RSC Advances

PAPER

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article OnlineView Journal | View Issue

aCollege of Chemistry and Molecular Engine

450001, China. E-mail: lizhongjun@zzu.ed

0371-67732205bInstitute of Environmental & Municipal Eng

Resources and Electric Power, Zhengzhou, 4

163.com; Fax: +86-0371-67732205; Tel: +86

Cite this: RSC Adv., 2014, 4, 16731

Received 30th November 2013Accepted 13th February 2014

DOI: 10.1039/c3ra47168c

www.rsc.org/advances

This journal is © The Royal Society of C

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

Yuanyuan Zhou,ab Pengwei Qi,a Zhihua Zhao,a Qian Liua and Zhongjun Li*a

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.

1. Introduction

A typical example of an inorganic and organic composite for aviable clinical alternative to bone autograing consists ofminerals (hydroxyapatite, HAP) and a biocompatible polymermatrix. The inorganic compounds in the composite exhibitsuperior mechanical properties, bone-bonding ability, andunique osteoconductive function, while the matrix polymersoffer exibility and stability of structure to the design. Thecomposites have been shown to combine features of HAP andthe polymers, including favorable mechanical strength, in vivoosteoconductivity, processability, and biodegradability.1–6 Inorder to mimic the intimate inorganic–organic structure inbone, HAP/polymer composites have now been extensivelyprepared using various fabrication methods such as electro-spinning, chemical precipitation, an alternate soaking method,and self-assembly. Among these, electrospinning has received a

ering, Zhengzhou University, Zhengzhou,

u.cn; Fax: +86-0371-67732205; Tel: +86-

ineering, North China University of Water

50011, China. E-mail: zhouyuanzy2004@

-0371-67732205

hemistry 2014

great deal of attention in recent years and is by far the mostprevalent method used. In this simple and continuous process,a charged polymer solution ows out of a syringe/needle setupand accelerates toward a collector, resulting in the formation ofnonwoven random nanobers. The electrospun bers have alarge surface area, good porosity, and a well interconnectedpore network structure for cell adhesion and growth.7–16 More-over, electrospun brous matrices with a nanoscale diametermimic the morphological nanofeatures of native extracellularmatrices. Representative biodegradable polymers, includingsynthetic ones such as poly(lactic acid) (PLA), poly(D,L-lactide-co-glycolide) (PLGA), polyvinylpyrrolidone (PVP), and natural onessuch as collagen, gelatin or chitosan, have been electrospuninto nanobers.17–31 For example, Derrick R. Dean et al. haveobtained aligned nanobrous scaffolds by an electrospinningprocess based on poly(D,L-lactide-co-glycolide) (PLGA) and nano-hydroxyapatite (nano-HAP) dispersed in 1,1,1,3,3,3-hexauoro-2-propanol (HFP) and sonicated to disrupt possible agglomer-ates.32 Gao CY et al. have successfully prepared biodegradablePLGA nanobrous composite scaffolds incorporated withhydroxyapatite particles using tetrahydrofuran (THF)–N,N-dimethylformamide (DMF) as a solvent by the electrospinningmethod.33 In short, to obtain improved biodegradable polymer/HAP composites with conventional methods, previous

RSC Adv., 2014, 4, 16731–16738 | 16731

RSC Advances Paper

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

researchers oen dissolve the prepared HAP particles orcommercial inorganic powder in an organic solvent, such asHFP, THF or DMF, for the preparation of the electrospinningsolutions. This inevitably introduces the toxic solvent residueinto the resultant product. Moreover, agglomeration betweenthe nanoparticles of the commercial inorganic powder or thedried powder tends to take place irreversibly due to their highsurface energy, which would be disadvantageous for the re-dispersion of the nanoparticles in the polymer solution.34 Forexample, Vinoy Thomas et al.35 have prepared nanostructuredbiocomposite scaffolds with type I collagen and nano-hydroxyapatite powder dissolved in HFIP by electrostaticcospinning. Here the minerals were intensively distributed onthe surface of the collagen ber matrix, which would lead todifficulty in shaping and poor mechanical properties due to theweak interfacial binding force between the two differentcomponents. To overcome the limitations and disadvantages ofthe above-mentioned methods, it is necessary, and also veryimportant, to nd a simple and benign solvent for theproduction of HAP/polymer nanobers.

