Cathodic Synthesis of Strontium-Substituted Hydroxyapatite Coatings

Calcium strontium phosphate was electro-crystallized on Ti along with hydroxyapatite (HA). Stock solutions of 167 mM CaCl₂, 167 mM SrCl₂, and 100 mM NH₄H₂PO₄ were initially prepared. Two different electrolyte formulations of equi-amounts of (CaCl₂ + SrCl₂) and NH₄H₂PO₄ were used. The Ca:Sr ratios were maintained as 9:1 and 3:1, respectively, in these two formulations. Using these solutions, a cathodic deposit was obtained on Ti using ultrasonic agitation of the electrolyte at the current density of 10 mA/cm² for 60 min. The electrolytes were heated at 50°C. X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM) analysis were utilized to characterize the phase compositions and surface morphology of the coatings. A surface roughness tester, an universal testing machine, and contact angle measurements were used to estimate the coating roughness, pull-off tensile adhesion strength, and wettability, respectively. The coatings were composed of hydroxyapatite (HA) and calcium strontium phosphate. The strontium (Sr) substitution was nearly 0.5% and 1% in the coatings obtained from the 9:1 and 3:1 Ca:Sr ratios, respectively. The coatings contained predominantly spherical crystals and occasional plate-like crystals attached to the substrate. Both the coatings displayed a similar surface roughness of about 450 nm. The coating obtained from the first electrolyte (Ca:Sr = 9:1) had a higher contact angle in simulated body fluid, but a lower pull-off adhesion strength than the coating obtained from the second (Ca:Sr = 3:1) electrolyte.