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A Novel Method for In Situ Synthesis ofNanostructure Hydroxyapatite–nylon 66Composites in Tissue EngineeringMehran Mehrabanian a , Mojtaba Nasr-Esfahani b & Majid Jafari ba Young Researchers Club, Najafabad Branch, Islamic AzadUniversity, Najafabad, Isfahan, Iranb Department of Materials Science and Engineering, NajafabadBranch, Islamic Azad University, Isfahan, Iran

Available online: 04 May 2012

To cite this article: Mehran Mehrabanian, Mojtaba Nasr-Esfahani & Majid Jafari (2012): A NovelMethod for In Situ Synthesis of Nanostructure Hydroxyapatite–nylon 66 Composites in TissueEngineering, International Journal of Polymeric Materials, 61:7, 558-570

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A Novel Method for In SituSynthesis of NanostructureHydroxyapatite–nylon 66Composites in TissueEngineering

Mehran Mehrabanian,1 Mojtaba Nasr-Esfahani,2 and

Majid Jafari2

1Young Researchers Club, Najafabad Branch, Islamic Azad University, Najafabad,Isfahan, Iran2Department of Materials Science and Engineering, Najafabad Branch,Islamic Azad University, Isfahan, Iran

An interfacial polymerization method for nylon 66 was adapted to produce nanostruc-tured composites with nanohydroxyapatite (n-HA) via in situ polymerization. Nylon 66was synthesized in n-HA slurry to increase homogeneity and biocompatibility. Syn-thesis powders was characterized by XRD, TEM, TGA, and FT-IR analysis. The resultsshowed a uniform dispersion in composites, whereas needle-like, n-HA crystals dis-persed in condensed polymer matrix. The samples were immersed in SBF for bioactiv-ity tests in different periods, and results were monitored by SEM and EDX in termsof forming an apatite layer. The mechanical properties of the n-HA=nylon66nanocomposites are discussed in terms of n-HA loading.

Keywords biomaterials, in situ polymerization, nano hydroxyapatite, nanocom-posites

Received 6 February 2011; accepted 22 May 2011.The authors gratefully acknowledge the Najafabad Branch, Islamic Azad University forthe financial support of this work. The authors wish to thank A. Chami and O. Torabifor the help in laboratory testing and figure preparation, respectively.Address correspondence to Mojtaba Nasr-Esfahani, Department of Materials Scienceand Engineering, Islamic Azad University-Najafabad Branch, Isfahan, Iran. E-mail:m-nasresfahani@iaun.ac.ir

International Journal of Polymeric Materials, 61:558–570, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0091-4037 print=1563-535X online

DOI: 10.1080/00914037.2011.593061

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1. INTRODUCTION

HA [Ca10(PO4)6(OH)2] has been successfully applied in orthopedics as a bone

substitute due to its excellent bioactivity and biocompatibility [1–4], which is

probably from its similarity to the main mineral component of human bone.

Nevertheless, the fracture toughness of HA ceramics do not exceed one

MPa �m1=2, as compared with 2–12MPa �m1=2 for human bone [5,6]. Therefore,

HA cannot serve as a bulk implanted material, such as artificial teeth or long

bones, under high physiological loading conditions. These shortcomings

greatly confine HA application as load-bearing biomaterials because the

implantable material properties should match the mechanical characteristics

of the surrounding bone tissue. Therefore, development of biocomposites with

good mechanical properties, excellent bioactivity, and biocompatibility similar

to a natural bone to meet the need of hard tissue repair has been a hot topic for

