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functionalized multi-walled carbon nanotube composite
Pressureless sintering and mechanical properties of atite/functionalized multi-walled carbon nanotube composite
hydroxyap-
M.J. Abden, J.D. Afroze, M.S. Alam, N.M. Bahadur PII: DOI: Reference:
S0928-4931(16)30444-1 doi: 10.1016/j.msec.2016.05.018 MSC 6508
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
24 December 2015 17 April 2016 5 May 2016
Please cite this article as: M.J. Abden, J.D. Afroze, M.S. Alam, N.M. Bahadur, Pressureless sintering and mechanical properties of hydroxyapatite/functionalized multiwalled carbon nanotube composite, Materials Science & Engineering C (2016), doi: 10.1016/j.msec.2016.05.018
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ACCEPTED MANUSCRIPT Pressureless sintering and mechanical properties of hydroxyapatite/functionalized multi-walled carbon nanotube composite M. J. Abdena,c,*, J. D. Afrozeb,c, M. S. Alamb, N. M. Bahadurb Department of Electrical and Electronic Engineering, International Islamic University
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a
Chittagong, Chittagong 4203, Bangladesh
Department of Applied Chemistry and Chemical Engineering, Noakhali Science and
Technology University, Noakhali 3802, Bangladesh c
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b
Development of Materials for Tools and Bio-Metallic Implant, Bangladesh Council of
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Science and Industrial Research, Dhaka 1205, Bangladesh *
Corresponding author: Cell: +880 1712030929, E-mail: [email protected]
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Abstract
This work aims to study the optimum sintering conditions of hydroxyapatite/functionalized multi-walled carbon nanotube (HA/f-MWCNT) composite with improved mechanical properties for bone implant applications using a pressureless sintering technique. The
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carboxyl functional group (–COOH) introduced by the acid treatment on the MWCNT
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surface by which HA molecules are grafted onto the surface of functionalized MWCNT with strong interfacial bonding. The composite exhibits a lower hemolytic rate of 1.27%. The flexible nature of f-MWCNT makes them bend and attached to the HA grains, indicates that
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f-MWCNT bear significant stress by sharing a portion of the load and it leads to improve their mechanical properties. The maximum Vickers hardness of 3.6 GPa is obtained for the HA/f-MWCNT composite sintered at 1100 °C, whereas the highest compressive strength of 481.7 MPa and fracture toughness of 2.38 MPa.m1/2 is achieved after sintering at 1150 °C. This study demonstrated that HA/f-MWCNT composite create suitable structures by vacuum pressureless sintering technique to satisfy the mechanical requirements for bone tissues. Keywords: Hydroxyapatite, Carbon nanotube, Pressureless sintering, Hemocompatibility, Mechanical properties
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ACCEPTED MANUSCRIPT Introduction Over the last few years, there have been growing demands for replacing hard tissue with artificial implant biomaterials as a result of the increasing incidence of various bone injuries
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and failures. An overriding guideline in the design of innovative biodegradable material for replacing failed hard tissue, such as bone tissue is the request of able to provide bone bonding
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with surrounding living tissue to serve as a long-lasting implant with sufficient mechanical strength [1, 2]. Ultimately, the implant is gradually eliminated from the body to be replaced
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by newly formed bone tissues as the implant biodegrades.
Hydroxyapatite (HA) is biologically active bioceramic has often been employed as a bone implant material due to its excellent biocompatibility, bioactivity and osteoconductivity [3,
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4]. It is chemically and structurally very similar to the mineralized matrix of natural bone [5, 6]. Formation of chemical bond actively with the living hard tissue offers HA a greater advantage in bone implant applications. However, due to inferior mechanical properties of HA the recent trend in bioceramic research is focused on improving their mechanical
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properties by incorporating second phase reinforcements, such as polymers [7-9] and hard
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ceramics [10-13] along with applying different sintering processes. However, a significant amount of the reinforcing phase is needed to achieve the desired properties as these phases are either bioinert, considerably less bioactive and the ability of the composite to form a
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stable interface with bone is poor compared with pure HA. Therefore, keeping in concern the bioactivity of the composite structure, the ideal reinforcement material is the one that can increase the mechanical integrity significantly with a low content of second phase. Carbon nanotube (CNT), especially multi-walled carbon nanotube (MWCNT) with their excellent stiffness and strength have stimulated their use possess great potential as a reinforcing agent for HA without offsetting their bioactivity [14, 15]. CNT-based biomaterials are proven to be suitable for cell growth and degraded by enzymes in the human body with the favorable degradation rate for bone regeneration [16, 17]. However, CNT tend to form bundles and the solubility in common solvents is very limited. With the aim to obtain an efficient mechanical reinforcement, the surface of CNT needs to be functionalized or chemically modified to achieve a uniform dispersion of CNT in the matrix material and also to induce an ideal interface between CNT and HA which is ultimately responsible for efficient load-transfer mechanism [18-20].
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ACCEPTED MANUSCRIPT In addition, a lot of methods, including hot pressing [21], hot isostatic pressing [22, 23], pressureless sintering [22] and spark plasma sintering [24, 25] have been employed for the fabrication of free standing HA–CNT composite. Among these methods, pressureless
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sintering is still a most favorable one due to its low cost and easy technique. Moreover, pressureless sintering technique might allow the retention of both CNTs and HA’s phase
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purity as well as OH groups, while providing enough densification for improved mechanical properties over HA [22]. There are only a few reports on HA/MWCNT composite have been reported by pressureless sintering technique. Mukherjee et al. [26] synthesized and
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characterized HA/MWCNT composite using shear mixing technique in argon to investigate the physical, mechanical, biological and in vitro bioactivity properties of composites
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containing different ratios of MWCNT. Another recent study they synthesized HA on pristine and functionalized MWCNT which was also densified using argon atmosphere, to report on the effects of functionalized MWCNT enhancing the mechanical properties of the composite [27]. Li et al. [28] investigated the fracture toughness of HA/MWCNT composite to be as
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high as 2.40 MPa.m1/2 with 3 wt.% MWCNT addition by pressureless sintering in a vacuum,
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which is 200% higher than the one sintered in argon atmosphere. However, they reported that the addition of MWCNT decomposed the HA into α-TCP and γ-Ca2P2O7 which make it unsuitable for real applications. In fact, to the best of author’s knowledge, there are no reports
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on the mechanical properties of the composite containing functionalized multi-walled carbon nanotube (f-MWCNT) by pressureless sintering in a vacuum. Hence this paper aims to study the optimum sintering conditions of the pressureless sintering in a vacuum to fabricate the HA/f-MWCNT composite that overcome the existing limitations and disadvantages. In addition, we perform an in-depth investigation of the mechanical properties and attempt to correlate them with structural parameters using a variety of techniques to check its suitability for biomedical applications, such as load bearing bone repair. 2. Experimental procedure 2.1 Materials Pristine MWCNTs (purity > 95%, 10-15 nm in diameter, 0.1-10 μm in length) that have been used during this experiment were purchased from Sigma Aldrich, USA. All other chemical reagents such as sulfuric acid (H2SO4, 97%), nitric acid (HNO3, 70%), calcium hydroxide
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ACCEPTED MANUSCRIPT (Ca(OH)2, 95%), phosphoric acid (H3PO4, 85%) , ammonium hydroxide (NH4OH, 28-30%) were of analytical grade and used as received. 2.2 Preparation of HA/functionalized MWCNT composite discs
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The carboxylic groups on pristine MWCNTs (purity > 95%, 10-15 nm in diameter, 0.1-10 μm in length) were introduced by refluxing them in a mixture of 3:1 concentrated H2SO4 and
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HNO3 under stirring at 70 °C for 4 h, left to sit for 12 h, and then stirring for an additional 2 h at the same temperature. The resulting solution was diluted with DI water and left overnight.
