Cik Rohaida C.H, Idris B., Rusnah M., Mohd Reusmaazran Y., Narimah A. B.

Transcription

1 26 EFFECTS OF SINTERING TIME ON THE PHYSICAL AND COMPOSITIONAL PROPERTIES OF POROUS BIPHASIC CALCIUM PHOSPHATE FOAM Cik Rohaida C.H, Idris B., Rusnah M., Mohd Reusmaazran Y., Narimah A. B. Materials technology group (MTEG), Industrial Technology Division, Malaysian Nuclear Agency, Bangi, 43000, Kajang, Selangor. ABSTRACT Phase composition of calcium phosphate ceramic is a characteristic directly related to the biological response of implants due to the differences in mechanical and biochemical properties of these compounds. The biodegradation rate of biphasic calcium phosphate (BCP) can be controlled by altering the HA to -TCP ratios. In this study the crystalline phase evolution of BCP synthesized via precipitation from aqueous solution of (NH 4 ) 2 PO 4 titrated into heated solution of Ca (NO 3 ) 2 was evaluated. The resulting powder was fabricated into porous scaffold using polyurethane foam method. Bulk powders were sintered from C to determine the most significant sintering temperature to obtain a stable and well crystallize BCP phases. The porous scaffolds were then sintered at selected temperature and the effects of various sintering times from 5,7,9,11,13 and 15 h were investigated. Bulk powders were characterized by dilatometer, IR analysis and XRD and porous scaffolds were analyzed by XRD and SEM- Edx. RIR method was performed to show that the HA to -TCP ratios were increased with increasing of sintering time and reached the maximum HA value at 11h. It is found that, the possibilities to manipulate the HA to -TCP ratios in BCP porous scaffold by just controlling the sintering time of the scaffold without controlling the starting powder characteristics. ABSTRAK Komposisi fasa seramik kalsium fosfat adalah satu ciri berkait terus untuk gerak balas biologi penanaman kerana perbezaan itu dalam ciri-ciri mekanikal dan biokimia sebatian-sebatian ini. Biodegradasi kadar kalsium fosfat dwifasa (BCP) boleh dikawal dengan meminda HA untuk nisbah-nisbah -TCP. Dalam kajian ini evolusi fasa berhablur BCP mensintesiskan melalui pemendakan daripada larutan berair (NH4)2PO4 dititrat kepada penyelesaian hangat Ca (NO3)2 telah dinilaikan. Mengakibatkan serbuk dipasang siap menjadi perancah berliang menggunakan kaedah busa poliuretan. Serbuk pukal adalah tersinter daripada C bagi menentukan paling penting pensinteran suhu memperoleh sebuah kandang kuda dan baik menghablur BCP berfasa. Aram-aram berliang itu adalah kemudian tersinter pada suhu terpilih dan kesan dari beberapa pensinteran masa-masa daripada 5,7,9,11,13 dan 15 j telah disiasat. Serbuk pukal berunsur dilatometer, analisis IR dan XRD dan aram-aram berliang dianalisis oleh XRD dan SEM-Edx. Kaedah RIR telah diusahakan bagi menunjukkan bahawa HA untuk nisbah-nisbah -TCP telah dinaikkan dengan bertambah pensinteran masa dan sampai maksimum nilai HA pada 11j. Ia didapati bahawa, kemungkinan itu untuk memanupulasi HA untuk nisbah-nisbah -TCP dalam BCP perancah berliang oleh baru sahaja mengawal pensinteran masa perancah tanpa mengawal permulaan ciri-ciri serbuk. Keywords: Biphasic calcium phosphate (BCP), porous scaffold, sintering time, wet precipitation 206

