Bioprinting of exosomes: Prospects and challenges for clinical applications

453Three-dimensional bioprinting (3DBP) is an additive manufacturing technique that has emerged as a promising strategy for the fabrication of scaffolds, which can successfully recapitulate the architectural, biochemical, and physical cues of target tissues. More importantly, 3DBP offers fine spatiotemporal control and high submicron scale resolution, which can be leveraged for the incorporation and directional gradient release of single or multiple biomimetic cues, including cell-derived exosomes (EXOs). EXOs are extracellular vesicles that originate from the endosomal compartment of various cell types, with sizes ranging from 30 to120 nm. They act as cell mediators and contain discrete cell constituents, including growth factors, cytokines, lipid moieties, nucleic acids, metabolites, and cell surface markers, depending on the cell type. Essentially, owing to their therapeutic potential, EXOs derived from mesenchymal stem cells (MSCs) have been recently investigated in several clinical trials for the treatment of various conditions, including cancer, diabetes, dry eyes, periodontitis, and acute ischemic stroke. The 3DBP strategy of EXOs is especially useful in tissue engineering and regenerative medicine applications, as tissues can be biofabricated to closely mimic the complex microarchitecture and developmental profiles of native heterogeneous tissues for restoring biological functions. Moreover, EXOs can be manipulated to carry exogenous cargo such as genes or proteins of therapeutic interest, confer multifunctional attributes, and further enhance their tissue regenerative potential. However, significant challenges, including the selection of appropriate bioink, pattern resolution, engineering-defined exosomal gradient, spatial presentation and modulation of EXO release kinetics, as well as EXO stability and storage conditions, must be addressed for the successful translation of therapeutic grade EXOs to clinical settings. In this review, we highlight the recent advances and offer future perspectives on the bioprinting of EXOs as regenerative biotherapeutics for the fabrication of complex heterogeneous tissues that are suitable for clinical transplantation.