Comparison of Mechanical Properties of Sheep Hydroxyapatite (SHA) and Commercial Synthetic Hydroxyapatite (CSHA)-MgO Composites

Comparison of Mechanical Properties of Sheep Hydroxyapatite (SHA)
and Commercial Synthetic Hydroxyapatite (CSHA)-MgO Composites
Nermin Demirkol1,2,a, Onur Meydanoglu2,b, Hasan Gokce2,c
Faik Nuzhet Oktar3,d and Eyup Sabri Kayali2,e
1Technical Prog. Dept., Vocational School of Degirmendere Ali Ozbay, Kocaeli University, Kocaeli,
Turkey
2Metallurgical & Materials Eng. Dept., Istanbul Technical University, Istanbul, Turkey
3Medical Imaging Techniques Dept., School of Health Related Professions, Marmara University,
Istanbul, Turkey
anermin.demirkol@kocaeli.edu.tr, bmeydanoglu@itu.edu.tr, cgokceh@itu.edu.tr,
dfoktar@marmara.edu.tr, ekayali@itu.edu.tr,
Keywords: Sheep hydroxyapatite, synthetic hydroxyapatite, mechanical properties, magnesium
oxide.
Abstract. In this study, microstructures and mechanical properties of sheep hydroxyapatite (SHA)
and commercial synthetic hydroxyapatite (CSHA)-MgO composites were investigated. The
production of hydroxyapatite (HA) from natural sources is preferred due to economical and time
saving reasons. The goal of development of SHA and CSHA based MgO composites is to improve
mechanical properties of HA. SHA and CSHA composites were prepared with the addition of
different amounts of MgO and sintered at the temperature range of 1000-1300 °C.
The physical and mechanical properties were determined by measuring density, compression
strength and Vickers microhardness (HV). Structural characterization was carried out with X-ray
diffraction (XRD) and scanning electron microscopy (SEM) studies.
In all composites, mean density values and mechanical properties increased with increasing
sintering temperature. The increase of MgO content in SHA-MgO composites showed better
mechanical properties in contrast to CSHA-MgO composites. Although the highest hardness and
compression strength values were obtained at the SHA-10wt% MgO composite sintered at 1300°C,
higher hardness and compression strength values were achieved with 5 wt% MgO addition at the
CSHA-MgO composites when compared to SHA-MgO composites sintered between 1000-1200°C.
Introduction
In the light of human life expectancy up to 90 years, the improvement of health care and the
increase of accidents (due to sport activities and car accidents), the need for effective and
inexpensive biomaterials available to everyone, such as HA produces from different sources such as
biologically derived and synthetic hydroxyapatite is in great demand [1]. HA possesses exceptional
biocompatibility and bioactivity properties with respect to bone cells and tissues, probably due to its
similarity with the hard tissues of the body. To date, calcium phosphate biomaterials have been
widely used clinically in the form of powders, granules, dense and porous blocks and various
composites [2]. HA from natural origins differs from synthetic HA in composition, crystal
morphology, size, shape and physico-chemical properties depending on the technology used to
obtain the synthetic HA. Synthetic HA can be prepared from an aqueous solution, by solid-state
reaction or by hydrothermal methods [3]. To prepare HA from those sources needs analytical pure
grade chemicals. The conventional methods for producing HA needs also time. All those make
commercial synthetic HA (CSHA) production costly and time consuming. CSHA ceramics do not
possess rare minerals like Sr, Si, Mg and many others. Natural HA’s have all kind of those rare
minerals. Interest has been increased to prepare rare natural HA’s from natural sources with many
Key Engineering Materials Vols. 493-494 (2012) pp 588-593
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methods. Calcination is one of the most used methods. Nowadays natural HA is prepared
(calcination method) from bovine, sheep, pig and goat bones. It is already known that the
mechanical properties of HA are poor, especially in wet environment. For this reason ceramics of
pure HA cannot be suggested for use in heavy-loaded implants, such as artificial bones or teeth.
