li. “Hopefully, we’ll at least be at the point in say five years where we’re able to generate human pieces of bone that are derived from your own stem cells.” Ameer envisions a future of “smart regenerative systems”: creating scaffolds that would allow tissues and cells to regrow, and that could integrate with telemedicine to relay information about the microenvironment of that implant. It’s something he’s currently working on with his team at Northwestern University. Yet while we wait for those developments to materialise, we need to keep doing what we’re doing, says Li. “We still need to work on our implants and our coatings.” ●
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US demand for knee and hip replacement revision is predicted to rise by 601% and 137% respectively.
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