Characterization of the Micro-Arc Coatings Containing β-Tricalcium Phosphate Particles on Mg-0.8Ca Alloy

Potential bioactive coating system for high-performance absorbable magnesium bone implants

Magnesium alloys are considered the most suitable absorbable metals for bone fracture fixation implants. The main challenge in absorbable magnesium alloys is their high corrosion/degradation rate that needs to be controlled. Various coatings have been applied to magnesium alloys to slow down their corrosion rates to match their corrosion rate to the regeneration rate of the bone fracture. In this review, a bioactive coating is proposed to slow down the corrosion rate of magnesium alloys and accelerate the bone fracture healing process. The main aim of the bioactive coatings is to enhance the direct attachment of living tissues and thereby facilitate osteoconduction. Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are six bioactive agents that show high potential for developing a bioactive coating system for high-performance absorbable magnesium bone implants. In addition to coating, the substrate itself can be made bioactive by alloying magnesium with calcium, zinc, copper, and manganese that were found to promote bone regeneration.

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Bioactive-coated magnesium implant could accelerate bone fracture healing time to match with magnesium degradation.

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Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are high potential bioactive coating materials.

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The incorporation of Ca, Zn, Cu, Sr, and Mn in Mg base-metal could further enhance bone formation.

Surface Modification of Biomedical MgCa4.5 and MgCa4.5Gd0.5 Alloys by Micro-Arc Oxidation

The aim of this work was to characterize the structure and corrosion properties of the MgCa_4.5(Gd_0.5) alloys surface treated by the micro-arc oxidation (MAO) process. The MgCa_4.5 and MgCa_4.5Gd_0.5 alloy samples were processed by MAO in an electrolyte composed of NaOH (10 g/dm³), NaF (10 g/dm³), NaH₂PO₄ (5 g/dm³), Na₂SiO₂·5H₂O (10 g/dm³) and water. Two different voltages (120 V and 140 V) were used in the MAO process. The alloys protected by an oxide layer formed in the MAO were then the subject of corrosion resistance tests in an environment simulating the human body (Ringer’s solution). After the experiments, the resulting samples were investigated using SEM, XPS and EDS techniques. The addition of Gd affected the fragmentation of the coating structure, thereby increasing the specific surface; higher voltages during the MAO process increased the number and size of surface pores. Corrosion tests showed that the MgCa_4.5Gd_0.5 alloys were characterized by low polarization resistances and high corrosion current densities. The studies indicated the disadvantageous influence of gadolinium on the corrosion resistance of MgCa_4.5 alloys. The immersion tests confirmed lower corrosion resistance of MgCa_4.5Gd_0.5 alloys compared to the referenced MgCa_4.5 ones. The MgCa_4.5 alloy with the MAO coating established at voltage 140 V demonstrated the best anticorrosion properties.

Tailoring the Surface Morphology and the Crystallinity State of Cu- and Zn-Substituted Hydroxyapatites on Ti and Mg-Based Alloys

Titanium-based alloys are known as a “gold standard” in the field of implantable devices. Mg-based alloys, in turn, are very promising biocompatible material for biodegradable, temporary implants. However, the clinical application of Mg-based alloys is currently limited due to the rapid resorption rate in the human body. The deposition of a barrier layer in the form of bioactive calcium phosphate coating is proposed to decelerate Mg-based alloys resorption. The dissolution rate of calcium phosphates is strongly affected by their crystallinity and structure. The structure of antibacterial Cu- and Zn-substituted hydroxyapatite deposited by an radiofrequency (RF) magnetron sputtering on Ti and Mg–Ca substrates is tailored by post-deposition heat treatment and deposition at increased substrate temperatures. It is established that upon an increase in heat treatment temperature mean crystallite size decreases from 47 ± 17 to 13 ± 9 nm. The character of the crystalline structure is not only governed by the temperature itself but relies on the condition such as either post-deposition treatment, where an amorphous calcium phosphate undergoes crystallization or instantaneous crystalline coating growth during deposition on the hot substrate. A higher treatment temperature at 700 °C results in local coating micro-cracking and induced defects, while the temperature of 400–450 °C resulted in the formation of dense, void-free structure.

