Hierarchical and Double-Network Hydrogels in Vat Photopolymerisation: Microgel Reinforcement, Interpenetration and Energy Dissipation
Segurola, Juan
2026-08-27 · Report · Version 1.2
Hydrogels combine high water content with a polymer network, but the same hydrated state that enables transport and compliance commonly limits strength, crack resistance and dimensional stability. Vat photopolymerisation adds a further constraint: the material must form a geometrically stable network on the optical timescale of printing, whereas many toughening mechanisms rely on slow physical association, secondary crosslinking, microstructural rearrangement or post-print equilibration. Hierarchical and double- network strategies separate structural continuity from energy dissipation. Demonstrated VPP routes include covalent networks reinforced by hydrophobic association, metal-carboxyl coordination, thermoreversible polysaccharide helices, preformed microgels and post-print dynamic networks; fibre-assisted bulk UV photocuring provides an adjacent architecture boundary rather than direct VPP evidence. Some systems form both networks during exposure; others print a primary network and create the secondary network by cooling, immersion, coagulation, dark polymerisation or another controlled step. Primary studies show that tough hydrogel architectures can be shaped by DLP, projection stereolithography and LCD-VPP into lattices, optical elements and other complex geometries. Microgel-reinforced VPP has demonstrated up to four-fold increases in stress at failure and work of fracture at a 30% microgel volume fraction, while temperature- controlled projection stereolithography has produced highly stretchable and compression-resistant acrylamide/κ-carrageenan double networks. These values remain formulation-specific and do not define universal hydrogel targets. This review develops a network-architecture and qualification framework for printed hydrogels. It separates double networks from semi-interpenetrating networks, particle-reinforced hydrogels, supramolecular single networks and self-healing systems; links sacrificial-bond dissipation to hysteresis, recovery and fatigue; and requires mechanical testing to be tied to hydration, swelling, cycle number and specimen geometry. The central conclusion is that printed hydrogel toughness is not a single material constant but the outcome of how multiple network levels share load, dissipate energy and recover in the hydrated state.