Programmable continuous gradient bioprinting for engineering spatially heterogeneous microenvironments

Native tissues exhibit spatial heterogeneity in mechanical, cellular, and biochemical properties, yet reproducing such gradients in extrusion-based bioprinting remains challenging. Existing multi-material bioprinting approaches can create spatial variations in material composition, but often rely on discrete material transitions rather than continuous gradients. Here, we present a dual ball-valve mixing module that generates continuous gradients through real-time control of the mixing ratio between two precursor inks within a single-nozzle configuration. Complementary actuation of paired ball valves continuously adjusts the mixing ratio while maintaining constant extrusion conditions, transforming gradient formation into a programmable feature of the printing process. The versatility of the platform was demonstrated through gradients in mechanical stiffness, cell density, and biochemical cues. Continuous mixing enabled gradual transitions in material properties, reducing abrupt interfacial changes associated with discrete material deposition. Spatially defined cell-density distributions and graded two-population cellular interfaces were achieved while maintaining cell viability comparable to conventional extrusion bioprinting. Furthermore, a TGF-β1 gradient induced location-dependent epithelial-mesenchymal transition responses, demonstrating the ability to translate programmed biochemical gradients into spatially regulated cellular behavior. Consequently, this work establishes dynamic bioink mixing as an effective strategy for generating continuous and spatially programmable gradients, enabling the integration of material composition, cellular organization, and biochemical signaling within a single extrusion-based bioprinting process.