Development of hydroxyapatite nanorods-polycaprolactone ...

Tissue Engineering and Regenerative Medicine, Vol. 9, No. 6, pp 295-303 (2012) DOI 10.1007/s13770-012-0002-z

｜Original Article｜

Development of Hydroxyapatite Nanorods–Polycaprolactone Composites and Scaffolds Derived from a Novel In-Situ Sol-Gel Process Amirreza Rezaei and Mohammad Reza Mohammadi* Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, Tehran, Iran. (Received: August 2nd, 2012; Revision: September 25th, 2012; Accepted: October 9th, 2012)

Abstract : Hydroxyapatite (HA) is the most substantial mineral constituent of a bone which displays splendid biocompatibility and bioactivity properties. Nevertheless, its mechanical property is not utmost appropriate for a bone substitution. Therefore, a composite consist of HA and a biodegradable polymer is usually prepared to generate an apt bone scaffold. In the present work polycaprolactone (PCL) was employed as a matrix and hydroxyapatite nanorods were used as a reinforcement element of the composite. HA/PCL nanocomposites were synthesized by a new in-situ sol-gel process using low cost chemicals. Chemical and physical characteristics of the nanocomposite were studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and Fourier transform infrared (FTIR) analyses. XRD analysis revealed that pure hydroxyapatite with no undesirable compound was formed within the nanocomposite. Moreover, hydroxyapatite had low crystallinity with the average crystallite size of 62.5 nm. FE-SEM images showed dispersion of HA nanorods in PCL matrix with suitable interaction were obtained. The average length and diameter of HA nanorods were calculated 167 nm and 53 nm, respectively. It was found that HA/PCL nanocomposite had marcoporous structure and high surface area which are essential parameters for cell attachment and protein absorption. Biological properties of HA/PCL scaffolds, prepared through a solvent casting process, were investigated under in vitro condition. Bioactivity of these scaffolds was studied in a saturated simulated body fluid (5×SBF). It was confirmed that HA/PCL scaffold was uniformly covered with a layer of calcium phosphate crystals with the thickness of few microns and phase composition of hydroxyapatite. Consequently, scaffolds met the requirements of materials for bone tissue engineering and could be used for many clinical applications in orthopedic and maxillofacial surgery. Key words: polymer-matrix composites, nanocomposites, sol-gel methods.

characteristics of nHA, such as being the major inorganic component of the bone matrix, its specific affinity toward many adhesive proteins, and its direct involvement in the bone cell differentiation and mineralization processes, make nHA especially attractive for applications in the bone regeneration field.7 The close chemical similarity of HA to natural bone has led to extensive research efforts to use synthetic HA as a bone substitute and/or replacement in biomedical applications.8, 9 However, they have not adequate biomechanical properties (i.e., high brittleness, low fatigue strength, and low flexibility) and do not encounter the mechanical requirements for directloading applications, as well as application of dynamic force during the in vitro bone tissue engineering process.10 Recent advances in nanoscience have reignited interest in the formation of nanosized HA and the study of its properties on the nanoscale.11 Recently, synthesis of HA with nanorod morphology has

1. Introduction Tissue engineering offers a new approach to regenerate diseased or damaged tissues such as bone.1 The rapidly growing research in the bone tissue engineering area thus provides a new and promising approach for bone repair and regeneration.2 Bone is a natural organic–inorganic ceramic composite consisting of collagen fibrils containing embedded, well-arrayed, nanocrystalline, rod-like inorganic materials 25-50 nm in length.3-5 Hydroxyapatite (HA) is chemically similar to the inorganic component of bone matrix, a very complex tissue with general formula Ca10(OH)2 (PO4)6. HA particles on nanometric scale (nHA) have been proved to be an osteoconductive material that also chemically binds to enamel and dentine.6 The biologically beneficial *Tel: +98-21-6616-5211; Fax: +98-21-6600-5717 e-mail: [email protected] (M.R. Mohammadi)

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Amirreza Rezaei and Mohammad Reza Mohammadi

