Plasma-Based Amorphous Carbon Coatings on Polymeric Substrates for Biomedical Applications: A Critical Review Focused on Adhesion

Material surfaces are of primary importance in biomaterial development, significantly influencing implant lifespan and clinical success. Consequently, coating technologies are frequently employed to modify surface properties and functionality. Plasma-based amorphous carbon coatings have been widely applied to all classes of substrates to improve their tribology, corrosion resistance, hardness, and even biological properties. Plasma technology is widely recognized to be effective, not only for the deposition of amorphous carbon coatings but also for substrate pre-treatment, in which it may play a key role in activating surfaces and enhancing interfacial adhesion. Amorphous carbon coatings can be classified into two major categories: diamond-like carbon (DLC) and polymer-like carbon (PLC), according to their mechanical properties. Regardless of their nature, the adhesion of both types of amorphous carbon coatings to the substrate has always represented a major challenge. Several strategies have been reported to enhance the adhesion of DLC coatings to silicon wafers, metals, and glass substrates. However, few studies report strategies aimed at controlling the adhesion of (both types of) amorphous carbon coatings to polymeric substrates, polymeric implants, and polymeric devices. Therefore, this work aims to provide a state-of-the-art review on the adhesion of amorphous carbon coatings to polymeric substrates for biomedical applications. Furthermore, this review presents the main techniques used to assess adhesion and the strategies available to improve adhesion between coatings and polymeric substrates.