ICMCTF 2023 Session D1-1-MoM: Surface Coatings and Surface Modifications in Biological Environments I

Magnesium is a promising metallic material for the manufacture of bioabsorbable orthopedic implants since in addition to being biodegradable and biocompatible, it has mechanical properties like to those of human bone. However, despite these properties, its poor resistance corrosion resistance has prevented its technological application. A solution to improve its corrosion resistance is to use a surface treatment, such as that provided by plasma electrolytic oxidation (PEO). This technique is used to apply ceramic-type coatings on metals and is widely used in biomedical applications. Therefore, in this work, we carried out a study on the effects generated by the square wave frequency of the pulsed DC mode on the surface properties of pure magnesium sheet by applying TiO₂-based coatings using the PEO technique.Anatase-TiO₂ powders were incorporated into the electrolytic solution due to their good biocompatibility and high corrosion resistance. Coatings were developed using 0.210 g of TiO₂, a current density of 200mA/cm², two frequencies of 2 Hz and 10 Hz were evaluated. Data acquisition of voltage transients in the PEO processes was carried out, showing that, in the first stage of the PEO process, as the frequency increases, there is a rapid growth of the voltage, indicating rapid passivation of the magnesium surface. A transversal section of the coatings was observed by optical microscope at 400x, and images showed three regions: the substrate, the substrate-coating interface, and the bulk of the coating. Through X-ray diffraction analysis (XRD), the presence of TiO₂ and MgO into the coatings was identified as a result of the PEO technique. The potentiodynamic polarization curves (Tafel) were employed to investigate the corrosion resistance of a pure Mg substrate and the coated ones, in Hanks’ balanced salts. Preliminary results show that Icorr is reduced from 1.471 μA to 454.127 nA, indicating an improvement on the corrosion resistance.

Keywords: Magnesium, corrosion, PEO, surface treatment.

This research focuses on the synthesis and characterization of bio-inspired artificial antimicrobial surfaces. The antimicrobial surfaces studied in this research are surfaces that are covered with nanoscale protrusions, which are observed in nature such as cicada wings and dragonfly wings [1]. These nanoscale protrusions, mostly nanopillars and nanocones, achieve antimicrobial by mechano-bactericidal: the bacteria are deformed by being punctured into the protrusions and eventually killed [2]. Even though research groups have published papers on mechano-bactericidal surfaces [1, 3, 4], there are still some challenges in this research: 1) the synthesis, scaling-up and fine control of the artificial nanoscale protrusions are still challenging; 2) the mechano-bactericidal mechanism is not universal. The challenge of the synthesis comes from the fact that the three dimensional protrusion features for puncturing the bacteria have to be much smaller than bacteria, as small as only a couple nanometers. The proper spacing of the protrusion features is also required to allow the deformation of the bacteria. However, the sub-100 nm, high aspect-ratio, three-dimensional (3D) features are challenging to obtain through traditional nanofabrication methods, especially by top-down nanofabrication techniques.

Our research group proposed to use glancing angle deposition (GLAD) to recreate cicada-wing-mimicry antimicrobial surfaces [5]. GLAD is a bottom-up process for achieving complex 3D nanofeatures. The use of seeds alters the distribution of the features, and hexagonally packed nanofeature arrays are recreated. However, the requirement of the nanosphere seeds adds complexity to the process: the preparation of a monolayer of nanospheres is challenging, and the area of the seeds can be limited if seeding is not properly conducted. In the current research, we discuss the possibility of synthesizing the mechano-bactericidal antimicrobial surfaces by GLAD without predetermined seeds. The design and control of the process for synthesizing the antimicrobial surfaces are addressed in the study. Multiple materials are used for creating nanoscale protrusions for uncovering the mechano-bactericidal mechanism. The characterization of the surfaces, including the morphology, the superhydrophobicity, and the effectiveness of the antimicrobial property are presented. Finally, potential integration methods of the surfaces with nanoscale protrusions and biomedical implants are discussed in the study.

Surface modification of biomaterials holds the potential to enhance the performance and lifespan of existing metallic implantable devices by utilizing metal oxide coatings. These coatings can effectively augment biocompatibility and various other biologically relevant characteristics, including biocorrosion resistance, of the metallic implants. Metal oxide coatings offer significant potential as a surface modification option for metallic implants due to their inherent advantages, including corrosion resistance, biologically inert nature, notable hardness, and natural formation on the implants.

This study examines various aspects of the response exhibited by thin films of TiO2 and ZrO₂ deposited on diverse substrates, encompassing crystalline pure Ti (cp-Ti), stainless steel (SS), Magnesium alloy (ZX11), and Si (100) substrates. The deposition of the thin films was accomplished using Magnetron Sputtering. The evaluation initially focused on distinguishing the effect of the film structure, leading to the conclusion that the nanocrystalline films facilitate osteogenic differentiation and reduce bacterial adhesion. One potential explanation for this observation is the enhanced protection against metallic corrosion provided by the amorphous films. However, it should be noted that the magnitude of this effect varies depending on the specific substrate used.

To further investigate the disparities between TiO₂ and ZrO₂ when deposited on the ZX11 alloy, the films were deposited on a non-corrosive substrate. This approach aimed to determine whether notable distinctions could be observed between the two materials. The results indicate that both TiO₂ and ZrO₂ exhibit similar rates of cell proliferation and promote osteogenic differentiation in a biological context. However, it was observed that the osteogenic differentiation process occurred slightly faster on ZrO₂ compared to TiO₂.

Under passive electrochemical conditions, both TiO₂ and ZrO₂ demonstrate similar behavior. However, due to the broader range of valency states available for Ti, it is more inclined to engage in electron transfer processes compared to Zr.