A compound-structured piezoelectret system and its applications in wearable health monitoring

Piezoelectrets, also called ferroelectrets, can exhibit promising piezoelectric properties and have plenty of applications in wearable health monitoring. Usually, the cellular structure of piezoelectrets is of outstanding importance for their sensing properties, and structure improvement and optimisation would be a possible way to realise high-performance piezoelectrets. The authors proposed a compound-structured piezoelectret system, where a layer of polypropylene foam was sandwiched between two layers of solid polytetrafluoroethylene, resulting in a combination of a foam-structured and a layer-structured piezoelectrets. The compound systems are thin and flexible, they can exhibit stable electrical outputs, they have relatively broader linear working range under pressure, and promising mechanical sustainability for multiple testing. The results reveal that the compound system can be considered as a simple addition of both components, and each component contributes linearly and independently to the whole system. The application potential of this proposed compound system has been demonstrated by sleep monitoring together with carotid and radial pulse recordings, where many useful physiological information including breath, heartbeat, and pulse details can be extracted from the signals acquired by the compound system. A type of flexible sensor system that is very competitive for future portable and wearable applications may be provided.