2 INTRODUCTION Calcium phosphate based bioceramics have proved to be attractive materials for biological applications [1]. Among these bioceramics, particular attention has been given to hydroxyapatite (HA) due to its bioactivity and -tricalcium phosphate (-TCP) due to its bioresorbability. The main attractive feature of these bioactive bone graft materials is their ability to form a strong direct bond with the host bone resulting in strong interface compared to other bionert or biotolerant materials which form a fibrous interface [2]. Whereas stoichiometric HA and TCP have been widely studied during the last decades, few works devoted to sintering study of biphasic calcium phosphate (BCP) that consists of an intimate mixture of HA and - TCP of varying HA to -TCP ratios has been done. BCP has been developed in order to combine the benefits of the properties of HA and -TCP. The term BCP was first used by Nery et al [3] to describe bioceramic that consisted of a mixture of HA and -TCP, after the x-ray diffraction analysis. Le Geros et al. (1986) had reported that the mixture of 20% HA and 80% -TCP was contained in the prepared porous TCP by XRD analysis. Recently, there is growing interest in developing BCP ceramic as scaffolding material because BCP is more effective in bone repair or regeneration than pure HA or pure -TCP, and has controllable biodegradation rate to a certain degree [4]. The degradability of this compound is depending on the Ca/P ratio with TCP being the highest ratio and usually results in the most extensive bone remodeling around the scaffold [5]. Controlling the HA to -TCP ratio is very important for controlling the bioreactivity of the porous BCP foam as reported by Le Geros et al. [3]. Many of the previous study reported on the methods of producing BCP powder/porous such as solid state mixing from commercial HA and TCP powders [6] and wet precipitation method from Ca(OH) 2 and H 3 PO 4 [7]. In this study BCP powder was synthesize using wet precipitation method as reported by Kivrak et al. [8]. Precipitation technique was chosen because of its low cost process and allowed a better control of composition and physical characteristics of powders. Some reports have shown the effect of reaction parameters (ph, temperature, Ca/P ratio and reagent concentration) and heat treatment conditions on the BCP powder properties. In the previous paper [9], we reported the possibilities to produce BCP powder with HA/-TCP ratios in range of 20-70wt% of HA (the rest being -TCP) by controlling the ph of the synthesis reaction and subjected to different synthesis temperature. However, it was very difficult to control the ph of the chemical synthesis process as it can vary with temperatures. In this present study, the BCP porous scaffold with different HA/-TCP ratios have been produced by employing different sintering time at constant sintering temperature. The most significant sintering temperatures and times which can produce a well crystallize with densified grain structure were determined. MATERIALS AND METHODS BCP powder was synthesized via wet precipitation method as discussed in C Rohaida et al. [9]. The synthesis process was prepared using auto titrator from Metrohm to control the flow rate of (NH 4 ) 2 PO 4 (Merck) dropped into Ca (NO 3 ) 2.4H 2 O (Merck) solution. The ph of the reaction was restricted at 4 in the initial stage and then increase to 7.5 before decrease to 6.5 at the end of the reaction by addition of NH 4 OH (R&M). The temperature was controlled from 40-65ºC. After the titration was finished, the stirred solution left overnight for ageing process for about 20h. The white precipitates were collected through vacuum filtration process, dried overnight in the oven and ground into powder with particle sized of 2-7μm. The bulk powders were sintered in air furnace for about 5 hours to study the phase transition from 700 ºC ºC using dilatometer (NETZSCH), XRD (PanAlytical) and FTIR (Perkin Elmer). The powder was then 207

