Application of a piezoelectric braided cord to sense vital signs during sleep

The final goal of this study is to provide a sleep-monitoring device that indexes sleep status and allows users to measure their sleep status and easily recognize how to improve their sleep. As a first step toward achieving this, we have developed a prototype system with a simple structure consisting of a bed sheet and a pillow case embraided with a piezoelectric poly-L-lactic acid (PLLA) braided cord, enabling in situ observation of sensing body movements and vital signs during sleep. A PLLA braided cord as a material for a new wearable sensor is flexible and feels soft, which can sense complex human motion without using a complex computer system. As a preliminary experiment, the pulse wave and breathing were observed in situ using the prototype system when the subject lay awake on a bed, where the subject first lay on their back and then lay on their side. Optimization of the piezoelectric PLLA embroidery was performed only on selected patterns. It was found that the pulse wave could usually be measured by in situ observation using a pillow case embraided with piezoelectric PLLA embroidery, and breathing could usually be measured using a bed sheet with piezoelectric PLLA embroidery. On the basis of this experimental result, we measured breathing and pulse waves during sleep for eight hours. We successfully measured breathing and pulse waves using the optimized system with a bed sheet and a pillow case embroidered with piezoelectric PLLA embroidery even though the subject, who wore pajamas, often turned over in his sleep.