Wang et al. 31 prepared porous nHA/PCL scaffolds with different composition ratios of nHA/PCL via an ex-situ method (i.e., melt-molding/porogen leaching technique). PCL with molecular weight 50000 and nano-hydroxyapatite with particle size 40-60 nm (Nanjing Emperor Nano Material Co. Ltd., China) were used as starting materials and poly-ethylene glycol (PEG20000) was used as the fugitive agent. PCL has an intrinsic hydrophobic chemical nature, and its poor surface wetting and interaction with biological fluids avoid cells adhesion and proliferation. For this reason, and in order to get enhanced mechanical properties, PCL is often used as polymer matrix in composites including osteogenic and osteoinductive inorganic phases, such as HA to confer its high bioactivity to the polymerbased composite promoting bone regeneration. Therefore, biomechanical characteristics of scaffold are higher than those of pure HA. 18, 33-38 Some chemical and physical properties of nHA/PCL scaffolds are evaluated in previous studies; accordingly, it is pointed out that by increasing nHA content of composite the degrading rate of composite will be increased due to facilitating water to infiltrate into the scaffolds. After employing HA/PCL scaffolds as part of a tissue, they are able maintain their suitable mechanical properties for a proper time.31 Once HA particles are dispersed in the polymer matrix; poor interfacial adhesion is often observed as a consequence of the different chemical nature of the components and their different surface energy, resulting in a too fast decay of the mechanical properties of the composite. For this reason, an in situ synthesis of HA by sol–gel process directly in the presence of the polymer solution, appears to be a promising strategy for the achievement of homogeneous hybrid materials. This procedure should avoid the extensive particles agglomeration typically seen in HA/PCL composites obtained by mechanical incorporation of preformed HA powders into the polymer melt or solution, causing non homogeneous materials.29 Moreover, in a sol-gel process, particle size is controlled directly by the means of the interaction between calcium and phosphate precursors under controlled temperature and pH conditions.24 So far, HA/PCL hybrid composite containing HA nanords has not been reported by an in-situ sol-gel process. Here, a new strategy for synthesis of HA/PCL composite, containing HA nanorods, is introduced by sol-gel method using non-alkoxide precursors (i.e., calcium hydroxide and phosphoric acid) and acetone as the solvent. One of the advantages of this method is the use of an alternative to alkoxide precursors as calcium and phosphorous sources. Besides controlling the phase structure, composition homogeneity, monodispersity and microstructure, the cost of the product is an important concern in developing technologies for synthesis of HA/PCL nanocomposites. There-

received considerable attention. For instance, in column chromatography application, rod shaped HA shows enhanced protein absorption due to their charging surface efficiency.12 HA crystals with nanorod features have shown desirable biocompatibility and bioactivity because of higher absorbability, since the underlying van der Waal’s interactions are proportional to surface area of the rods.13 Furthermore, HA in human tooth and bone is found in the form of nano-polycrystalline hexagonal nanorods.14 Polycaprolactone (PCL) is a biodegradable polymer with remarkable toughness and good biocompatibility.15, 16 It is a semi-crystalline aliphatic polymer with sustained biodegradability, good biocompatibility and expectant mechanical strength that has a slower degradation rate and higher fracture energy than most biocompatible polymers.17-20 Sol-gel process is one the appropriate methods for synthesis of biomaterials as the final product shows chemical uniformity.21 Moreover, high level of interaction makes it possible to carry out the procedure at the low temperature.22 The sol-gel derived HA compounds usually show small grain size in the range of submicrons or even nanometric which would be adopted to the host tissue more rapidly.23 Meanwhile, It has been reported that, dispersion of particles has impressive effects on mechanical and bioactivity properties of scaffolds.24 The ideal porous scaffolds for hard tissue engineering should be biocompatible, biodegradable and absorbable. In addition, a suitable microstructure of scaffold (including their porosity, pore size and interconnection between pores), the sufficient mechanical strength retaining for a period and good cell-scaffold interaction are also the necessary for bone tissue engineering application.25-27 The idea of combining bioactive ceramics and degradable polymers to produce three-dimensional (3D) scaffolds with high porosity is a promising strategy for the design and development of composite systems for hard tissue regeneration materials.28 Many bone tissue engineering scaffolds have been made as composites, either by the introduction of nHA or HA within polymeric matrices or by the mineralization of HA nanoparticles on the surface of polymeric substrates.28-32 For example, Fabbri et al. 29 synthesized PCL/HA composites by in situ generation of HA in the polymer solution starting from the precursors calcium nitrate tetrahydrate and ammonium dihydrogen phosphate and the solvent tetrahydrofuran (THF) via sol–gel process. XRD pattern of PCL/HA composite revealed a strong calcium deficiency in the inorganic phase generated in situ inside the polymer solution, being brushite (CaHPO4.2H2O). Raucci et al. 30 reported HA/PCL hybrid composite material using calcium nitrate tetrahydrate and di-phosphorous pentoxide as precursors and ethanol as the solvent. They observed a trace of calcium carbonate in the hybrid composite.

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Development of Hydroxyapatite Nanorods–Polycaprolactone Composites and Scaffolds Derived from a Novel In-Situ Sol-Gel Process

fore, starting with a low cost solvent such as acetone rather than tetrahydrofuran may reduce the total cost of production. HA/PCL porous scaffolds for bone tissue engineering were also prepared by salt leaching method. A careful investigation of the physicochemical properties of the nanocomposite was carried out by different analyses. The in vitro bioactivity was tested using a saturated Simulated Body Fluid which controls deposition of HA crystals.