Effect of the Porosity, Roughness, Wettability, and Charge of Micro-Arc Coatings on the Efficiency of Doxorubicin Delivery and Suppression of Cancer Cells

Porous calcium phosphate coatings were formed by the micro-arc oxidation method on the surface of titanium for the loading and controlled release of the anticancer drug doxorubicin. The coatings’ morphology and microstructure were examined by scanning electron microscopy. The phase composition was determined with the help of X-ray diffraction analysis. Studies of the hydrophilic properties of the coatings and their zeta potential were carried out. Data on the kinetics of doxorubicin adsorption-desorption were obtained. In addition, the effect of calcium phosphate coatings impregnated with doxorubicin on the viability of the Neuro-2a cell line was revealed. The coating formed at low voltages of 200–250 V contained a greater number of branched communicating pores, and therefore they were able to adsorb a greater amount of doxorubicin. The surface charge also contributes to the process of the adsorption-desorption of doxorubicin, but this effect is not fully understood and further studies are required to identify it.

Comparative Study of the Structure, Properties, and Corrosion Behavior of Sr-Containing Biocoatings on Mg0.8Ca

A comparative analysis of the structure, properties and the corrosion behavior of the micro-arc coatings based on Sr-substituted hydroxyapatite (Sr-HA) and Sr-substituted tricalcium phosphate (Sr-TCP) deposited on Mg0.8Ca alloy substrates was performed. The current density during the formation of the Sr-HA coatings was higher than that for the Sr-TCP coatings. As a result, the Sr-HA coatings were thicker and had a greater surface roughness R_a than the Sr-TCP coatings. In addition, pore sizes of the Sr-HA were almost two times larger. The ratio (Ca + Sr + Mg)/P were equal 1.64 and 1.47 for Sr-HA and Sr-TCP coatings, respectively. Thus, it can be assumed that the composition of Sr-HA and Sr-TCP coatings was predominantly presented by (Sr,Mg)-substituted hydroxyapatite and (Sr,Mg)-substituted tricalcium phosphate. However, the average content of Sr was approximately the same for both types of the coatings and was equal to 1.8 at.%. The Sr-HA coatings were less soluble and had higher corrosion resistance than the Sr-TCP coatings. Cytotoxic tests in vitro demonstrated a higher cell viability after cultivation with extracts of the Sr-HA coatings.

Modification of titanium surface via Ag-, Sr- and Si-containing micro-arc calcium phosphate coating

The current research is devoted to the study of the modification of the titanium implants by the micro-arc oxidation with bioactive calcium phosphate coatings containing Ag or Sr and Si elements. The coatings' microstructure, phase composition, morphology, physicochemical and biological properties were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Ag-containing and Sr-Si-incorporated coatings were formed in alkaline and acid electrolytes, respectively. The formation of the coatings occurred at different ranges of the applied voltages, which led to the significant difference in the coatings properties. The trace elements Ag, Sr and Si participated intensively in the plasma-chemical reactions of the micro-arc coatings formation. Ag-containing coatings demonstrated strong antibacterial effect against Staphylococcus aureus AТСС 6538-P. MTT in vitro test with 3T3-L1 fibroblasts showed no cytotoxicity appearance on Sr-Si-incorporated coatings.

Plasma Electrolytic Oxidation of Metals and Alloys

Porous oxide layers formed on metals and alloys via Plasma Electrolytic Oxidation (PEO) have been developed and used for decades in medicine and for technical purposes. [...]

Degradation Behavior of Micro-Arc Oxidized ZK60 Magnesium Alloy in a Simulated Body Fluid

Bio-ceramic coatings were synthesized on ZK60 magnesium alloys by micro-arc oxidation (MAO). The degradation behavior of the ZK60 alloys with and without MAO coating in the simulated body fluid (SBF) was studied. The samples were characterized by means of scanning electron microscopy (SEM), laser scanning confocal microscopy (CLSM), and X-ray diffraction (XRD). Electrochemical impedance spectroscopy (EIS) was used to study the degradation behavior. The results showed that the porous MAO coating mainly consisted of MgO, Mg2SiO4, Mg3(PO4)2, and CaCO3. The pH values of both coated and uncoated samples increased over time. However, the pH values of the SBF for coated samples always maintained a lower level compared with those for the uncoated samples. Thereby, the coated samples showed a much lower degradation rate. After immersion in SBF for 5 days, corrosion product containing Ca and P was found on both samples, while the deposition was more active on the coated samples. The degradation models for the uncoated and coated samples in the SBF are also proposed and discussed.