Every biological object such as a cell, tissue or organ, has the ability to impede alternating electrical current. This is called electrical bioimpedance. The prefix –bio means that it is the impedance of living objects. Other materials also have similar electrical properties and in these cases bioimpedance is just called impedance.
Calculating the impedance of biological objects
When measuring a cell’s bioimpedance, you first need to apply an alternating current to the biological object and then measure the voltage that is created. It is also possible to do it vice versa – first, apply the voltage on the object and then measure the induced current.
Coming back to the first case, the voltage that you measured has the same frequency as the initially applied excitation current. However, the amplitude and the phase shift differ depending on the biological object itself and the excitation frequency. Now, by knowing the voltage and the current it is possible to calculate the impedance. It is a complex quantity consisting of two parts. The real part is called resistance and the imaginary part is called reactance. The practical value of bioimpedance is usually hidden in the characteristics of these two parts, mainly the magnitude and phase of the complex impedance.
Cells and tissues are good conductors of electricity
Biological objects consist of different tissues that are made up of single cells. In electrical terms, all cells are encapsulated by a very thin dielectric membrane, which has a rather big electrical capacitance. The cell itself is filled with intracellular fluid and the space between the individual cells is filled by intercellular fluid. Both fluids are good ionic conductors.
All these elements form a simple electrical circuit in the cell. You first have a resistor, represented by the conductivity of intracellular fluid. This is connected in series with a capacitor, represented by the capacitance of cell membranes. The third component in this circuit is another resistor connected in parallel characterizing the conductivity of cell membranes.
Biological processes are visible at lower frequencies
Bioimpedance can be used to monitor a range of physiological processes, it can detect how biological cells are generated, for example in tumour progression and the healing of wounds and bone fractures. It can also detect the changes in the composition of cells and tissues, and thus help diagnose dehydration, and water accumulation in lungs. Finally, bioimpedance is also a cost-effective and straightforward way to monitor breathing and respiratory rate.
These different biological phenomena described above take place in the frequency range of 1 kHz to 1 MHz. This is also called the beta dispersion range. At lower frequencies, cell membranes isolate intercellular fluid and the electrical current flows around the cells.
At higher frequencies, the impedance of cell membrane is very low and electrical current flows evenly through the whole tissue.
The changes in the ratio between the inter- and intracellular volume can be monitored in the impedance.
Bioimpedance based medical devices
Bioimpedance is a well-known method for detecting and monitoring different biological processes. The medical industry is using bioimpedance but not nearly enough. The problem is in the complex nature of impedance measurement. However, for companies interested in developing a diagnostic medical device, it is definitely worth looking into. Eliko can help as we have developed impedance-based measurement devices for well over a decade already – take a look at our latest smart needle project.
