implanted within the body. Metallic glasses tout lower Young's Moduli and higher elastic limits than bioactive glass, and as such, will allow for more deformation of the material before fracture occurs. This is highly desirable, as a permanent implant would need to avoid shattering within the patient's body. Common materials which compose the metallic bulk include Zr and Ti, whereas some examples of the few key metals that shouldn't be used as bulk materials are Al, Be, and Ni. Laser-Cladding While metals are not necessarily inherently bioactive, bioactive glass coatings which are applied to metal substrates via laser-cladding introduce the bioactivity that the glass would express, but have the added benefits of having a metal base. Laser Cladding is a method by which bioactive glass microparticles are thrust in a stream at the bulk material, and introduced to a high enough heat that they melt into a coating of material. Sol-Gel Processing Metals can also be affixed with bioactive glass using a Sol-Gel Process, in which the bioactive glass is sintered onto metals at a controlled temperature that is high enough to perform the sintering, but low enough to avoid phase-shifts and other unwanted side effects. Experimentation has been done with sintering double layered, silica-based bioactive glass onto stainless steel substrates at 600 °C for 5 hours. This method has proven to maintain largely amorphous structure while containing key crystalline elements, and also achieves a remarkably similar level of bioactivity to bioactive glass. Mechanism of activity 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. Five inorganic reaction stages are commonly thought to occur when a bioactive glass is immersed in a physiological environment: Ion exchange in which modifier cations (mostly Na+) in the glass exchange with hydronium ions in the external solution. Hydrolysis in which Si-O-Si bridges are broken, forming Si-OH silanol groups, and the glass network is disrupted. Condensation of silanols in which the disrupted glass network changes its morphology to form a gel-like surface layer, depleted in sodium and calcium ions. Precipitation in which an amorphous calcium phosphate layer is deposited on the gel. Mineralization in which the calcium phosphate layer gradually transforms into crystalline hydroxyapatite, that mimics the mineral phase naturally contained with vertebrate