Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 144

The ability of a material to transmit an electric current is quantified by its electrical conductivity or, less commonly, by its electrical resistivity which is the inverse of the electrical conductivity. The resistivity ρ of a material is independent of its geometry but, for a wire of length l and uniform cross sectional area A, it is related to the measured electrical resistance R by the equation

(4.39)

As the unit of R is ohm (Ω), the unit of ρ is ohm‐meter (Ω m), and the electrical conductivity, equal to 1/ ρ, has the unit (Ω m)⁻¹, commonly written S/m, where S, the inverse of Ω, is the unit Siemens.

The electrical conductivity of materials covers an enormous range of ~28 orders of magnitude. Metals are good conductors, with a conductivity in the range ~10⁴–0⁸ S/m, whereas most ceramics and polymers are insulators with conductivity in the range ~10⁻²⁰–10⁻¹⁰ S/m. Materials with intermediate conductivity, ~10⁻⁶–10⁴ S/m, are called semiconductors (Figure 4.16).

Figure 4.16 Bar chart showing the range of electrical conductivity for different types of materials at room temperature.

Transmission of an electric current results from the motion of electrically charged particles in response to forces that act on them from an externally applied electric field. The high electrical conductivity of metals arises from the ease with which the sea of nearly free electrons that surround the metal cations move through the material (Chapter 2). Thus, metals are said to show electronic conductivity because electrons are essentially responsible for their ability to conduct an electric current. In comparison, a few ionic‐bonded ceramics can show a limited capacity to conduct an electric current, which arises from the migration of ions, referred to as ionic conductivity, plus any contribution from electronic conductivity. While the total electrical conductivity due to the electronic and ionic contributions increases with temperature, ceramics are essentially insulators at temperatures relevant to biomedical applications.

Platinum alloys (conductivity ~10⁷ S/m) are commonly used in cardiac pacemakers while silver alloys with a higher conductivity (~10⁸ S/m) find use in some implantable defibrillators. On the other hand, polyurethane (conductivity ~10⁻¹² S/m), is often used as a coating to isolate or insulate sensitive electronic devices from surrounding tissues and fluids.