Introduction: One of the challenges met by metallic implants in contact with bone is the direct attachment of bone with the implant. Most of the time, a direct growth of bone on the implant is possible thanks to an osteoconductive coating on the metallic parts. Such coatings are mostly made of calcium phosphates (hydroxyapatite) by plasma spray deposition. The objective of this study is to develop an alternative method, Electrostatic Spray Deposition (ESD) based on electrohydrodynamics laws, cheap, low temperature and that allows a larger variety of phases (including possibly out-of equilibrium ones) to be formed on the surfaces.
Materials and Methods: Hydro-alcoholic mixtures containing different calcium salts and phosphates were sprayed on polished and cleaned titanium surfaces using an electro-spraying deposition device. The initial pH of the liquids was controlled so that they were either under-saturated (thus spraying a solution, whose ions reacted during the flight) or over-saturated (thus spraying a suspension, in which both ions and small precipitates were present). Spraying distance, substrate temperature (around 400°C) and electrical field, spraying flow rate and spraying time were considered as the process variables.
The microstructures of the obtained coatings were characterized by scanning electron microscopy associated with energy dispersive X-ray spectroscopy. Their structure and phase were determined using X-ray diffraction and infra-red spectroscopy.
Results and Discussion: The results obtained so far show that ESD of calcium phosphate may be a quite versatile technique. For example, the infra-red spectra of a coating obtained by spraying a solution (continuous line) and by spraying a suspension (dotted line) are shown on the figure below, together with a tentative indexation of the bands.
It appears that the obtained coating is less carbonated when coming from a solution than when coming from a suspension, which may be the result either of a higher pH in the suspension or of the carbon being trapped in the particles. Other microstructural parameters may also vary depending on the ESD parameters and initial precursors.
Conclusion: Electrostatic Spray Deposition proved to be a viable and potentially interesting technique to coat metals (Ti) with calcium phosphate layers, thus rendering them osteoconductive. Its potential has yet to be developed to take full advantage of its characteristics: cheap process, low temperature that enables the retention of out-of equilibrium phases, variety of the obtained coatings…