Mg Biodegradation Mechanism Deduced from the Local Surface Environment under Simulated Physiological Conditions

Abstract

Although certified magnesium-based implants are launched some years ago, the not well-defined Mg degradation mechanism under physiological conditions makes it difficult to standardize its use as a degradable biomaterial for a wide range of implant applications. Among other variables influencing the Mg degradation mechanism, monitoring the pH in the corrosive solution and, especially, at the corroding interface is important due to its direct relation with the formation and stability of the degradation products layer. The interface pH (pH at the Mg/solution interface) developed on Mg-2Ag and E11 alloys are studied in situ during immersion under dynamic conditions (1.5 mL min -1 ) in HBSS with and without the physiological amount of Ca 2+ cations (2.5 × 10 -3 m). The results show that the precipitation/dissolution of amorphous phosphate-containing phases, that can be associated with apatitic calcium-phosphates Ca 10-x (PO 4 ) 6-x (HPO 4 or CO 3 ) x (OH or ½ CO 3 ) 2-x with 0 ≤ x ≤ 2 (Ap-CaP), promoted in the presence of Ca 2+ generates an effective local pH buffering system at the surface. Thus, high alkalinization is prevented, and the interface pH is stabilized in the range of 7.6 to 8.5.

Morphological and Surface Potential Characterization of Protein Nanobiofilm Formation on Magnesium Alloy Oxide: Their Role in Biodegradation

“…These include metal ion release, 14 adsorption of inorganic species (Ca 2+ , HPO 4 2– , H 2 PO 4 – , and HCO 3 – ), 15 , 16 and formation of complex corrosion products. 17 , 18 Mg and Mg alloys are promising materials for either macro- or miniscale medical devices 19 , 20 because they have superior biocompatibility 21 and bioresorbability or biodegradability, 22 , 23 alongside appropriate mechanical properties. 24 Nevertheless, these metals interact with protein molecules and require special attention when considering their corrosion and biodegradation behavior.…”

“…The adsorption of proteins onto Mg and its alloys is a complicated process because multichemical and electrochemical processes occur simultaneously. These include metal ion release, adsorption of inorganic species (Ca 2+ , HPO 4 2– , H 2 PO 4 – , and HCO 3 – ), , and formation of complex corrosion products. , Mg and Mg alloys are promising materials for either macro- or miniscale medical devices , because they have superior biocompatibility and bioresorbability or biodegradability, , alongside appropriate mechanical properties . Nevertheless, these metals interact with protein molecules and require special attention when considering their corrosion and biodegradation behavior. , It is reported that depending on their type, proteins can inhibit or accelerate (detrimental impact) Mg degradation processes .…”

Corrosion Behavior in Magnesium-Based Alloys for Biomedical Applications

“…[ 59 , 64 , 65 ]. These compounds with low solubility product constant (K sp ) precipitate as insoluble in the corrosion layer [ 66 , 67 , 68 , 69 ], which is beneficial for slowing the corrosion rate [ 59 , 70 ] and stabilizing the local pH [ 55 , 56 , 71 ]. However, in this process, the hydroxide film can gradually become unstable [ 72 ], facilitating the penetration of Cl − ions in this quasi-protective film [ 73 , 74 , 75 ].…”

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys—A Review: Part III—Corrosion Inhibitors and Combining Them with Other Protection Strategies

“…Multiple reports refer to a strong corrosion inhibition effect when Ca 2+ is combined with anions that form sparingly soluble precipitates in an alkaline environment, which is immediately (within the first minute of immersion) generated on Mg interface upon contact with aqueous electrolytes. These ions include phosphates (H 2 PO 4 − /HPO 4 2− /PO 4 3− ), carbonates (HCO 3 − and CO 3 2− ), and fluorides, F − [ 24 , 25 , 50 , 55 , 56 ]. Under the presence of Ca 2+ cations, the precipitation of several Ca-containing phases such as hydroxyapatite (Ca 5 (PO 4 ) 3 OH), CaMg(CO 3 ) 2 , and CaCO 3 occurs on magnesium surface, prompted by the alkalinization front due to cathodic HER and ORR.…”