Zulkifli, Muhammad Hafiz Zulhilmi, et al. “Exploring the Efficacy of Piezoelectric-Based Sensory Systems for Heart Rate Monitoring in Differentiating Stress Vs. Relax Conditions”. IIUM Engineering Journal, vol. 25, no. 2, July 2024, pp. 325-37, doi:10.31436/iiumej.v25i2.3017.