A Pneumatic McKibben Muscle-Based Limb Volume Phantom Model for Wearable Strain Sensor Validation

Wearable strain sensors are a promising technology for continuous health monitoring; however, existing validation approaches present a fundamental trade-off. Bench-top methods offer high resolution and repeatability but lack fidelity to real-life use conditions, whereas human-subject testing provides physiological relevance with limited resolution, repeatability and coverage across use cases. We present a novel validation platform - a limb volume phantom model (LVPM) - that bridges this gap by integrating a soft pneumatic artificial (McKibben) muscle with a compliant limb phantom and a closed-loop, camera-based control system for precise circumferential regulation. The LVPM provides a 24cm-long region of uniform circumference along its longitudinal axis and enables programable control of the circumference from 30 to 37 cm with an optical diameter resolution of approximately 1 mm, < 0.5% steady-state error, and settling times below 40s. To demonstrate its utility, a soft, highly stretchable capacitive strain sensor with an interdigitated electrode architecture was evaluated using the LVPM and compared against conventional uniaxial tensile testing and tape-based measurements on a human limb. The LVPM achieved a measurement range and resolution comparable to uniaxial testing while producing strain-response trends that more closely resemble in vivo behavior, without the resolution and repeatability limitations inherent to manual measurements. These results establish the LVPM as an effective and practical validation platform for wearable strain sensors, offering a balanced combination of biomechanical compliance, controllability, repeatability, measurement range, and resolution.