Biomedical Engineering Reference
In-Depth Information
Weaver, 1996). High voltage pulses up to 200 V decaying
in about 1 ms have also been used on human skin for
enhancement of transport by electroporation (Pliquett
andWeaver, 1996). The effect was found to be due to the
creation of aqueous pathways in the stratum corneum.
There are, for example, three membranes (meninges)
around the brain and CNS (pia mater, arachnoidea, dura
mater). There are membranes around the abdomen
(peritoneum), fetus, heart (pericardium), lungs (pleura),
inside the blood vessel (endothelium), around the nerves
(myelin, neurolemma in the hand). At some tissue in-
terfaces and at the electrodes the chemical composition
will gradually change.
A sensation will start either under one of the elec-
trodes (anode or cathode), or in the tissue between. The
chemical reaction at an electrode is dependent on the
electrode material and electrolyte, but also on the cur-
rent level. A sensation around threshold current level is
slowly developing and may be difficult to discern from
other sensations, for example, the mechanical pressure or
the cooling effect of the electrode. After the sensation is
clear, and the current is slowly reduced to avoid AC
excitation, the sensation remains for some time. That
proves that the current does not trigger nerve ends di-
rectly, but that the sensation is of a chemical, electrolytic
nature as described by the law of Faraday. The after
current sensation period is dependent on the perfusion of
the organ eliciting the sensation.
On palmar skin, with a surface electrode of varying
area
A
, the current
I
th
or current density
J
th
at the
threshold of perception follows the following equation
for a sensation within 3 minutes after current onset
(Martinsen et al., 2004).
4.1.16 Non-medical applications
Fingerprint detection as described in Section
4.1.15.1 is a measurement on humans, but not
with a medical purpose.
The monitoring of fermentation processes in beer
brewing or pharmaceutical industry is measuring on
different sort of cell suspensions.
Plant tissue is both strongly similar and very different
from animal tissue, the cell membranes for instance
are quite different.
Meat quality can be estimated from bioimpedance
measurements. See. e.g. Oliver et al., 2001 and
Guerro et al., 2004.
In geophysics impedance measurements were used as
early as in the 1920s for oil exploration (Schlumberger,
1920). Impedance measurements are also used for
monitoring volcanic activity (e.g. on Iceland).
4.1.17 Electrical safety
4.1.17.1 Threshold of perception
J
th
¼ J
0
A
0
:
83
I
th
¼ I
0
A
0
:
17
or
(4.1.16)
The perception of a current through human skin is
dependent on frequency, current density, effective elec-
trode area (EEA) and skin site/condition. Current dura-
tion also is a factor, in the case of DC determining the
quantity of electricity and thereby the electrolytic effects
according to Faraday's law.
The perception was only localized under the monop-
olar electrode, never in the tissue distal to the electrode.
Surprisingly, according to eq. (4.1.16) the threshold as
a function of electrode area
A
is more dependent on
current
than current density. There may be more than
one reason for this:
1.
A spatial summation effect in the nervous system.
The current
density
is reduced when the same
current is spread by a larger electrode, but at the
same time a larger number of nerve endings are
excited. Consequently the current threshold is not
so much altered when electrode area is changed.
2.
The DC current is not evenly distributed under
a plate electrode, the current density is higher at the
edge. The conductance of a surface sphere or plate
electrode is proportional to radius or circumference,
not to area.
3.
The DC current is probably concentrated to the
sweat ducts and the nerve endings there (Grimnes,
1984).
DC
If a DC source coupled to two skin surface electrodes is
suddenly switched on, a transient sensation may be felt in
the skin. The same thing happens when the DC current is
switched off. This proves that many nerve endings are
only sensitive to
changes
in a stimulus, and not to a
static
stimulus. At the moment a DC is switched on, it is not
only a DC, it also contains an AC component. DC must
therefore be applied with a slow increase from zero up to
the desired level, if the threshold of DC perception is to
be examined.
DC causes ion migration (iontophoresis) and cell/
charged particle migration (electrophoresis). These
charge carriers are depleted or accumulated at the elec-
trodes, or when passing ion-selective membranes in the
tissue. In particular, almost every organ in the body are
encapsulated in a
macromembrane
of epithelia tissue.
A practical use of DC perception is the old test of the
condition of a battery by placing the poles at the tongue.
