Victoria Wagener

Functionalization of metallic magnesium with protein layers via linker molecules.

We present an innovative method to cover pure magnesium with protein monolayers by utilizing the OH termination of the oxide surface and silane coupling chemistry. The protein of interest was albumin. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) were used to monitor the success of the treatment. The attachment of proteins via linker groups yielded smoother and more homogeneous surfaces than coatings produced by steeping magnesium in protein solution. A positive effect on the corrosion behavior of pure magnesium was also observed.

Albumin coating on magnesium via linker molecules—Comparing different coating mechanisms

As magnesium is a non-toxic and biodegradable metal, it is gaining more and more interest in the biomedical sector. The biodegradability is due to the corrosion of Mg in aqueous, chloride containing environment, as it is present in the body. However, corrosion of pure magnesium occurs too fast and takes place inhomogeneously on the metal surface. Moreover, Mg dissolution is connected with strong hydrogen evolution. Therefore alloying and/or coating of magnesium seem to be promising approaches to slow down corrosion and in return hydrogen evolution. This study explores coating of Mg with albumin via three different linker molecules, aminopropyl-triethoxysilane (APTES) plus ascorbic acid (VitC), carbonyldiimidazole (CDI) and stearic acid (SA). The metal samples were first passivated and after the pre-coating with the linker molecules the protein coating took place by soaking the pre-treated samples in an aqueous albumin solution. The immersion time was varied from 0.25 h up to 24 h. The success and the quality of the different coatings were documented by X-ray Photoelectron Spectroscopy (XPS) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Pre-coatings were additionally characterized by contact angle and surface roughness measurements. Electrochemical measurements were carried out in simulated body fluid (SBF) to characterize the coatings in view of their corrosion behavior. Albumin coatings could be produced with every linker molecule investigated. Certain protective effects were observed already for linker SAM coated Mg, overall the systems CDI-BSA and SA-BSA showed the best corrosion resistances.

Protective layer formation on magnesium in cell culture medium.

In the past, different studies showed that hydroxyapatite (HA) or similar calcium phosphates can be precipitated on Mg during immersion in simulated body fluids. However, at the same time, in most cases a dark grey or black layer is built under the white HA crystals. This layer seems to consist as well of calcium phosphates. Until now, neither the morphology nor its influence on Mg corrosion have been investigated in detail. In this work commercially pure magnesium (cp) was immersed in cell culture medium for one, three and five days at room temperature and in the incubator (37 °C, 5% CO2). In addition, the influence of proteins on the formation of a corrosion layer was investigated by adding 20% of fetal calf serum (FCS) to the cell culture medium in the incubator. In order to analyze the formed layers, SEM images of cross sections, X-ray Photoelectron Spectroscopy (XPS), X-ray diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX) and Fourier Transformed Infrared Spectroscopy (FTIR) measurements were carried out. Characterization of the corrosion behavior was achieved by electrochemical impedance spectroscopy (EIS) and by potentio-dynamic polarization in Dulbeccos Modified Eagles Medium (DMEM) at 37°C. Surface analysis showed that all formed layers consist mainly of amorphous calcium phosphate compounds. For the immersion at room temperature the Ca/P ratio indicates the formation of HA, while in the incubator probably pre-stages to HA are formed. The different immersion conditions lead to a variation in layer thicknesses. However, electrochemical characterization shows that the layer thickness does not influence the corrosion resistance of magnesium. The main influencing factor for the corrosion behavior is the layer morphology. Thus, immersion at room temperature leads to the highest corrosion protection due to the formation of a compact outer layer. Layers formed in the incubator show much worse performances due to completely porous structures. The existence of proteins in DMEM seems to hinder the formation of a corrosion layer. However, protein adsorption leads to similar results as concerns corrosion protection as the formed calcium phosphate layer.

Cell Adhesion on Surface-Functionalized Magnesium

The biocompatibility of commercially pure magnesium-based (cp Mg) biodegradable implants is compromised of strong hydrogen evolution and surface alkalization due to high initial corrosion rates of cp Mg in the physiological environment. To mitigate this problem, the addition of corrosion-retarding alloying elements or coating of implant surfaces has been suggested. In the following work, we explored the effect of organic coatings on long-term cell growth. cp Mg was coated with aminopropyltriehtoxysilane + vitamin C (AV), carbonyldiimidazole (CDI), or stearic acid (SA). All three coatings have been previously suggested to reduce initial corrosion and to enhance protein adsorption and hence cell adhesion on magnesium surfaces. Endothelial cells (DH1+/+) and osteosarcoma cells (MG63) were cultured on coated samples for up to 20 days. To quantify Mg corrosion, electrochemical impedance spectroscopy (EIS) was measured after 1, 3, and 5 days of cell culture. We also investigated the speed of initial cell spreading after seeding using fluorescently labeled fibroblasts (NIH/3T3). Hydrogen evolution after contact with cell culture medium was markedly decreased on AV- and SA-coated Mg compared to uncoated Mg. These coatings also showed improved cell adhesion and spreading after 24 h of culture comparable to tissue-treated plastic surfaces. On AV-coated cp Mg, a confluent layer of endothelial cells formed after 5 days and remained intact for up to 20 days. Together, these data demonstrate that surface coating with AV is a viable strategy for improving long-term biocompatibility of cp Mg-based implants. EIS measurements confirmed that the presence of a confluent cell layer increased the corrosion resistance.

Interface Chemistry and Molecular Bonding of Functional Ethoxysilane-Based Self-Assembled Monolayers on Magnesium Surfaces

The modification of magnesium implants with functional organic molecules is important for increasing the biological acceptance and for reducing the corrosion rate of the implant. In this work, we evaluated by a combined experimental and theoretical approach the adsorption strength and geometry of a functional self-assembled monolayer (SAM) of hydrolyzed (3-aminopropyl)triethoxysilane (APTES) molecules on a model magnesium implant surface. In time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS), only a minor amount of reverse attachment was observed. Substrate-O-Si signals could be detected, as well as other characteristic APTES fragments. The stability of the SAM upon heating in UHV was investigated additionally. Density-functional theory (DFT) calculations were used to explore the preferred binding mode and the most favorable binding configuration of the hydrolyzed APTES molecules on the hydroxylated magnesium substrate. Attachment of the molecules via hydrogen bonding or covalent bond formation via single or multiple condensation reactions were considered. The impact of the experimental conditions and the water concentration in the solvent on the thermodynamic stability of possible APTES binding modes is analyzed as a function of the water chemical potential of the environment. Finally, the influence of van der Waals contributions to the adsorption energy will be discussed.

Influence of Electrolyte Composition (Simulated Body Fluid vs. Dulbecco’s Modified Eagle’s Medium), Temperature, and Solution Flow on the Biocorrosion Behavior of Commercially Pure Mg

Magnesium (Mg) and Mg-based alloys are promising materials for biodegradable implant applications. Electrolyte composition, temperature, and dynamic conditions can differ strongly for in vitro and ...