Plasma Polymer Coating of Titanium for Improved Bone Implants

Abstract

Gas-discharge plasma processes could be very valuable for the modification of titanium surfaces to improve implant performance. Surface properties determine cellular acceptance, a crucial factor of implant healing. There already exists fundamental knowledge about the influence of physicochemical surface properties like roughness, titanium dioxide modifications, cleanness, and (mainly ceramic) coatings on cell behavior in vitro and in vivo. In this process, the cell adhesion and spreading via integrins was recognised to be a prerequisite for the occupancy of an orthopaedic implant surface. But recent findings indicate an additional adhesion mechanism. Hyaluronan seems to play a key role in the very first encounter with the substrate. Based on this adhesion mechanism of osteoblasts with their negativeley charged hyaluronan coat, a new surface functionalization strategiy was developed utilizing a positively charged metal surface. For this purpose, titanium was coated with a microwave plasma polymer made from allylamine (PPAAm) to boost the initial adhesion processes. The process development was accompanied by physicochemical surface analysis like XPS, FTIR, contact angle, SEM, and AFM. A very thin, adherent, cross-linked, pinhole- and additive- free PPAAm layer could be deposited, which is resistant to hydrolysis and delamination and equipped with a high density of positively charged amino groups. This titanium surface functionalization was found to be advantageous concerning osteoblastic focal adhesion formation and, in consequence in differentiated cell functions in vitro. Intramuscular implantation of test samples in rats revealed a reduced inflammation reaction compared to uncoated titanium in vivo.

This gas-discharge plasma-based PPAAm surface modification process could be an alternative to improve the biocompatibility of titanium implants.