Hanna Arstila

Liquidus Temperatures of Bioactive Glasses

Melts in the composition range of bioactive glasses have their liquidus temperatures within or close to the working range. Additionally, bioactive glasses show a strong tendency to crystallization due to their composition. Thus, many bioactive glass compositions are unsuitable for demanding working procedures such as fiber drawing. In this work we discuss the suitability of different methods for measuring liquidus temperature of bioactive glasses. Thermal analysis was used, and complemented by sintering curves as given by hot stage microscopy. Hot stage microscopy could be used to measure liquidus also when the thermal effect associated with liquidus was low and consequently the DTA was not suitable. Liquidus of some glasses was measured with optical microcopy for samples heat treated in a gradient furnace. The values indicating the crystallization of the samples during viscosity measurement with rotational viscosimeter were compared with the other measurements. The crystalline phases formed at liquidus were identified with XRD and SEM-EDXA. The measured liquidus temperatures were also tested by drawing fibers directly from molten glass and from preforms heated to typical fiber drawing viscosity values. For all compositions the fiber drawing viscosity values were below liquidus. Continuous fibers could be manufactured of compositions only with low rates of crystal nucleation and growth.

Measuring the Devitrification of Bioactive Glasses

The devitrification tendency during heat-treatment was measure d for four bioactive glasses in the system Na 2O-K2O-MgO-CaO-B2O3-P2O5-SiO2. The measurements where performed by high-temperature optical microscopy, X-ray diffraction, sca nning electron microscopy and differential thermal analysis. The combination of these methods proved t be a reliable way to define the devitrification temperatures and the composition of crysta l ph ses formed. This information is vital in choosing glasses for implant production, especi ally in cases where thermal treatment is required.

In vitro reactivity of bioactive glass fibers

Implants with long lasting bioactivity and mechanical sustainability would be of interest in several novel clinical applications. By processing bioactive glass fibers and biodegradable polymers into 3D structures, bone formation ability of glasses and flexibility of polymers can be combined. In order to achieve desired physiological response, reactivity of bioactive glass fibers must be specified. Bundles of fibers within the range of bioactivity were soaked in the simulated body fluid at stationary conditions for several time intervals after which the cross-sectional surfaces of the fibers were studied with SEM-EDXA. The reaction layers and precipitations formed on the fiber surfaces suggest that the fibers react according to three mechanisms depending on the glass composition. Fibers with a high in vitro bioactivity showed the formation of distinct and thick silica –rich and calcium phosphate –rich layers already at one day’s immersion. Fibers of medium bioactivity did not show any clear silica –rich layer but a formation of calcium phosphate precipitations or layers at one day’s immersion. Slow glasses showed sporadic calcium phosphate precipitation only after the longest immersion times. The results indicate that the medium and slow glasses are interesting alternatives for applications where a long term mechanical durability suggested by their slow reactivity in combination with their osteoconductive tendency is desired.

Bioactive Glass Compositions Suitable for Repeated Heat-Treatments

The crystallization tendency for 30 experimental glasses in the system Na2O-K2O-MgOCaO-B2O3-P2O5-SiO2 was studied with thermal methods, DTA, HSM and XRD. The glasses were also immersed into simulated body fluid for 8 and 72 hours. The formation of the silica-rich gel and calcium phosphate layer on the glasses were analyzed with SEM. The in vitro behavior and crystallization tendency for heat-treated glasses were then related. This information is essential for choosing glass compositions that can be manufactured to desired products with controlled bioactivity for different applications. In general, glasses with low alkali content can tolerate heattreatment without crystallization but have less initial Si-gel formation ability and show less in vitro bioactivity than glasses with high alkali content.

In vitro Behavior of Fiber Bundles and Particles of Bioactive Glasses

In vitro reactions of bundles of fibers with diameters 20-500 μm and crushed glasses of fractions 500-800 μm were compared with the reactions of plates of the same bioactive glasses. The samples were immersed in simulated body fluid (SBF) for 2-7 days. After immersion the changes on the surfaces of the samples were observed by SEM/EDXA. Layer formation on the glass surface was found to vary with glass composition, sample shape and local condition of single particle/fiber. However, only some fibers or particles formed similar in vitro reaction layers as the plates. The product form did not change the in vitro bioactivity of particles or fibers exposed to the bulk immersion solution. When the glasses were used as fiber bundles or particle beds, the packing degree and the flow of body fluids within the system interfered with the reactivity. Also a clear correlation between in vivo layer formation in bone and in vitro of the glass plates could be found.