Bioprinting Using Organ Building Blocks: Spheroids, Organoids, and Assembloids

Three-dimensional (3D) bioprinting, a promising advancement in tissue engineering technology, involves the robotic, layer-by-layer additive biofabrication of functional 3D tissue and organ constructs. This process utilizes biomaterials, typically hydrogels and living cells, following digital models. Traditional tissue engineering uses a classic triad of living cells, scaffolds, and physicochemical signals in bioreactors. A scaffold is a temporary, often biodegradable, support structure. Tissue engineering primarily falls into two categories: (i) scaffold based and (ii) scaffold free. The latter, scaffold-free 3D bioprinting, is gaining increasing popularity. Organ building blocks (OBB), capable of self-assembly and self-organization, such as tissue spheroids, organoids, and assembloids, have begun to be utilized in scaffold-free bioprinting. This article discusses the expanding range of OBB, presents the rapidly evolving collection of bioprinting and bioassembly methods using these OBB, and finally, outlines the advantages, challenges, and future perspectives of using OBB in organ printing. The organ building blocks (OBB) such as cell spheroids, organoids, and assembloids are advantageous due to their capacity to self-assemble and self-organize, enable quicker production of large tissue constructs, the potential for prevascularization, and achieve cell densities closer to native tissues. Despite the promising future of bioprinting human organs using OBB, various hurdles, including cost-effective biofabrication, assembling blocks into organs, biomonitoring, postimplantation monitoring, and creating larger blocks and connecting vascular systems, need to be addressed.