Rohit Chowdhury 1,*, Liam C. Cooper 1 and Chen Shen 1
1 School of Chemistry, University Technology of Sydney, Ultimo, New South Wales 2007, Australia; lico21087@gmail.com; s.chen99@outlook.com
* corresponding author e-mail address: chowdhury.rohit391@gmail.com
ABSTRACT
A series of bioactive glass composition were prepared, and the effects of TiO2 addition on the structure, bioactivity and hardness of the glasses were analyzed. This study consisted of glass characterization, simulated body fluid (SBF) trials, and hardness testing. Three glasses were formulated, where a SiO2-CaO-Na2O-P2O5 bioactive glass was used as control, with the addition of 5 and 10 wt.% TiO2 at the expense of CaO. X-ray diffraction patterns confirmed that an amorphous microstructure was obtained for all three glasses. Differential thermal analysis indicated an increase in the glass transition temperature of the glass series from 660°C to 721°C with the incorporation of TiO2. Hot stage microscopy results exhibited higher sintering and softening temperatures for TiO2 containing glasses. Each glass was then incubated in SBF for 1, 10, and 30 days. Scanning electron microscopy images confirmed that the calcium phosphate particulates were precipitated on control glass after 10 days, however, for TiO2 containing glasses, the deposition layer was only observed after 30 days. The hardness of SBF incubated samples were tested, where TiO2 containing glasses showed significantly higher hardness values at each incubation period, with 0.72 GPa for control and 1.71 GPa for the TiO2 containing glass, after 30 days of incubation.
Keywords: glass, characterization, TiO2, bioactive, SBF, hardness.
INTRODUCTION
Over the past few years, bioactive glasses and glass-ceramics have gained increasing attention because of their ability to induce direct bonding to living bone tissue (Baino et al., 2018, Jones, 2013). As many different compositions have been investigated to evaluate such bioactivity, it has been reported that some specialized compositions of bioactive glasses form a bond with soft tissues as well as hard tissues (Mokhtari et al., 2018). Bioactive glass was first discovered by Professor Larry L. Hench. Hench proposed a hypothesis of a synthetic material that can form hydroxyapatite (HA) structure on the material’s surface to be biocompatible compared to metals or polymers that are rejected in the human body, evident by the formation of fibrous tissue (Rahaman et al., 2011). This specific glass was later named Bioglass®, which is composed of 45SiO2-24.5Na2O-24.5CaO-6P2O5 in weight percent (Fiume et al., 2018). Although Bioglass® has been promising for the use of bone tissue engineering application, many studies were conducted to improve several disadvantages associated with these glasses, and further to increase bioactivity and cell attachment to bioactive glass (Roseti et al., 2017). One issue with regular bioactive glass composition is mechanical strength, especially when fabricated into forms such as scaffolds (Sanz-Herrera et al., 2011). Due to a less stable glass network, and high dissolution rates, the mechanical strength of such materials is compromised when implanted in a biological environment (Roseti et al., 2017).
As mentioned above, bioactive glasses have been widely researched in order to improve their biological performance and also to enhance their mechanical properties. Such studies include variations in fabrication methods, surface engineering, and composition (Rabiee et al., 2015, Mokhtari et al., 2019). Fabrication methods include both melt derived methods and sol-gel processing to determine their effect on the properties of bioactive glass (Hoppe et al., 2011). Compositional changes in bioactive glass can include incorporations of therapeutic ions such as Sr, Cu, and Zn to improve the glass behavior in biological environments (Lázaro et al., 2014, Mokhtari et al., 2017, Balamurugan et al., 2007).
