A magneto-mechanical bio-interface with ultra-low-power analog backscatter for inclusive cardiac monitoring

Pervasive cardiac monitoring demands wearable systems capable of long-term, reliable operation across diverse users and environments. Modern optical wearables can provide accurate pulse-rate estimates, but performance can still depend on optical coupling, contact conditions, and device-specific sensing strategies. Here we report an ultra-low-power wireless bio-interface tag coupling magneto-mechanical cardiac sensing with analog backscatter communication. The tag transduces heart-induced skin micro-vibrations into magnetic-field fluctuations that modulate the reflection of an incident RF carrier, bypassing on-tag digitization and active radio transmission. Across 30 participants spanning Fitzpatrick skin types I–VI and diverse body weights, the tag maintained ECG-referenced RR-interval agreement and accurate heart-rate estimation relative to a commercial smartwatch benchmark under the tested protocol. The tag consumes 1.26 mW in active mode and 113.9 μ W under duty-cycled operation, enabling over five months from a coin cell. Multi-site monitoring during daily activities and hardware-in-the-loop EEG, EMG, and EOG replay further demonstrate extensibility toward long-term multimodal health monitoring.