92

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4 Measurement of Biosignals and Analog Signal Processing

Tab. 4.1: Electrical conductivity of selected tissue types and physiological saline.

Tissue/Substance

Conductivity in S/m

Physiological saline

2

Body fluid

1,5

Blood

0,67

Heart muscle

0,1–0,2

Brain

0,17

Kidney

0,16

Skeletal muscle

0,08–0,25

Lung

0,07–0,1

Fat

0,02–0,1

Bone

0,006–0,02

to the biosignal source in time and space. In practice, one selects the leakage points

according to a standardized arrangement on the body surface, if possible.

Electrical contact with the skin is necessary for biosignal measurement. The skin

is composed of the three layers epidermis, dermis and subcutis (cf. Figure 4.2). The

total thickness is between 1.5 and 4 mm. Among other things, the skin serves as a fluid

barrier for the body. The horny layer, the outermost layer of the epidermis, plays a de-

cisive role in this. The stratum corneum consists of dead epithelial cells joined together

to form firm and compact layers with relatively high mechanical and chemical resist-

ance. Its function as a fluid barrier also provides an electrically insulating effect, since

electrolytes can hardly penetrate the epidermis. Nevertheless, an electric current can

flow through the skin. This is primarily due to sweat pores and hair ducts that begin

in the lower part of the dermis and subcutis and penetrate the epidermis. Ion flow is

possible along the pores and channels.

For electrical modeling, the skin can be understood as a parallel circuit of capa-

citor and resistor, where the largely insulating epidermis accounts for most of the ca-

pacitive part and the sweat pores and hair ducts for the resistive part. The total im-

pedance of the skin ZH is of the order of a few kΩfor low-frequency currents up to

100 Hz, although it is clear from what has been said previously, that this value de-

pends strongly on the sweat gland activity or the degree of hairiness. Furthermore, it

varies locally and is frequency dependent because of the capacitive property. For sim-

ulation purposes, in the equivalent circuit diagram according to Figure 4.2 (left), for

example, a capacitance value CH of 200 nF and a resistance value RH of 2 kΩcan be

used. For the electrical modeling of the lower skin region, an additional resistor of the

order of 100 Ωis inserted in series with the parallel circuit from Figure 4.2.