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Bioprinting, microfluidics, and organoids to defeat COVID-19
promising ones as they allow precise reproduction As the olfactory neuroepithelium located in
of tissue morphological and functional properties. the nasal mucosa is involved in virus entry and
Bioprinting is a complex technique that, smell dysfunction [89,111] , the rational model should
particularly, enables tissue fabrication using contain sustentacular cells expressing high levels
spheroids or organoids (microtissues) as building of ACE2 and TMPRSS2 and olfactory receptor
blocks. Hence, the appropriate spatiotemporal neurons expressing these enzymes at lower
status and cell-cell and cell-matrix contacts may levels. There are numerous efficient protocols to
be achieved . In bioprinting, cells distributed form olfactory neurospheres and to differentiate
[98]
in a hydrogel system (“bioink”) are usually olfactory neuroepithelial cells [112-115] ; therefore,
deposited by a bioprinter, which can be based on these cells and their self-aggregates can be a
different technologies such as extrusion , ink perfect cell component for a bioink to print a
[99]
jet [100] , laser-induced forward transfer (LIFT) [101] , “smell-sensitive” nasal mucosa construct.
stereolithography [102] . Extrusion-based bioprinting The lungs, particularly the alveoli, are the main
is the most widely used technique [103] ; however, target for the SARS-CoV-2 infection and remain
only LIFT bioprinter can enable precise deposition technically challenging. Only a limited number of
at high speed and resolution and is considered to studies achieved success in the 3D reconstruction
be the best option to print minor cell populations of alveolar epithelial-endothelial barrier, and most
within complex 3D tissue-like structures [101] . scientists attempt to model only the air-cell and
Regarding bioinks, the most promising cell fluid-cell biointerfaces. For instance, Horvath
components are spheroids or organoids establish et al. [116] bioprinted the epithelial/endothelial
intercellular junctions and newly synthetized ECM cell barrier system on a porous membrane and
compared to a single cell suspension and maintain showed that it is possible to create reproducible
cell phenotype [22,104-106] ; the biomaterial component thin homogenous cell layers. To date, the most
– hydrogel system – is usually presented by natural complex lung-like structure was fabricated
and synthetic polymers, including their conjugates by Grigoryan et al. using a stereolithographic
such as acellularized ECM, alginate, gelatin, bioprinter [117] . To reproduce the alveoli scale and
fibrin, hyaluronic acid, cellulose, polyethylene morphology, particularly their epithelial side,
glycol, and Pluronic-F127. [101,107-109] . Lewis et al. created hollow epithelial cysts using
As specific targets for SARS-CoV-2 are ACE2 the microsphere-based approach [118] . Such cysts
and TMPRSS2, it is rational to include those as a cell component of a bioink can be easily
tissues whose cells express these enzymes in hierarchically structured through bioprinting to
the COVID-19 test tissue platform. Particularly, achieve lung-like constructs.
there should be 3D models of the nasal mucosa Blood vessels containing ACE-expressing
(including the olfactory neuroepithelium), lungs endothelial cells are a common object for bioprinting
(particularly, the alveoli), blood vessels, heart, because they ensure the proper survivability and
kidney, and intestine (Figure 3). To date, scientists engraftment of tissue-engineered constructs.
have accumulated data on their fabrication Different approaches varying in fabrication
through bioprinting, and this experience is shortly method and bioink blends were offered and can
described further. be classified as follows: Sacrificial and core/shell
The nose is one of the main ports of SARS- techniques. The choice depends on the required
CoV-2 infection, and the nasal mucosa is the shape and sizes; vessels with bigger diameter can
first barrier tissue for the virus . Among the be fabricated using an extrusion-based bioprinter,
[81]
existing models mimicking the nasal mucosa, vessels with smaller diameter using a laser-assisted
none of them was fabricated through bioprinting. bioprinter, and multibranched vessels using
They are mainly presented by monolayers, air- stereolithographic bioprinter [101,117,119,120] .
fluid single or multilayered biointerfaces, and Another target organ for COVID-19 that
scaffold-based and explant-based cultures [110] . should be included in an integrated platform is

20 International Journal of Bioprinting (2020)–Volume 6, Issue 4