Journal focus: Corrosion Engineering, Science and Technology
Biodegradable metals, while having the potential to revolutionise the orthopaedic implant industry, must overcome several hurdles in the years to come. Nick Birbilis and Nicholas Kirkland explain more.
The days of sitting on a waiting list for removal of a metal pin could soon become a thing of the past. Metal implants currently used in orthopaedic surgery – such as knee caps to treat sports injuries, or pins to heal fractured limbs – are making way for biodegradable implants, which have the potential to solve a myriad of problems currently associated with traditional orthopaedic surgical techniques.
As such, interest in biodegradable implants has rapidly increased over the past five years. Nominally based on magnesium (Mg) and its alloys, their many potential benefits include:
- similar mechanical properties to bone, avoiding adverse stress-shielding and mechanical mismatch commonly found with harder permanent implant metals
- potential for complete biocompatibility, as Mg can be effectively processed via the kidneys and excreted in urine
- the biodegradation and bioresorbability that arises from the corrosion of Mg in aqueous saline environments, which avoids the need for a second operation to remove any implant – degradation forms part of healing process
- potential for functionalisation, either by bulkallying or surface treatment to facilitate bone growth, or by controlled dissolution
To date, initial interest has primarily stemmed from research institutions. However, the topic is nascent and while initial industrial interest, such as that from Magnesium Elektron, focused on bioresorbable stents, growth in R&D is expected to increase.
Several technical and practical challenges are in need of further research, and these remain hot topics in the field of bioresorbable magnesium. These include the role of alloying elements on the rate and morphology of bioresorbability, as well as the potential hazards of using alloying elements that may possess some adverse toxicity, or cumulatively build up in vivo. One example is hydrogen gas evolved during Mg dissolution.
Biomimetic coatings must perform multiple functional roles, making their design a demanding challenge. The design must incorporate:
- slow initial rate of biocorrosion, such that the implant can be accepted by the body
- function as a corrosion barrier for only a finite period, since the implant must, overall, be bioresorbable
- sustained biocompatibility
- the potential to stimulate bone growth
What must also be defined is the relationship between in vitro and in vivo biocorrosion. To date, correlations between the two are weak due to the rapid and dynamic corrosion of Mg alloys, which alters the local microenvironment surrounding the implant. Stagnant versus buffered (and flowing) experiments show significant differences, and mimicking the flow of body fluid and buffering capacity of the body is indeed a challenge for such a dynamic condition. Corrosion testing is nominally a short-term exercise, and scaling this to long-term predictions requires care. Additionally, greater collaboration between scientists and clinical physicians would see an immediate step-change in the alignment of research and its practical outlets.
Informing the industry
A number of international workshops and conferences deal with medical-orientated degradable metal development. An increasing number of large biomedical and corrosion conferences are now including biodegradable metal symposia as part of their programs, such as the recent 9th World Biomaterials Conference, held in Chengdu this year, and the annual North American Corrosion meeting (NACE). The NACE 2012 meeting included a one-day symposium, Corrosion of Biomaterials, which covered permanent and degradable implants, and the upcoming NACE 2013 meeting in Orlando will host the symposium Biocorrosion of Metals. Similarly themed symposia have also been included in past TMS (The Minerals, Metals & Materials Society) conferences.
More recently, smaller symposia have been formed, such as the annual Symposium on Biodegradable Metals (SBM) in Italy in 2012, where all participants work in the specific area. Typically hosting between 70–100 participants, this meeting is crucial in encouraging leading researchers in the field to collaborate in a more structured manner and in setting out research goals for the community as a whole. Future conferences will also look to work more closely with companies, spurring the commercialisation and further testing of current materials.
Dr Nick Birbilis is an Associate Professor in the Department of Materials Engineering at Monash University in Australia. His research involves the corrosion and protection of metals, with a particular interest in light metals.
Dr Nicholas Kirkland is an Assistant Professor with the Department of Advanced Technology and Science for Sustainable Development at Nagasaki University, Japan. He has worked in the bio-Mg field for the past five years. The authors contributed to a theme issue of the IOM3 journal, Corrosion Engineering, Science and Technology, which contained a selection of papers on Durability of Biomaterials and Biocorrosion, published in Vol.47 No.5 in August 2012.
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