many years. An ideal material for bone repair must be biocompatible and

bioactive, able to initiate osteogenesis, and should have mechanical properties

and composition similar to the bone. In order to obtain an advanced mechan-

ical performance of the bioactive composite, bioceramic particles were usually

incorporated into the polymer matrix using conventional plastics-processing

technology. Various composite systems have been explored as bone substitute

materials, including HA reinforced polyethylene, polylactide, collagen, and

others [7]. Most studies report using less than 45% of HA due to the large size

of the HA particles used and the melting process [8]. This can restrict the

bioactivity of the composite [9–12]. In some other reported studies, the organic

matrix used is not a polar polymer or its molecular structure is not similar to

collagen in natural bone. This can restrict the interface binding and the mech-

anical behavior of the composites [13–17]. The first n-HA=nylon 66 biocompo-

sites for bone repair were reported by Yubao Li [18], in which the research

found mechanical properties close to a natural bone. Nylon 66 is an important

engineering plastic with excellent mechanical property. It is also used as a

medical polymer owing to its toughness, such as medical thread, artificial

skin, etc. The degraded monomer of nylon 66, 1,6-hexamethylene-diamine

and hexanedioic acid, may have a role of an anti-bacterium in vivo [19], but

nylon 66 cannot form bone-bonding with bone tissues alone. Recent studies

on animals and clinical experiments have proven that the n-HA=nylon 66 com-

posites have good compatibility to the bone and can bond directly to the bone.

It is well known that nanoparticles are more difficult to disperse than micro-

particles, mainly because of their higher surface area and very small particle

size, which promote the particle aggregation. It can be affirmed that a nano-

composite with poor nanoparticle dispersion is going to have low mechanical

properties due to the formation of stress-concentration regions or weak points

on the polymer matrix by the nanoparticle aggregates that initiate and propa-

gate cracks. The chemical and physical properties of polymer materials and

In Situ Synthesis of Nanostructure HAp-nylon 66 Composites 559

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polymeric matrix composites are strongly influenced by their thermal and

crystallization behavior. Until now, only a few studies have been reported on

the thermal and crystallization behavior of n-HA=nylon 66 biocomposites

[20,21].

In this study, a new fabrication method of HA=nylon 66 nanocomposite

and the influence of n-HA crystals on the characterization of n-HA=nylon 66

were reported. This composite has excellent mechanical properties and bioac-

tivity, and is a promising biomaterial for load-bearing bone replacement.

2. MATERIALS AND METHODS

2.1. MaterialsThe polymerization system consists of a cyclohexan phase containing the

dichloric acid adipoyl chloride (C6H8Cl2O2, Acros Organics) and a dimethyl

formamide (DMF) phase containing the diamine 1,6-hexamethylene-diamine

(C6H16N2, Merck) and the base sodium hydroxide (NaOH, Merck). The slurry

of nano-hydroxyapatite (n-HA) used for composites was prepared by our

laboratory according to Ref. [22]. All solvents were purchased from Merck

Group, Germany.

2.2. Preparation of n-HA/Nylon 66 CompositesNano-apatite slurry was prepared by a chemical co-precipitation method

through aqueous solutions of the reactants according to the literature [22].

The reactants used were of analytical grade. (NH4)2HPO4 and Ca(NO3)2 were

first dissolved in distilled water to form 0.5M and 0.3M aqueous solutions,

respectively. The pH value of both solutions was rectified to more than 11 by

ammonia. A (NH4)2HPO4 aqueous solution was then slowly dropped into an

intensively stirred Ca(NO3)2 solution at room temperature, simultaneously

keeping the pH value more than 11. The suspension colloid was aged for

24h, and then boiled to 70–80�C. Nano-apatite slurry was finally obtained

by water-washing to 7 of pH values. Then, with replacement of DMF instead

of water in around 153�C (boiling point of DMF), the nano-apatite slurry in

DMF was obtained. XRD and TEM analysis confirmed the nano-scales,

needle-like shape, and apatite composition of dispersed n-HA crystals in

DMF organic solvent, of course after drying in 80�C.

For in situ polymerization of the neat nylon 66, equimolar monomer solu-

tions of 0.0244mol were made with 1ml adipoyl chloride in 30ml cyclohexan

and 0.71ml 1,6-hexamethylene-diamine in 30ml DMF, which also contained

0.50 gr. sodium hydroxide and 60wt% n-HA. The solutions were combined in

a hot plate with magnetic stirrer in 40�C and reacted for 5min. with agitation.