Numerous studies conducted on titanium and its alloys suggest its potential as a promising candidate for biomedical applications due to its ability to bond directly with bone (Elias et al., 2008). Titanium has been widely researched and is currently used as a bioactive material in medical devices (Liu et al., 2004). For instance, titanium can be processed into pure or alloyed metals, and foams as part of the acetabular cup used in hip joint replacement (Niinomi, 2008, Niinomi et al., 2015). Furthermore, titanium coating is often used as a coating material on medical implants (Liu et al., 2004). Piscanec et al. have studied bioactivity of TiN coating on medical implants where TiN coated hip prosthesis heads have shown spontaneous growth of calcium phosphate phases on the surface of the material (Piscanec et al., 2004). Feng et al. have studied osteoblast adhesion of titanium surfaces containing calcium, phosphate ions as well as carbonate apatite. Studies conducted on osteoblast adhesion to titanium samples with different surface characteristics showed that osteoblast cells are capable of adhering onto all the samples in both flattened and elongated morphology (Feng et al., 2004). Samudrala et al. studied the biological behavior of Titania doped calcium borosilicate bioactive glass compositions, where they reported that titanium doped glasses exhibited lower solubility of glass and an increase in cell viability (Samudrala et al., 2017).
Thus, with current knowledge of bioactive glass compositions, the hypothesis is that substitution of TiO2 within the glass system will increase the network stability of the glass and decrease the dissolution rates (Samudrala et al., 2017). Controlled dissolution rates of glass will stabilize localized pH and hence improves the cell viability, and attachment (Babu et al., 2019). Moreover, TiO2 glass compositions exhibit higher mechanical properties than Bioglass-based compositions (Babu et al., 2019). In this study we are interested in the influence of the TiO2 within the glass system with relatively high contents of TiO2. The following base glass system of SiO2-Na2O-CaO-P2O5 was produced as a control bioactive glass in this study, with 5, and 10% substitution of TiO2 for CaO. The structure, mechanical properties and biological behavior of glasses were studied.
MATERIALS AND METHODS
Glass Synthesis
Glasses in this study were prepared through the melt-quench method. Three glass compositions were designed, two titanium-containing glasses (T5, and T10) in addition to a titanium free SiO2-CaO-Na2O-P2O5 glass as control (T0). The titanium-containing glasses contain incremental concentrations of TiO2 at the expense of CaO (Table 1). The powdered mixes of analytical grade reagents of SiO2, TiO2, CaCO3, Na2CO3, NH4H2PO4 were milled for 1 hour, dried at 120°C for 1 hour and melted at 1350°C for 5 hours in an alumina crucible and shock quenched into water.
Table 1. Glass compositions (wt.%) where CaO is substituted with TiO2.
| SiO2 | CaO | Na2O | P2O5 | TiO2 | |
| T0 | 50 | 25 | 20 | 5 | 0 |
| T5 | 50 | 20 | 20 | 5 | 5 |
| T10 | 50 | 15 | 20 | 5 | 10 |
X-Ray Diffraction
Glass powders were analyzed for their diffraction patterns using a Bruker D8 Advance X-Ray Diffractometer (Bruker AXS, Germany). Diffractograms were collected at 40 kV and 30 mA utilizing CuKa radiation, from 10° to 60°, at a step size of 0.03° and step time of 10 s.
Particle Size Distribution
The particle size distribution of the powder specimens was measured by a laser particle size analyzer (SK-Laser Micron PRO-7000S). Glass powder samples were suspended in ultrapure water and evaluated in the range of 0.4–40.0 µm, with a run time of 60 s. Relevant volume statistics were calculated and reported for each glass.
Specific Surface Area
For determining surface area, the Brunauer-Emmett-Teller (BET) method was used. The surface area of selected powders was measured via N2 gas adsorption according to the BET method using instrument of micromeritics Tristar 3000. Prior to analysis powders were degassed in a micromeritics flow sample degas system at 150 °C for 24 h. The instrument was standardized prior to and after analysis using silica-alumina rod standard (201 ±5 m2/g).
Scanning Electron Microscopy & Energy Dispersive X-Ray Analysis
SEM Imaging was performed using scanning electron microscope (TEscan, Libusina, Czech Republic). Samples were gol coated for 60 seconds using a sputter coater. The EDX spectra were generated using an EDAX Genesis Spectrometer at 20 kV at a beam current of 26 nA.
