Sustainable highly stretchable and tough gelatin-alkali lignin hydrogels for scaffolding and 3D printing applications

Hydrogels and bioinks obtained from gelatin (Gel) generally present poor mechanical properties and require a series of time-consuming and stepwise chemical processes to exhibit improved elasticity and resistance to fatigue. Alkali lignin (AL) is an underutilized by-product of the paper and pulp industry. It is a widely available and inexpensive biomaterial that presents enormous potential for high-value applications owing to its ease of chemical modification and unique naturally occurring polyaromatic structure. This work aims to develop different GelAL hydrogel formulations with a single-step method that are innovative and sustainable. The results obtained from the mechanical, rheological, and degradation studies of the developed GelAL hydrogels demonstrated that their properties can be easily modified and tuned using straightforward processing techniques, allowing these stretchable and tough hydrogels to be used as bioinks in 3D printing. The modulation of mechanical properties through hydrogel formulations is a result of interactions between the Gel and AL which can be associated with the interplay of anionic sulfonates in AL and the arginine and lysine residues from Gel. The tensile stress at the break for the Gel20AL10 formulation was 32% higher than the value for Gel20AL5 and 157% higher than that of Gel10AL10. The elongation at break also decreased as it averaged 659 ± 149% for the Gel20AL10 formulation, which is 20% more than that of Gel20AL5 and 55% more than the average elongation at break of Gel10AL10. Further zeta potential measurements and quartz crystal microbalance with energy dissipation studies demonstrated that Gel and AL seem to form neutral complexes when mixed. These assays support the idea that AL and Gel are readily bound through weak interactions, and chemical crosslinking strategies need to be considered when degradability and mechanical properties tuning are envisioned. Altogether, these high-performance GelAL hydrogels display mechanical properties similar to soft tissues with high elasticity beyond that of natural hydrogels and fulfill the requirements of a broad range of biomedical and tissue engineering scaffolding applications.