Active implantable devices are conventionally powered with batteries. The miniaturisation of implants necessitates the need to find an efficient way to transfer power wirelessly. Ultrasound energy transfer with its short wavelength, small receiving transducer size, beamforming capability makes deep tissue penetration possible making it a good alternative for powering deep implants. There is also the advantage of using the reflected energy (ultrasound echo) from the implants to perform passive uplink data telemetry. The ultrasound power transfer efficiency depends on how well the energy is focused on the implant. So, the implant needs to be located first and then an initial contact needs to be established that can then be used to fine-tune beam forming attributes such as focus depth and steering angle.
In this work, a simple low power uplink data telemetry circuit to establish the 'first contact' that sends information on the implant's energy status to the transmitter is designed. The telemetry uses load impedance modulation to passively transmit back information. This principle is tested using an experimental setup with a 4 MHz burst frequency and different resistive loads varied from 0 Ω to 2000 Ω. Next, an algorithm was developed that automatically finds the optimal focus and steering angle settings so that the maximum amount of power is transferred to the ultrasound energy scavenger. This algorithm works based on the information sent from the uplink data telemetry circuit.
The results show that load impedance modulation can effectively be used for backscatter communication. A reflected energy change of up to 40% is observed between matched and mismatched load conditions. This load modulation was implemented in the uplink data telemetry circuit. A simple low power circuit that consumes less than 0.5 μW was designed for this. The uplink telemetry protocol uses pulse width modulation to send the storage capacitor voltage which powers the implant, and an almost linear correlation was found between the pulse width and the supply voltage. The algorithm produces a heat plot showing the maximum power transferred at the location where CMUT was placed and gives the optimal value of focus and steering angle.