emergent trends in the device development for neural prosthetics have focused on establishing stimulus localization, improving longevity through immune compatibility, reducing energy requirements, and embedding active control in the devices. Ultrasound stimulation can single-handedly address several of these challenges. Ultrasonic stimulus of neurons has been studied extensively from 100 kHz to 10 MHz, with high penetration but less localization. In this paper, a chip-scale device consisting of piezoelectric Aluminum Nitride ultrasonic transducers was engineered to deliver gigahertz (GHz) ultrasonic stimulus to the human neural cells. These devices provide a path towards complementary metal oxide semiconductor (CMOS) integration towards fully controllable neural devices. At GHz frequencies, ultrasonic wavelengths in water are a few microns and have an absorption depth of 10-20 µm. This confinement of energy can be used to control stimulation volume within a single neuron. This paper is the first proof-of-concept study to demonstrate that GHz ultrasound can stimulate neurons in vitro. By utilizing optical calcium imaging, which records calcium ion flux indicating occurrence of an action potential, this paper demonstrates that an application of a nontoxic dosage of GHz ultrasonic waves () ≥0.05 W cm 2 caused an average normalized fluorescence intensity recordings >1.40 for the calcium transients. Electrical effects due to chip-scale ultrasound delivery was discounted as the sole mechanism in stimulation, with effects tested at α = 0.01 statistical significance amongst all intensities and control groups. Ionic transients recorded optically were confirmed to be mediated by ion channels and experimental data suggests an insignificant thermal contributions to stimulation, with a predicted increase of 0.03 o C for 1.2 W cm 2 ⋅ This paper paves the experimental framework to further explore chip-scale axon and neuron specific neural stimulation, with future applications in neural prosthetics, chip scale neural engineering, and extensions to different tissue and cell types. Neural interfaces can be used to implement human assistive devices to improve the quality of life of patients by restoring nerve signal pathways, improving communications, and motor functions 1. These devices can be used for individuals that have undergone stroke, spinal cord injuries, brain disease related damage, peripheral nerve damage for which there is significant clinical burden. For example, there are an estimated 17,700 new cases of spinal cord injuries per year 2. Any technology used to for neuron activation should ideally be long-lasting, preferably with lifelong performance for the patient. Prosthetic microsystems range in size from nanometer to centimeter scale, and include optical probes for optogenetic methods, electrode arrays (micro and nanowire or planar electrodes), and ultrasonic transducers 3-7. However, major challenges exist in the development of long-term stable neural prosthetics, including (1) controlled neuron specific excitati...
