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Abstract: Smart materials possess a high potential for application in process engineering. Among these smart materials, stimuli-responsive hydrogels exhibit the chemically inherent characteristic to significantly change their macroscopic properties through shifts in environmental conditions. This enables response-triggered actuation caused by a reaction or process deviation. Thereby, smart process concepts are facilitated, which are capable of self-contained process control without external input. Through additive manufacturing of responsive hydrogels, intricate geometries can be generated, with which the response-triggered actuation can perform sophisticated control tasks. Periodic open-cell structures are such geometries, which improve the mass transport in multiphase flows through the distribution of the disperse phase. Responsive hydrogels fabricated as periodic open-cell structures enable the actuation of multiphase flows through an environmental switch allowing for adjustment of flow conditions. Herein, we demonstrate the application of switchable smart structures that facilitate the adaptation of fluid-dynamic properties and mass transfer in cocurrent gas–liquid flows depending on environmental conditions. Smart structures, which are additively manufactured from acrylate photoresist formulations, are applied for in situ and in operandi adjustment of phase distribution through expansion and collapse of these structures in flow channels. Further, diverse photoresist formulations with different associated response triggers are shown, which demonstrate the versatility for application as an in situ and in operandi switch for mass transfer in process units operating with multiphase flows.