This article presents the formation of a dextran nanofibrous layer by needleless electrospinning. Optimum process parameters such as polymer solution and addition (surfactant) concentration, voltage, distance, etc. were determined to obtain uniform and smooth dextran nanofibers. It was not possible to produce nanofibers from pure dextran/water solution. Instead, solution drops were deposited on the collector; therefore, anionic surfactant was added in various concentrations to start the nanofiber production. Also, the effects of surfactant concentration on the solution properties, spinnability and fiber properties were determined. Generally, uniform and fine nanofibers were obtained from the rod electrospinning method. The value of 2 wt% surfactant concentration was chosen as the optimum concentration to produce a dextran nanofibrous layer by roller electrospinning. According to the results, spinning performance was 0.6726 g/min per meter, average fiber diameter was 162 nm, diameter uniformity coefficient was 1.03 and the nonfibrous area was 0.5%. In conclusion, this methodology resulted in the production of good product properties such as good spinnability, fine and uniform nanofibers and high fiber density.
Electrospinning is the most common method of producing nanofibers, and one of the greatest potential uses of electrospun fiber is in the area of bioengineering (1). For many biomedical applications, natural polymers are often used because of their high biocompatible and biofunctional properties. Several natural polymers such as collagen (2), DNA (3), silk (4), chitosan (5, 6), gelatin (7), cellulose (8, 9) and dextran (10) were used to produce nanofibers by the electrospinning method to develop products for biomedical applications. Most natural polymers that have been electrospun are proteins and polysaccharides (1). Dextran is a bacterial polysaccharide that consists of α-1,6-linked d-glucopyranose residues with some α-1,2-, α-1,3- or α-1,4-linked side chains (10). Dextran has biocompatibility and biodegradability characteristics, which are very important properties in biomedical applications such as drug delivery (11, 12) and scaffolds (13).
In the literature, there has been very little work done on dextran nanofiber production by electrospinning. Jiang et al. prepared uniform dextran nanofibrous membranes by electrospinning (10). They used various solvent mixtures such as water, dimethyl sulfoxide (DMSO)/water and DMSO/dimethylformamide (DMF) to produce dextran nanofibers. When water was used as a solvent, up to 10% of bovine serum albumin or lysozyme could be directly incorporated into the dextran membrane without compromising its morphology. Composite electrospun membranes consisting of polylactic-co-glycolic acid and dextran were obtained using DMSO/DMF (50:50) solvents. Also, the effects of various process parameters on the membrane properties were investigated, and no significant effect of the electrospinning process on lysozyme activity was observed (10). In another study, Ritcharoen et al. researched the effects of solution concentration and applied electric field on the morphological appearance and size of the electrospun dextran fibers. They observed that fiber diameter increased with increasing electric field and solution concentration. Also, the effects of curing temperature, curing time and MgCl2 catalyst addition were investigated for cross-linking dextran membranes with glutaraldehyde. According to the results, cross-linking did not affect the membrane morphology and only slightly decreased fiber diameter. Swelling and weight loss in water of the cross-linked dextran membranes decrease with increasing curing temperature, curing time and MgCl2 addition (14). Shawki et al. studied the fabrication of antimicrobial dextran nanofibers by electrospinning (15). They used water as a solvent and loaded moxifloxacin antibiotic on the dextran nanofibers. They investigated moxifloxacin release from dextran-moxifloxacin and its antimicrobial effectiveness. They found that dextran-moxifloxacin nanofibrous mats showed very high and rapid antibacterial activity against Staphylococcus aureus and Escherichia coli strains and reported that dextran-moxifloxacin nanofibrous mats can be used in a wide range of biomedical applications (15). In recent years, Spano and Massaro obtained aligned dextran-based nanofibers using a special collector of electrospinning (16). Again in 2012, Unnithan et al. prepared an antibacterial scaffold by electrospinning of a solution that includes dextran, polyurethane and ciprofloxacin HCl drug. They specified that dextran addition into the polyurethane increased the cell attachment and viability (17). Kim et al. proposed a new type of smart nanofibers with adjustable properties for the “on-off” controlled release of the dextran. According to the results, this study will be useful to provide a platform for on-off drug delivery (18).
