3D bioprinting is a technology with applications in critical biomedical fields, including tissue engineering 1 and regenerative medicine. Bioprinted 3D tissue biomimetics are attractive options for replacing traditional 2D culture and animal models. 3D printed biomimetics can reproduce the physical and biochemical environment of human tissue and better capture signalling pathways and drug responses. In addition, these systems lend themselves to dynamic stimuli that mimic those experienced by cells during regular tissue function. For example, mechanical stimulation of lung cells through cyclic stretching of the lung during breathing is a crucial feature to consider when modelling this tissue, which provides a more accurate model over conventional static cultures performed in 2D. Thus, 3D bioprinting opens the door to the creation of dynamic biomimetic models that resemble the mechanical and biochemical microenvironment of in situ tissues. Several technologies are used for 3D bioprinting, with inkjet, 7-9 laser, and extrusion bioprinting being the most popular. In the latter approach, a bioink is placed in a container (e.g., cartridge, syringe) and extruded through a nozzle, producing filaments and allowing the fabrication of cell-laden 3D structures in a layer-by-layer fashion. The bioinks used in extrusion-based bioprinting are formulations containing hydrogels and embedded cells, and must meet specific requirements to simultaneously result in good printability and cell viability. To form stable filaments and consequently yield high structural integrity and shape fidelity structures, it is necessary to use bioinks with high viscosity, shear thinning behaviour, and fast thixotropic recovery. The elevated viscosity and thixotropic behavior avoid the formation of droplets after extruding the ink through the nozzle and cell sedimentation in the cartridge during printing, while the shear thinning behaviour allows the extrusion to occur at low pressure, protecting cells from high shear stress. Gelatin is the material of choice for many tissue engineering and bioprinting applications due to its natural origin, biocompatibility, low immunogenicity, biodegradability, gelation properties, and low cost, along with the presence of RGD (Arg-Gly-Asp) domains that promote cell adhesion and spreading. Gelatin is fabricated through the acidic (Type A) or basic (Type B) hydrolysis of collagen, which produce polypeptides with average molecular weights between 15 and 400 kDa that undergo reversible gelation at low temperatures (Tg = 20-30 ºC). Gelatin gels are thermoreversible because the noncovalent interactions between the protein chains can be disrupted by increasing the temperature. Gelatin methacryloyl (GelMA) is a modified form of gelatin that can undergo rapid initiator mediated photo-crosslinking. This permits its use in light-assisted 3D bioprinting, resulting in hydrogels that are covalently crosslinked and retain their shape independent of temperature, which is not the case with unmodified gelatin. GelMA retains many of the desirable properties of unmodified gelatin (i.e., biocompatibility, biodegradability, low immunogenicity, low cost, RGD motifs), which has made it a preferred material for bioink formulation. Importantly, the hydrogels obtained from crosslinked GelMA also promote cell adhesion, growth, and proliferation. Despite the numerous advantages that GelMA offers, there are challenges in direct 3D bioprinting of this material via extrusion. At low concentrations and/or physiological temperatures suitable for cell culture (i.e., 37 ºC), GelMA solutions have low viscosity and cannot be successfully printed by themselves with extrusion printers. Therefore, low temperatures (to promote physical gelation) and high total solids content are needed to print constructs with structural stability and good shape fidelity. Yet, using a high concentration of GelMA during printing leads to lower cell viability, adhesion, and migration. In addition, decreasing the temperature to improve the viscosity of the ink complicates the printing due to clogging of the extrusion nozzle, and tighter control over temperature is needed, as small fluctuations can lead to significant changes in viscosity that impact the pressure needed for optimal extrusion. The need for higher GelMA concentrations and tight temperature control affect the printing reproducibility and complicate the optimization of printing workflows. GelMA 3D bioprinting through direct extrusion. (A) Graph of total solids vs rheological modifier content for reported for GelMA 3D bioprinting. The total solid is defined as the sum of the GelMA and modifier content. (B) Temperatures used to 3D bioprint GelMA bioinks with the respective theological modifiers. Data extracted from References .