2. Materials and Methods HA/PCL (20/80 wt.%) nanocomposite scaffolds were prepared at 40oC from Ca(OH)2 (Acros) and H3PO4 (Merck 85%) as Ca and P precursors, respectively. The best performance of the nanocomposite has been reported with HA:PCL = 20:80 (wt%). 28 Acetone (Merck) was used as the solvent and PCL (Sigma-Aldrich Mw=80,000) was used as the nanocomposite matrix. The procedure consists of two steps (Fig 1). The first step involves the addition of 4 gr PCL to 40 cc acetone under the vigorous mixing condition for 5 hr using a magnetic stirrer and in a 3-neck flask with a reflux system. Then, 0.753 gr Ca(OH)2 was added to the solution and mixed for 5 hr. In a separate preparation, 0.688 gr H3PO4 was added to 5 cc acetone and vigorously mixed. Afterwards, this solution was added to the PCL solution drop by drop to achieve Ca/P molar ratio of 1.67. Ammonia was added to control the pH value around 10. This final solution was stirred vigorously for 24 hr. In the second step, the scaffold was prepared by salt leaching method using sodium chloride crystals (Merck) as porogens homogeneously mixed with the HA/PCL solution at a weight ratio of approximately 9/1 (NaCl/PCL), at which point the composite solution became viscous. The mixture was poured into a cylindrical mold and dried for 24 hr at RT. Finally, daily rinsing in double-distilled water for 7 days removed all NaCl as well as the extra ions formed during the reaction. The water was changed every 12 hr. Morphological analysis was performed on HA/PCL nanocomposites using a JEOL 6340 field emission scanning electron microscope (FE-SEM). Phase composition and crystallinity of synthesized nanocomposites were studied using Xray diffraction diffractometer (XRD) Philips X’pert PW3020. The mean crystallite size (D) of the sample was calculated from the XRD line broadening measurement using the Scherrer equation:39 0.9λ D = ------------------BCOSθ

Figure 1. Schematic image of preparation of HA/PCL nancomposites and scaffolds.

half maximum of HA (2 1 1) 2θ = 31.774o reflection which has the highest intensity among HA peaks and θ is the diffraction angle. The fraction of crystalline phase (Xc) of the HA powders was evaluated by the following equation:40 1 – v 112 ⁄ 300 X C = --------------------------I 300

(Eq. 2)

where v112/300 is the intensity of the hollow between (1 1 2) 2θ = 32.197o and (3 0 0) 2θ = 32.902o diffraction peaks of HA and I300 is the intensity of (3 0 0) diffraction peak. Fourier transform infrared (FTIR) spectroscopy (Bruker Optics Tensor 27 analyser) was used to identify the functional groups of HA and to determine the bonds between the ceramic and polymer phases in the composite material. FTIR spectra were recorded in the 500-4000 cm-1 region. In vitro cell tests of the nanocomposites were performed using supersaturated SBF solutions (5×SBF).41 Briefly, the treatment combines the preliminary use of a SBF solution (5×SBF1) to stimulate the nuclei formation, while a fresh chemically-modified solution (5×SBF2) is further used, in order to promote the growing of apatite nuclei, once formed. Meanwhile, 5×SBF1 was prepared by sequentially dissolving CaCl2, MgCl2.6H2O, NaHCO3 and K2HPO4.3H2O in deionized water. Solution pH was lowered to 6 by adding hydrochloric acid to increase the solubility. Na2SO4, KCl and NaCl were added until the previous solution become clear. The final pH was adjusted with 1 M NaOH to reach a final pH of 6.5 (5×SBF1). Conversely, Mg2+ and HCO3 free 5×SBF2 was prepared by adding CaCl2, K2HPO4.3H2O and NaCl subsequently to deionized water up to obtain a fresh solution (5×SBF2). In this case, the solution pH was lowered to 6.0 with hydrochloric acid to increase the solubility. Both solutions were buffered at pH 6.5 and 6 respectively, by using

(Eq. 1)

where λ is the wavelength (Cu-Kα), B is the full width at the

297


Table 1. Ionic concentration of human blood plasma, SBF, 5×SBF1 and 5×SBF2 30. Ion concentration (mM)