In this study, we have rstly synthesized translucent andstable HAP sol with calcium nitrate and ammonium phosphate,using sodium citrate as a dispersant. This wet HAP sol, withoutany dry processing, was liable to make a good blend withpolymers. Aer adding the biocompatible PVP and PEO poly-mer powder and ethanol into the HAP sol, continuous anduniform bers were successfully generated by the electro-spinning method, and featured a well-developed compositestructure of HAP nanoparticles dispersed in a PVP and PEOmatrix. The effect of the constituent of the spinning solution onits spinnability and on the morphology of the composite berswas investigated, and the inuence of HAP content on themechanical properties of the composite bers was also dis-cussed. Notably, this sol-electrospinning technique has severalfunctions and benets, including using the dispersant toproduce HAP sol with an enhanced dispersibility of HAP inaqueous solution, and obtaining the composite bers withoutusing any organic and toxic solvents. This study has opened anew and green avenue to the synthesis of biodegradable poly-mer/HAP composites that have great potential in orthopedicapplications and bone tissue engineering.

2. Materials and methods

Calcium nitrate (Ca(NO3)2$4H2O, AR) and ammonium phos-phate ((NH4)3PO4$3H2O, AR) used as the calcium and phos-phorus precursors, respectively, were purchased from TianjinChemical Reagent Factory, China. Sodium citrate(C6H5Na3O7$2H2O) as a dispersing agent was provided byShanghai Chemical Reagent Factory, China. Poly-vinylpyrrolidone (PVP; Mw ¼ 1.3 � 105, K88-96) and poly-ethylene oxide (PEO; Mw ¼ 1.0–1.3 � 105) were purchased fromAladdin Chemistry Co. Ltd. These chemicals were analyticalgrade reagents and were used as received, without furtherpurication.

16732 | RSC Adv., 2014, 4, 16731–16738

2.1. Synthesis of the HAP sol

HAP sol was rstly prepared as followed: 23.6 g of calciumnitrate and 20 g of sodium citrate were dissolved completely in100 ml of deionized water under continuous stirring and the pHwas adjusted to 12 with NH4OH solution. Prior to carrying outthe reaction, this sodium citrate solution was mixed into aCa(NO3)2$4H2O solution as a dispersing agent, to make theobtained HAP particles uniformly distribute during precipita-tion. Subsequently, a certain amount of ammonium phosphatewith a molar ratio of Ca/P ¼ 1.67 was dissolved in 100 ml ofdistilled water and then added dropwise to the above solutionfor a certain time. The pH of the reacting mixture was main-tained 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 vigorousstirring to produce a light blue translucent hydroxyapatite solwith a HAP concentration of 0.055 g ml�1. To conduct XRDcharacterization for the nanoHAP particles in the HAP sol, theHAP sol was puried and lyophilized to prepare a HAP powder.

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

The as-prepared HAP sol was mixed with PVP and PEO poly-mers, as well as anhydrous ethanol at the given proportion,under stirring to get the nal electrospinning solution. Theelectrospinning solution was fed from a 10 ml syringe with a6-gauge blunt-tip needle attached. The syringe was mountedonto 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 uid jet was injectedout at a rate of 1.0 ml h�1 and the resultant nanobers werecollected on an aluminum foil which was put at a distance of15 cm away from the needle. Aer electrospinning for 3 h at 45–50 �C, HAP/PVP/PEO nanobers were obtained.

2.3. Characterization of the samples

TEM images of the HAP nanoparticles in the HAP sol andelectrospinning solution, as well as the eletrospun nanobers,were obtained via transmission electronmicroscopy (TEM, JEM-2010; JEOL, Japan). The viscosity and conductivity of the spin-ning solution were measured by a rotational viscometer (ModelNDJ-79, Shanghai, China). Surface tension was determined bythe Wilhelmy plate method with a tensiometer (DCAT21,Dataphysics, Germany). All measurements were made at 25�C. Morphological characterization of the nanobers was per-formed using a scanning electron microscope (SEM, JSM-5600LV, JEOL, Japan) with a beam voltage at 10 kV. All sampleswere sputter-coated with gold before SEM observation. Thephase structure of HAP in sol and the composite nanobers wasanalyzed by X-ray diffraction (XRD, PANalytical X'Pert PRONetherland) using Cu Ka radiation, in the range 20�–60�. Thechemical bonding state of the HAP/PVP/PEO composite nano-bers was analyzed by Fourier-Transform Infrared Spectroscopy(FTIR) using a Thermo Scientic (Nexus 470) spectrometer. Thepristine HAP/PVP/PEO nanober mats were cut into 30 mm �10 mm rectangles with a thickness of 0.05 mm. The thickness of

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

these nanober mats was measured using a micrometer. Thesetest strips were measured using a YG-001N ber tensile tester at10 mm min�1 crosshead speed with a 10 mm gauge length atroom temperature, and the tensile strength and breakingelongation were calculated based on the obtained peak forcefrom the instrument data. An average of ve measurements wasreported as the mean � standard deviation for each sample.

