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Preparation and characterization of microcrystalline hydroxyapatite using sol gel method.

Hydroxyapatite ceramics have been recognized as substitute materials for bone and teeth in orthopaedic and dentistry field due to their chemical and biological similarity to human hard tissue. On implantation, the bioactive materials induce specific tissue response which results in the formation of a bonding interface between the implant and the natural bone tissue. Calcium phosphate is used as biomaterials such as synthetic bone graft, drug carrier and coating on metal prosthesis (in hip endoprosthesis and dental implants). Sol Gel technique has been developed and employed to prepare various materials because it has main advantage of easy control of chemical composition and low temperature synthesis that are very important for thin film formation. In this work, microcrystalline hydroxyapatite powder was synthesized from calcium acetate and tri ethyl phosphate in water and ethanol medium. The hydrolysis rate of tri ethyl phosphate in water and alcoholic medium was examined. The pH and gelation time of the batch containing with and without water was examined. The effect of calcinations on powder properties was studied. The XRD analysis was used to examine the phase transformation of hydroxyapatite at different temperature. At particular temperature calcium carbonate was observed and upon acid treatment the calcium carbonate peak at 29.399[degrees] was minimized and a stoichiometric hydroxyapatite was examined. The functional group change of hydroxyapatite at different temperature was observed by FT- IR spectroscopy. The morphological feature of the raw and sintered samples was examined. The synthesized powder was characterized through FT-IR, XRD and SEM analysis.

Introduction

Materials, which are introduced in medical practice for reconstruction and replacement of the diseased and damaged human bones, are called "Biomaterials". According to the tissue response behaviour at the implant interface, biomaterials have been divided in to inert, resorbable and bioactive materials. Among these bioceramics, hydroxyapatite ceramic is a biocompatible and bioactive material that can be used to restore damaged human calcified tissue. The use of sol gel routes to form a bioactive hydroxyapatite layer on metal substrates has recently attracted in the biomedical field. The sol gel method represents the low temperature way of the production of glasses, ceramic and composite materials with better purity and homogeneity than high temperature conventional processes. This process has been used to produce a wide range of compositions (mostly oxides) in various forms, including powders, org/inorg hybrids, fibers, coating, thin films, monoliths and porous membranes. One of the most attractive features of the sol gel process is that it can produce compositions that cannot be created by the conventional methods. The mixing level of the solution is retained in the final product.

In sol gel chemistry, the metal alkoxides convert to amorphous gels of metal oxides through hydrolysis and condensation reactions. Only limited attempts have been reported on the sol gel processing of hydroxyapatite material (1-4). The alkoxide solution route has been proven to be a good way to synthesis materials in different shapes such as films, fibers and powders (5).

It has been reported that the materials prepared with the sol gel process are efficient calcium phosphate absorbents in in vitro and in vivo studies (6-8). While those of the same composition prepared by traditional methods at high temperature are biologically inert (9), hydroxyl groups present on the sol gel process can be responsible for the bioactivity of these materials (10).

In the present investigation, tri ethyl phosphate and calcium acetate were used to develop a hydroxyapatite phase with simple procedures. The synthesized amorphous calcium phosphate powder can be sintered into a pure hydroxyapatite ceramic. The chemical and phase behaviours of the ACP powder have been examined after the thermal treatment. The pH and gelation time of the batch containing with and without alcohol was examined. Many reports have been focused on the sol gel derived hydroxyapatite and that showed the presence of secondary phase like calcium oxide and pyrophosphate. But in our study, we didn't see any calcium oxide peak instead we have calcium carbonate peak. This CaC[O.sub.3] is minimised by acid treatment. The synthesized sample was characterised through FT-IR, XRD and SEM analysis.

Methods and Materials:

Calcium acetate and tri ethyl phosphate was used as calcium and phosphorous precursors respectively. Solution preparation was conducted in a moisture free atmosphere because of the hygroscopic nature of the reactants. High purity calcium acetate was dissolved in water. The phosphate solution was added drop wise into the calcium containing solution with the aid of a magnetic stirrer to obtain a Ca : P ratio of 1.67 and the resultant mixture was stirred for additional 10min at ambient temperature. Preparation was carried out in a nitrogen atmosphere glove box. The solution was allowed to age for the period of 24 hrs. During this stage pH and gelation time of the sols was observed. The formed gels were then dried at 120[degrees]C for 16hrs. The same procedure was carried out in the presence of ethanol medium.

The dried powder was sintered at 900[degrees]C for 2hrs. The resultant powder was washed with 0.01M HCl and washed with water to eliminate the calcium carbonate and it was filtered through filter paper then the resultant powder was dried at 120[degrees]C for 3hrs.

