3D bioprinting modalities for tissue engineering and regenerative medicine.

Additive manufacturing (AM), particularly three-dimensional (3D) bioprinting, is advancing as a versatile biofabrication platform capable of spatially organizing cells, biomaterials, and biochemical cues, such as signaling molecules, in a precise manner to recapitulate the complex architecture of native tissues. Current 3D bioprinting modalities include extrusion-based, inkjet-based, laser-assisted, and light-based techniques, as well as scaffold-free bioprinting and bioassembly strategies involving cellular aggregates such as spheroids and organoids. Extrusion-based bioprinting utilizes pneumatic, piston, or screw-driven mechanisms, whereas inkjet-based systems employ thermal, piezoelectric, or electrostatic actuation. Laser-assisted bioprinting encompasses laser-induced forward transfer (LIFT) and other laser-direct writing (LDW) techniques, while light-based methods, such as stereolithography (SLA), digital light processing (DLP), and two-photon polymerization (2PP/TPP), involve vat photopolymerization. Furthermore, bioassembly approaches using spheroids or organoids facilitate the construction of higher-order biological structures. Native human tissues are characterized by intricate 3D arrangements, specific porosity, cellular heterogeneity, and specialized mechanical and physiological properties. Various 3D bioprinting modalities are tailored to address these tissue-specific requirements within regenerative medicine. Direct ink writing (DIW) and liquid/fused deposition modeling (LDM/FDM) provide robust scaffold architectures, while droplet-based and laser-assisted methods offer high-resolution cell patterning. Conversely, scaffold-free approaches, including the Kenzan method, aspiration-assisted bioprinting (AAB), and magnetic levitation (Mag-TE), utilize cellular self-organization to produce native extracellular matrix (ECM)-rich tissues. Moreover, bioprinting-assisted tissue emergence (BATE) leverages the principles of developmental biology. Within this paradigm, organoid-forming stem cells, deposited at defined geometries and cell densities into a permissive ECM, undergo directed morphogenesis to autonomously self-organize into centimeter-scale, native-like tissue architectures. In addition, emerging techniques such as volumetric bioprinting and embedded bioprinting, such as freeform reversible embedding of suspended hydrogels (FRESH), have been developed to enable the precise fabrication of soft biomaterials. Consequently, each 3D bioprinting modality provides distinct advantages for specific tissue engineering applications. This review comprehensively discusses major 3D bioprinting modalities, their underlying principles and working mechanisms, and biomaterial selection criteria for diverse applications in tissue engineering and regenerative medicine.