3D‐printed polymer‐based composites are promising for various engineering applications due to high strength‐to‐weight ratios and design flexibility. However, conventional matrix materials, such as polylactic acid and epoxy resin, often exhibit brittleness and limited impact resistance (< 10 kJ m ^{− 2} ). Herein, a universal strategy is reported for enhancing the ductility and impact energy absorption of 3D‐printed composites by leveraging the dynamic crosslinking of B─O dative bonds. To validate its effectiveness, a smart composite (PLA/SSG) comprising shear‐stiffening gel fillers embedded in a polylactic acid matrix is designed and its rate‐dependent mechanical adjustability along with 3D printability is evaluated. The resulting composite shows significant improvements in impact resistance, ductility, and strength‐ductility balance. Specifically, the multiple crack and localized plastic yielding of polylactic acid matrix induced by shear‐stiffening gel fillers enables PLA/SSG with a 40‐times increase in ductility; the “soft‐hard” phase transition of shear‐stiffening gel induced by B─O bonds endows PLA/SSG with a 330% improvement in impact energy absorption. This B─O bonds‐inspired strategy provides a universal approach for printing smart impact‐resistant composites and structures.

Underwater adhesion involves bonding substrates in aqueous environments or wet surfaces, with applications in wound dressing, underwater repairs, and underwater soft robotics. In this study, we investigate the underwater adhesion properties of a polyacrylic acid hydrogel coated substrate. The underwater adhesion is facilitated through hydrogen bonds formed at the interface. Our experimental results, obtained through probe-pull tests, demonstrate that the underwater adhesion is rapid and remains unaffected by contact pressure and pH levels ranging from 2.5 to 7.0. However, it shows a slight increase with a larger adhesion area. Additionally, we simulate the debonding process and observe that the high-stress region originates from the outermost bonding region and propagates towards the center, spanning the thickness of the target substrate. Furthermore, we showcase the potential of using the underwater adhesive hydrogel coating to achieve in-situ underwater bonding between a flexible electronic demonstration device and a hydrogel contact lens. This work highlights the advantages of employing hydrogel coatings in underwater adhesion applications and serves as inspiration for the advancement of underwater adhesive hydrogel coatings capable of interacting with a wide range of substrates through diverse chemical and physical interactions at the interface.