Surface modification and antibacterial activity of electrospun polyurethane fibrous membranes with quaternary ammonium moieties

https://doi.org/10.1016/j.memsci.2008.04.012Get rights and content

Abstract

A novel antibacterial material was developed by surface modification of electrospun polyurethane (PU) fibrous membranes, using a process which involved plasma pretreatment, UV-induced graft copolymerization of 4-vinylpyridine (4VP), and quaternization of the grafted pyridine groups with hexylbromide. The success of modification with poly(4-vinyl-N-hexyl pyridinium bromide) groups on these was ascertained by X-ray photoelectron spectroscopy (XPS). The morphologies and mechanical properties were investigated by scanning electron microscopy (SEM) and tensile test, respectively. The results showed that the morphologies of PU fibrous membranes changed slightly during the modification process and the fiber structures were maintained. The tensile strength of PU fibrous membranes decreased after surface modification, with the smallest decrease (<20%) observed in the membrane with largest diameter. The antibacterial activities of the modified PU fibrous membranes were assessed against Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli). The modified PU fibrous membranes possessed highly effective antibacterial activities and may have a wide variety of potential applications in high-performance filters, protective textiles, and biomedical devices.

Introduction

In recent years, considerable attention has been paid to electrospinning as a versatile technique for producing fibers with diameters ranging from submicron to a few microns. In the typical electrospinning process, nanofibers are produced from an electrostatically driven jet of polymer solution (or melt). The discharged polymer solution jet undergoes a whipping process wherein the solvent evaporates and the highly stretched polymer fiber deposits on a grounded collector. The non-woven mats of electrospun fibers have shown unique properties, such as dramatically increased surface/volume ratios, excellent mechanical strength, and highly open porous structures. In the past few years, a wide range of synthetic and natural polymers in pure or blend solutions, as well as melts, have been electrospun to form fibers [1], [2], [3], [4]. Thermoplastic polyurethane (PU) is a resilient elastomer of significant industrial importance which possesses a range of desirable properties such as elastomeric, resistant to abrasion, and excellent hydrolytic stability [5], [6]. Numerous studies on the electrospinning of PU have been conducted [7], [8], [9]. Electrospun PU nanofiber mats exhibiting good mechanical properties may have a wide variety of potential applications in high-performance air filters, protective textiles, wound dressing materials, sensors, etc. [10], [11], [12], [13].

With the growing public health awareness of the pathogenic effects, malodors and stain formations caused by microorganisms, there is an increasing need for antibacterial materials in many application areas like medical devices, health care, hygienic application, water purification systems, hospital, dental surgery equipment, textiles, food packaging, and storage [14], [15]. A number of functional nanofibers and composites have been developed through electrospinning by incorporating well-selected functional agents to achieve antibacterial properties. Two common methods are incorporation of antibacterial agents in the electrospinning solutions [16], [17], and electrospinning of polymers with intrinsic antibacterial properties such as quaternized chitosan [18]. Kim et al. reported that the sustained release of hydrophilic antibiotic drug (Mefoxin) from electrospun poly(lactide-co-glycolide)-based nanofibrous scaffolds was effective to inhibit Staphylococcus aureus growth (>90%) [19]. Electrospun cellulose acetate fibers containing silver nanoparticles showed strong antimicrobial activity against both Gram-negative and Gram-positive bacteria [20], [21]. Kenawy et al. modified the poly(vinyl phenol) either by sulphonation or by formation of lithium salt of the sulphonated species, and investigated the antibacterial activities of the modified poly(vinyl phenol) electrospun mats [22]. Polyurethane cationomers polymerized from base PU with chain extenders having a quaternary ammonium group were electrospun into non-woven nanofiber mats for antimicrobial nanofilter applications [23].

A more straightforward way is to modify the surface of polymer nanofibers without affecting bulk properties of the treated nanofibers. The methods used to impart surface modification usually depend strongly on the nature of the fiber-forming polymer and include, but are not limited to, covalent polymer grafting [24], plasma treatment [25], physisorption (e.g., hydrogen-bonding interactions) [26], chemisorption [27], and chemical derivatization [28]. Among the various methods, plasma treatment provides a clean and environmentally friendly way for surface modification [29]. The free radicals and electrons created in the plasma treatment could be used to modify the polymer nanofibers chemically. Covalent attachment of functional compounds to polymer fiber surfaces is the preferred approach to introduce functionalities permanently and at reasonably high efficiency.

There are numerous antimicrobials suitable for immobilization on polymer surfaces. Quaternary ammonium compounds seem attractive because their target is primarily the microbial membrane and they accumulate in the cell driven by the membrane potential [30]. To maximize efficiency, quaternary ammonium compound is used as monomeric link in the polymeric leash and poly(4-vinylpyridine) (PVP) is usually selected as the carrying polymer. Tiller et al. showed that the surfaces of commercial polymers treated with N-alkylated PVP groups were lethal on contact to both Gram-positive and Gram-negative bacteria, and it was also shown that N-alkyl chain of six carbon units in length was the most effective [31].

The purpose of this paper was to develop novel antibacterial PU fibrous membranes by electrospinning the polymer followed by plasma pretreatment, UV-induced graft copolymerization and quaternization reaction. Electrospun PU fibrous membranes were modified with poly(4-vinyl-N-hexyl pyridinium bromide) on the surfaces to achieve antibacterial activities. The modified PU fibrous membranes were subsequently characterized in terms of their morphologies, surface chemical compositions and mechanical properties. The antibacterial activities of the fibrous membranes were assessed against both Gram-positive Staphylococcus aureus (S. aureus) and Gram-negative Escherichia coli (E. coli).

Section snippets

Materials

Polyurethane elastomer, Elastollan® 1180A10, was received from BASF. 4-Vinylpyridine (4VP) monomer was obtained from Aldrich Chemical Co. and freshly distilled under reduced pressure before use. Hexylbromide and solvents, such as tetrahydrofuran (THF), N,N-dimethyl formamide (DMF), heptane, 2-propanol, were of reagent grade and used as received from Aldrich Chemical Co. Peptone, yeast extract, agar and beef extract were purchased from Oxoid. S. aureus (ATCC 25923) and E. coli (ATCC DH5α) were

Electrospinning of PU

While electrospinning has proven to be a versatile and powerful means of fabricating polymer micro/nanofibers, its applicability to obtain smooth, uniform fibrous structure is not straightforward. Among various parameters of electrospinning process, such as applied voltage, needle tip-to-receiver distance and solution delivery rate, concentration or corresponding viscosity of spinning solution is one of the most effective variables for controlling fiber morphology and diameter. Results obtained

Conclusions

The surface of electrospun PU fibrous membranes was successfully modified with poly(4-vinyl-N-hexyl pyridinium bromide) using a method involving plasma pretreatment, UV-induced surface graft copolymerization and N-alkylation reaction. The morphologies of PU fibrous membranes changed slightly and the fiber structures were maintained after the modification process. The tensile strength and elongation at break of modified PU fibrous membranes decreased, whereas the Young's moduli showed no

Acknowledgements

Project 50573011 and 50673019 supported by the National Natural Science Foundation of China. Grant no.: R 279000202112 from the National University of Singapore, Ministry of Education, Singapore.

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