Topography Control of Hydroxyapatite Coatings

The biomaterial surface represents the first contact point for proteins and cells and is thus critical to optimise the features to transmit the best signals for tissue and organ regeneration. Both chemistry and topography are surface characteristics that can be modified by the manufacturing process and provide signals to cells. While chemistry and crystallinity have received attention for thermally sprayed hydroxyapatite coatings, the control of the surface microtopography has not been addressed. This study collected a range of implants with a coating and assessed the topography on dental implants, orthopaedic screws and hip prostheses. The surface was found to be composed of large topographic features (angular particulate and smooth areas) and micron sized aspects (fine grains, cracks and ridges). Thermal spray experiments were designed to determine the influence of processing conditions on droplet spreading. This knowledge was then applied to see the effect of different parameters on the final coating topography. The parameters investigated for their influence on the surface topography included substrate roughness, substrate temperature, spray distance and particle size. The particle size showed the largest influence on altering the roughness, Ra of the coating. A two-fold increase in particle size doubled the roughness from Ra of 4.8 µm when the powder was sprayed under the same conditions.