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Afterward, the mixture was vacuum filtered and washed with DI water repeatedly until it had a neutral pH. The resulting carboxylated MWCNTs (f-MWCNTs) were dried in a vacuum at 60 °C for 12 h. The f-MWCNTs were then dispersed in DI water, mixed with a 1M aqueous
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solution of Ca(OH)2 and stirred for 1 h under ambient conditions. The stock solution of H3PO4 of 0.6 M, used as an initiator was added dropwise under constant stirring at room temperature. The solution was adjusted to ~10.5 pH by using NH4OH. The relative amounts of reactants were calculated to maintain a Ca/P ratio of 1.67 equal to that of stoichiometric
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HA. The HA grafted f-MWCNTs thus obtained was centrifuged and dried in a vacuum at 90
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°C for 6 h. The sample was ball milled by using a high-energy planetary ball mill (Retsch, PM 100 Japan) at 400 rpm for 3 h in ethanol. To minimize contamination during milling a zirconia pot and balls (10 mm diameter) with a ball-to-powder weight ratio of 10:1 were
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used. This solution was then dried at 50 °C for 24 h to remove the ethanol. The f-MWCNT content in these samples was 2 wt. % by controlling the amount of f-MWCNT and HA matrix. Powder compacts with dimensions of 10 mm × 6 mm were formed by pressing at 150 MPa. The pressureless sintering was performed at 950 to 1200 °C by maintaining at a heating rate of 5 °C/min for holding time 2 h in a vacuum atmosphere. The cooling rate was 5 °C/min. 2.3 Characterization The morphology and dispersion of f-MWCNT in HA matrix were carried out by using field emission scanning electron microscope (FESEM, JEOL JSM-7600F, Japan) and transmission electron microscope (TEM, LIBRA 120 Zeiss, Germany). The crystalline phase of the samples were conducted by X-ray diffractometer (XRD, D8 ADVANCE Bruker AXS Karlsruhe, Germany) with Cu-Kα radiation (λ = 0.15406 nm) in a continuous mode from 2θ range 25-55° at an angular step of 0.02° and a fixed counting time of 0.6 s/step. The voltage and current were set at 40 kV and 40 mA, respectively. The chemical structure and 4
ACCEPTED MANUSCRIPT interaction of f-MWCNT with HA matrix were detected with a Fourier transform infrared (FTIR, Perkin-Elmer Paragon 500) using KBr powder. Thermogravimetric analysis was carried out by using TG/DTA 6300 (SII NanoTechnology Inc.) under a nitrogen atmosphere
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between room temperature and 1000 °C at a heating rate of 10 °C/min. 2.4 Hemolysis assay
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Cytotoxicity of the HA/f-MWCNT composite was evaluated by studying their percent hemolysis in blood. The hemolytic activity was studied following standard protocols using
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human red blood cells (RBCs) as the samples [29]. The RBC were isolated by centrifugation, washed and then diluted via sterile phosphate buffered saline (PBS). The HA/f-MWCNT composite were washed with deionized water three times and then put into a test tube with 10
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mL 0.9% saline and incubated for 30 min at 37 °C under static conditions. After that, 0.2 mL of diluted blood was added into test tube and incubated for another 60 min at 37 °C. Similarly, 0.2 mL of diluted blood was added to 10 mL of deionized water and 0.9% saline solution using as a positive and negative controls, respectively. After the incubation, each
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tube was centrifuged at 2000 rpm for 10 min and the absorbance of the supernatant was
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measured at 545 nm by using a spectrophotometer (UV-2550, Japan). The hemolysis activity was calculated by using the equation (1): (1)
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Hemolytic rate (%) =
where A is the absorbance values of the test samples, B and C are the absorbance values of negative and positive control groups respectively. The hemolysis results were average of three measurements.
2.5 Statistical analysis The data was expressed as means ± standard deviation (SD) and was analyzed by SPSS (version 11.0). The P < 0.05 was considered statistically significant. 2.6 Mechanical properties The compressive strength of the cylindrical sample was measured using a universal tensile tester (Tenstometric FS-300KN, UK) at a cross-head speed of 0.5 mm/min. Five samples of each temperature were tested and the results were averaged. A Vicker’s micro-indentation instrument (Shimadzu Corporation HMV-2, Japan) was used to determine the hardness of the samples by applying a 19.614 N force for 6 s on the fracture surface. The indentation fracture 5
ACCEPTED MANUSCRIPT toughness of the sample was calculated using diagonal crack lengths produced at the indentation corners from the micro-indentation tests. The fracture toughness was measured by using the equation (2) [30]: KIC = 0.0726 (P/c3/2)
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(2)
where KIC is the fracture toughness, P is the applied load and c is the diagonal crack length.
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The hardness and fracture toughness values were averaged for three samples with five indents per sample.
