Electrospinning of chitosan–poly(ethylene oxide) blend nanofibers in the presence of micellar surfactant solutions

Abstract

Nanofibers were fabricated by electrospinning a mixture of cationic chitosan and neutral poly(ethylene oxide) (PEO) at a ratio of 3:1 in aqueous acetic acid. Chitosan ((1 → 4)-2-amino-2-deoxy-β-d-glucan) is a multifunctional biodegradable polycationic biopolymer that has uses in a variety of different industrial applications. Processing conditions were adjusted to a flow rate of 0.02 ml/min, an applied voltage of 20 kV, a capillary tip-to-target distance of 10 cm and a temperature of 25 °C. To further broaden the processing window under which nanofibers are produced, surfactants of different charge were added at concentrations well above their critical micellar concentrations (cmc). The influence of viscosity, conductivity and surface tension on the morphology and physicochemical properties of nanofibers containing surfactants was investigated. Pure chitosan did not form fibers and was instead deposited as beads. Addition of PEO and surfactants induced spinnability and/or yielded larger fibers with diameters ranging from 40 nm to 240 nm. The presence of surfactants resulted in the formation of needle-like, smooth or beaded fibers. Compositional analysis suggested that nanofibers consisted of all solution constituents. Our findings suggest that composite solutions of biopolymers, synthetic polymers, and micellar solutions of surfactants can be successfully electrospun. This may be of significant commercial importance since micelles could serve as carriers of lypophilic components such as pharmaceuticals, nutraceuticals, antimicrobials, flavors or fragrances thereby further enhancing the functionality of nanofibers.

Introduction

Electrospinning is a technique that involves applying a high voltage between the tip of a syringe and a collector plate, with a polymer solution being contained within the syringe. The voltage causes a jet of polymer solution to be expelled from the syringe and move towards the collector plate. As the solvent in the jet dries the remaining polymer solidifies and forms ultrafine nanofibers that are collected on the collector plate as a non-woven mesh or membrane. Electrospinning can produce polymer nanofibers ranging from 10 to several 100 nm in diameter whose properties depend on polymer type, solution properties and processing conditions. To date, a wide variety of polymers and blends of polymers have been electrospun, with synthetic polymers yielding the best results, i.e. fibers of high mechanical strengths and uniform morphologies. The generated nanofibers have very large surface area-to-mass ratios that may sometimes be as high as several 100 m²/g and may be engineered to have high porosities with small pore size. These properties have led to the development of numerous applications in the field of biomedical engineering [1], [2], drug delivery, biosensors, material science, pharmacy and increasingly food science. Fabrication of nanofibers from biopolymers has attracted increased interest due to the fact that biopolymers may have superior biocompatibility and biodegradability, are generally non-toxic, renewable and available at lost costs, and may have functionalities such as antioxidant, antimicrobial or enzyme activities [3]. However, electrospinning of nanofibers from biopolymers has proven to be challenging because they have limited solubility in most organic solvents, are often polyelectrolytes when dissolved, have poor molecular flexibilities, readily form three-dimensional networks via hydrogen bonds, and most importantly are insufficiently entangled to facilitate electrospinning [4].

Chitosan, a copolymer of (1 → 4)-2-amino-2-deoxy-β-d-glucan and (1 → 4)-2-acetamido-2-deoxy-β-d-glucan, capable of forming extensive intra- and intermolecular hydrogen bonds can be derived from chitin, one of the most abundant biopolymers, by deacetylation in NaOH [5], [6]. At pH below its pKa, chitosan behaves like a cationic polyelectrolyte. Chitosan molecules carry a high positive charge density due to the protonation of the amino groups attached to their backbone. The positive charge of chitosan gives rise to a number of useful functional properties. For example, chitosan may bind free fatty acids during digestion of fatty meals thereby preventing adsorption making it of interest to manufacturers of dietary weight loss supplements [7], [8]. In the food industry, its broad spectrum of antimicrobial activity against yeasts, fungi and bacteria have resulted in being investigated as a novel, naturally-occurring food preservative [9], [10], [11], [12]. Production of nanofibers from chitosan may thus enable the development of novel food, pharmaceutical, and cosmetic applications.

Electrospinning of chitosan has been investigated by several authors all of which found that the manufacture of pure chitosan nanofibers was extremely challenging [13], [14], [15], [16], [17], [18]. Some authors reported success in electrospinning homogeneous chitosan nanofibers from chitosan solutions of relatively high concentration and low molecular weight upon dispersion in solvents such as trifluoroacetic acid [19], 1,1,1,3,3,3-hexafluoro-2-propanol [20] where further extraction of the solvent became necessary or concentrated aqueous acetic acid [21], [22]. Unfortunately, low molecular weight chitosans have shown to exhibit lower biological activities and the presence of residues of the above mentioned solvents may prohibit their application in food systems.

Researchers have instead focused on electrospinning blends of chitosan with other compatible polymers such as PVA [16], [18], [23], [24], PEO [13], [25], [26], or others [27]. In blends, the fiber forming ability of the co-spinning agent is utilized to facilitate polymer entanglement and generation of a polymer jet. Electrospinning of polymer blends is also an efficient way to create composite nanofibers that have improved material properties such as higher tensile strengths. Based on previous studies, we chose to use poly(ethylene oxide) as a co-spinning agent due to its excellent electrospinning characteristics, its ability to form ultrafine fibers, its linear structure with flexible chains, its biocompatibility, its solubility in aqueous media, and its capability to form hydrogen bonds with other macromolecules. In a chitosan–PEO blend, PEO acts as a plasticizer facilitating orientation and flow of chitosan by uncoiling and wrapping around chitosan chains [28].

Surfactants are used in a wide array of applications because of their potential to lower surface or interfacial tension of the medium in which they are dissolved [29]. Each molecule contains both a hydrophilic and a hydrophobic part. An ionic surfactant, which has an ionic hydrophilic head, may also improve the electrical conductivity of the solution, promoting bending instability during the electrospinning process, thereby facilitating thinner fiber production with a higher degree of orientation [30]. Further, surfactants may self-assemble to form colloidal aggregates above a critical concentration, the so-called critical micellar concentration or cmc [31]. These micellar solutions are able to serve as solubilization vesicles to improve solubility and protect and deliver lypophilic functional ingredients. Incorporation of micelles into nanofibers could thus offer a novel means to further functionalize biopolymer nanofibers and their blends. Finally, polymer–surfactant interactions may modulate the molecular structure and interactions of polymer molecules thereby altering rheological and interfacial properties of polymer dispersions [30], which are critical factors in the successful preparation of nanofibers by electrospinning. For example, addition of small amounts of nonionic surfactant was found to improve both the onset voltage and the reproducibility of electrospinning [32]. In nonionic polymer solutions, nonionic surfactants did not stop bead formation but greatly reduced it, while cationic surfactants prevented beaded fibers and lead to fibers with smaller mean diameters [30].

In this study, we hypothesize that addition of surfactants to polymer solutions may prove to be a convenient means to (a) modulate the electrospinning conditions of biopolymer–polymer blends and (b) further functionalize fibers by either altering surface properties of fibers or by inclusion of micellar structures that could serve as vehicles for lypophilic functional ingredients. To test this hypothesis, various surfactants (anionic, cationic and nonionic) were added to chitosan–PEO solutions. Solutions were subjected to electrospinning and the influence of surfactant type and charge on solution properties and on the size and morphology of the nanofibrous structures generated were evaluated.