All of the studies mentioned above achieved their results by the needle electrospinning method. There are currently no studies in the literature that cover dextran nanofiber production by needleless electrospinning at an industrial scale. Therefore, this paper aimed to fill the current knowledge gap through studying dextran nanofibrous layer production by the needleless electrospinning method. Needleless electrospinning concerns self-organization of polymeric jets on free liquid surfaces and is a result of electrohydrodynamics in the liquid surface (19). Up to now, many needleless electrospinning methods are being developed to prevent production problems such as needle clogging, low throughput, etc. (20–25). In this study, rod and roller electrospinning methods were used to provide optimization of dextran nanofiber production at an industrial scale. To date, only a few studies have been completed with needleless electrospinning (rod and roller) using common polymer/solvent systems such as poly(vinyl alcohol), polyurethane, poly(vinyl butyral), etc. (26–30), and in the literature, this study will be the first to obtain dextran nanofibrous layers by needleless electrospinning. This study also investigated the effects of surfactant concentration on the electrospinning process and dextran nanofiber morphology. From the literature, it is well known that addition of surfactant can enhance electrospinnability. Wang et al. used nonionic surfactants to enhance electrospinnability of poly(vinyl pyrrolidone). They found that surfactant addition increased spinnability while decreasing solution surface tension and fiber diameter (31). Talwar et al. also reported that addition of nonionic surfactant provides defect-free fibers and a significant improvement in nanofiber morphology, such as reduced beading (32). In 2004, Lin et al. found that small amounts of cationic surfactant stopped the formation of beaded electrospun polystyrene fibers. They highlighted that cationic surfactant addition increased solution conductivity but had no effect on the viscosity. However, addition of nonionic surfactant did not eliminate the fiber beads but reduced the bead numbers and changed the fiber morphology. Also, nonionic surfactant addition slightly improved the solution conductivity and had a minor effect on the surface tension but no effect on the viscosity (33). In another study, Kriegel et al. suggested that surfactants may modulate polymer-polymer interactions, therefore affecting the fiber morphology and composition of deposited nanostructures. They specified that pure chitosan did not form fibers and was instead deposited as beads. After the addition of poly(ethylene oxide) and an increase of the surfactant concentration, electrospinnability was enhanced, yielding a larger fiber diameter (34).
The objective of this study is to produce a dextran nanofibrous layer by needleless electrospinning with the aim of developing the techniques used for the industrial production of biomedical products.
2 Materials and methods
Dextran 70 was used as a polymer [Mw, 70,000 Da (g/mol)], distilled water was used as a solvent and anionic synferol was used as a surfactant. Dextran was obtained from Pharma Cosmos (Denmark) and anionic synferol surfactant was purchased from Enaspol (Czech Republic). All solutions include 0.75 g/ml (0.75 g of dextran powder in 1 ml of distilled water) dextran and were prepared at various anionic synferol concentrations: 0, 2, 4, and 6 wt% surfactant. The aim of using this surfactant was to increase the electrospinnability of dextran solution.
All solutions were prepared under the same conditions (room temperature, stirring time, the same day, etc.). It is well known that polymer solution properties play a critical role in electrospinning and resultant fiber morphology (26, 35–37). Therefore, the solution properties such as conductivity, surface tension and viscosity were determined. Conductivity and surface tension properties were determined by a conductivity meter (Radelkis, OK-102/1) (Budapest, Hungary) and the Wilhelmy method (Krüss) (Germany) using a platinum plate and a high-precision electronic balance, respectively. The viscosity of the solutions was measured using a HAAKE RotoVisco 1 rheometer (Germany) at 25°C with a shear rate of 200 s-1.
In this work, first, the rod electrospinning method was used to optimize nanofiber production from dextran solutions. This method is a needleless electrospinning process invented by Lukas (24). As a main principle of rod electrospinning, a steel rod and collector are used to spin solutions directly from the solution surface (Figure 1A).
A high-voltage supplier is connected to the rod and the collector electrode. An electrostatic field forms between the rod and the collector electrode. The collector electrode is grounded, and the setup is placed in a chamber to control the relative humidity and temperature.
A droplet of dextran solution was placed on the rod electrode before the source of voltage was switched on and 30 kV was applied to the solution. Rod diameter is an important parameter for the Taylor cone number and the productivity of electrospinning. If the rod diameter is 3 mm or less, only one Taylor cone occurs on the surface. One to six Taylor cones usually formed on the rod’s surface during electrospinning if the rod diameter is higher than 8 mm (38) (Figure 1B). To obtain several Taylor cones on the rod surface, the rod diameter was adjusted to 10 mm for this study. The process parameters of the rod electrospinning method are shown in Table 1.
|Rod diameter (mm)||Distance between rod and collector (cm)||Voltage (kV)||Temperature (°C)||Relative humidity (%)|
After the rod electrospinning process, an optimum solution (2 wt% surfactant concentration) was chosen and spun by barbed roller electrospinning (Nanospider) (Figure 2).
