Electrical Impedance of Biological Tissues

active resistance

inductance

capacitance

filling

none

axis

Electrical impedance (or simply impedance or total resistance)

Z

measures opposition to a sinusoidal alternating current (AC). Electrical impedance extends the concept of resistance to AC circuits. Impedance is the complex generalization of resistance. In most cases, impedance is a function of the frequency (ω in radians/sec or

f

in hertz), resistance

R

(in ohms), inductance

L

(in henries), and capacitance

C

(in farads):

Z=

2

R

+

2

(

X

L

-

X

C

)

The reactance

X

L

-

X

C

is the imaginary part of the impedance,

X

L

=ωL

;

X

C

=1/(ωC)

. We consider the impedance to be a function of frequency only;

L

and

C

are constant parameters. The impedance over a range of excitation frequencies is plotted. In the case of biological tissues, the impedance is

Z=

2

R

+

2

X

C

.

The plots show the impedance

Z

(total resistance) in red, the biological tissue impedance in gray, and the capacitance

C

resistance in green.