Bioabsorbable magnesium alloys: pushing the boundaries of medical materials

medicalCompletely new materials in the field of implantable devices are a rare occurrence. This is because of the heavy regulatory burden placed on implantable medical devices to ensure that they are safe and effective for use in the specified application. There has to be a very good reason to invest in the cost of qualifying a new material over the ones that have a long standing history in the field. However, one group of materials that seem likely to cross that boundary and have been gaining increasing attention in recent years are bioabsorbable magnesium alloys. But what are these alloys and why are they gaining such attention?

Implants help the body repair itself

The natural materials that form the human body are excellent at achieving their function, as discussed in my previous blog article on bone. Not only that, but most of the natural tissues of the body are good at repairing themselves. Despite this, there are times when through injury, illness, or the natural aging process, a part of the body may need to be repaired or replaced.

Endeavors to repair or replace damaged parts of the body can be traced all the way back to ancient civilizations, such as the Egyptians, Mayans, and Chinese, with examples of limb replacement and even dental implants made from shells having been uncovered in archaeological studies. But the vast majority of medical advancements in this area have occurred in the last 60 years, and the use of synthetic materials to repair or replace body parts is now relatively common.

There are a number of implantable devices that are used to achieve a temporary function, then are either removed in a second procedure or have to remain in situ as the surgery to remove them is too difficult. Examples include cardiovascular stents, which help support a newly reopened blood vessel until healing has occurred but are left implanted permanently, and metallic bone fracture fixation plates and rods that are removed in a second operation once their function is complete. Both options carry risks. Stents left in the arteries can contribute to restenosis (recurrence of artery narrowing) over a period of time, and there are increased risks and costs associated with additional surgical procedures.

Viewing a record for a stent in Granta's Medical Materials Database. This comprehensive online resource supports medical device design, providing traceable materials properties, biocompatibility data, drug compatibilities, and surface treatment processes for materials, coatings, and drugs used in implantable devices.

Viewing a record for a stent in Granta’s Medical Materials Database. This comprehensive online resource supports medical device design, providing traceable materials properties, biocompatibility data, drug compatibilities, and surface treatment processes for materials, coatings, and drugs used in implantable devices.

Bioabsorbable materials ‘disappear’ when no longer needed

A major focus of modern implant research is in the development of materials that support the natural tissue healing initially, but then disappear once the tissue has repaired itself. The first materials used to achieve such a function were polymers, including poly-lactic acid and poly-glycolic acid. Many of us are familiar with bioabsorbable stitches that naturally disappear over time, removing the need to return to the doctor to have them removed. Another common application of these materials is for bone fracture fixation screws and rods. As these materials are relatively low strength materials, the use of these materials is typically limited to lower load bearing applications. It is for this reason that researchers are investigating the application of bioabsorbable metals, and in particular bioabsorbable magnesium alloys.

Magnesium has a high specific strength and a high affinity for oxidation. It is also an essential mineral in the body, and studies so far have not indicated any toxic effects from the degradation process. For magnesium alloys to be used successfully, in addition to having the appropriate mechanical requirements to achieve the initial function, they must corrode at an appropriate rate for mechanical strength to be retained long enough for healing to occur. They and their degradation products must also show excellent biocompatibility. A particular challenge that magnesium alloys present is that, if degradation of the alloy is too rapid, hydrogen gas bubbles can form around the implant. This can delay tissue healing and cause a localized pH increase that can severely affect the local physiological processes. The mechanical and degradation properties of the materials can be controlled with the choice of alloying elements. Magnesium alloys incorporating calcium, zinc, yttrium, zirconium, aluminum and other rare earth alloying elements have all been investigated in order to achieve the optimal balance of mechanical properties, degradation rate, and biocompatibility.

Putting these materials on trial

The application area where research is most advanced is in the use of the material for coronary stents, with the Biotronik DREAMS stent being one such device reaching the clinical trial stages. Traditional stents are typically manufactured from stainless steel, cobalt chromium alloys or nitinol; all non-absorbable metals. For the stent to serve its temporary role of opening the vessel and providing support, it is only required for a limited time during healing. If the stent were to disappear naturally with time, it would avoid the restenosis effect mentioned above and could allow the artery to return to its normal function without some of the adverse effects of the permanent presence of the device.

In addition to use in stents, magnesium alloys are also being researched in the orthopaedic industry for use in bone repair such as fracture fixation. The use of these alloys here could eliminate the need for surgery to remove devices used once healing has occurred. The Young’s Modulus of magnesium and its alloys is closer to bone than the stainless steels and titanium alloys presently used to achieve this function, an important consideration as materials with a higher Young’s Modulus can induce stress shielding causing the bone to weaken. Studies are also indicating that magnesium actively encourages new bone growth, which would clearly be an additional advantage for the use of these materials.

Approval for a new implantable material?

There are some significant hurdles that must be overcome for these materials to become widely used in the medical industry, both in the development of the materials and in proving them to be safe and effective for use in implantable applications. However, with bioabsorbable magnesium alloy stents being actively deployed in clinical trials, it may not be long before at least one of these materials joins the short list of those accepted for use as implantable materials.

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