Morphology of calcium phosphate coatings for biomedical applications deposited using Electrostatic Spray Deposition

Electrostatic Spray Deposition (ESD) of biomedical calcium phosphate (CaP) coatings was investigated by in situ measurements of droplet sizes and velocities using Phase Doppler Anemometry. Processing parameters, related to the ESD-apparatus as well as the composition of the precursor solutions, were varied (deposition temperature, nozzle-to-substrate distance, nozzle geometry, and composition of the precursor solution). Thereafter, morphological characteristics of these ESD-derived CaP coatings were correlated with measured droplet characteristics. By varying physical apparatus-related parameters such as the nozzle-to-substrate distance and the deposition temperature, it was observed that electrosprayed butyl carbitol droplets did not shrink during droplet flight towards the heated substrate. Nevertheless, coatings with a different surface morphology were obtained, varying from microporous structures with coalesced pore walls to isolated rings on top of dense or grainy underlayers. The chemical composition of the precursor solutions and the mixing characteristics of the calcium and phosphate precursor components strongly influenced the initial droplet sizes, precipitation kinetics of the CaP solute, and subsequent coating morphology. Unique, reticular coatings can be deposited at a deposition rate of 3.2 μm/h, which have a graded structure consisting of a dense underlayer, a submicron-porous intermediate layer, and a roughened toplayer revealing droplet-derived features such as isolated rings or coalesced, hollow surface pits.