Tough and durable hydrogels with robust skin layers formed via soaking treatment

Recent progress has seen significant advances in the mechanical performances of synthetic hydrogels. However, the mechanics of hydrogels usually suffer drastic degradation under repetitive mechanical and complicated environmental loadings. Here, we fabricate a class of biocompatible layered poly(vinyl alcohol) hydrogels by simply soaking a preformed poly(vinyl alcohol) gel in sodium silicate. The resulting hydrogels exhibit a combination of superior mechanical performances and good biocompatibility, along with exceptional chemical robustness. The layered structure is composed of a compact cover layer and a porous center layer. Both layers are composed of poly(vinyl alcohol). The network in the cover is crosslinked by ordered polarized H-bonds with swelling stability, while the porous center confines a large volume of interstitial water. The structural and crosslinking metric confers the hydrogels with capabilities to tolerate complicated mechanical and environmental loads. The optimized gel is tough (fracture energy >10 kJ m^-2) and strong (fracture stress ≈ 5 MPa). The strengthening mechanism can be correlated to the layered structure, which causes the impeded flow of interstitial water. Their mechanical performance is maintained in strong acidic/alkaline, and concentrated electrolytes, and in the presence of salting-in and H-bond-breaking reagents even at elevated temperatures. We speculate that the dipole-dipole pairings of O^δ--H^δ+ O^δ--H^δ+ in the cover layer probably generate hydrophobic microdomains. This swelling-resistant interaction may protect the hydrogels from swelling in complex aqueous envrionments. We also discuss the possible mechanism of the formation of layered structures and their crosslinkings.