Dual-frequency metastructured-hydrogel film for high-contrast ultrasound imaging

Contrast agents enhance the resolution and clarity of ultrasound imaging, thereby improving the accuracy of diagnosis and treatment. However, the imaging performance of existing contrast agents is limited by the non-specificity of B-mode ultrasound imaging, particularly when high-echo tissue interference is present. Here, we propose a metastructured hydrogel (metagel) film with a steep reflectance transition at a specific frequency, enabling differential imaging between the higher and lower frequencies to suppress background interference. The ultrasonic reflection field is investigated for parameter optimization. The metagel film exhibits shape-retaining properties, demonstrating approximately 4 times reflectivity contrast within a 40% strain range. Through a customized dual-frequency excitation mode combined with ultrasonic spectrum analysis, we propose a dual-frequency differential imaging workflow. Compared with the reflection amplitude of zinc metal, our method achieves a signal-to-noise ratio exceeding 50 dB and a 4 times signal enhancement. Combined with an adhesive hydrogel patch, we demonstrate the conformal and imaging enhancement capabilities of the metagel film on complex surfaces, providing an effective tool for interference-free ultrasound imaging.