Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 145

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).

Figure 4.17 Simplified explanation of the electrical conductivity of conducting polymers. (a) A dopant D removes or adds an electron from/to the polymer chain, creating a delocalized charge. (b) It is energetically favorable to localize this charge and surround it with a local distortion of the crystal lattice. (c) A charge surrounded by a distortion is known as a polaron P (a radical ion associated with a lattice distortion). (d) The polaron can travel along the polymer chain, allowing it to conduct electricity.

Source: From Balint et al. (2014).