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3. Results and discussion 3.1 Morphology and phase analysis
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The FESEM micrograph and XRD patterns of as-dried HA/f-MWCNT composite are shown in Fig. 1. As seen in Fig. 1(a), the unique flexible nature of f-MWCNT makes them bend and pass through space are attached to the HA grains, indicates that f-MWCNT bear significant stress by sharing a portion of the load, at the same time; strengthen and toughen the HA
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matrix. To further characterize the HA/f-MWCNT composite, X-ray diffraction analysis is
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performed and the patterns are shown in Fig. 1(b). The low diffraction peaks at 26.6° and 43.8° are assigned to graphite (002) and (100) planes of f-MWCNT. The remaining peaks at 25.9°, 28.1°, 29°, 31.9°, 33°, 34.2°, 40°, 46.7°, 48.1°, 49.4° and 53.2° are assigned to (002),
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(102), (210), (211), (300), (202), (310), (222), (312), (213) and (004) reflections of HA, respectively (JCPDS PDF No. 09-0432). The broad nature of the diffraction peaks which are seen in Fig. 1(b) suggests that the HA crystals are very small in size. 3.2 TEM analysis
Fig. 2 shows the TEM micrograph of f-MWCNT and HA/f-MWCNT composite. As seen in Fig. 2 (a), the f-MWCNTs tubular chains with a diameter 10-20 nm are found to be smooth and debundled structure. The f-MWCNT maintained their typical hollow structure in the HA/f-MWCNT composite, as shown in Fig. 2 (b), and would therefore be expected to act as an excellent reinforcement in the HA matrix. The TEM image confirmed that the fine HA nanorods are well attached to the f-MWCNT surface. This nano-HA has good bioactivity and excellent biocompatibility which are important for bone tissue engineering applications [31]. Shi et al. revels that individual 25-50 nm HA crystal is the essence of bone in terms of mechanical properties, bioresorbability and play an important role in biomineral formation [32]. The isolated appearance of HA nanorods in Fig. 2(b) most likely are an artifact related
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ACCEPTED MANUSCRIPT to the TEM specimen preparation. This requires the application of ultrasonic pulses with high energy densities leading to the ultimate detachment of formerly attached HA nanorods. 3.3 FTIR and TG/DTA analysis
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Fig. 3(a) shows the FTIR spectra of the HA/f-MWCNT composite in the range 400−4000 cm. The FTIR spectrum confirmed the bands corresponding to PO43− (ν3-1041, ν1-962, ν4-602,
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ν4-670, ν4-573 cm-1) and CO32− (ν3-1438, ν2-872 cm-1), respectively [33, 34]. The broad band at 3433 cm-1 is attributed due to the O−H stretch of the hydroxyl group which can be ascribed to the oscillation of carboxyl groups (O=C−OH and C−OH) [35]. The peak found at 1616 cmis due to C=C stretching of the f-MWCNTs. The prominent peak at 2360 cm-1 is associated
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with O-H stretch from strongly hydrogen bonded –COOH [35]. The carbonate containing HA
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is chemically and structurally more similar to that in natural bone. The spectrum in the ν4 PO43− domain exhibits the bands at 602 and 573 cm-1 is assigned to PO43− ions in apatite sites [36]. In the ν2 CO32− domain exhibits the band at 872 cm-1 which may improve the bioactivity of HA and similar to the characteristic ones observed in bone crystals [36]. The TG/DTA
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curves of HA/f-MWCNT composite are shown in Fig. 3(b). A gradual and continuous weight
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loss of 30.3% is observed at temperatures ranging from 25 to 1000 °C. The first step weight loss below 120 °C is due to the liberation of absorbed water and the second step in a temperature range of 120 to 500 °C is probably attributed to the elimination of water released
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from decarboxylation or condensation of HPO4−2 [37]. Finally, the third step is maybe due to the loss of constitution water of HA which can change to oxyapatite (Ca10(PO4)6O) [38]. In the DTA curve, no endothermic and exothermic peaks up to 1000 °C which are related to organic compound decomposition are detected. This confirms that the obtained HA sample with HA/f-MWCNT composite are sufficiently pure. 3.4 X-ray diffraction analysis Fig. 4 exhibits the XRD patterns of HA/f-MWCNT composite as a function of sintering temperature. As seen in Fig. 4, the major peaks in all three patterns are from HA. Low intensity graphite peak (2θ = 26.6°) is observed in the diffraction patterns of the composite. The intensity of graphite (002) peak increased substantially with the sintering temperature which indicates the more graphite structure is developed at higher temperature. No other peaks corresponding to Ca3(PO4)2 between 30° and 31° are detected at all sintering temperatures [24]. This implies that the reaction resulted in the formation of phase pure HA and at a higher sintering temperature does not lead to parts of HA decomposing. It is worth 7
ACCEPTED MANUSCRIPT noting that there are reports indicating that HA/CNT composite decomposed in to β Ca3(PO4)2 at 1100 °C [25]. A further observation of the curve (c) indicates that the diffraction peaks corresponding to HA become sharper and narrower in comparison with curves (a) and
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(b), evidence of higher crystallinity of HA. 3.5. Hemocompatibility study
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Hemolysis is a measure of cytotoxicity of a biomaterial towards red blood cells (RBCs). Cytotoxicity results in the rupture of RBC membranes and the release of hemoglobin.
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Spectrometry of the free hemoglobin allows the estimation of the extent of cytotoxicity. Table 1 shows that HA/f-MWCNT composite, fabricated with 2 wt.% f-MWCNT addition exhibit a lower hemolytic-rate of 1.27%. If the hemolytic-rate of any material is less than 5%,
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it is considered to be highly hemocompatible [29]. Therefore, the throughput of this study clearly reveals that the HA/f-MWCNT composite exhibits excellent hemocompatibility. 3.6 Mechanical properties and toughening mechanisms
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Fig. 5 shows the effect of sintering temperature on the compressive strength of HA/f-
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MWCNT composite. The maximum compressive strength of 481.7 MPa is reached at 1150 °C which is much higher than that of the HA/CNT composite (156.8 MPa) [39]. The significant differences are thought to result from the variations, both materials processing and
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in the sintering processes, such as sintering atmosphere, heating rate and holding time. Moreover, the use of as-dried HA/f-MWCNT composite (Fig. 1(a)) ensures the well distribution, flexible nature and good bonding of f-MWCNT with HA matrix. Hence, the effective load transfer from the matrix to f-MWCNT is possible and a significant load sharing of f-MWCNT mainly enhances the mechanical properties of HA/f-MWCNT composite. Fig. 6 shows the hardness and fracture toughness as a function of sintering temperature. As seen in Fig. 6, the hardness increases with an increase in sintering temperature to a maximum, then decreases to further increase in temperature. The composites sintered at 1100 °C shows a maximum hardness value of 3.6 GPa. In addition, it is observed that the fracture toughness of the composite sintered at 1150 °C is 2.38 MPa.m1/2, which is higher than the reported literature value of 1.9 MPa.m1/2 for the HA/MWCNT composite sintered in Ar atmosphere [26]. The fracture toughness improvement in the present study is partly attributable to the differences in their sintering atmosphere. It turns out that HA and MWCNT both are absorbed gas intensively. Therefore, in a vacuum that is favorable for gas expulsion and densification of hydroxyapatite, resulting improved their mechanical properties but it is 8
ACCEPTED MANUSCRIPT contrary in Ar. The cracks initiated by Vickers indentation in the HA/f-MWCNT composite layers are shown in Fig. 7. Detailed FESEM investigations inside the indentation cracks in the HA/f-MWCNT composite reveal that two factors are responsible for the improvement in
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fracture toughness. One of them is the resistance to crack propagation of HA/f-MWCNT composite. Observations at higher magnification images of arms of the indented sample
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which is marked as square in Fig. 7(a) is illustrated in Fig. 7(b) and (c) where the crack deflections are clearly evident. The crack deflection in HA matrix that creates a more tortuous path to release residual stresses which is an important mechanism that promotes
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increases in fracture toughness. Another mechanism is f-MWCNT bridging (Fig. 7(d)) that absorb more energy and provide resistance to crack growth. Thus, the functionalized
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MWCNT in the composite tend to break the crack from its propagation and yield the high toughness through crack bridging under the indentation load [40]. The decrease in mechanical properties from 1150 to 1200 °C in the present study can be ascribed to the effect of MWCNT. Peigney et al. [41] reported that the quantity of CNT retained in the massive
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composite is more dependent on the treatment temperature than of the nature of the oxide
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matrix. In agreement with previous observations [25], the f-MWCNT are damaged and broken when the treatment is at 1200 °C (Fig. 8), leading to a decrease or total loss of reinforcing effects of f-MWCNT, resulted in a drop in the mechanical properties.
