Electret Piezo and Pyroelectric Effects

Assemblies whose microscopic subunits and macroscopic whole both possess permanent electric dipoles are known as electrets. They exhibit properties known as piezoelectricity and pyroelectricity which may be useful in biological activities. In these crystals, dipolar elementary subunits are arranged in such a way that all the positive dipole ends point in one direction and all the negative dipole ends are oriented in the opposite direction. In microtubules, the positive ends of tubulin dimer subunits point away from microtubule organizing centers (MTOC) toward the cell periphery, and the negative ends point toward MTOC. Electrets can store charge and polarization and have now been identified in a variety of nonbiological materials such as ionic crystals, molecular solids, polymers, glasses, ice, liquid crystals and ceramics. Biological tissues demonstrating electret properties include bone, blood vessel wall materials, keratin, cellulose, collagen, gelatin, artificial polypeptides, keratin, DNA, cellulose and microtubules

(Athenstaedt, 1974). An electret effect accounts for specific properties such as anti-blood clotting in biomaterials and the non-stickiness of teflon. Sources of polarization or charge storage in macromolecules are dipoles, ionic space charges, or ordered surface water.

Electret materials are piezoelectric (Gubkin and Sovokin 1960). Piezoelectric materials change their shape or conformation in response to electrical stimuli, and change their electrical state in response to mechanical stimuli. Koppenol (1980) has shown that the dipole moment orientation of an enzyme changes in concert with its functional activity. Electrets are also "pyroelectric," in that any change in temperature alters the electrical and conformational characteristics of the molecule. The permanent electric dipole moment of pyroelectric bodies results from the parallel alignment of elementary fixed dipole moments. Any change in temperature modifies the length of the pyroelectric body and alters its elementary dipole moments (pyroelectric effect). In the same way, any mechanical change in length or any deformation of a pyroelectric body produces a modification of its dipole moment (piezoelectric effect). Thus every pyroelectric body is at the same time piezoelectric. The two requisites for such behavior are an electric property which causes a permanent dipole moment in the molecules and a morphological property that favors a parallel alignment. Microtubules and other cytoskeletal structures appear to be appropriately designed electret, pyroelectric, piezolectric devices.

The electret state within bone has been well studied and is able to store large amounts of polarization of the order of 10-8 coulombs per square centimeter (Mascarenhas, 1974, 1975). The limit of charge separation (equivalent to maximal information density) has been calculated (Gutman, 1986) to be about 1017 electronic charges per cubic centimeter, while there may be about 1021 total molecules per cubic centimeter. One cubic centimeter of parallel microtubules densely arrayed 100 nanometers apart contains about 1017 tubulin subunits and therefore may contain 1017 dipoles equivalent to the maximal density of charge! Electret and related properties can impart interesting and potentially useful properties to biomolecules including cytoskeletal polymers. Among these are the potential capacity to support the propagation of conformational waves such as solitons.

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