Stretchable and compliant films can be prepared from edible biopolymers such as hydrocolloids and proteins. Their mechanical performance under stress affects their potential applications. This study investigates the uniaxial and biaxial extensional properties of various edible hydrogel films plasticized to various degrees. Hydrogel films consisting of single biopolymer networks (SN), crosslinked networks (CN) and interpenetrating networks (IPN) were prepared using gelatine, gelatine-caseinate and alginate-agar, respectively, with different concentrations of glycerol. Their extensional properties were characterized with computer-vision-based biaxial extension testing and compared to standard uniaxial tensile testing. The IPN films exhibited strain-hardening and the highest stiffness and tensile strength. Adding plasticizer reduced the stiffness of all film types, and also reduced the tensile strength in SN and CN films. The influence of the plasticizer on the tensile strength of IPNs is governed by the architectures of the first and second networks. While standard, uniaxial extension of IPN films showed no relation between glycerol content and fracture properties, our biaxial methods showed that plasticization by glycerol reduced both fracture strain and stress. Our results highlighted the necessity of combining uniaxial and biaxial tensile tests to evaluate film properties for practical applications relying on different types of stress load. The insight obtained may be used to design edible films with improved extensional performance, and our biaxial method could potentially be used as a screening tool for evaluating tensile properties in novel materials. • Plasticizers decrease elastic modulus and tensile strength of hydrogel films • Interpenetrating network films respond differently to uniaxial and biaxial loads • Interpenetrating network films possess very high tensile strength • Bulge deformation is more accurately approximated by ellipsoids than spheres • Combined uniaxial and biaxial tensile tests offer a more complete insight in film behaviour
Steffens et al. (Sun,) studied this question.