The Effect of C 3 S and Hardener on Setting Properties and Compressive Strength of Calcium Phosphate Cement

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1 Biomaterials Research (2012) 16(2) : Biomaterials Research C The Korean Society for Biomaterials The Effect of C 3 S and Hardener on Setting Properties and Compressive Strength of Calcium Phosphate Cement Ekavianty Prajatelistia, Sungsu Chun, and Sukyoung Kim* School of Materials Science & Engineering, Yeungnam University, Gyeongbuk , Korea (Received April 23, 2012/Acccepted May 2, 2012) Tetracalcium phosphate (TTCP) is one of the most common starting materials of calcium phosphate-based ceramic bone cement. However, the ceramic bone cement has a critical drawback such as poor mechanical properties and slow setting time. In this study, the thermodynamically unstable tricalcium silicate (C 3 S) was synthesized and introduced into TTCP-based bone cement to enhance the mechanical and setting properties of the bone cement. The cement properties and compressive strength of hardened TTCP-cement with the addition of C 3 S were examined in terms of the contents of C 3 S and hardener. First, TTCP and DCPD with C 3 S (0; 5; 10 wt%) were ball milled for 24 hrs and then dried at 60 o C for 1 day. The sample powder was mixed with the solution of a hardener (Na 2 HPO 4.nH 2 O) with various concentrations (0, 0.5, 1.0 mol/l). The hydration of TTCP and DCPD resulted in the precipitation of HA crystallites and is the major driving force of setting for the TTCP-based bone cements. The XRD peaks of the hardened cements were identified as a mainly HA. With the addition of C 3 S, the cement showed a longer setting reaction time and lower setting temperature. However, the addition of hardener caused a higher setting temperature and faster setting time. The Vicat setting time decreased with the addition of hardener, but showed anomalous results with the C 3 S content. The incorporation of both hardener and C 3 S to cements would come with both the increase of mechanical strength and the reduction of setting time. However, the effect of C 3 S addition was not significant. Key words: TTCP, CPC, Bone Cement, Compressive strength, Tricalcium Silicate C Introduction alcium phosphate has been widely used as a bone substitute with various forms in dental and orthopedic fields. It was chosen not only because of its unrivaled compatibility with human bone but also due to its various shape formability for any complicated geometry. 1) Calcium phosphate cement (CPC) can be handled in paste form, however, many CPC pastes tend to disintegrate upon early contact with blood or other aquaeous fluids. To solve this problem, fast setting and mechanical property maintaining are important in clinical applications. Numerous CPC formulations have been proposed for over 25 years. 2) Tetracalcium phospate (TTCP, 4CaO.P 2 O 5 ) is one of the calcium phosphate salts that has a Ca/P ratio higher than hydroxyapatite (HA). It plays an important role as a cement component in that only TTCP can be combined with other components because of its high solubility. The ultimate product of the reaction TTCP with others is HA which is a major inorganic phase (such as carbonated and substituted biological apatites) of human bones and teeth. *Corresponding author: sykim@ynu.ac.kr Thermodynamically unstable TTCP is usually synthesized by the solid-state reaction of calcium carbonate with dicalcium phosphate anhydrate (DCPA, CaHPO 4 ) or dicalcium phosphate dihydrate (DCPD, CaHPO 4 2H 2 O) with a Ca/P ratio of 2. 3) Brown and Chow, 4) Chow and Takagi, 5) and Matsuya et al 6] heat-treated the mixture at 1450~1500 o C for 6~12hrs. The reaction is shown in the following equation: 2CaCO 3 +2CaHPO 4 Ca 4 (PO 4 ) 2 O+2CO 2 +H 2 O (1) After heat-treatment, the mixtures were rapidly quenched into room temperature. In case of slow cooling instead of quenching, it forms undesired stable secondary phases, such as HA, CaO, and β-tcp(3cao.p 2 O 5 ). Studies about the incorporation of Si to CPC has been seriously undertaken by many scientists recently because of the biomimetic effect of Si and the enhanced osteoconductivity of the materials. The addition of thermodynamically unstable tricalcium silicate (C 3 S, 3CaO.SiO 2 ) to CPC has been studied recently because C 3 S is known as a main component of Portland cement and is responsible for initial strength of the Portland cement. 7) When C 3 S is mixed with water, it will react with water to create calcium silicate hydrate (C-S-H) that will 67

