Soft bio-metamaterials with high acoustic transparency and gradient refractive index for tunable acoustic beamformer

Dolphins utilize soft forehead tissues for dynamic detection by efficiently transmitting and manipulating broadband acoustic waves. Imparting high acoustic transparency, gradient refractive index, and softness to acoustic functional materials can significantly enhance sound wave manipulation. Conventional metamaterials, consisting of solid and impedance-mismatched meta-atoms, have limitations in achieving acoustic transparency and flexibility. Here, we overcome these constraints with a new class of acoustic soft bio-metamaterials (SBMs) composed of solid microparticles or liquid microdroplets dispersed in a hyper-elastomer. The SBMs exhibit an extraordinary combination of high acoustic transparency with transmission coefficients near 1, a gradient refractive index above 1.25, and a low elastic modulus under 100 kPa comparable to biological tissues. Furthermore, we developed a dolphin-inspired tunable subwavelength acoustic beamformer using SBMs, which can dynamically modulate acoustic beam patterns through mechanical stretching. SBMs offer a new design paradigm for acoustic soft metamaterials and have potential applications in underwater monitoring and biomedical ultrasound.