Fabrication and characterization of fibrous HAP/PVP/PEO composites prepared by sol-electrospinning

Abstract

Fibrous composites of hydroxyapatite (HAP)/polyvinylpyrrolidone (PVP)/polyethylene oxide (PEO) with good mechanical properties were successfully prepared by a sol-electrospinning process. As HAP nanoparticles could not disperse well in polymer aqueous solution, a modified procedure was presented to firstly prepare HAP sol. The PVP and PEO polymers were then directly dissolved in the HAP sol solution for electrospinning, which was based on completely miscible solutions. The morphology and structure of the electrospun fibrous composites were investigated by XRD, SEM, TEM and FTIR. It was of interest to observe that large numbers of HAP nanoneedles were preferentially oriented parallel to the longitudinal direction of the electrospun PVP–PEO nanofibers when a relatively small amount of HAP was used. When there was a large amount of HAP, the agglomeration of needle-like HAP particles could stretch out of the fibers. This could reduce the electrical charge carried by the liquid jet, and make some agglomerated HAP particles randomly arrange on the edge of the jet, with the result that some protuberances existed on the surface of the fibers. Mechanical testing demonstrated that the incorporation of HAP into the PVP–PEO matrix led to significantly better tensile properties compared to those of the pure electrospun polymer membrane, and the incorporation of 60 wt% HAP into the matrix of PEO–PVP nanofibers led to a higher tensile strength of 19.20 ± 0.09 MPa and a percentage of elongation of 11.64 ± 0.31%, respectively. Based on the study, the combination of PVP–PEO and HAP could be promising for application as scaffolds for bone tissue engineering.

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1. Introduction

A typical example of an inorganic and organic composite for a viable clinical alternative to bone autografting consists of minerals (hydroxyapatite, HAP) and a biocompatible polymer matrix. The inorganic compounds in the composite exhibit superior mechanical properties, bone-bonding ability, and unique osteoconductive function, while the matrix polymers offer flexibility and stability of structure to the design. The composites have been shown to combine features of HAP and the polymers, including favorable mechanical strength, in vivo osteoconductivity, processability, and biodegradability.^1–6 In order to mimic the intimate inorganic–organic structure in bone, HAP/polymer composites have now been extensively prepared using various fabrication methods such as electrospinning, chemical precipitation, an alternate soaking method, and self-assembly. Among these, electrospinning has received a great deal of attention in recent years and is by far the most prevalent method used. In this simple and continuous process, a charged polymer solution flows out of a syringe/needle setup and accelerates toward a collector, resulting in the formation of nonwoven random nanofibers. The electrospun fibers have a large surface area, good porosity, and a well interconnected pore network structure for cell adhesion and growth.^7–16 Moreover, electrospun fibrous matrices with a nanoscale diameter mimic the morphological nanofeatures of native extracellular matrices. Representative biodegradable polymers, including synthetic ones such as poly(lactic acid) (PLA), poly(D,L-lactide-co-glycolide) (PLGA), polyvinylpyrrolidone (PVP), and natural ones such as collagen, gelatin or chitosan, have been electrospun into nanofibers.^17–31 For example, Derrick R. Dean et al. have obtained aligned nanofibrous scaffolds by an electrospinning process based on poly(D,L-lactide-co-glycolide) (PLGA) and nano-hydroxyapatite (nano-HAP) dispersed in 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) and sonicated to disrupt possible agglomerates.³² Gao CY et al. have successfully prepared biodegradable PLGA nanofibrous composite scaffolds incorporated with hydroxyapatite particles using tetrahydrofuran (THF)–N,N-dimethylformamide (DMF) as a solvent by the electrospinning method.³³ In short, to obtain improved biodegradable polymer/HAP composites with conventional methods, previous researchers often dissolve the prepared HAP particles or commercial inorganic powder in an organic solvent, such as HFP, THF or DMF, for the preparation of the electrospinning solutions. This inevitably introduces the toxic solvent residue into the resultant product. Moreover, agglomeration between the nanoparticles of the commercial inorganic powder or the dried powder tends to take place irreversibly due to their high surface energy, which would be disadvantageous for the re-dispersion of the nanoparticles in the polymer solution.³⁴ For example, Vinoy Thomas et al.³⁵ have prepared nanostructured biocomposite scaffolds with type I collagen and nanohydroxyapatite powder dissolved in HFIP by electrostatic cospinning. Here the minerals were intensively distributed on the surface of the collagen fiber matrix, which would lead to difficulty in shaping and poor mechanical properties due to the weak interfacial binding force between the two different components. To overcome the limitations and disadvantages of the above-mentioned methods, it is necessary, and also very important, to find a simple and benign solvent for the production of HAP/polymer nanofibers.

