Fig. 1: Design and fabrication of HHPC hydrogels.

a Schematic illustrations of fabrication of HHPC hydrogel composed of a compact sheath and porous core by directional freezing assembly and stretch-assisted salting out method. Firstly, the DF-PVA/CNF gel with a honeycomb structure was obtained through aggregation of PVA chains and CNFs repelled by ice crystals. By stretching the gel along the channel direction, a temporarily elongated architecture was constructed with the aligned PVA and CNFs species on the cellular wall. Immersing in salt solution, the aligned structure was fixed and heterogeneous HHPC hydrogel was finally fabricated. The δ⁺ and δ^- symbols around the water molecules indicated the polarization of the water molecules by adjacent anions during salting out. b Hierarchically aligned nanofibril structures by PVA crystallization and interactions with CNFs via hydrogen bonding on the internal cellular wall, that is, belt-like fibril bundles at microscale, fibril bundles at submicroscale and fibrils at nanoscale. c Evolution of heterogeneous structure. Upon the Hofmeister effect, the gel surfaces were rapidly crosslinked arising from strong self-coalescence and phase-separation of amorphous PVA chains and CNFs. The impeded inward permeation of ions by dense surfaces led to a gradient concentration of kosmotropic ions. The dashed blue arrow represented the trajectory of the ion. Aging with salting out, the gel was thinner, and heterogeneous core-sheath structured hydrogel was finally constructed with distinct density gradient.