Cheaper, tougher, less toxic: new alloys show promise in developing artificial limbs
Titanium is a strong, resilient and relatively light metal. Its properties have also been well studied; scientists know a great deal about it. All of this makes it the ideal base for fashioning artificial limbs—particularly knees and hips—and teeth. It is less likely than other metals to rust and, as research has shown, it is more compatible with the human body than, for instance, stainless steels and cobalt based materials.
But there's a major problem: titanium is not cheap. Precise data is hard to come by, but a conservative average cost of titanium-based prostheses is between US$3,000 and US$10,000. That's expensive for most people, and prohibitively so for the majority of people in middle- and low-income countries like those in Africa.
Again, data is scarce, but a recent study about sub-Saharan Africa (excluding South Africa, which has better facilities for such procedures than most other countries on the continent) found that 606 hip and 763 knee replacements were performed between 2009 and 2018. Many more people in the region likely need replacements but will go without because they simply can't afford the procedure. And, with the global population of those aged 65 and older rising, the demand for implants is set to increase; this age group is prone to diseases like osteoporosis and osteoarthritis.
That's why we are working to produce cheaper titanium based materials that can be used to make affordable limbs. In our latest research my colleagues and I experimented with metallic elements like titanium, aluminium, iron and vanadium to create new alloys. We tested each in a solution that mimics humans' bodily fluids.
We found that the new alloys showed negligible rust in the solution. The new alloys, which are slightly cheaper than the commercial grade alloy, performed as well as it does—and one alloy even outperformed it.
Pure titanium vs titanium alloys
The biggest benefit of titanium for making artificial hips, knees and teeth is that it's safe for use in the human body because it doesn't degrade easily when exposed to body fluids.
However, when titanium is used in its pure form, it lacks the necessary strength and wear resistance required to cope with the rigours of human activity.
That's why other metallic elements are added. Examples include aluminium, vanadium, zirconium, tantalum, niobium, molybdenum and iron. Scientists use these and other elements to create new alloys that are stronger and resistant to wear.
Currently the most utilised alloy in artificial hips and knees is Ti-6Al-4V: 90% titanium, 6% aluminium and 4% vanadium. Though it is effective, it has two major drawbacks. The first is the cost. Vanadium is nearly as expensive as titanium. The second is toxicity: aluminium and vanadium are toxic in large quantities. When the material degrades through corrosion, ions are released into the body and can cause chronic inflammation. These ions have also been linked to Alzheimer's disease.
For this study we reduced the amount of aluminium and vanadium that are added to Ti-6Al-4V to make new titanium based materials. We also excluded aluminium and replaced vanadium fully with iron to make another, cheaper, titanium based material.
Then we investigated whether these new implant materials would degrade quickly when immersed in the human body fluid. We used a solution called Hanks Balanced Salt Solution which contains the main ingredients in the human body fluid. We compared the new titanium materials with the commercial grade Ti-6Al-4V that is commonly used.
Almost all the new alloys performed better than Ti-6Al-4V in the salt solution. Those that fared worse in the solution were still on a par with Ti-6Al-4V. And none of the new alloys degraded more than 0.13 millimetres per year, the maximum permissible degradation rate allowed for implant material.
The alloys without vanadium and aluminium performed well, meaning they are potentially safer than Ti-6Al-4V because they have lower toxicity levels.
And, crucially, the new alloys are cheaper to produce than Ti-6Al-4V. We are not working on the actual manufacturing of artificial limbs—this research focuses on the chemical composition of the alloys. So we can't say what the ultimate cost-saving would be if these alloys were to be used. But, merely by altering the starting materials as we did, replacing aluminium and vanadium fully or partially with iron, up to 10% cost savings can be achieved.
A promising step
From 2030 and beyond, more older adults will reside in developing countries such as those across the African continent. As this population increases, the demand for artificial limbs may also rise. That's why identifying affordable, safe materials is so important. Our research is a promising step towards meeting that goal.
More information: Mogomotsi Leshetla et al, Corrosion resistance of iron‐containing experimental titanium alloys exposed to simulated body fluids, Materials and Corrosion (2022). DOI: 10.1002/maco.202213076
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