Fiber-integrated hydrogels: a versatile platform to improve structural and biological performance in 3D biofabrication

Hydrogels emerged as versatile biomaterials for tissue engineering due to their extra cellular matrix similarity and mechanical and biochemical properties. Still, hydrogels expose limited stiffness, anisotropy and nutrient diffusion. By reinforcing hydrogels with synthetic and natural fibers, these drawbacks can be effectively addressed, thereby enabling the modeling of advanced biomimetic tissue. This review discusses recent progress in the fabrication of fiber-integrated hydrogels and brings together developments from biomaterials, biofabrication, mechanobiology, and organ-model engineering. Fiber-addition impact on viscoelastic, time-dependent und nonlinear material properties, on multiscale and hierarchical constructs and on mechanical and biological readouts are analyzed. Specifically, the integration of both synthetic and natural fibers into hydrogel matrices is highlighted which significantly broaden their structural and biochemical versatility. These fiber-added hydrogels display improved properties including enhanced stiffness (up to 10-fold increase), anisotropy (>80 % alignment) and nutrient diffusion (4-fold increase). Moreover, the incorporation of fibers directly impacts cellular behavior by promoting adhesion, migration, proliferation and differentiation. Finally, bone, muscle and nerve tissue are exemplary presented in more detail to highlight the broad potential of these composite materials. In conclusion, fiber-embedded hydrogels represent a decisive step toward enhanced 4D-metamaterials.