Titania doped bioactive ceramics prepared by solid state sintering method

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 8964–8972 www.elsevier.com/locate/ceramint

Titania doped bioactive ceramics prepared by solid state sintering method Rehana Ziaa, Madeeha Riaza,n, Sitara Maqsooda, Safia Anjuma, Zohra Kayania, Tousif Hussainb a Department of Physics, Lahore College for Women University, Lahore 54000, Pakistan Center for advance studies physics, Government College University, Lahore 54000, Pakistan

b

Received 9 February 2015; received in revised form 22 March 2015; accepted 23 March 2015 Available online 1 April 2015

Abstract A bioactive ceramic in (48
x) SiO2–36CaO–4P2O5–12Na2O–xTiO2 (where x ¼ 0, 3.5, 7, 10.5 and 14 mol%) system was prepared by solid state sintering method. The in vitro bioactive properties of bodies were evaluated using stimulated body fluid under static condition at 37 1C. The formation of hydroxyl-carbonated apatite layer on the surface of samples was examined by XRD, FTIR, SEM and AAS. It was found that partial substitution of SiO2 with TiO2 produced a positive impact on the bioactivity of the specimens. Addition of TiO2 r 10.5 mol% to the system not only enhanced the bioactive properties but also accelerates the apatite forming process. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: In vitro bioactivity; Titania; Bioceramics; Solid state sintering

1. Introduction Bioceramics are frequently used in medical industry to replace the faulty organs and diseased tissues in living organisms. This field has benefited a large number of people. The history of implantation is not new but with the development of new technologies, the use of bioceramics inside human bodies has now become widespread [1]. Bioactive materials are of special interest for their wide range of properties including their fast reaction rates responsible for their attachment to bone, their nontoxic nature and ease to control their reaction rates by modifying their composition according to particular function. Bioactivity is an essential property of these materials; a spontaneous communication between the material and physiological environment that in consequences led to bonding of the material to surrounding bone and tissues [2]. Bioactive materials utilizes the fact that apatite is an important mineral phase of bone and these materials also form tight bonding to bone via formation of apatite layer on their surface. Bioactive materials are the most effective materials to be used as bone grafts to fill the gaps between the neighboring bones. On implantation inside the body these materials develop a biologically n

Corresponding author. E-mail address: [email protected] (M. Riaz).

http://dx.doi.org/10.1016/j.ceramint.2015.03.170 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

active surface layer of apatite which bonds tightly to bony apatite and in this way the gap is filled by newly formed tissues. It means that the formation of apatite is the most important requirement for a synthetic material to be bioactive [3]. Before actual transplantation of the biomaterial, it is safe to pre-check their in-vivo bioactivity in a cellular solution (SBF) having ionic concentration similar to human blood plasma [4]. Bioactivity is affected not only by the type of material but also the type of the solution in which it is soaked. Kokubo et al. observed that A/W glass ceramics did not produce any apatite layer when tested in Tris buffer solution, while upon soaking in SBF, an apatite layer was observed over the surface. Various experiments have been performed and it was found that SBF can be used as a standard to evaluate the in-vitro bioactivity [5]. The bioactive materials used as bone grafts are most commonly obtained from the basic system of oxides CaO–P2O5– SiO2 [6]. The addition of Na2O to CaO–P2O5–SiO2 had been reported to enhance bioactivity of the system; it disturbs the network connectivity by increasing the number of non-bridging oxygen atoms that result in prompt degradation rate [7]. Peitl et al. reported P2O5–Na2O–CaO–SiO2 the highly bioactive composition, because the presence of phosphorous in solid solution and combeite (Na2Ca2Si3O9), known for high level of bioactivity [8], were responsible for accelerating the apatite forming process [9,10]. Ti ions are not part of the human body, it has bioinert

