Highly aligned narrow diameter chitosan electrospun nanofibers
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- Haider, S., Al-Zeghayer, Y., Ahmed Ali, F.A. et al. J Polym Res (2013) 20: 105. doi:10.1007/s10965-013-0105-9
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Random and highly aligned bead-free chitosan nanofibers (NFs) were successfully prepared via electrospinning by keeping the applied voltage (22 kV), flow rate (0.4 mL h−1), needle diameter (0.8 mm), and needle to collector distance (100 mm) constant while varying the solution concentration and collector rotation speed. No electrospinning was observed for lower solution concentrations, i.e., 1–3 wt% (w/v), whereas a decrease in the number and size of beads and microspheres, and bead-free NFs were obtained when the concentration of solution was increased from 4 to 6 wt%. Increase in the polymer concentration increased the solution viscosity (from 3.53 to 243 mPa s) and conductivity (from 29.80 to 192.00 μs cm−1) to critical values, which led to beadless NFs. The optimized conditions (i.e., concentration of solution 6 wt%, applied electrical potential 22 kV, flow rate 0.4 mL h−1, needle diameter 0.8 mm, and needle to collector distance 100 mm) were further used for the alignment of chitosan NFs. The alignment of the NFs increased from 35.6 to 94.4 % and the diameter decreased from 163.9 to 137.4 nm as the rotation speed of the cylindrical collector drum was increased from 2.09 to 21.98 m s−1. The aligned and small diameter chitosan NFs might find potential applications in biomedical, environmental, solar fuel cell applications, etc. Several target devices and polymer systems in the literature have been used to obtain aligned NFs; however, almost no work has been reported on individual chitosan alignment.
Electrospinning is a versatile technique for producing fibers with diameters between 10 and 200 nm and in some cases to micrometers . Although the first ever patents on the electrical dispersion of fluids and fabrication of textile yarns from electrically dispersed fluids were registered in 1902 and 1934 , however true efforts using this technique started in the 1990s [3–5]. In 2011, a variety of organic and inorganic materials were not only successfully electrospun but were also used in industrial processes and products . A typical electrospinning assembly consists of four components: (i) a pump, which holds a syringe containing polymer solution, and allows controlled outflow of polymer solution; (ii) a high voltage supply; (iii) a metallic capillary (needle), connected to syringe and a positive voltage; and (iv) a metallic collector (flat or rotation drum), connected to negative voltage . During electrospinning, as the voltage is applied, the solution drop moves to the tip of the needle, the hemispherical shape of the droplet is destabilized by charges that accumulate on its surface, and it is converted to a Taylor’s cone. At a critical voltage, the electric forces overcome the surface tension on the droplet and a jet of ultra-fine fibers in the range of 10–200 nm or in some cases up 600 nm is produced from the tip of the Taylor’s cone . The bending instability of the electrified polymer jet during the stretching process causes the nonwoven NFs to be collected in a random orientation on the collector . Owing to their fine diameter , large surface area per unit mass, high porosity, high gas permeability, and small interfibrous pore size , the randomly oriented NFs mats have found applications in filtration , wound dressing , drug delivery systems , and tissue engineering scaffolds . More recently, researchers have shown that in addition to random orientation, NFs could also be electrospun in an aligned orientation. The aligned NFs showed enhanced mechanical, electrical, and optical properties when compared to randomly oriented NFs. Therefore these could find more advanced applications in solar cells, fuel cells [15–17], biomedical engineering for bioactive protein delivery , improve growth of nerve or muscle cells [19–21], wound dressing, and chemical and biological sensors . A number of modified electrospinning assemblies were proposed [23–29] and used for the alignment of mainly synthetic polymers such as polycaprolactone (PCL), polyethylene oxide (PEO), poly(methyl methacrylate) (PMMA), polystyrene (PS), nylon-6, polyvinyl alcohol (PVA), polyacrylonitrile (PAN), and their blends with chitosan and chitosan derivatives (e.g., chitosan/PCL, chitosan/PEO, chitosan/PVA, and carboxymethyl chitosan/PVA [20, 21, 29, 30]). They showed excellent cellular biocompatibility which proves that chitosan and chitosan-based materials could be used in biomedical engineering [20, 21]. Chitosan, a cationic polysaccharide polymer, has been intensively studied owing to its abundance in nature, cheap availability, and potential biomedical applications . However, there are almost no reports on its individual alignment. In some instances it has been blended with synthetic polymers [29, 30]. In this work we studied the alignment of chitosan NFs. We believe that our work will give researchers further insight into the utilization of chitosan NFs for more advanced applications.
