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Abstract:
In this research, the effect of the presence of living cells (SaOS-2) on in vitro degradation of Mg-2.0Zn-0.98Mn (ZM21) magnesium alloy was examined by two methods simple immersion/cell culture tests and electrochemical measurements (electrochemical impedance spectroscopy and potentiodynamic polarization) under cell culture conditions. In immersion/cell culture tests, when SaOS-2 cells were cultured on ZM21 samples, pH of cell culture medium decreased, therefore weight loss and Mg2+ ion release from the samples increased. Electrochemical measurements revealed the presence of living cells increased corrosion rate (Icorr) and decreased polarization resistance (Rp) after 48 h of incubation. This acceleration of ZM21 corrosion can mainly be attributed to the decrease of medium pH due to cellular metabolic activities.

Keywords:
biodegradable metals, biomaterials, electrochemical impedance spectroscopy, immersion, cell culture condition

Abstract:
Four kinds of biodegradable polymers were employed to prepare bioresorbable coatings on Mg-2.0Zn-0.98Mn (ZM21) alloy to understand the relationship between polymer characteristics, protective effects on substrate corrosion, cytocompatibility and cell functionality. Poly-l-lactide (PLLA), poly(3-hydroxybutyrate) (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) or poly(lactic-co-glycolic) acid (PLGA) was spin-coated on ZM21, obtaining a smooth, non-porous coating less than 0.5 μm in thickness. Polymer coating characterization, a degradation study, and biocompatibility evaluations were performed. After 4 w of immersion into cell culture medium, degradation of PLGA and PLLA coatings were confirmed by ATR-FTIR observation. The coatings of PLLA, PHB and PHBV, which have lower water permeability and slower degradation than PLGA, provide better suppression of initial ZM21 degradation and faster promotion of human osteosarcoma cell growth and differentiation.

Keywords:
Biodegradable metal, Magnesium alloy, Biodegradable polymer, SaOS-2 differentiation, Calcification

Abstract:
In vitro degradation behavior of squeeze cast (CAST) and equal channel angular pressed (ECAP) ZM21 magnesium alloy (2.0 wt% Zn-0.98 wt% Mn) was studied using immersion tests up to 4 w in three different biological environments. Hanks' Balanced Salt Solution (Hanks), Earle's Balanced Salt Solution (Earle) and Eagle minimum essential medium supplemented with 10% (v/v) fetal bovine serum (E-MEM + 10% FBS) were used to investigate the effect of carbonate buffer system, organic compounds and material processing on the degradation behavior of the ZM21 alloy samples. Corrosion rate of the samples was evaluated by their Mg2 + ion release, weight loss and volume loss. In the first 24 h, the corrosion rate sequence of the CAST samples was as following: Hanks > E-MEM + 10% FBS > Earle. However, in longer immersion periods, the corrosion rate sequence was Earle > E-MEM + 10% FBS ≥ Hanks. Strong buffering effect provided by carbonate buffer system helped to maintain the pH avoiding drastic increase of the corrosion rate of ZM21 in the initial stage of immersion. Organic compounds also contributed to maintain the pH of the fluid. Moreover, they adsorbed on the sample surface and formed an additional barrier on the insoluble salt layer, which was effective to retard the corrosion of CAST samples. In case of ECAP, however, this effect was overcome by the occurrence of strong localized corrosion due to the lower pH of the medium. Corrosion of ECAP samples was much greater than that of CAST, especially in Hanks, due to higher sensitivity of ECAP to localized corrosion and the presence of Cl−.

The present work demonstrates the importance of using an appropriate solution for a reliable estimation of the degradation rate of Mg-base degradable implants in biological environments, and concludes that the most appropriate solution for this purpose is E-MEM + 10% FBS, which has the closest chemical composition to human blood plasma.

Keywords:
ZM21 magnesium alloy, ECAP, Simulated body fluids, In vitro degradationBicomponent nanofibers, Biodegradation, Biopolymer

Abstract:
Application of a biodegradable polymer coating is one of the methods to improve the initial corrosion resistance and cytocompatibility of magnesium (Mg) alloys. However, bulging of the coating film during long term immersion has been reported. Therefore, improvement of interface strength between the coating and the substrate surface is a key for the success of this method. Combination of surface modification [silanization with 3- (glycidyloxypropyl) triethoxysilane (GPTES)] and biodegradable polymer coating [poly-L-lactide (PLLA)] were applied to a Mg- 2.0Zn-0.98Mn (ZM21) cast alloy. Results of a cell proliferation assay show that PLLA and GPTES+PLLA coating successfully improved cell growth during 7 days of incubation and suppressed Mg2+ release after 4 days of incubation. The silanization process had no impact on suppression of corrosion. Calcification was observed on all samples after 1 week of incubation with calcification medium, but the calcified area was much larger on the GPTES+PLLA coated sample than on the uncoated sample.

Keywords:
biodegradable metal, ZM21, cytocompatibility, silane-coupling, PLLA