Effect of electrospinning process parameters of polycaprolactone and nanohydroxyapatite nanocomposite nanofibers

Electrospinning is a relatively cost-effective technique for producing nanofibers in the nanometer to micron diameter range. In the electrospinning process, a polymer solution held by its surface tension at the end of a capillary tube is subjected to an electric field and an electric charge is included on the liquid surface due to this electric field. When the applied electric field reaches a critical value, the repulsive electrical forces overcome the surface tension forces. Eventually, a charged jet of the solution is ejected from the tip of the Taylor cone and an unstable jet occurs in the space between the capillary tip and collector, which leads to polymer fiber formation by solvent evaporation.^1,2

The main feature of the electrospinning process is that it is a simple means to prepare continuous fibers with unusually large surface to volume ratios and porous structures. ³ Electrospun fibers have been applied in various fields, such as nanocatalysis, tissue engineering scaffolds, filtration and biomedical, pharmaceutical, optical and environmental engineering.^4–6 Several reports have shown that the electrospun nanofibrous scaffolds serve as a better environment for cell attachment and proliferation, since they resemble the extracellular matrix (ECM).^7–11 This special architecture can affect the tissue-specific cell morphology, function and mechanical properties. Mimicking the nanoscale structure of the ECM is an effective strategy to design and develop tissue-engineered scaffolds. ¹² Electrospinning is a well-known technique to produce nanofibers and has been used by many researchers to make a nanofibrous matrix for tissue engineering applications.^13–16 Nanocomposites based on hydroxyapatite (HA) nanoparticles and biopolymers have attracted attention for their good osteoconductivity, osteoinductivity, biodegradability and high mechanical strengths. The nano-sized HA (nHA) may have other special properties due to its small size and huge specific surface area. Webster et al. ¹⁷ reported a significant increase in protein adsorption and osteoblast adhesion on nHA. Electrospun polymer/HA composites, such as collagen, gelatin and poly(L-lactic acid), are being investigated by various research groups for bone tissue engineering.^18,19 The co-precipitation of HA nanocrystals in soluble collagen has met with partial success in the fabrication of HA–collagen nanocomposites similar to the nanostructure of real bone, although with weaker mechanical properties. On the other hand, Wang and Li ²⁰ produced carbonate-substituted HA–chitosan silk fibroin composites with better compressive strength, mimicking real bone.

Polycaprolactone (PCL) is a semi-crystalline aliphatic polyester with sustained biodegradability, good biocompatibility and expectant mechanical strength. ²¹ nHA is a ceramic material with a composition and structure close to natural bone mineral. ²² Therefore, there are increasing interests in the preparation and investigation of porous PCL/nHA composite as scaffolds for bone tissue engineering.^22,23 A number of recent studies have reported the differentiation of Mesenchymal stem cells (MSCs) into mineralizing osteoblasts when cultured on electrospun nanofibers of PCL/nHA.^23,24 Prabhakaran et al. ²⁵ produced poly-L-lactide (PLLA)/collagen/HA nanofibrous scaffolds and the in vitro biocompatibility of them was assessed by growing human fetal osteoblasts (hFOBs). The results of the study showed that prepared nanocomposite scaffolds could be a potential substrate for the proliferation and mineralization of osteoblasts, enhancing bone regeneration. Considering the influence of fiber size and morphology on the hydrophobic behavior of polymers, ²⁶ controlling the mean fiber diameter (MFD), which is a function of process parameters, is essential. Several parameters, including polymer solution parameters (molecular weight, concentration, etc.), processing parameters, such as applied voltage, flow rate and tip to collector distance, and also ambient conditions affect the morphology and fiber diameter of nanofibers. ²⁷

Numerous studies have reported that the processing parameters affect the morphology and fiber diameter of electrospun fibers. You et al. ²⁸ investigated the effect of solution parameters on poly(lactic-co-glycolic acid) (PLGA) nanofibrous mats. Demir et al. ²⁹ reported that the fiber diameter increased with increasing polymer concentration according to a power law relationship and Deitzel et al. ³⁰ reported a bimodal distribution of fiber diameter for fibers spun from higher concentration solution. Boland et al. ³¹ obtained a strong relationship between fiber diameter and concentration in the electrospinning of poly(glycolic acid) (PGA). Reneker and Chun ³² obtained a result that fiber diameter did not change much with electric field during the electrospinning of polyethylene oxide. However, Demir et al. ²⁹ reported that fiber diameter increased with increasing applied voltage when they electrospun polyurethane fibers. In this study, poly(ɛ-caprolactone) polymer solutions with different amounts of nHA were used to produce nanofiber mats (NFMs) by electrospinning. The effects of process parameters on average fiber diameter were investigated. The reported ranges of parameters were selected from a previous study. ³³

Results and discussion

nHA concentration effect

The electrospun composite nanofibers were successfully prepared at different nHA concentrations. SEM images of composite fibers with different amounts of nHA are shown in Figure 1. nHA concentrations from 1 to 20 w/w% have been studied to find their effect on MFD of prepared nanofibers. Spinning distance, voltage and flow rate were kept constant at 15 cm, 19 kV and 0.5 mL/h, respectively. The average diameter of fibers decreased from 765 to 387 nm with increasing nHA concentration from 1 to 15 w/w% and then increased to 422 nm at 20 w/w%. Increasing the nHA content initially decreased the fiber diameter, probably due to the increase in conductivity of the solution and the surface charge density of the solution jet. However, at higher concentrations, the diameter increased. This is probably due to the nHA agglomeration, which prevents the formation of continuous fibers and also results in thicker fiber production. For example, at nHA concentration of 30%, no uniform fiber can be seen, because of nHA agglomeration (Figure 2). At higher concentrations, however, there are extensive chain entanglements, resulting in higher viscoelastic forces, which tend to resist against the electrostatic stretching force. OPEN IN VIEWER

Figure 1.

