A mm-sized acoustic wireless implantable neural stimulator based on a piezoelectric micromachined ultrasound transducer

Implantable medical devices are increasingly being used to perform electrophysiological stimulation, thus calling for wireless implantable neural stimulators for which wireless power transfer is needed. Compared with radio frequency and inductive coupling wireless power transfer methods, acoustic wireless power transfer features lower attenuation, a smaller size and a higher safe power threshold. This paper demonstrates the design, fabrication, assembly and characterization of a mm-sized acoustic wireless implantable neural stimulator based on a piezoelectric micromachined ultrasound transducer. Ex vivo experiments in water are conducted to characterize the power transfer link, showing improvement of the power transfer efficiency by more than two times with a matching circuit. Following FDA guidelines, the wireless implantable neural stimulator achieves a 3.24 mu W output power in tissue. The feasibility for rat sciatic nerve stimulation is successfully demonstrated in vivo by the implantable prototype with a size of 5 x 5.5 x 2.5 mm3. The proposed solution has the potential to shrink the entire implanted system to a monolithic transducer-IC chip, paving the way toward an acoustic wireless implantable neural stimulator with higher levels of biocompatibility, integration and miniaturization.