Differential Thermal Analysis
Differential Scanning Calorimetry (DSC) experiments were performed in a DSC 404 F1 Pegasus (Netzsch, Germany) at 10 K/min from 25°C to 1200°C in air, with Pt pans and lids, sample mass of 50 mg and one empty lidded Pt pan as a reference. To prevent the crucible from sticking to the sample holder at high temperatures, we used an alumina 200-mm-thick disk below each pan.
Hot Stage Microscopy
To analyze the samples softening and sintering temperatures, A hot stage microscope (MISURA, Modena, Italy) was used. The samples were heated to 1200˚C at a heat rate of 20˚C/min.
Simulated Body Fluid Trials
Simulated Body Fluid (SBF) was used to study the in-vitro bioactivity of samples. SBF solution was prepared following the method proposed by Kokubo et al (Kokubo et al., 2006). Glass discs were submerged in the SBF solution and kept in an incubator at 37˚C for 1, 10, and 30 days. Each glass disk, with dimensions of 1.5 × 5 mm, was immersed in 10ml of SBF solution.
Vickers Hardness
Hardness testing was completed on each glass disc, incubated in SBF for 1, 10, and 30 days. The microhardness was measured by using a microhardness tester (Model: SHIMADZU, HMV-G20S, Japan) at the room temperature an applied load of 500g and a dwell time of 10s. 10 measurements, and within 3 different regions were performed on each glass disks.
RESULTS AND DISCUSSIONS
This study considers replacement of 5 and 10 wt.% TiO2 for CaO in the glass composition of a 50SiO2-25CaO-20Na2O-5P2O5 wt.% system. The purpose of this work is to analyze the effects of TiO2 addition on the structure of glasses, as well as their in-vitro bioactivity, and hardness. Initial characterization on the glass powders was to perform X-ray Diffraction (XRD) to confirm that a fully amorphous glass structure was obtained. The X-ray diffractograms for each of the glass powders are presented in Figure 1. The characteristic amorphous hump for silicate glasses at angles between 20°-30° is present for each sample, verifying that an amorphous structure was obtained for each glass composition. However, it can be noticed that by increasing titanium content in the glass series, the intensity of the amorphous hump is decreasing. This indicates the possibility of crystallization at higher titanium content in the glass composition, as TiO2 has been widely known as a nucleating agent in the silicate glass compositions (Lee et al., 2001). Presence of the crystalline phases can significantly alter the degradation process in physiological solutions and therefore delays the onset of bioactivity and bone formation.
Figure 1. X-ray Diffractograms of (a) T0, (b) T5, And (c) T10 glasses.
Figure 2 shows the particle size analysis (PSA) and surface area measurements on synthesized glass powders. The mean surface area of each glass composition was determined using BET method. The BET surface area for T0 glass powder was at 0.61 m2/g. For the TiO2 containing glasses, the surface area was found to be at 0.68 m2/g and 0.47 m2/g for T5 and T10 glasses, respectively. To find the mean particle size distribution of the glass powders, particle size analysis was performed. The mean particle size of the glass series was found to range between 8.4 to 9.7 µm, with T0 having a mean diameter of 9.5 µm, T5 with 8.4 µm, and 9.7µm for T10. Moreover, the size distribution of glass particles determined using PSA and it was found to be similar, with D10 ranging between (5.2-5.7 μm), D50 (8.5–9.1 μm), and D90 (19.6–20.8μm). The results of PSA measurements are summarized in Table 2. The results from the PSA and BET surface area analysis suggest that the particle size distribution was consistent in all three compositions without a significant change in the mean diameter or the size distribution of glass powders with respect to TiO2 addition. The effects of particle size on ion-release have been an important factor when testing the antibacterial effect of bioactive glasses. Waltimo et al. have shown that bioactive glass with nanoparticles displayed higher antibacterial effects due to the higher release of alkaline species compared to glass with micron-sized particles (Waltimo et al., 2007). Thus, it is important to note that all three glasses produced in this study show no significant difference in particle size distributions and safe assumptions can be made that any significant change in their in-vitro bioactivity is not influenced by differences in the mean particle sizes.