Those HA’s can only be used at non-loading applications, such as graft materials. For improving
the mechanical reliability of HA-ceramics, i.e. to increase their fracture toughness, incorporation of
metallic materials, ceramic oxides, whiskers or fibers, have been suggested [4-7]. For example
MgO is one of the most successful candidates of reinforcement oxides. Magnesium (Mg) is also a
very important element in human body, related to mineralization of calcined tissues, apatite
crystallization, destabilization of HA and the thermal conversion of HA to β-tricalcium phosphate
(β-TCP, Ca3(PO4)2). Mg seemingly reduces risks of cardiovascular diseases, promotes catalytic
reactions and controls biological functions of human body[8].
The aim of this study is to compare the mechanical properties of sheep HA (SHA) and
commercial synthetic HA (CSHA)-MgO composites to determine the effects of the source and
production process of HA.
Materials and Methods
The SHA used in this study was prepared from calcinated sheep bones. Firstly, fresh cut femurs
were deproteinized with NaOH and after reirrigation the samples were subjected to calcination at
750°C. Then calcinated sheep bones were wet ball milled for 24 hours and they were dried at the
drying oven. SHA powders had an average particle size of 10 μm. The CSHA and SHA powders
were mixed with (seperately) 5 and 10 wt% magnesium oxide powder for 4 hours. The samples
were prepared according to a British Standard for compression tests (BS 7253) [9]. The powder
portions were pressed at 350 MPa between hardened steel dies. Pressed samples were subjected to
sintering at different temperatures between 1000°C and 1300°C (with the heating rate of + 5°C min
-1) for 4 h. Compression strength, Vickers microhardness as well as density were measured. SEM
and X-ray diffraction studies were also conducted. The compression tests were done with an
universal testing machine, at the crosshead speed of 3 mm/min. Microhardness values were
determined under 200 g. load. SEM images were taken with Scanning Electron Microscope (JEOL
JSM-5410).
Results and Discussion
Table 1 shows the experimental results of density, compression strength and Vickers
microhardness of the SHA samples sintered at different temperatures.
Table 1. Experimental results of density, compression strength and Vickers microhardness of the
SHA samples sintered at different temperatures.
Temperature (°C) Density (g/cm3) Compression Strength
(MPa)
Vickers Microhardness
(HV)
1000 2,09 31 49
1100 2,16 38 67
1200 2,40 50 138
1300 2,59 69 189
The mean density, compression strength and Vickers microhardness values of SHA increase
with increasing sintering temperature, as seen in Table 1. The maximum values were achieved at
1300 °C sintering temperature. Table 2 and 3 show mean values of density, compression strength
and Vickers microhardness of SHA-MgO and CSHA-MgO composites at the different sintering
temperature, respectively. Fig. 1 and 2 show the results of Table 2 and 3 to compare the properties
of SHA and CSHA composites.
Key Engineering Materials Vols. 493-494 589
Table 2. Influence of magnesium oxide content and sintering temperature on density, compression
strength and Vickers microhardness of composites made of sheep hydroxyapatite and magnesium
oxide (SHA-MgO).
Temperature (°C) Density (g/cm3) Compression Strength
(MPa)
Vickers Microhardness
(HV)
5wt% 10wt% 5wt% 10wt% 5wt% 10wt%
1000 2,11 2,13 32 42 72 79
1100 2,17 2,26 51 63 89 106
1200 2,57 2,72 70 71 165 212
1300 2,96 2,99 109 116 263 458
Table 3. Influence of magnesium oxide content and sintering temperature on density, compression
strength and Vickers microhardness of composites made of magnesium oxide and commercial
synthetic hydroxyapatite (CSHA-MgO).
Temperature (°C) Density (g/cm3) Compression Strength
(MPa)
Vickers Microhardness
(HV)
5wt% 10wt% 5wt% 10wt% 5wt% 10wt%
1000 2,15 2,07 47 38 83 76
1100 2,26 2,18 71 65 156 124
1200 2,77 2,73 78 73 316 282
1300 2,90 2,84 85 78 370 358
(a) (b)
Fig.1. Comparison graphics of compression strength of (a) SHA-5 wt% MgO and CSHA-5 wt%
MgO (b) SHA-10 wt% MgO and CSHA-10 wt% MgO composites at different sintering
temperatures.
