## 2 Biodegradable Metals and Their Alloys  
The most basic criteria for choosing a biomedical device is from observing the compatibility response of the body tissues towards the device [1]. Hence, the most commonly-used biodegradable metals are iron, magnesium, zinc, and their alloys. Not only are these metals compatible with the human body, but are also considered essential elements for the body to function properly. The corrosion rate of iron is usually lower than that of magnesium while zinc corrodes at a rate between iron and magnesium and lastly, magnesium being the quickest to corrode as evident by [PERSON] et al. [5] where a comparison of their corrosion rates can be arranged in an ascending order with pure iron, pure zinc, and pure magnesium corroding at rates of 0.2, 0.6, and 4 mm/year respectively after immersion in a 37 \({}^{\circ}\)C saline solution for 168 h.  
Iron is considered as one of the essential elements to allow for normal function inside the human body as it plays an important role of in the transport of oxygen [6]. Its daily intake allowance is set at 20 mg [2]. Exceeding this amount would generate lesions in the digestive tract, induce pain in the abdominal area, cause fatigue, as well as harm the liver [7]. One of the benefits of utilizing iron as a biodegradable metal implant is its mechanical property which is similar to that of 316L stainless steel, which is the actual base standard for biomaterials involving metals. The superior mechanical properties of pure iron compared to pure zinc and pure magnesium makes it an appropriate candidate for constructing implants that require significant structural strength such as coronary stents [5]. In addition, when a complex iron phosphate layer is formed, it causes iron to very slowly degrade in-vivo. Several attempts have however been made to increase its corrosion rate by alloying it with manganese and palladium or adopting a vapor vacuum arc technique to implant silver. Additional efforts such as  
Figure 1: Examples of biodegradable metal implants: (**a**) headless screws developed by U&I Corporation, Korea, (**b**) compression screw from a German company; and (**c**) a biodegradable stent designed by a German company [2]. making it composites with its oxides or bioceramics have shown minor improvement towards iron's bioactivity [2]. Zinc is considered the latest addition proposed as a biodegradable metal [5] based on its vital function in numerous enzymes and cell metabolic activities, cell reproduction, and cellular differentiation. Its daily allowance is the lowest compared to iron and magnesium which is at 15 mg. In excessive amounts, it poses a threat of neurotoxicity, muscle cramps, diarrhea [8], anemia, and reduced bone formation [6]. The compatibility of zinc-based metals amongst cells has been studied to evaluate its application for bone and vascular implants. The physical properties (i.e., toughness and flexibility) of zinc can be improved by alloying it with a small amount of magnesium (up to 3 wt. %) and further enhanced by adopting severe plastic deformation processing techniques [2] or through thermomechanical treatment (e.g., extrusion) [5]. Since it is relatively new, further research is required to upgrade its ultimate tensile, compressive and yield strengths in order to improve their function in surrounding tissues [9]. Magnesium is the lightest engineering metal possessing a mass to volume ratio of 1.74g/cm\({}^{3}\)[10] and has a low ratio of force, about 40 to 45 GPa exerted on the body which is similar to that of human bone [5] making it compatible with bone tissues [11]. Similarly to the aforementioned two metals, it is also an essential element but with a higher intake tolerance level of up to 700 mg and the ability to mimic a protein coagulator, magnesium has influence in numerous bodily reactions involving enzymes [2]. However, exceeding the daily intake causes hypermagnesemia which is linked with diseases such as arrhythmia and asystole.