Quasi-surface ultrasound enabled by nonlocal metasurfaces resolves the penetration safety trade-off in sonotherapy

Ultrasound is widely used in biomedical imaging and therapy because its strong penetration enables deep-tissue visualization and treatment of neurological, cardiovascular, and musculoskeletal disorders. However, this same property becomes a fundamental limitation for diseases confined to biological surfaces, where deep ultrasound penetration leads to off-target exposure and safety concerns. Here, we report quasi-surface ultrasound enabled by a nonlocal metasurface, which reshapes transmission channels to convert a propagating wave into a surface-confined mode. The metasurface suppresses forward bulk-wave transmission while sustaining laterally extended propagation along the interface, resulting in strong confinement of ultrasound energy within subwavelength distances from the surface. By engineering the ultrasound wave state rather than relying on frequency scaling or exposure parameter tuning, this approach intrinsically resolves the penetration safety trade-off in sonotherapy. Using hair follicle stimulation as a representative surface-confined model, we demonstrate enhanced transdermal drug efficacy and accelerated in vivo hair regeneration, while minimizing ultrasound exposure to deep brain tissue and preserving hippocampal neuronal integrity. These results establish nonlocal metasurfaces as a versatile platform for wave-state engineering of ultrasound, enabling spatially selective and safe biointerfacing.