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.
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.
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.
Superconductivity
In a normal material, electrons moving through the lattice encounter resistance from defects,
impurities, and lattice vibrations. A superconductor is a material in which electrons flow without
experiencing any resistance. A mathematical theory has been developed (BCS theory) that explains
superconductivity on the basis of pairing of some of the free electrons to form Cooper pairs (50).
Superconductivity was generally thought to be a phenomenon associated only with metals at
temperatures below about 20°K. Beginning in the mid-1960's, however, theoreticians predicted the
ELECTROMAGNETISM & LIFE - 61