Characterization of gel:

Fourier transform infrared spectroscopy (FT-IR)

The FT-IR spectral studies were conducted using Hitachi 270-50 spectrophotometer by KBr disks in the range of 4000-400 [cm.sup.-1]

X-Ray Diffraction (XRD)

Sintered gel was powdered and the XRD spectrum was recorded using SEIFERT JSO-DEBYEREX-2002 using a step size of 0.02[degrees]. The scan rate of 1[degrees] Per minute and a scan range between 10-60[degrees] in flat plate geometry with Cu radiation was performed.

Scanning Electron Microscope (SEM)

The samples were investigated with a scanning electron microscope (Philips 501 model) to investigate the powder morphology and microstructure of the powders.

Chemical analysis

The phase constitution and the chemical homogeneity of the sample were examined by quantitative chemical analysis via EDTA titration, gravimetry. The molar Ca/P ratio was found to be 1.67, which indicates the formation of hydroxyapatite.

Results and Discussion:

Organic phosphorous derivatives of various ester functional group exhibit different hydrolysis rate upon exposure to water and alcoholic medium. The hydrolysis rate can be monitored by testing its pH values. After preparation of the sols, pH =11.25 was detected for the aqueous based sol and pH=9.47 was observed for the ethanol based sol. Decrease in pH values was monitored after aging in an oil bath for 24hrs. The pH values of aging precursors were measured and the values before and after aging are summarized in Table 1. The decrease in pH values after aging indicating hydrolysis of TEP. Upon aging the hydrolysis products form a complex with [Ca.sup.2+] ions dissolved in the solution. Both the sols were transformed into clear and transparent solution. The viscosity of the sols was increased after the solvents were removed during heating at 60[degrees]C. After cooling the viscosity of the sols was further increased for both viscous sols. Further heating causes removal of the solvents accompanied by thermal dehydration and condensation in the formation of more (-Ca-O-P-) bonds in the dried gels.

FT-IR Spectroscopy:

The FT-IR spectra of as prepared powder with and without treated powders are shown in Fig 1(a, b, c). FT-IR spectrum of the raw sample showed a strong band at 3390cm-1 was associated to the O-H stretching vibration of hydrogen bonded molecules. The IR bands at 1430 and 870[cm.sup.-1] corresponded to the C[O.sub.3.sup.2-] groups. These vibrations are characteristic of a carbonate groups engaged in an amorphous solid. The amorphous solid had a strong band at 1060, 570 and 606[cm.sup.-1] which were associated to the P-O vibration modes respectively. The other featureless peaks are associated to the P[O.sub.4.sup.3-] band in the disordered manner.

[FIGURE 1 OMITTED]

The sample heated at 900[degrees]C was exhibited a strong characteristic peak with respect to hydroxyapatite. When the temperature increased the carbonate bands became less intense and the stretching and vibrational modes of OH- groups became more intense. The peak at 1430[cm.sup.-1] was minimized but not negligible. From the figure, it was confirmed that the hydroxyapatite constitute of calcium carbonate after thermal treatment at 900[degrees]C.

In order to eliminate the carbonate groups from the hydroxyapatite powder acid treatment was performed. The sintered hydroxyapatite particles were suspended in a 0.01M hydrochloric acid medium and stirred for 3 hrs with the aid of magnetic stirrer. After acid treatment, the resultant slurry was filtered and dried in an oven for 3hrs at 80[degrees]C. Then the dried powder was analysed by FT-IR, XRD and SEM.

Figure 1b and 1c represents the FT-IR Spectra of the sample after acid treatment. The acid treated samples had a strong band at 873,1430[cm.sup.-1] that was indicative of the carbonate ion substitution. But after the treatment the intensity of carbonate band was decreased. These spectra suggest that hydroxyapatite samples used in our study are with carbonate substitution. The peaks at 3650[cm.sup.-1] are associated with adsorbed hydrate. The triplet with well resolved bands at 1096, 1085 and 1056 [cm.sup.-1] was identical to phosphate band. The triply degenerate bending vibration of the PO43- ions at 570, 602, and 632[cm.sup.-1] was indicated of the presence of hydroxyapatite phase.

