An Acoustic Isolator-Type Metamaterial for Ultrasound Attenuation at MHz Frequencies.

Acoustic metamaterials (AMMs) offer significant promise for ultrasound probe backing layers due to their capacity to enhance acoustic energy dissipation through tailored sub-wavelength structures. However, practical implementation remains challenging due to difficulties in reliably reproducing the micron-scale features required for MHz-frequency operation, and the lack of quality assurance processes linking design intent to fabricated performance. This work presents the evaluation of a 3D-printed acoustic isolator-type metamaterial (AI-MM) backing designed for MHz operation using a custom aluminum oxide resin. Directional transmission intensity measurements revealed frequency-dependent asymmetry in forward and backward wave propagation (in both experiments and simulations), consistent with passive acoustic isolator behavior. X-ray micro-CT imaging of AI-MM samples revealed dimensional deviations, apex rounding, and local density variation. Attenuation spectra showed that AI-MM backings consistently outperformed homogeneous controls in both simulation and experiment, with frequency-dependent trends indicating enhanced scattering and viscous losses. A local attenuation peak near 2.6 MHz was within the operational range estimated from the measured geometry (2.22-2.94 MHz), underscoring the importance of linking performance to real-world fabrication. These findings support the potential of AI-MMs as tunable passive components in ultrasound systems and highlight the need for integrated design, fabrication, and validation workflows.