4D-printed polysaccharide composite hydrogel labels with hygroscopic actuation and CO₂-responsive colorimetric function for fresh produce monitoring

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This study developed a 4D-printed intelligent label based on an HPMC/SA/KGM composite hydrogel system, integrating humidity-driven deformation with CO₂-responsive colorimetric functionality. By systematically tuning printing parameters, including layer number, infill density, and infill pattern, the coupling mechanisms among moisture absorption behavior, deformation kinetics, and mechanical constraints were elucidated. The results demonstrated that a “dense constraint layer-low infill patterned layer” configuration could generate a stable hygroscopic swelling strain gradient under humidity stimulation, enabling programmable and directional bending. Among the tested structures, the three-layer configuration achieved an optimal balance between response sensitivity and structural stability. Infill density further regulated deformation amplitude and response rate by influencing pore structure and moisture absorption kinetics. Under 100% RH conditions, the material exhibited a transition from differential (layered) swelling to overall swelling, accompanied by a characteristic “bending–unfolding” response behavior. Meanwhile, the label showed excellent reversibility and structural stability during the moisture absorption–desorption–rehydration cycle. Application in fruit and vegetable systems demonstrated that the label could effectively respond to variations in humidity and CO₂ microenvironments induced by different respiratory metabolisms, enabling synergistic visual indication through both deformation and color change. Specifically, fresh-cut produce exhibited rapid responses followed by partial unfolding, whereas whole fruits showed slower yet more stable responses. This study provides a novel design strategy and theoretical basis for the application of 4D-printed intelligent packaging in fruit and vegetable quality monitoring.