In this study, we have firstly synthesized translucent and stable HAP sol with calcium nitrate and ammonium phosphate, using sodium citrate as a dispersant. This wet HAP sol, without any dry processing, was liable to make a good blend with polymers. After adding the biocompatible PVP and PEO polymer powder and ethanol into the HAP sol, continuous and uniform fibers were successfully generated by the electrospinning method, and featured a well-developed composite structure of HAP nanoparticles dispersed in a PVP and PEO matrix. The effect of the constituent of the spinning solution on its spinnability and on the morphology of the composite fibers was investigated, and the influence of HAP content on the mechanical properties of the composite fibers was also discussed. Notably, this sol-electrospinning technique has several functions and benefits, including using the dispersant to produce HAP sol with an enhanced dispersibility of HAP in aqueous solution, and obtaining the composite fibers without using any organic and toxic solvents. This study has opened a new and green avenue to the synthesis of biodegradable polymer/HAP composites that have great potential in orthopedic applications and bone tissue engineering.

2. Materials and methods

Calcium nitrate (Ca(NO₃)₂·4H₂O, AR) and ammonium phosphate ((NH₄)₃PO₄·3H₂O, AR) used as the calcium and phosphorus precursors, respectively, were purchased from Tianjin Chemical Reagent Factory, China. Sodium citrate (C₆H₅Na₃O₇·2H₂O) as a dispersing agent was provided by Shanghai Chemical Reagent Factory, China. Polyvinylpyrrolidone (PVP; M_w = 1.3 × 10⁵, K88-96) and polyethylene oxide (PEO; M_w = 1.0–1.3 × 10⁵) were purchased from Aladdin Chemistry Co. Ltd. These chemicals were analytical grade reagents and were used as received, without further purification.

2.1. Synthesis of the HAP sol

HAP sol was firstly prepared as followed: 23.6 g of calcium nitrate and 20 g of sodium citrate were dissolved completely in 100 ml of deionized water under continuous stirring and the pH was adjusted to 12 with NH₄OH solution. Prior to carrying out the reaction, this sodium citrate solution was mixed into a Ca(NO₃)₂·4H₂O solution as a dispersing agent, to make the obtained HAP particles uniformly distribute during precipitation. Subsequently, a certain amount of ammonium phosphate with a molar ratio of Ca/P = 1.67 was dissolved in 100 ml of distilled water and then added dropwise to the above solution for a certain time. The pH of the reacting mixture was maintained in the range of 10–11 by the addition of NH₄OH solution. The resultant system was kept at 80 °C for 2.5 h under vigorous stirring to produce a light blue translucent hydroxyapatite sol with a HAP concentration of 0.055 g ml⁻¹. To conduct XRD characterization for the nanoHAP particles in the HAP sol, the HAP sol was purified and lyophilized to prepare a HAP powder.

