Ultrasound neuromodulation is a rapidly developing, minimally invasive technique for brain stimulation. It holds great promise for treating neurological disorders, psychiatric conditions, and enabling brain– computer interfacing. A key challenge is achieving fine-grained, dynamic control over the acoustic focus within tissue. This requires focusing mechanisms that are both electronically tunable and scalable, in order to support precise and addressable focal regions. Current beamforming strategies based on phased arrays face limitations in spatial resolution and power efficiency.

This thesis explores a fundamentally different approach: shaping ultrasound fields through magnetic microparticle-based acoustic holography. These systems can be fabricated at the microscale and offer potential for control with high spatiotemporal precision. To assess the feasibility of this concept, this work presents the design, fabrication, and acoustic characterization of a planar, soft-lithography-based Fresnel Zone Plate (FZP). The FZP incorporates integrated microfluidic channels and is developed to manipulate ultrasound fields using magnetic microparticle suspensions.

The fabricated FZP consists of alternating concentric rings of PDMS and fluid-filled microchannels, where the acoustic properties of the microfluidic zones can be tuned by varying particle types and concentrations. Multiple layouts and methods were evaluated to enable reliable injection and exchange of different media into the sub-millimeter-thick microfluidic device. The finalized design was successfully replicated via soft lithography, and experimental measurements were conducted using five media: air, deionized water, and suspensions of three types of magnetic microparticles. Acoustic measurements revealed that the microparticle suspensions exhibited reduced transparency compared to water, but greater than that of air. Pressure amplitude decreased with increasing particle concentration, consistent with enhanced acoustic attenuation. Phase analysis showed a non-monotonic relationship between concentration and the effective speed of sound, deviating from trends reported in bulk suspensions. This highlights the complex interplay between confinement, concentration, particle settling, and wave propagation in microfluidic geometries.

The results demonstrate that magnetic microparticles enable simultaneous modulation of both the phase and amplitude of ultrasound. Combined with earlier demonstrations of digitally controlled spatial distribution using microfabricated current lines, this approach offers a promising route toward dynamic acoustic field steering. As such, it marks a step toward programmable acoustic holography at the microscale.