Measurement of Skin Surface pH with a Non-invasive Dry pH Sensor

Recent reports suggest that skin barrier function and atopic dermatitis are related to skin surface pH. We are developing a portable multifunctional skin measurement system for evaluating data such as skin pH and water content. The system uses a non-invasive method for measuring the skin’s surface pH and does not require addition of water. Previously, we reported a flat, dry pH sensor based on the Nernstian response that was fabricated by ion plating. When the sensor was used on the skin, the voltage was constantly elevated because of water evaporation from the skin. Herein, we measured the skin surface pH with the dry pH sensor and a glass electrode to compare non-invasive measurements and measurements with added water. The sensor was tested by using a standard pH solution and a correlation between pH and voltage was found. All experiments were carried out on the same region of the left forearm of healthy male subjects at constant room temperature and humidity (23 °C, 30% RH). Skin surface pH was measured for 2 min with the dry electrode and with a glass electrode, and then the skin water content was measured. The increase in voltage caused by water evaporation from the skin was subtracted from the dry pH sensor results. The skin surface pH was different between the non-invasive dry sensor and the conventional sensor. The response of the pH sensor voltage was affected by skin water evaporation and by water-soluble substances in the stratum corneum.