Study of physical and mechanical properties of BG / HA / TiO 2 biocomposite for bone implantation

Sunil Prasad. In the present study composition of HA, TiO2 and bioglass were successfully synthesized and biocomposites were prepared by powder metallurgy method. Microstructure and morphology of the biocomposites were examined by scanning electron microscopy (SEM). Analytical, thermal and microstructural investigations were carried out by XRD, SEM, DTA/TGA techniques, and results shows considerable higher rates of apatite formations. The dissolution behavior of the bioactive glasses and biocomposites in SBF showed in vitro test for all samples. Addition of X%HA and X%TiO2 in (100-2X%) 45S5 bioglass improves mechanical properties such as hardness, compressive strength, elastic modulus of these biocomposites.

Experimental:-2.1 material and methods:-2.1.1 Preparation of Bioglass:-Bioglass (45S5) with the chemical compositions 45%SiO 2 , 24.5%CaO, 24.5%Na 2 O and 6%P 2 O 5 in (wt%) was prepared from reagent grade chemicals. Chemicals were weighout, properly mixed and melted in 100 ml platinum crucible at 1400± 5˚C with air as furnace atmosphere for 4 hours. Melted glasses were poured in water to prepare frit and it was milled to a powder form in a porcelain ball mill for 24 h.

Preparation of BG/HA/TiO 2 composites:-
The HA and TiO 2 powder were mixed (5, 10, 15 and 20 wt%) with bioglass (45S5) powder, compacted at 1500 MPa pressure into cylindrical samples (1 cm,1 cm) and were sintered at 1150˚C to prepare the composites as shown in Table 1.

Preparation of SBF:-
Kokubo and his colleagues developed simulated body fluid that has an inorganic ion concentrations similar to those of human body fluid in order to reproduce in vitro formation of apatite on bioactive materials (Kokubo T et al.,2006

Physical Analysis:-
Differential thermal analysis (DTA) was carried out on bioactive glass samples which were examined at the temperature of 250˚C to 900˚C, using alumina as a reference material and the heating rate was 10˚C/min. Identification of the crystalline phases after heat treatment of biocomposite samples was carried out by X -ray diffraction (XRD) analysis, adopting Ni filter and Cu target with voltage of 40 KV and a current of 25 mA.

Powder X-ray diffraction (XRD) measurements:-
The biocomposite samples were ground to 75 microns and the fine powders were subjected to X-ray diffraction analysis (XRD) with RIGAKU-Miniflex II diffractometer adopted Cu-Kα radiation (λ = 1.5405A°) with a tube voltage of 40 kV and current of 35mA in a 2θ range between 20 o and 80 o . The step size and measuring speed was set to 0.02˚ and 1° per min respectively, in the present investigation. The JCPDS-International Centre for Diffraction Data Cards were used as a reference.

Density and Mechanical Properties Measurements:-
Archimedes principle was applied to obtain the density of biocomposite samples using distilled water as buoyant. All the weight measurements were taken using a digital balance (Sartorius,Model: BP221S, USA) having an accuracy of ± 0.0001 g. Density (ρ) of sample was obtained by employing the relation (1) as given below: where W a is the weight of sample in air, W b is the weight of sample in buoyant and ρb is the density of buoyant. Micro indentations were made on the polished surfaces of bioactive glass composite using a diamond Vickers indenter of a micro hardness testing machine (Future -Tech Corp, Tokyo, Model FM -7e, Japan). The size of the specimen was 20 mm x 20 mm x 20 mm according to ASTM Standard: C730 -98. The indentations have been made for loads ranging between 30 mN and 2000 mN, applied at a velocity of 1 mm/s and allowed to equilibrate for 15 seconds before measurement. Microhardness (H) (GPa) of specimen is calculated using the formula (2) as given below: Where H v hardness value, P (N) applied load on specimen and D (m) is the diagonal of the impression.

Mechanical behaviour measurements:-
Pellets were prepared by hydraulic press machine in the form of rectangular shape pellets and the resultant biocomposite samples were sintered at 1150˚C and polished for required dimension using grinding machine. Samples were subjected to three-point bending test. The test was performed at room temperature using Instron Universal Testing Machine (AGS 10kND, SHIMADZU) of cross-head speed of 0.5 (mm/ min) and full scale load of 2500 kg. Flexural strength was determined according to ASTM Standard: C158 02(2012). Polished bioactive glass samples were subjected to hardness testing machine, size of sample was 10 × 10 × 10 mm according to the ASTM Standard: C730-98. The indentations have been made for loads ranging between 30 and 2000 mN, applied at a velocity of 1(mm/s) and allowed to equilibrate for 16 s before measurement. The densities of casted biocomposites were measured by the Archimedes principle with water as the immersion fluid. The measurements were taken at room temperature. Thin copper wire was used for immersing the samples into water. The density was determined by using ASTM: B962-14. Compressive strength of the biocomposite samples having size of 3×2×1 cm −1 dimension according to ASTM D3171 were subjected to compression test. The test was performed using Instron Universal Testing Machine at room temperature (cross speed of 0.05 cm min −1 and full scale of 5000 kgf).