2 68 Ekavianty Prajatelistia, Sungsu Chun, and Sukyoung Kim increase the strength of the material, according to the following equation: Ca 3 SiO 5 +zh 2 O Ca x Si(OH) y.nh 2 O+(3 x) Ca(OH) 2 (2) Recent studies demonstrated that the presence of silicate had a good biocompatibility, enhanced the cell proliferation, and activates bone-related gene expression. 8) In general, the set CPC showed the excellent bone regeneration, but the setting time of the CPC paste was too long (> 60 min). 7) Faster set time and higher mechnical strength are needed for clinical applications. In order to enhancing the setting property and mechnicla strength, C 3 S was selected as a potential additive for injectable calcium phosphate cement in this study. Various concentration of C 3 S and hardener were mixed with TTCP-based bone cements and the setting time and compressive strength were measured to find the most appropriate composition for the clinical application of CPC. Materials and Methods Synthesis of TTCP and C 3 S TTCP and C 3 S were prepared by a quenching method. First, in order to produce the TTCP, DCPD (CaHPO 4.2H 2 O, Aldrich Co. U.S.A) and Calcium Hydroxide (Ca(OH) 2, Aldrich Co. U.S.A), as the starting raw materials, were mixed with a molar ratio of 1:1, stirred for 4 hours and dried. Afterwards the samples were sintered at 1500 o C for 24 hours and then quickly quenched to get a high purity of TTCP. C 3 S (3CaO.SiO 2 ) was prepared by a sol-gel method, Tetraethyl orthosilicate (TEOS, SiC 8 H 20 O 4, Aldrich Co. U.S.A) and Calcium Nitrate Tetrahydrate (Ca(NO 3 ) 2 4H 2 O, Duksan, South Korea), as the starting raw materials, and Nitric Acid (HNO 3, Aldrich Co. U.S.A) as a catalyst were mixed together, dried, sintered at 1450 o C for 8 hours, and then quenched to obtain a high purity of C 3 S. Table 1. Composition of Various Calcium Phosphate Cement Mixtures Samples Cement-a Cement-b Cement-c Additives Hardener (Na 2 HPO 4 ) 1 distilled water mol/l mol/l 1 distilled water mol/l mol/l 1 distilled water mol/l mol/l Starting materials C 3 S TTCP DCPD 0 wt% 1 mol 1 mol 5 wt% 1 mol 1 mol 10 wt% 1 mol 1 mol Preparation of calcium phosphate cement For the CPC preparation, TTCP, DCPD with a molar ratio of 1:1 and C 3 S with various contents (0, 5, 10 wt%) were ball milled for 24 hours and then dried in 60 o C for 1 day to obtain the three types of powder mixture. As a hardener, an aqueous solution of disodium phosphate (Na 2 HPO 4 ) was prepared by dissolving the Na 2 HPO 4 with distilled water. Subsequently, each powder was mixed with various concentrations of hardener (0, 0.5 and 1.0 mol/l) with a constant liquid/powder (L/P) ratio (0.5~0.6 ml/g). This L/P ratio was the proper ratio for the forming of a paste-like sample. Characterization of samples The surface microstructure and morphology of the hardened cements were observed using the SEM (S-4200, Hitachi, Japan) at a magnification of 10,000. The SEM images were collected at an operating voltage of 15 kv and a working distance of 15 cm. The surface composition, phase changes of the sample before and after hardened were analyzed by X-Ray Diffraction (XRD, MPD-Pro, PANalytical Co., Netherland) with Cu Kα radiation in a continuous scan mode and 2θ range of 10~90 o at a scanning speed of 10 o /min. Setting temperature and time In order to measure the setting time, the cements were kept in a 95% relative humidity chamber at 37 o C, because ISO requires the cement to be maintained at a temperature of 37 o C and a relative humidity of at least 30%. A