The study of hydroxyapatite and hydroxyapatite-chitosan composite coatings on stainless steel by electrophoretic deposition method

Abstract

Hydroxyapatite is a salt of calcium and phosphorus having the Ca/P ratio as 1.6. Hydroxyapatite is one of the major mineral components of the bone due to which it shows excellent biocompatibility. Even though it is biocompatible, hydroxyapatite cannot be used directly in load bearing structures because of its poor mechanical properties. An alternative approach to utilize the biocompatibility of hydroxyapatite is by using it as a coating material. The biocompatibility of hydroxyapatite needs to be complemented by coating it on a substance having desirable mechanical properties. A suitable material for this purpose is medical grade stainless steel, which has been used to make prosthesis, bone fixation screws, guide wires etc. It has been shown that coating medical grade stainless steel (SS 316L) implants with hydroxyapatite leads to better biocompatibility. For the coating process various techniques can be used. Electrophoretic deposition is a procedure of mild nature not involving extreme temperatures and has the advantage of producing a stoichiometric, uniform coating of the desired thickness. In this study a practical approach to coating was undertaken and a HA and composite HA-Chitosan coating obtained by varying the processing parameters was studied.
A coating of HA and composite coating of stoichiometric hydroxyapatite and chitosan were produced on medical grade stainless steel (SS 316L). Industrially pure chitosan was bought for the coating purposes and hydroxyapatite nanoparticles were synthesized by a chemical reaction of calcium nitrate and ammonium phosphate. In the synthesis process of HA nanoparticles the pH was controlled by the addition of ammonium hydroxide. The coating was carried out stainless steel as the cathode and graphite rod as the counter electrode, the distance between electrodes being 2 cm at a current density of 0.11 Ampere per metre square.
The SEM analysis showed that at a constant voltage the coating deposition increases with an increase in the coating time, whereas when the coating time is kept constant, the fineness of the coating increases with an increase in the voltage. From the XRD analysis it was confirmed that the phase purity of chitosan and HA is maintained even after the coating.