Document Type : Original Research Article
Subjects
Introduction
Electrospinning and electrospray are electrohydrodynamic techniques which are controlled by electromechanical and fluid-dynamic forces. In both methods, a strong electric field is applied to a liquid at a capillary nozzle which charges and deforms the droplet at the tip of the nozzle. Once the field exceeds a critical value, the droplet makes a Taylor cone that emits a fine, charged jet. In electrospray, Rayleigh–Plateau instability occurs that breaks the jet into fine charged particles (due to Coulomb repulsion), while in electrospinning, sufficient viscoelasticity of the polymer solution suppresses the breakup, thus, ends in formation of fibers [1]. Between these extremes, intermediate conditions can end in formation of beaded fibers, a morphology which combines features of both methods [2].
In the fabrication of polymer fibers, the formation of beads is typically considered a defect that compromises the structural integrity and uniformity of the material [3]. Although beaded morphologies are desirable in specific applications (e.g. controlled drug release systems) [4], they are predominantly regarded as detrimental by-products. A major drawback of beading is the reduction of effective surface area, which diminishes the performance of nanofibrous mats. This is particularly important in filtration, where beads impair filtration efficiency [5]. Also, in tissue engineering applications, beads can reduce cell adhesion and growth [6]. In a report, straight nanofibers from solution of polyacrylonitrile (PAN) in dimethylformamide (DMF) were prepared via electrospinning by adding potassium iodide to the solution. The submicron fibers (below 300 nm in diameter) were aimed for improved aerosol filtration [7].
Reviewing the literature, several parameters have been suggested that can affect beading on fibers: nozzle to collector distance, polymer flow rate, solution conductivity, applied voltage [5] and the surface tension of the solution [8] are examples of these parameters. However, the majority of works in this field are using one-factor-at-a-time approach. The tendency to converge on local rather than global optima, along with the restriction of the factor space inherent to this approach, are two major limitations that justify the use of alternative methods when it is required to obtain a deeper understanding of the phenomenon [9]. To fully exploit the potential of electrospinning, one should have precise control over the morphology of the produced nanofibers. Here in, we investigated the influence of some processing and solution properties (i.e. applied voltage, feed rate and polymer concentration) on formation of beads in polyurethane/ polyethylene glycol nanofibers. These three parameters have been investigated in our previous studies, where their significant effects on the electrospray process were demonstrated [10].
Polyurethane (PU), Polyethylene glycol (PEG, 1000 kDa) and Ploylactic acid (PLA) were from Sigma (USA). Dimethylformamide (DMF) was from Neutron co (Iran). Tetrahydrofuran (THF) and chloroform were purchased from Dr Mojallali co (Iran).
Preparation and characterization of PU/PEG nanofibers
DMF and THF (1:1) were mixed, then, 9 % PU was added and stirred overnight. Subsequently, different concentrations of PEG were added and stirred for 1 h. The solution was then electrospun using Electroris ® (Fanavarn Nano Meghyas co, Iran) as detailed earlier ([11]). In brief, using a 5 mL plastic syringe, having a 21G needle, the solution was sprayed on the grounded drum of the device (i.e. collector). The needle to collector distance was 20.0 cm. A high voltage device was connected to the needle. The rotation speed of the drum was 20 rpm and the electrospinning time was ~ 10 min. The electrospinning process was performed at ambient temperature and humidity. The obtained mat was collected on the drum was investigated for formation of bead and fiber on the mat using scanning electron microscopy (SEM, model S-3000N, Hitachi, Japan). The SEM images were analyzed using ImageJ (National Institute of Health, USA) to find the area occupied by formed fibers and beads. Then, the ratio of bead/fiber was calculated for each sample.
Here in, the influence of three experimental factors on the formation fiber and bead from PU/PEG mixture was assessed. The independent factors included feed rate (injection rate, mL/h), PEG concentration (% W/V) and voltage (kV). The ratio of bead/fiber area was taken as the dependent variable.
To model the relation between the independent and dependent variables, we used Box–Behnken experimental design which is a response surface methodology [12]. This design requires lesser experiments compared with commonly used designs such as central composite design (CCD). The modeling process was done using Design-Expert (Version 7.0.0, Stat-Ease, Inc., USA). The independent (input) factors were defined in low, basal and high levels, coded as -1, 0 and +1, respectively (see Table 1).
17 experiments were designed having 12 factorial points and 5 replicates (at the center point to estimate the pure error sum of squares). The details of the variables have been given in Table 2.
The relations of the outputs with the inputs were modeled by the equation (1) :
Y= β0 + β1X1 + β2X2 + β3X3 + β11X12 + β22X22 + β33X32 + β12X1X2 + β13X1X3 + β23X2X3 (1)
Where Y is the predicted response (bead/fiber ratio), β0, intercept, β1 to β3, linear coefficients, β11, β22 and β33, squared coefficients, β12 to β23, the interaction coefficients and X1, X2 and X3 are the independent variables (concentration, voltage and feed rate). The equation enables the evaluation of the linear, quadratic and interactive effects of the independent variables on the dependent variable. Using the software, the relations and interactions between the independent and the dependent variables were visualized using the response surfaces (3D graphs). To find the optimized preparation, constraints for bead/fiber ratio were set to minimum. Subsequently, the suggested preparation was experimentally prepared and characterized.
