Highly stretchable and wearable strain sensors using conductive wool yarns with controllable sensitivity

Wearable strain sensors for various applications, such as human motion detection, soft robotics, and healthcare have recently received extensive attention. Although a number of strain sensors based on various active materials have been proposed, a simple and practical method to obtain both high stretchable and sensitive strain sensors remains challenging. This paper presents a simple, scalable and environment-friendly fabrication method for wearable strain sensors, based on wool yarns, as abundant, lightweight, and stretchable natural materials. A simple coating technique was used to achieve highly conductive wool yarns using a conductive ink. Different types of strain sensors were then fabricated by changing the shape of the active material within the elastomer to tune their sensitivity. In detail, the conductive yarns were sandwiched within the Ecoflex in the form of a straight line (CWY-1 strain sensors) or serpenoid curves (CWY-2 and CWY-3 strain sensors). The strain sensors were fully characterized up to 200% of applied tensile strain. Gauge factors of 5 and 7.75 were found within the percentage stretch ranges of 0–127 and 127–200 %, respectively, for the CWY-1 strain sensors. We have also demonstrated the ability of the strain sensors to monitor human muscle and joints movements.