Abstract—Accurate noninvasive quantification of subcutaneous edema remains a clinical challenge due to the lack of objective tools beyond conventional subjective methods such as pitting edema scoring. We present a wearable fringing field capacitive sensor system for real-time monitoring of interstitial fluid (ISF) accumulation in the lower extremities. The sensor features a coplanar capacitive electrode encapsulated in thermoplastic polyurethane (TPU), integrating a passive LC tank, a capacitance-to-digital converter ASIC, and a wireless MCU, forming a compact and lightweight prototype (2.65 cm × 3.35 cm, 7.78 g). The proposed sensor utilizes fringing electric field interactions to noninvasively penetrate the epidermis and reach the subcutaneous tissue layer. Changes in local dielectric properties due to ISF volume variations modulate the measured capacitance, producing a resonant frequency shift. To ensure measurement robustness against environmental and physiological variability, a dual-frequency resonance strategy is employed to compute a Health index (HI) for edema quantification. A three-tier validation framework was conducted, including benchtop beaker phantom testing, ex vivo porcine tissue studies, and a pilot clinical trial with 10 healthy participants and 15 dialysis patients. The sensor effectively distinguished healthy from edematous subjects and quantified pre- to post-dialysis changes. A linear correlation was observed between HI variation and ultrafiltration volume (Pearson’s r = 0.803). In conclusion, this study presents a fringing field-based capacitive sensing strategy for quantitative assessment of subcutaneous edema, supporting early detection of fluid overload in home settings and enabling personalized treatment planning and endpoint adjustment during clinical dialysis in the future.

Index Terms— Capacitive sensor, Dialysis, Edema, Fringing field, Interstitial fluid, Resonant frequency.