synthesis and characterization of nano-hydroxyapatite/mpeg-b-pcl composite coating on nitinol alloy

In this study the bioactivity of hydroxyapatite/poly(ε-caprolactone)–poly(ethylene glycol) bilayer coatings on nitinol superelastic alloy was investigated. the surface of nitinol alloy was activated by a thermo-chemical treatment and hydroxyapatite coating was electrodeposited on the alloy, followed by applying the polymer coating. the surface morphology of coatings was studied using fe-sem and sem. the data revealed that the hydroxyapatite coating is composed of one-dimensional nano sized flakes and the polymer coating is uniformly covered the sublayer. also, high resolution tem studies on the hydroxyapatite samples revealed that each flake contains nano-crystalline grains with a diameter of about 15 nm. the hydroxyapatite monolayer coating was rapidly covered by calcium phosphate crystals (ca/p=1.7) after immersion in simulated body fluid confirming the bioactivity of the nanostructured flakes. however, the flakes were weak against applied external forces because of their ultra-fine thickness. scratch test was applied on hydroxyapatite/ polymer coating to evaluate delamination of the coating from substrate. it was shown that, the polymer coating has a great influence on toughening the hydroxyapatite coating. to assess the degradation effect of the polymer layer on hydroxyapatite coating, samples were immersed in phosphate-buffered saline at 37 °c. sem studies on the samples revealed that the beneath layer of hydroxyapatite appears after 72 h without any visible change in morphology. it seems that, application of a biodegradable polymer film on the nanostructured hydroxyapatite coating is a good way to support the coating during implantation processes.