Outbreaks of infections are disturbingly persistent in our headline news. The efficacy of antibiotics and biocides are being challenged at a time when we need breakthrough innovations to combat resistant pathogens ("superbugs"). We asked ourselves if a continuously killing surface that maintained its potency over many months without intervention be useful? Our investigations of potent antimicrobials without significant downsides quickly led us to copper. We sought a material that maintains the antimicrobial efficacy of copper metal while minimizing the downsides - cost, appearance, and metallic properties - that limit application. We made and tested oxide glasses containing copper(I) as per the literature since copper(I) ions are highly bioactive and known to kill pathogens under realistic "dry-test conditions". Our hope was that the positively charged water species present at the surface would ion-exchange with monovalent copper to provide a copper (I) extraction mechanism. However, we quickly realized that the small glass network sites that contain the copper (I) ions prevent one-for-one exchange with larger positively charged water species such as hydronium ions.
To overcome the antimicrobial inefficacy of copper (I) glasses, we considered a new approach. The concept was to design a glass that phase separates into a highly durable matrix phase and a lower durability second phase that contains the monovalent copper. By reducing the alumina content of the base glass composition and introducing phosphorous, boron, and potassium we were able to achieve the desired phase separating glass. Within the discontinuous, lower durability phase, crystalline cuprite precipitates as shown in Fig. 1. Upon exposure to water, the phosphate-rich discontinuous phase dissolves to provide access to these antimicrobial cuprite crystals as shown in Fig. 2.
Fig.1 SEM image of the surface of the copper glass ceramic. The glassy phases consist of a high durability continuous phase and lower durability discontinuous phase. Within the discontinuous phase cuprite crystal formation takes place.
Fig 2. SEM image of the surface of the copper glass ceramic following exposure to water. The discontinuous phase dissolves and releases cuprite crystals into solution. Other cuprite cystals remain trapped in the cavities with surfaces exposed. The cavitation leads to tunneling into the subsurface with separated discontinuous phases becoming connected and providing additional access to cuprite crystals. Our results shed light on the fundamentals of ion-exchange in glasses in aqueous media and a fundamentally new mechanism to cause the tailored release of otherwise trapped ions in conventional glasses.
To demonstrate this new material as a sustainable delivery system for copper (I) ions, we chose water based paint coatings as first-case application due to their ubiquity and the challenges of maintaining copper(I) in aqueous formulations. The paint coatings containing copper-glass ceramic powder showed extraordinary antimicrobial efficacy with 99.9% reduction in colony counts for S. aureus, P. aeruginosa, K. aerogenes, and E. Coli. The copper-glass ceramic is advantaged over existing organic antimicrobial paint additives owing to a favorable toxicological profile, copper's broad-spectrum efficacy, and low probability for development of resistant strains. Unlike air and moisture sensitive cuprous compounds, the copper-glass ceramic is environmentally stable (resistant to bulk dissolution and color change) and disperses easily in water. Our data shows this new material to be an effective and versatile antimicrobial additive and can be incorporated in a wide variety of coatings and plastics. We look forward to the development of products incorporating this material.
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