The present challenge of Mg alloy implants is in controlling their degradation rate in the physiological environment (Gu et al. 2010; Wang et al. 2010a, b; Janning et al. 2010; Song et al. 2010; Witte et al. 2010; Zheng et al. 2010; Lorenz et al. 2009; Zhang et al. 2009, 2010; Xin et al. 2008; Kannan and Raman 2008; Li et al. 2008; Levesque et al. 2008; Xu et al. 2008). In the body, Mg ions dissolve into solution during corrosion and bind to other ions before precipitating back onto the metal surface to form a reaction layer. Chlorides bind with Mg to form MgCl2, which is highly soluble in aqueous solutions, leaving no resistance to further corrosion. In high pH solutions, Mg(OH)2 can form on the surface creating a passivation layer protecting Mg from further corrosion (Lorenz et al. 2009). It has also been suggested that Mg-containing phosphates contribute to the good osteoconductivity of Mg alloys (Zhang et al. 2009; Ye et al. 2010; Witte et al. 2007c). Phosphate ions also play a role in protecting the corrosion by controlling the surface pH by regulating the hydrogen ion concentration. Proteins in solution also influence corrosion resistance. In addition to being dependent upon local pH and ions, the corrosion rate is also dependent on the metals that Mg is alloyed with. Manganese (Mn) and zinc (Zn) are biocompatible; aluminum has lower biocompatibility. All these alloys improve corrosion resistance, particularly when Ca2+ ions are present in the solution creating an insoluble protective phosphate layer (Song et al. 2010). Protective corrosion layers are generally not uniform and do not protect the surface in all areas. Corrosion also depends on the location of the implant (compact bone vs. cancellous bone) and the local environment (Witte et al. 2006). For moderate corrosion rates, excessive Mg cations are efficiently filtered by the kidneys and passed through the urine. Corrosion of Mg also leads to hydrogen gas evolution that can cause necrosis due to a separation of tissue layers, extended healing time, or obstructing the blood stream (Witte 2010; Hort et al. 2010; Witte et al. 2005, 2006). Hydrogen evolution is also associated with crack formation on the surface of the Mg, which reduces the strength of the alloy. Hydrogen evolution is directly related to the rate of corrosion. Therefore, if the corrosion rate can be controlled, the rate of hydrogen evolution can be kept at a safe value.
Ideally, Mg-based implants or devices should be designed in a way that Mg alloys could safely dissolve into the blood stream while cells migrate and replace the metal with host tissue. To achieve this goal, biosensors that can monitor hydrogen generation, ion concentrations, and the integrity of implants or even control the degradation rate with a responsive loop are needed.
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