skinimpedance

Skin impedance refers to the opposition that biological skin presents to the flow of alternating electrical current. It is a key parameter in bioelectrical sensing and transcutaneous stimulation, influencing measurement quality and safety in devices such as ECG, EEG, and wearable sensors.

The skin is a layered, highly inhomogeneous medium. The outermost layer, the stratum corneum, provides substantial resistance and capacitance, while the underlying epidermis and dermis offer additional conductive pathways. Because these properties are frequency dependent, skin impedance changes with the applied signal frequency. At low frequencies, impedance is dominated by the resistive barrier of the stratum corneum; at higher frequencies, capacitive pathways allow more current to pass, reducing the overall impedance.

Commonly, skin impedance is modeled with equivalent circuits that combine resistive and capacitive elements, sometimes including a constant-phase element to reflect non-ideal behavior of the electrode–skin interface. In practice, a two-electrode or four-electrode configuration may be used, and impedance is often measured across a spectrum of frequencies (impedance spectroscopy) to characterize both contact quality and tissue properties.

Factors influencing skin impedance include moisture and sweat, temperature, age, skin thickness and condition (e.g., calloused skin), electrode type and contact area, gel or interface materials, and mechanical pressure. These factors introduce variability across individuals and over time, presenting challenges for consistent sensing and stimulation.

Applications rely on managing skin impedance through proper electrode design and preparation, such as conductive gels, skin cleaning, pressure control, and moisture management, to improve signal quality and safety in biomedical devices.