Wearable Skin Sensors and understanding the Signal

The purpose of this note is to give a brief synopsis of some preliminary work performed at ZP for measuring the electrical properties of skin.

At Zimmer and Peacock we develop wearable biosensors for our clients and customers, including: lactate, glucose, potassium, sodium, hydration, perspiration/sweat.


In this note we have taken one of our off the shelf sensors and investigated them for measuring the conductivity of the skin, applications of such sensors can be used to monitor the surface of the skin or depending on the excitation signal we use we can penetrate into the skin and determine the conductivity below the top layer of skin.


The sensors we used for these conductivity based sensors are shown in the adjacent figure.

Having decided which electrode to use one can choose to investigate the different layers of the skin through the  judicial use of frequency. In simplest terms the higher a frequency the more it penetrates into the skin.


For this investigation we used one of our off the shelf electrochemical impedance spectrometers.

At Zimmer and Peacock we investigated frequencies up to 1 MHz but the useful information really stopped at approximately 100 kHz. We were able to extract signal from the frequency spectrum by viewing it in the Bode Plot and the Nyquist Plot and using a modelling technique called Equivalent Circuits.

The actual data from the study of putting a ZP electrode on the skin is shown in the adjacent image; what we show is that the data obtained on the skin could be modelled with a simple equivalent circuit, but that was in fact a poor model for the skin and a better model was the Advanced Equivalent Circuit shown in the adjacent figure.

The adjacent image shows both data gathered on the skin and the circuit necessary to understand all the elements that contribute to the signal.

The experimental set-up is shown in the adjacent figure.


What was finally interesting was that we were able to get some interesting insight by moving the sensor around the body, which is shown in the adjacent image.