## 5 Implant-Tissue Interaction during In Vivo Corrosion  
After implantation has been performed, there is continuous interaction between the biodegradable metal implant and the extracellular tissue fluid. For this reason, an electrochemical dissolution of the biodegradable metal implant occurs. This causes a continual process of protein absorption, coagulation, acute, and chronic as a result of the body response to a foreign entity. All these biological processes occur right after implantation thus, inducing space between the implant and tissues to be filled with body fluid. After the first week of post-implantation, cellular inflammation follows which decreases after a period of time, followed by surrounding tissue proliferation to fill the interspace [2]. An illustration of a biodegradable metal interacting with the local tissue followed by the response of the body is presented in Figure 3. Corrosion begins after implantation, and then, the tissue responds to the foreign object. Corrosion products are formed and discharged into the tissue at various sizes; the blood cells transport these products into several other tissue areas depending on location of implant (e.g., muscle skin, bone, etc.) [3]. Figure 2: Ideal balance between an implant’s mechanical integrity and corrosion rate [2]. Corrosion occurs when a material gradually deteriorates after undergoing electrochemical reactions as a result of being placed in the body's harsh electrolytic environment. The fluid medium in the body is made up of various anions, cations, and other components. Biological molecules, such as proteins, interfere with the equilibrium of corrosion reactions by binding themselves to the ions discharged from the metal whereas those absorbed on the surface reduce the oxygen diffusion at certain regions thus, causing corrosion [1]. In general, corrosion stems from either metal imbalance such as phase variation, grain boundaries, and impurities or from shape and environmental changes such as crevices, scratches, defects in coating, amount of oxygen, and others. For a metal (M), the general corrosion reactions that would occur in a physiological environment is represented below [4]:  
Anodic reaction:  
\[\mathrm{M}\rightarrow\mathrm{M}^{n+}+\mathrm{ne}^{-} \tag{1}\]  
Cathodic reaction:  
\[2\mathrm{H}_{2}\mathrm{O}+2\mathrm{e}^{-}\rightarrow\mathrm{H}_{2}+2\mathrm{ OH}^{-} \tag{2}\]  
or  
\[2\mathrm{H}_{2}\mathrm{O}+\mathrm{O}_{2}+4\mathrm{e}^{-}\to 4\mathrm{OH}^{-} \tag{3}\]  
Product formation:  
\[\mathrm{M}^{n+}+\mathrm{nOH}^{-}\rightarrow\mathrm{M}(\mathrm{OH})_{\mathrm{n}} \tag{4}\]  
The physiological solution comprises numerous discharged ions, dissolved oxygen, organic compounds, proteins, and other components. These ions discharged from metal will interact with water and other species to construct a corrosion product layer (e.g., metal hydroxides or metal complexes as illustrated in Figure 4). Simultaneously, water reduction and/or oxidation by electrons released from the metal causes the formation of hydrogen gas or hydroxide ions. Surrounding the implantation site will be a decrease in pH in the range of 5.3 to 5.6 because of the inflammatory response which accelerates the corrosion rate of the metal and reduces the local oxygen concentration. For inorganic ions such as Cl\({}^{-}\), decomposition of M(OH)\({}_{\mathrm{n}}\) layer occurs, resulting in perpetual and localized corrosion attack. This effect is similar for SO\({}_{4}\)\({}^{2-}\) whereas other ions such as HPO\({}_{4}\)\({}^{2-}\)/PO\({}_{4}\)\({}^{3-}\), HCO\({}_{3}\)-/CO\({}_{3}\)\({}^{2-}\), and Ca\({}^{2+}\) help to protect magnesium and iron due to the precipitation of phosphate and carbonate salts on the surface of the metal. Other methods of protection involve delaying the  
Figure 3: Illustration of biodegradable metal interacting with the local tissue, followed by the body’s response [4].