Materials and methods
Medium molecular weight chitosan powder, trifluoroacetic acid (CF3COOH, TFA), absolute ethanol (C2H5OH), and acetone (C3H6O) were purchased from Sigma-Aldrich, Alfa Aesar, Paneac Quimica SAU, and Scharlab S.L., respectively. All the chemicals were of analytical grade and were used without further purification. The basic electrospinning setup NANO-1A from Japan was used for the alignment of chitosan NFs.
Electrospinning of solutions
Chitosan solutions at various concentrations (e.g., 1, 3, 4, 6, 8 wt% (w/v)) were prepared by dissolving chitosan in TFA using a sonicator bath (Branson 2510) at 55 °C for 90 min. After dissolution, the solutions were stirred for 15 min using magnetic stirrers (Cerastir 30539) and finally filtered through a mesh with 0.063 mm pore size to obtain homogeneous solutions and remove any undissolved particles. For electrospinning the solutions were added to a 5-mL plastic syringe of 10 mm diameter equipped with a stainless steel needle of 0.8 mm diameter. The syringe was placed in a syringe pump and the needle was connected to a high voltage supply, which could generate voltages of up to 30 kV. In this study, the applied voltage was 22 kV, flow rate was 0.4 mL h−1, the distance between needle tip and collector was 100 mm, and the speed of the cylindrical collector was changed from 2.09 to 21.98 m s−1. After electrospinning the NFs mats were removed from the rotating drum, dried in a vacuum oven (DP63, Yamata Scientific Co. Ltd) at 60 °C and at −0.1 MPa, and then stored in a desiccator until characterization.
Morphology of NFs
Morphologies of the aligned NFs samples were studied by field emission scanning electron microscopy (FE-SEM) (JSM-7600F). To study the surface morphology of the electrospun NFs via FE-SEM, NFs samples were fixed onto a holder with the aid of carbon tape and then placed in the sputtering machine for platinum coating (to increase their electrical conductivity). After platinum coating the electrospun NFs were examined by FE-SEM under high vacuum.
Measurement of diameter and alignment angle
To measure the diameter and alignment angle of the NFs, Adobe Photoshop software was used. Approximately 250 NFs were randomly selected and their diameter and angle of alignment were measured. Photoshop was used to measure the diameter which was numerically converted using the scale bar on the FE-SEM micrograph. The alignment angle was considered as a reference angle (zero angle) and all deviations were measured relative to that angle.
Results and discussion
Effect of solution concentration
Relationship between chitosan concentration and morphology of NFs at constant flow rate, needle diameter, applied voltage, and needle tip to collector distance
Optimal parameters for the fabrication of random and aligned NFs
0.8 mL h−1
Needle to collector distance
243.00 mPa s
192.00 μs cm−1
Collector design and alignment of NFs
Chitosan NF alignment
Effect of rotation speed on NF diameter
Aligned and bead- free individual chitosan NFs were successfully prepared via electrospinning by controlling solution concentration and collector rotation speed, while keeping the applied voltage, flow rate, needle diameter, and needle to collector distance constant. No electrospinning was observed for lower solution concentration samples, i.e., 1–3 wt%, whereas a decrease in the number and size of the beads and microspheres, and bead-free NFs were obtained when the concentration of solution was increased to 6 wt%. Increase in the polymer concentration increased the solution viscosity (from 3.53 to 243 MPa s) and conductivity (from 29.80 to 192.00 μs cm−1) to critical values, which led to beadless NFs. The optimized conditions were further used to align individual chitosan NFs. The alignment of the NFs increased from 35.6 to 94.4 % and the diameter decreased from 163.9 to 137.4 nm as the rotation speed of the cylindrical collector drum increased from 2.09 to 21.98 m s−1. The aligned and small diameter chitosan NFs might find potential applications in solar cells, fuel cells, biomedical engineering for bioactive protein delivery, improve growth of nerve or muscle cells, wound dressing, and chemical and biological sensors, etc.
Financial support from the National Plan for Science and Technology, King Saud University, Saudi Arabia under the grant 09-NANO869-02 is greatly acknowledged.