Na+

K+

Ca2+

Mg2+

HCO-3

Cl-

HPO-4

SO42-

Blood Plasma

142

5

2.5

1.5

27

103

1

0.5

SBF

142

5

2.5

1.5

4.2

148

1

0.5

5×SBF1

710

25

12.5

7.5

21

740

5

2.5

5×SBF2

710

10

12.5

-

-

735

5

-

Tris and chloric acid ((CH2OH)3CNH2 and HCl). In table 1, the ionic concentration of blood plasma, SBF, 5×SBF1 and 5×SBF2 were reported. All solutions were prepared freshly before the use. The biomimetic treatment consists in two steps in pHcontrolled environment: during the first step, samples with preordered size were soaked into 5×SBF1 at pH 6.5 where the 5×SBF solution volumes have been calculated respect to the total scaffold material surface using an exposed surface to SBF volume ratio equal to 10 mm2/ml, as reported in literature.42 The solution temperature was fixed at 37oC during the treatment by putting the samples in an incubator. After the sequential immersion in 5×SBF1 (3 days) and in 5×SBF2 (4 days), all scaffolds were dried overnight. To examine the distribution of HA particles in the scaffolds and the presence of coating after treatment in 5×SBF, the samples were stained with a hydrophilic dye (0.5% w/v trypan blue) as reported elsewhere.43 The residual dye was eliminated by washing with 100% ethanol. Determination of the elemental constituents of the formed layer on the nanocomposites scaffolds after the biomimetic treatment and their percentage was carried out by X-ray energy dispersive spectroscopy (VEGA\TESCAN, 15.00 kV). Coincidently, the morphological characteristics were studied from SEM images. Ultimately, to reassure the formation of HA layer on the samples the XRD analysis was conducted under the previously pointed conditions for the characterization of nanocomposites.

Figure 2. FE-SEM micrographs of HA/PCL nanocomposite prepared by an in situ sol-gel process. HA/PCL composite with marcoporous structure containing HA nanorods with diameter of 53 nm is prepared.

of nanostructures refers to their large surface area.45 Based on Clemex Vision PE software, the average length and diameter of HA nanorods were calculated 167 nm and 53 nm, respectively. Therefore, the aspect ratio of HA nanorods was found to be 3.15. The aspect ratio of HA nanorods is an important parameter since it determines their absorbability. The higher the aspect ratio, the better the cell attachment and bioactivity of the scaffold. It is evident that HA/PCL nanocomposite shows marcoporous structure. Such porosity increases the surface area of the nanocomposite and, therefore, the cell attachment and protein absorption will be improved.46-49 The optimum porosity size of a scaffold for tissue growth and nutrient transportation into the tissues has been reported in the range 300-400 µm.50, 51 In order to increase the porosity size of HA/PCL nanocomposite a solvent casting/particulate leaching method was employed to prepare the corresponding scaffold. The nucleation and growth mechanism of HA nanorods could be explained by two facts. Firstly, it is attributed to the relative specific surface energies associated with the different planes of HA crystals or nucleus. The different planes with different surface energies have various amounts of adsorbed

3. Results and Discussion A sol-gel method was used in order to acquire a nanocomposite material with a ceramic phase nanorods dispersed homogeneously in a polymeric matrix. In order to evaluate the dispersion of organic and inorganic phases, a qualitative analysis by FE-SEM on the HA/PCL composite (without scaffold preparation procedure) material was performed. FE-SEM image of HA/PCL nanocomposite washed with double distilled water several times is shown in Fig 2. It can be seen in Fig 2B and Fig 2C that a splendid dispersion of HA nanorods in PCL matrix with suitable interaction was obtained. It has been reported that HA clustering happens in polymeric matrix due to hydrophilicity of HA and hydrophobicity of PCL.44 Moreover, the agglomeration

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OH- groups from the solution.52, 53 The planes contain high concentration of OH- groups; have low concentration of Ca2+ and PO43- ions. Therefore, the growth rate of HA nuclei would be limited on these planes, providing the favored condition for one-dimensional growth (i.e., anisotropic growth). Secondly, it is related to the dielectric constant of the solvent (i.e., acetone). Solvents with low dielectric constant are usually used for onedimensional growth.54 Fig 3 shows the XRD pattern of HA/PCL nanocomposites. PCL is a semi-crystalline polymer which can be detected by three specific XRD peaks at 2θ= 15.7o, 21.5o and 23.8o, as labeled by ■ mark28, 29. Meanwhile, the HA specific peaks were labeled by * mark and were matched with database in JSPDS card numbers of 74-566 and 09-432. It can be observed that HA peaks were broad; accordingly, it could be inferred that they have small particle size and low crystallinity. This can be related to the synthesis process at the low temperature. It has been reported that no new bone can be formed in highly crystalline porous HA ceramics, whereas bone generation has been observed in poorly crystalline HA ceramics.55 As a result, HA with low crystallinity has high bioactivity. As can be seen in XRD pattern, calcium carbonate phase which is usually formed during preparation of HA compound was not detected. Apparently one of privileges of this synthesis route is disappearance of any residual calcium carbonates which can be found in the most sol-gel derived hydroxyapatite. The average crystallite size of HA was calculated 62.5 nm. Moreover, the percentage

Figure 4. FTIR spectrum of HA/PCL nanocomposite prepared by in situ sol-gel process. All characteristic structural bands of HA and PCL are observed in the obtained spectrum.

