In-vitro dissolution of magnesium-calcium binary alloys: clarifying the unique role of calcium additions in bioresorbable magnesium implant alloys.

A systematic investigation of a series of magnesium-calcium binary alloys is presented to reveal the influence of increasing calcium (Ca) additions on the in vitro degradation of magnesium (Mg). Because of its prevalence in structural tissues, Ca is among the most biologically viable additions to orthopedic-intended Mg-based biomaterials. Hence, a fundamental electrochemical study of Ca additions to Mg biomaterials is essential to its continued role as an alloying addition. In this work, in vitro degradation conditions closer to the physiological environment were implemented through the addition of proteins to simulated body fluid and maintenance of a constant pH, with tests conducted using Hanks solution, minimum essential medium (MEM), and MEM containing fetal bovine serum. Alloying with Ca leads to the formation of Mg2Ca intermetallic particles that result in systematically enhanced dissolution kinetics. This observation is rationalized via microelectrochemical tests upon the Mg2Ca intermetallic in isolation, which reveals rapid anodic kinetics. Hence, the extent of Mg-Ca alloy dissolution can be modified depending on the amount of Mg2Ca present, suggesting that Ca can be deployed as a functional addition capable of not only enhancing biodissolution of the alloy, but being able to do this in a systematic, controllable manner depending on its volume fraction. In addition, up to a 3-fold reduction in the corrosion rate is observed with corrosion testing in an albumin-containing medium when compared to Hanks solution, the results highlighting that the use of a physiologically "correct" medium is essential for the in vitro screening of Mg-based alloys suitable for orthopaedic applications.