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A bioactive glass is a glass designed to elicit a useful biological response at its interface, notably bonding with bone for some compositions and uses. It is a class of biomaterials, not a claim that every glass has the same response. Related glass-ceramics contain crystalline phases and should be distinguished from wholly glassy materials.

A motivating early experiment compared glass-ceramic solubility and interfacial hydroxyapatite crystallization with observations around implants in rat femurs. Larry L. Hench and colleagues examined histology and ultrastructure as well as laboratory interfacial changes. The question was how bonding developed at the material–bone interface, beyond the bulk material's mechanical strength.^[ref]

Contact with fluid can change the material's surface and release dissolved constituents. Geometry therefore matters alongside composition. For an original monodisperse spherical-powder model, radius $r$ and density $\rho$ give surface area per massAt stipulated $\rho =2500\mathrm{kg}{\mathrm{m}}^{-3}$ and $r=10\mu \mathrm{m}$, this is $120{\mathrm{m}}^2\mathrm{k}{\mathrm{g}}^{-1}$, or $0.12{\mathrm{m}}^2{\mathrm{g}}^{-1}$. Increasing radius to $100\mu \mathrm{m}$ reduces that area per mass tenfold. The example neglects pores and aggregation and is not a particular product's specification.

If initial release flux per unit area were equal in two such powders, their initial release rates per mass would differ by the same factor. Actual flux can change with solution composition and surface evolution; a biological response need not scale linearly with geometric area. Comparisons should therefore specify particle dimensions, exposed area, fluid conditions and time. Surface-layer formation, cytotoxicity, mechanical behavior and tissue bonding remain distinct observations rather than interchangeable proof of suitability.

T. K. Greenlee is a coauthor of the 1971 study of bonding between a glass-ceramic biomaterial and bone. The publisher’s byline lists Greenlee at the Veterans Administration Hospital in Gainesville, Florida.^[ref]

The study connected an interfacial chemical model with observations in biological tissue. Laboratory studies of material solubility and hydroxyapatite crystallization were compared with histology and ultrastructure around implants in rat femurs. This comparison addresses a concrete problem: a material can have suitable bulk mechanical properties while developing an unsuitable interface with living bone. Microscopic observations of that interface help test a proposed bonding mechanism instead of inferring it from bulk strength alone.^[ref]

In his historical account, Larry L. Hench identifies Greenlee as an orthopaedic surgeon involved in the early work.^[ref]