Pre-Charged Collapse-Mode CMUTs

Abstract

Advancements in healthcare technology are driving innovation through the decentralization and personalization of medicine. The convergence of Pharma, MedTech, and ECS industries has led to new medical domains, including bioelectronic medicines and personalized ultrasound. Ultrasound, particularly through Micromachined Ultrasonic Transducers (MUTs), is a key technology in these fields. MUTs, which include CMUTs (Capacitive Micromachined Ultrasonic Transducers) and PMUTs (Piezoelectric Micromachined Ultrasonic Transducers), offer benefits over traditional PZT transducers, such as lower costs and the absence of toxic materials.
This thesis focuses on pre-charged collapse-mode CMUTs, which operate without a DC bias voltage, making them suitable for in-body applications. This is achieved by embedding a charge storage layer in their dielectric. The research involved designing and testing a first and second generation of CMUTs with different charge storage layers. The second generation showed improved performance and comparable results to standard externally biased CMUTs.
The primary application explored was using pre-charged CMUTs as ultrasonic power receivers for implantable devices. The impact of lateral and angular misalignment on power efficiency was studied, with angular misalignment being most detrimental. Solutions proposed included decreasing the frequency, partitioning the receiving transducer, and using a beamforming algorithm.
Experimental evaluations showed power conversion efficiencies up to 80% with optimal load and about 50% with a resistive load at 2.5 MHz. These efficiencies are comparable to those achieved with PZT transducers but with better biocompatibility.
Finally, the thesis demonstrated the functionality of the first ultrasonically powered implantable device using pre-charged CMUTs for battery charging. The device achieved a total system efficiency of 21%, significantly better than other implantable devices using PMUT or PZT transducers.