The in vivo degradation
A biodegradable metal implant is continuously exposed to the extracellular tissue fluid once implanted into the human body. This exposure causes an electrochemical dissolution of metal at which the body responds via a cascade process of protein absorption, coagulation, acute inflammation, chronic inflammation, and foreign body response, as illustrated in Fig. 2.3. Acute interaction occurs shortly after implantation related to the corrosion process that induces the formation of an interspace (dead space) between the implant and the tissue filled with a large volume of body fluid. Cellular inflammation follows during the first-week postimplantation, which then decreases over a prolonged time, where the dead space is filled with proliferation of the surrounding tissue [46,47].
Corrosion is the typical process of degradation for biodegradable metals. It is initiated at any site on the surface that has a potential difference created from metallurgical inhomogeneity such as phase variation, grain boundaries, impurities, or from geometrical and environmental variation such as crevice, notch, scratch, coating defect, oxygen level gradient, etc. In the physiological environment, corrosion of metal (M) generally involves the following reactions:
Figure 2.3 Illustration of possible interaction between a degrading metal implant with the local tissue and systemic body response.
The physiological solution contains various anions and cations, dissolved oxygen, organic compounds, amino acids, proteins, and so on. The released metal ions interact with water and other species, creating a layer of degradation products from as simple as metal hydroxides (M(OH)w) to metal complexes. Meanwhile, the produced electrons flow in the metal and are consumed by the reduction of water and/or oxygen to form hydrogen gas or hydroxide ions. A decrease in pH to 5.3-5.6 around the implantation site, due to inflammatory response, may accelerate the corrosion process of the metal and reduce local oxygen concentration . The inorganic ions in physiological solutions such as Cl-, one of the most abundant solutions, aggressively breaks down the M(OH)n layer, leading to continuous corrosion and mostly localized attack . Similar effect as that of Cl- is also observed for SO42 -, meanwhile other ions such as HPO42 - /PO43 -, HCO3 - /CO32 -, and Ca2+ help to passivate magnesium and iron due to the precipitation of phosphate and carbonate salts on the metal surface [50,51]. Adsorption of organic molecules such as albumin on the metal surface delays the corrosion process of magnesium in the initial stage only ; meanwhile, amino acid reduces the barrier effect of the insoluble salt layer against the dissolution of magnesium .
The nature of the corrosion process that produces a flow of electrons (Eq. (3.1)) has been long exploited as a tool for measuring kinetics of corrosion by means of electroanalytical tools such as potentiostat/galvanostat. However, this measurement, which requires electrical connection wires and the insertion of reference and counter electrodes, is not applicable for in vivo corrosion monitoring. On the other side, the nature of metal ion release to the extracellular fluid around the implant can be used to approximate the degradation rate by measuring their concentration over time. By combining a kind of fluid sampling tool and electroanalytical apparatus, this approach can become an effective system for monitoring in vivo degradation of biodegradable metal implants. Details of this approach are discussed in the next section.