Fig. 2 An XRD pattern of the HAP sol.

3. Results and discussion3.1. Characterization of the HAP sol

Fig. 1a shows that the HAP sol synthesized by the dispersantexhibited a translucent state, and could be le to stand for up toseveral days without any obvious segregation of its initial solstate. The HAP nanoparticles had a needle-like morphology(Fig. 1b) with a uniform width of about 10 nm and lengths of50 nm or so. The crystallographic structure of the HAP nano-crystals was investigated by XRD as shown in Fig. 2, whichshows that the peak intensity was proportional to the standardspectrum of Ref. Pattern Hydroxyapatite 00-001-1008. Thecrystallite size was calculated using Scherrer's equation asfollows:

s ¼ kl

b1=2 cos q

where s is the average diameter in A, b is the broadening of thediffraction line measured at half of its maximum intensity inradians, l ¼ 0.1542 nm, k ¼ 0.9, and q is Bragg's diffractionangle. Taking into account the broadening of each peak in theXRD, the crystallite size of the (002) and (310) planes of the HAPnanoparticles were calculated as 43.6 nm and 14.2 nm,respectively.

3.2. Effect of the constituent of the spinning solution on itsspinnability and on the morphology of the bers

Many factors including electrospinning conditions (feedingrates, voltage, and collector distance, etc.) and spinning solu-tion constituent (solution concentration, solvent compositionand so on) may affect the electrospinnability of spinning solu-tions and the morphology of the resulting nanobers. Thisstudy focused on the inuences of the spinning solutionconstituent on nanober formation, which included the volumeof ethanol added, the mass ratio of PEO and PVP and theamount of HAP.

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

This journal is © The Royal Society of Chemistry 2014

3.2.1. The volume ratio of solution to ethanol. Because ofthe existence of salts introduced by the dispersant, theconductivity of the solution was assumed to be very high andtherefore, its effect on the morphology of the electrospun berscould be ignored. The viscosity and surface tension of the HAP/PVP/PEO electrospinning solution with different volume ratiosof solution to ethanol are presented in Table 1. The resultsobtained show that the surface tension of the solutionsdecreased with an increase in the amount of ethanol present. Asis well known, the surface tension coefficient of water at 293 K is72.75 � 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 thevolume of ethanol for different surface tensions to obtain goodquality bers. The viscosity of the solutions showed a tendencyto decrease with an increase in the amount of water they con-tained, changing from 210 mPa s to 90 mPa s when the volumeratio of solution to ethanol was varied from 1.5 : 1 to 3 : 1.

Fig. 3 shows the SEM images of the electrospun bers withvolume ratios of solution to ethanol of 2 : 1 and 1.5 : 1. whilekeeping xedmass ratios of PEO to PVP (4 : 6) and HAP to PEO +PVP (1 : 1). With ethanol volumes lower than 2 : 1, most beadswere formed along the ber. When the volume ratio of solutionto ethanol was 2 : 1, a well-developed brous morphology couldbe obtained (as shown in Fig. 3a). As the ethanol volumeincreased, the ber became thicker, and the bead-like forma-tion was less pronounced, but the bers seemed to form moreadhesions (as shown in Fig. 3b). In the electrospinning process,there are two competing factors affecting the ber morphology.On the one hand, the electric repulsive force makes the bersthinner; on the other hand, the solvent surface tension makesthe bers thicker and causes many beads to form. The twofactors have opposite effects on the formation of the bers. Aspresented in Table 1, the surface tension of the spinning solu-tion decreased from 44.2 mN m�1 to 29.6 mN m�1, while the

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

SampleVolume ratio ofsolution to ethanol

Viscosity(mPa s)

Surface tension(mN m�1)

