3D printing functional materials with extreme regulation of mechanical performances from hydrogel to engineering plastic

• Vat photopolymerization 3D printed PVA that can be stepwise transformed from hydrogel to plastic by controlling dehydration was realized. • The mechanical properties of the PVA can be continuously modulated in the range of orders of magnitude by controlling dehydration. • The adjustment of the mechanical properties of PVA was reversible by cycles of rehydration-dehydration. Poly(vinyl alcohol) (PVA) is an important component of functional materials exhibiting excellent mechanical properties due to its remarkable hydrogen bonding interactions and high crystallinity. Despite the successful development of PVA materials with enhanced mechanical properties through various methods, challenges remain in achieving an extreme regulation from hydrogel to engineering plastic, as well as in incorporating diverse functionalities such as shape memory, adhesion, and 3D printability with freeform design. In this study, inspired by the unique brittle-to-tough transition of the Discinisca tenuis shell, we developed a facile strategy to dramatically and reversibly modulate the mechanical properties of 3D printable PVA/Acrylamide (AAm) materials. By simply controlling hydration and dehydration, we achieved in situ continuous tuning of mechanical properties across several orders of magnitude. Specifically, the tensile strength varied from 0.02 ± 0.002 MPa to 104.58 ± 4.4 MPa, the modulus ranged from 0.002 ± 0.0004 MPa to 592 ± 7.8 MPa, and the toughness increased from 14 ± 1 kJ/m 3 to 24,671 ± 469 kJ/m 3 . To the best of our knowledge, these results represent the broadest range of tunable mechanical properties reported to date, marking the first successful transition from hydrogels to elastomers and even to engineering plastics. Furthermore, by integrating digital light processing 3D printing, the prepared material in its hydrogel state can be constructed into various architectures with the amazing double reversible shape memory behaviors, alongside excellent adhesion and conductivity, making it suitable for flexible sensors. Taking advantage of the transformation from hydrogel to engineering plastic, we have successfully integrated flexible sensors and rigid dislocated joint fixation, both fabricated from a single PVA/AAm material, into one smart system via continuous hydration and dehydration, thereby addressing the contradiction of using rigid materials for high-sensitivity flexible sensors. In summary, this study presents an efficient strategy for fabricating functional material with large-order regulation of mechanical performances and provides a solution for integrating flexible sensors with high-strength hydrogels into a cohesive platform.