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increased hardening with respect to maturation. Studies conducted by Tyas et al58 and Mitsuhashi et al59 on the setting, compressive strength and fracture toughness on GPCs and resin modified GPCs both conclude that GPCs benefit from having a distribution of particles sizes, where the finer particles initial the setting and the larger unreacted glass particles result in increasing the fracture energy of the materials.
The structure of the glasses was analyzed using a number of techniques including Differential Thermal Analysis (DTA), X-ray Photoelectron Spectroscopy (XPS) and Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). DTA profiles show that the glass transition temperature (Tg) of the Control (661°C) is significantly lower than that of the Cu containing glasses (Cu6C 773°C, Cu12C 786°C, Figure 3.3). The increase in Tg is indicative of increasing the network connectivity within the glass resulting in a less soluble material. This could be due to the role Cu plays in the glass. Cu has previously been reported to exist in glass systems as network modifier60. The increase in Tg is indicative of increasing the Bridging Oxygen (BO) concentration within the glasses. The addition of 6mol% and 12mol% Cu increased the Tg by approximately 110-120°C, which suggests that the role Cu assumes in the glass is independent with respect to concentration. Both XPS and MAS-NMR are techniques routinely employed to investigate the structure of glasses. High resolution O1s XPS results show that the binding energy increases from 531eV to 534eV with as the concentration of Cu in the glass increases (Figure 3.5). This is indicative of increasing the concentration of BO within the glass structure and the region of 534eV can be attributed to bridging oxygen in P2O5 and P2O5/ZnO. The binding energy of Cu2p at 952.53eV can be attributed to valence state of Cu like Cu1+ and Cu2+. MAS- NMR data presented in Figure 3.6 and Figure 3.7 also suggest that with the addition of 12mol% CuO, the bridging oxygen concentration in the glass is increasing as this technique probes the local environment of the Si atom. tThese trends are indicative of increasing the BO connectivity within the glass network. Glasses that have Q-structures with lower BO concentrations, or higher non bridging oxygen concentrations (NBOs), facilitate a higher degree of acid degradability when mixed with polyacrylic acids, thus providing ion dissolution required for crosslinking the acid chains48.
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Ion release profiles of Si4+, Zn2+, Ca2+ and Sr2+, each as a function of incubation time in fluids are presented in Figure 3.8. With respect to Cu and P, no ion release was recorded, or they were below the instruments detection limit. Si4+ levels were found to increase with respect to incubation time, and the effect of Cu replacement for Si4+ result in similar levels of Si release which ranged from 20-30mg/L. Si4+ release in this range are cited to increase osteoblast cell proliferation rate and stimulates genes in osteoblasts that play a critical role in bone metabolism29. Zn2+ levels were found to be low (<3mg/L) for each time period, however, toxicity limits for Zn2+ are cited to be 16mg/L for cytotoxic effects, or even lower at 8 mg/L have been reported to cause oxidative stress in human osteoblasts29, however Zn2+ at lower levels can have important roles in bone formation and protein synthesis61. Ca2+ release was found to increase over time for each glass, and similarly to Si4+, the addition of 6mol% and 12mol% CuO slightly reduced the release rates, however, each ranged between 13-16mg/L. Ca2+ release is essential for processes such bioactive glass surface mineralization, GPC formation and from physiological perspective Ca2+ has been attributed to increasing osteoblast proliferation, differentiation and extracellular matrix mineralization29. The Ca2+ levels reported here are low compared to cited cytotoxicity limits which can be up to 120mg/L29. Sr2+ release is known to have excellent bone metabolic potential and has been implicated for treating osteoporosis29. Studies by Gentleman et al from silicate glasses (SiO2-P2O5-Na2O-CaO), where the Ca2+
content was substituted with Sr2+, showed enhanced osteoblast activity and osteoblast activity and inhibited osteoclast activity in the range of 5-23mg/L29. The Sr2+ release from this glass system is within the region of 2-10mg/L and is highest with the highest Cu containing glass, CuG12. In addition, the solution pH of the glass series was found to be slight higher for the Control glass compared to CuG6 and CuG12. However, all glasses were in the region of 9.4-9.6 until 1000 hours had expired. At this stage, the pH of the Control reduced to 8.3 while CuG12 reduced to 7.7, close to neutral. This is a positive attribute as it may suggest that mineral solubility limits are reached and that precipitation reactions are occurring within the solution which would be a positive attribute as precipitation of minerals on bioceramics in vitro, via Simulated body fluid (SBF) testing, is widely regarded as a precursor to bone bonding in vivo5, 26.