liquid-to-powder (L/ P) volume/weight ratio of 0.5~0.6 was used in preparing the cement specimens. The mixed cement pastes were packed into a plastic mold at room temperature and then pressed with constant loading to maintain the flat surface. After that, the cement was stored in a humidity chamber. The setting temperature was measured in a 95% relative humidity chamber at 37 o C for 1 hour. The setting time of cements was measured by vertically lowered a Vicat needle onto the surface of the setting cements. In this method, the cement is considered to be set when a needle with a tip diameter of 1 mm and loaded with 400 g weight fails to make a perceptible circular indentation on the surface of the cement. The Vicot indentation was repeated at a 30-second interval until the cement was hardened. Compressive strength measurement In order to prepare the samples for compressive strength measurement, the cement pastes were poured on a mold with a 6 mm inner diameter and 12 mm height (based on ISO 5833). The cements were pressed with about 700 kpa for 5 seconds to eliminate the large pores or air bubbles. Then the sample were stored in an incubator at 37 o C and 97% humidity for 24 hours. After the pastes set, they were soaked in acetone for 1 hour to stop hydration, and then air dried. Then samples Biomaterials Research 2012

3 The Effect of C3S and Hardener on Setting Properties and Compressive Strength of Calcium Phosphate Cement were ready for a mechanical test. The compressive strength was measured according to ASTM D using an universal testing machine (R&B Co, Korea) at a cross head speed of 0.5 mm/min and a load speed of 0.5 kgf/min with 500 kgf load cell. 69 The purity of synthesized TTCP and C3S was more than 95% and 90%, respectively. After ball milling the TTCP (1 mol) and DCPD (1 mol) with C3S (0, 5, 10 wt%), the average particle size of the three kinds of bone cement powder was about 2 µm. Each powder was mixed with various concentrations of hardener (Na2HPO4, 0, 0.5 and 1.0 mol/l) together with a constant liquid/powder (L/P) ratio (0.5~0.6 ml/g). The pastes were injectable as shown in Figure 2(a) and formed to obtain cylinder-type samples for a compressive strength experiment after hardening (Figure 2(b)). The SEM micrographs of the hardened cement samples were given in Figure 3. The surfaces of the three types of cement (a, b, c) were all rough and uneven with macropores. The surfaces were composed of large ake-like particles with a 3~10 µm Figure 1. Flow chart of cement preparation and characterization. Figure 2. (a) Injectability of TTCP-based bone cement pastes; (b) rod-type samples of calcium phosphate-based bone cement after incubation in a 95% relative humidity chamber at 37oC for 1 day. Results and Discussions Figure 3. Surface morphologies of hardened cements: (a) Cement-a, (b) Cement-b, (c) Cement-c (where 1, 2, 3 represent with 0 (water, no hardener), 0.5 and 1.0 mol Na2HPO4 solution, respectively). Vol. 16, No. 2

4 70 Ekavianty Prajatelistia, Sungsu Chun, and Sukyoung Kim were identified as a mainly HA (Ca5(PO4)3(OH), ICSD ). The hydration of TTCP and DCPD results in the precipitation of HA crystallites and is the major driving force of setting the TTCP-based bone cements. The hydration reaction is given by the following equation: 4CaO.P2O5 + CaHPO4 + nh2o Ca5(PO4)3(OH) + n H2O (3) Figure 4. Small needle-like crystallites on the cement with C3S and hardener. size. The ake-like particles were agglomerated together to form larger clusters. With increasing C3S and hardener contents, the size of the flake-like agglomerated clusters decreased. Many small needle-like grains with about 1 µm length and 50 nm diameter were entangled with 3-dimensional network structure on the cluster surface. The tendency of needle-like HA grain formation increased with increasing hardener content, as shown in Figure 4. Figure 5 showed the XRD patterns of the hardened TTCPbased bone cements with various contents of C3S. XRD peaks With increasing C3S content, mainly HA peaks with similar