VitalLog: Development of a Compact Multimodal Physiological Signal Acquisition System for Tele-Monitoring Applications

Abstract This study addresses the limitations of bulky and costly clinical monitoring systems by developing VitalLog, a compact multimodal physiological signal acquisition device for tele-monitoring applications, with a fabrication cost of approximately US$45 per unit. The aim was to design and validate a portable system capable of real-time, continuous tracking of autonomic nervous system activity through simultaneous acquisition of electrocardiography (ECG), photoplethysmography (PPG), and galvanic skin response (GSR). The device integrates an ESP32-S2 microcontroller for analog-to-digital conversion, task scheduling with FreeRTOS, and secure wireless data transmission via MQTT, while server-side software performs signal processing, feature extraction, and visualization through desktop and mobile interfaces. Validation experiments compared ECG-derived heart rate variability (HRV) with recordings from a BIOPAC MP160 reference system, showing strong agreement with 98% of paired data points within the 95% limits of agreement and minimal bias; PPG-derived intervals correlated with ECG R–R intervals (R = 0.955, MAPE = 3.19%), and GSR measurements achieved a mean error of 1.25% relative to theoretical resistance values. These results demonstrate that the system achieves clinical-grade fidelity while maintaining portability and low cost, providing a scalable platform for long-term physiological monitoring, digital diagnostics, and integration into telehealth workflows.