of crystalline phase was estimated 5-10% which confirms that HA nanorods have low crystallinity. FTIR spectrum of the HA/PCL nanocomposite is shown in Fig 4 and all characteristic structural bands of both HA and PCL were observed and it means that HA nanorods and PCL are well-incorporated. However, a small change in their position is negligible according to chemical bonds which are formed between HA and PCL during sol-gel processing. The C=O, C-O, and C=H bands corresponded to PCL and the P–O and O–H bands were attributed to HA. Although, according to Kim et al. because of small weight percent of HA, its characteristics bands has bigger transmittance amount.56 The first indication of the hydroxyapatite formation is the form of broad FTIR band centered at about 1000-1100 cm-1.57 The bands at 960-965 cm-1 and at 565–601 cm-1 correspond to n1 and n4 symmetric P–O stretching vibration of the PO3-4 ion, respectively.58, 59 As a major peak of phosphate group, the n3 vibration peak could be identified in the region between 1100 cm1 and 960 cm-1, which is the most intensified peak among the phosphate vibration modes. The band between 565 cm-1 and 601 cm-1 belongs to n4 vibration mode of phosphate group which occupies two sites in the crystal lattice (at 601 cm-1 and 567 cm-1).30 Moreover, the peaks at 1420 and 1465 cm-1 are related to the OH- groups.60 The presence of PCL is verified by typical peaks at 1720 cm-1, due to carboxyl group, and at 2800 and 2900 cm-1, due to the presence of CH2 and CH3 groups.30 High transmittance percentage of PCL peaks can be related to effects of HA crystals on hydrogen bonds density of PCL and possible bonds between HA and PCL as reported previously.28 After sequential immersing of HA/PCL nanocomposite in 5×SBF solutions for 7 days, SEM and EDAX analyses were conducted on the samples without any pre-washing and their

Figure 3. XRD pattern of HA/PCL nanocomposite prepared by in situ sol-gel process (■= PCL, * = HA). Formation of highly pure HA compound in PCL matrix is confirmed from their corresponding reflections.

299


Figure 7. Marco image of scaffolds after trypan blue treatment: (A) HA/PCL hybrid scaffold, (B) untreated, (C) after 3 days treatment in 5×SBF1 , (D) after 7 days treatment in 5×SBF solutions. The escalation of blue color from left side to right side denotes this fact that more HA was formed by increasing the immersing time in 5×SBF solutions.

on the internal pore walls, verifying that the high concentration of 5×SBF solution induced a fast and uniform deposition of hydroxyapatite on the nanocomposite scaffolds. The previous studies described nucleation and growth mechanisms of HA in SBF solutions.61, 62 Based on EDAX results, the Ca\P ratio has been increased with increasing of immersing time in 5×SBF. Although, the Ca\P molar ratio was estimated approximately 1.67 but to verify it XRD analysis and Trypan blue treatment on pre-washed samples were carried out. Fig 7 depicts the HA/PCL nanocomposite scaffold trypan blue treatment before and after soaking in 5×SBF solutions. The escalation of blue color from left side to right side denotes this fact that more HA has been formed by increasing the immersing time in 5×SBF solutions. Furthermore, Fig 8 shows XRD pattern of HA/PCL scaffold immersed in 5×SBF for 7 days. The HA pattern is matched with JCPDS card numbers of

Figure 5. SEM images of HA/PCL scaffolds (A, B) after 3 days immersion in 5×SBF1 (C, D) after 3 days immersion in 5×SBF1 and subsequent immersion in 5×SBF2 for 4 days. The image shows that the nanocomposite material was uniformly covered with a calcium phosphate crystals layer with a thickness of few microns.

Figure 6. EDAX analysis of HA/PCL scaffolds (A) after 3 days immersion in 5×SBF1 and (B) after 3 days immersion in 5×SBF1 and subsequent immersion in 5×SBF2 for 4 days. It can be observed that the Ca\P ratio was increased with increasing of immersing time in 5×SBF.

results are shown in Fig 5 and Fig 6. In the SEM images nucleation and growth of a homogenous structure is obvious. The image shows that the nanocomposite material was uniformly covered with a calcium phosphate crystals layer with a thickness of few microns. It is also possible to clarify the presence of HA deposit

Figure 8. XRD pattern of HA/PCL scaffolds after immersion in 5×SBF for 7days ( ■ : PCL, *: HA). The intensity of HA peaks was extensively increased after immersion in SBF solution.

