Parallel addressing of biosensing ultrasonic implants using plane wave imaging

Ultrasonic backscatter communication allows wireless and battery-free biosensing and has been proposed to monitor various physiologic signals. These sub-mm implants serve as antennae deeply seated in tissue, and are typically addressed using external single-element transducers, preventing parallel sensing of multiple implants. Their basic principle is that their acoustic reflectivity is modulated by the electrical impedance of a transistor tuned on a specific physiologic signal. Here, we investigate plane wave imaging to simultaneously and selectively detect the modulation of multiple implants. An L11-5v ultrasound array operating at 4.4 MHz was connected to a Verasonics programmable scanner and used to observe 350-μm piezoceramic cubes connected to a variable impedance shunt load. We also observed the variation of the amplitude of the radiofrequency signals picked up by the implants and radiated acoustically (an active signal from the passive implants) after beamforming of the received signals. Accurate detection of a change in the amplitude of this emitted wave was assessed for two piezoceramic transducers separated by 5 mm in water both with a shunt load and a transistor sensitive to neural activation, and below 15 mm turkey breast. Successful detection of the signal variation validates this approach for simultaneously addressing multiple neural implants.