(Invited) ALD to Prevent Metal Transfer from Implants
Authors: Borgese Laura , Bilo Fabjola , Zacco Annalisa , Bontempi Elza , Pasquali Michela , Federici Stefania , Prost Josef , Rauwolf Mirjam , Turyanskaya Anna , Streli Christina , Kregsamer Peter , Wobrauschek Peter , Depero Laura Eleonora 
Published: February 2020

The interest about coatings devoted to biomedical applications has been continuously increasing during the last years. Several literature studies address the modification of surface properties, and the improvement of biocompatibility of implant surfaces by coatings. Among the other properties, adhesion and uniform coverage are desired. Another challengeable issue for biomedical applications is metal release from implant alloy into tissue environment. Indeed, severe adverse reactions may occur, including inflammation, fibrosis, thrombosis, infections and, last but non least, metallosis. Clinical cases of this pathology have been already observed with orthopaedic implants made with CoCrMo alloys.
On these purposes several materials and coating techniques have been investigated. Recently, nanomaterials have also been considered. Atomic Layer Deposition (ALD) is a synthesis technique for nanomaterials, that is raising interest in the field of biomedical application, due to its unique properties. ALD allows conformal coating by ultrathin layers with high adhesion and well-defined thickness, at the nanometer scale. Oxide compounds may be easily deposited by ALD, even at low temperature. Among these, TiO2 is one of the most attractive functional oxide for such application, due to its high durability, good corrosion resistance, and well-tested biocompatibility. Indeed, TiO2 is already used to coat biomedical alloys and implants to improve short and long term body responses.
In this work we study the effect of TiO2 layers deposited by ALD against metal release from CoCrMo alloy implants. Implant samples are coated with different thicknesses of TiO2, ranging from 10 to 200nm. Micro X-Ray Fluorescence is used to map the metal distribution in the bone surrounding the implant, after 1 and 2 months respectively from implantation. It is observed that the presence of TiO2 may prevent the metal transfer into the bone. Results show that Co and Cr has cluster distribution in the tissue surrounding the uncoated implant, with a number of particles that significantly increases with time. While, a decreasing gradient distribution from the alloy surface is evident in all the coated implant samples. In particular, the thicker the coating layer, the lower is the metal concentration in the surrounding tissue. The distribution of Mo is much uniform and extended with respect to Co and Cr. Metal transport increases with time, only in uncoated samples. It is also observed that corrosion of coated implants is delayed.
The content of metals at a fixed distance from the implant surface increases with time. However, the mechanism of metal transport through the coating layer is not clear. For this reason a further "proof of concept" test is designed, to study the transport kinetics of ionic species into water through TiO2 coatings, deposited by ALD. Results show zero order kinetics. This model experiment demonstrates that the release of ionic species occur by a mechanism of diffusion through a continuous layer.
This work figures out two important benefits of TiO2 coating obtained by ALD on the surface of CoCrMo implant: i. it shows a barrier effect against metal movement from the implant toward the tissue; ii. it protects the implant material against corrosion in this biological environment. Clinical benefits or antibacterial effects should be further investigated. From another point of view, this study highlights that ALD allows a very precise control of ionic species diffusion by means of layer thickness. This ability can be strongly exploited in the field of controlled release. The combination of these effects may be successfully used to obtain layer by layer structures with tunable properties, making ALD a powerful tool for biomedical applications.