Control of the forming properties of bioactive glasses

Bioactive glass products used in clinical applications are fairly simple inform, being mostly plates or crushed glass. Novel applications require a versatile palette of products ranging from fibres and porous bodies to composites or moulded implants with special product dimensions. However, the low silica and the high lime content of bioactive glasses make them sensitive to the forming process used within the viscosity range similar to 10(8)-10(3) dPas. Bioactive glasses show two distinct responses depending on the primary phase formed on crystallisation. Glasses which form sodium calcium silicate crystals possess low glass stability, and crystallise around 100 C above the glass transition. Glasses, which form wollastonite crystals, crystallise above 800 C, thus allowing for example viscous flow sintering of crushed glass to produce porous implants. Fibre drawing also strongly interferes with crystallisation. Crystalline phases at liquidus are the same as those formed on heating the glass. Sodium calcium silicate phases formed through bulk crystallisation at low viscosities inhibit fibre drawing. Wollastonite crystals are formed through surface nucleation at viscosities high enough to allow drawing of continuous fibres. Wollastonite type glasses are interesting especially for special products with a high surface area to volume ratio. The lower bioactivity and higher chemical durability of wollastonite glasses in such special products may be valuable for some clinical applications.