Electrospun fibers containing metal-releasing particles for microbial growth control

Resum

Electrospinning is the only general technique available for the production of nanofibers. It proved suitable to provide nonwovens with high surface to volume ratio, tunable porosity, plasticity to adapt to a variety of sizes and shapes and almost unlimited possibilities of chemical functionalization. The materials used to prepare electrospun fibers range from synthetic to natural polymers, also allowing the possibility of preparing ceramic nanofibers. There is also the possibility of creating hierarchical structures such as core/sheath and surface decorated fibers, which offer the possibility of electrospinning non-spinnable substances and surface complex functionalization. Electrospun fibers may contain a variety of organic or inorganic fillers offering physical reinforcement or the controlled delivery of chemicals included within fibers. The most common filler materials are inorganic nanoparticles that included in nanofibers of decorating their surface, benefit from a support to which they may impart different capacities. Many applications have been developed to take advantage of the unique features of nanofibers, among which the possibility of preparing composite mats with antimicrobial action stands out. The incorporation of antimicrobial compounds in electrospun fibers offers the possibility of developing new antibiotic surfaces that can be used as antibacterial scaffolds for biomedicine, biofouling resistant membranes for environmental uses and materials for active food packaging among many other uses. The aim of this work was to produce biocidal electrospun nanocomposite fibers by incorporating metal carriers to environmentally friendly polymers. For this purpose, the biopolymers polylactic acid and cellulose acetate and the water soluble polyvinylpyrrolidone were used to incorporate metal nanoparticles either free or in supported form. The carriers for metals were a modified sepiolite, mesoporous silica and hybrid organicinorganic materials belonging to the class of metal-organic frameworks. Electrospun fibers were physically characterized by scanning electron microscopy, transmission electron microscopy, X-Ray diffraction and energy dispersive X-Ray spectroscopy, among others. Special attention was paid to the release of metals, which was assessed by inductively coupled plasma mass-spectroscopy measurements. The organisms used for testing the biocidal behaviour of mats were the bacteria Pseudomonas putida, Staphylococcus aureus and Escherichia coli, the yeast Saccharomyces cerevisiae and the fungus Aspergillus niger. Confocal microscopy and microplate readings with fluorochromes measuring cell viability or integrity allowed determining the biocidal effect of composite fibres. For it, the cell-permeant esterase substrate fluorescein diacetate, the nucleic acid stains SYTO 9 and propidium iodide, a luciferin-based ATP determination system and the viability probe for yeasts and fungi FUN 1 were used. These methods were complemented by colony counting and the measurement of cell biomass. The results showed that nanometals, with or without carrier particles can be successfully included in all the polymeric matrixes tested using blended electrospinning. Fibres were produced in all cases as smooth nonwovens without beading and flaws and a fibre diameter of a few hundreds of nanometres. The rate of metal discharge showed an initial peak followed by a period or slower and relatively constant rate of release. Supported metals or structured frameworks resulted in slower metal release and more prolonged biocidal effect with respect to free nanometals. Silver-loaded fibres were particularly effective preventing bacterial colonization and biofilm formation, but copper and cobalt loaded membranes also displayed significant antibiocidal behaviour. The results generally showed a decrease in the number of microorganisms attached to the fibres, an increase in non-viable cells and a parallel decrease in viable microorganisms with the occasional finding of viable but non-culturable microorganisms