2.2. Preparation of the HAP/PVP/PEO composite nanofibers

The as-prepared HAP sol was mixed with PVP and PEO polymers, as well as anhydrous ethanol at the given proportion, under stirring to get the final electrospinning solution. The electrospinning solution was fed from a 10 ml syringe with a 6-gauge blunt-tip needle attached. The syringe was mounted onto a syringe pump (LongerPump LSP02-1B, Hebei, China), and the needle was connected to a high-voltage power supply (Dingtong High Voltage Power Supply, DPS-100(50KV/50w), Dalian, China). Under 17 kV voltage, the fluid jet was injected out at a rate of 1.0 ml h⁻¹ and the resultant nanofibers were collected on an aluminum foil which was put at a distance of 15 cm away from the needle. After electrospinning for 3 h at 45–50 °C, HAP/PVP/PEO nanofibers were obtained.

2.3. Characterization of the samples

TEM images of the HAP nanoparticles in the HAP sol and electrospinning solution, as well as the eletrospun nanofibers, were obtained via transmission electron microscopy (TEM, JEM-2010; JEOL, Japan). The viscosity and conductivity of the spinning solution were measured by a rotational viscometer (Model NDJ-79, Shanghai, China). Surface tension was determined by the Wilhelmy plate method with a tensiometer (DCAT 21,Dataphysics, Germany). All measurements were made at 25 °C. Morphological characterization of the nanofibers was performed using a scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan) with a beam voltage at 10 kV. All samples were sputter-coated with gold before SEM observation. The phase structure of HAP in sol and the composite nanofibers was analyzed by X-ray diffraction (XRD, PANalytical X'Pert PRO Netherland) using Cu Kα radiation, in the range 20°–60°. The chemical bonding state of the HAP/PVP/PEO composite nanofibers was analyzed by Fourier-Transform Infrared Spectroscopy (FTIR) using a Thermo Scientific (Nexus 470) spectrometer. The pristine HAP/PVP/PEO nanofiber mats were cut into 30 mm × 10 mm rectangles with a thickness of 0.05 mm. The thickness of these nanofiber mats was measured using a micrometer. These test strips were measured using a YG-001N fiber tensile tester at 10 mm min⁻¹ crosshead speed with a 10 mm gauge length at room temperature, and the tensile strength and breaking elongation were calculated based on the obtained peak force from the instrument data. An average of five measurements was reported as the mean ± standard deviation for each sample.

3. Results and discussion

3.1. Characterization of the HAP sol

Fig. 1a shows that the HAP sol synthesized by the dispersant exhibited a translucent state, and could be left to stand for up to several days without any obvious segregation of its initial sol state. The HAP nanoparticles had a needle-like morphology (Fig. 1b) with a uniform width of about 10 nm and lengths of 50 nm or so. The crystallographic structure of the HAP nanocrystals was investigated by XRD as shown in Fig. 2, which shows that the peak intensity was proportional to the standard spectrum of Ref. Pattern Hydroxyapatite 00-001-1008. The crystallite size was calculated using Scherrer's equation as follows:
where τ is the average diameter in Å, β is the broadening of the diffraction line measured at half of its maximum intensity in radians, λ = 0.1542 nm, k = 0.9, and θ is Bragg's diffraction angle. Taking into account the broadening of each peak in the XRD, the crystallite size of the (002) and (310) planes of the HAP nanoparticles were calculated as 43.6 nm and 14.2 nm, respectively.

Fig. 1 A photograph of the HAP precursor sol (a), and a TEM micrograph of the HAP nanoparticles (b).

Fig. 2 An XRD pattern of the HAP sol.

3.2. Effect of the constituent of the spinning solution on its spinnability and on the morphology of the fibers

Many factors including electrospinning conditions (feeding rates, voltage, and collector distance, etc.) and spinning solution constituent (solution concentration, solvent composition and so on) may affect the electrospinnability of spinning solutions and the morphology of the resulting nanofibers. This study focused on the influences of the spinning solution constituent on nanofiber formation, which included the volume of ethanol added, the mass ratio of PEO and PVP and the amount of HAP.

