Synthesis and Sintering-wet Carbonation of Nano-sized Carbonated Hydroxyapatite

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ScienceDirect Procedia Chemistry 19 (2016) 98 – 105

5th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015

Synthesis and sintering-wet carbonation of nano-sized carbonated hydroxyapatite W. Y. Wong and Ahmad-Fauzi Mohd Noor*** School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 11400 Nibong Tebal, Penang, Malaysia

Abstract Synthetic carbonated apatite ceramics are considered as promising alternative to auto- and allograft materials for bone substitute. In this study, Carbonated hydroxyapatite (CHA) was synthesized by nanoemulsion method. The powder produced was B-type CHA in nano-sized and had 8.25% carbonate content. The CHA samples were made into pellets and were sintered to 800°C. Upon cooling down to 150, 200, 250 and 300°C, carbonation with wet CO2 was performed on the CHA in a desiccator to recompensate the carbonate loss due to sintering and improve densification. The aim of this study was to investigate and compare the effect of cooled down temperatures on dense CHA with two kind of wet CO2 atmospheres: direct wet CO2 and dry CO2 through water. Sintered CHA carbonated by using dry CO2 through water had overall higher amount of carbonate content as compared to carbonation from wet CO2 directly from tank. D200, sample undergone carbonation by carbonated by dry CO2 through water at 200ºC had the highest carbonate content (3.35%). © Published by by Elsevier B.V.B.V. This is an open access article under the CC BY-NC-ND license ©2016 2016The TheAuthors. Authors. Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords: Bioceramic ; carbonation ; carbonated hydroxyapatite

1. Introduction Bone is a rigid hard organ in human body and susceptible to fracture due to injury or diseases1. One of the treatment to assist in healing is to use synthetic biomaterials as the replacement of the bone lost part2–5. Human bone consists of inorganic substance which was formed mostly of calcium phosphate5–8. Hydroxyapatite (Ca5(PO4)3OH)

* Corresponding author. Prof., Ph.D.; Tel.: +60 4 5996174; fax: +60 45941011. E-mail address: [email protected]

1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi:10.1016/j.proche.2016.03.121

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(HA) is a naturally occurring mineral which is chemically similar to the mineral component of bones and hard tissues in mammals, it is often being used as a substituent of loss part of bone6,9,10. Currently, the trend of bone substitute material is inclined to regenerative approach where the fast resorption of the implant material and simultaneous filling with newly formed bone and the substituted part, instead of permanently stay at the replaced side1,11,12. However, a stoichiometric of pure HA is chemically stable which results in its bioresorption rate being slow for the replacement of new growing bone tissue4,8,11,13. Thus, carbonated substituted hydroxyapatite ceramics are considered as promising alternative to HA-based bone substitute11,14,15. Inorganic component of human bone contains about β to 8 wt. % of carbonate ions depending on the individual‘s age15,16. Substitution of carbonate ion promotes the solubility or resorption of HA17,18. Hence, carbonated hydroxyapatite (CHA) is potentially better as its chemical composition approximates bone tissue more accurately. Carbonate ions can substitute at two sites in HA structure, namely hydroxyl and phosphate ions positions, give rise to A-type and B-type of CHA10,15,19. The amount of A/B ratio in calcium phosphate of human bone mineral is depending on the age, A-type CHA is found in old bone tissue while B-type is in young bone tissue10,17. B-type of carbonate substitution is preferred due to it is commonly found in young tissue, and responsible for the decrease of crystallinity and subsequently increases its solubility18,20. CHA powder can be produced by nanoemulsion method, which is able to produce nano size CHA particles21. Carbonate is incorporated into the structure by using some carbonate ions source during this process22. Sintering CHA at high temperature will cause carbonate to decompose into pure HA and calcium oxide with the released of carbon dioxide11,14,23. The elimination of carbonate ions affect both the biological and mechanical properties, so the processing condition chosen should hinder the decomposition or reduce the amount of carbonate loss23. In recent research work, the effect of carbonate content and sintering atmosphere has been undertaken to investigate thermal stability of CHA. Heat treatment were performed in various gas atmosphere, such as nitrogen, carbon dioxide, air, water vapour and wet oxygen. The gas atmosphere was revealed to affect the decomposition significantly20,24. Barinov et al.15 had reported that B-type CHA did not show evidence of decomposition by sintering in wet CO2. Consequently in research work by others, the effect of carbonate content and sintering atmospheres has been undertaken to investigate thermal stability of CHA. Heat-treatments were performed in various gas atmospheres, including nitrogen, carbon dioxide, air, water vapor and wet oxygen. The gas atmosphere used was revealed to affect the decomposition significantly24. According to Landi et al.24, thermal treatment in wet CO2 gave the best result in terms of high carbonate residue with low A/B ratio in the range of the biological CHA. In previous work done by Yanny-Marliana25, CHA with addition of Mg(OH)2 as the sintering aid and sintered to 800º achieved highest relative density25. Besides, carbonation at 200ºC during cooling stage with wet CO2 was to recompensate the carbonate loss. In this study, two different wet CO2 atmospheres were used: (1) Wet CO2 direct from cold tank and (2) Dry CO2 flowing through water, during cooling at 150, 200, 250 and 300ºC. 2. Experimental 2.1. CHA Powder Synthesis Nanoemulsion method was used to synthesis CHA powder, adopted from Zhou et al.21, where it can produce powder in very fine and uniform droplet sizes, typically in the range of 20-200nm. To synthesis the powder, Ca(NO3)2.4H2O (Merck, Germany) provides calcium source and (NH4)2.HPO4 (Merck, Germany) provides phosphate source while carbonate source was provided by NH4.HCO3 (Sigma-Aldrich, USA). The molar ratio of Ca/P was fixed to 1.67 according to the molar ratio in the biological bone, and while PO 3-4 /CO32- is 1. The synthesis was done in room temperature. The powder obtained was added with Mg(OH)2 sintering aid. Dry mixing was performed for 5 hours to make sure the powder has a homogeneous composition of mixture. Next, the CHA powder was compacted into pellets using a 13 mm diameter of stainless steel die. The powder was pressed at a pressure of 100MPa and held constant for 120 seconds so that the pressure was uniformly applied on the pellets.

