Soft armour-like layer-protected hydrogels for wet tissue adhesion and biological imaging

Hydrogels with high water contents have been widely used as ultrasound couplants for medical imaging due to their acoustic impedance compatibility with biological tissues. However, their functional reliability is compromised by unstable adhesion to wet tissues, as surrounding water disrupts the surface bonding. Herein, we report a novel strategy to fabricate a bioadhesive hydrogel by assembling a soft armour-like hydrophobic surface as the outermost adhesive layer to achieve instant and robust wet tissue adhesion. The external catechol-containing hydrophobic layer generates a water depletion region at the contact interface, which promotes rapid adhesion (within 5 s) and prevents the weakening of interfacial bonds from water penetration even under high hydraulic pressures. The internal hydrogel matrix reduces the signal attenuation in ultrasound imaging due to its high water content of 86.7 wt%. The porcine model further demonstrates that the as-prepared hydrogel adhesive enables both the stable attachment and high-contrast imaging for ultrasound diagnosis in wet intraoral environments. This work devises a new pathway for building the human-machine interface for biomedical applications.