Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 125

potential gradients on the order of 0.1-1 mV/cm and are oriented such that
sites distal from the center of the trunk are relatively electropositive with
respect to more central sites. Open epiphyseal growth plates and other
areas of growth, including tumors, are electronegative with respect to
surrounding tissues. Tissue injury, such as skin abrasion or bone fracture,
produces a short-term (<1 h) local positive polarization, ascribed to an
“injury current,” followed by a strong negative polarization that dimin-
ishes but persists until healing and remodeling activity have stopped.
The second of these electrical effects is a group of polarizations
called variously piezoelectric, stress-generated (SGP), or strain-related
(SRP) potentials. These potentials exist in tissue, whether living or dead,
and are produced by deformation in shear of the tissue. Thus, a popular
way to elicit them is by bending a slab of cortical bone or a whole bone.
Figure 5.14 shows the results of such an experiment. When a bending
stress is applied and shortly thereafter released, the following sequence
of events is observed. As the strain develops, an initial or forward polar-
ization develops, oriented such that the concave or compression side of
the specimen is electronegative with respect to the convex side. If the
strain is maintained constant for several seconds, the forward potential
rapidly decays. When the strain is released, the polarization rapidly dis-
appears and a small reverse polarization is seen, which decays to zero
in a short time. If the initial strain is maintained, suitable measurement
instruments reveal that the forward polarization decays to a very low
value (but not to zero), which is called the offset potential.
The forward potential is typically 10-20 mV in wet cortical bone for
surface strains of 500-1000 με. It increases linearly with strain at a con-
stant strain rate (Figure 5.14, upper right). For a given peak strain, it
increases with an increasing strain rate to a plateau value (Figure 5.14,
lower right). The offset potential is linearly related to the peak strain but
is not dependent on the initial strain rate.
There is some debate as to the origin of SRPs but the consensus is that
in vivo , they are related to the forced flow of fluid (secondary to strain)
over surfaces with net fixed charge densities.

-

∆ V

-

+

+
Time

ε

∆ V

ε

ε (fixed peak ε)

FIGUre 5.14 Production of strain-related potentials (srPs).