Design and Optimization of Ultrasonic Links With Phased Arrays for Wireless Power Transmission to Biomedical Implants
Zeinab Kashani, Sheikh Jawad Ilham, Mehdi Kiani
Abstract
Ultrasound (US) is an attractive modality for wireless power transfer (WPT) to biomedical implants with millimeter (mm) dimensions. To compensate for misalignments in WPT to a mm-sized implant (or powering a network of mm-sized implants), a US transducer array should electronically be driven in a beamforming fashion (known as US phased array) to steer focused US beams at different locations. This paper presents the theory and design methodology of US WPT links with phased arrays and mm-sized receivers (Rx). For given constraints imposed by the application and fabrication, such as load ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R<sub>L</sub></i> ) and focal distance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> ), the optimal geometries of a US phased array and Rx transducer, as well as the optimal operation frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f<sub>c</sub></i> ) are found through an iterative design procedure to maximize the power transfer efficiency (PTE). An optimal figure of merit (FoM) related to PTE is proposed to simplify the US array design. A design example of a US link is presented and optimized for WPT to a mm-sized Rx with a linear array. In measurements, the fabricated 16-element array (10.9×9×1.7 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) driven by 100 V pulses at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f<sub>c</sub></i> of 1.1 MHz with optimal delays for focusing at <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> = 20 mm generated a US beam with a pressure output of 0.8 MPa. The link could deliver up to 6 mW to a ∼ 1 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Rx with a PTE of 0.14% ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R<sub>L</sub></i> = 850 Ω). The beam steering capability of the array at -45 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> to 45 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> angles was also characterized.