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As-synthesized CHA powder was characterized to evaluate its conformation to the formation of B-type CHA in nano scale. X-ray diffraction (XRD: Bruker Advanced X-ray Solution D8) was used to determine the phase form of the CHA powder produced while the types of bonding were determined by Fourier Transform Infra-Red (FTIR: Perkin Elmer Spectrum One). Transmission Electron Microscope (TEM: Carl Zeiss Libra 120) was use to to observe the shape and size of the synthesized CHA particles. Carbon, Hydrogen, and Nitrogen analyzer (CHN: Perkin Elmer series 2, 2400 CHNS/O Analyzer) was used to determine the carbonate content of the synthesized CHA powder. Carbonate content of as-synthesized powder was characterized before and after sintering so that the carbonation ability of different wet CO2 can be compared. 2.2. CHA Sintering After the powder was compacted into desired shape, the powder would be sintered. In this study, all the CHA pellets were heated with heating rate of 10°C per minute, followed by soaking at 800°C for 120 minutes in Lenton tube furnace. The sintering profile is shown in Fig. 1. However, during cooling, the samples were taken out from the furnace at temperature 150°C, 200°C, 250°C and 300°C, and put in a desiccator. Subsequently carbon dioxide (CO2) gas was pumped into the desiccator. The gas was pumped into the desiccator for 20 minutes the samples were left in it for 24 hours. The summary of samples codes of the sintered CHA pellets which were studied to indicate different cooling atmosphere and different evacuation temperature from the furnace are shown in Table 1.

Fig. 1 Sintering profile of CHA samples. Table 1 Sample codes of sintered CHA pellets in different cooling atmosphere and different temperature evacuated from furnace.

Cooling Atmosphere

Temperature evacuated from furnace (°C) 150

200

250

300

Wet CO2

W150

W200

W250

W300

Dry CO2 Through Water

D150

D200

D250

D300

In this study, two types of wet CO2 gas were pumped into the desiccator as carbonation atmosphere: (1) Wet CO2 supplied directly from gas tank and (2) dry CO2 gas flow through water before enter the desiccator. After the sintering and carbonation, effect of different temperatures during cooling stage of sintering where the CHA pellets were taken out from furnace and treated with different wet CO2 was investigated.

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3. Results and Discussions 3.1. CHA Powder Synthesis 3.1.1. XRD Analysis XRD analysis is one of methods to evaluate the types of substitution, where carbonate ions can be substituted at hydroxyl site and it is known as A-type CHA, while substitution at phosphate site is known as B-type17,27. The peaks formed in the XRD analysis (Figure 2) conformed to the reference XRD pattern of pure HA with the ICDD file number of 00-009-0432. HA was used as a reference to indicate the types of substitution carbonate ion substitute HA, A-type or B-type of CHA can be indicated by comparing to the expansion and contraction of lattice parameter, a- and c-axis of HA24. The XRD spectrum of the powder in Fig. 2 shows that the synthesized CHA powder has low degree of crystallinity as observed from its broad peaks. The peaks at 2ϴ = 25.83° (002), 2ϴ = 28.81° (210), 2ϴ = 31.97° (211), 2ϴ = 34.04° (202), 2ϴ = 39.94° (310), 2ϴ = 43.86° (113), 2ϴ = 46.71° (222), 2ϴ = 41.45° (213), 2ϴ = 53.10° (004) and 2ϴ = 41.45° (304) matches the major peaks of HA. The XRD analysis of CHA powder revealed single phase of CHA was formed with no other secondary phase such as calcium oxide (CaO). Reference XRD pattern of HA was used instead of CHA because the expansion and contraction of lattice parameter, a- and c-axis cannot be compared with reference CHA. A decrease in a-axis and increase in c-axis relative to lattice parameter of HA (ICDD 00-009-0432) indicates that the formation of B-type CHA, and vice versa indicates the formation of A-type CHA24. However, the results in Table 2 shows there was expansion in both a-axis and c-axis relative to HA. In addition, the c/a ratio of the powder was 0.7323 which is higher than 0.7309 (HA), indicating that the formation of the CHA was B-type. So, to confirm the type of substitution, FTIR result was evaluated.

