Orthopaedic implants that bear loads while bones heal, then disappear once they’re no longer needed, could become a reality thanks to a new technique for enhancing the mechanical properties of zinc alloys. Developed by researchers at Monash University in Australia, the technique involves controlling the orientation and size of microscopic grains in these strong yet biodegradable materials.
Implants such as plates and screws provide temporary support for fractured bones until they knit together again. Today, these implants are mainly made from sturdy materials such as stainless steel or titanium that remain in the body permanently. Such materials can, however, cause discomfort and bone loss, and subsequent injuries to the same area risk additional damage if the permanent implants warp or twist.
To address these problems, scientists have developed biodegradable alternatives that dissolve once the bone has healed. These alternatives include screws made from magnesium-based materials such as MgYREZr (trade name MAGNEZIX), MgYZnMn (NOVAMag) and MgCaZn (RESOMET). However, these materials have compressive yield strengths of just 50 to 260 MPa, which is too low to support bones that need to bear a patient’s weight. They also produce hydrogen gas as they degrade, possibly affecting how biological tissues regenerate.
Zinc alloys do not suffer from the hydrogen gas problem. They are biocompatible, dissolving slowly and safely in the body. There is even evidence that Zn2+ ions can help the body heal by stimulating bone formation. But again, their mechanical strength is low: at less than 30 MPa, they are even worse than magnesium in this respect.
Making zinc alloys strong enough for load-bearing orthopaedic implants is not easy. Mechanical strategies such as hot-extruding binary alloys have not helped much. And methods that focus on reducing the materials’ grain size (to hamper effects like dislocation slip) have run up against a discouraging problem: at body temperature (37 °C), ultrafine-grained Zn alloys become mechanically weaker as their so-called “creep resistance” decreases.
Grain size goes bigger
In the new work, a team led by materials scientist and engineer Jian-Feng Nei tried a different approach. By increasing grain size in Zn alloys rather than decreasing it, the Monash team was able to balance the alloys’ strength and creep resistance – something they say could offer a route to stronger zinc alloys for biodegradable implants.
In compression tests of extruded Zn–0.2 wt% Mg alloy samples with grain sizes of 11 μm, 29 μm and 47 μm, the team measured stress-strain curves that show a markedly higher yield strength for coarse-grained samples than for fine-grained ones. What is more, the compressive yield strengths of these coarser-grained zinc alloys are notably higher than those of MAGNEZIX, NOVAMag and RESOMET biodegradable magnesium alloys. At the upper end, they even rival those of high-strength medical-grade stainless steels.
The researchers attribute this increased compressive yield to a phenomenon called the inverse Hall–Petch effect. This effect comes about because larger grains favour metallurgical effects such as intra-granular pyramidal slip as well as a variation of a well-known metal phenomenon called twinning, in which a specific kind of defect forms when part of the material’s crystal structure flips its orientation. Larger grains also make the alloys more flexible, allowing them to better adapt to surrounding biological tissues. This is the opposite of what happens with smaller grains, which facilitate inter-granular grain boundary sliding and make alloys more rigid.
The new work, which is detailed in Nature, could aid the development of advanced biodegradable implants for orthopaedics, cardiovascular applications and other devices, says Nei. “With improved biocompatibility, these implants could be safer and do away with the need for removal surgeries, lowering patient risk and healthcare costs,” he tells Physics World. “What is more, new alloys and processing techniques could allow for more personalized treatments by tailoring materials to specific medical needs, ultimately improving patient outcomes.”
The Monash team now aims to improve the composition of the alloys and achieve more control over how they degrade. “Further studies on animals and then clinical trials will test their strength, safety and compatibility with the body,” says Nei. “After that, regulatory approvals will ensure that the biodegradable metals meet medical standards for orthopaedic implants.”
The team is also setting up a start-up company with the goal of developing and commercializing the materials, he adds.
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