1 3 : 1 90 44.22 2 : 1 160 33.43 1.5 : 1 210 29.6

RSC Adv., 2014, 4, 16731–16738 | 16733

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

RSC Advances Paper

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

volume ratio of solution to ethanol was varied from 3 : 1 to1.5 : 1. Hence, the different volume ratios of solution to ethanolcould change the surface tension of the electrospinning solventand therefore inuence the spinnability and morphology ofbers. Only with a suitable ethanol content could uniform andcontinuous bers be obtained successfully. Lower ethanolvolumes meant that the solution contained more water whichneeded to be evaporated, causing many beads to form. Highervolumes of ethanol in the solution reduced the surface tensionof the spinning solutions, causing the water solvent fraction todrop and the solidication time to become shorter, andfavoring the formation of smooth nanobers. However, if theethanol volume was too high (for example, 1.5 : 1, as shown inFig. 3b), the viscosity of the spinning solution increased to210 mPa s, and the electrospun bers became unstable due tothe great amount of adhesion occurring between the bers, sothe uniformity of the bers also deteriorated.

3.2.2. The content of PEO and PVP. Fig. 4 gives the SEMimages of bers obtained with mass ratios of PEO to PVP of1 : 4, 4 : 6, 2 : 1 and 5 : 1, while keeping the volume ratio ofsolution to ethanol (2 : 1) and the mass ratio of HAP to PEO +PVP (1 : 1) constant. Fig. 4a shows the apparent shrinkage ofbers when the mass ratio of PEO–PVP was 1 : 4. Higherpercentage loading of PVP resulted in severe moisture absorp-tion, which led to adhesion between the bers. A few bersprobably then stuck together to one, resulting in an increase of

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

16734 | RSC Adv., 2014, 4, 16731–16738

the average diameter. When the mass ratio of PEO–PVP was4 : 6, a relatively smooth morphology of the HAP/PVP/PEOcomposite nanobers occurred, as shown in Fig. 4b. With anincrease of PEO content, intermittent and uneven surfaces wereobserved on the composite bers (Fig. 4c–d). A number ofparticles were found on the ber surfaces when the mass ratioof PEO to PVP was 2 : 1 (Fig. 4c), which were salt crystals fromthe hydroxyapatite sol. Interestingly, salt accumulation on bersurfaces also increased as the PEO content was increased. At amass ratio of 5 : 1, the surface of the bers was entirely coveredby a layer of salt particles (Fig. 4d).

In this study, PVP and PEO polymers blended withhydroxyapatite were used as the co-spinning agents to facilitatethe formation of HAP composite bers. However, the mass ratioof PEO to PVP distinctly inuenced the spinnability of thespinning solution and themorphology of the electrospun bers.With a lower mass ratio of PEO and PVP, the viscosity of thespinning solution was relatively low, which was bad for berformation; furthermore, the composite bers quickly showedobvious shrinkage at the relative humidity of 40–70%. Onepossible explanation for the phenomenon is that PVP, a kind ofhydrophilic polymer, gave the prepared composite bers astrong water-absorbing ability at ambient humidity, therebyresulting in the apparent shrinkage of the bers. In order toovercome the limitation of moisture absorption by thecomposite bers, more PEO polymer powder was added as a co-spinning agent. However, a higher PEO content can makeparticles occulate in aqueous solution, and the higher theconcentration of PEO, the more serious the occulation ofparticles was. In addition, PEO and inorganic salts can form acomplex with a high ionic conductivity,36 which increases thecharge density and electrical conductivity of the electrospunpolymer solution. Along with the volatiling of solvent in theelectrospinning process, this caused more and more salt crys-tals to attach to the ber surfaces (Fig. 4d). Salt accumulationwas a direct result of phase separation of the spinning solutionduring electrospinning. Most likely, this occurred at a criticalconcentration as the solvents evaporated during the process ofelectrospinning. Only at the right mass ratio of PEO–PVP (4 : 6),did the resultant composite nanobers have a uniform andsmooth morphology and could be kept from adsorbing mois-ture for a long time.

3.2.3. The content of HAP. Fig. 5 presents the SEM imagesof electrospun bers 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 bermorphology varied slightly with the different mass ratios of HAPto PVP + PEO, although the surface of the bers became rela-tively rough with increasing HAP content. The roughness of theber surface was caused by an incomplete alignment of theneedle-like HAP nanoparticles dispersed in the polymer matrix,which made part of the HAP nanoparticles stretch out of thebers, resulting in the presence of some protuberances on thesurface. This deduction can be conrmed by the TEM micro-graphs of electrospun composite bers with and without addi-tion of HAP, as given in Fig. 6. Compared to the smooth surfaceof the pure PVP–PEO bers (Fig. 6a), incorporating HAP nano-

This journal is © The Royal Society of Chemistry 2014

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

Fig. 7 A TEM micrograph for HAP/PVP/PEO electrospinning solutionswith 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.