3 fabricated into porous scaffold using polyurethane (PU) foam method. This method was already established and renowned of its ability to produce homogeneous porous structure. The slip consisting of distilled water, 4% binder and 47wt% solid loading of BCP powder was prepared prior to immersion process. Then, the PU foams were completely immersed in the slurry until the sponge being completely coated with the ceramic slurry. The excess slurry from the foams surfaces were removed to ensure proper drying process in the oven at 50-60ºC overnight. The dried foams were sintered at 1100ºC with 6 different holding times from 5 to 15 hours. The polymer burn off was set at 450ºC with heating rate at 1 C/min. The final scaffold obtained was crushed into powder for phase analysis using XRD and RIR method was performed to measure the phase composition. The micro structural properties of the porous scaffolds were observed using SEM (Philips) EDX has been used to determine the ca/p ratios. RESULTS AND DISCUSSION Powder analysis In this study, the BCP powder was synthesized via wet precipitation method as described by Kivrak et al. [8] and has been modified as described in Rohaida et al. [9]. This preparation method has been chosen because it conserves the bioactivity and biodegradability, increase the purity of the ceramics and displays interesting mechanical properties. The manufacturing of BCP powder is dependent on the simultaneous, in situ precipitation of both phases as a function of the overall solution ph and cation concentrations was achieved. The boiling step added to the synthesis process for higher HA compositions by conversion of TCP phase to HA [8]. In this study, thermal analysis of as synthesized BCP powder has been done by shrinkage analysis as a function of temperature using dilatometer in air atmosphere as shown in Figure1. There are three steps being identified; in the first step, about 0.25% shrinkage occurs at 190 C corresponds to the effect of water elimination resulting in cell volume contraction [6]. The water elimination correspond to the dehydroxylation of the HA that is believed to happen according to this reaction : Ca 10 (PO 4 ) 6 (OH) 2 3Ca 3 (PO 4 ) 2 + CaO + H 2 O (1) However, at this stage the powder is not yet sintered. The second step is starting from 700 C until 1190 C, indicates about 12% of shrinkage, represent the sintering period of the powder [6]. During sintering process, the decomposition of HA to -TCP phases occurs due to the elimination of hydroxyl groups. The formation of BCP phases (both HA and -TCP phases present intimately) can be detected at this stage as proved by XRD pattern in Figure2b. The third step indicates further sintering period from 1200 C to 1400 C associated with decomposition of HA into α-tcp and ά-tcp. At about 1190 C, the curve shows a slightly increased in the dimension change (showed by arrow in Figure1), indicates that the decomposition of HA to -TCP is reversible due to the presence of moisture in the air [10]. 208

4 Figure 1: Thermal analysis of BCP powder using Dilatometer. In development of calcium phosphate materials, phase identification is very important. Different phases may contribute to the different chemical properties. XRD patterns of as synthesized BCP powder is shown in Figure 2a, indicates no formation of BCP phases detected, only single phase HA is present. Figure 2b shows that, the sintering process of BCP begins only after 700 C when the well- crystallize phases of HA and -TCP are present. 209

5 Figure 2: X-Ray diffraction patterns of (a) ; as synthesized powder, and (b) ; BCP powder sintered from 700ºC to 1400 ºC Table 1 shows the summary of the XRD analysis. Powders sintered from 700 C to 1100 C show the same apatite structure consisting of intimate mixture of HA and -TCP phases. The most intense peak corresponds to -TCP peak, shows the similar d-spacing value of The peaks are very sharp indicating a highly crystalline material. As the temperature increased to 1200 C, further sintering process occurs with transformation of -TCP into α-tcp. The gradual transformation of -TCP to α-tcp becomes dramatic at 1300 C [6]. HA becomes the most intense peak with d-spacing value of 2.81, but the intensity is reduced due to the limited recrystallization during cooling [6]. As the temperature increased to 1400 C, α-tcp phases started decompose into meta-stable α -TCP phase. Table 1: X-Ray diffarction analysis of BCP powder sintered from C Sintering T (ºC) Highest peak 2θ (º) d- spacing Highest intensity (cps) FWHM Space group 210 Reference code 700 -TCP R3-C

