Instrumented Socks with Novel Sensors for Fluid Accumulation Monitoring

Abstract

The overarching goal of this dissertation is to develop wearable sensors that can be integrated onto an instrumented sock for home-based monitoring of lower leg fluid accumulation. Swelling in 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. Three types of inexpensive and noninvasive wearable sensors are developed: leg size sensor, tissue elasticity sensor and water content sensor. The innovative leg size sensor developed has unique features of being drift-free, and capable of misalignment-rejection. It has an accuracy of being able to differentiate 1mm changes in diameter, much smaller than any changes that can be detected by the human eye. These features were achieved by using dual magnetic sensors, an inductor for generating alternating magnetic fields, and an unscented Kalman filter estimation algorithm. Elasticity is also an important indicator of fluid accumulation and defines how soft the leg is. The novel elasticity sensor has a simple architecture of two thin-film force transducers and two 3D-printed components, which form a cantilever mechanism. Mathematical models were established for the sensor to estimate tissue elasticity. Lab tests conducted on rubber samples with slightly different softness and human body showed promising results. Several generations of instrumented socks with the leg size sensor and the tissue elasticity sensor were fabricated in the lab. These socks were tested and validated to be accurate and useful in an IRB-approved study on healthy volunteers at Mayo Clinic. The leg size sensor was also integrated into a commercially available pneumatic compression medical device for treating lymphedema. A redesigned and miniaturized leg size sensor was sewed onto a wearable band, which was then attached to the pneumatic pump-based wearable system for monitoring lymphedema treatment progress. Finally, a compact water content sensor was developed. Ultrasound velocity in animal and human tissue has been found to change with water content. A novel integration of magnetic sensing and ultrasonic sensing was utilized to measure ultrasound velocity, and renders the previous bulky device wearable.