Taking Hydroxyapatite-Coated Titanium Implants Two Steps Forward: Surface Modification Using Graphene Mesolayers and a Hydroxyapatite-Reinforced Polymeric Scaffold.

Abstract

Coating with hydroxyapatite (HAP) presents a mainstream strategy for rendering bioinert titanium implants bioactive. However, the low porosity of pure HAP coatings does not allow for the infiltration of the surface of the metallic implant with the host cells. Polymeric scaffolds do enable this osseointegration effect, but their bonding onto titanium presents a challenge because of the disparity in hydrophilicity. Here, we demonstrate the inability of a composite scaffold composed of carbonated HAP (CHAP) nanoparticles interspersed within electrospun ε-polycaprolactone (PCL) nanofibers to bind onto titanium. To solve this challenge, an intermediate layer of graphene nanosheets was deposited in a pulsed laser deposition process, which facilitated the bonding of the scaffold. The duration of the deposition of graphene (0, 5, 10, 15, and 20 min) and the thickness of its mesolayer affected numerous physical and chemical properties of the material, including the surface atomic proportion of carbon bonds, the orientation and interlinking of the polymeric nanofibers, and the surface roughness, which increased in direct proportion with the thickness of the graphene mesolayer. Because the polymeric scaffold did not adhere onto the surface of pure titanium, no cells were detected growing on it in vitro. In contrast, human fibroblasts adhered, spread, and proliferated well on all the substrates sputtered with both graphene and the composite scaffold. The orientations of cytoskeletal filopodia and lamellipodia were largely determined by the topographic orientation of the nanofibers and the geometry of the surface pores, attesting to the important effects that the presence of a scaffold has on the cellular behavior. The protection of titanium from corrosion in the simulated body fluid (SBF) was enhanced by coating with graphene and the composite scaffold, with the most superior resistance to the attack of the corrosive ions being exhibited by the substrate subjected to the shortest duration of the graphene deposition because of the highest atomic ratio of C-C to C-O bonds detected in it. Overall, some properties of titanium, such as roughness and wettability, were improved monotonously with an increase in the thickness of the graphene mesolayer, while others, such as cell viability and resistance to corrosion, required optimization, given that they were diminished at higher graphene mesolayer thicknesses. Nevertheless, every physical and chemical property of titanium analyzed was significantly improved by coating with graphene and the composite scaffold. This type of multilayer design evidently holds a great promise in the design of biomaterials for implants in orthopedics and tissue engineering.