Biologic Significance of Piezoelectricity

Piezoelectricity is a phenomenon which has increasing importance in calcified tissue research. I t now is established that piezoelectric behavior is an inherent property of bone, of other mineralized structures and of non-mineralized connective tissues. Since the extensive data leading to this conclusion are of rather recent origin, they have not yet been integrated generally into the thinking of many engaged in the study of mineralized and other connective tissues. I t is the purpose of this editorial review, therefore, to focus attention of investigators in calcified tissue research on piezoelectric phenomena in bone and to indicate some areas of "po ten t ia l " importance in present and future research. Piezoelectricity may be defined as electricity resulting from pressure on crystals. In the purest sense, the term is restricted to single, inorganic crystals that lack a center of symmetry (CADu 1946). When such materials are deformed, charges of opposite polarity appear on opposite faces of the crystal. Basically, the effect results from a separation of the centers of neutrali ty of the positive and negative charges in the crystal lattice as the material is deformed along certain axes. The term, piezoelectric, also has been applied to a class of polycrystalline, inorganic materials (JAFFE, 1960) and to some organic substances (ATItENSTAEDT, 1961 and DUCHESNE et al., 1960). In polycrystalline materials and in "piezoelectric tex tures" (SHuBNIKOV, 1946), piezoelectricity results from a summation of charges produced by aggregations of oriented, crystalline or quasi-crystalline regions within the material. Piezoelectric ceramics fall into this polycrystalline category as may bone. Initially, these ceramics are not piezoelectric since there is random crystal orientation. They become active, however, after the crystals are polarized by the application of a d.c. field (JAFFE, 1960). Wood is an example of a "piezoelectric t ex tu re" in which the molecular order of the oriented cellulose fibers, embedded in amorphous lignin, approaches crystallinity (FUKADA, 1955; BAZE~HOV, 1961 ; GALLIGAN and BERTHOLF, 1963).