Layer-by-layer shear densification for multiscale hierarchical alignment in bulk hydrogels

Natural structural tissues achieve exceptional performance through precisely aligned hierarchical architectures that extend across multiple length scales. However, realizing such multiscale long-range alignment in synthetic bulk hydrogels remains challenging because of the difficulty in constructing a uniformly dense and highly oriented structure that extends throughout the full bulk matrix. Here, we introduce a scalable and versatile Layer-by-Layer Shear Densification (LBSD) strategy that integrates flocculation-induced aggregation with shear-driven progressive alignment, precisely driving the architectural evolution toward compact and uniformly ordered lamellar structures across multiscales. The resulting poly(vinyl alcohol) (PVA) hydrogels with a hierarchical network exhibit a Herman's orientation factor of 0.91, surpassing previously reported values for bulk hydrogels. The structural orientation enables the hydrogel to exhibit excellent mechanical properties, including a tensile strength of 41.29 ± 2.10 MPa and toughness of 159.37 ± 28.15 MJ·m⁻³. To demonstrate the versatility, this strategy is further used to fabricate gelatin hydrogels, resulting in a 32-fold enhancement in toughness. Anisotropic thermal conductivity, another representative physical property originating from molecular-level alignment, is also demonstrated. This work establishes a generalizable technology for developing high-performance bulk polymeric materials through molecular-level engineering, offering substantial potential for applications in load-bearing components, bioelectronic devices, thermal management systems, etc.