Self-healable and biodegradable soy protein-based protective functional film with low cytotoxicity and high mechanical strength

Abstract Soy protein isolate (SPI) is a natural organic polymer, being used to prepare protective functional materials due to its sustainability, abundance, and easy processing. However, the protective effect of unmodified SPI films is significantly limited due to its poor mechanical properties, irreversible damage, and monotone functionality. Herein, high-flowability polyethyleneimine (PEI) was used to improve the dynamic properties of the protein matrix and cross-linking, and the phenolic hydrogen bonding of polydopamine (PDA) was used to develop a facile strategy for fabricating SPI-based films with satisfactory self-healing properties, high strength, excellent UV barrier performance, and remarkable fluorescence for the heavy metal detection. PEI disrupted the strong hydrogen bonds between protein molecules, causing them to be aggregated due to the PDA cross-linking. Thus, the phenols of PDA formed hydrogen bonds and constructed a reversible network, which endowed the SPI-PEI-PDA films with excellent self-healing properties. Because the hyperbranched PEI imparted the films with more cavities, the microcracks caused by external forces were blunted. Due to the further PDA crosslinking, the SPI-PEI-PDA films exhibited high mechanical strength. The SPI-PEI-PDA films also showed outstanding UV barrier performance and fluorescence properties for the heavy metal detection due to the bionic melanin-like characteristic of PDA and the Michael addition reaction between the reactants. Additionally, SPI-PEI-PDA films demonstrated the low toxicity in vitro and in vivo, as well as the favorable biodegradability. These films improved the adaptability and safety of protective materials and could greatly promote functional applications of other biopolymer-based materials.