Biomimetic and electrolytic calcium phosphate coatings on titanium alloy: physicochemical characteristics and cell attachment

Biomimetically deposited octacalcium phosphate (OCP) and carbonate apatite (BCA) as well as electrolytically deposited carbonate apatite (ECA) were considered as promising alternatives to conventional plasma spraying hydroxyapatite. This study compared their physicochemical characteristics and cell attachment behavior. The physicochemical characteristics included scanning electron microscopy observation, X-ray diffraction analysis, Fourier transform infrared spectroscopy analysis, surface roughness, coating thickness, dissolution test and scratch test. Cell attachment tests included morphology observation with stereomicroscopy and scanning electron microscopy as well as cell number count with DNA content assay. The OCP coating had 100% crystallinity and was about 40 μm thick, composed of large plate-like crystals of 30 μm, with the lowest surface roughness (Ra=2.33 μm). The BCA coating had 60% crystallinity and was approximately 30 μm in thickness, composed of small crystals of 1–2 μm in size, with the highest surface roughness (Ra=4.83 μm). The ECA coating had intermediate characteristics, with 78% crystallinity, 45 μm thickness, crystals of 5–6 μm and an average roughness of 3.87 μm. All coatings could be seen by eyes dissolving quickly and completely into acidic simulated body fluid (simulated physiological solutions—SPS, pH 3.0) but slowly and incompletely into neutral SPS (pH 7.3). It was suggested that the main factor determining coating dissolution in acidic SPS was the solubility isotherm, while some other factors including crystallinity and crystal size joined to determine coating dissolution in neutral SPS. In regard to adhesive strength, results of scratch test showed the critical load at the first crack of coating (Lc1) was tightly related to crystal size as well as their arrangement, while the critical load at the total delamination of coating (Lc2) was also related to the coating thickness. The ECA coating had the highest values. Owing to higher dissolution rate and globular appearance, BCA coating demonstrated the best goat bone marrow stromal cells attachment at 1 day or 3 days, followed by OCP and ECA coating.