Table 2. Particle size distribution of glass powders.
| Mean (µm) | D10 (µm) | D50 (µm) | D90 (µm) | |
| T0 | 9.5 (±0.2) | 5.7 (±0.1) | 8.7 (±0.1) | 20.2 (±0.2) |
| T5 | 8.4 (±0.3) | 5.2 (±0.1) | 8.5 (±0.3) | 19.6 (±0.3) |
| T10 | 9.7 (±0.1) | 5.7 (±0.4) | 9.1 (±0.2) | 20.8 (±0.2) |
Figure 2. Particle size (µm) and BET surface area (m2/g) of T0, T5, and T10 glass powder.
Further characterization included scanning electron microscopy (SEM) of glass particles and corresponding EDX spectra of semi-quantitative chemical analysis. The morphology of glass particles is presented in SEM images in Figure 3. Glasses show a distribution of small and large particulates, with a majority of fine agglomerated particulates measuring approximately less than 10 micrometers in diameter. The presence of larger particles measuring approximately less than 50 micrometers can also be seen from these images. The SEM images of glass series showed a similar morphological structure for each glass composition. No phase separation was observed under SEM images for any of the glass powders tested. EDX was employed to confirm the elemental composition of the glass series. Regarding the control glass (T0), the EDX spectrum showed presence of Na, Si, P, and Ca. Whereas TiO2 containing glasses, also consist of Na, Si, P, and Ca as their base constituents, in addition to Ti, with a higher concentration of Ti in T10 glass compared to T5. Elements detected in EDX spectra of glasses presented in Figure 3, are in agreement with initial glass batch composition, eliminating concerns of contamination within the glass chemistry.
Figure 3. SEM images and corresponding EDX spectra of glass particles.
Differential thermal analysis (DTA) was conducted on each glass composition to determine the glasses thermal properties, and also to analyze the effects that TiO2 incorporation has on the glass thermal history. Results for DTA of glasses are presented in Figure 4. The glass transition temperature (Tg) is marked on each thermogram. An increase in Tg from 660°C (T0) to 693°C (T5) and to 721°C (T10) can be observed when TiO2 was substituted with CaO. It also shows the absence of crystalline peaks in T5 and T10 glasses during heating. The increase in the Tg of glasses from 660°C to 720°C when TiO2 was substituted for CaO, indicates the increase in stability of the glass network when Ti4+ ions are introduced into the glass. Ti4+ may act as an intermediate interacting with both network formers and modifiers (Wang et al., 2014). Unlike previous studies where Ti4+ was substituted with network formers such as Si4+ resulted in the decrease of Tg, observations made here indicates that the substitution of Ti4+ for network modifiers such as Ca2+ results the Tg and network stability of glasses to increase. Additionally, hot stage microscopy was performed to determine sintering (Ts), softening (Tf), and melting (Tm) temperatures of T0, T5, and T10. The results from the HSM are presented in Figure 5. Three temperature points were determined based on the shape of the prepared sample as a function of temperature. From Figure 5, it can be observed that T10 shows the highest Ts, Tf, and Tm among three glasses, while T0 shows lowest Ts, Tf, and Tm when compared to the other two glasses. It is evident that the substitution of Ti4+ for different network modifiers can have a different impact on the thermal properties of bioactive glasses. The sintering temperature (Ts) of T0 was found to be at 752°C. However, with the substitution of Ti4+ for Ca2+, the Ts increased to 761°C and to 858°C for T5, and T10 glasses, respectively. Such sintering temperatures of each glasses were indicated as the temperature range at which densification will occur during the synthesis of glasses. The softening (Tf) temperature of glasses increased with the incorporation of Ti4+, from 971°C for T0 to 1008°C for T5 and to 1112°C for T10. Similarly, melting temperature was also affected by glass chemistry, and presented an increase with 1087°C, 1114°C, and 1165°C, for T0, T5, and T10, respectively. Results from DTA and HSM analysis, showed higher glass transition, sintering, and softening temperatures for titanium containing glasses. This is indicative of higher network connectivity and therefore higher percentile of bridging oxygens when TiO2 is incorporated. This is due to the role of Ti4+ plays in the glass network. As discussed earlier, TiO2 is an intermediate glass-forming oxide. Depending on the concentration of TiO2, it can either form [TiO6] or [TiO4] structural units (Kashif et al., 2009). In the proposed study, the substitution of Ti4+ with Ca2+ in the glasses resulted in an increase in network connectivity. This indicated that the addition of Ti4+ would likely result in the formation of [TiO4] structural units, which can essentially bond to [SiO4] tetrahedra and would decrease the percentage of non-bridging oxygens to bridging oxygen in the glass. The change in the network connectivity and strengthening the silicate polyhedral structure will significantly affect the solubility of glass.
