A Washable and Reusable Sensor System for Moisture Detection

<br/><br/><b>This presentation investigates the integrity and repeatability of a washable system for moisture detection that integrates a polymer optical fiber (POF) with a consumer optical time-of-flight (ToF) sensor at the edge of an absorbent textile. Throughout the COVID-19 pandemic, disposable paper products have been in short supply, including diapers, paper towels, respirators, tampons, and incontinence supplies. Interest in washable and reusable products has soared. At the same time, there is a growing product roster of disposable electronic sensors for healthcare, such as moisture sensors in disposable diapers. These devices typically interface with a computer using resistive electronic traces and are not intended to be reused. Non-noble metal thin films, including the silver plating used in many e-textiles, corrode over time with exposure to water, losing their conductivity. All-polymer optical fiber based sensing methods promise a better washability outcome, as long as the materials can detect the signal of interest and can handle the mechanical stress of the wash-and-dry cycle. Better printable electronic materials, alloys, and soft composites are being developed, but soft optical sensors also have two key advantages versus electronic sensors: lower temperature sensitivity, and immunity to electromagnetic interference. The materials are deformable and often intrinsically stretchable, with similar mechanical properties to wearable stretch fabrics and soft robotic systems. </b><br/><br/><b>In the work reported here, a moisture sensor created from a plastic optical fiber (POF) is embedded in an absorbent bedding system that connects to a detachable optoelectronic readout module. This solution addresses the supply-chain challenge through reusability. The most popular type of plastic optical fiber (POF), polymethyl methacrylate (PMMA), is more suitable than glass fiber because of its flexibility and ability to recover from few-percent strains. However, PMMA is mildly hygroscopic. Fibers that absorb water can become brittle during the machine wash cycle, which soaks, rinses, and tumbles the fibers, resulting in fatigue on the fibers and ultimately breakage. Our approaches include three upgrades to commercial PMMA fibers. First, a waterproof coating is developed to mitigate damage to PMMA optical fiber caused by water absorption during the washing cycle. Second, a commercial embroidery machine is used to attach the fibers along paths in a nonwoven stabilizer layer with closely-spaced stitches, for well-distributed mechanical stress along the fiber. Third, PMMA is compared to alternative urethane-based fibers that have lower optical clarity but better stretchability for mechanical resilience.</b><br/><br/><b>A magnetic housing was also developed for quickly and uniformly assembling connectors onto the ends of soft waveguides, increasing the optical transmission of soft materials by improving their alignment with the optical source and detector. The connector provides a quick and repeatable reconnection to the sensor pad after the wash.</b><br/><br/><b>In the tests to be presented, PMMA fibers and other POFs are integrated into fabric pads, connected to junctions whose optical transmission changes in the presence of moisture, and the optical transmission in wet and dry conditions is tested with the magnetically-coupled optical source/sensor pair both before and after wash cycles. The integrity of the fiber is validated by comparing the wet/dry contrast intensity of the fibers before and after the wash cycle.</b>