Wearable Strain Sensor Using Conductive Yarn Sewed on Clothing for Human Respiratory Monitoring

In this study, we fabricated a wearable strain sensor to monitor human respiration by lock-stitching stretchable conductive yarn onto a textile substrate. We developed the strain sensor by using conductive yarn consisting of silver-plated and unplated nylon yarns twisted around a lycra fiber with a lock wrinkle structure that could stretch after sewing. The slopes of the strain-stress curves of three fabricated strain sensors with a 10% strain load increased to 121 kPa, 189 kPa, and 241.5 kPa, and the relative change in resistance ( $\Delta \text{R}/\text{R}_{\sf 0}$ ) of the strain sensor increased from 67% to 116% and from 8% to 109% by respectively applying more tension and fewer locks to the conductive yarn. In addition, as we strained the strain sensor to more than 4,000 cycles, the relative change in resistance decreased by only 6% from its initial value. A respiration measurement test was conducted with the developed sensor to monitor four different respiration patterns: slow (10 bpm), fast (30 bpm), deep, and shallow respiration, and the results were compared to those of reference respiration monitoring devices. Consequently, the proposed stretchable conductive yarn-based strain sensor worked as well as commercial respiration sensors, and its feasibility was demonstrated for use in respiration monitoring for daily living.