Paper RSC Advances

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

needles into PVP–PEO had a signicant inuence on themorphology of the composite bers (Fig. 6b). It can be seen thatmost of the HAP needles were held along the inside of the PVP–PEO, and that the majority of HAP needles were arranged in theber direction. However, some of the HAP nanoparticles wereout of order and stretched out of the bers, forming protuber-ances on the ber surfaces and making the ber surfacesroughen.

The increase in the degree of roughness of the compositebers with an increasing mass ratio of HAP to PVP + PEO (asshown in Fig. 5) can be related to the extent of the aggregationof the needle-like HAP nanoparticles in the electrospinningsolution, which was characterised by TEM, as shown in Fig. 7.When the mass ratios of HAP to PVP + PEO was 1 : 1, it wasshown that the discrete needle-like HAP nanocrystallinesuniformly dispersed within the PVP–PEOmatrix (Fig. 7a). Whenthe HAP content increased to 1.4 : 1, the electrospinning solu-tion was shown to have obviously agglomerated particles(Fig. 7b). The agglomeration of the HAP particles was evengreater when the HAP content increased from 1.4 : 1 to 1.8 : 1(Fig. 7b–c). This phenomenon was most pronounced at a HAPcontent of 2.2 : 1, where the HAP nanoparticles agglomeratedinto much larger ones (Fig. 7d).

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

This journal is © The Royal Society of Chemistry 2014

The agglomeration of needle-like HAP particles could reducethe electrical charge carried by the liquid jet and thus induce alow stretching force in the process of electrospinning. Thiscould cause some agglomerated HAP particles to becomerandomly arranged at the edge of jet, with the result that partsof the HAP particles were exposed on the ber surfaces as thesolvent evaporated. From the above results, it can be concludedthat a good dispersion of HAP particles in the PVP–PEO solutionfavored a smooth surface for the electrospun bers, andfurthermore, the content of HAP in the polymer solutionsubstantially inuenced the nal morphology of the electro-spun bers.

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

Fig. 8 shows the XRD spectra of HAP/PVP/PEO composite berswith different mass ratios of HAP to PVP + PEO. Overall, theXRD patterns obtained from both composite bers corre-sponded with the standard spectrum of Ref. Pattern Hydroxy-apatite 00-001-1008, andmost of the characteristic peaks of HAPwere distinctly observed. When the mass ratio of HAP to PVP +PEO was 2.2/1, peaks of salt crystals were detected, which couldpossibly be attributed to an additional introduction of saltsderived from the dispersing agent with the increase in HAP sol

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

RSC Adv., 2014, 4, 16731–16738 | 16735

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

RSC Advances Paper

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

content. The XRD patterns presented notable line broadeningand an overlap of peaks, implying that the HAP crystals have asmall size and low crystallinity.

SampleHAP contents(%)

Tensile strength(MPa)

Percentage ofelongation (%)

1 0 5.362 � 0.21 10.87 � 2.242 30 12.64 � 0.24 16.24 � 0.323 50 14.68 � 0.31 12.56 �1.984 55 16.42 � 0.27 9.92 � 0.425 60 19.20 � 0.09 11.64 � 0.316 70 17.36 � 0.15 8.49 � 1.48

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

The FTIR spectra of PVP, PEO, HAP sol and 1.4 HAP/PVP + PEOcomposite bers are given in Fig. 9. There are obvious absorp-tion 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 characteristictriplet (1148, 1110, and 1062 cm�1) with a maximum at 1110cm�1, which is associated with C–O–C vibration. This tripletdepends strongly on the crystallinity of PEO and the intermo-lecular interactions between C–O–C and other groups in PEO(Fig. 9b). In the FTIR spectrum of HAP sol (Fig. 9c), bandscorresponding to PO4

3� are observed at 1051, 603 and 561 cm�1,while the peak at �3569 cm�1 can be assigned to the hydroxylgroup of HAP. Importantly, it was evident that in the spectrumof the HAP/PVP/PEO composite bers (Fig. 9d) the character-istic peaks for PVP and HAP appeared, but the triplet of PEOcompletely disappeared. This disappearance could be attrib-uted to the lack of PEO crystalline structures in the compositebers, due to the strong interactions between HAP and PEO.Moreover, the HAP characteristic peak of the PO4

3� stretchingband in the range 450–700 cm�1 was found in spectra Fig. 9cand d.

