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Branched polymers exhibit a unique three-dimensional (3D) molecular architecture and distinctive physical/chemical properties, and thus have been applied in a wide variety of fields.

The generation of human induced pluripotent stem cell (hiPSC)‐derived kidney organoids have facilitated novel insights into renal developmental processes and have the potential to provide personalised treatment strategies for patients with end‐stage renal disease. Traditional protocols for the directed differentiation of hiPSC‐derived kidney organoids have predominantly focused on biochemical cues to specify hiPSCs towards a mesonephric lineage, but have not considered the influence of the surrounding biophysical properties of the extracellular environment on organoid formation. We hypothesised that Gelatin methacryloyl (GelMA), a derivative of collagen with mechanically amendable hydrogel stiffness profiles, could be used to investigate the influence of extracellular matrix stiffness on kidney organoid specification. hiPSC‐derived kidney organoids were differentiated within photo‐crosslinked GelMA hydrogels of defined mechanical strengths. Enrichment of renal cell types in response to the various mechanical microenvironments was subsequently investigated using immunocytochemistry, qRT‐PCR, transmission electron microscopy (TEM) and histological staining methods. Rheological analysis of formulated hydrogels confirmed the generation of matrices with distinct stiffness (Young’s Modulus, G’) profiles. Hydrogels comparable to the stiffness of the gastrulation‐stage embryo (G’ = 400 Pa) and adult human kidney tissue (G’ = 5‐8 kPa) were generated. Importantly, the mechanical properties of the hydrogels showed remarkable stability even with prolonged time in culture. PCNA proliferation and cleaved caspase‐3 apoptotic staining of organoids embedded within scaffolds demonstrated high cell proliferation and viability in the hydrogel conditions by day 24 of differentiation. The formation of glomerular, proximal tubular and distal tubular structures, supported by basement membrane and interstitial cells were confirmed in conditions using immunofluorescent imaging. Interestingly, qRT‐PCR analysis revealed significant upregulation of nephron‐associated genes (including PAX8, NPHS2, NPHS1, SLC3A1 and AQP1) in organoids differentiated within extracellular environments that approximated adult human kidney tissue, when compared to organoids differentiated within the much softer microenvironments. Our results illustrate the influence of the extracellular environment on appropriate cell fate determination and propose GelMA hydrogels as faithful extracellular supports for the specification of hiPSC‐derived kidney organoids.

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