Batteryless Internet of Things (IoT) devices powered by energy harvesting enable sustainable and maintenance-free operation, but face challenges in achieving synchronised bidirectional communication between intermittently-powered nodes. This thesis presents CardioSync, a novel framework that leverages the human heartbeat as a shared clock to synchronise Bluetooth Low Energy (BLE) connections between battery less devices. CardioSync integrates a low-power optical heart rate sensor to capture real-time heartbeat data. A peak detection algorithm identifies distinct heart rate peaks, establishing synchronisation points for connection-setup events. By scheduling BLE advertising and scanning activities timed with detected peaks, CardioSync aligns connection attempts between intermittently-powered devices. This heartbeat-based synchronisation is integrated into the existing FreeBie architecture for intermittent BLE communication. Experimental evaluations demonstrate CardioSync's ability to successfully establish synchronised connections between batteryless nodes, reducing average connection setup time by up to 1.8x compared to an asynchronous FreeBie system. However, these gains incurred increased power consumption due to the integrated sensor. CardioSync enhances FreeBie's capabilities, enabling efficient intermittent-to-intermittent BLE connections. The proposed technique shows promise for advancing body sensor networks through sustainable and maintenance-free operation. Further work should optimise sensor utilisation and explore adaptive synchronisation to improve energy efficiency.