Feature: Medical
Figure 1: Block diagram of the bioimpedance measurement system
Bioelectrical impedance – a tool for assessing the composition of the human body
By Cosimo Carriero, Field Applications Engineer, Analog Devices
T
he electrical properties of biological tissues are classified as active or passive depending on the source of electricity. We refer to “active response” when biological tissues generate electricity due to the ions inside their cells. These electrical signals are referred to as
biopotentials, and the best-known examples are found in electrocardiography and electroencephalography. The response is “passive” when the biological tissue responds
to an external electrical stimulus, like a current or voltage generator, in which case we are dealing with bioimpedance.
22 March 2021
www.electronicsworld.co.uk
Bioelectrical impedance analysis Bioelectrical impedance analysis is a low-cost, non-invasive technique for measuring the composition of the human body and evaluating its clinical conditions. Impedance is a complex quantity composed of a resistive value R (real part), mainly due to the total volume of water in the body, and a reactive value Xc
(imaginary
part), mainly due to the capacitance created by the cell membrane. Te impedance can also be represented as a vector, with module | Z | and phase angle φ, where φ plays a fundamental role in determining the composition of the body:
(1) (2) (3)
Te resistance R of a conductor with cross-sectional area S and
length l, and the capacitance C of a flat parallel plate capacitor with surface area S at a distance d are given by the following equations:
(4) (5)
As Equations 4 and 5 show, resistance and capacitance depend
and the measuring instrument. RE of extracellular fluids, RI Cm
on parameters like length, distance and surface area, linked to the adopted measurement system and parameters like resistivity ρ and dielectric constant ε, which are closely related to the type of material (in this case biological tissue) to be measured. Figure 1 shows a simplified electrical model of bioimpedance takes into account the resistance
is the resistance of intracellular fluids, and
is the capacitance of the cell membrane. Instrument and body are connected via electrodes applied to the skin. Te instrument
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