Biomaterials
Traditional titanium screws and plates used in implants may need to be removed at a later date, resulting in explant surgery.
for sole support in all larger, high-load-bearing fractures, such as certain types of femoral fractures, where metallic implants often remain the preferred option. With PGA, because it degrades – and therefore, loses its mechanical strength – faster, it’s been used in rods and screws for fractures of cancellous bone, the less dense, spongy inner layer of bone. You can also combine materials to give an implant its desired properties, Oosterbeek explains. He gives an example: PLA can be used alongside hydroxyapatite. “The hydroxyapatite as a ceramic particle will have some mechanical role, it’ll make the material stiffer. But it’s also there as an osteo-conductive material to trigger that behaviour in the physiological environment. So, it has both a mechanical and a biological role.”
Repairing tissue
Resorbable polymers can be used to create scaffolds for tissue regeneration, too. Here, the implant acts as a sort of framework for cells to grow on. These scaffolds “are often porous, because you want some kind of diffusion of oxygen and biological molecules”, says Hatton. “And you want those cells to start forming the tissue and taking over the functions you want.” In dentistry, while the mouth’s sensitivity necessitates careful consideration of inflammation, PLA and PGA are nonetheless utilised in tissue scaffolds and other applications. The risk of inflammation is managed through optimised material design and appropriate clinical application, rather than by broadly avoiding these polymers. Work is under way to develop resorbable membranes, but the path to market is challenging.
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“The evidence is that it really would work. The problem is that to get a tissue engineering product to market is ten times the cost of just the membrane on its own,” Hatton explains. A limited number of scaffold products are currently available. There are also a handful of bone scaffolds and resorbable stents, which can support the blood vessel as it heals while encouraging regrowth of injured tissue. Because stents need to be quite strong, the material requirements are similar to orthopaedics, says Oosterbeek. Yet polymers like PLA don’t tend to be strong or ductile enough to use in cardiac stents, he adds. So, the struts of the stents tend to be bulkier, “which leads to an increased risk of clotting down the line”. It’s a challenge many are working on: other materials such as polycaprolactone (PCL) are currently being investigated.
While some have used PLA to create scaffolds,
it’s easier to create porous architecture using PCL, Oosterbeek explains: “For a pure scaffold application, it’s generally less about the mechanics and more about the biocompatibility. You would be looking at: do cells grow on this material? While PCL might not be what we call osteo-inductive – it might not induce cells to produce bone – it will be biocompatible. It will allow growth on it, and then people will try to incorporate other elements into a composite to induce that cellular behaviour, by using things like glasses.” It’s something Oosterbeek is currently exploring – creating implants that combine resorbable polymers with bioglass. There are bioglasses that bond very well to bone, and that can help induce growth of both bone and blood vessels, he explains. “All of this is very beneficial for an implant.” ●
www.medicaldevice-developments.com
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