Wearable Water Content Sensor Based on Ultrasound and Magnetic Sensing

Fluid accumulation in the lower extremities is an early indicator of disease deterioration in cardiac failure, chronic venous insufficiency and lymphedema. At-home wearable monitoring and early detection of fluid accumulation can potentially lead to prompt medical intervention and avoidance of hospitalization. Current methods of fluid accumulation monitoring either suffer from lack of specificity and sensitivity or are invasive and cost-prohibitive to use on a daily basis. Ultrasound velocity in animal and human tissue has been found to change with water content. However, previous prototype fluid monitoring sensors based on ultrasound are cumbersome and not wearable. Hence, in this research a compact water content sensor based on a wearable instrumented elastic band is proposed. A novel integration of magnetic sensing and ultrasonic sensing is utilized, where the magnetic sensor provides distance measurement and the ultrasonic sensor produces time-of-flight measurement. Magnetic field modeling with a Kalman filter and least squares linear fitting algorithms are employed to ensure robust sensor performance on a wearable device. The combination of the two measurements yields ultrasound velocity measurement in tissue. The water content sensor prototype was tested on a tissue phantom, on animal tissue and on a human leg. The error in velocity measurement is shown to be small enough for early detection of tissue edema.