Bioactive Glass has been used for centuries; the scientific study of its structure began only in the 1920s. Early glass development relied on trial and error and practical experience. Today, a better understanding of how composition and structure affect glass properties allows scientists to design glasses for specific applications and interpret experimental results more effectively. (1)
Glasses share two key characteristics: an amorphous structure and a distinct glass transition temperature range (Tg) the interval during which a material transforms from a supercooled liquid into a rigid glass. Unlike crystalline solids, glasses lack long-range atomic order. When heated, glasses exhibit a gradual and significant decrease in viscosity, making them easily moldable into various shapes. (2)
Oxide glasses are typically produced by melting raw materials such as inorganic oxides, carbonates, or fluorides in ceramic or precious metal crucibles at high temperatures, generally between 1200°C and 1500°C for bioactive glasses, depending on the specific composition. Rapid cooling of the melt prevents crystallization, resulting in glass formation. An alternative method for synthesizing bioactive glasses involves a polycondensation reaction using organic precursors like alkoxides (e.g., tetraethyl orthosilicate). (3)
Bioactive glass refers to a class of surface-reactive glass-ceramic biomaterials renowned for their ability to form strong bonds with both bone and soft tissues. Understanding the structure of bioactive glass involves examining its atomic arrangement, network connectivity, and how these factors influence key properties such as degradation rate and ion release behavior.
In general, glass formation involves three primary types of oxides:
- Network-forming oxides:
- Silicon dioxide (SiO₂) – the most common and important glass former, especially in silicate glasses.
- Boron oxide (B₂O₃) – used in borosilicate glasses for thermal and chemical resistance.
- Phosphorus pentoxide (P₂O₅) – used in phosphate glasses.
These oxides contain elements (like Si, B, or P) that form strong covalent bonds with oxygen, resulting in a stable, interconnected network. This network structure gives glass its rigidity, transparency, and resistance to chemical attack.
Other types of oxides, like modifiers (e.g., Na₂O, CaO), can be added to alter the properties of glass, but the network formers are essential for the glass structure itself.(4)
- Network-modifying oxides:
These oxides added to glass compositions that do not form part of the primary glass network, but instead alter its structure and properties. These oxides break the continuous network formed by network formers like SiO₂, creating non-bridging oxygens, which reduce the connectivity of the glass network.
Common network modifiers include:
- Sodium oxide (Na₂O) – lowers the melting temperature and viscosity of the glass.
- Caxlcium oxide (CaO) – improves chemical durability and mechanical strength.
- Potassium oxide (K₂O) and magnesium oxide (MgO) – also serve similar modifying roles.
By disrupting the network, these modifiers make glass easier to melt and shape, and they can tailor properties like hardness, thermal expansion, and chemical resistance. However, excessive amounts can reduce glass durability and stability. (5)
- Intermediate Oxides
Intermediate oxides are oxides that cannot form a glass network on their own, but they can enter and modify the glass network when combined with network-forming oxides. They act as a bridge between network formers and modifiers, influencing the structure and properties of the glass.
Common intermediate oxides include:
- Aluminum oxide (Al₂O₃) – improves chemical durability, mechanical strength, and thermal stability.
- Titanium dioxide (TiO₂) – enhances refractive index and UV resistance.
- Zirconium dioxide (ZrO₂) – increases hardness and chemical resistance.
Intermediate oxides often replace or bond with oxygen atoms in the network, stabilizing the structure without significantly disrupting it. They are crucial for fine-tuning the performance of technical and specialty glasses.
These oxide components collectively determine the physical, chemical, and biological performance of bioactive glasses.
Conclusion
Bioactive glass has an amorphous (non-crystalline) structure composed primarily of silicon dioxide (SiO₂), along with sodium oxide (Na₂O), calcium oxide (CaO), and phosphorus pentoxide (P₂O₅). The core structural unit is the silicate tetrahedron (SiO₄⁴⁻). Unlike pure silica glass, bioactive glass contains a high number of non-bridging oxygens (NBOs) due to the presence of Na⁺ and Ca²⁺ ions, which act as network modifiers, disrupting the silicate network and enhancing reactivity.
Phosphorus exists mainly as isolated phosphate (PO₄³⁻) units, which contribute to apatite formation. This open, disordered structure enables ion exchange with bodily fluids, promoting the formation of a surface layer of hydroxycarbonate apatite (HCA) — similar to natural bone mineral — thus enabling strong bonding with bone tissue.
References:
1) A. C. Wright, The Constitution of Glass, Society of Glass Technology, Sheffield, 2012; b) W. E. S. Turner, J. Soc. Glass Technol. Trans. 1925, 9, 147–166; c) G. Tammann, J. Soc. Glass Technol. Trans. 1925, 9, 166–185.
2) J. E. Shelby, Introduction to Glass Science and Technology, 2nd ed., The Royal Society of Chemistry, Cambridge, 2005.
3) J. R. Jones in Bio-glasses. An Introduction (Eds.: J. R. Jones, A. G. Clare), Wiley, New York, 2012, pp. 29–44; b) J. R. Jones, J. Eur. Ceram. Soc. 2009, 29, 1275–1281; c) W. Stçber, A. Fink, E. Bohn, J. Colloid Interface Sci. 1968, 26, 62–69.
4) Firas Hmood*, Oliver Goerke, and Franziska Schmidt (2018) Chemical Composition Refining of Bioactive Glass for Better Processing Features, Part I
5) Fábián, M., Kovács, Z., Lábár, J.L. et al. Network structure and thermal properties of bioactive (SiO2–CaO–Na2O–P2O5) glasses. J Mater Sci 55, 2303–2320 (2020). https://doi.org/10.1007/s10853-019-04206-z