Our work comprised of formation of PU/PEG nanofibers using electrospinning. Nanofibers with mean diameter, bead area and fiber area of 274 to 519 nm, 5.4 to 20.3 (%) and 25.2 (%) to 47.6 (%) were obtained (see Figure 1). We then used response surfaces to study the effects of independent variables on the bead/fiber ratio and find the optimum values for each independent variable to minimize the bead/fiber ratio. This will lead to prepare “bead-less nanofibers”.
To find the best matching model, analysis of variance was performed (i.e. F-value). Quadratic second-order polynomial equation was fitted:
Bead/Fiber = +0.027544 × Voltage - 0.088834 × Feed rate - 5.56607×10-4 Voltage2 + 0.055497 × Feed rate2 (equation 2)
The F-value of the model was 4.31, indicating a significant model and the lack of fit F-value was calculated as 2.73, which shows a not significance relative to the pure error. Table 3 briefs the results of ANOVA study.
The software was then used to visualize the effect of independent variables on the dependent variable using the response surfaces (see Figures 1-3). In each plot, the effect of two interacting independent variables has been illustrated when the third variable has been fixed in a mid-level value.
Fig. 2 shows the effects of concentration and voltage on the bead/fiber ratio. From the details, concentration of the polymer does not appear to influence the bead/fiber ratio, a fact which can be determined from equation 2. Also, a value of ~ 17 kV is required for the voltage as the optimum value, above or below which the bead/fiber ratio tends to increase.
Figure 3 shows the effect of concentration and feed rate on the bead/fiber ratio. The findings show that concentration is not influencing the ratio while feed rate has an optimal value (~ 1.1 mL/h), above or below which the bead/fiber ratio tends to increase.
From Figure 4 which shows the effect voltage and feed rate, to obtain bead/fiber ratio close to 0, the two independent parameters should be fixed at their optimum values.
Following to generation of the response surfaces, the model was used to suggest the optimum values for the dependent variables. The goal was to obtain nanofibers with minimum beads (preferably no bead). The model suggested voltage and feed rate values of 14.84 (kV) and 1.14 (mL/h), respectively. The model predicted bead/fiber ratio of 0.0005, which can be considered as zero.
To validate the generated model, the suggested sample was prepared and analyzed using SEM. As the Figure 5 shows, the bead is hardly detectable, confirming the capability of the generated model in predicting the optimum conditions.
A prevalent challenge in electrospinning is the formation of beads which is commonly considered as morphological defects. Various parameters have been reported to affect the bead formation during an electrospinning process. Concentration of the polymer solution appears to be important: In an SEM-based study, it was indicated that indicated that bead-on-string nanofibers could be successfully produced only within a specific range of PLGA solution concentrations [4]. Polyethylene oxide (PEO) solutions (3.0% and 3.5% (W/V), with viscosities of 617 ± 6 cP and 995 ± 6 cP, respectively) yielded beaded nanofibers, while increasing the concentration to 4.0%, resulted in formation of bead-free morphology [13]. The effect of polymer concentration on size of nanoparticles has also been highlighted in electrospray [14]. High viscosity (due to high polymer concentration) is a critical requirement for the production of continuous, bead-free PEO nanofibers. In our study, concentration demonstrated no significant effect on formation of bead on electrospun nanofibers. This is likely because the concentration ranges investigated (i.e. > 1%) were sufficient to provide the viscosity required for forming defect-free fibers. Consequently, further increases in concentration (up to 9%) did not yield a measurable reduction in beads number.
Applied voltage can also influence the bead formation. In a PEO/water system, increasing the voltage beyond 5.5 kV transformed the fiber morphology from defect-free to high density beaded fibers. When the voltage is low, a stable Taylor cone forms, which produces cylindrical, defect-free fibers. By increasing the voltage, the droplet recedes, and the jet initiates from within the syringe tip, which leads to formation of bead defects [15]. On the other hand, bead formation is not exclusive to high voltage values: low voltages can also cause beaded fibers, indicating a non-monotonic relationship. For instance, in a polystyrene/THF system, beaded fibers were produced at both low (5 kV) and high (12 kV) voltages, whereas mid-range voltages (7.5 and 10 kV) yielded bead-free morphologies [16]. In our study, an optimum value for applied voltage was obtained for preparation of bead-free fibers, which agrees with the above findings.
Flow rate has been reported to directly increase the bead density on fibers: polystyrene fibers demonstrated beading when the flow rate was 0.10 mL/min and higher [16]. In another report, PLGA electrospun nanofibers showed controversial effects from voltage: The bead number initially increased and then decreased with increasing flow rate from 0.5 mL/h to 1.5 mL/h. The increase in flow rate enhances the net charge density, which suppresses bead formation. Conversely, high flow rate may result in a larger volume of solution being ejected within the same time period. The excess solution that cannot be adequately stretched by the electric field may form beads due to surface tension [17]. Therefore, the flow rate can affect the bead formation through different approaches, a possible explanation for our finding (i.e. decrease, followed by increase, in bead count when the flow rate increased).
Although several previous studies have reported some types of linear relationships between applied voltage or feed rate and bead formation, the present study revealed a more complex influence of these parameters on bead development. Optimal values of each parameter are required to achieve bead-free fiber formation.
This research has been supported by North Khorasan University of Medical Sciences grant No. 4010088.
Conflict of Interest Statement
Authors declare that no competing financial interests exist.
The artificial intelligence–based chatbot ChatGPT (OpenAI, USA) was used to assist in proofreading the manuscript. All suggested corrections were carefully reviewed/revised by the authors, and only the selected modifications were incorporated into the final version.
