## 2 Biodegradable Metals and Their Alloys
In addition, corroding magnesium creates hydrogen bubbles which have the potential to clog blood vessels, resulting in death [9]. Other side effects include nausea, kidney failure, and ineffective breathing [8]. When utilizing a biodegradable metal implant that is constituted of the alloys of magnesium, an advantage is that there is no presence of unwanted side effects from stress shielding which would otherwise occur in common implants made of metals [12]. It is these factors that permits magnesium to be a material that is osteoconductive [13]. However, there are challenges when adopting a magnesium-based implant due to its relatively low strength and ductility along with the production of hydrogen gas during its degradation. Further improvement on the properties of magnesium-based metals can be accomplished by advanced alloying (e.g., with calcium, zinc, zirconium, and rare earth metals), purifying the metal from impurities such as copper and nickel, switching its microstructure to nanocrystalline and amorphous state [2] and utilizing treatments such as thermomechanical treatment and severely deforming the material [5]. As opposed to iron, applying a coating on the surface of magnesium with bioceramics or biopolymers can delay its corrosion rate [2]. Being the eighth most plentiful element present in the earth's crust [10] has earned its place in being the most researched amongst all the three biodegradable metal materials [5]. This is mainly due to the fact that the requirement for a secondary removal surgery is not compulsory with the assistance of such material [14]. Comparisons between all three biodegradable metals are summarized in the table below (Table 1). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Category** & **Magnesium** & **Zinc** & **Iron** \\ \hline \multirow{4}{*}{Applications} & Tissue engineering, orthopedic (e.g., hip joints, screws/pins and dental implants [8]), micro clips for laryngeal microsurgery and cardiovascular applications [3], & & \\ \hline Corrosion rate & Fastest & Medium & Slowest \\ \hline Modulus/GPa [2] & 30 (Pure Mg) & 60 (Pure Zn) & 150 (Pure Fe) \\  & 45 (Mg-Based) & 100 (Zn-Based) & 200 (Fe-Based) \\ \hline Tensile Strength/MPa [5] & 100 (Pure Mg) & 90 (Pure Zn) & 200 (Pure Fe) \\ \hline Maximum elongation/\% [5] & 7 (Pure Mg) & 8 (Pure Zn) & 40 (Pure Fe) \\ \hline Corrosion rate \(\uparrow\)/(mm/year) [5] & 8 (Pure Mg) & 0.16 (Pure Zn) & 0.1 (Pure Fe) \\ \hline \multirow{4}{*}{Strengths [9]} & Its alloys are compatible with the human body and have elastic moduli, strength and compressive strength that are similar to trabecular bone. & Anti-inflammatory and anti-proliferative properties. Effective in reducing risk of atherosclerosis. & Fe stents have promising mechanical properties and biocompatibility [15]. \\ \hline \multirow{4}{*}{Weaknesses [14]} & Quickly corrodes before complete tissue reconstruction due to lack of resistance to chloride elements present in the bodily fluids. & Compared to other alloys, it suffers from very low radial strength. & Corrodes excessively slowly to be practical for bioreshorbable applications. \\ \hline \multirow{4}{*}{Solution [9]} & Surface treatment to reduce corrosion rate. & Considering that zinc is relatively new to the biodegradable metal family, further studies are required to conclude its potential as vascular scaffold and in orthopedic applications. & Alloy elements such as manganese to increase iron’s corrosion rate. \\ \cline{1-1}  & Potential techniques such as alkaline heat treating, microcar oxidation (e.g., FHAMAO coated Mg implant [16]), calcium phosphate deposition, fluoride treatment [17], electro deposition and polymer coating can be used. & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary comparison of the three biodegradable metals for implants. \({}^{*}\) Corrosion rate data obtained from identical experiments, i.e., conducting polarization test in 37 \({}^{\circ}\)C simulated body fluid. Values might change depending on testing condition and parameters.