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nature, but due to its extraordinary bone-healing and anticorrosive abilities that trigger the cell mediated immune response in the body, makes it more biologically attractive material to be used for restorative substitute [11]. This research work represent, a comphrehensive study on effect of increasing amount of TiO2 on invitro bioactivity of bioceramics prepared by solid state sintering method, which includes the effect of titania concentration on crystal structure, surface morphology and ionic release rates and bioactivity of bioceramics. 2. Materials and method Five different compositions (given in Table 1) of bioceramics are prepared in the system CaO–P2O5–SiO2–Na2O with varying amounts of titania by solid state sintering method. The batches were prepared using appropriate amount of analytical grade (99% pure) silicon dioxide (Merck), calcium carbonate (Merck), sodium carbonate (Merck), phosphorous V oxide (RDH) and titanium oxide (Fluka), subsequently milled in agate mortar and pestle for several hours. The pulverized batches were sevied to obtain r 40 μm desired particle size. Particle size has great importance because it exhibit sinterability, enhanced densification due to their greater surface area, which may improve fracture toughness as well as other mechanical properties and better bioactivity due to high reactvity [12]. The pulverized batches were compacted into disc shape of diameter 15 mm under hydrostatic pressure of 50 MPa, then sintered at 1000 1C for four hours holding time Table 1 The chemical composition of samples in mol%. Sample

SiO2

CaO

TiO2

P2O5

Na2O

BC1 BC2 BC3 BC4 BC5

48 44.5 41 37.5 34

36 36 36 36 36

0 3.5 7 10.5 14

4 4 4 4 4

12 12 12 12 12

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and cooled to room temperature at 5 1C/min rate. The X-Ray Diffraction (XRD) data was obtained by Bruker D-8 Discover diffractometer, JCPDS – International Centre for Diffraction Data cards was used as a reference data for interpretation of peaks. The average crystallite size was calculated by using the Scherrer's formula [13]: Dp ¼ Kλ=β cos θ

ð1Þ

where Dp is crystallite size, K is shape constant (1), β is full width half maximum (FWHM), θ is Bragg's angle. In vitro bioactivity test was performed in SBF for 30 days, the pellets were soaked in solution for 30 days under static condition at 37 1C. The formation of apatite on the surface of the samples were eludicated by XRD, Fourier transform infrared (FTIR; Midac, M2000), Scanning electron microscope (SEM; JSM6480VL, Jeol) and Atomic absorption spectroscopy (AAS; Z-5000, Hitachi). 3. Result and discussion 3.1. In vitro bioactivity assays Bioactive materials have a general property of forming a biological layer that eventually bonds to bone. Fourier transform infrared spectroscopy (FTIR) is the best tool to investigate the sequence of reaction kinetics of formation of hydroxyl-carbonated apatite (HCA) on the surface of implant [14,15]. The chemical composition, solubility rates and phases of ceramics greatly affects the formation of apatite [16]. So, the bioactivity of the samples was elucidated by FTIR, AAS, XRD and SEM after soaking in SBF for 30 days. 3.1.1. Phase analysis In Fig. 1(A) the X-Ray Diffraction peaks corresponding to parawollastonite [(CaSiO3) (PDF 10-0489)], sodium calcium silicate [(Na2Ca3Si6O16) (PDF 23-0671)], combeite [(Na2Ca2Si3O9) (PDF 22-1455)] and oxyapatite [(Ca10(PO4)6O) ;(PDF 89-6495)] were indentified for all heat treated bioceramic

Fig. 1. XRD pattern for bioceramics (a) BC1, (b) BC2, (c) BC3, (d) BC4 and (e) BC5 before (A) and (B) immersion in SBF solution for 30 days.