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From the above, it can be summarized that the improvement of the compressive strength, hardness and fracture toughness are attributed to the well distribution and strong bonding of f-MWCNTs with HA matrix. The load sharing and bridging effect of f-MWCNTs result in the composite with significant enhancement of mechanical properties. Moreover, the hemocompatibility evaluation result shown that the HA/f-MWCNT composite exhibited very good hemocompatibility. Thus, the prepared composite with the addition of 2 wt.% fMWCNT might be a promising bone repair implant material. 4. Conclusion Hydroxyapatite/functionalized multi-walled carbon nanotube (HA/f-MWCNT) composite is successfully fabricated by in-situ chemical precipitation of HA on modified MWCNT and followed by vacuum pressureless sintering at 950 °C to 1200 °C. The carboxyl functional group (–COOH) is introduced on the MWCNT surface by the acid treatment are favorable for chemical bonding between the HA and MWCNT. The prepared composite exhibited excellent hemocompatibility. The strengthening mechanism is based on the significant load
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ACCEPTED MANUSCRIPT sharing between the HA matrix and well distributed modified MWCNT. At the same time, the fracture toughness is strongly related to the crack deflection at the HA/f-MWCNT interface and crack bridging effect by MWCNT during the crack propagation. The maximum
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Vickers hardness of 3.6 GPa is obtained at the sintering temperature of 1100 °C. Moreover, the highest compressive strength and fracture toughness of the HA/f-MWCNT composite
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sintered at 1150 °C are 481.7 MPa and 2.38 MPa.m1/2, respectively. Based on the findings of this study, it can be summarized that vacuum pressureless sintering technique have been successfully utilized to fabricate the HA/f-MWCNT composite with improved mechanical
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properties without any chemical decomposition of HA matrix for a wide variety of bone implant applications.
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Acknowledgments
The authors wish to express their appreciation to Prof. Dr. Md. Fakhrul Islam for providing FESEM facilities.
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[27] S. Mukherjee, B. Kundu, A. Chanda, S. Sen, Effect of functionalisation of CNT in the preparation of HAp–CNT biocomposites, Ceram. Int. 41 (2015) 3766-3774. [28] A. Li, K. Sun, W. Dong, D. Zhao, Mechanical properties, microstructure and histocompatibility of MWCNTs/HAp biocomposites, Mater. Lett. 61 (2007) 1839–1844. [29] Z.X. Meng, W. Zheng, L. Li , Y.F. Zheng, Fabrication and characterization of three dimensional nanofiber membrance of PCL–MWCNTs by electrospinning, Mater. Sci. Eng. C 30 (2010) 1014–1021. [30] M.J. Abden, J.D. Afroze, M.A. Mamun, M.M. Haque, Microstructure and mechanical properties of ZrO2-40 wt% Al2O3 composite ceramics, Mater. Exp. 4 (2014) 317-323. [31] J. Huang, Y.W. Lin, X.W. Fu, S.M. Best, R.A. Brooks, N. Rushton, W. Bonfield, Development of nano-sized hydroxyapatite reinforced composites for tissue engineering scaffolds, J. Mater. Sci.: Mater. Med. 18 (2007) 2151–2157. [32] Z. Shi, X. Huang, Y. Cai, R. Tang, D. Yang, Size effect of hydroxyapatite nanoparticles on proliferation and apoptosis of osteoblast-like cells, Acta Biomater. 5 (2009) 338–345.
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ACCEPTED MANUSCRIPT [33] S. Tavakol, M.R. Nikpour, A. Amani, M. Soltani, S.M. Rabiee, S.M. Rezayat, P. Chen, M. Jahanshahi, Bone regeneration based on nano-hydroxyapatite and hydroxyapatite/chitosan nanocomposites: an in vitro and in vivo comparative study, J. Nanopart. Res. 15 (2013) 1373-
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1388. [34] J.K. Han, H.Y. Song, F. Saito, B.T. Lee, Synthesis of high purity nano-sized
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[35] W.M. Davis, C.L. Erickson, C.T. Johnston, J.J. Delfino, J.E. Porter, Quantitative fourier
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[36] S. Padilla, I. Izquierdo-Barba, M. Vallet-Regi, High specific surface area in nanometric carbonated hydroxyapatite, Chem. Mater. 20 (2008) 5942–5944. [37] G.M. Neelgund, K. Olurode, Z. Luo, A. Oki, A simple and rapid method to graft hydroxyapatite on carbon nanotubes, Mater. Sci. Eng. C 31 (2011) 1477-1481.
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[38] K.D. Son, D.J. Yang, K.S. Kim, I.K. Kang, S.Y. Kim, Y.J. Kim, Effect of alginate as
132 (2012) 1041-1047.
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polymer matrix on the characteristics of hydroxyapatite nanoparticles, Mater. Chem. Phy.
[39] X.Y. Lu, N.Y. Zhang, L. Wei, J.W. Wei, Q.Y. Deng, X. Lu, K. Duan, J. Weng,
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Fabrication of carbon nanotubes/hydroxyapatite nanocomposites via an in situ process, Appl. Surf. Sci. 262 (2012) 110-113. [40] Z. Xia, L. Riester, W.A. Curtin, H. Li, B.W. Sheldon, J. Liang, B. Chang, J.M. Xu, Direct observation of toughening mechanisms in carbon nanotube ceramic matrix composites, Acta Mater. 52 (2004) 931–944 [41] A. Peigney, S. Rul, F. Lefevre-Schlick, C. Laurent, Densification during hot-pressing of carbon nanotube-metal-magnesium aluminate spinel nanocomposites, J. Eur. Ceram. Soc. 27 (2007) 2183-2193.
