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Bioactive glasses are a group of surface reactive glass-ceramics and include the original bioactive glass, Bioglass. The biocompatibility of these glasses has led them to be investigated extensively for use as implant materials in the human body to repair and replace diseased or damaged bone.
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History
Larry Hench and colleagues at the University of Florida first developed these materials in the late 1960s and have been further developed by his research team at the Imperial College London and other researchers worldwide.
Compositions
There have been many variations on the original composition which was Food and Drug Administration (FDA) approved and termed bioglass. This composition is known as 45S5. Other compositions are in the list below.
- 45S5: 46.1 mol% SiO2, 26.9 mol% CaO, 24.4 mol% Na2O and 2.5 mol% P2O5. Bioglass
- 58S: 60 mol% SiO2, 36 mol% CaO and 4 mol% P2O5.
- S70C30: 70 mol% SiO2, 30 mol% CaO.
Mechanism of bioactivity
The underlying mechanisms that enable bioactive glasses to act as materials for bone repair have been investigated since the first work of Hench et al. at the University of Florida. Early attention was paid to changes in the bioactive glass surface. A reaction sequence consisting of five steps is commonly thought to occur when a bioactive glass is immersed in a physiological environment: 1) Ion exchange in which modifier cations in the glass exchange with hydronium ions in the external solution, 2) Hydrolysis in which the glass network is disrupted, 3) Condensation in which the disrupted glass network changes its morphology to form a porous, gel-like surface layer, 4) Precipitation in which an X-ray amorphous calcium phosphate layers is deposited on the gel, and 5) Mineralization in which the calcium phosphate layer gradually transforms into hydroxyapatite-like substance that mimics the mineral phase naturally contained with vertebrate bones. Later, it was discovered that the morphology of the gel surface layer was a key component in determining the bioactive response. This was supported by studies on bioactive glasses derived from sol-gel processing. Such glasses could contain significantly higher concentrations of SiO2 than traditional melt-derived bioactive glasses and still maintain bioactivity (i.e., the ability to form a mineralized hydroxyapatite layer on the surface). The inherent porosity of the sol-gel-derived material was cited as a possible explanation for why bioactivity was retained.
Subsequent advances in DNA microarray technology enabled an entirely new perspective on the mechanisms of bioactivity in bioactive glasses. Previously, it was known that a complex interplay existed between bioactive glasses and the molecular biology of the implant host, but the available tools did not provide a sufficient quantity of information to develop a holistic picture. Using DNA microarrays, researchers are now able to identify entire classes of genes that are regulated by the dissolution products of bioactive glasses, resulting in the so-called "genetic theory" of bioactive glasses. The first microarray studies on bioactive glasses demonstrated that genes associated with osteoblast growth and differentiation, maintenance of extracellular matrix, and promotion of cell-cell and cell-matrix adhesion were up-regulated by conditioned cell culture media containing the dissolution products of bioactive glass.
Structure
Solid state NMR spectroscopy has been very useful in elucidating the structure of amorphous solids. Bioactive glasses have been studied by 29Si and 31P solid state MAS NMR spectroscopy. The chemical shift from MAS NMR is indicative of the type of chemical species present in the glass. The 29Si MAS NMR spectrscopy showed that Bioglass 45S5 was a Q2 type-structure with a small amount of Q3; i.e., silicate chains with a few crosslinks. The 31P MAS NMR indicated a Q0 species; i.e., PO44- with predominately sodium cations. (Lockyer et al. 1995)
Medical Applications
Bioactive glasses have many applications but these are primarily in the areas of bone repair and bone regeneration via tissue engineering.
- Synthetic bone graft materials for general orthopaedic, craniofacial (bones of the skull and face), maxillofacial and periodontal (the bone structure that supports teeth) repair. These are available to surgeons in a particulate form.
- Cochlear implants.
- Bone tissue engineering scaffolds. These are being investigated in many forms, in particular as porous (contains pores into which cells can grow and fluids can travel) 3-dimensional scaffolds.
- Treating Dentine hypersensitivity and promoting enamel remineralization in the form of NovaMin.
References Lockyer, M. W. G., Holland, D. & Dupree, R., NMR investigation of the structure of some bioactive and related glasses. J. Non-Crys. Sol., 1995, 188, 207-219.
External links
Orthovita distributes Vitoss Bioactive Foam, bone graft line. Visit: www.orthovita.com NovaBone Products in Alachua, FL manufactures and markets bioactive glass. The Musculoskeletal Transplant Foundation distributes NovaBone internationally. Visit: www.novabone.com and www.mtf.org
Vivoxid Ltd in Finland manufactures, markets and sells bioactive glass. Visit: www.vivoxid.com
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Wikipedia content modification information:
- This page was last modified on 4 December 2008, at 06:50.
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