Scanning electron microscopy images of polycaprolactone/nanohydroxyapatite composite nanofibers at different concentrations: (a) 1 w/w%; (b) 5 w/w%; (c) 10 w/w%; (d) 15 w/w%; (e) 20 w/w% (voltage = 19 kV, distance = 5 cm, flow rate = 0.5 mL/h).

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Figure 2.

Scanning electron microscopy image of composite nanofibers at concentration of 30% (w/w).

Applied voltage effect

Figure 3 shows SEM images of prepared nanofibers at different voltages. Spinning distance, nHA concentration and flow rate were kept constant at 5 cm, 5 w/w% and 0.5 mL/h, respectively. Applied voltage changed from 10 to 22 kV. MFD decreased from 489 to 410 nm with increasing applied voltage from 10 to 19 kV and then increased to 437 nm at 22 kV. Increasing the applied voltage draws much more solution out of the capillary. So, the fiber diameter will be increased with increasing applied voltage, as reported by Demir et al., ²⁹ Baker et al. ³⁴ and Zhang et al. ³⁵ On the other hand, if electrostatic force causes more extension of the jet in comparison to the flow rate, the fiber diameter would decrease with increasing applied voltage.^36,37 The balance between these two effects will determine final diameter of electrospun fibers. OPEN IN VIEWER

Figure 3.

Scanning electron microscopy images of polycaprolactone/nanohydroxyapatite composite nanofibers at different voltages: (a) 10 kV; (b) 13 kV; (c) 16 kV; (d) 19 kV; (e) 22 kV (concentration = 5 w/w%, distance = 5 cm, flow rate = 0.5 mL/h).

Spinning distance effect

Figure 4 shows the effect of spinning distance (5–25 cm) on the morphology and fiber diameter of composite nanofibers. MFD decreased from 597 to 306 nm with increasing spinning distance from 5 to 25 cm. Varying the distance has a direct influence on the jet flight time as well as electric field strength. Longer spinning distance will provide more time for the jet to stretch in the electric field before it is deposited on the collector. Furthermore, solvents will have more time to evaporate. Hence, the fiber diameter will be prone to decrease. Decrease in fiber diameter with increasing spinning distance was reported in the literature.^34,38 There were also some cases in which spinning distance did not have a significant influence on fiber diameter.^39–41 OPEN IN VIEWER

Figure 4.

Scanning electron microscopy images of polycaprolactone/nanohydroxyapatite composite nanofibers at different spinning distances: (a) 5 cm; (b) 10 cm; (c) 15 cm; (d) 20 cm; (e) 25 cm (concentration = 5 w/w%, voltage = 19 kV, flow rate = 0.5 mL/h).

Flow rate effect

Figure 5 shows the SEM images of PCL/nHA nanofibers produced at various flow rates. Concentration, voltage and spinning distance were fixed at 5 w/w%, 19 kV and 5 cm, respectively. With increasing the flow rate from 0.1 to 5 mL/h, the average fiber diameter slightly increased from 391 to 652 nm. Zong et al. ⁴² showed the same effect in the electrospinning of poly-D-lactide (PDLA). Increasing flow rate caused the suspended droplet at the needle tip to become larger; thus, the diameter of the polymer jet and fibers increased. At low flow rates, solvent would have sufficient amount of time to evaporate until the fibers are collected on the plate, and thinner and uniform fibers were produced. At high feed rate, which seems above the quasi-stable point, the solution was not completely carried away to the collector, which resulted in an unstable jet and larger fibers. ⁴³ OPEN IN VIEWER

Figure 5.

Scanning electron microscopy images of polycaprolactone/nanohydroxyapatite composite nanofibers at different flow rates: (a) 0.1 mL/h; (b) 0.5 mL/h; (c) 1 mL/h; (d) 3 mL/h; (e) 5 mL/h (concentration = 5 w/w%, voltage = 19 kV, distance = 5 cm).

FTIR analysis

FTIR spectra of electrospun nHA, PCL and PCL/nHA nanofibrous scaffolds are shown in Figure 6. Typical infrared bands for PCL-related stretching modes are observed for the PCL and PCL/HA scaffolds. These include 2923 cm⁻¹ (asymmetric CH2 stretching), 2857 cm⁻¹ (symmetric CH2 stretching), 1720 cm⁻¹ (carbonyl stretching) and 1293 cm⁻¹ (C–O and C–C stretching in the crystalline phase). PO₄⁻³ absorption bands attributed to HA nanoparticles can be seen for each of the nHA and PCL/nHA scaffolds. These PO₄⁻³ bands have appeared at 569 cm⁻¹ and 1031 cm⁻¹. For PCL/nHA nanofibers, all characteristic peaks corresponding to PCL and nHA appear, indicating successful incorporation of nHA into PCL/nHA composite nanofibers. OPEN IN VIEWER

Figure 6.

Fourier transform infrared spectra for (a) nanohydroxyapatite (nHA), (b) polycaprolactone (PCL)/nHA and (c) PCL.