A sprayable and rapidly cross-linked hydrogel membrane for fruit preservation

Hydrogel membranes (HMs) exhibit great potential for fruit preservation due to their excellent biocompatibility and scalability. However, most HMs cannot meet the requirements of rapid and large-scale preparation. In this work, one type of hydrogel paint that can form a uniform membrane on fruit surfaces by either spraying, brushing, or dipping methods has been developed. The hydrogel paint, which utilizes ethanol to dynamically modulate the hydrogen-bond cross-linking between tannic acid (TA) and polyvinyl alcohol (PVA), enables the rapid and uniform formation of a membrane (PVA@TA₁ HM) on fruit surfaces within 50 s by ethanol evaporation. The PVA@TA₁ HMs can not only effectively extend the freshness of fruits to 8 days (25 °C, 40 % relative humidity) but also possess properties such as easy cleaning, recyclability, and antioxidation. Besides, the natural antibacterial properties of TA endow the PVA@TA₁ HMs with long and effective antibacterial activity (antibacterial rate ≥ 99.7 %, lasting for 7 days) against both Staphylococcus aureus and Escherichia coli. Furthermore, the dense hydrogen-bonds cross-linking between PVA and TA molecules endow the PVA@TA₁ HMs with high tensile strength (≥ 7 MPa) and significantly reduces their water and oxygen transmission rate. Moreover, the adhesion of hydrogel paint has been significantly improved with the introduction of TA, as validated by both atomic force microscopy and lap-shear adhesion tests, at both micro- and macro- scales. This work provides a method for the rapid and large-scale preparation of HMs under different working environments, which shows great potential for fruit preservation.