Salt-induced secondary reinforcement of fibre-reinforced hydrogel composites via sodium phytate post-treatment

Hydrogels have been regarded as highly promising load-bearing tissue engineering materials. However, their inherent insufficient mechanical strength and poor interfacial adhesion with the reinforcing phase limit their applications. To simultaneously overcome these critical drawbacks, we propose a novel physical dual-network crosslinking strategy based on the Hofmeister effect of a biocompatible polyphosphate salt, sodium phytate (NaPA). This strategy induces a strong salting-out effect, causing the polyvinyl alcohol (PVA) hydrogel matrix to form microcrystals through dehydration and densification, while establishing dynamic interfacial interactions and physical interlocks at the fibre-matrix interface. As a result, the tensile strength of the obtained NaPA-induced PVA-silk fibre composite hydrogel reached 3.98 MPa, with toughness reaching 8.71 MJ m⁻³, and the elongation at break extending to approximately 430%. More importantly, the interfacial shear strength was quadrupled to 0.27 MPa, thus achieving efficient load transfer between the matrix and the fibres. In addition, these hydrogels demonstrate good dimensional stability, tuneable degradation behaviour, reversible mechanical reinforcement, and beneficial in vitro biocompatibility under the tested conditions. The universality of this strategy has been verified across multiple representative natural and synthetic matrices reinforced with organic and inorganic fibres. The simple and multifunctional strategy established in this work enables the construction of strong and tough hydrogel composites with tailored interfacial architectures, making them suitable for biomedical and broader engineering applications.