A Wearable Reflectance PPG Optical Sensor Enabling Contact Pressure and Skin Temperature Measurement

In the long term we have been focused on mapping the influence of examination using magnetic resonance imaging (MRI) technique on the mental and physiological state of a tested person. The stress effect during this examination manifests in a current state of a human cardiovascular system including changes in the arterial stiffness and the heart rate and can be monitored by wearable optical sensors based on photoplethysmography (PPG) principle.

It is well known that the actual state of the skin surface including a color, a temperature, and other factors have an influence on properties of the sensed PPG signals. We have developed the wearable PPG sensor with a contact thermometer to carry out a measurement of the skin temperature. The precision of the determined PPG wave features depends also on the position of the optical sensor and the contact pressure exerted in the place of sensor. Force-sensitive resistors (FSR) can be successfully used to measure the localized physical pressure.

Motivation of our work is to analyze influence of the adjusted contact pressure on the quality and temporal features of sensed PPG waves. This paper describes realization of a wearable PPG sensor including also an integrated contact thermometer and an FSR element for contact pressure measurement. To enable application of this sensor in the weak magnetic field environment, all parts of sensor are shielded by aluminum boxes. The FSR sensor film is mounted on surface of the cover box near hole for the light source and the photodetector. For different contact forces on a finger/wrist, we will realize measurement of PPG signals to collect a database of PPG wave records. Then, we will statistically analyze the determined PPG wave features to make practical recommendation about setting the contact force which is important for long-time measurement experiments inside the MRI device.