Alginate-gelatin hydrogel scaffolds for establishing physiological barriers on a gut-brain-axis microchip

Organ-on-chips (OOCs) technique has transferred real animal models to the in vitro models, especially for the exploration of the gut-brain-axis. In this work, we proposed a novel hydrogel-based microchip system to simulate two of the most indispensable physiological barriers in the gut-brain-axis, providing a feasible tool for studying the bacteria-induced regulations of tryptophan metabolism on cells. Using UV-light-induced hydrogel of alginate-gelatin mixture filled within the microchip as the cell scaffold, different cell layers were successfully inoculated and formed an intestinal barrier (IB) and a blood-brain barrier (BBB) with specific morphology and fundamental functions. Tryptophan-kynurenine pathway, a significant metabolic process in the gut-brain-axis, has been analyzed using this model by liquid chromatography-mass spectrometry and presented a possible explanation of this metabolic mechanism. Under bacterial conditions, the tryptophan in IB was about 1.46 times higher than that in BBB, while this value was about 3.14 times higher for kynurenine, indicating that the BBB has selective permeability to tryptophan and hinders the diffusion of kynurenine. This alginate-gelatin hydrogel scaffold based OOCs system has successfully simulated the dual barriers without moral dilemmas of animal models and analyzed tryptophan metabolism among different physiological barriers, providing an important tool for simulating physiological models with multi-barrier structures.