300


74-566 and 09-432. This finding corroborates that the formed layer after biomimetic treatment on HA/PCL scaffold consists of HA material. Keeping in mind Fig 3 and Fig 8, it is clear that the intensity of HA peaks was extensively increased after immersion in SBF solution. As mentioned earlier, this phenomenon can be related to high surface area of HA nanorods, resulting in high absorption of desired molecules from SBF solution for HA formation. Consequently, HA nanorods show higher bioactivity than HA nanoparticles. As can be seen in Fig 8, PCL peaks are shortened and broadened in comparison with PCL peaks in Fig 3. This means after biomimetic treatment PCL weight percent and crystallinity were reduced. Meanwhile, In spite of this fact that cell attachment on PCL is poor due its hydrophobic character, the incorporation of the HA nano-crystals into the PCL matrix on the nano-scale can improve the biocompatibility of the resulting PCL/HA nanocomposites significantly.18, 63

5. C Hellmich, FJ Ulm, Average hydroxyapatite concentration is uniform in the extracollagenous ultrastructure of mineralized tissues: evidence at the 1–10 um scale., Biomech Model Mechanobiol, 2, 21 (2003). 6. LL Hench, DL Wheeler, DC Greenspan, Molecular control of bioactivity in sol–gel glasses, J Sol-Gel Sci Technol, 13, 245 (1998). 7. Y Liu, G Wang, Y Cai, et al., In vitro effects of nanophase hydroxyapatite particles on proliferation and osteogenic differentiation of bone marrow-derived mesenchymal stem cells, J Biomed Mater Res, 90, 1083 (2009). 8. DW Hutmacher, JT Schantz, CXF Lam, et al., State of the art and future directions of scaffold based bone engineering from a biomaterials perspective, J Tissue Eng Reg Med, 1, 245 (2007). 9. W Habraken, JGC Wolke, JA Jansen, Ceramic composites as matrices and scaffolds for drug delivery in tissue engineering, Adv Drug Deliv Rev, 59, 234 (2007). 10. HL Nichols, N Zhang, J Zhang, et al., Coating nanothickness degradable films on nanocrystalline hydroxyapatite particles to improve the bonding strength between nanohydroxyapatite and degradable polymer matrix, J Biomed Mater Res, 82A, 373 (2007). 11. H Zhou, J Lee, Nanoscale hydroxyapatite particles for bone tissue engineering, Acta Biomaterialia, 7, 2769 (2011). 12. G Kawachi, S Sasaki, K Nakahara, et al., Porous apatite carrier prepared by hydrothermal method, Key Eng Mater, 309-311, 935 (2006). 13. M Zandi, H Mirzadeh, C Mayer, et al., Biocompatibility evaluation of nano-rod hydroxyapatite/gelatin coated with nanoHAp as a novel scaffold using mesenchymal stem cells., J Biomed Mater Res, 92, 1244 (2010). 14. F Cuisinier, C Robinson, The structure of teeth: human enamel crystal structure: in handbook of biomineralization: medical and clinical aspects, m epple, e baeuerlein, (eds). Wiley-VCH, Weinheim, 177 (2007). 15. SR Chastain, AK Kundu, S Dhar, et al., Adhesion ofmesenchymal stemcells to polymer scaffolds occurs via distinct ECM ligands and controls their osteogenic differentiation, J Biomed Mater Res, 78, 73 (2006). 16. JJ Mao, X Xin, M Hussain, Continuing differentiation of human mesenchymal stem cells and induced chondrogenic and osteogenic lineages in electrospun PLGA nanofiber scaffold, Biomaterials, 28, 316 (2007). 17. APD Elfick, Poly([var epsilon]-caprolactone) as a potential material for a temporary joint spacer, Biomaterials, 23, 446 (2002). 18. L Shor, S Cuceri, X Wen, et al., Fabrication of three-dimensional polycaprolactone/hydroxyapatite tissue scaffolds and osteoblastscaffold interactions in vitro, Biomaterials, 28, 5291 (2007). 19. SC Baker, G Rohman, J Southgate, et al., The relationship between the mechanical properties and cell behaviour on PLGA and PCL scaffolds for bladder tissue engineering, Biomaterials, 30, 1321 (2009). 20. CM Agrawal, RB Ray, Biodegradable polymeric scaffolds for musculoskeletal tissue engineering, J Biomed Mater Res, 55, 141 (2001). 21. G Bezzi, G Celotti, E Landi, et al., A novel sol–gel technique

4. Conclusion A new in-situ sol-gel process was introduced for fabrication of HA/PCL nanocomposite and scaffolds containing HA nanorods. Starting with the low cost precursors and solvent such as calcium hydroxide, phosphoric acid and acetone respectively reduced the total cost of production. FE-SEM images showed that the HA nanorods were hemogenously dispersed in PCL polymeric matrix. XRD analysis confirmed that highly pure hydroxyl apatite with no undesirable phase such as calcium carbonate was formed within the nanocomposite. Moreover, biomimetic treatments on prepared HA/PCL scaffolds revealed that they can absorb bone cells effectively and form a HA layer uniformly. The bioactivity of the scaffolds was such high that the intensity of HA reflections in XRD pattern was increased two times. Acknowledgement: The authors would like to thank Iran Nanotechnology Initiative Council for the financial support.