Roller electrospinning is another needleless method used to produce nanofibrous layers at an industrial scale. This method was invented by Jirsak et al. from the Technical University of Liberec (25) and commercialized by the Elmarco Company (http://www.elmarco.cz. Accessed 2012). Roller electrospinning consists of a roller and a movable collector electrode. The roller slowly rotates in a tank filled with a polymer solution. The electrostatic field organized between the roller and the oblong collector enables the self-organization of jets along the upper surface of the roller (19). Then, a high voltage is connected to the roller and collector electrode while the collector electrode is grounded. Various shapes of rollers can be used in roller electrospinning such as plain cylindrical, tine roller or barbed roller (39). For this study, a barbed roller was used, which has six rows of barbs, and each row has nine barbs on it (a total of 54 barbs). A picture of the barbed roller during the spinning process is shown in Figure 2B.
While rotating, the barbed roller is covered by a film of dextran polymer solution, and many Taylor cones (40) are created after the voltage is switched on. The jets of polymer solution move toward the collector electrode and settle down on the supporting material as solid nanofibers owing to rapid solvent evaporation before reaching the collector electrode. The optimum process parameters of the roller electrospinning applied during the spinning experiments are presented in Table 2.
|Roller length (cm)||Roller diameter (cm)||Roller speed (rpm)||Take-up cylinder speed (cm/min)||Distance between electrodes (cm)||Voltage (kV)||Temperature (°C)||Relative humidity (%)|
Conditioned air is blown into the spinning device from a suitable air conditioner. The air conditioner is able to keep the relative humidity between 18% and 60% and the temperature between 18°C and 30°C. Inside the chamber, optimum values of relative humidity and temperature for spinning dextran nanofibers by barbed roller electrospinning are 35% and 24°C, respectively, which were determined from the preliminary experiments. Dextran nanofibers were collected on polypropylene spunbond nonwoven antistatic material.
After the dextran nanofibrous layers were obtained by barbed roller electrospinning, the spinning performance (SP) value, which is the most important parameter of the roller electrospinning method, was calculated. SP, or polymer throughput, is the amount of nanofiber material in grams per minute produced using a 1-m-long roller (g/min per meter). The equation of spinning performance is given as (41):
where SP is the spinning performance (g/min per meter), V is the take-up cylinder speed (m/min), W is the width of nanofiber web on collected nonwoven material (m), M is the area weight of nanofiber web (g/m2) and l is the length of the spinning roller (m).
The fiber morphology and diameter of the dextran nanofibers were analyzed with fiber pictures under 1000× and 10,000× magnification by scanning electron microscopy (SEM) (TESCAN) (Czech Republic). From the SEM pictures, average fiber diameter (nm), diameter uniformity and nonfibrous area (NFA) percentage (%) were determined with the aid of Lucia 32G computer software. Average fiber diameter was calculated using 100 different diameter values for each sample. The fiber diameter uniformity coefficient was calculated using the number and weight average calculation method. Number average has been used as an arithmetic mean in mathematical science, and the method that was used to calculate the uniformity coefficient has the same principle as the molar mass distribution in macromolecular chemistry (28). Both of these values were calculated using Eqs. (2) and (3):
where di is the fiber diameter and ni is the fiber number.
The fiber diameter uniformity coefficient (FDUC) was determined using Eq. (4); the optimum value should be very close to 1 for uniform fibers.
In addition, the NFA percentage value was calculated from SEM pictures of nanofibers under 1000× magnification. The NFA percentage refers to the quality of the spinning process. NFA is the area fraction of nonfibrous area in a membrane in relation to the total area of product (27). The equation is given below:
NFA=(total area of nonfibrous area/total area of nanofibrous membrane)×100
3 Results and discussion
3.1 Results of solution properties
Measurement of solution properties (conductivity, surface tension, viscosity, etc.) showed that the polymer-solvent and polymer-polymer interactions are key factors in the electrospinning process and that surfactants are able to adjust these interactions via electrostatic or hydrophobic interactions as well as hydrogen bonding (34). Therefore, dextran solution properties such as conductivity, surface tension and viscosity at various surfactant concentrations were determined. Figure 3 shows the effect of surfactant concentration on the solution conductivity.
The conductivity of the dextran solution increased with surfactant concentration and there was a high linear regression between the conductivity and the surfactant concentration. In the literature, similar results were also observed (33).
When the effect of surfactant concentration on the solution surface tension and viscosity was analyzed, it was observed that the surface tension decreased with surfactant, whereas viscosity was not affected by surfactant concentration (Figure 4).
It is well known in the literature that solution surface tension decreases with surfactant addition (34, 42). Also, surfactant addition has no effect on viscosity, and this result is concordant with the literature (33). Therefore, these results confirm what has been previously observed by other groups. The effect of surfactant concentration on the surface tension was found to be statistically significant using a one-way analysis of variance (ANOVA) test.
