Polyacrylic Acid-Reinforced gelatin hydrogels with enhanced mechanical properties, temperature-responsiveness and antimicrobial activity for smart encryption and salmon freshness monitoring

Abstract

Hydrogels hold great potential for use in intelligent packaging, yet they often suffer from limited functionality and inadequate mechanical strength when applied to anticounterfeiting and freshness monitoring. In this study, we present a straightforward method to create a multifunctional hydrogel by in-situ polymerizing acrylic acid (PAA) within a gelatin-Al³⁺ system. The resulting hydrogels exhibited an elongation at break of over 1200 %, a tensile stress of 1.20 MPa, and impressive toughness reaching 5.15 MJ/m³, significantly outperforming traditional gelatin-based hydrogels that typically achieve less than 800 % strain and below 1 MPa stress. These hydrogels also showed exceptional antifatigue and tear resistance, with a tearing energy of 5200 J/m², greatly exceeding the 1000 J/m² standard of typical double network hydrogels, and were capable of supporting weights 1560 times their own mass. The strong hydrogen bonding between the –COOH groups of PAA and the –NH₂ groups of gelatins contributed to an upper critical solution temperature above 40°C, with adaptable PAA content allowing for anticounterfeiting applications. The hydrogel could encode information such as self-erasing numbers, QR codes, and ASCII binary codes, changing its encoded data with temperature shifts and erasing at room temperature to enhance data security. Additionally, it exhibited potent antibacterial properties against S. aureus and E. coli, immobilized anthocyanin as an ammonia-responsive indicator, and accurately tracked salmon spoilage by correlating color changes with total volatile basic nitrogen content. These characteristics make the hydrogel highly suitable for smart packaging applications within the food industry.