Omnidirectional elastic constraint-based embedded 3D printing in non-yield stress fluids for engineering complex tissues

Embedded 3D printing, relying on yield-stress fluid-based supporting baths, has significantly expanded our ability to create complex tissues and organs. However, many extracellular matrix-based hydrogels, despite their excellent biological performance, such as gelatin and hyaluronic acid (HA) hydrogels, lack yield-stress properties, limiting their use in embedded 3D printing and hindering functional maturation of complex tissue constructs. To address this challenge, we present a method for embedded printing in non-yield stress fluids using sequential printing in a reversible ink template (SPIRIT) strategy. By leveraging an omnidirectional elastic constraint provided by conventional supporting baths, we can precisely fabricate GelMA and HAMA hydrogels into functional tissue constructs with both intricate external structures and internal vessel systems, which cannot be achieved with extant embedded printing techniques. As a proof of concept, we successfully printed a complex ventricle model with high cell density and internal channels using GelMA hydrogel, which demonstrated high cell viability and synchronous beating performance. The omnidirectional elastic constraint-based SPIRIT technique holds promise for advancing organ printing with remarkable structural complexities and biomaterials affinity akin to natural tissue constructs by decoupling from yield-stress fluids.