bioactive glass
Bioactive glass is a special glass, invented by Larry Hench in 1969, that does something ordinary glass never does: when you put it in the body, living bone bonds chemically and directly to its surface, and it can even stimulate cells to make new bone. Instead of being walled off as foreign like a shard of window glass, it is embraced by tissue. It is used to fill bone defects, to repair the small bones of the middle ear, in toothpaste to rebuild worn tooth mineral, and as a scaffold for regenerating bone — a rare case of a man-made material that actively coaxes the body to heal.
The classic composition, called 45S5 Bioglass, is a soda-lime-phosphosilicate glass — roughly 45 percent SiO2 with substantial sodium oxide, calcium oxide, and some phosphorus pentoxide. What makes it bioactive is deliberately low silica and high modifier content, which makes the glass more reactive than a durable window glass. When it meets body fluid, a sequence of surface reactions unfolds: sodium and calcium ions leach out, the silica surface reorganises into a gel, and a layer of carbonated hydroxyapatite — the same mineral as bone — precipitates on top. Bone-forming cells recognise that biological apatite layer and bond new bone straight to it. Remarkably, the dissolved silicon and calcium ions also signal bone cells to switch on genes for making bone, so bioactive glass is not just accepted but osteostimulatory — it actively promotes new bone growth.
That is why bioactive glass is prized where you want the body to regenerate, not merely tolerate: bone-graft granules, scaffolds for tissue engineering, middle-ear implants, and remineralising toothpaste that plugs the tiny tubes in dentine and rebuilds mineral to soothe sensitive teeth. The honest limits are those of a glass. It is brittle and mechanically weak — far too fragile to bear structural load, so it is used as a filler, coating, or scaffold, not as a load-bearing implant. And its bioactivity comes precisely from being reactive and partly soluble, so, unlike inert alumina, it is meant to change and dissolve in service — a feature when you want resorption and regeneration, but the reason it cannot be a permanent, strong, stand-alone part.
Packed into a bony defect, bioactive-glass granules leach sodium and calcium, grow a hydroxyapatite layer on their surface within days, and bond directly to the surrounding bone — while the released silicon and calcium ions signal the patient's own cells to lay down fresh bone, so the graft becomes living bone rather than an inert plug.
Bioactive glass reacts in the body to grow a bone-like apatite surface and even switch on bone-making genes — but it is a weak, brittle glass, used as filler and scaffold.
Bioactive glass works because it is reactive and partly soluble — the opposite of inert alumina. That means it is meant to change and dissolve in service, so it is a scaffold and filler, never a strong permanent load-bearing part.