Fig.2. Comparison graphics of Vickers microhardness of (a) SHA-5 wt% MgO and CSHA-5 wt%
MgO (b) SHA-10 wt% MgO and CSHA-10 wt% MgO composites at different sintering
temperatures.
590 Bioceramics 23
In all composites, mean density values and mechanical properties increased with increasing
sintering temperature. Density, compression strength and hardness values of CSHA-5 wt% MgO
composites are higher than CSHA-10 wt% MgO composites and higher Vickers microhardness
values were achieved with CSHA-MgO composites than SHA-MgO composites at all sintering
temperatures. Although, the highest hardness and compression strength values were obtained at the
SHA-10wt% MgO composite sintered at 1300°C, the highest hardness and compression strength
values were achieved with 5 wt% MgO addition at the CSHA-MgO composites sintered between
1000-1200°C. The MgO addition to SHA improved to strength properties of composites up to 70%.
(a) (b)
(c) (d)
Fig.3. XRD diagrams of (a) SHA-5wt% MgO (b) SHA-10wt% MgO (c) CSHA-5wt% MgO (d)
CSHA-10wt% MgO at 1000 and 1300°C sintering temperature.
Fig.3 shows the XRD diagrams of composites at 1000 and 1300°C sintering temperatures.
Hydroxyapatite and MgO phases were achieved in the SHA-composites at all sintering temperatures
(Fig.3a,b). CSHA-MgO composites include calcium hydrogen phosphate hydrate (CHPH) phase at
1300°C sintering temperature in addition to the phases hydroxyapatite (HA), calcium magnesium
phosphate (CMP) and periclase-syn MgO (P) phases present between sintered composites at 1000-
1200°C. The lower compression strengths obtained at CSHA-MgO composites compared to SHAMgO
composites sintered at 1300°C, may be related to calcium hydrogen phosphate hydrate phase
formed at 1300°C in CSHA-MgO composites.
Key Engineering Materials Vols. 493-494 591
(a) (b)
(c ) (d)
(e) (f)
(g) (h)
Fig.4 Microstructures of MgO containing composites (a) SHA-5 wt% MgO, 1000°C, (b) SHA-5
wt% MgO, 1300°C, (c) SHA-10 wt% MgO, 1000°C, (d) SHA-10 wt% MgO, 1300°C, (e) CSHA-5
wt% MgO, 1000°C, (f) CSHA-5 wt% MgO, 1300°C, (g) CSHA-10 wt% MgO, 1000°C, (h) CSHA-
10 wt% MgO, 1300°C.
The microstructures of both MgO containing SHA and CSHA composites sintered at different
temperatures are given in Fig.4. The microstructures sintered at 1300 °C show better densification
as seen in Fig.4.
592 Bioceramics 23
Summary
In this study, the microstructural and mechanical properties of sheep HA and commercial
synthetic HA composites with MgO addition were compared to determine the effects of the source
and production process of HA.
The following conclusions were obtained.
1. In all composites, mean density values and mechanical properties increased with increasing
sintering temperature.
2. The increase of MgO content in SHA-MgO composites showed better mechanical properties
in contrast to CSHA-MgO composites.
3. The highest hardness and compression strength values were obtained at the SHA-10wt%
MgO composite sintered at 1300°C.
4. The higher hardness and compression strength values were achieved with 5 wt% MgO
addition at the CSHA-MgO composites for 1000-1200°C sintering temperatures.
5. The MgO addition to SHA improved to strength properties of composites up to 70%.
Biocompatibility studies are going on. If the results of biocompatibility tests are positive, MgO
containing HA composites seems to be very good material for orthopedic applications.
Acknowledgement
The authors would like to thank Prof. Dr. Serdar Salman, Prof.Dr. Mustafa Urgen and Research
Assist. Serdar Pazarlıoglu for their support during experimental studies.
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