Mechanically robust non-swelling cold water fish gelatin hydrogels for 3D bioprinting

Three-dimensional (3D) bioprinting of hydrogels allows embedded cells to be patterned and hosted in an extracellular matrix (ECM)-mimicking environment. This method shows great promise for the engineering of complex tissues on account of the facile spatial control over materials and cells within the printed constructs. Hydrogels, which represent extensively explored and employed biomaterials for 3D bioprinting, are characterized by both their high water content and swelling behavior. Post-printing swelling inevitably alters the geometrical and mechanical properties of printed features, thus causing a deviation from the original design and affecting both cellular function and tissue structure. Despite substantial effort being dedicated to the development of non-swelling hydrogels, their application in 3D encapsulation and bioprinting of living cells is yet to be realized, owing to limitations imposed by their often tedious material syntheses and complex network structures. Herein, we describe a new type of non-swelling hydrogel based fully on cold water fish gelatin (cfGel-Hydrogel) consisting of only a single network formed via thiol-ene "click" chemistry. We show that such cfGel-Hydrogels enable 3D patterning of living cells in a shape-retaining and mechanically robust matrix. These cfGel-Hydrogels show negligible swelling (<2 %) under physiologically relevant conditions (simulated by 37 °C PBS buffer), while also being able to withstand large cyclic deformations (80 % compressive strain) by dissipating around 40 % of the imposed loading energy. Human dermal fibroblast (HDF)-laden cfGel-Hydrogels could be fabricated via extrusion-based 3D printing, allowing for the <i>in vitro</i> culturing of cells in shape-retaining constructs, thus offering new opportunities for hydrogel-based applications in tissue engineering and regenerative medicine.