Optimal Ultrasonic Pulse Transmission for Miniaturized Biomedical Implants

This paper presents optimal ultrasonic pulse transmission that could be used for data transmission to/from millimeter-sized biomedical implants in general or the self-image-guided ultrasonic (SIG-US) wireless power transfer (WPT), which we have recently proposed. To reduce the power consumption of the implant, data bits can be applied as sharp pulses across the implant's ultrasonic transducer, resulting in a signal in the form of a short ringing across the external unit transducer (or array). In SIG-US WPT, also short pulses are transmitted by the implant periodically. The relative delays in received signal by each external transducer in an array are then used to guide the beamformer for optimal steering of the power beam towards the implant. Through simulations and measurements, the effect of transmitted number of pulses (N_p) on the amplitude of the received signal (V_DRx) has been studied, which is vital for low-power robust transmission. In measurements with two identical disc-shaped transducers (spaced by d = 50 mm inside water) with the diameter of 1.2 mm and thickness of 0.9 mm (series resonance frequency of ~1.3 MHz), V_DRx increased almost linearly with N_p (50% duty cycle with the period of 1/1.3 MHz ~ 770 ns) up to N_p = 5 beyond which VDRX increase was negligible and started to saturate at N_p = 10. In the SIG-US context, it was also shown in measurements that for up to 25 mm misalignment of the implant at d = 50 mm, the relative delay in receiving 4 pulses increased from 40 ns to 4.75 μs, which are quite measurable.