Deposition of Antioxidant and Cytocompatible Caffeic Acid-Based Thin Films onto Ti6Al4V Alloys through Hexamethylenediamine-Mediated Crosslinking

A promising approach for advanced bone implants is thedepositionon titanium surfaces of organic thin films with improved therapeuticperformances. Herein, we reported the efficient dip-coating depositionof caffeic acid (CA)-based films on both polished and chemically pre-treatedTi6Al4V alloys by exploiting hexamethylenediamine (HMDA) crosslinkingability. The formation of benzacridine systems, resulting from theinteraction of CA with the amino groups of HMDA, as reported in previousstudies, was suggested by the yellow/green color of the coatings.The coated surfaces were characterized by means of the Folin-Ciocalteumethod, fluorescence microscopy, water contact angle measurements,X-ray photoelectron spectroscopy (XPS), zeta-potential measurements,and Fourier transform infrared spectroscopy, confirming the presenceof a uniform coating on the titanium surfaces. The optimal mechanicaladhesion of the coating, especially on the chemically pre-treatedsubstrate, was also demonstrated by the tape adhesion test. Interestingly,both films exhibited marked antioxidant properties (2,2-diphenyl-1-picrylhydrazyland ferric reducing antioxidant power assays) that persisted overtime and were not lost even after prolonged storage of the material.The feature of the coatings in terms of the exposed groups (XPS andzeta potential titration evidence) was apparently dependent on thesurface pre-treatment of the titanium substrate. Cytocompatibility,scavenger antioxidant activity, and antibacterial properties of thedeveloped coatings were evaluated. The most promising results wereobtained in the case of the chemically pre-treated CA/HMDA-based coatedsurface that showed good cytocompatibility and high reactive oxygenspecies' scavenging ability, preventing their intracellularaccumulation under pro-inflammatory conditions; moreover, an anti-foulingeffect preventing the formation of 3D biofilm-like bacterial aggregateswas observed by scanning electron microscopy. These results open newperspectives for the development of innovative titanium surfaces withthin coatings from naturally occurring phenols for bone contact implants.