The qualities of any given material – reactivity, adhesion, conductivity, and toxicity – can be extensively modified by altering its surface. In some cases, this isn’t surprising; if we plate a reactive metal with an inert one like silver, we would expect it to exhibit the qualities of silver as long as the top layer remains intact. However, it turns out that just altering the structure of surfaces can radically improve their potential.
Many surface treatments exploit both approaches. For example, anodising an aluminium surface produces molecular columns of aluminium oxide. It is the porous structure of this surface that then allows other chemicals, such as sealants, to penetrate and form an effective bond, providing more desirable properties.
In other cases, texture alone can radically enhance a material’s useful properties.
Surface modification techniques
Anodising works by accelerating and deepening oxidation. Oxidisation is something to avoid in most materials, but the oxides of aluminium, titanium and magnesium are stable ceramics, and their porous structure allows things like PTFE to be diffused into the surface. PTFE provides a low friction surface ideal for bearings, prosthetic or otherwise.
Anodising can be a preparation before other surface treatments. These can include electroplating, painting, immersion chemical deposition, and thermal sprays.
Thermal spraying is a general term for several techniques, including electric arc spraying, high-velocity oxygen fuel (HVOF), wire spraying and plasma spraying. They provide well-packed layers of metallic or semi-metallic surfaces, such as chrome, tungsten carbide, alumina-polyester, molybdenum or yttria-zirconia. The preferred technique can be chosen to achieve a particular surface structure (see https://www.poeton.co.uk/treatments).
Advanced techniques can also sculpt the structure directly by chemical action, laser-peening or even shot blasting, and there is much wide-ranging research in this area.
Suitably textured surfaces – both of metals and synthetics – can significantly enhance the ability of natural tissues to bond with implants and prosthetics. Currently, orthopedic pins and titanium dental implants take months to osteointegrate, and it is rarely fully achieved. When it isn’t, repairs are weaker and complications like thrombosis are more likely.
Sculpted surfaces can improve biocompatibility, adhesion, elasticity, lubricity, sterility, asepsis, thrombogenicity and hydrophilicity.
Chemical treatments, laser-peening and shot blasting can be used to achieve dense concave surfaces. Convex textures can be manufactured using plasma-spraying, solid-state diffusion, chemical vapour deposition, or PVD.