3D PRINTING AND PATTERNING VASCULATURE IN ENGINEERED TISSUES - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 168

structures were formed after rinsing the first layer with buffer, changing the mask, using a thicker
spacer, and injecting prepolymer solution into the chamber followed by UV exposure. Multilayered
hydrogel microstructures containing encapsulated living cells were developed by adding cells to the
prepolymer solution and following the same procedure. Using this approach, cellular tissue constructs
were created in which placement of cells could be spatially controlled in 3D configurations throughout
a thick construct. The authors were able to demonstrate fidelity of the patterns and cell viability in the
fabricated hydrogel microstructures.
Further efforts from the same group resulted in a more refined apparatus to fabricate structures
with macroporous architecture ( Liu Tsang et al., 2007 ). Due to their interest in fabricating hepatic tis-
sues, the authors tailored the chemistry and architecture of hydrogels to support hepatocyte survival
and liver-specific function. The authors observed that coculture of hepatocytes with fibroblasts stabi-
lized the hepatocytes, allowing for better incorporation into biomaterials by increasing their affinity
to ligands. Multilayered cellular hydrogels fabricated with controlled 3D microarchitecture facilitated
the transport of oxygen and nutrients, resulting in higher cell viability due to the high metabolic
demands of encapsulated hepatocytes. Additionally, the authors demonstrated the importance of in-
corporating macroporous structures within the fabricated multilayered constructs after the constructs
were transferred to a perfused bioreactor, resulting in higher cellular metabolic activity for 2 weeks
compared to static constructs. Although the system utilized was able to achieve 3D microarchitectures
that were reminiscent of native tissue, the approach involved manually switching out spacers, masks,
and adding prepolymer solution which can be time consuming and increase the chances of problems
with architectural fidelity. Similar 3D constructs were automatically fabricated in a layer-by-layer
fashion by using 3D projection stereolithography to fabricate an entire layer under one single UV
exposure ( Gauvin et al ., 2012 ). This approach resulted in rapid fabrication of tissue constructs with
controlled microarchitecture. The custom designed projection stereolithography (PSL) system devel-
oped consists of a digital light processing (DLP) chip to create dynamic photomasks from computer-
aided design (CAD), which are projected onto the prepolymer solution, and a servo stage which can
increment the z -axis for patterning of subsequent layers ( Figure 8.1 ). Additionally, a syringe which
dispensed prepolymer solution was connected to the system. The authors demonstrated that the physi-
cal characteristics of the gelatin methacrylate (GelMA) scaffold, such as porosity and interconnectiv-
ity, can be tailored by controlling pore geometry and architecture. The authors also demonstrated that
dynamic seeding of cells on the hydrogel structure remained viable, spread, and proliferated for an
extended period of incubation, indicating biocompatibility of the fabrication process. Recently, simi-
lar liver-mimetic 3D structures were fabricated by 3D printing as a potential detoxification device.
In this work, poly(diacetylene) nanoparticles were incorporated into hydrogels with precise micro-
structures and were shown to attract, capture, and sense toxins while the 3D matrix trapped the toxins
( Gou et al., 2014 ).
8.1.2 FABRICATING FLUIDIC NETWORKS WITHIN BIOMATERIALS
The fabrication of macroporous scaffolds, such as those described earlier, provided early insights into
the relationship between structure and function in native tissues, though they failed to mimic the car-
diovascular system responsible for the bulk of convective mass transport in the body. Recent efforts
have turned toward the introduction of vascular networks into engineered tissues, able to support larger
cell populations, and able to sustain the flow of complex fluids such as whole blood. Importantly, blood