ICMCTF 2022 Session DP-ThP: Coatings for Biomedical and Healthcare Applications (Symposium D) Poster Session

Inside a spacecraft, the suitable temperature and humidity, designed for onboard human crew, also creates an ideal breeding environment for the proliferation of bacteria and fungi, which can present hazard for both the safety running of equipment and human health. To address this issue, it is proposed to coat the key parts and components with wear-resistant antimicrobial thin films prepared by magnetron sputtering. Silver and copper are among the most studied active bactericidal materials for a long history. In this work we investigate the antibacterial properties of silver doped, silver-copper doped, and silver cluster doped amorphous carbon coatings for aerospace applications and their longevity, which is heavily influenced by the metal diffusion rate to the coating surface. Samples have been prepared with different silver and copper volume fractions via co-sputtering, while Ag cluster samples are prepared using a cluster source to better control the metal particle size distribution in the amorphous carbon coating matrix. Antibacterial tests are performed under both terrestrial gravity and microgravity conditions, to simulate the condition in the space. Results show that silver doped coatings are very effective in terms of antimicrobial property but with a faster silver atom diffusion, while those silver-copper doped samples have generally a slightly lower antimicrobial activity, but no obvious metal diffusion observed. For the silver cluster doped samples, they show a high antimicrobial activity despite a much lower silver volume fraction, and a seemingly slow Ag metal diffusion rate.

Titanium alloys are propitious materials in biomedical implants for their mechanical properties and biocompatibility. Nitinol (Ni-Ti), especially, has been used because of its superelasticity (up to 12% recoverable strain) due to a reversible martensitic transition. Ni, however, has been proven allergenic and so in the past decade, research has focused on β-type titanium alloys with non-toxic and non-allergenic elements such as Nb, Sn, Ta or Zr with the aim of replacing Ni-Ti. In this study, novel coatings based on ternary and quaternary titanium alloys have been elaborated. Their properties have been studied for bulk materials; however, the structural and mechanical properties for their thin film counterpart remain to be elucidated, which is the aim of the present study.

Ti-Nb-Zr and Ti-Nb-Zr-Sn coatings have been elaborated at room temperature by magnetron sputtering at a working pressure of 0.26 Pa. By using different targets and by changing the power applied to them during deposition, different chemical compositions have been obtained, with Nb content ranging from 0 to 33 at.%. The focus has been on identifying the crystallographic structure and the mechanical properties of the films depending on their chemical composition. XRD, TEM and resistivity measurements were performed to study the phase formation with respect to the Nb content. The stress induced during film growth has been evaluated in-situ using wafer curvature measurements and the mechanical properties were evaluated using nano-indentation and tensile tests. To assess the biocompatibility of the films, early cell behavior (cell adhesion, spreading and morphology) will be characterized and standardized cytotoxicity assay will be conducted.

Using XRD θ/2θ scans, the films have been found to be highly textured: the main peaks are 002 α and 110 β. XRD, TEM and resistivity show that with increasing Nb and Zr content, the phase evolves from hexagonal α phase to orthorhombic α’’ martensite and then to cubic β phase. The curvature measurements have shown that at low Nb content, the film first develops a compressive stress that evolves during continuing growth into tensile stress. At high Nb content, the stress remains compressive throughout deposition. Preliminary in vitro biocompatibility tests show that osteoblast cells well adhere on the quaternary coatings after 72 h and manifest cell functional activity.

Graphene can be an effective antibacterial candidate owing to its bactericidal property. In this study, graphene coatings were prepared on Nitinol substrate through a cost-effective electrophoretic deposition method (EPD). The antibacterial activity of graphene-coated samples was investigated against two strains of gram-positive (S.Aureus) and gram-negative (E.Coli) bacterias by classic colony counting method, live/dead fluorescent microscopy and scanning electron microscopy (SEM) techniques. Here, bare nitinol substrate was chosen as a control sample.The results showed that the coatings exhibited stronger antibacterial activity against E. coli bacteria with thin membrane than S. aureus bacteria with thick membrane. Furthermore, the antibacterial test suggested that oxidative stress mechanism is the main factor of antibacterial activity of EPD prepared graphene coatings.