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The obtained n-HA=nylon 66 composite powders were filtered through a

Buchner fitted-disk funnel and washed repeatedly with distilled water and

acetone. After washing, the obtained 60wt% n-HA=nylon 66 composite was

dried in a vacuum oven at 80�C for 48h. The measured molecular weight of neat

nylon 66, which was synthesized by the in situ polymerization method in the

presence of the n-HA with the reagent ratios as described above, was 14kDa.

2.3. Characterization and Analysis of n-HA/Nylon66 CompositesTransmission electron microscopy (TEM, Philips CM 10) was used to

examine the microscopy morphology of n-HA in the slurry and in the com-

posite. The samples for IR, XRD, and TGA analysis were dried in a vacuum

oven at 80�C for 24h before testing. IR spectra of all the samples were mea-

sured using an FT-IR spectrometer (FT-IR 6300, JASCO, Japan) with KBr

prisms in the range of 400–4000 cm�1. All spectra were treated using the base

line correction. XRD measurements were performed with a Philips X’pert XRD

analyzer. In these cases, CuKa radiation from a Cu X-ray tube was used. The

samples were measured in the 2h range from 10� to 90�. In order to determine

the effect of n-HA crystals on the thermal stability and decomposition tem-

perature of nylon 66, thermogravimetric analysis (TGA, 401 SA Instruments)

was used. The samples were analyzed at a heating rate of 10�C=min in the air.

The assessment of the in vitro bioactivity was carried out by soaking the nano-

composites in simulated body fluid (SBF), which was prepared according to

Ref. [23]. The samples with 50mg nanocomposite content were immersed in

50ml of SBF in closed plastic containers and kept at 37�C for 7 and 14 days.

After each time, the nanocomposites were washed in deionized water, dried at

30�C for 12 hours, and stored in a drier. The formation and growth of the apa-

tite layer on the nanocomposite surfaces were verified by a scanning electron

microscope coupled with Energy-Dispersive Spectroscopy (SEM=EDX, Philips

XL30). For mechanical tests, the samples were directly injection-molded into

standard testing specimens according to ASTM procedures. Tensile tests were

performed using an Instron machine series 1122 according to ASTM D 638 at a

crosshead speed of 50mm=min. Bending tests were carried out using the same

instrument according to ASTM D 790 at a crosshead speed of 5mm=min.

3. RESULTS AND DISCUSSION

3.1. XRD AnalysisFigure 1 shows relevant XRD patterns, in which 1a is for pure nylon 66, 1b

is for 60% n-HA=nylon 66 composite powder, and 1c is for n-HA needle crystals

In Situ Synthesis of Nanostructure HAp-nylon 66 Composites 561

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in DMF. Figure 1c exhibits an apatite structure; the obvious peak of (0 0 2) at

2h¼ 25.9� indicates a trend to grow along the c-axis direction. The crystalline

peaks of n-HA in Fig. 1c should belong to a poorly crystallized apatite struc-

ture according to apatite in natural bone. This means that dispersing n-HA

crystals in DMF solvent also have a similarity in crystallinity to apatite in

bone [24,25]. Crystal sizes of n-HA are calculated by the Scherrer formula

from XRD patterns of different specimens [26]:

Dhld ¼ 0:89kffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffib2 � b20 cos h

q

where k stands for wavelength, h stands for Bragg angle, b stands for half

width of the characteristic peak, and b0 is the half width of XRD machine

proofread by multicrystal silicon powder, giving a value of b0¼ 0.1215. The

data indicate that the size of n-HA crystals in DMF is 25nm and the size of

HA crystals in composite are 40nm approximately, which is bigger than pure

n-HA. This may be attributed to the influence of the process of nanocomposite

powder preparation or=and the interaction between HA and nylon 66.

