Novel antibacterial fibers of quaternized chitosan and poly(vinyl pyrrolidone) prepared by electrospinning

Abstract

The preparation of continuous defect-free fibers from quaternized chitosan derivative (QCh) has been achieved by electrospinning of mixed aqueous solutions of QCh with poly(vinyl pyrrolidone) (PVP). The average fiber diameter significantly decreases from 2800 to 1500 nm on increasing the polyelectrolyte content. In order to impart to QCh/PVP electrospun fibers stability to water and water vapor, the fibers have been crosslinked by incorporation of photo-crosslinking additives into QCh/PVP spinning solutions and subsequent UV irradiation of the electrospun fibers. Photo-crosslinked QCh-containing electrospun mats show high antibacterial activity against the Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Escherichia coli.

Introduction

Chitosan is a polysaccharide which is typically produced by partial N-deacetylation of the natural polymer chitin. Chitosan may be considered as a copolymer built of β(1 → 4)-linked 2-acetamido-2-deoxy-β-D-glucopyranose and 2-amino-2-deoxy-β-D-glucopyranose residues. Chitosan is widely studied because of its biological properties including wound healing and antibacterial properties [1], [2]. It is known that chitosan derivatives with quaternary ammonium groups possess high efficacy against bacteria and fungi. It is now widely accepted that the target site of these cationic polymers is the cytoplasmic membrane of bacterial cells [3]. Micro- and nano-fibrous materials are suitable for preparing wound dressings. Electrospinning is a promising technique for producing continuous polymer fibers with diameters down to nano-scale range using the action of an external electric field imposed on a polymer solution or melt [4]. The morphology and diameters of electrospun fibers depend on a number of parameters, that include properties and composition of the spinning solution such as polymer type, conformation of polymer chain, viscosity (concentration) of the solution, conductivity, polarity and surface tension of the solvent; electrospinning conditions: applied field strength, distance between the capillary and collector, and feeding rate [5], [6], [7]. Recently, the nano-fibrous polymer materials prepared by electrospinning have drawn great interest because of their unique properties, such as high surface area-to-volume and aspect ratios. These properties make the non-woven mats composed of electrospun fibers excellent candidates for various applications, e.g., as wound-dressings, and drug delivery systems [8].

Until recently, the preparation of continuous fibers of polyelectrolytes by electrospinning of their aqueous solutions was considered impossible due to the repulsive forces between ionogenic groups [9]. Nano-fibers from ionogenic polymers have been successfully electrospun from mixed solutions of ionogenic polymer and non-ionogenic polymer. The preparation of electrospun chitosan-containing nano-fibers has been achieved from mixed chitosan/polyoxyethylene solutions [10], [11], from chitosan/silk fibroin solutions [9], as well as from mixed chitosan/poly(vinyl alcohol) solutions [12], [13]. Recently, we have prepared non-woven nano-fibrous mats from N-carboxyethylchitosan and polyacrylamide [14]. It was shown that the presence of ionizable low-molecular-weight compounds (8-quinolinol derivatives) in chitosan- and N-carboxyethylchitosan-containing spinning solutions led to the decrease in the diameter of the nano-fibers [10], [14]. The drug-containing nano-fibers showed good antimicrobial and antimycotic activity. Recently, we have reported on the preparation of nano-fibers containing quaternized chitosan (QCh) by electrospinning of mixed aqueous solutions QCh/poly(vinyl alcohol) [15]. The electrospun fibers had diameters in the range 60–200 nm and showed good antibacterial activity against Gram-positive and Gram-negative bacteria.

A suitable candidate as a non-ionogenic partner for electrospinning of chitosan and its derivatives is poly(vinyl pyrrolidone) (PVP). Until now, PVP has found a wide variety of applications in the biomedical field because of its useful properties including non-toxicity, biocompatibility, high hydrophilicity, good complexation properties, and film-forming ability [16]. Recently, PVP has been successfully electrospun into nano-fibers from ethanol, DMFA, dichloromethane or their mixtures [17], [18]. To the best of our knowledge, until now no data about electrospinning of PVP from its aqueous solution have been reported.

The present work reports on the successful preparation of electrospun quaternized chitosan-containing fibers. PVP has been used as the non-ionogenic partner facilitating the electrospinning. The effects of the spinning solution composition and of the applied field strength on the average diameters and morphology of the QCh/PVP electrospun fibers have been studied. In order to impart to electrospun QCh/PVP mats resistance to water and water vapor, the appropriate conditions for photo-crosslinking of the fibers in the solid state have been found. The antibacterial activity of the electrospun mats against the Gram-positive bacteria Staphylococcus aureus and Gram-negative bacteria Escherichia coli has also been assessed.