Figure 4. Differential Thermal Analysis (DTA) of (a) T0, (b) T5, and (c) T10 glass compositions.
One of the key features of silicate bioactive glass is the low SiO2 content, which makes the glass soluble in the biological environment. Through the initial stages of glass dissolution, network modifier ions such as Na+ and Ca2+ will be leached into the solution, this would break down the Si-O-Si backbone of glass and a SiO2-rich layer forms on the surface of glass. The SiO2 gel layer, then attracts Ca and P ions from the solution, leading to the precipitation of a calcium-phosphate (CaP) layer on the surface. This amorphous CaP layer will later crystallize into crystalline apatite (HCA) (Cormier et al., 2010). Glasses were incubated under simulated body fluid (SBF) to determine surface reactivity of bioactive glass with respect to composition, and incubation periods, which was 1, 10, and 30 days. SEM images of incubated control glass samples (T0) and corresponding semi-quantitative chemical EDX analysis of glass surface incubated at 1, 10 and 30 days are presented in Figure 5. Surface SEM images of T0 when incubated in SBF for 1-day show presence of sparse precipitations. CaP precipitants can be seen on the surface even at 1 day. At 30 days, the glass surface is covered by CaP depositions. Dehydration cracks can also be seen on the surface. The EDX analysis of corresponding glass, confirmed the presence of Ca at ~42 wt.% and P at ~54 wt.% while Si was detected at ~4 wt.% as shown in Figure 5.
Figure 5. Hot stage microscopy results of glasses with corresponding sintering, softening, and melting temperature.
SEM images of SBF incubated samples of T5 and T10, with their corresponding EDX spectra are represented in Figures 6, 7 and 8. At SBF incubation of 10 days, T5 shows partial deposition of CaP but at 1-day traces of CaP deposition cannot be observed as shown in Figure 6. However, the T5 glass samples showed a considerable amount of CaP deposition at 30 days. Corresponding EDX of 30 days incubated T5 samples show the presence of Ca at ~36 wt.%, P at ~42 wt%, while Si was detected at ~19 wt.%. Compared to control glass, lower amounts of Ca and P, with higher amounts of Si content, were present at the surface of T5 glass.
Figure 6. SEM images of T0 samples, incubated in SBF for (a) 1 day, (b) 10 days, (c) 30 days, and (d) the corresponding EDX spectra of T0 incubated for 30 days.