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GPCs were formulated with a range of PAA concentrations (40, 50, 60wt%) and the Tw and Ts were evaluated. The addition of higher concentration of PAA typically reduced the rheological properties as the increase in COO- groups facilitates chelation of the metal cations from the polysalt matrix at a much more rapid rate. The Tw for each GPC were found to follow this trend, the addition of Cu was found to reduce the Tw. The Ts,
however, did not follow this expected trend, conversely the setting times were found to increase as the PAA concentration increased. This effect is independent of Cu addition as the ConC also experienced this trend. The addition of Cu did however, reduce the Ts to levels that are more representative of the Ts outlined by the ISO9917 standard for water based cements51. This effect may be related to changes in local pH of the cement matrix through dissolution of the glass particles in response to PAA. The acid-base setting reaction attributed to these cements setting and mechanical properties can be heavily influenced by the setting pH48.
Mechanical properties were evaluated using compressive strength testing and shear adhesion bond strength testing which were also testing according to ISO standards.
Compressive strength testing (Figure 3.11) was conducted after 7 days’ incubation in deionized water. The mechanical properties of conventional GPCs are known to increase with an increasing the polyacrylic acid concentration, however, this effect is highly dependent on the solubility of the glass and the time it takes for the GPCs to set48. With regard to the GPC series tested here, little trends were evident and the strengths were in the region of 18-35MPa, far less than the required 50MPa for luting and lining applications and even more compared to the 100MPa for restorative applications. The strengths recorded for this work were found to be both independent of PAA concentration and of Cu addition. Further bond strength testing was conducted to provide insight into the failure mechanism. The adhesive bond strengths were 0.85, 1.32 and 0.79 MPa for ConC, Cu6C and Cu12C respectively (Figure 3.12). These values are lower than reported bonding agents Clearfil New Bond and Histocaryl, which reported values of 5.2MPa and 1.9MPa respectively44. The failure of the mechanical properties in these GPCs is likely due to the reduced dissolution rate of the glass particles. SEM imaging of the shear bond strength fracture samples suggest that the Cu12C in particular does not fail adhesively, but fails
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cohesively. Upon examining the fracture surface, a high concentration of unreacted glass particles is present in addition to a high degree of porosity. Both of these factors may account for the reduced strengths of this GPCs series.
Antibacterial evaluation was conducted using the 40wt% addition of PAA with ConC, Cu6C and Cu12C (Figure 3.13 and Figure 3.14). The presence of Cu in Cu6C and Cu12C presented antibacterial properties in E. coli, which were concentration dependant.
The Cu-GPCs showed the greatest inhibition in S. epidermidis and presented notable effect in Vancomycin Resistant S. aureus and UMAS-1. This suggests that the addition of Cu to GPC based skeletal adhesives or other bone cements may be an adequate alternative to loading cements with antibiotics such as vancomycin, streptomycin and gentamycin. The overuse of these antibiotics has led to the evolution of resistant strains of microbes which pose serious issues in immunocompromised patients62-64. The mechanism of action of Cu ion is cited to be related to the ions ability to adhere to the negatively charges bacterial cell wall causing it to rupture, thereby leading to protein denaturation and eventual cell death65.