References 1. JM Karp, PD Dalton, MS Shoichet, Scaffolds for tissue engineering, MRS Bull Cell Solid, 28, 301 (2003). 2. JR Porter, TT Ruckh, KC Popat, Bone tissue engineering: a review in bone biomimetics and drug delivery strategies, Biotechnol Progress, 25, 1539 (2009). 3. GE Poinern, RK Brundavanam, N Mondinos, et al., Synthesis and characterisation of nanohydroxyapatite using an ultrasound assisted method, Ultrason Sonochemistry, 16, 469 (2009). 4. S Weiner, HD Wagner, The material bone: structure-mechanical function relations, Annu Rev Mater Res, 28, 271 (1998).

301


for hydroxyapatite preparation, Mater Chem Phys, 78, 816 (2003). 22. DM Liu, T Troczynski, WJ Tseng, Water-based sol gel synthesis of hydroxyapatite: process development, Biomaterials, 22, 1721 (2001). 23. A Jillavenkatesa, RA Condrate, Sol–gel processing of hydroxyapatite, J Mater Sci, 33, 4111 (1998). 24. LY Huang, KW Xu, J Lu, A study of the process and kinetics of electrochemical deposition and the hydrothermal synthesis of hydroxyapatite coatings, J Mater Sci Mater Med, 11, 667 (2000). 25. DW Hutmacher, Scaffolds in tissue engineering bone and cartilage, Biomaterials, 21, 2529 (2000). 26. SH Oh, IK Park, JM Kim, et al., In vitro and in vivo characteristics of PCL scaffolds with pore size gradient fabricated by a centrifugation method, Biomaterials, 28, 1664 (2007). 27. AC Jones, CH Arns, DW Hutmacher, et al., The correlation of pore morphology, interconnectivity and physical properties of 3D ceramic scaffolds with bone ingrowth, Biomaterials, 30, 1440 (2009). 28. MG Raucci, V D’Antò, V Guarino, et al., Biomineralized porous composite scaffolds prepared by chemical synthesis for bone tissue regeneration, Acta Biomaterialia, 6, 4090 (2010). 29. P Fabbri, F Bondioli, M Messori, et al., Porous scaffolds of polycaprolactone reinforced with in situ generated hydroxyapatite for bone tissue engineering, J Mater Sci Mater Med, 21, 343 (2010). 30. MG Raucci, V Guarino, L Ambrosio, Hybrid composite scaffolds prepared by sol–gel method for bone regeneration, Compos Sci Technol, 70, 1861 (2010). 31. Y Wang, L Liu, S Guo, Characterization of biodegradable and cytocompatible nano-hydroxyapatite/polycaprolactone porous scaffolds in degradation in vitro, Polym Degrad Stab, 95, 207 (2010). 32. JV Araujo, A Martins, IB Leonor, et al., Surface controlled biomimetic coating of polycaprolactone nanofiber meshes to be used as bone extracellular matrix analogues, J Biomater Sci Polym Ed, 19, 1261 (2008). 33. P Wutticharoenmongkol, P Pavasant, P Supaphol, Osteoblastic phenotype expression of MC3T3–E1 cultured on electrospun polycaprolactone fiber mats filled with hydroxyapatite nanoparticles, Biomacromolecules, 8, 2602 (2007). 34. AP Marques, RL Reis, Hydroxyapatite reinforcement of different starch-based polymers affects osteoblast-like cells adhesion/spreading and proliferation, Mater Sci Eng C, 25, 215 (2005). 35. F Causa, PA Netti, L Ambrosio, et al., Poly-epsilon-caprolactone/ hydroxyapatite composites for bone regeneration: in vitro characterization and human osteoblast response, J Biomed Mater Res A, 76A, 151 (2005). 36. V Guarino, F Causa, PA Netti, et al., The role of hydroxyapatite as solid signal on performance of PCL porous scaffolds for bone tissue regeneration, J Biomed Mater Res B, 86B, 548 (2008). 37. SJ Heo, SE Kim, YT Hyun, et al., In vitro evaluation of poly ecaprolactone/hydroxyapatite composite as scaffolds for bone tissue engineering with human bone marrow stromal cells, Key Eng Mater, 342, 369 (2007). 38. D Verma, K Katti, D Katti, Bioactivity in in situ

