Tightening of gelatin chemically crosslinked networks assisted by physical gelation

Developing the use of polymers from renewable sources to build hydrogels with tailored mechanical properties has become an increasing focus of research. The impact of the thermo-reversible physical networks of gelatin (arising from the formation of triple-helices) on the structure formation of a chemical network, obtained by cross-linking with glutaraldehyde (a non-catalytic cross-linker), was studied using optical rotation, oscillatory rheology and large strain mechanical deformation. We observed a direct correlation between the storage shear modulus of the chemical network grown in the gel state (i.e. simultaneously with the physical network) and the amount of gelatin residues in the triple-helix conformation (χ). Since χ is directly affected by temperature, the value of the storage modulus is also sensitive to changes in the temperature of gel formation. χ values as low as 12% lead to an increase of the shear storage modulus of the cross-linked gel by a factor of 2.7, when compared to a chemical network obtained in the sol state (i.e. in the absence of a physical network). Our results show that the physical network acts as a template, which leads to a greater density of the chemical cross-links and a corresponding higher elastic modulus, beyond what is otherwise achieved in the absence of a physical network.