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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.

      4.5.2 Electrical Conductivity of Conducting Polymers

      Conducting polymers have attracted considerable interest in recent years because they can show an electrical conductivity as high as some metals (Figure 4.16). In addition to a high electrical conductivity, conducting polymers have an attractive combination of properties that make them suitable for use as biomaterials in applications such as biosensors, neural probes, tissue engineering and drug delivery (Guimard et al. 2007). These properties include ease of synthesis, flexibility of forming into desirable shapes, which are characteristic of polymers in general, and the potential for functionalizing their surface with appropriate molecules.

      Polyacetylene has shown one of the highest electrical conductivities, up to ~10⁷ S/m, whereas polypyrrole is the most widely studied conducting polymer. These and other conducting polymers typically show a characteristic structure composed of alternating single and double bonds in the polymer chain (Chapter 3). While conducting polymers can show an electrical conductivity as high as some metals, the source of their electrical conductivity is not the sea of almost free electrons that surround the cations in a metal. Instead, their high conductivity arises from a combination of factors that depend on the atomic bonding and structure of the polymer.

A key factor is the alternating single and double bonds in the polymer chain backbone, allowing the π electrons of the double bond to be more easily delocalized and move more freely between the atoms of the chain (Figure a). However, this type of atomic bonding alone is not sufficient to endow the polymers with a high conductivity. Another key factor is the ability of these polymers to be doped with appropriate molecules that introduce a charge carrier into the system by removing electrons from, or adding electrons to, the polymer chain. While the mechanism is more complex, Figure 4.17 illustrates a simplified explanation of the electrical conductivity (Balint et al. 2014).