3.2.1. The volume ratio of solution to ethanol. Because of the existence of salts introduced by the dispersant, the conductivity of the solution was assumed to be very high and therefore, its effect on the morphology of the electrospun fibers could be ignored. The viscosity and surface tension of the HAP/PVP/PEO electrospinning solution with different volume ratios of solution to ethanol are presented in Table 1. The results obtained show that the surface tension of the solutions decreased with an increase in the amount of ethanol present. As is well known, the surface tension coefficient of water at 293 K is 72.75 × 10⁻³ N m⁻¹, and that of ethanol is 22.32 × 10⁻³ N m⁻¹, which is lower than that of water. Therefore, we could adjust the volume of ethanol for different surface tensions to obtain good quality fibers. The viscosity of the solutions showed a tendency to decrease with an increase in the amount of water they contained, changing from 210 mPa s to 90 mPa s when the volume ratio of solution to ethanol was varied from 1.5 : 1 to 3 : 1.

Table 1 Viscosity and surface tension of the electrospinning solution with different volume ratios of solution to ethanol

Sample	Volume ratio of solution to ethanol	Viscosity (mPa s)	Surface tension (mN m^−1)
1	3 : 1	90	44.2
2	2 : 1	160	33.4
3	1.5 : 1	210	29.6

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Fig. 3 shows the SEM images of the electrospun fibers with volume ratios of solution to ethanol of 2 : 1 and 1.5 : 1. while keeping fixed mass ratios of PEO to PVP (4 : 6) and HAP to PEO + PVP (1 : 1). With ethanol volumes lower than 2 : 1, most beads were formed along the fiber. When the volume ratio of solution to ethanol was 2 : 1, a well-developed fibrous morphology could be obtained (as shown in Fig. 3a). As the ethanol volume increased, the fiber became thicker, and the bead-like formation was less pronounced, but the fibers seemed to form more adhesions (as shown in Fig. 3b). In the electrospinning process, there are two competing factors affecting the fiber morphology. On the one hand, the electric repulsive force makes the fibers thinner; on the other hand, the solvent surface tension makes the fibers thicker and causes many beads to form. The two factors have opposite effects on the formation of the fibers. As presented in Table 1, the surface tension of the spinning solution decreased from 44.2 mN m⁻¹ to 29.6 mN m⁻¹, while the volume ratio of solution to ethanol was varied from 3 : 1 to 1.5 : 1. Hence, the different volume ratios of solution to ethanol could change the surface tension of the electrospinning solvent and therefore influence the spinnability and morphology of fibers. Only with a suitable ethanol content could uniform and continuous fibers be obtained successfully. Lower ethanol volumes meant that the solution contained more water which needed to be evaporated, causing many beads to form. Higher volumes of ethanol in the solution reduced the surface tension of the spinning solutions, causing the water solvent fraction to drop and the solidification time to become shorter, and favoring the formation of smooth nanofibers. However, if the ethanol volume was too high (for example, 1.5 : 1, as shown in Fig. 3b), the viscosity of the spinning solution increased to 210 mPa s, and the electrospun fibers became unstable due to the great amount of adhesion occurring between the fibers, so the uniformity of the fibers also deteriorated.

Fig. 3 SEM images of the electrospun fibers with different volume ratios of solution to ethanol: (a) 2 : 1; (b) 1.5 : 1.

