Biomedical Engineering Reference
In-Depth Information
impedance. Furthermore, the current and potential dis-
tribution in the skin will also be determined by the
electrode geometry, which must be taken into account.
The impedivities of the stratum corneum and the viable
skin converge as the measuring frequency is increased.
Measurements at high frequencies will hence normally
be largely influenced by the deeper layers of the skin. The
frequency must therefore be kept low in order to achieve
isolated measurements on the stratum corneum. A fre-
quency scan (i.e. impedance spectroscopy) cannot be
utilized in stratum corneum hydration measurements,
owing to the problems of interacting dispersion mecha-
nisms. Contrary to certain opinions (Salter, 1998), the
mere fact that the current distribution in the different
skin layers will differ between different measuring fre-
quencies, is enough to discard the multiple frequency
approach on stratum corneum in vivo (Martinsen et al.,
1999). Further complications are introduced by the dis-
persions of the electrode impedance and deeper skin
layers, and also by the Maxwell-Wagner type of disper-
sion that is due to the interface between the dry stratum
corneum and the viable epidermis.
Since the sweat ducts largely contribute to the DC
conductance of the skin, the proper choice of electrical
parameter for stratum corneum hydration assessment is
consequently low frequency AC conductance (where
DC conductance is removed) or susceptance.
There are a number of instruments for skin hydration
assessment on the market. Most of them measure at
rather high frequencies, which mean that they measure
deep into the viable skin. Some instruments use closely
spaced interdigitated microelectrodes. This reduces
somewhat the contribution from viable skin layers, but
the chance of only measuring in redundant moisture on
the skin surface is obvious for such systems. Rationales
for using a low frequency electrical susceptance method
for skin hydration assessment and description of
a method for absolute calibration of the measurements
can be found in (Martinsen et al., 1998, 2008; Martinsen
and Grimnes, 2001).
parameter sensitive to these physiological changes could
serve as a possible parameter for the assessment of skin
irritation. As for other diagnostic bioimpedance mea-
surements, the parameter should be immune to other,
irrelevant changes in the skin. To eliminate the large var-
iations in interpersonal electrical impedance baseline,
normalization by means of indexes are often used rather
than absolute impedance values.
A depth-selective skin electrical impedance spec-
trometer (formerly called SCIM) developed by
S. Ollmar at the Karolinska Institute is an example of
a commercial instrument intended for quantification and
classification of skin irritation. It measures impedance at
31 logarithmically distributed frequencies from 1 kHz to
1 MHz, and the measurement depth can to some extent
be controlled by electronically changing the virtual sep-
aration between two concentric surface electrodes
(Ollmar, 1998).
Ollmar and Nicander (1995), Nicander et al. (1996),
Nicander (1998) used the following indices:
Magnitude index
ð
MIX
Þ¼jZj
20 kHz
=jZj
500 kHz
Phase index
ð
PIX
Þ¼
4
20 kHz
4
500 kHz
Real part index
ð
RIX
Þ¼R
20 kHz
=jZj
500 kHz
Imaginary part index
ð
IMIX
Þ¼X
20 kHz
=jZj
500 kHz
where
Z
,
R
,
X
and 4 have their usual meaning. The au-
thors found significant changes in these indexes after
treatment with sodium lauryl sulfate, nonanoic acid and
benzalkonium chloride, and the measured changes cor-
related well with the results from subsequent histological
examinations. The stratum corneum is soaked with saline
before the measurements in order to provide good con-
tact between the electrode system and the skin surface
and to focus the measurements on the viable skin, al-
though the barrier function of intact stratum corneum
will still give a considerable contribution. The choice of
frequencies for the indices hence seems reasonable. The
group have also extended their impedance spectroscopy
technology to further applications. Examples are the
detection of other conditions and diseases in the skin or
oral mucosa (Emtestam and Nyr´n, 1997; Lindholm-
Sethson, et al., 1998; Norl´n et al., 1999), the early de-
tection of transplanted organ complications (Ollmar,
1997; Halldorsson and Ollmar, 1998) and assessment of
skin cancer (Emtestam et al., 1998).
After publication of a paper by Nicander et al. (1996)
where it was demonstrated that skin reactions elicited by
three irritants of different polarity created three differ-
ent histopathological patterns and that each pattern
could be correlated to corresponding patterns in the
impedance indices, the Ollmar group has taken steps
away from the data reduction technique based on the
4.1.15.3 Skin irritation and skin diseases
including skin cancer
Irritant contact dermatitis is a localized, superficial, non-
immunological inflammation of the skin resulting from
the contact with an external factor. The dermatitis may be
acute, for example, if the influence from the external
source was strong and of short duration, or of a more
chronic kind if the influence is weaker but prolonged. The
difference between irritant and allergic contact dermatitis
is subtle, and depends mainly on whether the immune
system is activated or not. Established signs of irritation
are edema, erythema and heat, and any electrical
