image: The graphic shows how researchers turned a porous hydrogel into a soft acoustic metapad that can both transmit ultrasound efficiently and focus it over a broad frequency range. Designed with a gradient acoustic refractive index, the material is intended for applications that require acoustic transparency, flexible fit and wave control, including biomedical ultrasound and underwater acoustics.
Credit: ©Science China Press
Acoustic metamaterials have become powerful tools for shaping sound, but they come with a built-in limitation: most are rigid.
That has not been a minor inconvenience. In many cases, rigidity is exactly what makes precise wave control possible. But in biomedical ultrasound and underwater acoustics, rigidity also becomes a problem. These applications do not just need sound to be focused or steered. They also require efficient transmission, low reflection loss, broad operating bandwidth, and, in many cases, a material that can adapt to soft or irregular interfaces. Those demands are difficult to meet with conventional rigid acoustic structures.
This mismatch has created a persistent gap in acoustic materials design. Traditional soft coupling media such as liquids, gels, and elastomers can fit skin or other soft boundaries well, but they usually do little more than make contact. By contrast, Conventional metamaterials can manipulate wavefronts with high precision, yet their matrix absorption, multiple scattering from large structural units, and poor impedance matching with tissue or water have limited their usefulness in exactly the settings where transparency and adaptability matter most.
To address that gap, researchers from Xiamen University and collaborators at the First Affiliated Hospital of Xiamen University and Institut Langevin in France developed a different kind of acoustic material. Reported in National Science Review, their hydrogel metapad combines three properties rarely united in one platform: high acoustic transparency, broadband focusing, and mechanical flexibility.
The idea begins with hydrogel, a material already known for its softness, water-rich composition, and biocompatibility. But the key advance is structural. By precisely tailoring subwavelength pores inside the hydrogel, the researchers introduced multiple scattering effects that make the acoustic parameters continuously tunable. This allowed them to build a gradient acoustic refractive-index profile directly into a flexible material. In other words, the hydrogel does not simply sit between probe and target as a passive layer. Its internal structure actively reshapes the wavefront while preserving the material advantages of a soft interface.
That combination matters because each of the needed properties solves a different part of the problem. Acoustic transparency reduces the energy that would otherwise be lost at the interface. Broadband focusing makes the device useful across a wide frequency range rather than at one narrow operating point. Flexibility allows the material to conform to biological tissue and other complex boundaries instead of forcing the interface to adapt to the device. Put together, those features point to a different design logic from the one that has dominated acoustic metamaterials for years.
The researchers tested whether these material advantages translated into better imaging. Ultrasound simulations and human experiments showed that when the metapad was integrated with a standard probe as a transparent coupling layer, it significantly increased echo intensity and image contrast in the focal region without sacrificing spatial resolution. In carotid artery and cardiac imaging, vessel boundaries, blood-flow signals, and internal heart structures became clearer. Cell and animal experiments further indicated good biosafety, with no obvious cytotoxicity, low hemolysis, and no skin irritation.
The implications extend beyond biomedical imaging. Underwater acoustics faces a closely related materials problem. Efficient sound propagation in water also depends on impedance matching, low-loss transmission, and controllable wavefront shaping. Yet many underwater acoustic devices still rely on rigid materials or large structural elements that introduce mismatch, scattering, and insufficient transparency. Because the hydrogel metapad is water-based and supports gradient wave control with limited added reflection and loss, the researchers say it could inspire new soft materials for sonar, underwater imaging, and beam shaping in complex aquatic environments.
More broadly, the work suggests that the future of acoustic metamaterials may not lie only in making rigid structures more sophisticated. It may also depend on rethinking what an acoustic interface can be. By showing that high transparency, broadband wave control, and flexibility can be engineered into the same hydrogel platform, the study opens a path beyond the traditional rigid-metamaterial paradigm for both medicine and underwater acoustics.