Properties of natural materials - Orthopaedic Biomaterials in Research and Practice - page 124

If this experiment is done with a battery (which is a nearly constant
voltage source at low current) and the current is measured, it will be seen
to diminish with time. This is due to a process called polarization : the
accumulation of charges in the vicinity of the electrodes and the separa-
tion of tissue-bound charges of opposite signs. If nonpolarizable elec-
trodes are used, such as pressed silver-silver chloride pellets, the current
will still decline with time because of tissue polarization.
This polarization ability, which is seen as an apparent increase in
resistivity with time in response to a fixed voltage, is governed by the
following equation:

Δ V = q / K

where q is the density of charge (coulombs/m 2 ) and K is the dielectric
constant (farad/m).
This effect produces a lower initial resistivity (and thus a higher cur-
rent); the final steady value is called a resistive current, whereas the ini-
tial contribution by polarization is called a displacement current, since a
fixed charge is displaced rather than mobile charges being moved.
If the direct current is replaced by an alternating current, then the
contribution of the (dielectric) polarization is greater. Thus, in gen-
eral, apparent tissue resistivity decreases with increasing frequency of
applied voltage. The dielectric constant of water decreases with increas-
ing frequency, which tends to diminish the decrease of resistivity with
frequency above 100 kHz.
The resistivity and the dielectric constant thus completely character-
ize the passive electrical properties of tissues. Unless current is driven
through a cell or tissue membrane, both resistivity and dielectric con-
stants tend to reflect the water content of tissue. For resistivity, the low-
est values obtained are perhaps 0.6 (ohm·m) for blood and 1 for muscle,
and they range up to 30-40 for cortical bone (fully wet). The dielectric
constant of water is nearly 90 (air = 1), and the various tissues are frac-
tions of this value.
Thus, imposed currents, either injected by electrodes or produced
internally by various processes, tend to flow through fluid-filled spaces
and high-water-content tissues such as articular cartilage, whereas poten-
tial gradients are larger in tissue with lower water contents, such as bone.

active electrical
properties

In addition to these passive materials properties, musculoskeletal tissues
are themselves the source of a number of electrical signals. Those that
arise in nerve tissue resulting from membrane depolarization and repo-
larization, secondary to ion flows, need not be recited here as they are
well known.
However, nonexcitable tissues are the source of two types of electrical
potential difference whose significance is now just being appreciated. The
first of these is seen in a pattern of constant polarization throughout the
bodies of living mammals. They are called bioelectric potentials or more
simply biopotentials and have been shown to be dependent on the pres-
ence and magnitude of coupled oxidative phosphorylation. They produce