A team of scientists and medical professionals is collaborating on research into new measures to combat the problem of corrosion, writes Dick Ahlstrom.
Oil rigs and medical implants have much in common, at least for one researcher at NUI Galway's National Centre for Biomedical Engineering Science. The link between the two is corrosion and how to stop it.
Dr Liam Carroll, lecturer in chemistry and a research director at the centre, used to study ways of slowing the decay of offshore oil rigs, caused by chloride in sea water eating into the metal. Now he is trying to find ways to stop or at least slow the degradation of medical implants in the almost-as-salty environment of the human body.
The centre received a €32million award from the Programme for Research in Third-Level institutions, administered by the Higher Education Authority. Dr Carroll heads the biomaterials group, which includes a team of 25 researchers. They are looking at all types of implants, he says. "That covers everything from polymers to metals that go inside the body. One of the areas we are working on is looking at stainless steel and a new metal, nitinol."
Nitinol, a 50/50 blend of nickel and titanium, is a valuable new material for use in implants. It is already used widely in spectacle frames because of its "metal memory", the ability to take twisting and distortion but return to its original shape, explains Dr Carroll.
This makes it ideal for use in stents, implants used in a range of medical procedures to support damaged blood vessels and to open up clogged arteries.
"We are looking at stents made from stainless steel and nitinol and used in the body. All of these things are subject to attack from chloride ions," says Dr Carroll.
Medical implants must undergo rigorous testing to ensure they are safe to use, he says. This includes ongoing tests for corrosion damage and an assessment of any possible consequences for the patient.
It isn't all about protecting the implant from damage, he says. Nickel is a carcinogen and 25 per cent of the population has some level of allergic reaction to the metal. While the implant has to be protected from gradual corrosion, the patient also has to be protected from the unintentional release of nickel, particularly when using the nitinol alloy, which is half nickel.
"The test methods are exactly the same," whether working on oil platforms or implanted joints, says Dr Carroll. The goal is to isolate the metal, finding ways of keeping chloride ions away from the surface.
On an oil rig or ship's hull you bolt on "sacrificial anodes", metals such as copper or zinc that protect the steel but gradually dissolve away in the process. He was experimenting with other metals, including aluminium, to find alloys that could serve as a sacrificial anode but corrode more slowly than copper or zinc.
With implants it involves getting a very thin oxide layer onto the metal surface, says Dr Carroll. Surgical stainless steel has an oxide layer only three to five nanometres thick. "As long as that layer is intact everything is all right," he says.
He and his team are looking at ways of chemically enhancing the oxide layer in what is known as a "passivation treatment". While the approach can make the oxide layer more resistant to corrosion, the treatment also "tends to thin the oxide", says Carroll.
The Galway team researches novel ways to blocking corrosion but also characterises new materials being developed by companies that manufacture implants.
The work has become more complicated in recent years given the increasing number of "drug-eluting" stents coming onto the market. These implants carry a coating that gradually releases a drug into the tissues adjacent to the implant.