Investigation of Halloysite Nanotube Effect in Poly- (Lactic Acid)/Hydroxyapatite Coatings on Ti-6al Biomedical Alloy

Abstract

Due to their biodegradable nature and enhanced adhesive strength at the interface, poly- (lactic acid) (PLA) based coatings are frequently used in biomedical alloys. But it is unable to create metal-tissue interaction because of its very hydrophobic surface feature. Researchers are therefore looking at composite coatings that are reinforced with hydroxyapatite (HA). In this study, the effect of halloysite nanotube (HNT) was investigated to hydroxyapatite (HA) reinforced PLA coatings synthesized on Ti-6Al-4V (Ti64) biomedical alloy. Investigations were conducted on the hybrid coatings' characterization such as SEM/EDS, FTIR, contact angle tests, and XRD analyses. Adhesion tests at alloy-coating interface were revealed with ASTM D3359-09 standards. In-vitro electrochemical corrosion tests (ASTM-G59) and immersion tests (ISO-23317) were conducted on coatings for reveal both biodegradability and osteoconductivity behaviours of coatings. SEM morphologies revealed that surfaces have micron-sized pores on both coatings. But HA/HNT reinforced coatings have more homogenous morphology with decreased pore size. The elemental distribution of coatings was confirmed by the EDS analyses. While FTIR research demonstrated a variation in the influence of HNT on the coating of PLA depending on the bond status, XRD results showed that there were no phase changes in the coatings. It was found that coatings with HNT reinforcement, have higher adhesion strength at the substrate-coating interface (1B for HA and 4B for HA/HNT reinforced coatings). Following in-vitro electrochemical corrosion experiments, it was discovered that the corrosion current density (I-corr) value reduced even more with the PLA/HA-HNT coating (1.784, 0.245, and 0.032 mu A cm(-1)) than it did with the PLA/HA coating alone. The results of the impedance spectroscopy (Nyquist curves) confirmed that the coatings' corrosion resistance increased as the I-corr value decreased. Possible corrosion mechanism was discussed with equivalent circuit model which consist two-constant elements. It was found that both coatings exhibited a characteristic that induced calcium phosphates (CaPs) on their surfaces after in-vitro immersion tests. The contact angle tests revealed that the high hydrophobic effects of coatings would have a positive effect on the alloy-tissue interaction in the long term due to the bioactive compounds that they have. The dense and homogeneous CaPs in the PHH coatings increased with HNT reinforcement, leading to more effective interaction with surrounding tissues during in-vitro tests. The findings led to the conclusion that the hybrid coatings produced with the reinforcement of HA/HNT will not readily delaminate during application and will improve the interaction between the alloy and the tissue under prolonged physiological conditions. [GRAPHICS] .