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samples. Combeite and wollastonite phase both posses high bioactivity index because of rapid dissolution rate [8,17,18] are the two most expected phases in calcium-silicate ceramic systems [19]. With addition of titania a new crystalline peak of whitlockite [(Ca3(PO4)2) (PDF 09-0169)] appeared. Whitlockite a bioresorbable material show slow degradation rates but posses high biactivity in physiological fluids is effectively used in medical industry [20]. With addition of TiO2 4 3.5 mol % a new minor peak of sodium titanium phosphate [(NaTi2(PO4)3) (PDF 33-1296)] phase at 2θ of 24.251 appeared, and its intensity increased with increase in TiO2 content. The development of new crystalline phases with increasing amount of TiO2 is due to the reason that the oxides of transition metals increases the trend towards separation of phases and hence increases crystallization [21]. After immersion in SBF for 30 days, all other crystalline phases were decomposed only a single phase of HCA [(Ca10H2O26P6) (PDF 04-0697)] with reflections at 25.881, 28.971, 31.71, 33.01, 34.21, 39.91, 43.51, 47.31 and 50.21 in 2θ range were observed (shown in Fig. 1(B)), indicating the formation of HCA layer on the surface of the samples. The intensity of peak at 25.881 corresponding to reflection from (002) plane increases for r 10.5 mol% TiO2, but decrease in peak intensity was observed for 14 mol% in sample BC5 this result may be attributed to controlled dissolution of this sample. The average crystallite size of HCA phase; measured by Scherer's formula for bioceramics after immersion in SBF for 30 days (shown in Table 2). It was observed that the HCA crystallite size increases with increasing titania content due to increased number of nucleation sites (Si–OH and Ti–OH) as reported by Shyu et al [22]. The electrostatic interaction between negatively charged functional group (Si–OH and Ti–OH) and positively charged calcium ions is the main factor for formation of apatite layer on the surface of the sample [23]. 3.1.1.1. XRD instrumental broadening. Broadening in X-Ray Diffraction lines occurs when the particle size is less than 100 nm, this broadening is due to particle size and strain. The crystallite size measured by Scherer’s formula does not consider lattice strain and instrumental factors on broadening of X-Ray Diffraction lines. Williamson–Hall [24] plot provide the total broadening represented as follows: Bt ¼ BD þ Bi þ Bs

ð2Þ

Table 2 Crystallite size measured from Scherer's formula and Williamson–Hall method. Sr.

BC1 BC2 BC3 BC4 BC5

Diffraction angle (2θ) deg

β crystallite size crystallite size (FWHM) (Scherer’s formula) (William–Hall (radians) (nm) method) (nm)

25.946 25.909 25.909 25.936 25.884

0.00506 0.002772 0.002575 0.00233 0.0019037

3.12441 5.70287 6.13916 6.78507 7.52498

10.9 31.6 35.3 20 23

where Bt is the total broadening, BD is the broadening due to crystallite size and lattice strain and Bi is the broadening due to instrumental factors. The instrumental broadening is presented in Fig. 2(A). The sample broadening is described by Kλ β  cos θ ¼ þ 4 sin θ  strain ð3Þ Dp The William–Hall plot gives the particle size and strain, βcos θ plotted with respect to 4sin θ and straight line (y¼ mxþ c) with y-intercept refers to crystallite size and slope refers to strain. 3.1.2. FTIR analysis Fig. 3(A) and Table 3 show the IR transmission spectra of all bioceramics before immersion in SBF revealed the presence of Si–O–Si stretching vibration of non-bridging oxygen atoms (940–860 cm
1), Si–O–Si bend (500–400 cm
1) and Si–O–Si tetrahedral stretch (1175–710 cm
1) which are the characteristics bands of silica network [25–27]. The presence of these vibrational bands indicated that the silica is the basic building component of the system. The spectra also indicated the presence of phosphate bands i.e. P–O bend (600–560 cm
1) and P–O stretch (1043–1024 cm
1) in all bioceramics which may be due to the presence of phosphate containing phases in bioceramics [28]. The IR spectra also showed the vibrational modes corresponding to C–O stretch (1465–1415 cm
1) [29]. This may be due to use of sodium carbonate in the green composition and secondly firing of bodies in air. The bands in the region 1600–1650 cm
1 represents the H–O–H bonding vibration and 3000–3600 cm
1 represent the stretching vibration of absorbed water molecule [30]. Fig. 3(B) and Table 4 show FTIR spectra after 30 days immersion in SBF, it was observed that the silicate transmission bands disappeared and all samples showed characteristics vibrational bands of biological apatite along with a new band of P–O bend (490 cm
1) at 489.2 cm
1 which is a typical band of HCA layer that is well crystallized [19]. The maximum intensity of these bands is shown by BC4, which means it possesses the highest bioactivity of all prepared samples. Fig. 3(B) shows that the bioactivity progressively increases with addition of TiO2 r 10.5 mol% due to high reaction rates whereas bioactivity decreases for sample BC5 having 14 mol% TiO2. This result may be due to the fact that the chemical durability of system usually increases with addition of oxides of transition metals as studied by Sarivasta et al. [31]. The leaching rate of ions from BC5 is suppressed due to its low reaction rates, which suppresses the formation of silica layer, which in turn delays the development of CaO–P2O5 rich layer. Thus, it became important to study the solubility rate of specimens and its effect on pH of physiological fluid used. 3.1.3. AAS analysis It is important to study the solubility rate of specimens and its effect on pH of physiological fluid used, in order to determine the factors responsible for increase or decrease in apatite forming rates. The change in concentration of Si, Ca, and P, along with variation in pH of solution and weight of specimens after

