A Universal Toughening and Energy-Dissipating Strategy for Impact-Resistant 3D-Printed Composites

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.