Metal and Electronic Conductor Properties

This is a much used electrode metal (silver covered by silver chloride) in biology and medicine for DC applications both because it is simple and because it has a well-defined DC potential not very dependent on DC current flow. It is therefore a nonpolarizable DC reference electrode. It usually consists of silver metal covered by an AgCl layer, often electrolytically deposited. Ag and AgCl are toxic and cannot be used in long-term living tissue contact. A salt bridge is often used to remove the electrode metal from direct tissue contact. 

The platinum electrode can be improved by an active electrolytic process forming a platinum black surface. The reduced polarization impedance is due to an increased effective metal surface area (fractal surface). The electrode is prepared in an electrolyte containing (e.g., 3% platinum chloride), with the platinum as the cathode. Platinum black is deposited on the surface, and also here there are optimum values for current density and quantity of electricity a current density of about 10 mA/cm and a quantity of electricity (charge) of about 30,000 mA s/cm is recommended (Schwan, 1963). Best results are obtained if the platinum surface is sandblasted before platinum black deposit. However, the surface may be fragile, and a protein layer formed with tissue contact may easily smooth the micro-rough surface and increase polarization impedance. Platinum black electrodes are best stored in distilled water and short-circuited (Schwan, 1963). 

Typically, titanium alloys have been the materials of choice for medical implants. The Ti-6A1-4V alloy is generally considered chemically inert, compatible with human tissue, and resistant to corrosion by human body fluids. However, the small percentages of vanadium and aluminum contained in the alloy are potentially toxic. Pure titanium is chemically and biologically more compatible with human fluids and tissue, but it is too weak for prostheses that must bear heavy loads, such as leg or hipbone implants. 

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