Figure 7 shows surface images of T10 where no CaP precipitants were found on the surface of samples incubated over 1, and 10 days. However, after 30 days, scattered precipitants of CaP were present on the surface. In comparison with the other two compositions, the lowest amount of Ca and P was found on T10 glass. The corresponding EDX spectrum of T10, identified Ca at ~19 wt.%, P at ~11 wt.% and Si at ~44 wt.%. It is evident that the control glass of T0, exhibited the highest amount of CaP deposition on the surface over incubation time. The morphology of the CaP percipitans on the surface of the glass disks, are similar to earlier studies on TiO2 containing bioactive glasses (Babu et al., 2019). After 10 days, CaP deposition took a shape of microspheres spread across the surface of the material as well as stacked upon each other. At 30 days, it is clearly visible that the deposition formed a layer on top of the surface as shown in Figure 5. Also, cracks on the surface of control can be attributed to dehydration during preparation for analysis. T5 and in particular T10 glasses showed a lower amount of CaP depositions. In the proposed glass system, where the Ti4+ replaces Ca2+ in the glass, the overall network connectivity of glasses increases with TiO2/CaO substitution. As discussed earlier, the dissolution rate of glass was expected to decrease with TiO2 substitution. Silicate bioactive glass networks exhibit promising properties for the use of bone tissue engineering applications. The ability to form an amorphous CaP deposition, which later then crystallizes into HCA layer for osteoblast colonization is a significant advantage regarding biomedical applications. However, high dissolution rates of the glass correspond to higher amounts of ions released in the surrounding physiological solution. Release of ions will greatly alter the localized pH values which have significant effects on the cell viability. Although the addition of TiO2 in the glass, decreased the kinetics of CaP layer formation, but it could be beneficial in terms of pH control which hence could enhance cell viability (Samudrala et al., 2017).
Figure 7. SEM images of T5 samples, incubated in SBF for (a) 1 day, (b) 10 days, (c) 30 days, and (d) the corresponding EDX spectra of T5 incubated for 30 days.
Aside from the high dissolution rate for silicate bioactive glasses, another major concern is the change in mechanical properties when these materials are immersed within a biological environment, which is indicated by any changes in hardness due to reactions with a physiological environment (Kashyap et al., 2011). The relationship between mechanical properties and dissolution characteristics needs to be further investigated. In order to overcome such limitations of regular bioactive glass series, hardness testing was conducted on TiO2 containing glasses, and the results were compared with the control glass. Figure 9 presents the Vickers hardness values for the glass disks incubated in SBF for 1, 10, and 30 days. All glass compositions showed a decrease in their surface hardness as a function of incubation time. The hardness of T0 glass ranged from 1.25 to 0.72 GPa after 30 days of incubation and was significantly lower compared to both T5 and T10 glasses, at all time periods. T5 showed higher values compared to T0. The hardness of T5 glass slightly decreased from 2.68 to 2.56 GPa after 1 to 10 days of incubation but it significantly dropped to 1.40 after 30 days. On the other hand, T10 showed the highest hardness of all incubation times compared to T0 and T5. Hardness of T10 at 1 day of incubation was at 2.80 GPa, with a slight decrease after 10 days of incubation time to 2.71 GPa, and to 1.71 GPa after 30 days of incubation. TiO2 containing glasses obtained higher hardness values when compared to the control. The Vickers Hardness is useful technique for the measurements of surface hardness, which can provide information about the surface structure of the glasses. Substitution of TiO2 for CaO, resulted in an increase in network connectivity and a more compact glass structure, which consequently increased the microhardness of glass. Among the all three glasses 10 wt.% TiO2 glasses showed the highest microhardness.
Figure 8. SEM images of T10 samples, incubated in SBF for (a) 1 day, (b) 10 days, (c) 30 days, and (d) the corresponding EDX spectra of T10 incubated for 30 days.
Figure 9. Vickers Hardness testing of all three glasses after incubation under SBF for 1, 10, and 30 days.
CONCLUSIONS
The main goal of this work was to study the influence of TiO2 addition in SiO2-CaO-Na2O-P2O5 bioactive glass system. The addition of TiO2 did not affect initial crystallinity of the glass in powder form and thermal analysis showed absence of crystallization (vitrification) temperatures. TiO2 containing glasses showed higher glass transition, softening and sintering temperatures compared to control glass. Results showed that TiO2 containing glass structures have higher network connectivity. This was further evidenced by Simulated body fluid trials, which explains lower content of calcium phosphate precipitation on the surface of the TiO2 containing glass after SBF incubation. Addition of TiO2 significantly improved the mechanical stability of glasses, where T5 and T10 showed greater hardness values even after 30 days of incubation time.
CONFLICTS OF INTEREST
The authors declare no conflict of interest.
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