resulting in major alterations in cell function (49).
Bone
An electromagnetic property may be intrinsic in the structure of biological materials, where it
plays a specific functional role. This was observed as a result of the search for a simple, easily
isolatable growth system with well-defined input and output parameters. Healing processes as
previously described are complex, involving major cellular reactions and a variety of biochemical
changes within the entire organism. Morphogenetic growth in the embryo is even more complex,
involving, in addition, interactions between structures as they form and genetically preprogrammed
factors.
There is however, a growth phenomenon, unique to bone, that is ideal for such an analysis. In
1892., Wolff systematized the growth response of living bone to mechanical stress into a specific law
that subsequently has become known as "Wolff's Law." Simply stated, bone grows in response to
mechanical stress so as to produce an anatomical structure best able to resist the applied stress. For
example, should a fracture of a weight-bearing long bone heal with an angulation, each step that the
patient subsequently took would result in a bending stress with compression on the concave side at the
angulation and tension on the convex side. Rather than progressively weaken the bone structure at this
site, such repeated mechanical stress results in a "remodeling," with new bone growth on the concave
side and bone resorption on the convex side. If the patient is young enough, the bone will ultimately
through this process grow straight. In control system terms, the applied mechanical stress causes a
growth response that negates the applied stress; a closed-loop negative-feedback control system. Such
systems imply the presence of transducers producing a signal proportional to the stress and indicating
its direction. The system may be schematicized as:
In certain inorganic crystalline materials having a nonsymmetrical lattice, the application of
mechanical stress results in the displacement of charges within the lattice that can be sensed as a pulse
of electricity on the exterior surface of the crystal. With release of the mechanical stress, a pulse equal
in magnitude but opposite in polarity is produced. This property is called piezoelectricity, and in 1954
it occurred to a Japanese orthopedic surgeon, Iwao Yasuda, that bone might be piezoelectric, with the
mechanically produced electrical signals being the stimulus that produced bone growth according to
Wolff's law. He was able to actually demonstrate piezoelectricity in whole bone that year (50) and in
the following year he stimulated the growth of bone in experimental animals by the application of
electrical currents. Later, in conjunction with another Japanese scientist, Eiichi Fukada, he was able to
show that the piezoelectric property also existed in the collagen fibers of tendon (51). Subsequently,
this same property has been found in the collagen fibers of many different tissues.
In 1962, Bassett and Becker extended these observations using fresh whole bone subjected to
bending stress (52). They noted under these conditions that the signal produced by the application of
ELECTROMAGNETISM & LIFE - 40