Current Opinion in Solid State and Materials Science
Degradable biomaterials based on magnesium corrosion
Introduction
Biodegradable metals are breaking the current paradigm in biomaterial science to develop only corrosion resistant metals. In particular, metals which consist of trace elements existing in the human body are promising candidates for this approach. The purpose of biodegradable implants and coatings is to support tissue regeneration and healing in a specific application by material degradation and concurrent implant replacement through the surrounding tissue. Biodegradable metals have an advantage over existing biodegradable materials such as polymers, ceramics or bioactive glasses in load bearing applications that require a higher tensile strength and a Young’s modulus that is closer to bone [1] (Table 1).
In this review we will focus on biodegradable magnesium and its alloys. Preliminary and most recent advances will be reviewed. Magnesium and its alloys are generally known to degrade in aqueous environments via an electrochemical reaction (corrosion) which produces magnesium hydroxide and hydrogen gas. Thus, magnesium corrosion is relatively insensitive to various oxygen concentrations in aqueous solutions which occur around implants in different anatomical locations. The overall corrosion reaction of magnesium in aqueous environments is given below:This overall reaction may include the following partial reactions:Magnesium hydroxide accumulates on the underlying magnesium matrix as a corrosion protective layer in water, but when the chloride concentration in the corrosive environment rises above 30 mmol/l [2], magnesium hydroxide starts to convert into highly soluble magnesium chloride. Therefore, severe pitting corrosion can be observed on magnesium alloys in vivo where the chloride content of the body fluid is about 150 mmol/l [3], [4], [5]. In magnesium and its alloys, elements (impurities) and cathodic sites with low hydrogen overpotential facilitate hydrogen evolution [6], thus causing substantial galvanic corrosion rates and potential local gas cavities in vivo. The corrosion morphology of magnesium and its alloys depends on the alloy chemistry and the environmental conditions [4], [6]. Currently investigated magnesium alloys were obtained off-the-shelf, purchasable standard alloys or alloys which can be easily cast.
As discussed in the field of biodegradable materials, there is at least a two-way relationship between the material and the biological host response i.e. the degradation process or the corrosion products can induce local inflammation and the products of inflammation can enhance the degradation process. The complexity of this relationship is generally unknown for biodegradable metals, even though first results have shown that fast corroding magnesium alloys respond with a mild foreign body reaction [7].
The major recent advances in magnesium alloys as temporary biomaterials have been in understanding the interface and interaction of magnesium alloys and their biological environment. In contrast to previous technical alloy developments aiming on the improvement of mechanical properties, corrosion resistance and production costs, the main focus is shifting to the influence of the alloying elements on the formation of the corrosion protective interfaces and on the surrounding biological environment in vitro and in vivo. However, currently available magnesium alloys were investigated in different biomedical applications. Indisputably the most advanced clinical applications are biodegradable cardiovascular magnesium stents which have been successfully investigated in animals [8], [9] and first clinical human trials have been conducted [10], [11], [12]. Magnesium alloys were also investigated as bone implants [3], [4], [13] and can be applied in various designs e.g. as screws, plates or other fixture devices. Magnesium chips have been investigated for vertebral fusion in spinal surgery of sheep [14] and open-porous scaffolds made of magnesium alloys have been introduced as load bearing biomaterials for tissue engineering [7], [15], [16], [17]. However, high extracellular magnesium concentrations have been found beneficial for cartilage tissue engineering [18].
Section snippets
Magnesium alloys
The magnesium alloys currently under investigation as implant materials are mostly commercial alloys which have been developed for the needs in transportation industry [19]. The designation system of magnesium alloys is generally following the nomenclature of the American Society for Testing and Materials (ASTM) [20], [21] and uses a typical letter-figure combination (Table 2). The magnesium alloys can be divided into three major groups: pure magnesium (Mg) with traces of other elements,
Effect of the solution and organic content
Many authors performed systematic corrosion studies on magnesium alloys with different corrosion media (± proteins) [23], [24], [26]. The composition of the corrosive medium influenced the magnesium corrosion behavior, which was additionally altered by the presence or absence of proteins. Proteins such as albumin have been demonstrated to form a corrosion blocking layer on the magnesium alloys in in vitro experiments [23], [26], [34], [63]. This layer is enriched by calcium phosphates in vitro [26]
How to choose the right magnesium alloy?
Current investigated magnesium alloys are used “off-the-shelf” or are known for their properties in technical applications. The empirical approach in biodegradable stent development lead to magnesium alloys containing rare earth elements. This approach seems to be obviously one successful way to obtain usable implant materials [11], [12]. Most rare earth elements show a beneficial effect on magnesium corrosion in vivo [4]. However, the rare earth elements are used as mischmetal in alloy
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