Page 465 - IJB-9-2

International Journal of Bioprinting Bioprinting of exosomes

In another study, therapeutic growth factors, clinical applications . Moreover, as EXOs directly activate
[72]
vascular endothelial growth factor (VEGF), and bone signaling processes to regulate cell function via the gene
morphogenetic protein 2 (BMP2) were incorporated transcription machinery, they act upstream of protein
as spatiotemporally defined patterns in implants using synthesis , and thus can be leveraged to modulate tissue
[73]
extrusion-based bioprinting for simultaneous induction of repair and regeneration. With the above considerations,
angiogenesis and osteogenesis . The results demonstrated the 3DBP of cell-derived EXOs offers great potential
[67]
that implants with distinct spatial presentation of VEGF in the engineering of implantable constructs for the
promoted an increase in vessel invasion compared to localized delivery of EXO-based therapeutics with precise
implants that have been homogenously loaded with VEGF. spatiotemporal control.
Furthermore, bioprinted implants with both spatial VEGF As a proof-of-concept model, EXOs have been
gradients and defined BMP2 localization accelerated bone incorporated in a bioink to engineer scaffolds with spatially
defect healing with minimal heterotopic bone formation, well-defined patterns for promoting cell differentiation
suggesting that spatiotemporally defined growth factor in vitro . With regard to that, EXOs derived from different
[74]
delivery can be employed for the regeneration of large macrophage subsets have been incorporated into scaffolds
bone defects in vivo. In a different study, extrusion- fabricated via inkjet-based bioprinting, and their influence
based 3D-printed scaffolds with physical (microgrooves) on C2C12 mouse myoblasts has been investigated .
[74]
and biochemical (spatiotemporal gradients of nerve In order to facilitate the bioprinting process, glycerol
growth factor and glial cell line-derived neurotrophic was used as an additive in the bioink to reduce EXO
factor) cues were employed to provide axonal guidance agglomeration, increase the viscosity of the bioink, and
and chemoattractant/chemokinetic function for also serve as a humectant to reduce solvent evaporation
neuroregeneration . In a rat model of nerve injury, the at the tip of the nozzle. Cellular uptake studies showed
[68]
3D-printed scaffolds facilitated nerve regeneration across a that fluorescently labeled ECM-bound EXOs were readily
10-mm nerve gap and demonstrated functional restoration taken up by C2C12 cells within 15 min, suggesting that a
of the regenerated nerve 12 weeks after implantation bioprinted solid-phase ECM environment did not affect
in vivo. EXO membrane integrity for effective delivery of cargo
Concurrently, these studies show the potential of 3DBP into the cell cytoplasm. More importantly, ECM-bound
as a promising methodology for spatiotemporal patterning EXOs derived from proinflammatory M1 macrophages
of growth factors, cell modulators, and therapeutic moieties demonstrated spatial inhibition of myogenesis, whereas
to tightly regulate tissue development for engineering EXOs from pro-regenerative M2 macrophages promoted a
complex functional tissues for tissue transplantation or microenvironment that spatially induced myogenesis in a
drug screening applications. dose-dependent manner in vitro. These results corroborate
the evidence showing the compatibility of 3DBP with
4. Bioprinting of exosomes for regenerative EXOs and the potential use of the spatially defined patterns
therapy of EXOs to spatially trigger intended biological functions.
Although gradient patterning of growth factors offers In another study, BMMSC EXOs were incorporated
biomimetic path-specific biochemical cues for tightly in a decellularized cartilage ECM/gelatin methacrylate
regulated tissue regeneration, the supraphysiological bioink and 3D printed using the dynamic projection
release of protein therapeutics has been shown to induce stereolithography technique to yield EXO-functionalized
adverse effects without significant therapeutic benefits. scaffolds . The final bioprinted structure, which
[75]
For instance, major adverse events such as heterotopic possessed radially oriented channels to promote cartilage
ossification, osteolysis, infection, and cancer have been repair and regeneration, was crosslinked in the presence
implicated with INFUSE® bone graft, which consists of a of a photoinitiator, lithium acylphosphinate, and 405-nm
[75]
Col sponge, loaded with recombinant human BMP2 [69,70] . wavelength visible light . Subsequent implantation of
Alternatively, directional gradient release of EXOs at the these scaffolds in a rabbit osteochondral defect model
target site can be availed to modulate tissue regeneration demonstrated that the EXOs released from the bioprinted
and function. The advantages of employing EXOs over scaffolds rescued cartilage mitochondrial dysfunction,
recombinant therapeutic growth factor (GF) proteins are promoted chondrocyte migration, and supported
multifold. As EXOs are naturally produced in the body, M2 macrophage polarization, thereby facilitating the
[75]
they possess unique membrane proteins that facilitate regeneration of cartilage in vivo . Although this study
the internalization by recipient cells . Furthermore, as did not aim to spatially distribute EXOs in the bioprinted
[71]
EXOs are native to the body, they are associated with low structure, it provided evidence that the 3DBP of cell-
immune responses and thus safe even at high dosages for derived EXOs can be used for tissue regeneration.

Volume 9 Issue 2 (2023) 457 https://doi.org/10.18063/ijb.690