The long-term reliability of pre-charged CMUTs for the powering of deep implanted devices

Abstract

Recently, focused ultrasound has been proposed to power deeply implanted medical devices. Almost exclusively, lead zirconate titanate (PZT) transducers are used to convert acoustic energy into electrical energy. Unfortunately, these lead containing devices cannot be hermetically encapsulated since that would block the ultrasound. We propose the use of biocompatible Capacitive Micromachined Ultrasonic Transducer (CMUT) elements to replace traditional PZT transducers. In addition, to eliminate the external bias voltage, we introduced a charge trapping Al2O3 layer inside the CMUT to create a built-in bias voltage. These devices can be pre-charged and used as a receiver for US power. In this work, the viability of charged CMUTs to power deep implants was explored by investigating the effect of the charging parameters and by performing Accelerated Lifetime Tests (ALT). The estimated lifetime at body temperature ranges between 2.5 to 3.5 years at body temperature, which significantly depends on the charging parameters. Keywords-capacitive micromachined ultrasound transducers, pre-charged CMUT, zero-bias transducers, ultrasound power transfer, accelerated lifetime test

PMUT and CMUT Devices for Biomedical Applications: A Review

“…Commonly, PZT-based transducers are used to convert acoustic energy into electrical energy. Unfortunately, these lead containing devices require hermetic encapsulation for them to be implanted inside the body, and this severely degrades their energy-efficiency [136]. Other lead-free piezoelectric materials that are biocompatible, such as polyvinylidene fluoride (PVDF) and barium titanate (BaTiO 3 ), have been investigated, however their piezoelectric constants are significantly lower compared to the conventional PZT.…”

“…Other lead-free piezoelectric materials that are biocompatible, such as polyvinylidene fluoride (PVDF) and barium titanate (BaTiO 3 ), have been investigated, however their piezoelectric constants are significantly lower compared to the conventional PZT. Additionally, also AlN-based PMUTs and CMUTs have been studied, although AlN PMUTs feature good piezoelectric properties, their operation frequency is larger, thus determining a higher ultrasound attenuation along its travelling path; CMUTs, on the other hand, support traditional ultrasound frequencies, in the range of a few-to-tens of MHz while also allowing seamless integration in CMOS processes [136]. The main disadvantage of CMUTs is the required higher bias voltage, which can reach more than 100 V and is clearly not suitable for wearable or implantable applications where a low-voltage source like a battery needs to be used instead: the need for high external bias voltages, however, can be eliminated by creating a built-in bias voltage through charge trapping Al 2 O 3 layer inside the CMUT [136].…”

cMUT technology developments

“…Because cMUTs (1) may have shorter lifetimes than piezoelectric transducers [25] , [65] , (2) can be batch-processed and monolithically integrated with front-end electronics necessary for 3D imaging [52] , and (3) are well-suited for high-frequency imaging arrays [66] , there has been ongoing interest in lower-cost cMUT-based devices for single-use applications. These devices and applications have included catheters, endoscopes, and “patch” arrays (rigid sub-arrays linked by flexible interconnects) for monitoring.…”