3.5. Effect of HAP content on the mechanical properties ofthe electrospun bers

The results of the mechanical tests are shown in Table 2. Thetensile strength and percentage of elongation were 5.362 �0.21 MPa and 10.87 � 2.24%, respectively, for pure PVP–PEOnanobers. The tensile strength of HAP/PVP/PEO compositenanobers increased with an increase in HAP content. However,it should be noted that the mechanical properties of HAP/PVP/PEO composite bers do not vary monotonically with thenanoHAP content. A maximum occurs for the tensile strength at60 wt% HAP content. The highest tensile strength of the HAP/PVP/PEO composite nanobers and percentage of elongationwere 19.20 � 0.09 MPa and 11.64 � 0.31%, respectively. Theincorporation of HAP ranging from 10% to 60% increased themechanical strength. This may be attributed to an increase in

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

16736 | RSC Adv., 2014, 4, 16731–16738

rigidity over the pure polymer, because HAP is a hard inorganiccomponent. Furthermore, by increasing the amount of HAP upto 70 wt%, the tensile strength decreased to 17.36 � 0.15 MPaand the percentage of elongation decreased to 8.49 � 1.48%.This decrement is attributed to the agglomeration of HAPnanoparticles at higher content, which resulted in poor inter-face bonding of the nanoHAP particles with polymers.Furthermore, failure of the composite materials usually occursat the interface of the polymer and the HAP.

4. Conclusions

This paper reported the fabrication of HAP/PVP/PEO compositenanobers with HAP sol and polymer PEO–PVP by a sol-elec-trospinning process. Sodium citrate was chosen for thedispersing agent to enable an enhanced dispersion of HAP inthe aqueous solution; this yielded a homogeneous electro-spinning solution with PVP and PEO polymer, and alsoimproved the spinnability of the spinning solution. PEO–PVPadditive could greatly reduce the hygroscopicity of thecomposite nanobers, and continuous and uniform bers witha good tensile strength could be obtained. A volume ratio ofsolution to ethanol of 2 : 1 and a mass ratio of PEO–PVP of 4 : 6were most suitable to ensure production of relatively good bersunder certain electrospinning conditions. The nanocompositebers containing up to 60% HAP nanoparticles showed thehighest tensile strength, of 19.20 � 0.09 MPa with a percentageof elongation of 11.64� 0.31%. It was of interest to observe thatthe needle-like HAP nanocrystals were mainly aligned along thePVP–PEO bers, which is very similar to the HAP in living bone.Therefore, it can be summarized that the electrospun HAP/PVP/PEO composite nanobers could be a promising biomaterial forbone tissue engineering.

Notes and references

1 A. Saenz, W. Brostow, E. Rivera and V. M. Castano, Ceramicbiomaterials: an introductory overview, J. Mater. Educ., 1999,21(5/6), 267–276.

2 J. N. Wang, Q. Wang, X. Q. Liu and H. Q. Liu, Synthesis andcharacterization of hydroxyapatite on hydrolyzedpolyacrylonitrile nanober templates, RSC Adv., 2013, 3,11132–11139.

This journal is © The Royal Society of Chemistry 2014

Paper RSC Advances

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

3 M. Kikuchi, S. Itoh, S. Ichinose, K. Shinomiya and J. Tanaka,Self-organization mechanism in a bone-like hydroxyapatite/collagen nanocomposite synthesized in vitro and itsbiological reaction in vivo, Biomaterials, 2001, 22, 1705–1711.

4 S. C. Rizzi, D. T. Heath, A. G. A. Coombes, N. Bock, M. Textorand S. Downes, Biodegradable polymer/hydroxyapatitecomposites: surface analysis and initial attachment ofhuman osteoblasts, J. Biomed. Mater. Res., 2001, 55, 475–486.

5 K. M. Woo, J. H. Jun, V. J. Chen, J. Seo, J. H. Baek, H. M. Ryooand G. S. Kim, Nano-brous scaffolding promotes osteoblastdifferentiation and biomineralization, Biomaterials, 2007,28, 335–343.