6 800 -TCP R3-C TCP R3-C TCP R3-C TCP R3-C HA P63/m α-tcp P21/a Theoretically, TCP has 3 polymorphs; α, and α. Among these three polymorphs -TCP is the most preferred phase due to its chemical stability, relatively high mechanical strength and favorable bioresorption rate [11]. Therefore, the aim of this study is to maintain the processing of BCP ceramics consisting of intimate mixture of HA and -TCP. Previous literature indicated that, the notation used for the low-temperature polymorph of TCP to be designated as -TCP and high-temperature polymorph to be designated as α -TCP. According to Kivrak et al. [8] the transformation from low-temperature form to high-temperature form occurs at ~ 1180 C, whereas according to Raynaud et al. [10], the allotropic transformation of -TCP to α-tcp starts from 1150 C and this transformation was not totally reversible since both α and phases were present after cooling. The decomposition of HA to TCP occurs in accordance with the polymorphic transformation observes in the TCP phase. When the temperature is <1200 C, the HA decomposed to -TCP, while when temperature >1200 C, HA decomposed to α-tcp [8 ]. This study has indicated that, to retain structural integrity of BCP with intimate mixture of HA and -TCP, the powder must be sintered between 700 C to 1100 C. Figure 3 : FTIR analysis of BCP powder Porous scaffold analysis Normally, sintering temperature of calcium phosphate material is chosen in the range of ºC depending on the starting powder features. Sintering at lower temperature is defined as the excess in residual porosity at grain boundaries and sintering at high temperature may lead to the formation of decomposition products [6], which can impair the densification and biological response of the scaffolds [9]. However, in this study the porous BCP ceramics were obtained upon sintering at 1100ºC. The porosities and densities of the BCP porous as measured with Archimedes principles are in the range of % and g/cm -3 respectively. The porosities are increased with sintering time and started to decrease at 15h sintering. This 211

7 reduction is caused by the complete densification achieved at 15h sintering as shown in Figure 4d. The micrographs given in Figure4 illustrate the morphology of micro-porosities of BCP porous sintered at different temperatures. For sintering time below 7h, there are a lot of microporosities distributed uniformly on the BCP porous surface indicating that the grain coalescence without further densification. These micro-porosities are reduced when the sintering time is increased from 11h to 15h (Figure 4b-d) due to the crystallization and grain growth contributes to densification. The sizes of micro-porosities are in the range of µm as determined by image analyzer using Axiovision 4.4 (Carl Zeiss) software. These micro-porosities are required for nutrient delivery and cellular response in tissue ingrowths. The BCP grains are homogeneously distributed on the surface and the sizes of the grains are increased as sintering times are increased. The grain growth and densification are believed to begin only after 9h sintering and it continues until it achieves the final densification at 15h sintering (Figure 4d), causing the reduction in porosity. Figure 4: SEM micrographs for BCP scaffold sintered at 4 a)7h, b)11h, c)13h and d) 15h. 212

8 213 Figure 5: X-Ray diffraction patterns for BCP porous sintered from 5h-15h.

9 Phase composition ( wt%) C a/p ratio HA TCP Ca/P ratio Sintering time (h) Figure 6: Graph showing the HA/-TCP phase composition analysis (wt%) and the Ca/P ratio as a function of sintering time. The XRD patterns done on BCP porous sintered at 6 different times were shown in Figure 5. The patterns show no other phases presented except for both HA and -TCP phases, indicating that increasing of sintering time does not induce any new phase formation. All BCP porous sintered from 7, 9, 13 and 15h exhibit similar patterns except for sample sintered at 11h. Samples sintered from 7, 9, 13 and 15h signify the -TCP peak as the major peak with d- spacing values of 2.88, whereas the latter sample shows the highest peak corresponds to HA peak with d-spacing value of At this point, the HA peak shows the most highest peaks with intensity is approximately 2299 cts compared to other samples with -TCP as the highest peak, the intensity is in the range of 1513 cts to 1682 cts. In addition to that, this sample consists of the highest HA phase composition ie: 84wt% and the rest being -TCP phase as proved in Figure 6. Phase composition of BCP porous was determined with RIR methods obtained in the Xpert Hi- Score Plus software (PanAlytical). The reference peaks were (211) and (2 0 10) for HA and b- TCP respectively. Furthermore, the Ca/P ratio being determined from SEM-Edx analysis with the 3-4 spots for each sample. Figure 6 shows the relationship between phase composition in wt% and Ca/P ratio as a function of sintering time. All samples exhibit higher HA composition than -TCP except for sample sintered at 7h which is having 50-50wt% of HA and -TCP. The HA phase composition is increased approximately 27.4% when the sintering time increased from 9 to 11h but slightly decreased when time increased to 13 and 15h. In overall, BCP porous sintered at elevated time from 5 to 11h showing an increase in HA phase compositions until it reaches the highest values of HA at 84wt% after 11h sintering. It seems reasonable to say that during cooling, while -TCP remains nearly constant, low content and unstable α-tcp at this temperature tends to disappear making total concentration of TCP lower as compared to slight increased in HA [6]. HA increased due to the partially conversions of -TCP to HA at low temperature due to the phenomenon occurs during cooling stage. The Ca/P ratios give the similar trends with HA phase composition. The Ca/P values are in the range of 1.59 to 1.85 with 214