Physical Analysis:-3.1.1 Differential Thermal Analysis (DTA):-
The DTA traces of biocomposites show that the incorporation of HA and TiO 2 in the base bioactive glass (45S5) causes a decrease in its endothermic peak temperature as well as its exothermic peak temperature. In the differential thermal analysis (DTA) traces of bioactive glasses (Fig.2), endothermic peak shows the nucleation region and the exothermic peak corresponding to the crystallisation process.

Phase analysis:-
The prepared biocomposite samples are BGHATi1, BGHATi2, BGHATi3 and BGHATi4 (all of the samples in Table 1) were characterized by XRD. X-ray powder diffraction data of the prepared biocomposites are shown in Fig.3 Fig.3 shows the increase of the intensity of calcium titanium silicate oxide peaks denoting more reaction found between the BG and TiO 2 (Long M et al.,1998)  . As a result, the apatite layer forms onto the composite surface after soaking in SBF in a short period (3days) and this phenomenon is confirmed by SEM of BG/HA/titania composites post-immersion as shown in Fig.5(a-d). BGHATi1 biocomposites, shows that this composite has many particles on its surface proving slight formation of apatite layer due to the composite contains high content of silica characterizing melted and dense structure that reduced nucleation of apatite layer compared to other composites. In this domain, the simultaneous dissolution of silicates results in the formation of silanol groups on material's surface, which are essential for nucleation sites resulting in HA formation (C. Ti-OH groups at the expense of Si-OH groups resulting in high nucleation of apatite (Fig. 5d). In this study, it was observed that the rutile form of titania is the main phase in four composites and is essential for improvement of apatite nucleation especially BGHATi3 and BGHATi4 composites compared to BGHATi1 composites containing low content of titania. The catalytic effect of the Si-OH groups and Ti-OH groups for the apatite nucleation has proven by the observation that silica and titanium will form apatite on their surfaces in SBF and are abundant on the composite surfaces (C. Sarmento  EDAX point analyses show that Ca, P, and Ti coexist in different properties of sintered pellet as shown in Fig. 4  (e-h) conferming the interfusion between HA and TiO 2 particles before their impinging into the substrate.

pH behavior in SBF:-
The variation in pH values of simulated body fluid (SBF) after soaking of biocomposite for various time periods is shown in Fig. 6

Mechanical Properties:-3.3.1 Density, Compressive Strength and Hardness of biocomposites:-
The density increased rapidly when the pellet samples were sintered at 1150˚C and 1250˚C due to the partial HA decomposition into α-TCP and TTCP. Density of biocomposite sintered at 1150˚C increased with increasing HA and TiO 2 content as shown in Fig.6 because density of titanium is more than 45S5 bioglass and HA. Variation of compressive strength depending on reinforcement content and sintering temperature has shown in Fig.7. The figure shows that increasing reinforcement content from (5, 10, 15 and 20) wt.% increased the compressive strength from 41 to 104 MPa for sintering at 1150˚C. Such a phenomenon can be attributed to the occurrence of a new phase among bioglass, HA and TiO 2 for higher sintering temperatures. Fig.7 Variation of compressive strength depending on reinforcement content of pellet samples.

Degradation of compressive strength of biocomposite during in vitro test:-
The compressive strength and elastic modulus of the scaffolds after immersion in SBF in vitro are shown in Fig.8(a) as a function of immersion time or implantation time. The strength and modulus decreased rapidly during first 3 weeks but more slowly thereafter. This trend was independent of the in vitro environment. The strength decreased from the as fabricated value of 104±8MPa to 84±5MPa after 3 weeks in SBF in vitro test. After 12 weeks, the strength of the scaffolds immersed in SBF was 73±8MPa. The elastic modulus of the scaffolds decreased from the as fabricated value of 82±8GPa to 75±5GPa after 3 weeks in SBF in vitro Fig.8(b). The modulus of the scaffolds was 66±8GPa, respectively after 12 weeks in SBF and in vitro environment.

Elastic properties of HA and TiO 2 reinforcement of biocomposites:-
The results indicate that the elastic moduli showed an anomalous with an initial addition of HA ,TiO2 and it increases with further addition of HA,TiO 2 content as shown in Fig.9. In BGHATi1 and BGHATi4 biocomposite, the measured young's and shear moduli ranges respectively from 47 to 82 GPa and 25 to 33 GPa. Similarly, the young's and bulk moduli ranges from 47 to 82 GPa and 33 to 53 GPa

Conclusion:-
Biocomposites with addition of HA and TiO 2 in Bioactive glass (45S5) are prepared respectively using sintering process. The presence of TiO 2 in silicate based 45S5 bioactive glass network results in higher rigidity which is explored from the observed increase in glass density. Due to the higher bonding ability of TiO 2 , its results an increase in ultrasonic velocity. The elastic constants results support the above observation. The thermal treatment of silicate based glasses results in the release of stresses from the glass and the possible formation of crystalline phases along with the residual glassy phases.
Ti 2+ ion was introduced in the glass composition for Si 4+ ion in different concentrations (0-20 wt%) to yield a non charge valence series of bioglass based biocomposites. The increase of HA and TiO 2 content in bioglass composites result in increase of density, compressive strength, youngs, shear and bulk modulus while the poisson's ratio remained nearly constant. Mechanical property of the samples can be measured without any destruction of the biocomposites, since the biomaterials are very expensive to prepare.