Low‐Fraction Irregular Inclusions Trigger Large Compression Stiffening in Biopolymer Hydrogels

Materials with adaptive mechanical responses are increasingly sought for load‐bearing, tissue‐mimetic biomaterials and mechanically responsive soft devices. Most soft biological tissues stiffen under compression, whereas isolated biopolymer networks typically soften. Here a morphology‐controlled contact network strategy is demonstrated to enable pronounced, smooth compression stiffening in soft hydrogel composites using only low fractions of rigid inclusions (∼10 vol%). In biopolymer networks containing carbonyl iron particle aggregates or irregular coffee‐ground inclusions, the shear modulus increases by multiple folds under modest axial strains, far below the loadings required by classical spherical‐particle models. Experiments across inclusion geometries and matrices, together with complementary simulations, show that morphological irregularity lowers the contact‐percolation threshold of those stiff inclusions. Upon compression, the resultant connected inclusion network rearranges non‐affinely, stretching surrounding fibers and promoting tension‐dominated stress propagation, and simultaneously provides parallel load‐bearing pathways. These coupled effects yield tissue‐like stiffening profiles without requiring high solid fractions or invoking jamming. This work establishes a physical framework linking polymer‐network mechanics to tissue‐level compression responses and provides a morphology‐guided design rule for engineering mechanically functional hydrogels with physiologically relevant compression behaviors for biomaterial scaffolds and soft devices.