R. Zia et al. / Ceramics International 41 (2015) 8964–8972

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0.0055 0.005

y = 0.0001x + 0.0002

0.0045

FWHM

0.004 0.0035 0.003 0.0025 0.002 0.0015 0.001 25

30

35

40

45

50

2 theta 0.0045 0.004

0.0034

β cos theta

β cos theta

0.0036

0.0032 0.003 0.0028 y = 0.008326x- 0.002384 R² = 0.309990

0.0026 0.0024 0.0022 0.87

1.07

1.27

1.47

1.67

β cos theta

β cos theta

0.003 y = 0.0008x + 0.0022 R² = 0.6104

0.002 0.0015

1.07

1.27

1.47

1.67

1.87

4 sin theta

Sample: BC3

y = 0.0017x + 0.0012 R² = 0.9893

1.07

0.005 0.0045 0.004 0.0035 0.003 0.0025 0.002 0.0015 0.001 0.0005 0 0.87

Sample: BC4

1.27

1.47

1.67

1.87

4 sin theta

Sample: BC2

0.004

0.001 0.87

0.002

0.001 0.87

1.87

0.0035

0.0025

0.003 0.0025

0.0015

4 sin theta

Sample: BC1

0.0035

y = 0.0025x + 0.0001 R² = 0.9440

1.07

1.27

1.47

1.67

4 sin theta

0.005 0.0045

β cos theta

0.004 0.0035 0.003 0.0025 0.002 y = 0.0018x + 0.0004 R² = 0.1166

0.0015 0.001 0.0005 0 0.87

Sample: BC5

1.07

1.27

1.47

1.67

4 sin theta

Fig. 2. (A) A typical instrumental broadening. (B) Williamson–Hall plot.

soaking in physiological fluid (SBF) were determined for 0, 1, 3, 7,15, 21, 25 and 30 days. It can be seen in Fig. 4, that the ceramic BC4 showed the highest release rates of ions whereas BC5 exhibited the lowest release of ions. The concentration of Si increased rapidly during the initial days and then it attained a constant value showing only a small increment with time. The Ca concentration was observed

to be increased during initial soaking periods and then it rapidly decreased due to its precipitation over the sample surface [32]. In case of BC4 the concentration calcium ion starts decreasing after 15 days, whereas for all other ceramics it decreased after 21 days. It shows that HCA developed earlier on the surface of BC4 as compared to others and thus it possesses highest bioactivity of all. The initial increase in amount of Ca are due to their exchange

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After 30 days Before

Si-O(2NBO) P-O (stretch)

3500

3000

2500

2000

Wavenumber

1500

H-O-H C-O (stretch) P-O(bend)/Glass

Transmittance %

Transmittance %

Si-O-Si (stretch)

BC1 BC2 BC3 BC4 BC5

H-O-H C-O(stretch)

4000

C-O(stretch)

O-H(stretch)

Si-O-Si(bend)

P-O(stretch) P-O(bend)

BC1 BC2 BC3 BC4 BC5

P-O (stretch) P-O(bend)

1000

500

4000

3500

3000

(cm-1)

2500

2000

Wavenumber

P-O (stretch) P-O(bend)/Crystal

1500

1000

500

(cm-1)

Fig. 3. FTIR transmission spectra of bioceramics before (A) and after (B) soaking in SBF solution.

Table 3 FTIR band spectra for all bioceramics post-immersion in SBF. Vibration mode

Si–O–Si bend H–O stretch C–O stretch PQO stretch Si–O–Si stretch (2NBO) Si–O–Si sym. stretch P–O bend/ crystal P–O bend/ amorphous P–O stretch

Table 4 FTIR band spectra for all bioceramics after immersion in SBF.

