Shelf-Life Optimisation of Plasma Polymerised (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPOpp) Coatings; A New Possible Approach to Tackle Infections in Chronic Wounds.

Chronic wounds fail to heal and are accompanied by an ongoing infection. They cause suffering, shorten lifespans, and their prevalence is increasing. Unfortunately, the medical treatment of chronic wounds has remained unchanged for decades. A novel approach to break the biological vicious cycle is the long-lived radical (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO). TEMPO can be plasma polymerised (TEMPOpp) into thin coatings that have antimicrobial properties. However, due to its radical nature, quenching causes it to lose effectiveness over time. Our aim in this study was to extend the shelf-life of TEMPOpp coatings using various storage conditions: Namely, room temperature (RT), room temperature & vacuum sealed (RTV), freezer temperature & vacuum sealed (FTV). We have analysed the coatings' quality via the surface analytical methods of X-Ray Photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR); finding marked differences among the three storage conditions. Furthermore, we have compared the antimicrobial efficacy of the stored coatings against two major bacterial pathogens, Staphylococcus aureus and Staphylococcus epidermidis, commonly found in chronic wounds. We did so both qualitatively via live/dead staining, as well as quantitatively via (2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino) carbonyl]-2H-tetrazolium (XTT) viability assay for up to 15 weeks in 5 weeks increments. Taken all together, we demonstrate that samples stored under FTV conditions retain the highest antimicrobial activity after 15 weeks and that this finding correlates with the retained concentration of nitroxides.

To be a radical or not to be one? The fate of the stable nitroxide radical TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] undergoing plasma polymerization into thin-film coatings.

The stable nitroxide radical TEMPO [(2,2,6,6-Tetramethylpiperidin-1-yl)oxyl] has a multitude of applications in fields ranging from energy storage to biomedical applications and many more. However, to date, the processes of incorporating nitroxide radicals into thin-film coatings are laborious and not cost-effective, which hinders their wider use in many applications. In contrast, the authors have recently demonstrated the facile method of plasma polymerization of TEMPO into thin-film coatings that retain the stable nitroxide radicals. In this work, we are using three types of mass spectroscopic methods (plasma-mass spectrometry, time of flight secondary ion mass spectrometry, and high-performance liquid chromatography-mass spectrometry) and electron spin resonance to track the fate of the TEMPO molecule from monomer flask through the plasma and inside the resulting coatings. The results of this study demonstrate that TEMPO is a versatile monomer that can be used across different plasma reactors and reliably retain the stable nitroxide radical in the resulting thin-film coatings if certain process conditions are observed, namely, higher process pressures and lower powers.