Impedance is measured by applying a small electric current e.g. via 2 electrodes and picking up the resulting small voltage with another pair of electrodes: The lower the voltage the lower the tissue impedance for a given current. Fig. 7.43 shows an example with electrodes around a cylinder, for instance skin surface band electrodes around an arm or a leg.
Fig. 7.43 in Grimnes & Martinsen (2008): "Bioimpedance and Bioelectricity Basics", 2. ed., Academic Press.
Tissue consists of cells and membranes, and membranes are thin but have a high resistivity and do electrically behave as small capacitors. By using high measuring frequencies the current passes right through these capacitors, and the result is dependent on tissue and liquids both inside and outside the cells. At low frequencies, however, the membranes impede current flow, and the results are dependent only on liquids outside the cells.
In an ideal model like Fig. 7.43 the impedance Z is given by:
Z = ρ L / A
where ρ is the resistivity of the tissue, L is the distance between the pick-up electrodes, and A is the cross-sectional area (the equation is actually only correct if the tissue is homogeneous and the current is applied by band electrodes far away from the pick-up electrodes. For the real setup with 4 band electrodes the mathematics are more complicated.)? It is clear that by measuring impedance, either r or L or A may be found if the two other quantities are known. Bioimpedance can therefore be used to measure volumes, shapes or tissue electrical properties. Geddes and Baker (Principles of Applied Biomedical Instrumentation, 1989) wrote about bioimpedance:
"The elegantly simple technique requires only the application of two or more electrodes, and it has been used successfully for many years to detect a remarkable variety of physiological events."
Bioimpedance applications include e.g.:
- Skin water content
- Impedance imaging (tomography)
- Body composition (training, nutrition)
- Impedance Cardiography (ICG)
- Cardiac Output monitoring
- Ablation monitoring
- Blood volume
- Meat quality assessment
- Tissue ischemi monitoring
- CO2 catheter transducer
- Single cell motion microscope
- Single cell counting and characterisation
- Fingerprint sensors
A complete introduction to the field of bioimpedance can be found in Grimnes & Marinsen (2008): Bioimpedance and Bioelectricity Basics, 2. ed., Academic Press.