crystallinity were observed in all samples, but C3S peaks (ICSD024625) were found in bone cement with 10 wt% of C3S. It is believed that the observation of C3S pecks at the 10 wt% C3S addition is due to the residue of un-reacted C3S of the cement (Figure 5c1). As the hardener content increased, sharper XRD patterns of HA were observed. The results showed that the hardener did the major role for the hydration of cements, even at the 10 wt% C3S added cement (Figure 5c). All cements of the mixture of TTCP and DCPD with various contents of C3S and hardener were transformed into HA phase which is the same as a human bone mineral. The results of the setting temperature of cement pastes measured in a humidity chamber at 37oC for 1 hour are showed in Figure 6. The curves observed were related to exothermic hydration reaction. Where, the reference in legends means the temperature of a humidity chamber and the water is the temperature of a cement paste with 0 mol/l hardener. Figure 5. XRD patterns of hardened TTCP-based bone cements with various contents of C3S; (a) Cement-a (0 wt%), (b) Cement-b (5 wt%), (c) Cement-c (10 wt%) (where, 1, 2, 3 represent with 0 (water, no hardener), 0.5 and 1.0 mol Na2HPO4 solution, respectively). Biomaterials Research 2012

5 The Effect of C 3 S and Hardener on Setting Properties and Compressive Strength of Calcium Phosphate Cement 71 Figure 6. The setting reaction temperature of each cement at 37 o C for during 1hour (after 1min mixing) ; (a) Cement-a, (b) Cement-b, (c) Cement-c. The setting temperatures increased with the content of hardener (example, 37, 43, 49 o C at 0 wt% C 3 S), but slightly decreased with increasing C 3 S content (example, 49, 48, 47 o C at 1.0 mol/l hardener). The time of maximum setting temperature was longer with increasing C 3 S content (from 7 to 12min at 1.0 mol/l hardener). However, the time of maximum setting temperature was shorter with increasing hardener content (from 10 and 9 to 7min at 0 wt% C 3 S). That is, with the addition of C 3 S, the cement showed a longer setting reaction time and lower setting temperature. However, the addition of hardener enhanced the hydration rection of TTCP and DCPD and caused higher setting temperature and faster setting time. The fastest setting time was obtained at 1 mol/l hardener added cements without C 3 S addition. The setting time of cements was measured by a Vicat method with a tip diameter of 1 mm and a load of 400 g as in Table 2. The Vicat setting time showed a similar tendency with the setting time measured in a humidity chamber. In this experiment, the setting time decreased with the addition of hardener, but showed anomalous results with the C 3 S content. The fastest setting time amongst the cements was at 1 mol/l hardener added cement. As shown in Table 2, the compressive strength of hardened cements tends to be slightly increased from 6 to 7 MPa with the increasing of content of C 3 S (with no addition of hardener). However, the compressive strength significantly increased from 6 to 14 MPa with the increasing of the content of hardener (with no addition of C 3 S). Cement with 10 wt% of C 3 S and Table 2. Optimum L/P ratio, setting time, compressive strength and phases of hardened cements Samples Cement-a Cement-b Cement-c Hardener Optimum L/P (vol/wt ratio) Phases Setting time (min) Compressive strength (MPa) 1 water 0.6 HA mol /L 0.6 HA mol/l 0.6 HA water 0.5 HA mol /L 0.5 HA mol /L 0.5 HA water 0.5 HA, C 3 S mol /L 0.5 HA mol /L 0.5 HA Vol. 16, No. 2

