Page 83 - Magnesium-based supports for stem cell therapy of vascular disease - Mónica Echeverry Rendón

STERILIZATION PROCEDURES IMPACT PURE MAGNESIUM MODIFIED BY PLASMA ELECTROLYTIC OXIDATION
1. Introduction
During the past years, the use of magnesium (Mg) in the biomedical field has increased significantly [1–5]. Its high biocompatibility and the fact that it is degradable, rendered magnesium a promising and versatile option for tran- sient implants[6–8]. In constrast, the chemical element Mg is highly reactive and upon its oxidation causes an in- crease of pH and releases hydrogen which is cytotoxic. Upon implantation, increased pH and hydrogen may affect the stability of the implant and also of the surrounding tissue [9–11]. This problem can be managed by generation of a protective coating that decreases the rate of degradation of the base material. Plasma electrolytic oxidation is a technique in which the material is oxidized in a controlled way [12–15]. The PEO generates an oxide/hydroxide Mg layer which by itself degrades slowly and also reduces the degradation of the underlying than pure Mg. Previous research suggests that this surface modification as a versatile way to improve the corrosion properties of the Mg and currently is used in different medical devices [16–18]. Since Mg is highly reactive in aqueous solutions, it is important to assess the influence of the sterilization process on the material, in particular those that employ steam (autoclaving). The aim of this work was to show the influence of the most commonly used sterilization processes on the integrity of the PEO coatings formed on commercial pure magnesium (c.p Mg), looking to understand how this can lead changes at biological level.
2. Material and Methods 5
2.1 Anodization of samples
Square samples of c.p Mg (99.9%) of 1cm x 1 cm and a thickness of approximately 1 mm were mechanically polished with silicon carbide paper 1000 grade. All specimens were cleaned in water and then degreased with acetone in an ultrasonic bath for 30 minutes. After that, samples were anodized in a two-electrochemical cell where the c.p Mg was set as anode and a stainless steel beaker was used as cathode. The experiment was carried out under poten- tiostatic mode at 320V during 600s by using a DC power supply (Kepco BHK 500-0.4 MG). As electrolyte solution, 0.0352 M NaSiO3.9H2O/0.07 M KOH was used. The voltage data were recorded electronically by Labview 8.1 software (National Instruments) interfaced with a personal computer. After anodizing, the specimens were removed immedi- ately from the electrolyte; subsequently, they were rinsed with water and dried at room temperature. AFM analysis was performed in order to know the morphology and roughness of the obtained coating.
2.2 Sterilization protocols
Samples were sterilized by using four techniques: Autoclaving (A), dry-heat (DH), UV-irradiation (UV) and formalde- hyde (F) under the following conditions:
- Steam Autoclaving (A): 121 oC during 45 min at 1bar (CISA Autoclave Model: 4270).
- Dry-heat (DH): 170 °C during 1 hour after the desired temperature is reached
- UV-irradiation (UV): Samples were exposed to UV –light (40 W.cm-2) in a biosafety cabinet during 1 hour per side and in total 2 hours per sample.
- Steam Formaldehyde (F): 55oC, 0.35 bar during 6-7 hours. (CISA Autoclave Model: 4270).
- A set of samples without sterilization were used as a control.
2.3 Characterization of the samples
After the respective sterilization, each group of samples were observed by SEM to identify changes in the surface morphology and in the cross-sections. Changes in the composition of the coating were initially studied by EDS in which the samples are stimulated with electrons and the outgoing spectrum gives the composition and the per- centage of each element. As an additional technique, XRD was performed for analysis of the crystal structure of the material which were compared against a database of known structures
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