Recent decades have witnessed the development of sustainable food and beverage packaging technologies, which constitutes a critical societal concern globally, holding high relevance to public health and environment protection. [1-8] Let us take the insulating coffee container components as an example. In the USA alone, nearly two thirds of adults drink coffee daily and ~140 billion cups of coffee are consumed annually [9,10]. How to deal with these coffee container components becomes an important topic and a notorious sustainability challenge. [11-14] Consequently, researchers around the world are actively trying to find packaging solutions that exhibit the low manufacturing costs, desirable recyclability for reducing the carbon footprints, adaptive heat management for appropriate serving temperature and/or favorable form factors for food and beverage containers.
Within this context, our laboratory has focused on the engineering of bioinspired platforms with user-tunable infrared or thermoregulatory functionalities [15,16]. One judicious source of inspiration is the unique skin of the common squid, for which changes in appearance result, in part, from the muscle-controlled switching of embedded organs called chromatophores between contracted and expanded states [17,18]. On this basis, we have engineered reconfigurable metallized composite materials, for which changes in infrared reflectance and transmittance result from stretching-induced switching of the surface metal domains between abutted and separated states .
In this study, we draw inspiration from biological chromatophore organs in squid skin and leverage our previously established technical foundations for reconfigurable composite materials to develop the scalable manufacturing of squid-skin-inspired sustainable packaging materials with tunable heat-management properties.
We began our efforts by substantially modifying the procedures previously reported for the preparation of analogous small-area composite materials to manufacture large-area composite materials. We then characterized our composite materials’ mechanical properties via tensile testing, while also evaluating their surface morphology change at different applied strains. Amazingly, our composite materials could withstand extreme deformation before irreversible failure. Also, the electron microscopy images exhibited the transition of surface microstructures from resembling a nearly continuous Cu layer consisting of abutting domains to resembling a fractured Cu layer consisting of separated metal domains. Both characteristics resembled those of the reported much smaller-area materials , despite the distinct fabrication methodologies.
Next, we investigated the adaptive infrared functionality of our composite materials upon applied mechanical stimuli, which can be expected from their strain-reconfigurable morphologies. The mid-infrared spectroscopic measurements revealed that, under an increased applied strain, the total reflectance of our composite materials show a non-linear decrease, while the transmittance shows a non-linear increase. The total reflectance and transmittance switching ratios reach ~2 and ~23, which are comparable to that we previously reported . Moreover, we measured and analyzed the effect of the changes in the reflectance and transmittance components. We found that the substantial reduction of the specular reflectance component contributed more to the decrease of total reflectance, while comparable enhancements in both specular and diffuse transmittance components lead to the increase of total transmittance.
Having confirmed our composites’ tunable infrared properties, we evaluated their potential for adaptive thermal management in packaging applications. Toward this end, we first used a guarded hot plate to perform standardized thermal testing. Our composite materials can efficiently modulate an ~30 W m-2 heat flux with a minimal ~3 W m−2 energy input, which show the dual advantages of their on-demand tunability and energy efficiency. We further assessed the tunable heat-management capability of our composite materials in a practical scenario - the cooling of a hot-coffee-filled paper cup. Thermocouples were used to record and compare the temperature change of the covered coffee cup during the cooling process. We found that our composite materials at relaxed state exhibited heat-management properties similar to those of the space blanket. Moreover, upon application of the increasing applied strain we observed increase in the cooling rate, therefore demonstrating dynamic heat-management properties of our large area composite materials. Altogether, these experiments further underscore the potential of our composites as highly effective user-tunable insulation in various food and beverage packaging applications.
Summary and Future Directions
In summary, by drawing inspiration from state-switching chromatophore organs and dynamic color-changing ability of the squid skin, we fabricated and studied large-area composite materials with tunable heat-management properties. Our work holds significance for the development of sustainable beverage and food packaging technologies. First, the fabrication of our composites uses the scalable techniques of electron-beam evaporation, spray-coating, and delamination to reach the size comparable to those of ubiquitous metallized films [19-21]. Second, the structure–function relationships in our composites were demonstrated through the connection of their reconfigurable morphologies and adaptive mid-infrared functionalities, which would be important for further modification and optimization. Third, the function stability of our composites under severe or repeated deformation will be favorable for their applications in food and beverage packing. Moreover, the tunable heat-management capabilities of our composites are proved in both standard thermal testing and daily application. These results highlight the potential of our composites in packaging applications, which may influence a huge global market . To further address the sustainability pressures facing the food and beverage packaging industry, more efforts are required, including transferring the manufacturing process from laboratory scale to the factory-level production scale, and designing a sustainable and convenient method to recycle our composites. Overall, our continued work will solve these challenges and enable our composites to be widespread deployed and adopted and impact the packaging industry.
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