Biomedical Engineering Reference
In-Depth Information
Frequency dependence
For sine waves the maximum sensitivity of our nervous
system is roughly in the range 10-1000 Hz (cf. also
Fig. 4.1-31
). At lower frequencies each cycle begin to be
discernible, and during each cycle it may be charge enough
to give electrolytic effects. At frequencies
>
1000 Hz the
sensitivity is strongly reduced, and at
>
100 kHz no per-
ception remains, because the levels are so high that elec-
tric stimulation are shadowed by the heat effect of the
current. That is the frequency range for electrosurgery.
A single pulse or a repetitive square wave may give
both DC and AC effects. In both cases the duration of
the pulse or square wave is an important variable
(cf. rheobase and chronotaxi).
The exponential decaying discharge waveform is the
case of electrostatic discharges (see below).
Skin
Conductor
Figure 4.1-28 Electrovibration perception mechanism.
This test is actually also done clinically:
electrogus-
tometry
is the testing of the sense of taste by applying
a DC to the tongue.
Sine waves
The lowest level
(
<
1
m
A) of 50/60 Hz perception is caused
by
electrovibration
(Grimnes,1983d;
Fig. 4.1-28
). It is
perceived when the current carrying conductor slides on
dry skin. Dry skin is a poor conductor, so that potential
differences of several tenths of volts may exist across the
dielectric which is the stratum corneum of the epider-
mis. With dry skin only a small microampere current
flows. The electric field sets up an electrostatic com-
pression force in the dielectric, pressing the stratum
corneum to the metal plate. In the stratum corneum
there are no nerve endings, and consequently no per-
ception. However, if the skin is made to slide along the
metal, the frictional force will be modulated by the
electrostatic force and be felt as a lateral mechanical vi-
bration synchronous with the double frequency of the
AC voltage. Even if the voltage across the dielectric is
>
20 V at threshold, the corresponding (mainly capaci-
tive) current may be
<
1
m
A. If the skin is at rest, or if the
skin is wet, no sensation is felt.
The second level
(1 mA) is due to the
direct electric
excitation
of nerve endings, which must be a function
of the local current (density). The electric current
threshold of perception with firm hand grip contact and
contact area several square centimeters, is around 1 mA.
Threshold current has a surprisingly small dependence on
contact area. The reason for this is mentioned above in
the section on DC perception. With a small area contact
around 1 mm
2
, the threshold of perception is around
0.1 mA, corresponding to 100 A/m
2
.
Interpersonal variations and the dependence on age
and sex are small. Skin condition is not important as long
as there are no wounds. Skin site may be important. On
the fingertips the density of nerve ending is large, but the
stratum corneum is thick and the current will be rather
uniformly distributed. Other skin sites may have much
thinner skin and lower density of nerve endings, but
conductive sweat ducts that canalize the current.
Electrostatic discharge pulse
The perception of an electrostatic discharge is an an-
noyance, and in some situations a hazard. It is particularly
troublesome indoor during the winter with low relative
humidity (RH). Low RH reduces the conductivity of
most dielectrics (e.g. the stratum corneum) and also the
conductivity of clothing, construction materials, tree,
concrete, etc. A person may be charged up to more than
30 kV under such circumstances, and with a body ca-
pacitance to the room (ground) of about 300 pF, the
electrical energy of the person is of the order of 0.1 J. A
smaller discharge, near the threshold of perception, is
typically with a time constant of a few
m
s, and the peak
current around 100 mA (
Fig. 4.1-29
). It is obtained by
discharging a capacitor of 100 pF charged to 1.4 kV. The
point electrode is approached to the skin until an arc is
formed, heard and perceived in the skin.
The maximum current is determined by the voltage
drop in the arc (probable less than 100 V) and the re-
sistance in the skin. The arc probably has a very small
60
40
1400 V, 100 pF
palmar skin
20
0
2
4
6
8
10
t
(µS)
Figure 4.1-29 Capacitor discharge current flow through palmar
skin. Monopolar electrode: 1.3 mm diameter pin of steel,
sharpened at the tip. Indifferent electrode on the underarm.