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Figure captions: (a) FESEM images, (b) XRD pattern of HA/f-MWCNT composite.
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TEM images of (a) f-MWCNTs, (b) HA/f-MWCNT composite.
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(a) FTIR spectra, (b) TG/DTA curves of HA/f-MWCNT composite.
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XRD patterns of HA/f-MWCNT composite at different temperatures: (a) 1100, (b)
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1150, (c) 1200 °C. Fig. 5
Compressive strength of HA/f-MWCNT composite as a function of sintering
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Vickers hardness and fracture toughness of HA/f-MWCNT composite as a function
Fig. 7
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of sintering temperatures.
FESEM images of HA/f-MWCNT composite at 1150 °C showing (a) Vickers indentation, (b-c) crack deflection, (d) bridging action of f-MWCNT chains across the cracks in HA matrix.
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FESEM image of HA/f-MWCNT composite at 1200 °C.
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Fig. 1 (a) FESEM images, (b) XRD pattern of HA/f-MWCNT composite.
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Fig. 2 TEM images of (a) f-MWCNTs, (b) HA/f-MWCNT composite.
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Fig. 3 (a) FTIR spectra, (b) TG/DTA curves of HA/f-MWCNT composite.
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1150, (c) 1200 °C.
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Fig. 4 XRD patterns of HA/f-MWCNT composite at different temperatures: (a) 1100, (b)
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Fig. 5 Compressive strength of HA/f-MWCNT composite as a function of sintering
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Fig. 6 Vickers hardness and fracture toughness of HA/f-MWCNT composite as a function of
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Fig. 7 FESEM images of HA/f-MWCNT composite at 1150 °C showing (a) Vickers indentation, (b-c) crack deflection and (d) bridging action of f-MWCNT chains across the cracks in HA matrix.
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Fig. 8 FESEM image of HA/f-MWCNT composite at 1200 °C.
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Table 1. Hemolytic rate of the HA/f-MWCNT composite Hemolytic rate (%)
Standard deviation
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± 0.08
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HA/2 wt.% f-MWCNT
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Highlights:
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Prepared HA grafted modified MWCNTs by vacuum pressureless sintering. Carboxyl group (–COOH) is introduced by the acid treatment on the MWCNT surface. HA/f-MWCNT strong interfacial bonding and well f-MWCNT dispersion. And the ultimate mechanical properties are near to the natural bone.
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hydroxyap-
M.J. Abden, J.D. Afroze, M.S. Alam, N.M. Bahadur PII: DOI: Reference:
S0928-4931(16)30444-1 doi: 10.1016/j.msec.2016.05.018 MSC 6508
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
24 December 2015 17 April 2016 5 May 2016
Please cite this article as: M.J. Abden, J.D. Afroze, M.S. Alam, N.M. Bahadur, Pressureless sintering and mechanical properties of hydroxyapatite/functionalized multiwalled carbon nanotube composite, Materials Science & Engineering C (2016), doi: 10.1016/j.msec.2016.05.018
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ACCEPTED MANUSCRIPT Pressureless sintering and mechanical properties of hydroxyapatite/functionalized multi-walled carbon nanotube composite M. J. Abdena,c,*, J. D. Afrozeb,c, M. S. Alamb, N. M. Bahadurb Department of Electrical and Electronic Engineering, International Islamic University
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a
Chittagong, Chittagong 4203, Bangladesh
Department of Applied Chemistry and Chemical Engineering, Noakhali Science and
Technology University, Noakhali 3802, Bangladesh c
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b
Development of Materials for Tools and Bio-Metallic Implant, Bangladesh Council of
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Science and Industrial Research, Dhaka 1205, Bangladesh *
Corresponding author: Cell: +880 1712030929, E-mail: [email protected]
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Abstract
This work aims to study the optimum sintering conditions of hydroxyapatite/functionalized multi-walled carbon nanotube (HA/f-MWCNT) composite with improved mechanical properties for bone implant applications using a pressureless sintering technique. The
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carboxyl functional group (–COOH) introduced by the acid treatment on the MWCNT
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surface by which HA molecules are grafted onto the surface of functionalized MWCNT with strong interfacial bonding. The composite exhibits a lower hemolytic rate of 1.27%. The flexible nature of f-MWCNT makes them bend and attached to the HA grains, indicates that
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f-MWCNT bear significant stress by sharing a portion of the load and it leads to improve their mechanical properties. The maximum Vickers hardness of 3.6 GPa is obtained for the HA/f-MWCNT composite sintered at 1100 °C, whereas the highest compressive strength of 481.7 MPa and fracture toughness of 2.38 MPa.m1/2 is achieved after sintering at 1150 °C. This study demonstrated that HA/f-MWCNT composite create suitable structures by vacuum pressureless sintering technique to satisfy the mechanical requirements for bone tissues. Keywords: Hydroxyapatite, Carbon nanotube, Pressureless sintering, Hemocompatibility, Mechanical properties
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ACCEPTED MANUSCRIPT Introduction Over the last few years, there have been growing demands for replacing hard tissue with artificial implant biomaterials as a result of the increasing incidence of various bone injuries
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and failures. An overriding guideline in the design of innovative biodegradable material for replacing failed hard tissue, such as bone tissue is the request of able to provide bone bonding
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with surrounding living tissue to serve as a long-lasting implant with sufficient mechanical strength [1, 2]. Ultimately, the implant is gradually eliminated from the body to be replaced
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by newly formed bone tissues as the implant biodegrades.
Hydroxyapatite (HA) is biologically active bioceramic has often been employed as a bone implant material due to its excellent biocompatibility, bioactivity and osteoconductivity [3,
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4]. It is chemically and structurally very similar to the mineralized matrix of natural bone [5, 6]. Formation of chemical bond actively with the living hard tissue offers HA a greater advantage in bone implant applications. However, due to inferior mechanical properties of HA the recent trend in bioceramic research is focused on improving their mechanical
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properties by incorporating second phase reinforcements, such as polymers [7-9] and hard
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ceramics [10-13] along with applying different sintering processes. However, a significant amount of the reinforcing phase is needed to achieve the desired properties as these phases are either bioinert, considerably less bioactive and the ability of the composite to form a
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stable interface with bone is poor compared with pure HA. Therefore, keeping in concern the bioactivity of the composite structure, the ideal reinforcement material is the one that can increase the mechanical integrity significantly with a low content of second phase. Carbon nanotube (CNT), especially multi-walled carbon nanotube (MWCNT) with their excellent stiffness and strength have stimulated their use possess great potential as a reinforcing agent for HA without offsetting their bioactivity [14, 15]. CNT-based biomaterials are proven to be suitable for cell growth and degraded by enzymes in the human body with the favorable degradation rate for bone regeneration [16, 17]. However, CNT tend to form bundles and the solubility in common solvents is very limited. With the aim to obtain an efficient mechanical reinforcement, the surface of CNT needs to be functionalized or chemically modified to achieve a uniform dispersion of CNT in the matrix material and also to induce an ideal interface between CNT and HA which is ultimately responsible for efficient load-transfer mechanism [18-20].
