Washable Smart Threads for Strain Sensing Fabrics

We present washable smart sensing threads for creation of smart wearable fabrics. More specifically, we report the fabrication of a facile stretchable and washable thread-based strain/force sensor, which can be sewn into wearable textiles and is capable of measuring human physiological motion. We use polybutylene terephthalate (PBT) puffy threads as our substrate which is made strain-sensitive using a simple reel-to-reel coating process. While PBT in itself is not very stretchable, threads made from them have coiled helical filaments. Stretching such threads uncoils these filaments imparting them stretchability. To convert them into strain sensor, we coat these PBT threads with resistive carbon ink. The straightening (uncoiling) of the helical filaments changes its resistance. There is a negligible change in resistance after repeated washing steps. Such washable immunity is achieved by coating thin layer of PDMS on them. The sensor exhibits a constant gauge factor of ~2.5 within its working range (50% strain) and exhibits a linear response to applied strain up to 50% of its initial length. Such strain sensing threads can form the basis of smart fabrics that can track human motion and for use in real-time physiological monitoring. We present promising results on the detection of the human form (seated versus standing) and human motion (running versus walking) using these threads sewn onto trousers.