*4.3. Electrochemical Signals*

Metabolic factors, such as glucose level, have been linked to increased risk for arrhythmias. However, monitoring of metabolite levels has traditionally been performed

with invasive devices. Recently, wearable glucose-detection devices have allowed for repeated measurement of glucose levels without the burden of an implantable device [70]. Bandodkar et al. reported a tattoo-based noninvasive glucose-monitoring system based on Ag/AgCl ink electrodes and a reagent layer [71]. In vitro characterization proved the system's ability to detect micromolar levels of glucose, and on-body evaluation proved its ability to detect a rise in glucose after a meal. Sempionatto et al. improved this by creating a stretchable patch for both hemodynamic and metabolic monitoring by combining PZT ultrasound transducers and printed polymer composites [72]. Blood pressure could be monitored through the sensor array, while chemical sensing was realized through sweat detection. This study showed the potential of the integration of acoustic and electrochemical sensing.
