Water-abundant and tough structured composite hydrogels via ion transfer printing

Hydrogels with high water content and toughness are essential to various applications in smart materials and biomimetic systems. However, there exists a conflict between water content and toughness. To enhance toughness, high polymer chain density or water-free reinforcements are usually introduced into hydrogel matrices, which inevitably lead to a reduction in water content. In this study, we present a facile method for preparing water-abundant and tough hydrogels through ion transfer printing. By utilizing sodium alginate/polyacrylamide (Alg/PAAm) hydrogels as a flexible matrix and Fe³⁺ ions as stiffening agents, we selectively introduce Fe³⁺ ions into predefined regions of the hydrogel matrix, resulting in well-structured composite hydrogels comprising soft Alg/PAAm matrix and hard Fe³⁺-crosslinked Alg/PAAm (Fe-Alg/PAAm) fibers. As both the matrix and fibers are stretchable and water-abundant, the composites exhibit impressive stretchability (ε∼1000 %) and water content (p∼95 %). Notably, the alternating arrangement of the soft and hard fiber/matrix architecture effectively prevents crack propagation during loading by inducing stress deconcentration at the crack tip, thereby leading to exceptional toughness (Γ∼22000 J/m²). This simple method introduces a universal design strategy for constructing stretchable, water-abundant, and tough hydrogels, considering that ionic crosslinking with multi-valent cation crosslinkers is widely used in hydrogels. Beyond the Fe³⁺ and Alg/PAAm hydrogel system discussed here, this concept can be extended to various combinations of multi-valent ions and hydrogel networks containing opposite charges.