3.2.2. The content of PEO and PVP. Fig. 4 gives the SEM images of fibers obtained with mass ratios of PEO to PVP of 1 : 4, 4 : 6, 2 : 1 and 5 : 1, while keeping the volume ratio of solution to ethanol (2 : 1) and the mass ratio of HAP to PEO + PVP (1 : 1) constant. Fig. 4a shows the apparent shrinkage of fibers when the mass ratio of PEO–PVP was 1 : 4. Higher percentage loading of PVP resulted in severe moisture absorption, which led to adhesion between the fibers. A few fibers probably then stuck together to one, resulting in an increase of the average diameter. When the mass ratio of PEO–PVP was 4 : 6, a relatively smooth morphology of the HAP/PVP/PEO composite nanofibers occurred, as shown in Fig. 4b. With an increase of PEO content, intermittent and uneven surfaces were observed on the composite fibers (Fig. 4c–d). A number of particles were found on the fiber surfaces when the mass ratio of PEO to PVP was 2 : 1 (Fig. 4c), which were salt crystals from the hydroxyapatite sol. Interestingly, salt accumulation on fiber surfaces also increased as the PEO content was increased. At a mass ratio of 5 : 1, the surface of the fibers was entirely covered by a layer of salt particles (Fig. 4d).

Fig. 4 SEM images of the as-spun fibers with different mass ratios of PEO to PVP: (a) 1 : 4; (b) 4 : 6; (c) 2 : 1; (e) 5 : 1.

In this study, PVP and PEO polymers blended with hydroxyapatite were used as the co-spinning agents to facilitate the formation of HAP composite fibers. However, the mass ratio of PEO to PVP distinctly influenced the spinnability of the spinning solution and the morphology of the electrospun fibers. With a lower mass ratio of PEO and PVP, the viscosity of the spinning solution was relatively low, which was bad for fiber formation; furthermore, the composite fibers quickly showed obvious shrinkage at the relative humidity of 40–70%. One possible explanation for the phenomenon is that PVP, a kind of hydrophilic polymer, gave the prepared composite fibers a strong water-absorbing ability at ambient humidity, thereby resulting in the apparent shrinkage of the fibers. In order to overcome the limitation of moisture absorption by the composite fibers, more PEO polymer powder was added as a co-spinning agent. However, a higher PEO content can make particles flocculate in aqueous solution, and the higher the concentration of PEO, the more serious the flocculation of particles was. In addition, PEO and inorganic salts can form a complex with a high ionic conductivity,³⁶ which increases the charge density and electrical conductivity of the electrospun polymer solution. Along with the volatiling of solvent in the electrospinning process, this caused more and more salt crystals to attach to the fiber surfaces (Fig. 4d). Salt accumulation was a direct result of phase separation of the spinning solution during electrospinning. Most likely, this occurred at a critical concentration as the solvents evaporated during the process of electrospinning. Only at the right mass ratio of PEO–PVP (4 : 6), did the resultant composite nanofibers have a uniform and smooth morphology and could be kept from adsorbing moisture for a long time.

3.2.3. The content of HAP. Fig. 5 presents the SEM images of electrospun fibers with different mass ratios of HAP to PVP + PEO, while keeping the volume ratios of water to ethanol (2 : 1) and the mass ratio of PEO to PVP (4 : 6) constant. The fiber morphology varied slightly with the different mass ratios of HAP to PVP + PEO, although the surface of the fibers became relatively rough with increasing HAP content. The roughness of the fiber surface was caused by an incomplete alignment of the needle-like HAP nanoparticles dispersed in the polymer matrix, which made part of the HAP nanoparticles stretch out of the fibers, resulting in the presence of some protuberances on the surface. This deduction can be confirmed by the TEM micrographs of electrospun composite fibers with and without addition of HAP, as given in Fig. 6. Compared to the smooth surface of the pure PVP–PEO fibers (Fig. 6a), incorporating HAP nano-needles into PVP–PEO had a significant influence on the morphology of the composite fibers (Fig. 6b). It can be seen that most of the HAP needles were held along the inside of the PVP–PEO, and that the majority of HAP needles were arranged in the fiber direction. However, some of the HAP nanoparticles were out of order and stretched out of the fibers, forming protuberances on the fiber surfaces and making the fiber surfaces roughen.

Fig. 5 SEM images of electrospun fibers with different mass ratios of HAP to PVP + PEO (a) 1 : 1 (b) 1.4 : 1, (c) 1.8 : 1, and (d) 2.2 : 1.