The nylon 66 in Fig. 1a had two characteristic peaks at 2h¼ 20.4� and 24.1�,

Figure 1: XRD patterns of nylon 66 (a), 60% n-HA=nylon 66 nanocomposite (b), and n-HApowder (c).

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indicating a-crystal structure. In composite, the crystallinity of nylon

66 phase decreased as shown in Fig. 1b, indicating that the crystal structure

of nylon 66 was changed after forming the composite with n-HA crystals.

The hydrogen bonds in nylon contribute to its crystallinity when forming

the composite. The interface binding between n-HA and nylon 66 may

result in a decreased number of hydrogen bonds, thus lessenening nylon

crystallinity.

3.2. Microscopic Morphology of n-HA and CompositeFigure 2 shows the TEM photographs of the n-HA crystals (a) and the com-

posite powders (b). Figure 2a indicates that acicular n-HA crystals are in nan-

ometer grade and have a crystal size of 15–25 nm in diameter by 35–75nm in

length approximately. The composite particles shown in Fig. 2b have mor-

phology with a size of 20–40nm in diameter by 45–80nm in length. It can

be seen that the composite particles have a larger diameter and longer length

than n-HA crystals, with the nylon 66 component present on the surface of the

n-HA needle crystals. The apatite crystals in natural hard tissues are formed

as thin needles, with a size of 5–20 nm by 60nm and over 100nm long in

enamel [27]. The shape and size of the prepared n-HA crystals are similar

to the apatite crystals in natural bone; this similarity is beneficial for making

a biomimetic composite. When using the n-HA=nylon 66 composite powders to

make bulk products, the n-HA crystals can be ensured to disperse uniformly in

the polymer matrix.

3.3. IR AnalysisThe interactions between n-HA and nylon 66 can be easily formed through

chemical bondings, i.e., hydrogen-bonding, since both n-HA and nylon 66 are

Figure 2: TEM photographs of n-HA crystals (a) and composite particles (b).

In Situ Synthesis of Nanostructure HAp-nylon 66 Composites 563

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polar compounds. The IR spectra of n-HA, nylon 66, and n-HA=nylon 66 are

shown in Figure 3. The corresponding wavenumbers of characteristic groups

for n-HA, nylon 66, and n-HA=nylon 66 are listed in Table 1 and the values

indicate that:

1. The absorption peaks of the �OH groups of HA in composites are lower

than typical peaks of HA dues to the hydrogen bonds between HA and

nylon 66, which show a slight shift in the composite.2. No obvious shifts have been observed for the absorptions of phosphate

bands in composites compared to that in typical peaks of HA, offering

almost no bonding between phosphate bands of HA and nylon 66. Atomic

structure of PO�34 possibly does not permit the formation of hydrogen

bonds between HA and amide. Three negative charges above three oxygen

atoms trend intensively to attract calcium ions for reaching the stable

structure form; on the other hand, the orientation property of the hydro-

gen bonds between OH of HA and amide would hamper the formation of

hydrogen bonds between them. It also can be seen from Table 1 that the

vibration intensity of PO�34 in these composites becomes weaker compared

to that of HA.

Figure 3: IR spectra of n-HA (a), nylon 66 (b), and n-HA= nylon 66 nanocomposite (c)powder. (Figure is provided in color online.)

564 M. Mehrabanian et al.

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3. The stretching vibration frequency of N-H groups in nylon 66 at 3303 cm�1

