3D PRINTING AND PATTERNING VASCULATURE IN ENGINEERED TISSUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 175

conduit with a root consisting of SMCs, which was deposited first, and then a leaflet region consisting
of VICs, to demonstrate fabrication of living valve conduits with anatomical resemblance to the native
valve based on alginate/gelatin hydrogel system via 3D bioprinting, obtaining 3D hydrogel constructs
that can maintain high cell viability with clinically relevant thickness. This approach allowed the re-
searchers to obtain anatomically complex, mechanically heterogeneous valve scaffolds rapidly, without
any processing post-fabrication.
8.1.5 PATTERNING MULTISCALE VASCULATURE WITH ENDOTHELIAL FUNCTION
Although fluid flow through perfusable microvasculature can help mimic physiologic transport on a
small scale, the mammalian cardiovascular system is comprised of a wide range of diameters of blood
vessels connected through a well-organized fractal hierarchy. Researchers are also investigating ways
to generate multiscale vasculature—vessel networks containing an array of channel diameters and their
accompanying fluidic junctions.
Work utilizing a direct-writing approach involved fabricating microchannels with barrier function
by casting a hydrogel precursor over bioprinted agarose fibers, which serve as template material ( Ber-
tassoni, Cecconi, et al . 2014 ). After photopolymerization of constructs, the template fibers can be re-
moved to result in perfusable networks ( Figure 8.2 E). The authors demonstrated the ability to fabricate
microchannel networks with various architectural features inside a range of photopolymerizable hydro-
gel systems. Microchannels with diameters ranging from 1000 m m down to 150 m m could be obtained
using this approach. By parallel overlapping of multiple template fibers over one another, the authors
demonstrated versatility of the approach to fabricate even larger channel diameters that branched out
into lateral individual channels of narrower diameters. The authors also demonstrated high efficiency
of bioprinted microchannels to form cell-laden tissue constructs with improved functionality due to the
channels allowing for improved nutrient transport, resulting in increased cell viability and cell differ-
entiation. Furthermore, formation of endothelial monolayers inside hydrogel constructs was observed,
allowing the constructs to remain fully perfusable.
We have also demonstrated an approach to rapidly cast patterned vascular networks in engineered
tissue, a technique compatible with a wide range of cell types, synthetic and natural ECMs, and cross-
linking strategies ( Miller et al., 2012 ). In this approach, rigid 3D filament networks of carbohydrate
glass are printed, cast into an ECM, and then dissolved to obtain a monolithic cellularized tissue con-
struct. Multiscale filament architecture was obtained by varying only the translational velocity of the
extrusion nozzle. After sacrificing the carbohydrate glass, smooth elliptical intervessel junctions were
left behind by the glass interfilament fusions. As a result, fluidic connections between adjoining vascu-
lar channels can be obtained. Encapsulation of cells in the ECM was achieved by mixing a suspension
of cells in the ECM prepolymer, while endothelialization was achieved by injecting a suspension of
ECs into the vascular architecture. Encapsulated cells were shown to be viable, spread, and migrate
normally in the channeled scaffolds. Seeded ECs quickly lined the walls of the network, even in con-
structs containing vessels of differing diameters ( Figure 8.3 F). Complete endothelialization of the vas-
cular scaffolds resulted in the ability to perfuse human blood under high pressure by either laminar
or pulsatile flow. Additionally, we found that highly sensitive cells, such as primary hepatocytes, can
maintain metabolic activity at or near physiologic cell densities in tissue constructs with perfusable
vascular channels and junctions.