Flexible circuit-free system via passive modulated ultrasound for wireless thoracic pressure monitoring.

Implantable medical devices (IMDs) provide effective medical solutions for diverse health care applications. Electrical circuits are crucial for implantable devices due to the requirement of intended functions, such as communication with external devices. Circuits have several risks, such as biocompatibility issues, power limitations, or size constraints. In this work, we propose a passive modulated ultrasound (PMU) principle for IMDs and develop a circuit-free ultrasonic system (CUS) for thoracic pressure monitoring. The PMU principle can passively modulate monitored physiological signals into ultrasound pulses without using electrical circuits or power supply. The size of the developed CUS is only 2.5 millimeters in radius and 850 micrometers in height. Animal experiments demonstrated that the CUS, with a high sensitivity (-22.96 millivolts per kilopascal), can monitor thoracic pressure to assist in diagnosing different heart diseases, including cardiac arrest and myocardial infarction. The PMU provides a human-friendly wireless sensing and communication strategy for IMDs, which promotes advancements in health care applications within the human body.