6 E. Martinez, E. Engel, J. A. Planell and J. Samitier, Effects ofarticial micro- and nano-structured surfaces on cellbehaviour, Ann. Anat., 2009, 191, 126–135.

7 X. L. Deng, G. Sui, M. L. Zhao, G. Q. Chen and X. P. Yang,Poly(L-lactic acid)/hydroxyapatite hybrid nanobrousscaffolds prepared by electrospinning, J. Biomater. Sci.,Polym. Ed., 2007, 18(1), 117–130.

8 G. Sui, X. Yang, F. Mei, X. Hu, G. Chen, X. Deng and S. Ryu,Poly-L-lactic acid/hydroxyapatite hybrid membrane for bonetissue regeneration, J. Biomed. Mater. Res., Part A, 2007,82(2), 445–454.

9 H. W. Kim, H. H. Lee and J. C. Knowles, Electrospinningbiomedical nanocomposite bers of hydroxyapatite/poly(lactic acid) for bone regeneration, J. Biomed. Mater.Res., Part A, 2006, 79A, 643.

10 P. Wutticharoenmongkol, P. Pavasant and P. Supaphol,Osteoblastic phenotype expression of MC3T3-E1 culturedon electrospun polycaprolactone ber mats lled withhydroxyapatite nanoparticles, Biomacromolecules, 2007, 8,2602–2607.

11 V. Thomas, D. R. Dean, M. V. Jose, B. Mathew, S. Chowdhuryand Y. K. Vohra, Nanostructured biocomposite scaffoldsbased on collagen coelectrospun with nanohydroxyapatite,Biomacromolecules, 2007, 8, 631–637.

12 H.W. Kim, J. H. Song and H. E. Kim, Nanober generation ofgelatin/hydroxyapatite biomimetics for guided tissueregeneration, Adv. Funct. Mater., 2005, 15, 1988–1994.

13 Y. Zhang, J. R. Venugopal, A. El-Turki, S. Ramakrishna, B. Suand C. T. Lim, Electrospun biomimetic nanocompositenanobers of hydroxyapatite/chitosan for bone tissueengineering, Biomaterials, 2008, 29, 4314–4322.

14 M. S. Rehmann and A. M. Kloxin, Tunable and dynamic somaterials for three-dimensional cell culture, So Matter,2013, 9, 6737–6746.

15 J. N. Wang, Q. Wang, X. Q. Liu and H. Q. Liu, Synthesis andcharacterization of hydroxyapatite on hydrolyzedpolyacrylonitrile nanober templates, RSC Adv., 2013, 3,11132–11139.

16 A. Areias, C. Ribeiro, V. Sencadas, N. Garcia-Giralt, A. Diez-Perez, J. L. Go'mez Ribelles and S. Lanceros-Me'ndez,Inuence of crystallinity and ber orientation onhydrophobicity and biological response of poly(L-lactide)electrospun mats, So Matter, 2012, 8, 5818–5825.

17 A. Cipitria, A. Skelton, T. R. Dargaville, P. D. Daltonac andD. W. Hutmacher, Design,fabrication and characterization

This journal is © The Royal Society of Chemistry 2014

of PCL electrospun scaffolds—a review, J. Mater. Chem.,2011, 21, 9419–9453.

18 P. Gowsihan, B. B. Yu, J. R. Jones and T. Kasugab,Electrospun silica/PLLA hybrid materials for skeletalregeneration, So Matter, 2011, 7, 10241–10251.

19 A. Polini, S. Pagliara, R. Stabile, G. S. Netti, L. Roca,C. Prattichizzo, L. Gesualdo, R. Cingolani and D. Pisignano,Collagen-functionalised electrospun polymer bers forbioengineering applications, SoMatter, 2010, 6, 1668–1674.

20 R. B. Ng, R. Zang, K. K. Yang, N. Liu and S. T. Yang, Three-dimensional brous scaffolds with microstructures andnanotextures for tissue engineering, RSC Adv., 2012, 2,10110–10124.

21 L. H. Lao, H. P. Tan, Y. J. Wang and C. Y. Gao, Chitosanmodied poly(L-lactide) microspheres as cell microcarriersfor cartilage tissue engineering, Colloids Surf., B, 2008, 66,218–225.

22 J. C. Johnson, T. LaShanda and J. Korley, Enhancedmechanical pathways through nature's building blocks:amino acids, So Matter, 2012, 8, 11431–11442.