3.2 Spinning and analysis of fiber properties
Dextran nanofibrous layers were fabricated by needleless rod and roller electrospinning methods. It was not possible to produce nanofibers from pure dextran/water solution by needleless electrospinning because solution drops were deposited on the collector. As has been previously shown in the literature, surfactants may modulate polymer-polymer interactions to enhance electrospinnability and influence fiber morphology (34). Therefore, anionic surfactant was added in various concentrations to start the nanofiber production process. First, the rod electrospinning method was used to obtain dextran nanofibers at various surfactant concentrations to determine the optimum surfactant value for roller electrospinning. Figure 5 shows the SEM images and diameter distributions of the dextran nanofibers that were obtained by the rod electrospinning method.
As can be seen in Figure 5, uniform and very fine nanofibers and also smooth fiber morphology were obtained from the dextran solution at various surfactant concentrations. In particular, the nanoweb structure of dextran at 2 wt% surfactant was very smooth and without fiber stickiness. The 2 wt% surfactant solution was the chosen optimum for the roller electrospinning method because of its excellent fiber properties. Also, fiber diagram histograms were obtained and analyzed (Figure 5). Generally, uniform and unimodal curves were obtained from all histograms. Fiber diameter range was between 102 and 357 nm.
The effect of the surfactant concentration on the dextran nanofiber diameter and diameter uniformity coefficient is given in Figure 6.
According to Figure 6, fiber diameter and diameter uniformity increased with surfactant concentration. Fiber diameter uniformity coefficient values were very close to 1. Therefore, it can be said that uniform dextran nanofibers were obtained from the rod electrospinning method. Also, the effect of surfactant concentration on the fiber diameter was statistically significant, whereas uniformity coefficient was not (ANOVA test).
Spinning nanofibers by the barbed roller electrospinning technique using the dextran solution with 2 wt% surfactant was tried. The SEM images of dextran nanofibers produced by roller electrospinning at 1000× and 10,000× magnifications are given in Figure 7.
As can be seen from Figure 7, there is high fiber density and low NFA under 1000× magnification. Also, very fine and uniform nanofibers were obtained. Average fiber diameter was 162 nm as shown in the fiber diameter histogram. The lowest fiber diameter was 91 nm and the highest value was 243 nm, and a unimodal curve was obtained. The fibers obtained from this method were very fine compared to previous findings in the literature (10, 14–16). Table 3 shows the product properties of dextran nanofibers from roller electrospinning.
|Dextran solution||Product properties|
|Spinning performance (g/min per meter)||Fiber diameter (nm)||Fiber diameter uniformity coefficient||NFA (%)|
|2 wt% surfactant||0.6726||162||1.03||0.5|
From this table, spinning performance, which is the most important parameter for a roller electrospinning system, is 0.6726 g/min per meter, average fiber diameter is 162 nm, fiber diameter uniformity coefficient is 1.03 and NFA percentage is 0.5%. Therefore, it can be said that good product properties were obtained. Lastly; when the fiber properties were compared for rod and roller methods, it is possible to explain that finer fibers were obtained from roller electrospinning, whereas fiber morphology and fiber diameter uniformity are similar.
In this study, formation of dextran nanofibrous layers by needleless electrospinning was achieved successfully. First, solution properties such as polymer concentration, surfactant concentration, conductivity, surface tension and viscosity were determined. Then, optimum process parameters for rod and roller electrospinning, such as voltage, distance, humidity, etc., were determined to obtain uniform and smooth dextran nanofibers. It was not possible to produce nanofiber from a pure dextran/water solution. Instead, solution drops were deposited on the collector, so anionic surfactant was added in various concentrations to start nanofiber production. In addition, the effect of surfactant concentration on solution properties such as conductivity, surface tension and viscosity was determined. According to the results, conductivity increased while surface tension decreased with increasing surfactant concentration. However, solution viscosity was not affected by the surfactant concentration. Uniform and very fine nanofibers were obtained from the rod electrospinning method. The value of 2 wt% surfactant concentration was chosen as the optimum for producing a dextran nanofibrous layer by roller electrospinning. The product properties of roller electrospinning are as follows: spinning performance, 0.6726 g/min per meter; average fiber diameter, 162 nm; fiber diameter uniformity coefficient, 1.03; and NFA percentage, 0.5%. In summary, it can be said that good product properties such as good spinnability, very fine and uniform nanofibers, and high fiber density were obtained. The results obtained in this study have potential future implications in the production of biomedical materials and should be investigated further at the industrial scale.
The author would like to thank the Nonwoven Department of the Textile Engineering Faculty, Technical University of Liberec in the Czech Republic for providing work facilities. Thanks are also offered to Fatma Yener, Baturalp Yalçınkaya and Prof. Dr. Oldrich Jirsak for their assistance.
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