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 produced poly(vinyl alcohol) (PVA) hydrogels with hierarchical network exhibit a Herman orientation factor of 0.91, surpassing all previously reported bulk hydrogels. The structural orientation enables the hydrogel with outstanding 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 was further used to fabricate gelatin hydrogel, resulting in a 32-fold enhancement in mechanical strength. Anisotropic thermal conductivity, another representative physical property originating from molecular-level alignment, was also demonstrated. This work establishes a generalizable technology in developing high-performance bulk polymeric materials through molecular level engineering, offering substantial potential for creating new materials for superior load-bearing components, bioelectronic devices, and thermal management systems, etc.