ICMCTF2019 Session D1-2-MoA: Surface Coating and Modification for Use in Biological Environments II

Recently, gallium ions have been studied for its multi-functional bio-activity. While gallium has no known function in human physiology, the chemical properties that it shares with iron allow it to bind to iron-containing proteins (Trojan Horse effect), including the iron transport protein transferrin. Thus, malignant cells and microorganisms may be tricked into incorporating gallium in place of iron for iron-dependent processes essential for bacterial viability and growth. Hence, rather than facilitate iron-dependent cellular function, gallium disrupts it. As a result, the interaction between gallium and iron-proteins can be exploited for therapeutic purposes in cancers and bactericidal treatments. Accordingly, it is of great interest to know how Ga can be doped into surface coatings.

In this study, the deposition processes of tunable TaOxNy-Ga thin film coatings were studied systematically. Following this, the resultant mechanical, structural, and, bio-related properties were examined, in terms of O/N ratio, surface roughness and Ga contents. However, due to low melting point of Ga (~29 ^oC), it is relatively difficult to sputter Ga into those oxynitride coatings. Therefore, Ga₂O₃ target was used in this study to provide oxygen and Ga. According to the results, it was learned that small amount of oxygen (2~5%) and gallium (1~3%) in the coatings could be beneficial to the improvement of mechanical and antibacterial properties. The enhanced dissolution of Ga ions was observed with the addition of oxygen. The highest hardness could reach 30 GPa, while an antibacterial efficiency of >90% could be found after 48 hrs of immersion. The films showed good biocompatibility with MG63 cells.

Nanotubular structures were produced on the c.p. Titanium surface by anodization in an aqueous electrolyte that contains carboxymethylcellulose (CMC) and NaF. The internal diameter obtained at voltages of 20, 10 and 2 V was about 100, 48 and 9.5 nm, respectively. Those diameters were measured using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Scientific reports about nanotube produced by anodization with internal diameters upper than 30 nm are very common, however, reports about lower nanotube diameters (<20 nm) are scarce. Several heat treatments at 200, 350 and 600 °C were made to produce nanotubes with different TiO₂ polymorphs (Anatase, Rutile), at 200 °C no phase change was observed, at 350 °C the nanotubes change from amorphous phase to Anatase, and at 600 °C the rutile phase was predominant. These phases were corroborated by XRD and Micro-Raman microscopy. All the tested surfaces were superhydrophilic (high surface free energy), and the superhydrophilic behavior is maintained after at least 25 days, regardless of the heat treatment. The aim of produce nanotubes with different diameters and various heat treatments was to correlate the nanotube characteristics (morphology, internal diameter, composition) and the biologic behavior (cell adhesion and proliferation and antibacterial properties). The heat treated samples showed higher antibacterial properties in contrast to the as anodized samples. All nanotube coatings of TiO₂ were non-cytotoxic, nevertheless the anodization parameter or electrolyte composition used. However, some differences in terms of cell adhesion were found. Based on those results, these coatings can be applied as drug carriers, surface modification of biomaterial devices and catalyst, among others.

Titanium oxide (TiO₂) has been recognized as the active layer responsible for the good biocompatibility and osteogenic properties of the Ti-based medical alloys used for dental and orthopedic applications. Meanwhile, magnesium (Mg) and its alloys are currently widely researched for orthopedic applications, since their mechanical properties are more adequate to balance load transfer between bone and implant, but also due to its biodegradability. Extensive mechanical, in-vitro and in-vivo studies have been done to improve the biomedical performance of Mg alloys through alloying, processing conditions and surface modifications, including coating deposition. The main purpose of such modifications is to extent the degradation rate of the alloy in order to match it with bone self-healing time. In this work, we are investigating the use of titanium oxide coatings deposited by reactive magnetron sputtering on high purity Mg alloys.

Here, we present the electrochemical response of TiO₂-coated Mg-alloys in simulated physiological fluids. The results indicate that independently of the TiO₂ film thickness (60 - 250 nm) and the use of a Ti-buffer layer, the corrosion rate of the Mg alloy is not significantly reduced. Such response is probably associated to specific chemical reactions occurring between Mg and Ti that were not expected.

Metallization of polymers has been of interest for years, as a means to achieve barrier coatings in packaging, biocompatible interfaces for medical implants, or to produce light-weight alternatives to metal parts in the automotive and aerospace industries, to cite a few examples. As the performance requirements of metalized plastics increase, so too must our understanding of plasma-polymer and metal-polymer interactions that occur in coating processes. High Power Impulse Magnetron Sputtering (HiPIMS) is a physical vapor coating method that allows one to span a whole range of energetic regimes; close to direct current sputtering, with low metallic ion discharge, up to almost fully ionized discharges. This versatile technique is our method of choice to study and tailor the substrate-film interphase region responsible for ‘good’ and long-lived coating adhesion.

In our study we vary the pulse-on/off time, thus controlling the time allowed for surface processes to occur between metal-species fluxes. We evaluate the chemical state of our interface using XPS and gauge the practical adhesion of the resulting films using a modified ASTM D4541 pull-off test. In addition, we study the effect of sample pre-treatment on adhesion, also using the above methods. The test metal-polymer system for this study is titanium on PEEK, which has shown exemplary adhesion in the case of orthopedic implants for use in spinal fusion surgery.

INTRODUCTION: Mg and its alloys possess a biodegradable nature that has recently made them attractive for developing biomedical devices that are expected to degrade and bioresorb completely in-vivo after the healing of the body tissue. In addition to their biodegradable nature, Mg alloys are biocompatible, and they have low density and modulus of elasticity, close to the bone. We have developed a patent-pending alloy, Mg-1.2Zn-0.5Ca-0.5Mn produced using biocompatible alloying elements, and a heat treatment process that is likely to provide the needed mechanical stability during bone healing and reliably resorb following the healing of a reconstructed skeletal segment. Our goal is to investigate the use of a composite ceramic-based coating to delay device corrosion (weakening) for 4-6 months.

METHODS: We heat-treated Mg-1.2Zn-0.5Ca-0.5Mn alloy samples to achieve significant mechanical strength and then coated them using plasma electrolyte oxidation (PEO) and layer-by-layer coating techniques to achieve corrosion resistance. The coated samples were characterized for their morphological and chemical properties using SEM, EDS and XRD methods. Finally, the corrosion characteristics before and after coating were determined using Potentiodynamic polarization test conducted in a simulated body fluid (SBF).

RESULTS: PEO-coated samples show pitting and pores, a common observation following PEO coating, and a tight junction with the subsequent layer-by-layer coatings. The composite coated samples showed a major enhancement in the corrosion resistance compared to the only PEO-coated and uncoated samples. The XRD patterns showed the presence of the incorporated compounds on the coating surface in addition to the presence of biocompatible corrosion products on the surface after corrosion such as hydroxyapatite (HA) and magnesium hydroxide (Mg(OH)₂). The use of heat treatment followed by composite ceramic coating may be beneficial for the fabrication of Mg-1.2Zn-0.5Ca-0.5Mn skeletal fixation devices with predictable resorption rates.