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Bioactive materials are a class of biomaterials designed to elicit a specific, positive biological response at the material-tissue interface, resulting in the formation of a bond between them. Unlike bioinert materials that aim for minimal interaction, bioactive materials actively participate in healing processes. A classic example is Bioglass, which forms a hydroxyapatite layer on its surface, bonding directly to bone.
The concept of bioactivity was pioneered by Larry Hench with his 1969 invention of Bioglass® 45S5, a specific composition of 45 wt% SiO2, 24.5% CaO, 24.5% Na2O, and 6% P2O5. The mechanism of bone bonding involves a sequence of five surface chemical reactions. When exposed to bodily fluids, (1) rapid ion exchange of Na+ from the glass with H+ from solution occurs, creating a silica-rich layer. (2) This increases local pH, which causes (3) the glass network to break down, releasing silicic acid (Si(OH)4) and forming silanols (Si-OH) at the surface. (4) A high local concentration of calcium and phosphate ions, both from the glass and surrounding fluid, leads to the formation of an amorphous calcium-phosphate layer. (5) This layer crystallizes into hydroxycarbonate apatite (HCA), which is chemically and structurally similar to the mineral phase of bone. This HCA layer provides a bioactive scaffold for osteoblasts to colonize, differentiate, and deposit new bone matrix, creating a strong, direct, and seamless bond with the host bone. This process, known as osseointegration, is crucial for the long-term stability of implants. Furthermore, the dissolution products, particularly silicic acid, have been shown to stimulate genes within osteoprogenitor cells, upregulating the production of growth factors. This makes the material not only osteoconductive (a scaffold for bone growth) but also osteoinductive (actively stimulating new bone formation).
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