Additive Manufacturing of Hydrogel-Polymer Combinations for Applications in Smart Reactors

Abstract The increasing use of renewable raw materials with fluctuating properties creates a need for process equipment that can adapt to changing operating conditions. As components of smart reactors, self-adaptive structures could respond to reaction conditions such as temperature, pressure drop, or pH to maintain selectivity and yield within a desired range. Stimuli-responsive hydrogels are promising materials as they undergo reversible swelling and shrinkage when exposed to external stimuli, thereby enabling changes in reactor geometry and flow conditions. However, their low mechanical stability limits their use as structural reactor elements. This paper therefore proposes a multi-material additive manufacturing approach combining material extrusion of thermoplastics with a second extrusion system for hydrogel precursor. Hydrogel formulations with different water contents are investigated regarding printability and curing behavior, and response surface methodology is used to identify a robust processing parameter set. The selected formulation exhibits an average mass-based shrinkage of 54.22 ± 3.36% between 25 and 40 °C, while image-based measurements show that the majority of the volume change occurs within the first 5–10 min, with an apparent volumetric plateau reached after 16–18 min. Thermoresponsive actuation is further demonstrated using hydrogel-thermoplastic polyurethane bilayer specimens, with all specimens exhibiting a complete reversal in bending direction and curvature changes between 0.0343 and 0.0550 mm⁻¹. Based on these results, potential hydrogel-polymer design elements for smart reactors are proposed, including adaptive baffles, variable flow cross-sections, flow redirecters, and adaptive structured reactor internals.