A promising approach for advanced bone implants is the
deposition
on titanium surfaces of organic thin films with improved therapeutic
performances. Herein, we reported the efficient dip-coating deposition
of caffeic acid (CA)-based films on both polished and chemically pre-treated
Ti6Al4V alloys by exploiting hexamethylenediamine (HMDA) crosslinking
ability. The formation of benzacridine systems, resulting from the
interaction of CA with the amino groups of HMDA, as reported in previous
studies, was suggested by the yellow/green color of the coatings.
The coated surfaces were characterized by means of the Folin–Ciocalteu
method, fluorescence microscopy, water contact angle measurements,
X-ray photoelectron spectroscopy (XPS), zeta-potential measurements,
and Fourier transform infrared spectroscopy, confirming the presence
of a uniform coating on the titanium surfaces. The optimal mechanical
adhesion of the coating, especially on the chemically pre-treated
substrate, was also demonstrated by the tape adhesion test. Interestingly,
both films exhibited marked antioxidant properties (2,2-diphenyl-1-picrylhydrazyl
and ferric reducing antioxidant power assays) that persisted over
time and were not lost even after prolonged storage of the material.
The feature of the coatings in terms of the exposed groups (XPS and
zeta potential titration evidence) was apparently dependent on the
surface pre-treatment of the titanium substrate. Cytocompatibility,
scavenger antioxidant activity, and antibacterial properties of the
developed coatings were evaluated. The most promising results were
obtained in the case of the chemically pre-treated CA/HMDA-based coated
surface that showed good cytocompatibility and high reactive oxygen
species’ scavenging ability, preventing their intracellular
accumulation under pro-inflammatory conditions; moreover, an anti-fouling
effect preventing the formation of 3D biofilm-like bacterial aggregates
was observed by scanning electron microscopy. These results open new
perspectives for the development of innovative titanium surfaces with
thin coatings from naturally occurring phenols for bone contact implants.