Nano-layered inorganic-organic hybrid materials for the controlled delivery of antimicrobials to protect against healthcare associated infections

Kinninmonth, Malcolm Allan (2012) Nano-layered inorganic-organic hybrid materials for the controlled delivery of antimicrobials to protect against healthcare associated infections. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

Healthcare associated infections (HCAI) are a significant problem facing modern healthcare settings; therefore it is important to develop strategies aimed at reducing the number of patients contracting HCAIs. This study investigates the use of essential oils (EO) as antimicrobial additives for polymer materials, including surface coatings (paints), silicone elastomer and a thermoplastic. Conferring antimicrobial properties to surfaces made from such materials, would help to reduce the number of bacteria, reduce the risk of cross infection and therefore limit the number of HCAIs. The antimicrobial efficacies of five EOs (manuka (MO), oregano (OO), rosewood (RO), lavender (LO) and geranium (GO)) were examined individually against three microorganisms associated with HCAI (MRSA, Acinetobacter baumannii, and Pseudomonas aeruginosa). Oregano oil provided the highest level of antimicrobial activity, inhibiting the growth of all three bacteria at 2.5 % (v/v). In an attempt to reduce the concentration of oil required for inhibition of growth, blends of the EO have been explored. A blend of oregano oil and rosewood oil exhibited the strongest synergistic antimicrobial effect at a ratio of 3:1, and was selected as the antimicrobial to be added to the polymer materials. EOs are volatile, direct addition to polymers may allow the EO to bloom to the surface and rapidly evaporate from the polymer, resulting in loss of antimicrobial activity. Furthermore thermoplastics are processed at high temperatures which can result in degradation of the EO. Encapsulation strategies for the EO were therefore investigated in order to promote both sustained release of the EO and protection against degradation / volatilisation during blending in to thermoplastic materials. Considering the latter in particular, inorganic adsorbents were selected and the adsorption characteristics of the EOs onto a range of natural and synthetic montmorillonite type layered silicates were investigated. Using gas chromatography and flow micro-calorimetry their suitability as controlled release reservoirs was investigated. Rockwood Additives Laponite® RD provided the highest levels of adsorption, achieving approximately 170 mg.g-1 for oregano oil and 140 mg.g-1 for rosewood oil. Organic modification (via cation exchange) of the layered silicates with alkyl ammonium surfactants did not give the anticipated increase in the level of EO adsorption from solution. However the strength of adsorption was in some cases increased. Laponite® RD with 50 % 2HT2M organic modification (modified via cation exchange using dihydrogenated tallow dimethyl ammonium chloride) was selected as the adsorption substrate for the EO based antimicrobial. Samples of linear low density polyethylene (LLDPE), silicone elastomer (SE) and a solvent based paint were formulated containing the oregano oil:rosewood oil blend pre-adsorbed onto Laponite® RD with 50 % 2HT2M organic modification. These materials were tested for antimicrobial efficacy against MRSA and A. baumannii, and the results were compared to control polymers where the EO blend had been added directly. P. aeruginosa was found to be unsuitable as a test organism for the polymer samples. All of the polymer samples containing the EO blend exhibited antimicrobial activity, with SE containing the pre-adsorbed EO blend performing the best achieving a 3 log reduction in MRSA cells. The SE and paint materials containing the preadsorbed EO blend outperformed the directly added EO controls; however the reverse was true for LLDPE. The release rates of the EO molecules from the polymers were investigated using headspace gas chromatography, and it was found that pre-adsorption of the EO onto the organically modified Laponite® RD more than doubled the time the EO molecules were retained in comparison to the directly added controls. It has been shown that EO can be used to obtain antimicrobials with broad spectrum antimicrobial activity, and that said activity can be successfully conferred onto polymer materials. Adsorption of the EO onto layered silicates before addition to polymers improved the retention of the EO molecules, thereby increasing the antimicrobial lifetime. Further work will be undertaken to optimise the EO polymer formulations, to combine the best possible antimicrobial activity with EO molecule retention that will make the materials a viable commercial product. The mechanical properties of the polymers; such as hardness, strength and toughness, will be optimised to ensure commercial viability.