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

Abstract

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.

A Prototype Sensor System Using Fabricated Piezoelectric Braided Cord for Work-Environment Measurement during Work from Home

“…Therefore, there is no need for a circuit system or a personal computer (PC) that requires complex calculations for processing because sensing is the only required procedure. Taking these features together, piezoelectric PLLA braided cords can be used as sensors that can detect expansion, contraction, shear, and bending, depending on how they are tied or embroidered with the type of stich pattern [ 3 , 4 , 5 , 20 , 21 , 22 ].…”

“…We considered attaching a piezoelectric PLLA braided cord as a sensor to a cushion placed on a chair used at home when working from home. The idea was to embroider cushion covers with the piezoelectric PLLA braided cord, which had been studied already for a long time (an example of embroidery with the PLLA braided cord is shown in Figure 2 and used as a sensor) [ 3 , 4 , 5 , 20 , 21 ] However, with this ideas, in order to embroider the sheets, it is necessary to collect them from each home and then prepare them according to the shape of each chair before experiments on vital sign sensing during telework can be conducted. This takes too much time and effort to prepare, and clinical trials cannot be conducted.…”

“…Therefore, we attempted to develop a piezoelectric PLLA braided cord sensor on a tape that can be attached to each cushion cover as it is. The knowledge gained from our studies on piezoelectric PLLA embroidery is useful for the design of the motion sensor used here [ 3 , 4 , 5 , 20 , 21 ]. When the piezoelectric PLLA braided cord is attached to clothing and used as a sensor, the sensor does not directly bear the weight; however, in this setup, a static load such as human weight is applied to the cushion of the chair.…”

“…That is, the load is always applied vertically to the cord sensor. This makes a large difference in the design of the sensor [ 3 , 4 , 5 , 20 , 21 ]. For example, if the piezoelectric PLLA braided cord is under the weight of the person sitting on the chair, the braided cord may be displaced from its initial position on the chair for measurements, which should cause abnormal conditions that should be prevented [ 3 , 4 , 5 , 20 , 21 ].…”

“…This makes a large difference in the design of the sensor [ 3 , 4 , 5 , 20 , 21 ]. For example, if the piezoelectric PLLA braided cord is under the weight of the person sitting on the chair, the braided cord may be displaced from its initial position on the chair for measurements, which should cause abnormal conditions that should be prevented [ 3 , 4 , 5 , 20 , 21 ]. The basic condition that ensures the stability of sensing is to provide a safe, easy, and reliable means of mounting so that anyone can use it easily.…”