The Ticklish Gene
123
the negative charge from bending stimulated the adaptive growth of
Wolff's law. To find out, we tested both living and dead bones from a
variety of animals, and found that bending produced an immediate po-
tential, as expected. The compressed side became negative; the stretched
PIEZOELECTRICITY—MECHANICAL STRESS INTO ELECTRICITY
Furthermore, the reversed potentials that appeared when we released
the stress weren't nearly as large as the first ones. This was just as it
should be. If a negative voltage was the growth stimulus, there had to
be some way to cancel out the positive rebound voltage; otherwise it
would have negated the growth message. In electronic terms, there had
to be a solid-state rectifier, or PN junction diode.
PIEZOELECTRICITY IN BONE
*After writing up this experiment, we found that it had been done before. Iwao Yasuda,
a
Japanese orthopedist,
had
shown that bone was piezoelectric back in 1954; he and
Eiichi Fukada, a physicist, had confirmed the fact in 1957. We made note of their prior
observations but published our paper anyway, since our techniques were different and
ours was the first report in English.