Wave number (cm
1) published data

Before immersion in SBF BC1

BC2

BC3

BC4

BC5

470–455

479.9

479.9

479.9

479.9

479.9

1650–1600 1460–1415 1350–1025

1637.1 1637.1 1641.2 1637.1 1641.2 1419.1 1419.1 1419.1 1419.1 1419.1 1038.2 1031.1 1045.3 1045.3 1024.8

940–860

876.5

879.6

879.6

876.5

876.5

1175–710

762.6

725.9

735.4

717.1

785.9

610–600

–

–

–

–

–

600–560

573.0

565.9

580.1

573.0

580.1

960

957.0

–

–

957.0

957.0

Vibration mode

with H þ and H3O þ ions from solution, in order to form Si–OH necessary for apatite growth [33]. A decrease in concentration of P ions was also observed. This decrease in Ca and P ions concentration with an increase in Si concentration is an indication of formation of Ca–P rich layer [34]. Fig. 5 shows that the pH value of the solution was found to be increased rapidly during first 7 days of immersion, which is in agreement with previous studies according to which apatite formation is always encouraged by a higher pH value [34]. After day 7, the pH of solution became nearly constant and increased with comparatively slower rate till 30th day. After 30 days of immersion, the change in pH value was maximum for BC4 (from 7.25 to 9.01), while this increment was minimum for BC5 (from 7.25 to 7.86). The bioceramics BC1, BC2 and BC3 showed values 8.109, 8.266 and 8.684 respectively that lie in between of BC4 and BC5. The pH values obtained for ceramics

Si–O–Si bend H–O stretch C–O stretch PQO stretch P¼ O stretch C–O stretch Si–O–Si sym. stretch P–O bend/ crystal P–O bend/ amorphous P–O bend/ crystal H–O–H stretch P–O bend P–O stretch

Wave number (cm
1) published data

After 30 days immersion in SBF BC1

BC2

BC3

BC4

BC5

470–455

–

–

–

–

–

1650–1600

1639.1

1631.9 1625.4 1628.5 1628.5

1460–1415

1412.09 1413.1 1417.1 1417.1 1420.2

1350–1025

1054.0

1046.9 1061.9 1039.0 1054.0

1350–1025

1054.0

1046.9 1061.9 1039.0 1054.0

890–800

875.0

875.0

875.0

882.9

876.0

1175–710

–

–

–

–

–

610–600

606.9

600.0

606.9

606.9

606.9

600–560

–

–

–

–

–

560–500

–

–

–

554.8

–

3550

3418.0

3432.2 3418.0 3411.1 3418.0

490 960

489.2 –

489.2 957.0

489.2 957.0

489.2 964.9

489.2 957.0

were found higher than Zhang et al. [35] observed values for titania substituted glass ceramics which may be assumed to be due to higher dissolution rate of ceramic bodies as compared to glass ceramics. The release of alkaline ions are actually responsible for rise in pH values as they are exchanged with H þ ions of solution due to which concentration of H þ ions get reduced while increasing the OH
concentration in solution. The increase in pH became slower with time as the solution became

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240

140

230

BC1 BC2 BC3 BC4 BC5

220

120

Ca concentration (mg/l)

Si concentration (mg/l)

210 100

80

60 BC1 BC2 BC3 BC4 BC5

40

20

200 190 180 170 160 150 140 130 120 110

0

100 0

5

10

15

20

25

30

0

5

10

Reaction time in SBF (days)

15

20

25

30

Reaction time in SBF (days) 0.18

35

BC1 BC2 BC3 BC4 BC5

0.14 0.12

Ti concentration (mg/l)

P concentration (mg/l)

30

BC1 BC2 BC3 BC4 BC5

0.16

25

20

15

0.10 0.08 0.06 0.04 0.02

10

0.00 5

-0.02 0

5

10

15

20

25

30

0

5

10

Reaction time in SBF (days)

15

20

25

30

35

40

45

50

Reaction time in SBF (days)

Fig. 4. Change in concentration of Si, Ca, P and Ti with immersion time in SBF solution. 1.0

9.0

BC1 BC2 BC3 BC4 BC5

8.8

0.8

Weight loss(mg/cm2)

8.6 8.4

pH

BC1 BC2 BC3 BC4 BC5

0.9

8.2 8.0 7.8

0.7 0.6 0.5 0.4 0.3 0.2

7.6

0.1

7.4

0.0 0

5

10

15

20

25

30

Reaction time in SBF (days)