changes to 3304 cm�1, whereas the bending vibration frequency of N-H

at 1538 and 688 cm�1 moves to 1541 and 686 cm�1. The stretching peak

of –C¼O in the nylon 66 rises to 1639 cm�1 in the composite. The peaks

of C-H in nylon 66 at 2935 and 2859 cm�1 increase slightly to higher wave-

numbers of 2937 and 2861 cm�1 in the composite. These spectral shifts

indicate that the composition and structure of both components hardly

changes after incorporation with each other, and that hydrogen bonds

mainly exist between the hydroxyl of HA and the amido group of nylon

66 because of the variations in their absorption peaks. Moreover, the

addition of n-HA crystals into the condensed polymer matrix disturbs

the strong interaction in polyamide molecules and influences the chemical

environment of them. Furthermore, the electronegativity of oxygen atom

is stronger than that of the nitrogen atom; the attraction of the oxygen in

OH� group of HA to hydrogen is stronger than that of the nitrogen on the

amide of nylon 66; that is, the positive charge on the hydrogen of OH� is

relatively more than that of the hydrogen of amide, which makes the for-

mation of hydrogen-bonds easier between hydrogen of OH� and nitrogen

of the amido group than that between oxygen of OH� and hydrogen of

amide. Figure 4 shows the two forms of hydrogen bonds existing between

n-HA and nylon 66.

3.4. Effect of n-HA on the Thermal Stability of Nylon 66Thermal degradation of polymers is a major problem at temperatures

above the melting point and inevitably occurs in polymer melts during proces-

sing. The study of thermal degradation can be best complimented or corrobo-

rated by such techniques as thermogravimetric analysis (TGA), which

Table 1: The changes of wavenumber of n-HA (a), nylon 66 (b), and n-HA=nylon66 nanocomposites (c).

Characteristicgroups

n-HA(cm�1)

nylon6,6(cm�1)

n-HA/nylon6,6(cm�1)

HA –OH 3571 – 3564633 – 626

�PO�3A 1093 – 1091

603 – 6021031 – 1032563 – 565

Nylon 66 N–H stretching vibration – 3303 3304N–H bending vibration – 688 686–C¼O stretching vibration – 1636 1639N–H bending vibration – 1538 1541–CH2– stretching vibration – 2935 2937–CH– stretching vibration – 2859 2861

In Situ Synthesis of Nanostructure HAp-nylon 66 Composites 565

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measures the weight loss as a function of temperature. Therefore, we have

investigated the effect of n-HA on the thermal decomposition characteristics

of nylon 66.

Figure 5 shows TG curves for neat nylon 66 and n-HA=nylon 66 nanocom-

posites in heating rate between 10–900�C. It is evident from this figure that

the thermal stability of n-HA=nylon 66 nanocomposite (Fig. 5b) is higher than

that of neat nylon 66 (Fig. 5a). The onset of thermal decomposition (tempera-

ture of 50% weight loss) of the nanocomposite sample containing 60wt% n-HA

is roughly 35�C higher as compared to neat nylon 66. The improved thermal

stability features of the nanocomposite samples are attributed to the effective

interfacial interaction between the polymer matrix and the inorganic filler,

which in turn points to homogeneous filler dispersion.

Figure 4: The scheme patterns of hydrogen bonding between n-HA and nylon 66.

Figure 5: Thermogravimetric curves of (a) nylon 66 and (b) 60wt% n-HA=nylon 66nanocomposite.

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3.5. Mechanical PropertiesTable 2 gives the mechanical properties of nylon 66, n-HA=nylon 66 com-

posite, and cortical bone. Compared with pure nylon 66, it can be seen that the

bending strength, tensile strength, and bending modulus of nanocomposite

(prepared by in situ polymerization, with 60wt% n-HA) are increased con-

siderably by the incorporation of n-HA, indicating a good reinforcing effect

of n-HA on nylon 66, which can be attributed to the large aspect ratio of

n-HA, good interfacial interaction, and nanoscale dispersion of n-HA. For a

biomaterial to be suitable for bone repair or fixation, mechanical compatibility

is very important, i.e., the material should have mechanical properties similar

to those of bone. The tensile strength and bending strength of n-HA=nylon 66

composite are 85.1 and 140.7MPa, respectively, which are close to those of

natural bone (60–120 and 80–210MPa, respectively). The bending modulus

of nanocomposite is 5GPa, also in the range of that of natural bone (3–

25GPa) [28–30]. From the comparison of the mechanical properties

of 60wt% n-HA=nylon 66 composite and natural bone, it can be said that

composites display good mechanical compatibility with natural bone.

