MEMS ultrasound for active implantable devices

Abstract

During the past decade, Implantable biomedical devices have emerged as an efficient treatment for neurological disorders. Among them, low-intensity focused ultrasound (LIFU) has recently gained interest due to its focusing ability, depth of penetration and reversible effects. Although the exact biophysical mechanism that leads to the interaction between ultrasound and neurons is not clear yet, many successful applications have been reported both in vitro and in vivo in animals, raising prospects for clinical applications. Nevertheless, treatments were mainly delivered transcranially due to the lack of implantable technologies, constraining the frequency range to below 1 MHz. Thanks to the development of biocompatible capacitive micromachined ultrasound transducers (CMUTs), ultrasound has become implantable, opening the way for high frequency ultrasound neuromodulation. In addition, implantable biomedical devices are becoming smaller and smaller, requiring their power supply to follow this trend. Batteries can be a limiting factor for their miniaturisation and other powering methods have been investigated. Among them, ultrasound as a means to wirelessly transfer power represents the best trade-off between the amount of available power, implanted receiver size, and depth of penetration. Also in this case, biocompatible CMUTs could be used as transducers to power an implanted device. Yet they require a DC bias source in the order of tens of volts which is not practical for an implanted device. In this thesis the application of ultrasound for implantable biomedical devices is investigated. First, the modulation of the excitability of in vitro mice hippocampal brain slices (normal and epileptic) using high frequency sonication protocols is explored. Secondly, wireless power transfer using CMUT devices with charges trapped in the dielectric which do not require a DC bias voltage is investigated. The results indicate that high frequency ultrasound clearly affects the excitability of neurons, having an excitatory effect when delivered using a 5 MHz continuous wave sonication protocol, while it inhibits the neurons when they are exposed to pulsed wave ultrasound at 13 MHz. The intensities used were in line or much lower compared to values used by other groups of researchers. In addition, ultrasound power transfer with an efficiency higher than 48% was achieved using CMUT devices with charges permanently trapped in their Al2O3 layer. This efficiency was in line with values obtained by other groups using piezoelectric transducers.
Based on these results, it is clear that ultrasound, specifically using implantable CMUT devices, has a great potential to be used in bioelectronic medicine applications for both the treatment of neurological disorders as well as a means of power transfer.