Piezoelectricity
The piezoelectric effect is the production of electrical polarization in a material by the application of mechanical stress. Piezoelectric materials also display the converse piezoelectric effect-mechanical deformation upon application of electrical charge. Polarization and stress are vector and tensor properties respectively, and in general, arbitrary components of each can be related via the piezoelectric effect. For this reason, piezoelectricity is a complicated property and up to 18 constants may be required to specify it (17).
Fig. 4.2. A. On the right side, an ascorbate molecule meets an oxygen molecule and passes on to it one of its electrons. With this exchange, the oxygen molecule gains an electron and the ascorbate molecule becomes a highly reactive free radical. On the left side, methylglyoxal lies in contact with the protein molecule. At this stage methylglyoxal is a very weak acceptor unable to pull electrons from the protein molecule. B. On the right side, the oxygen molecule moves on with its gained electron. On the left side, the highly reactive ascorbate moves to lie against the methylglyoxal molecule. In this position it shares and pulls electrons from methylglyoxal, which in turn pulls electrons from the protein molecule. This sets off a chain reaction, electronically desaturating the protein molecule, making it very active and conductive. C. Methylglyoxal and ascorbate are incorporated into the protein molecule; thus, the protein is activated by incorporating into it the acceptor. (Reproduced, by permission, from Nutrition Today, P. O. Box I829, Annapolis, Maryland 2.1404, September/October, 1979.)
Many biological materials have been found to be piezoelectric, including tendon, dentin, ivory, aorta, trachea, intestine, silk, elastin, wood, and the nucleic acids. Bone, however, has been the most frequently studied tissue. Piezoelectricity in bone was discovered (at least in the modern era) by Fukada and Yasuda, and their work was subsequently verified by many others (18-23). The most important piezoelectric constant in bone is d14-it relates a shear stress applied along the long axis of a bone to a polarization voltage that appears on a surface at right-angles to the axis. The discovery of piezoelectricity in bone aroused great interest because it seemed to provide an important key in understanding bone physiology. Bone was known to adapt its architecture to best carry out its functions, including that of providing skeletal support (24-27) (see chapter 2), and piezoelectricity became a candidate for the underlying physical mechanism. For example, we hypothesized a mechanism by which bone's piezoelectric signal could regulate bone growth (28) (Fig. 4.3). In support of it we showed that the piezoelectric property of bone arose from the protein moiety (23), changed with age (29), and existed in fully hydrated frozen bone (30). But despite the continuing effort of many investigators (31-40), the possible physiological role of piezoelectricity has not been fully evaluated, because practical techniques for studying it under physiological conditions of temperature and moisture have not yet been developed.
Fig. 4.3. Charge distribution (in pcoul/cm2) in a human femur. The indicated piezoelectric charges were measured when a load was applied to the femur (21). We displaced the medial surface at each charge location (left for growth, right for resorption) by an amount proportional to the measured charge (28). The lateral surface was similarly displaced (except left for resorption, and right for growth). Our result (the dotted femoral outline) revealed a self-consistent change in architecture, thereby lending support to the theory of a link between piezoelectricity and bone function.
The converse piezoelectric effect is a possible molecular mechanism by which an organism could detect an external field. Successful experiments based on this hypothesis have been reported by McElhaney, Stalnaker and Bullard (41), and Martin and Gutman (42) (see chapter 8).
Pyroelectricity is the development of electric charges
on the surface of a material when it is heated; all pyroelectric materials
are piezoelectric (but the converse is not true). Lang showed that both
bone and tendon exhibited the pyroelectric effect (43). Ferroelectricity
is the existence of a spontaneous electric dipole moment in material of
macroscopic size-it is the electrical analog of the more familiar phenomenon
of ferromagnetism. Athenstaedt presented evidence for the existence of ferroelectricity
in bone (44, 45). Some electrical characteristics of ferroelectric materials
are similar to those of an electret-a material that has an external electric
field because of its specific electrical and thermal history. Mascarenhas
showed that bone can be made into an electret (46, 47), and Fukada, Takamaster
and Yasuda (48), and Fukada (49) reported that plastic electrets applied
to bone produced alterations in growth.