Biomedical Engineering Reference
In-Depth Information
recovery time. They also give extensive statistical data
for these parameters in different age groups. The use of
absolute values for the electrical properties of tissue is
hazardous due to their liability to measurement error and
their dependence on, for example, electrode size, gel
composition and ambient environment. The use of
indexes or other relative parameters would presumably
prove beneficial also in EDR measurements. Mørkrid and
Qiao (1988) analyzed the use of different parameters
from the Cole admittance equation in EDR measure-
ments and proposed a method for calculating these
parameters from measurements at only two frequencies.
another wet sponge. The current is hence driven locally in
the hand, and only one hand is treated at a time.
How tap water iontophoresis can impede excess
sweating is still not fully understood. One theory sug-
gests abnormal keratinization in the epidermis as a result
of the current being shunted through the sweat ducts,
leading to a plugging of the sweat orifices. This plugging
cannot be found on micrographies of the skin, however,
and a more plausible theory is presumably that the cur-
rent leads to a reversible destruction of the sweat glands.
4.1.15.7 Iontophoresis and transdermal
drug delivery
4.1.15.5 Sweat measurements
The transport of charged substances through the skin was
shown early by the famous experiment by Munk (1873).
He applied an aqueous solution of strychnine in HC1 under
two electrodes attached to the skin of a rabbit. Without
current flow nothing happened to the rabbit; with appli-
cation of a DC current for 45 minutes, the rabbit died.
Abramson and Gorin (1939, 1940) found that timo-
thy pollen could be transported into the skin by elec-
trophoresis. They studied the transport of dyes into
human skin by electrophoresis. Without the application
of electricity, no particular skin marks were seen after the
dye had been in contact with the skin for some minutes.
With an applied DC current, and after the superfluous
dye had been wiped off, small dots were seen corre-
sponding to the pores of the skin. Positively charged
methylene blue was transported into the skin under an
anode, and negatively charged eosin under a cathode.
Some pores were colored with only one of the types.
Iontophoresis of pilocarpine
is the classical method for
obtaining sweat for the cystic fibrosis test (Gibson and
Cooke, 1959). The penetration of pilocarpine in the
skin enhances sweat production. The test is usually
performed on children with both electrodes placed on
the underarm (for safety reasons the current should not
pass the thorax). A 0.5% solution of pilocarpine is placed
under the positive electrode, and the DC current is
slowly increased to a maximum of about 1.5 mA. The
iontophoresis time is about 5 minutes.
A skin surface
negative
electrode attracts water from
deeper layers, a positive electrode repels water. This is an
electro-osmotic
effect and not iontophoresis (Abramson
and Gorin, 1939; Grimnes, 1983b).
The conductivity of human skin is very unevenly dis-
tributed. The current pathways have been found to be
the pores of the skin, particularly the sweat ducts, only to
a small extent through the hair follicles (Abramson and
Gorin, 1940; Grimnes, 1984).
Transdermal drug delivery through iontophoresis has
received widespread attention. A long term delivery with
transdermal DC voltage of
<
5 V is used (Pliquett and
Quantitative assessment of sweat activity is of great im-
portance also for other purposes than the measurement of
EDR. Tronstad et al. (2007) have reported on the de-
velopment of a portable multichannel instrument for long
term logging of sweat activity. The instrument is based on
conductance measurements and has been developed pri-
marily for the assessment before and after treatment of
patients with hyperhidrosis (see Section 4.1.15.6). Since
the instrument is portable it can also be used in sports and
during other kinds of physical activities, and with four
independent channels it can monitor different body parts
simultaneously. Their measurements indicate that the
sweat activity of different skin sites behaves differently
under physical stress. Hence, such measurements will be
valuable for obtaining a better understanding of the
physiology controlling thermal sweating.
4.1.15.6 Iontophoretic treatment
of hyperhidrosis
Hyperhidrosis is a state of extreme sweat secretion in
palmar, plantar or axilliary skin sites. The disorder can be
treated with drugs (e.g. anticholinergica), tap water ionto-
phoresis or surgical sympathectomy. Tap water iontopho-
resis has been in use at least since the beginning of this
century, and represents a simple, effective, but somewhat
painful cure. In its simplest form, the setup used comprises
two water filled metal tubs and a DC supply. In case of
palmar hyperhidrosis, the hands are placed in the two tubs
and a DC current is driven from one hand to the other
through the upper body. This treatment has now been
stopped in Norway because the current is driven through
the heart region. The Drionic is an example of an alternative
device for the treatment of palmar hyperhidrosis where this
problem is solved. When one hand is placed on the Drionic,
half the hand is connected to one electrode through a wet
sponge, and the other half to the other electrode through