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ACCEPTED MANUSCRIPT In addition, a lot of methods, including hot pressing [21], hot isostatic pressing [22, 23], pressureless sintering [22] and spark plasma sintering [24, 25] have been employed for the fabrication of free standing HA–CNT composite. Among these methods, pressureless
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sintering is still a most favorable one due to its low cost and easy technique. Moreover, pressureless sintering technique might allow the retention of both CNTs and HA’s phase
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purity as well as OH groups, while providing enough densification for improved mechanical properties over HA [22]. There are only a few reports on HA/MWCNT composite have been reported by pressureless sintering technique. Mukherjee et al. [26] synthesized and
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characterized HA/MWCNT composite using shear mixing technique in argon to investigate the physical, mechanical, biological and in vitro bioactivity properties of composites
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containing different ratios of MWCNT. Another recent study they synthesized HA on pristine and functionalized MWCNT which was also densified using argon atmosphere, to report on the effects of functionalized MWCNT enhancing the mechanical properties of the composite [27]. Li et al. [28] investigated the fracture toughness of HA/MWCNT composite to be as
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high as 2.40 MPa.m1/2 with 3 wt.% MWCNT addition by pressureless sintering in a vacuum,
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which is 200% higher than the one sintered in argon atmosphere. However, they reported that the addition of MWCNT decomposed the HA into α-TCP and γ-Ca2P2O7 which make it unsuitable for real applications. In fact, to the best of author’s knowledge, there are no reports
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on the mechanical properties of the composite containing functionalized multi-walled carbon nanotube (f-MWCNT) by pressureless sintering in a vacuum. Hence this paper aims to study the optimum sintering conditions of the pressureless sintering in a vacuum to fabricate the HA/f-MWCNT composite that overcome the existing limitations and disadvantages. In addition, we perform an in-depth investigation of the mechanical properties and attempt to correlate them with structural parameters using a variety of techniques to check its suitability for biomedical applications, such as load bearing bone repair. 2. Experimental procedure 2.1 Materials Pristine MWCNTs (purity > 95%, 10-15 nm in diameter, 0.1-10 μm in length) that have been used during this experiment were purchased from Sigma Aldrich, USA. All other chemical reagents such as sulfuric acid (H2SO4, 97%), nitric acid (HNO3, 70%), calcium hydroxide
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The carboxylic groups on pristine MWCNTs (purity > 95%, 10-15 nm in diameter, 0.1-10 μm in length) were introduced by refluxing them in a mixture of 3:1 concentrated H2SO4 and
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HNO3 under stirring at 70 °C for 4 h, left to sit for 12 h, and then stirring for an additional 2 h at the same temperature. The resulting solution was diluted with DI water and left overnight.
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Afterward, the mixture was vacuum filtered and washed with DI water repeatedly until it had a neutral pH. The resulting carboxylated MWCNTs (f-MWCNTs) were dried in a vacuum at 60 °C for 12 h. The f-MWCNTs were then dispersed in DI water, mixed with a 1M aqueous
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solution of Ca(OH)2 and stirred for 1 h under ambient conditions. The stock solution of H3PO4 of 0.6 M, used as an initiator was added dropwise under constant stirring at room temperature. The solution was adjusted to ~10.5 pH by using NH4OH. The relative amounts of reactants were calculated to maintain a Ca/P ratio of 1.67 equal to that of stoichiometric
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HA. The HA grafted f-MWCNTs thus obtained was centrifuged and dried in a vacuum at 90
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°C for 6 h. The sample was ball milled by using a high-energy planetary ball mill (Retsch, PM 100 Japan) at 400 rpm for 3 h in ethanol. To minimize contamination during milling a zirconia pot and balls (10 mm diameter) with a ball-to-powder weight ratio of 10:1 were
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used. This solution was then dried at 50 °C for 24 h to remove the ethanol. The f-MWCNT content in these samples was 2 wt. % by controlling the amount of f-MWCNT and HA matrix. Powder compacts with dimensions of 10 mm × 6 mm were formed by pressing at 150 MPa. The pressureless sintering was performed at 950 to 1200 °C by maintaining at a heating rate of 5 °C/min for holding time 2 h in a vacuum atmosphere. The cooling rate was 5 °C/min. 2.3 Characterization The morphology and dispersion of f-MWCNT in HA matrix were carried out by using field emission scanning electron microscope (FESEM, JEOL JSM-7600F, Japan) and transmission electron microscope (TEM, LIBRA 120 Zeiss, Germany). The crystalline phase of the samples were conducted by X-ray diffractometer (XRD, D8 ADVANCE Bruker AXS Karlsruhe, Germany) with Cu-Kα radiation (λ = 0.15406 nm) in a continuous mode from 2θ range 25-55° at an angular step of 0.02° and a fixed counting time of 0.6 s/step. The voltage and current were set at 40 kV and 40 mA, respectively. The chemical structure and 4
ACCEPTED MANUSCRIPT interaction of f-MWCNT with HA matrix were detected with a Fourier transform infrared (FTIR, Perkin-Elmer Paragon 500) using KBr powder. Thermogravimetric analysis was carried out by using TG/DTA 6300 (SII NanoTechnology Inc.) under a nitrogen atmosphere
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between room temperature and 1000 °C at a heating rate of 10 °C/min. 2.4 Hemolysis assay
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Cytotoxicity of the HA/f-MWCNT composite was evaluated by studying their percent hemolysis in blood. The hemolytic activity was studied following standard protocols using
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human red blood cells (RBCs) as the samples [29]. The RBC were isolated by centrifugation, washed and then diluted via sterile phosphate buffered saline (PBS). The HA/f-MWCNT composite were washed with deionized water three times and then put into a test tube with 10
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mL 0.9% saline and incubated for 30 min at 37 °C under static conditions. After that, 0.2 mL of diluted blood was added into test tube and incubated for another 60 min at 37 °C. Similarly, 0.2 mL of diluted blood was added to 10 mL of deionized water and 0.9% saline solution using as a positive and negative controls, respectively. After the incubation, each
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tube was centrifuged at 2000 rpm for 10 min and the absorbance of the supernatant was
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measured at 545 nm by using a spectrophotometer (UV-2550, Japan). The hemolysis activity was calculated by using the equation (1): (1)
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Hemolytic rate (%) =
where A is the absorbance values of the test samples, B and C are the absorbance values of negative and positive control groups respectively. The hemolysis results were average of three measurements.