10 the highest value corresponds to sample sintered at 11h. Therefore, the above observations demonstrating that the HA/-TCP phase composition and Ca/P ratio can be controlled by employing different sintering time at constant temperature. This technique is much easier as compared to previous methods which need us to control the synthesis parameters including of solution molarity, ph, and temperature and sintering temperature. CONCLUSION In conclusion, this study has proved that BCP ceramics with intimate mixture of HA and -TCP can be obtained at sintering temperature of ºC. However -TCP retained their structural integrity only at 1100ºC and beyond this temperature, HA tends to decompose into α- TCP and α -TCP. Hence, 1100ºC is selected as the most preferred sintering temperature to produce BCP with HA and -TCP accessible intimately. BCP powder obtained can be used to fabricate porous scaffold for bone graft substitutes with the porosity and density values in the range of % and g/cm -3 respectively. This study investigated the effects of sintering time to the formation of HA and -TCP phases intimately presented in the BCP porous. It is found that, the HA/-TCP phase composition and Ca/P values are varies with sintering time up to highest values at 11h of sintering and then decrease slightly when the time increase to 13 and 15h. The HA phase compositions obtained are in the range of 50-84wt% with the rest being -TCP and the Ca/P ratios are in the range of Sintering at 7h and below is not appropriate to produce BCP porous with preferred densified micro-structure. The most preferred sintering time obtainable from 9-15h depending on the HA/-TCP composition or Ca/P ratio required. Different sintering times give different HA/-TCP phase compositions and Ca/P values. The sintering profile can be designed as per final BCP porous required. Therefore, it is possible to produce BCP porous ceramics with various HA/-TCP compositions and Ca/P ratios just by controlling the sintering time at constant temperature. ACKNOWLEDGMENTS Authors would like to acknowledge Agensi Nuklear Malaysia for allowing them to perform this research, providing all the materials and facilities, to all supporting staffs for their supports and helps, and to MOSTI for granting this research. REFERENCES [1] Jarcho M. (1981), Calcium phosphate ceramics as hard tissue prosthetics, Clin. Ortho. Rel Res; 157: [2] Daculsi G. (1998), Biphasic calcium phosphate concept applied to artificial one, implant coating and injectable bone substitutes, Biomaterials, , [3] Daculsi G., Le Geros R. (2006), Encyclopedia of Bimaterials and Bomedical Engineering. [4] Kohri M, Miki K, Waite DE, Nakajima H, Okabe T. (1993), In vivo stability of biphasic calcium phosphate, Biomaterials, 14: [5] Kleinn CPAT, de Blieck-Hogervorst JMA, Wolke JGC, de GrootK. (1990), Biomaterials, 11: [6] Tampieri A., Celloti G., Szontagh F., and Landi E. (1997), Sintering and characterization of HA and TCP bioceramics with control of their strength and phase purity, J. Mat Sci; Mat. Med., 8: [7] Victoria E.C., Gnanam F.D. (2002), Synthesis and characterization of biphasci calcium phosphate, Trends Biomater Ertif. Organs. Vol 16 (1):

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