Assessing Applied Pressure in Impedance Probe by Single-zone Force Sensing Resistors

Electrical impedance spectroscopy (EIS) is widely used for tissue characterization. Most EIS systems use tetrapolar electrode probes for taking the measurements, which are converted into the electrical properties of the material under study. One of the drawbacks is the applied pressure by the probe, resulting in a change in the measured impedance. The objective of this work is to develop a sensing mechanism for measuring the applied pressure by an electrode probe while measuring the impedance spectra of skin tissue. A system for both measuring the impedance spectra and the equivalent applied force by the probe was set. A measuring circuit of the force was developed by using single-zone force sensing resistors. Calibrations were made by using a sheet of hydrogel under different applied pressures in the frequency range 10 Hz to 1 MHz. Firm and soft pressure was applied in the skin of the distal forearm from a male volunteer of 46 years old. Results showed that the higher the applied pressure the higher the modulus of the impedance is, especially at lower frequencies. This might be important for better characterized tissue with different stiffness and elasticity.