Anodization produces a deeper profile, but is most effective with low-viscosity primers and adhesives. Otherwise, the deep profile remains unfilled and the mechanical advantage is lost. Problems arise when the oxide layers rehydrate, forming unstable phases or friable hydroxides that can promote failure at the interface of the oxide and the metal, not at the adhesive-surface interface. The function of a surface preparation such as anodization on a metal is to provide a stable oxide layer that will remain constant, even following inevitable rehydration. Ideally, the surface energy of the adhesive will match that of the substrate.
Metals have naturally high surface energies provided by their oxide layers.
The most common adhesives used in medical bonding are the acrylics, cyanoacrylates, epoxies, urethanes, and silicones. Their relative properties are presented in Table I . Acrylics are among the fastest curing adhesives, with excellent substrate versatility, good strength and humidity resistance, and a wide formulating range. Gap filling is modest, with joint thicknesses of 0.010 in. These materials fixture rapidly to polar surfaces, especially those that have hydroxyl groups or residual moisture.