XRD pattern:

Figure 2 showed the XRD spectrum for the treated and untreated samples in water and ethanol medium. XRD patterns of the as synthesized powder showed the presence of an amorphous phase. The sample heated at 900oC showed broad peaks of an apatite phase. When the temperature was increased, the apatite peaks became sharper, because of crystal growth. Alternatively, calcium carbonate peak at 29.3990 was present together with HAP phase; however, the [beta]-TCP phase was not detected at any temperature. Also, the CaO peak at 37.469[degrees] and 54.029[degrees] was not detected at any temperature. The presence of hydroxyapatite was confirmed by a strong diffraction peak at (31.773[degrees]) (211) plane. The accompanying two peaks at 32.196 and 32.902[degrees] of equal intensities were also detected.

[FIGURE 2 OMITTED]

After acid treatment, the carbonate peak at 29.399[degrees] was minimized. The powder patterns do not indicate any peaks corresponding to CaO. The calcium carbonate peak was minimized and the intensity of hydroxyapatite peaks at 211, 112, 300 plane was increased. The crystallization of the carbonate hydroxyapatite was prepared. From the figure, the secondary impurity phases such as CaO and [beta]-TCP were not detected at the sintered samples.

Scanning Electron Microscope (SEM):

Microstructural changes were commensurate with thermal crystallization as shown in Figure 3. The morphology of the as prepared sample showed needle like crystal particles. The size of aggregates is decreased after calcinations of the sample at 900[degrees]C. The collapse of aggregate was more obvious for the sample after acid treatment, which results in the formation of a large particles network.

[FIGURE 3 OMITTED]

Conclusion:

Hydroxyapatite can be synthesized using the sol-gel route with proper heat and acid treatment. There will be no significant differences observed for the powder with and without alcohol medium excluding the pH and gelation time. The powder calcined at higher temperature did not show any secondary phases like CaO, [Ca.sub.2] [P.sub.2] [O.sub.7], [beta]-TCP respectively. The acid treated powder showed a higher resolved HAP peaks at 211,112,300 planes respectively. The acid consumes all the dissolvable impurities. The calcium carbonate peak at 29.399[degrees] was minimized by mild acid washing method.

References:

(1.) Y. Masuda, K. Matubara and S. Sakka, " Synthesis of hydroxyapatite from metal alkoxides through sol gel technique" J. Ceram. Soc. Jpn., 98, 1266-77, 1990.

(2.) A. Deptula, W. Lada, T. Olezak, A. Borello, C. Avani and A. Dibartolomea "Preparation of sperical powders of hydroxyapatite by sol gel processing" J. Non-Cryst. Solids., 147, 537-41, 1992.

(3.) T. Brendel, A. Engel and C. Russel " A Polymeric route for the synthesis of hydroxyapatite coatings" J. Mater. Sci. Mater. Med., 3, 175-79, 1992.

(4.) C. Chai, B. Ben-Nissan, S. Pyke and L. Evans, " Sol gel derived hydroxyapatite coatings for biomedical applications" Mater. Manuf. Processes., 10, 205-16, 1995.

(5.) D. Segal., "chemical synthesis of advanced ceramic materials" (Cambridge university press, Cambridge, PP 62,144, 1989.

(6.) P. Li , K. de. Groot., " Better bioactive ceramics through sol-gel process" J. Sol-gel Sci and Technol., 2, 797-801, 1994.

(7.) T. Peltola, M. Patsi, H. Rahiala, I. Kangasniemi, A. Yli-Urpo, " Ceramic phosphate induction by Sol-gel derived titania coatings on titanium substrate in vivo" J. Biomed. Mater. Res., 48(3), 504-10, 1998.

(8.) P. Li, C. Qhtsuki, T. Kokubo, K. Nakanishi, N. Soga, K. de .Groot, " A Role of hydrated silica, titania and alumina forming biologically active bone-like apatite on implant" J. Biomed. Mater. Res., 28, 7-15, 1994.

(9.) S.F. Holbter, L.L. Hench, L.S. Forbes Bowmann, P. In. Vincenzini, Editor, "ceramics in surgery", Amsterdam, Elsevier, P-3, 1983.

(10.) D.B. Haddow, P.F. James, R. Van Nooot, " Characterisation of Sol-gel surfaces for biomedical applications" J. Mater. Sci. Maters. Med, 7, 255-60, 1996.

U. Vijayalakshmi * and S. Rajeswari

Department of Analytical Chemistry

University of Madras

Guindy Campus, Chennai-600 025

E-mail: lakesminat@yahoo.com
Table 1. pH values of aging precursors before and after aging

Medium pH Before Aging pH After Aging

Without Alcohol 11.25 10.01
Ethanol 9.47 8.47
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Author:Vijayalakshmi, U.; Rajeswari, S.
Publication:Trends in Biomaterials and Artificial Organs
Geographic Code:9INDI
Date:Jan 1, 2006
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