BY ANTHONY KING
A group in Manchester has developed an antimicrobial coating by including silver ions inside layers of graphene oxide. The new coating boasts a slow and long-release of silver ions, providing a sustained antimicrobial barrier for at least ten days.
Nanoparticles of silver are often used as a source of silver ions and used in wound dressings and medical implants to reduce infection risk. ‘This gives good performance of antimicrobial activity in the first few days, but this declines rapidly,’ says Rahul Nair, a material scientist at the University of Manchester, UK, who led the research.
His group stacked layers of graphene oxide, each one atom thick, to create a 3-D laminate structure a few micrometres thick. Between the layers are nanometre-sized gaps filled with water, which act as tiny reservoirs. Silver ions, which carry a positive charge, get trapped in these water spaces because the graphene oxide layers themselves carry a negative charge.
‘When we stack the individual graphene oxide layers together, we can control the coating thickness from tens of nanometres to hundreds of micrometres,’ says Nair. The amount of silver ions held in the layered structure depends on its thickness and surface area. These silver ions slowly leak out, driven by differences in the concentration of silver within and outside the coating. The Manchester team collaborated with Smith & Nephew, a medical equipment firm headquartered in Watford, UK.
The team in Manchester used pure foetal bovine serum, which is packed with protein macromolecules, as a test medium for its studies. The graphene coating sustainably released silver ions at a minimum dose of 10μm/cm/day, ‘providing an effective and sustained antimicrobial barrier for over ten days,’ the team report.
‘Nanoparticles of silver obtained considerable fame and are used in wound clinics worldwide,’ notes Ray Turner, a microbial biochemist at the University of Calgary, Canada. ‘There are other formulations with various graphene and carbon nanoparticles, so the novelty here is how this material was put together that allowed for the sustained activity.’
Silver is not without its problems, as textiles impregnated with silver have resulted in release of the metal into wastewater and the environment. A further issue is that resistance to silver develops quite quickly, says Turner. We cannot repeat the same mistakes we did with antibiotics and need better stewardship and be wary of fancy marketing, he warns.
Nair says that the challenge for their coating in wound healing is to compete in a crowded antimicrobial marketplace. For now, silver is not allowed by the Food and Drug Administration to be used on implants, however. ‘People who have orthopaedic implants all face infection. Controlling that through a better coating could save lives,’ says Nair.
To coat an orthopaedic implant will requires the Manchester group to do more R&D and team up with a lab to do safety tests in animals.
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