Fig. 6 TEM micrographs for electrospun PVP–PEO fibers with a mass ratio of 4 : 6 (a) and composite fibers with a mass ratio of HAP to PVP + PEO equal to 1.4 : 1 (b).

The increase in the degree of roughness of the composite fibers with an increasing mass ratio of HAP to PVP + PEO (as shown in Fig. 5) can be related to the extent of the aggregation of the needle-like HAP nanoparticles in the electrospinning solution, which was characterised by TEM, as shown in Fig. 7. When the mass ratios of HAP to PVP + PEO was 1 : 1, it was shown that the discrete needle-like HAP nanocrystallines uniformly dispersed within the PVP–PEO matrix (Fig. 7a). When the HAP content increased to 1.4 : 1, the electrospinning solution was shown to have obviously agglomerated particles (Fig. 7b). The agglomeration of the HAP particles was even greater when the HAP content increased from 1.4 : 1 to 1.8 : 1 (Fig. 7b–c). This phenomenon was most pronounced at a HAP content of 2.2 : 1, where the HAP nanoparticles agglomerated into much larger ones (Fig. 7d).

Fig. 7 A TEM micrograph for HAP/PVP/PEO electrospinning solutions with different mass ratios of HAP to PVP + PEO (a) 1 : 1 (b) 1.4 : 1, (c) 1.8 : 1, and (d) 2.2 : 1, respectively.

The agglomeration of needle-like HAP particles could reduce the electrical charge carried by the liquid jet and thus induce a low stretching force in the process of electrospinning. This could cause some agglomerated HAP particles to become randomly arranged at the edge of jet, with the result that parts of the HAP particles were exposed on the fiber surfaces as the solvent evaporated. From the above results, it can be concluded that a good dispersion of HAP particles in the PVP–PEO solution favored a smooth surface for the electrospun fibers, and furthermore, the content of HAP in the polymer solution substantially influenced the final morphology of the electrospun fibers.

3.3. XRD of the HAP/PVP/PEO composite fibers

Fig. 8 shows the XRD spectra of HAP/PVP/PEO composite fibers with different mass ratios of HAP to PVP + PEO. Overall, the XRD patterns obtained from both composite fibers corresponded with the standard spectrum of Ref. Pattern Hydroxyapatite 00-001-1008, and most of the characteristic peaks of HAP were distinctly observed. When the mass ratio of HAP to PVP + PEO was 2.2/1, peaks of salt crystals were detected, which could possibly be attributed to an additional introduction of salts derived from the dispersing agent with the increase in HAP sol content. The XRD patterns presented notable line broadening and an overlap of peaks, implying that the HAP crystals have a small size and low crystallinity.

Fig. 8 XRD patterns of composite fibers with different mass ratios of HAP to PVP + PEO (a) 1 : 1 (b) 2.2 : 1.

3.4. FTIR of the HAP/PVP/PEO composite fibers

The FTIR spectra of PVP, PEO, HAP sol and 1.4 HAP/PVP + PEO composite fibers are given in Fig. 9. There are obvious absorption peaks at 2955, 1658, 1459 and 1288 cm⁻¹ for PVP (Fig. 9a), which can be assigned to the C–H, C=O, C–H (cyclic groups) and C–N vibrations, respectively.³⁷ PEO has a characteristic triplet (1148, 1110, and 1062 cm⁻¹) with a maximum at 1110 cm⁻¹, which is associated with C–O–C vibration. This triplet depends strongly on the crystallinity of PEO and the intermolecular interactions between C–O–C and other groups in PEO (Fig. 9b). In the FTIR spectrum of HAP sol (Fig. 9c), bands corresponding to PO₄³⁻ are observed at 1051, 603 and 561 cm⁻¹, while the peak at ∼3569 cm⁻¹ can be assigned to the hydroxyl group of HAP. Importantly, it was evident that in the spectrum of the HAP/PVP/PEO composite fibers (Fig. 9d) the characteristic peaks for PVP and HAP appeared, but the triplet of PEO completely disappeared. This disappearance could be attributed to the lack of PEO crystalline structures in the composite fibers, due to the strong interactions between HAP and PEO. Moreover, the HAP characteristic peak of the PO₄³⁻ stretching band in the range 450–700 cm⁻¹ was found in spectra Fig. 9c and d.

