Hydrogel metapad with ultrasound transparency and broadband focusing for biomedical imaging.

Imparting broadband transparency and focusing on flexible ultrasound bioelectronics can significantly enhance the capabilities for precise evaluation of tissues and organs as well as treatment of diseases. While existing wearable ultrasound devices, whether rigid or stretchable, often merge rigid ultrasound element arrays with soft materials to achieve a reliable interface on the human skin, they lack controlled artificial microstructures, resulting in compromised ultrasound transparency and challenges in achieving broadband focusing. Here, we report a metapad made of hydrogel metamaterials with high ultrasound transparency and broadband focusing by controlling the hydrogel porosity at a sub-wavelength scale. The hydrogel metapad achieves near-perfect acoustic impedance matching with tissues, low attenuation loss, broadband transmission, and high focusing intensity gain. Ultrasound imaging simulations further show a significant improvement in imaging contrast near the focal region when using the hydrogel metapad. Practical applications demonstrate that it enhances the imaging capabilities of ultrasound probes for vital human organs, including blood vessels and the heart. Our hydrogel metapad holds great potential for advancing soft acoustic functional devices, capable of seamlessly interfacing with both biological tissues and aquatic environments.