Engineering heterogeneous tissues and organs via multi-material bioprinting: Advances, challenges, and opportunities

Native tissues and organs exhibit intrinsic heterogeneity in their structural, compositional, and functional properties, which poses significant challenges towards the development of effective functional substitutes. Multi-material three-dimensional (3D) bioprinting has emerged as a promising strategy to address these challenges by enabling enhanced spatial control over the deposition of biomaterials, signaling factors, and diverse cell types. This capability facilitates the fabrication of heterogeneous tissue constructs that aim to approach the complexity of native tissues, although achieving sub-micron biomimicry remains restricted by current hardware limitations. Recent advances, such as the development of multi-nozzle systems, multi-ink platforms, and the incorporation of heterogeneous cell aggregates, have substantially enhanced the capacity to reproduce tissue heterogeneity with greater precision and scalability. This review provides a comprehensive overview of the fundamental concepts and classifications of multi-material bioprinting. Four representative heterogeneous tissues/organs-osteochondral tissue, trachea, liver, and heart-were selected for in-depth discussion, given their rising clinical significance and escalating bioprinting challenges imposed by their structural and physiological complexity. Furthermore, bioprinting of these tissues/organs covers critical concerns including stiffness, interface connection, biological function, and vascularization. In addition, this Review critically examines the current challenges in the field and offers a forward-looking perspective on emerging trends and potential breakthroughs in technologies in the context of multi-material bioprinting. STATEMENT OF SIGNIFICANCE: This review elaborates on multi-material bioprinting strategies for fabricating heterogeneous tissues and organs. It proposes a classification and definition of heterogeneity at multiple levels, including structural, compositional, and functional aspects, in both native tissues and 3D bioprinting approaches. Moreover, various 3D bioprinting modalities are discussed, encompassing both nozzle-based and nozzle-free techniques, together with their applications in representative heterogeneous organs. This work offers critical insights by addressing current limitations and future opportunities. Together, this review comprehensively categorizes heterogeneous tissue fabrication strategies across diverse multi-material bioprinting approaches, providing a holistic framework for advancing the design and fabrication of next-generation heterogeneous tissues and organs.