Design and Fabrication of Negative-Refractive-Index Metamaterial Unit Cells for Near-Megahertz Enhanced Acoustic Transmission in Biomedical Ultrasound Applications

Abstract

We present the design of negative-refractive-index acoustic metamaterials operating at near-megahertz frequencies, intended for the eventual aim of enabling enhanced acoustic transmission through highimpedance-contrast biological layers. Leveraging the concept of complementary acoustic metamaterials, the negative effective properties of the metamaterials are designed to match the magnitude of an ultrasound-blocking, high-impedance-contrast layer's properties. The negative properties are obtained using a linear array of unit cells containing Helmholtz resonators and membranes. Using finite-element and analytical models, we calculate the band structure and the effective modulus and density of the proposed negative metamaterials. Using the full three-dimensional model of the metamaterials, we then simulate the enhancement of ultrasound transmission, through layers with high-impedance contrast to water. For instance, we see an improvement from 80% transmission through a high-impedance layer alone to 98% transmission through the metamaterial-plus-high-impedance layer combination. Scaling arguments are provided to estimate the system dimensions needed to provide higher operational frequencies appropriate for imaging and high-intensity ultrasound applications. Finally, as a proof of feasibility, a preliminary experimental realization of the unit-cell structure, created using the nanofabrication approach, is presented and tested in the near-megahertz regime. These results provide a step toward metamaterial-enhanced devices for noninvasive biomedical ultrasonic imaging and therapy.

Complementary Acoustic Metamaterial for Penetrating Aberration Layers

“…Higher frequency and precision ultrasound therapy require finer processing techniques to create structures. Related research was carried out using plasma-enhanced chemical vapor deposition (PECVD) nanofabrication steps and patterned by UV lithography and Si dry etching processes to fabricate Helmholtz cavities and necks. These fabricated structures can reach frequencies of about 1–10 MHz that are characteristic of biomedical ultrasound technology.…”

Acoustic Holograms for Bilateral Blood-Brain Barrier Opening in a Mouse Model

“…Recently, acoustic holograms have been proposed for FUS to overcome these cost and time limitations [26]. An acoustic hologram is a material designed to spatially modulate the phase and, in some cases, the magnitude of a transmitted wavefront to synthesize a complex acoustic field [27]- [32]. These structures allow the synthesis of acoustic images, i.e., areas where the acoustic energy is high, combined with areas were the media is at rest.…”

Graphene-Integrated Plasmonic Metamaterial for Manipulation of Multi-Band Absorption, Based on Near-Field Coupled Resonators

“…By designing it reasonably, the MM can exhibit unusual properties which are not available in the constituent elements. Thanks to that, the unusual properties, such as the inversion of Snell's law [1], negative refractive index [2,3] and electromagnetic (EM)-wave absorption [4,5], are obtained easily. Therefore, the MM is promising for the potential applications such as sensors [6][7][8], superlens [9,10], filters [11,12], antennas [13,14], and so on.…”