Fig. 2 XRD pattern of synthesized CHA powder. Table 2 Lattice parameters, c/a ratio and crystallite size of HA and synthesized CHA Sample a-axis (Å) c-axis (Å)

c/a ratio

Crystallite size (nm)

HA (ICDD 00-009-0432)

9.4180

6.8840

0.7309

-

CHA powder

9.4277

6.9042

0.7323

9.38

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FTIR Analysis FTIR identifies chemical bonds in a molecule by producing an infrared absorption spectrum. Fig. 3 shows the detected bands originating from stretching vibration of CO 3β− at 873, 1420, and 1452 cm-1 which was the characteristic of B-type CHA 27,28. Typical peaks of A-type substitution of CHA normally occur at 880, 1450 and 1540 cm-1, and they were not detected in the FTIR spectra. Bands at 474, 571, 601, 962, 1032-1072, 1087 cm-1 assigned to the vibrations of the phosphate group, PO 3-4 27,28. The bands at 474, 565, 604, 960, and 1036 cm-1 show the existence of phosphate group. The stretching bands observed around at 1637 and 3456 cm-1 reveals the incorporation of water molecules, and this was similarly reported by Ibrahim et al. and Liu et al.18,29.

Fig. 3 FTIR pattern of synthesized CHA powder

3.1.2. TEM Observation TEM analysis is used to observe and quantify the synthesized CHA powder. Prior to TEM observation, the CHA powder was sonicated in ethanol for 15 minutes to disperse the agglomerated CHA particles. Fig. 4 shows the TEM images of synthesized CHA powder.

Fig.4 TEM images of synthesized CHA powder

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As can be seen in the TEM images, the synthesized CHA powder of current study has irregular shape of particles. From the observation, the particles have angular shape morphology with the dimension of 10-30 nm in length and 520 nm in width. It has been reported by others that by using wet synthesis process, HA forms needle like shape crystallites30. The images did not show the formation of needle like shape due to the substitution of carbonate ions. Carbonate ions substituted in HA causes the change in the size and shape of the crystal: from acicular crystals to rods and then to equiaxed crystals with an increasing carbonate content from 2.5 to 17.5 wt%30. 3.1.3. CHN Analysis The amount of carbonate substitution in the synthesized CHA powder was determined and it can then be subsequently compared with the CHA after sintering and carbonation. Table 3 Carbon and carbonate content in synthesized CHA powder. Sample

Weight percentage of carbon (wt. %)

CHA

Weight percentage of carbonate (wt. %)

1.65

8.25

The carbonate content usually found in human bone is about 2-8% depending on the age16. In this study, the carbonate contained in the powder was slightly higher than the 2-8% range (Table 3). So, the synthesized CHA powder fulfills the requirement to be used as synthetic bone substitute. 3.2. Sintering and Carbonation of Sintered CHA 3.2.1. CHN Before the sintering process, the carbonate content was 8.25%. After the sintering, the carbonate content of the sintered samples treated with different condition of wet CO2 is shown in Table 4 and Fig. 5. Table 4 Carbon and carbonate content of sintered CHA treated by different condition of CO2. CO2 Condition Sample Carbonate content (%) W150 2.65 W200 2.80 Wet CO2 W250 3.00 W300 2.65 D150 2.80 D200 3.35 Dry CO2 through water D250 3.20 D300 2.65

Carbonate content (%)

3.50

3.00

2.50

2.00 150

200 Temperature(°C)250

Fig. 4 Graph of carbonate content of sintered CHA pellet and treated with dry CO2 flow through water ( different temperature.

300 ) and direct wet CO2 (

) at

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Based on the results, there was reduction of carbonate content after sintering. All the carbonate content in sintered CHA had the carbonate content in the range of 2.65-3.35%. The amount of carbonate content retained in the CHA was within the range required in human bone (2-8%). In this work, wet carbonation was intended to recompensate the carbonate ions loss during sintering. The introduction of dry CO2 flow through water during cooling had overall highest level of carbonate content as compared to direct wet CO2 atmosphere. This shows that using dry CO2 through water will re-compensate more carbonate ions to the CHA lattice compared to the direct wet CO2. Typically, sintering of CHA in normal atmosphere would result in decomposition of carbonate and affect the biocompatibility of the product, while other sintering procedures, involve of continuous supply of CO2 gas in firing atmosphere. 4. Conclusion Carbonation in wet CO2 is intended to re-compensates the loss of carbonate after sintering at high temperature. In this experiment, XRD and FTIR results showed that as-synthesized CHA powder was B-type. From CHN results, carbonation using both wet CO2 was able to retain the carbonate content in CHA within the range of 2.65-3.35%. The amount of CHA retained was within the range required in human bone (2-8%). Dry CO2 flow through water have overall higher amount of carbonate content as compared by direct wet CO2. Acknowledgements The authors sincerely acknowledge the financial support from Fundamental Research Funds Scheme (6071285). References 1.

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