3.4: Environments Encountered in Biomedical Applications

3.4.1 Surgical Implants

The compositions of body fluids are complicated. However, from the corrosion perspective, the most important characteristics are the chloride, dissolved oxygen and pH levels. A 0.9% NaCl solution is considered to be isotonic with blood, and under normal conditions most body fluids have a pH of 7.4 and a temperature of 37 C. In these respects, body fluids appear to be slightly less aggressive than seawater. This is reflected by the fact that for stainless steels, a pitting resistance number (PREN) of greater than 26 is recommended for surgical implants as compared to the value of 40 usually required for stagnant seawater [23]. However, the dissolved oxygen levels in blood are lower than those of saline solutions exposed to air atmospheres, by a factor of about 2 for arterial blood and 6 for venous blood. Conversely, bicarbonate levels are about 20 times higher in blood than in seawater [53].

The many other components in body fluids, e.g., phosphates, cholesterols, phospholipids, etc. are usually thought to either play no role in the corrosion process or exist at inconsequential levels. As a result most in vitro experiments have been conducted in either 0.9% NaCl solution or standard isotonic solutions (such as Ringer s or Hank s solution) in which the presence of bicarbonate and calcium chloride poses as the main difference from the NaCl solution. A review by Solar [76] in 1979 concluded that inorganic solutions based on diluted NaCl were indeed satisfactory substitutes...