hydroxyapatitepolycaprolactone composites, J Biomed Mater Res A, 78A, 772 (2006). 39. LA Azaroff, Elements of X-ray crystallography. McGraw-Hill, New York, USA (1968). 40. E Landi, A Tampieri, G Celotti, et al., Densification behaviour and mechanisms of synthetic hydroxyapatites, J Eur Ceram Soc, 20, 2377 (2000). 41. YF Chou, WA Chiou, Y Xu, et al., The effect of pH on the structural evolution of accelerated biomimetic apatite, Biomaterials, 25, 5323 (2004). 42. M Tanahashi, K Hata, T Kokubo, et al., Effect of substrate on apatite forming by a biomimetic process: in bioceramics, T Yamamuro, T Kokubo,T Nakamuro (eds.) Kobunshi Kankokai, Kyoto, 57 (1992). 43. V Guarino, P Taddei, M Di Foggia, et al., The influence of hydroxyapatite particles on in vitro degradation behaviour of PCL based composite scaffolds, Tissue Engineering A, 15, 3655 (2009). 44. I Mobasherpour, MS Heshajin, A Kazemzadeh, et al., Synthesis of nanocrystalline hydroxyapatite by using precipitation method, J Alloys Compd, 430, 330 (2007). 45. SK Padmanabhan, A Balakrishnan, M-C Chu, et al., Sol–gel synthesis and characterization of hydroxyapatite nanorods, Particuology, 7, 466 (2009). 46. PX Ma, R Zhang, G Xiao, et al., Engineering new bone tissue in vitro on highly porous poly(a-hydroxyl acids)/hydroxyapatite composite scaffolds, J Biomed Mater Res, 54, 284 (2001). 47. JM Orban, KG Marra, JO Hollinger, Composition options for tissue-engineered bone, Tissue Eng, 8, 529 (2002). 48. RC Thomson, MJ Yaszemski, JM Powers, et al., Hydroxyapatite fiber reinforced poly(a-hydroxy ester) foams for bone regeneration, Biomaterials, 19, 1935 (1998). 49. R Zhang, PX Ma, Poly(a-hydroxyl acids)/hydroxyapatite porous composites for bone-tissue engineering. I. Preparation and morphology, J Biomed Mater Res, 44, 446 (1999). 50. E Tsuruga, H Tahita, H Itoh, et al., Pore size of porous hydroxyapatite as the cell-substratum controls BMP-induced osteogenesis, J Biochem, 121, 317 (1997). 51. WJ Li, R Tuli, XX Huang, et al., Multilineage differentiation of human mesenchymal stem cells in a three-dimensional nanofibrous scaffold, Biomaterials, 26, 5158 (2005). 52. WJ Li, EW Shi, WZ Zhong, et al., Growth mechanism and growth habit of oxide crystals, J Cryst Growth, 203, 186 (1999). 53. J Liu, K Li, H Wang, et al., Rapid formation of hydroxyapatite nanostructures by microwave irradiation, Chem Phys Lett, 396, 429 (2004). 54. P Dalvand, MR Mohammadi, Controlling morphology and structure of nanocrystalline cadmium sulfide (CdS) by tailoring solvothermal processing parameters, J Nanopart Res, 13, 3011 (2011). 55. J Dong, T Uemura, H Kojima, et al., Application of lowpressure system to sustain in vivo bone formation in osteoblast/ porous hydroxyapatite composite, Mater Sci Eng, C, 17, 37 (2001). 56. HW Kim, JC Knowles, HE Kim, Hydroxyapatite/poly(ecaprolactone) composite coatings on hydroxyapatite porous bone scaffold for drug delivery, Biomaterials, 25, 1279 (2004). 57. HK Varma, SS Babu, Synthesis of calcium phosphate bioceramics

302


by citrate gel pyrolysis method, Ceram Int, 31, 109 (2005). 58. M Kawata, H Uchida, K Itatani, et al., Development of porous ceramics with well-controlled porosities and pore sizes from apatite fibers and their evaluations, J Mater Sci Mater Med, 15, 817 (2004). 59. F Miyaji, Y Kono, Y Suyama, Formation and structure of zinc– substituted calcium hydroxyapatite, Mater Res Bull, 40, 209 (2005). 60. JP Chen, YS Chang, Preparation and characterization of composite nanofibers of polycaprolactone and nanohydroxyapatite for osteogenic differentiation of mesenchymal stem cells, Colloid Surf

B, 86, 169 (2011). 61. T Kokubo, HM Kim, M Kawashita, Novel bioactive materials with different mechanical properties, Biomaterials, 24, 2161 (2003). 62. M Brink, T Turunen, RP Happonen, et al., Compositional dependence of bioactivity of glasses in the system Na2O–K2O– MgO–CaO–B2O3–P2O5–SiO2, J Biomed Mater Res, 37, 114 (1997). 63. WY Choi, HE Kim, SY Oh, et al., Synthesis of poly(åcaprolactone)/hydroxyapatite nanocomposites using in-situ coprecipitation, Mater Sci Eng C, 30, 777 (2010).

303