Fabrication of films with tailored properties by regulating the swelling of collagen fiber through pH adjustment

Acid swelling collagen fibers have been commonly used to prepare casing or film, but how the swelling degree regulates the properties of collagen fiber dispersions (CFD) and their related films are still unclear. Herein, variant swelling degrees of collagen fiber were obtained by controlling the pH of CFD. The swelling ratio of collagen fiber increased and then decreased with the decrease of pH from 4.0 to 1.5. The maximum viscosity, modulus and denaturation temperature appeared at pH 3, where the fibers interconnected to form a dense network structure. This sufficient swelling enabled excellent mechanical and thermodynamic properties of films, whereas insufficient swelling of CFD led to inferior properties. Films from collagen fiber swelled at pH 3 further showed a more ordered layer structure, but cross-sectional microstructures with disintegrated tiny fibers along with the reduction of triple helix content for those at low pH were observed. This study provided a better understanding and guidance of pH regulation in actual production of collagen films using bovine dermis.