23 J. Fletcher, D. Walsh, C. E. Fowler and S. Mann, Electrospunmats of PVP/ACP nanobres for remineralization of enameltooth surfaces, CrystEngComm, 2011, 1, 3692–3697.

24 J. Mitra, G. Tripathi, A. Sharma and B. Basu, Scaffolds forbone tissue engineering: role of surface patterning onosteoblast response, RSC Adv., 2013, 3, 11073–11094.

25 R. Sahay, P. Suresh Kumar, R. Sridhar, J. Sundaramurthy,J. Venugopal, S. G. Mhaisalkar and S. Ramakrishna,Electrospun composite nanobers and their multifacetedapplications, J. Mater. Chem., 2012, 22, 12953–12971.

26 F. Y. Zheng, S. G. Wang, M. W. Shen, M. F. Zhu and X. Y. Shi,Antitumor efficacy of doxorubicin-loaded electrospun nano-hydroxyapatite–poly(lactic-co-glycolic acid) compositenanobers, Polym. Chem., 2013, 4, 933–941.

27 G. Sui, X. Yang, F. Mei, X. Hu, G. Chen, X. Deng and S. Ryu,Poly-L-lactic acid/hydroxyapatite hybrid membrane for bonetissue regeneration, J. Biomed. Mater. Res., Part A, 2007, 82(2),445–454.

28 W. Zhang, T. Liu, X. L. Hu and J. M. Gong, Novel nanobrouscomposite of chitosan–CaCO3 fabricated by electrolyticbiomineralization and its cell biocompatibility, RSC Adv.,2012, 2, 514–519.

29 N. Michelle, S. S. Liao, J. Avinash and S. Patilc, Ramakrishna,The fabrication of nano-hydroxyapatite on PLGA and PLGA/collagen nanobrous composite scaffolds and their effectsin osteoblastic behavior for bone tissue engineering, Bone,2009, 45, 4–16.

30 A. H. Rajabi-Zamani, A. Behnamghader and A. Kazemzadeh,Synthesis of nanocrystalline carbonated hydroxyapatitepowder via nonalkoxide sol–gel method, Mater. Sci. Eng., C,2008, 28, 1326–1329.

31 N. Hild, O. D. Schneider, D. Mohn, N. A. Luechinger,F. M. Koehler, S. Hofmann, R. J. Vetsch, B. W. Thimm,R. Muller and W. J. Stark, Two-layer membranes ofcalcium phosphate/collagen/PLGA nanobres: in vitrobiomineralisation and osteogenic differentiation of humanmesenchymal stem cells, Nanoscale, 2011, 3, 401–409.

RSC Adv., 2014, 4, 16731–16738 | 16737

RSC Advances Paper

Publ

ishe

d on

14

Febr

uary

201

4. D

ownl

oade

d by

Dal

hous

ie U

nive

rsity

on

15/0

7/20

14 2

1:43

:31.

View Article Online

32 M. V. Jose, V. Thomas, K. T. Johnson, D. R. Dean andE. Nyairo, Aligned PLGA/HA nanobrous nanocompositescaffolds for bone tissue engineering, Acta Biomater., 2009,5, 305–315.

33 H. P. Tan, L. Wan, J. D. Wu and C. Y. Gao, Microscale controlover collagen gradient on poly(L-lactide) membranesurfacefor manipulating chondrocyte distribution, Colloids Surf.,B, 2008, 67, 210–215.

34 S. Bose and S. K. Saha, Synthesis and Characterization ofHydroxyapatite Nanopowders by Emulsion Technique,Chem. Mater., 2003, 15, 4464–4469.

16738 | RSC Adv., 2014, 4, 16731–16738

35 V. Thomas, D. R. Dean, M. V. Jose, B. Mathew, S. Chowdhuryand Y. K. Vohra, Nanostructured Biocomposite ScaffoldsBased on Collagen Coelectrospun withNanohydroxyapatite, Biomacromolecules, 2007, 8, 631–637.

36 D. E. Fenton, J. M. Parker and P. V. Wright, Complexes ofalkali metal ions with poly(ethylene oxide), Polymer, 1973,14, 589–591.

37 A. H. Rajabi-Zamani, A. Behnamghader and A. Kazemzadeh,Synthesis of nanocrystalline carbonated hydroxyapatitepowder via nonalkoxide sol–gel method, Mater. Sci. Eng.,2008, 28, 1326–1329.

This journal is © The Royal Society of Chemistry 2014