The metals and their alloys

Since their first introduction in the early 2000s, there have been three groups of metallic materials studied as biodegradable metals: pure metals, metal alloys, and metal matrix composites. Iron, magnesium, and zinc are the three types of metal and their alloys that have been studied so far. Nonetheless, every element in a biodegradable metal implant must not cause adverse effect to the healing process and the body system. The most recent development of these three metals is described in the following paragraphs. More detailed discussion can be found in a recent review by Zheng et al. [7].

Iron is one of the essential elements in the human body that is involved in oxygen transport and is a component of metalloprotein with a daily allowance level up to 20 mg [9,10]. Iron has attractive mechanical properties approaching those of stainless steels, which makes it suitable for implants that require high strength such as coronary stents [6,11]. However, iron degrades very slowly in in vivo setting due to the formation of a complex iron phosphate layer [12,13]. Attempts to accelerate its degradation rate have been done by alloying with elements such as manganese and palladium [14], by implanting silver using a vapor vacuum arc technique [15], and by making composites with its oxides or bioceramics [16,17], resulting in a little improvement. A different direction has been taken by utilizing iron in its porous structure for potential scaffold applications [18,19] and is recently enhanced by creating a nanoporous surface via a dealloying zinc-diffused process on Fe-Mn alloys [20].

Magnesium is an essential element with a high daily allowance level up to 700 mg, and it acts as protein coregulator and cofactor in more than 300 enzymatic reactions [21,22]. Magnesium is a lightweight metal with a density of 1.74 g/cm3 and has a low elastic modulus of 45 GPa, near to that of bone (~30 GPa), which makes it very attractive for bone applications [23]. The main challenges with magnesium are its relatively low strength and ductility, and rapid corrosion rate producing hydrogen gas in physiological conditions [24,25]. Improvements have been made by purifying the metal from detrimental trace elements like iron, copper, and nickel [26]; by alloying with calcium, zinc, zirconium, and rare earth elements [27-29]; or by changing the microstructure to nanocrystalline and amorphous state [30]. Other methods for delaying its corrosion include surface coating with bioceramics and biopolymers [31,32].

Zinc has been proposed as a biodegradable metal, considering its essential role in many enzymes and in cell metabolic activity, proliferation, and differentiation [33]. It has a medium level of daily allowance at 15 mg, and possible neurotoxicity occurs if it is present in excess [34]. The cytocompatibility of zinc and its alloy was studied in view of its applications for bone and vascular implants [35,36]. Pure zinc has a less attractive mechanical property, but it degrades at a rate between magnesium and iron [37]. Alloying zinc with a low amount of magnesium (up to 3 wt%) improved mechanical properties to the level of some magnesium alloys and were further enhanced with severe plastic deformation processing techniques [38,39].

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