4. IN VITRO BIOACTIVITY EVALUATION

SEM micrographs of the surface of the nanocomposites, before and after

immersion in the SBF for 7 and 14 days, are shown in Figures 6, 7, and 8.

The figures also show the EDX analysis of the samples’ surfaces. As

the immersion time is increased, the apatite’s nucleation through the

nanocomposite surface is appreciated. The morphology of the apatite’s layer

is of spherical particle aggregates with relatively small crystals, which suggest

a high nucleation rate. These results confirm that a calcium phosphate layer,

of bone apatite, can efficiently grow on the surface of the nanocomposite by

incubation of the sample in a solution with an ionic composition similar to

human blood plasma [23]. EDX curves in Figures 7 and 8 indicate the increase

of phosphorous and calcium contents after soaking in SBF for 7 and 14 days.

The EDX analysis suggested that these spherical particles could be

Table 2: Mechanical properties of pure nylon 66, 60wt% n-HA=nylon 66composite, and cortical bone.

Material

Bendingstrength(MPa)

Tensilestrength(MPa)

Bendingmodulus(GPa)

Nylon 66 88.2 64.1 1.9860wt% n-HA=nylon 66composite

140.7 85.1 5

Cortical bone [27–29] 80–210 60–120 3–25

In Situ Synthesis of Nanostructure HAp-nylon 66 Composites 567

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Figure 7: SEM micrograph and EDX analysis of the 60wt% n-HA=nylon 66 compositesurface after immersion in the SBF for 7 days. A layer of spherical particles covered thesurface with Ca=P ratio of 1.56.

Figure 6: SEM micrographs of the 60wt% n-HA=nylon 66 composite before soaking in SBF.

Figure 8: SEM micrograph and EDX analysis of the 60wt% n-HA=nylon 66 compositesurface after immersion in the SBF for 14 days. A layer of spherical particles covered thesurface with Ca=P ratio of 1.64.

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calcium-deficient and non-stoichiometric apatite with Ca=P ratio of 1.56 and

1.64 for 60wt% n-HA=nylon 66 composites after immersion in SBF for 7 and

14 days, respectively. This ratio is close to that of natural apatite in bone. This

suggests that, under these conditions, the n-HA=nylon 66 nanocomposite

behaves as a bioactive material. Other studies have reported that the induced

apatite layer on the surfaces of different bioactive materials during their incu-

bation in SBF was also calcium-deficient [31,32].

5. CONCLUSIONS

We have adapted an interfacial in situ polymerization method to the fabrication

of bone-like n-HA=nylon 66 nanocomposites. This versatile fabrication method

incorporates n-HA suspended in DMF, and determines to a large extent the

n-HA dispersion in the resulting nanocomposite. The acicular n-HA crystals

used to prepare the compositeswere similar to bone apatite inmorphology, phase

composition, and crystal structure. Both nylon 66 and collagen contain –NHCO–

groups in their molecular chains, which means that nylon 66 has a similarity to

collagen in chemical composition to a certain extent. Amide groups cannot endow

n-HA=nylon 66 composites with bioactivity; however, they can guarantee strong

interface interactions between n-HA and nylon 66 matrix. In addition to hydro-

gen bonding and mutual attraction between anions and cations, some nylon 66

chains were grafted to the surface of n-HA by covalent bonding.

The various properties of the n-HA=nylon 66 composites prepared by in situ

polymerization were explored and results indicate that n-HA characteristics in

polymer composites have a tremendous influence on both the bioactivity and

mechanical properties, which can be attributed to nanoscale dispersion, the

large aspect ratio of needle-like n-HA and the good chemical binding at inter-

faces. The n-HA=nylon 66 nanocomposite with high n-HA content shows a simi-

larity to natural bone in bioactivity and mechanical properties, which makes it

a possible candidate biomaterial suitable for bone repair or fixation.

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