0

5

10

15

20

25

30

Reaction time in SBF (days)

Fig. 5. Variation in pH and weight loss of specimen in SBF solution with reaction time.

supersaturated and ion exchange reaction eventually cease. Weight loss in bioceramics measured by formula Weight loss ¼ M 0
M t =A

ð4Þ

where ‘M0’ is initial weight (mg), ‘Mt’ is weight loss (mg) at any time t and ‘A’ is surface area (cm2). The degree of weight lost has pronounced effect on development of apatite. The variation of weight loss with immersion time is shown in fig. 5. It was

observed that weight loss of BC4 showed the highest rates of solubility during the whole period of immersion and reached 0.9325 mg/cm2 after 30 days. The solubility rate possessed by ceramic BC5 was found the lowest so its weight loss was 0.2027 mg/cm2 observed after 30 days. The sample BC1 (0.569 mg/cm2), BC2 (0.605 mg/cm2) and BC3 (0.8002 mg/cm2) exhibited intermediate rates of solubility lying between BC4 and BC5. It is very important to control the dissolution rates of implant

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Fig. 6. SEM micrographs of bioceramics before and after immersion in SBF for 30 days.

R. Zia et al. / Ceramics International 41 (2015) 8964–8972

because very high rates produce high concentration of ions which is not effective. The cellular proliferation is also not possible to be stimulated if by low ionic concentrations resulted from very slow solubility rates [36]. There is a close relationship of bioactivity with the degradation rates of bioactive materials. Network connectivity is a major factor controlling the degradation rates and hence bioactivity. Decreased network connectivity is a positive sign for bioactivity due to increased dissolution. Na2O has been reported to disrupt the network effectively [37], whereas Ti being a tetravalent cation and having a high charge can be present both at former and modifier positions. It modifies the system by loosening the connectivity of Si–O–Si network with introduction of non-bridging oxygen (NBO) [38]. Both, Si þ 4 and Ti þ 4 ions are tetravalent cations, so substitution of silica with suitable amount of titania would assists in network formation. If Ti is present in former position then Ti þ 4 ions will tend to effectively cross link between NBO of two different chains in network and produce bridging oxygen (BO). This results in close packing of system due to which solubility decreases [39,40]. 3.1.4. SEM analysis The SEM confirmed the formation of apatite layer on the surface of the samples. Fig. 6 shows the images taken before and after soaking in SBF for 30 days. It was seen from morphological images that with the addition of TiO2 in the system the formation of HCA layer increased. For BC1 a few precipitates of HCA was seen while with progressive addition of TiO2 from 3.5% to 10.5% the HCA precipitates increased and for BC4 (having 10.5% TiO2) it was found that after 30 days immersion in SBF the surface was almost fully covered by HCA layer (shown in fig. 6) which indicates the higher bioactivity of the specimen and confirmed the results of XRD, FTIR and AAS. However, in case of BC5 only a few precipitates of HCA were seen scattered homogenously on the surface of the sample. Thus, bioactivity decreased when TiO2 content increased from 10.5 mol% It was observed that Ti (up to 10.5 mol%) appeared in modifier position resulting in softening of network and then it (14 mol%) entered in network as former, thus strengthening the network by making it more compact. This is confirmed by weight loss measurements which also showed that solubility rates increased up to BC4 and then it decreased for BC5. This study showed that adding TiO2 up to 10.5 mol% in base composition enhances the bioactivity and further increase in TiO2 amount decreases the process of apatite formation and hence bioactivity. Thus, with partial substitution of silica to titania (r10.5 mol%) in the system leads to increase in Ti–OH groups, that prompt apatite growth by assembling of remaining calcium, phosphate and carbonic acid ions from the physiological fluid around the nuclei. 4. Conclusion Bioceramics based on composition CaO–P2O5-SiO2-Na2O– TiO2 with varying quantities (0, 3.5, 7, 10.5 and 14 mol%) of TiO2 were successfully prepared by solid state sintering method.

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The addition of TiO2 enhanced the bioactivity, addition of TiO2 r10.5 mol% showed the superior bioactivity. These materials can be effectively used as bone grafts in human body.

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