Ultrasound Tracking and Data Telemetry using CMUTs

Abstract

Implantable Medical Devices (IMDs) offer a wide range of applications including but not limited to blood pressure monitoring, bladder pressure sensing, glucose level monitoring, neurostimulation etc. For such applications, it is often desired that an IMD is as small as possible while meeting the required objective. Consequently, using a battery as a power source for this miniaturized IMD is inhibited as it occupies huge space. A good alternative for the battery as the power source is the Wireless Power Transfer (WPT) method. Among the WPT techniques, ultrasound (US) has seen increasing popularity in the past decade owing to its unique advantages such as better resolution, lower propagation attenuation, better health benefits etc. Ultrasound WPT methods involve an ultrasound transducer that converts ultrasonic energy into electrical energy to power the IMD. One such transducer is the Capacitive Micromachined Ultrasound Transducer (CMUT). As the name suggests it is a capacitive ultrasound transducer that is micromachined on silicon substrates. Accordingly, this technology offers advantages such as the ability to integrate with CMOS technology, larger bandwidth and the ability to fabricate large arrays of these transducers. Previous research in CMUTs has proven that the CMUTs can be used to efficiently harvest US energy at depths of more than 100 mm. Consequently, this paves the foundation for miniaturized US-based implants that can be placed deep inside the human body, using CMUT technology. Building on this foundation, this thesis work explores the next steps in realizing a CMUT based implant. Two research questions are the main focus of this work: 1. How can we locate such small sized deep implants in order to transfer power efficiently? 2. How can we communicate with this device using ultrasound waves? Addressing the first issue, a Time Reversal beamforming based tracking algorithm is developed that is able to track CMUTs occupying a surface area of about 6.3 mm2 moving at speeds up to 1.25 mm/s in water. Addressing the second issue, a communication protocol is formulated to establish ultrasound communication. This protocol makes use of the focused US beam achieved during tracking to communicate while offering simultaneous power transfer. A data rate of 2 kb/s is achieved using this protocol when the transmitter (TX) and the receiver (RX) are separated by a distance of 130 mm with a phantom filling up the gap. US based IMDs can be built to occupy only a few mm2 of area, especially when using micromachined transducers such as CMUTs. This work, the tracking algorithm and the communication protocol further provide proof of concept for reliable localization of CMUTs and low power data communication, hence providing good prospects for CMUT applications in the field of Implantable Medical Devices.