2.5 Statistical analysis The data was expressed as means ± standard deviation (SD) and was analyzed by SPSS (version 11.0). The P < 0.05 was considered statistically significant. 2.6 Mechanical properties The compressive strength of the cylindrical sample was measured using a universal tensile tester (Tenstometric FS-300KN, UK) at a cross-head speed of 0.5 mm/min. Five samples of each temperature were tested and the results were averaged. A Vicker’s micro-indentation instrument (Shimadzu Corporation HMV-2, Japan) was used to determine the hardness of the samples by applying a 19.614 N force for 6 s on the fracture surface. The indentation fracture 5
ACCEPTED MANUSCRIPT toughness of the sample was calculated using diagonal crack lengths produced at the indentation corners from the micro-indentation tests. The fracture toughness was measured by using the equation (2) [30]: KIC = 0.0726 (P/c3/2)
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where KIC is the fracture toughness, P is the applied load and c is the diagonal crack length.
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The hardness and fracture toughness values were averaged for three samples with five indents per sample.
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3. Results and discussion 3.1 Morphology and phase analysis
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The FESEM micrograph and XRD patterns of as-dried HA/f-MWCNT composite are shown in Fig. 1. As seen in Fig. 1(a), the unique flexible nature of f-MWCNT makes them bend and pass through space are attached to the HA grains, indicates that f-MWCNT bear significant stress by sharing a portion of the load, at the same time; strengthen and toughen the HA
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matrix. To further characterize the HA/f-MWCNT composite, X-ray diffraction analysis is
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performed and the patterns are shown in Fig. 1(b). The low diffraction peaks at 26.6° and 43.8° are assigned to graphite (002) and (100) planes of f-MWCNT. The remaining peaks at 25.9°, 28.1°, 29°, 31.9°, 33°, 34.2°, 40°, 46.7°, 48.1°, 49.4° and 53.2° are assigned to (002),
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(102), (210), (211), (300), (202), (310), (222), (312), (213) and (004) reflections of HA, respectively (JCPDS PDF No. 09-0432). The broad nature of the diffraction peaks which are seen in Fig. 1(b) suggests that the HA crystals are very small in size. 3.2 TEM analysis
Fig. 2 shows the TEM micrograph of f-MWCNT and HA/f-MWCNT composite. As seen in Fig. 2 (a), the f-MWCNTs tubular chains with a diameter 10-20 nm are found to be smooth and debundled structure. The f-MWCNT maintained their typical hollow structure in the HA/f-MWCNT composite, as shown in Fig. 2 (b), and would therefore be expected to act as an excellent reinforcement in the HA matrix. The TEM image confirmed that the fine HA nanorods are well attached to the f-MWCNT surface. This nano-HA has good bioactivity and excellent biocompatibility which are important for bone tissue engineering applications [31]. Shi et al. revels that individual 25-50 nm HA crystal is the essence of bone in terms of mechanical properties, bioresorbability and play an important role in biomineral formation [32]. The isolated appearance of HA nanorods in Fig. 2(b) most likely are an artifact related
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Fig. 3(a) shows the FTIR spectra of the HA/f-MWCNT composite in the range 400−4000 cm. The FTIR spectrum confirmed the bands corresponding to PO43− (ν3-1041, ν1-962, ν4-602,
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ν4-670, ν4-573 cm-1) and CO32− (ν3-1438, ν2-872 cm-1), respectively [33, 34]. The broad band at 3433 cm-1 is attributed due to the O−H stretch of the hydroxyl group which can be ascribed to the oscillation of carboxyl groups (O=C−OH and C−OH) [35]. The peak found at 1616 cmis due to C=C stretching of the f-MWCNTs. The prominent peak at 2360 cm-1 is associated
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with O-H stretch from strongly hydrogen bonded –COOH [35]. The carbonate containing HA
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is chemically and structurally more similar to that in natural bone. The spectrum in the ν4 PO43− domain exhibits the bands at 602 and 573 cm-1 is assigned to PO43− ions in apatite sites [36]. In the ν2 CO32− domain exhibits the band at 872 cm-1 which may improve the bioactivity of HA and similar to the characteristic ones observed in bone crystals [36]. The TG/DTA
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curves of HA/f-MWCNT composite are shown in Fig. 3(b). A gradual and continuous weight
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loss of 30.3% is observed at temperatures ranging from 25 to 1000 °C. The first step weight loss below 120 °C is due to the liberation of absorbed water and the second step in a temperature range of 120 to 500 °C is probably attributed to the elimination of water released
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from decarboxylation or condensation of HPO4−2 [37]. Finally, the third step is maybe due to the loss of constitution water of HA which can change to oxyapatite (Ca10(PO4)6O) [38]. In the DTA curve, no endothermic and exothermic peaks up to 1000 °C which are related to organic compound decomposition are detected. This confirms that the obtained HA sample with HA/f-MWCNT composite are sufficiently pure. 3.4 X-ray diffraction analysis Fig. 4 exhibits the XRD patterns of HA/f-MWCNT composite as a function of sintering temperature. As seen in Fig. 4, the major peaks in all three patterns are from HA. Low intensity graphite peak (2θ = 26.6°) is observed in the diffraction patterns of the composite. The intensity of graphite (002) peak increased substantially with the sintering temperature which indicates the more graphite structure is developed at higher temperature. No other peaks corresponding to Ca3(PO4)2 between 30° and 31° are detected at all sintering temperatures [24]. This implies that the reaction resulted in the formation of phase pure HA and at a higher sintering temperature does not lead to parts of HA decomposing. It is worth 7
ACCEPTED MANUSCRIPT noting that there are reports indicating that HA/CNT composite decomposed in to β Ca3(PO4)2 at 1100 °C [25]. A further observation of the curve (c) indicates that the diffraction peaks corresponding to HA become sharper and narrower in comparison with curves (a) and
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(b), evidence of higher crystallinity of HA. 3.5. Hemocompatibility study
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Hemolysis is a measure of cytotoxicity of a biomaterial towards red blood cells (RBCs). Cytotoxicity results in the rupture of RBC membranes and the release of hemoglobin.