Fig. 9 FTIR spectra of PVP (a), PEO (b), HAP sol (c) and composite fibers with mass ratios of HAP to PVP + PEO equal to 1.4 (d).

3.5. Effect of HAP content on the mechanical properties of the electrospun fibers

The results of the mechanical tests are shown in Table 2. The tensile strength and percentage of elongation were 5.362 ± 0.21 MPa and 10.87 ± 2.24%, respectively, for pure PVP–PEO nanofibers. The tensile strength of HAP/PVP/PEO composite nanofibers increased with an increase in HAP content. However, it should be noted that the mechanical properties of HAP/PVP/PEO composite fibers do not vary monotonically with the nanoHAP content. A maximum occurs for the tensile strength at 60 wt% HAP content. The highest tensile strength of the HAP/PVP/PEO composite nanofibers and percentage of elongation were 19.20 ± 0.09 MPa and 11.64 ± 0.31%, respectively. The incorporation of HAP ranging from 10% to 60% increased the mechanical strength. This may be attributed to an increase in rigidity over the pure polymer, because HAP is a hard inorganic component. Furthermore, by increasing the amount of HAP up to 70 wt%, the tensile strength decreased to 17.36 ± 0.15 MPa and the percentage of elongation decreased to 8.49 ± 1.48%. This decrement is attributed to the agglomeration of HAP nanoparticles at higher content, which resulted in poor interface bonding of the nanoHAP particles with polymers. Furthermore, failure of the composite materials usually occurs at the interface of the polymer and the HAP.

Table 2 Mechanical properties of HAP/PVP/PEO composite fibers with different contents of HAP while keeping a fixed mass ratio of PEO to PVP (4 : 6)

Sample	HAP contents (%)	Tensile strength (MPa)	Percentage of elongation (%)
1	0	5.362 ± 0.21	10.87 ± 2.24
2	30	12.64 ± 0.24	16.24 ± 0.32
3	50	14.68 ± 0.31	12.56 ±1.98
4	55	16.42 ± 0.27	9.92 ± 0.42
5	60	19.20 ± 0.09	11.64 ± 0.31
6	70	17.36 ± 0.15	8.49 ± 1.48

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4. Conclusions

This paper reported the fabrication of HAP/PVP/PEO composite nanofibers with HAP sol and polymer PEO–PVP by a sol-electrospinning process. Sodium citrate was chosen for the dispersing agent to enable an enhanced dispersion of HAP in the aqueous solution; this yielded a homogeneous electrospinning solution with PVP and PEO polymer, and also improved the spinnability of the spinning solution. PEO–PVP additive could greatly reduce the hygroscopicity of the composite nanofibers, and continuous and uniform fibers with a good tensile strength could be obtained. A volume ratio of solution to ethanol of 2 : 1 and a mass ratio of PEO–PVP of 4 : 6 were most suitable to ensure production of relatively good fibers under certain electrospinning conditions. The nanocomposite fibers containing up to 60% HAP nanoparticles showed the highest tensile strength, of 19.20 ± 0.09 MPa with a percentage of elongation of 11.64 ± 0.31%. It was of interest to observe that the needle-like HAP nanocrystals were mainly aligned along the PVP–PEO fibers, which is very similar to the HAP in living bone. Therefore, it can be summarized that the electrospun HAP/PVP/PEO composite nanofibers could be a promising biomaterial for bone tissue engineering.

Notes and references

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