6 72 Ekavianty Prajatelistia, Sungsu Chun, and Sukyoung Kim 1.0 mol/l hardener showed the highest compressive strength among them. It is well-known that the C 3 S is responsible for the initial strength of Portlans cements for first 4 weeks. It is believed that the C 3 S increased the compressive strength with setting time and will further contribute to high compressive strength after being set. From the observed results (Table 2), it is clear that the incorporation of both hardener and C 3 S to cements would result in both the increase of mechanical strength and the reduction of setting time. However, the effect of C 3 S addition was not significant. The higher compacted cement body showed higher compacted structure and tended to higher mechanical strength of the hardened cement. Because the TTCP-based bone cement was set with water and other liquid contains water, the different weight ratio between water to cement powder lead to the different setting time and compresisve strength. It is obvious that the setting behavior is definitely dependent on the water content. In this experiemnt, when the water/powder weight ratio was less than 0.2, the cements had the shortest setting time. However, the hardened cement had a loose structure and relatively low strength. Thus, the water/powder ratio was optimally set as 0.5~0.6. Conclusions In order to prepare TTCP-based bone cements, the high purity TTCP and C 3 S as starting materials were synthesized (> 95% of TTCP amd > 90% of C 3 S). The surfaces of hardened cements were composed of large, ake-like particles with a 3~10 µm size. These ake-like particles were agglomerated with small, needle-like grains to form larger clusters. The tendency of the needle-like HA grain formation increased with increasing hardener content. The XRD peaks of the hardened cements were identified as a mainly HA. The hydration of TTCP and DCPD resulted in the precipitation of HA crystallites and is the major driving force of setting for the TTCP-based bone cements. The setting temperatures increased with the content of hardener, but slightly decreased with increased C 3 S content. The time of maximum setting temperature was longer with increased C 3 S content, but shorter with increased hardener content. That is, the addition of C 3 S, the cement showed the longer setting reaction time and lower setting temperature. However, the addition of hardener caused a higher setting temperature and faster setting time. The Vicat setting time showed a similar tendency with the setting time measured in a humidity chamber. The Vicat setting time decreased with the addition of hardener, but showed anomalous results with the C 3 S content. The compressive strength of hardened cements tended to be slightly increased with increased content of C 3 S (with no addition of hardener). However, the compressive strength significantly increased with increased content of hardener (with no addition of C 3 S). It is clear that the incorporation of both hardener and C 3 S to cements would come with both the increase of mechanical strength and the reduction of setting time. However, the effect of C 3 S addition was not significant. Acknowledgements This research was supported by a Yeungnam University research grant in References 1. W. E. Brown and L. C. Chow, A new calcium phosphate setting cement, J Dent Res., 62, 672 (1983). 2. Huan Zhiguang, Jiang Chang, Calcium-phosphate-silicate composite bone cement: self-setting properties and in vitro bioactivity, J Mater Sci., 20, (2009). 3. C. Jeon, S. Chun, S. Lim, S. Kim, Synthesis and characterization of TTCP for calcium phosphate bone ement, Biomaterials Research 15(1), 1~6 (2011). 4. W. E. Brown and L. C. Chow, Dental restorative cement pastes, U.S. Patent No.4, 518, 430, May 21, (1985). 5. L. C. Chow and S. Takagi, Self-setting calcium phosphate cements and methods for preparing and using them, U.S. Patent No. 5, 525, 148, June 11, (1996). 6. Y. Matsuya, S. Matsuya, J. M. Antonucci, S. Takagi, L. C. Chow, and A. Akamine, Effect of powder grinding on hydroxyapatite formation in a polymeric calcium phosphate cement prepared from tetracalcium phosphate and poly(methyl vinyl ether maleicacid), Biomaterials, 20, (1999). 7. Zhiguang Huan, Jiang Chang, Self-setting Properties and vitro Bioactivity of calcium sulfate hemihydrate-tricalcium silicate composite bone cements, Acta Biomaterialia, 3, , (2007). 8. Zhao W. Y., Chang J., Sol-gel synthesis and in vitro bioactivity of tricalcium silicate powders, Materials Letter, 58(19), , (2004). Biomaterials Research 2012