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Spectrometry of the free hemoglobin allows the estimation of the extent of cytotoxicity. Table 1 shows that HA/f-MWCNT composite, fabricated with 2 wt.% f-MWCNT addition exhibit a lower hemolytic-rate of 1.27%. If the hemolytic-rate of any material is less than 5%,
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it is considered to be highly hemocompatible [29]. Therefore, the throughput of this study clearly reveals that the HA/f-MWCNT composite exhibits excellent hemocompatibility. 3.6 Mechanical properties and toughening mechanisms
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Fig. 5 shows the effect of sintering temperature on the compressive strength of HA/f-
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MWCNT composite. The maximum compressive strength of 481.7 MPa is reached at 1150 °C which is much higher than that of the HA/CNT composite (156.8 MPa) [39]. The significant differences are thought to result from the variations, both materials processing and
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in the sintering processes, such as sintering atmosphere, heating rate and holding time. Moreover, the use of as-dried HA/f-MWCNT composite (Fig. 1(a)) ensures the well distribution, flexible nature and good bonding of f-MWCNT with HA matrix. Hence, the effective load transfer from the matrix to f-MWCNT is possible and a significant load sharing of f-MWCNT mainly enhances the mechanical properties of HA/f-MWCNT composite. Fig. 6 shows the hardness and fracture toughness as a function of sintering temperature. As seen in Fig. 6, the hardness increases with an increase in sintering temperature to a maximum, then decreases to further increase in temperature. The composites sintered at 1100 °C shows a maximum hardness value of 3.6 GPa. In addition, it is observed that the fracture toughness of the composite sintered at 1150 °C is 2.38 MPa.m1/2, which is higher than the reported literature value of 1.9 MPa.m1/2 for the HA/MWCNT composite sintered in Ar atmosphere [26]. The fracture toughness improvement in the present study is partly attributable to the differences in their sintering atmosphere. It turns out that HA and MWCNT both are absorbed gas intensively. Therefore, in a vacuum that is favorable for gas expulsion and densification of hydroxyapatite, resulting improved their mechanical properties but it is 8
ACCEPTED MANUSCRIPT contrary in Ar. The cracks initiated by Vickers indentation in the HA/f-MWCNT composite layers are shown in Fig. 7. Detailed FESEM investigations inside the indentation cracks in the HA/f-MWCNT composite reveal that two factors are responsible for the improvement in
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fracture toughness. One of them is the resistance to crack propagation of HA/f-MWCNT composite. Observations at higher magnification images of arms of the indented sample
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which is marked as square in Fig. 7(a) is illustrated in Fig. 7(b) and (c) where the crack deflections are clearly evident. The crack deflection in HA matrix that creates a more tortuous path to release residual stresses which is an important mechanism that promotes
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increases in fracture toughness. Another mechanism is f-MWCNT bridging (Fig. 7(d)) that absorb more energy and provide resistance to crack growth. Thus, the functionalized
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MWCNT in the composite tend to break the crack from its propagation and yield the high toughness through crack bridging under the indentation load [40]. The decrease in mechanical properties from 1150 to 1200 °C in the present study can be ascribed to the effect of MWCNT. Peigney et al. [41] reported that the quantity of CNT retained in the massive
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composite is more dependent on the treatment temperature than of the nature of the oxide
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matrix. In agreement with previous observations [25], the f-MWCNT are damaged and broken when the treatment is at 1200 °C (Fig. 8), leading to a decrease or total loss of reinforcing effects of f-MWCNT, resulted in a drop in the mechanical properties.
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From the above, it can be summarized that the improvement of the compressive strength, hardness and fracture toughness are attributed to the well distribution and strong bonding of f-MWCNTs with HA matrix. The load sharing and bridging effect of f-MWCNTs result in the composite with significant enhancement of mechanical properties. Moreover, the hemocompatibility evaluation result shown that the HA/f-MWCNT composite exhibited very good hemocompatibility. Thus, the prepared composite with the addition of 2 wt.% fMWCNT might be a promising bone repair implant material. 4. Conclusion Hydroxyapatite/functionalized multi-walled carbon nanotube (HA/f-MWCNT) composite is successfully fabricated by in-situ chemical precipitation of HA on modified MWCNT and followed by vacuum pressureless sintering at 950 °C to 1200 °C. The carboxyl functional group (–COOH) is introduced on the MWCNT surface by the acid treatment are favorable for chemical bonding between the HA and MWCNT. The prepared composite exhibited excellent hemocompatibility. The strengthening mechanism is based on the significant load
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Vickers hardness of 3.6 GPa is obtained at the sintering temperature of 1100 °C. Moreover, the highest compressive strength and fracture toughness of the HA/f-MWCNT composite
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sintered at 1150 °C are 481.7 MPa and 2.38 MPa.m1/2, respectively. Based on the findings of this study, it can be summarized that vacuum pressureless sintering technique have been successfully utilized to fabricate the HA/f-MWCNT composite with improved mechanical
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properties without any chemical decomposition of HA matrix for a wide variety of bone implant applications.
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Acknowledgments
The authors wish to express their appreciation to Prof. Dr. Md. Fakhrul Islam for providing FESEM facilities.
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Figure captions: (a) FESEM images, (b) XRD pattern of HA/f-MWCNT composite.
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TEM images of (a) f-MWCNTs, (b) HA/f-MWCNT composite.
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(a) FTIR spectra, (b) TG/DTA curves of HA/f-MWCNT composite.
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XRD patterns of HA/f-MWCNT composite at different temperatures: (a) 1100, (b)
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Compressive strength of HA/f-MWCNT composite as a function of sintering
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Vickers hardness and fracture toughness of HA/f-MWCNT composite as a function
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FESEM images of HA/f-MWCNT composite at 1150 °C showing (a) Vickers indentation, (b-c) crack deflection, (d) bridging action of f-MWCNT chains across the cracks in HA matrix.
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FESEM image of HA/f-MWCNT composite at 1200 °C.
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Fig. 1 (a) FESEM images, (b) XRD pattern of HA/f-MWCNT composite.
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Fig. 2 TEM images of (a) f-MWCNTs, (b) HA/f-MWCNT composite.
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Fig. 3 (a) FTIR spectra, (b) TG/DTA curves of HA/f-MWCNT composite.
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Fig. 4 XRD patterns of HA/f-MWCNT composite at different temperatures: (a) 1100, (b)
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Fig. 5 Compressive strength of HA/f-MWCNT composite as a function of sintering
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Fig. 6 Vickers hardness and fracture toughness of HA/f-MWCNT composite as a function of
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Fig. 7 FESEM images of HA/f-MWCNT composite at 1150 °C showing (a) Vickers indentation, (b-c) crack deflection and (d) bridging action of f-MWCNT chains across the cracks in HA matrix.
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Fig. 8 FESEM image of HA/f-MWCNT composite at 1200 °C.
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Table 1. Hemolytic rate of the HA/f-MWCNT composite Hemolytic rate (%)
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Highlights:
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Prepared HA grafted modified MWCNTs by vacuum pressureless sintering. Carboxyl group (–COOH) is introduced by the acid treatment on the MWCNT surface. HA/f-MWCNT strong interfacial bonding and well f-MWCNT dispersion. And the ultimate mechanical properties are near to the natural bone.
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