Apatite-forming PEEK with TiO₂ surface layer coating

Abstract

Polyetheretherketone (PEEK) is widely used in orthopedic implants, such as spinal fusion devices, because of its moderate elastic modulus, as well as relatively high mechanical strength. However, it does not bond to living bone, and hence it needs autograft to be fixed to the bone. In this study, we attempted to add bone-bonding properties to PEEK by coating with TiO₂ synthesized by the sol–gel process. When a TiO₂ sol solution consisting of titanium isopropoxide, water, ethanol, and nitric acid was deposited on a PEEK substrate without any pretreatment, the formed TiO₂ gel layer was easily peeled off after subsequent treatments. However, when the same solution was deposited on PEEK that was preliminarily subjected to UV or O₂ plasma treatment, the deposited TiO₂ gel layer strongly adhered to the substrate even after subsequent treatments. The strong adhesion was attributed to the interaction among the C–O, C=O, and O–C=O groups on the PEEK owing to the UV or O₂ plasma treatment and the Ti–O bond of the TiO₂ gel. Apatite did not form on the as-formed TiO₂ gel layer in a simulated body fluid (SBF) even within 3 days; however, apatite formed after soaking in 0.1 M HCl solution at 80 °C for 24 h. This apatite formation was attributed to positive surface charge of the TiO₂ gel layer induced by the acid treatment. The PEEK with the TiO₂ gel layer coating formed by the proposed process is expected to bond to living bone, because a positively charged titanium oxide which facilitates the formation of apatite in SBF within a short period is known to bond to living bone.