Page 44 - IJB-4-2

Bioprinting of artificial blood vessels

it was also reported that the printed cells proliferated thus forming perfusable micro-channels without the
over a course of one week and were comparable to cell need for any additional dissolution process [53] . It was
viability of the control groups (~82% cell viability). The further reported that such a method prevents unnecessary
authors suggested that the initial decrease in cell viability osmotic damage to encapsulated cells and further
could be attributed to shear stress exerted on the cells prevents interactions of dissolved sacrificial material
during the printing process, which is also in consistence that could potentially modify the structural properties of
[54]
[50]
with reports and observations made by other studies . the main scaffoldsas reported by others . Interestingly,
Another recent study that uses similar technique this study studied on the feasibility of endothelization
biofabricated a thrombosis-on-a-chip model with cell within the micro-channels and that seeding of human
laden GelMA and Pluronic-F127 as the sacrificial umbilical vascular endothelial cells (HUVECs) and
ink [51] . Similarly, this study demonstrated that vascular formation of endothelial mono-layers were found to
channels can be biofabricated and endothelized using be significantly faster in larger channels of 1000 μm
this method. Further studies had also demonstrated that a and 500 μm as compared to narrower channels of 250
thrombosis and thrombolytic situation can be simulated μm. In addition, it was reported that 250 μm has the
in this model and initial results showed that fibroblasts lowest perfusion capability as compared to the rest of
encapsulated in the GelMA hydrogels migrated to the the larger channels. Most critically, it was reported that
area of the clots and deposited collagen Ⅰ locally, which mouse calvarial pre-osteoblasts cells (MC3T3) that were
is a similar phenomenon seen in humans. encapsulated in constructs with micro-channels had
A similar study by Suntornnond R, et al. combined significantly higher cellular viability at days 1 and 7, as
Pluronic-F127 with GelMA to form Plu-GelMA for well as significantly higher differentiation as determined
bioprinting of constructs [52] . NMR results showed the by alkaline phosphatase activity (ALP) levels on day 14.
presence of methacrylate groups, thus making Plu- On the other hand, MC3T3 encapsulated in constructs
GelMA both thermo-responsive and photo-crosslinkable. with no micro-channels only showed 60% cell viability
In this study, it was reported that simple hollow at the same time points. This result is consistent with
cylindrical structures of 50 layers and mluti-layered other similar vascularization studies fabricated via self-
structures can be printed with 2:1 Plu-GelMA without assembly of endothelial cells which also demonstrated
any external structural support. However, structural enhanced tissue functionality which would be further
support and dual-nozzle system were required for more discussed below.Droplet-based indirect extrusion
complex structures. In addition, Plu-GelMA were non- was also used in biofabricating perfusable scaffolds.
toxic to L292 cells with 2:1 Plu-GelMA having the Using the micro-valve bioprinting technique, this team
highest cell viability and proliferation as compared to successfully bioprinted gelatin (sacrificial ink) into a
others. It might be due to the higher concentration of cylindrical shape structure before depositing cell laden
Pluronic-F127 which causes higher swelling rate and hydrogels over it. Post modification involves removal
well defined pores which allowed better diffusion of of gelatin via thermal de-crosslinking, leaving behind a
nutrients and provided more surface area for cellular tubular channel which was subsequently endothelized
attachment. In addition, 2:1 Plu-GelMA were shown and perfused [55] . It was demonstrated that the vascular
to be a good platform for cellular differentiation. channels were covered a monolayer of confluent
HUVECs cultured in 2:1 Plu-GelMA showed signs of endothelial cells and this structure was stable for a
differentiation with expression of CD31 and VWF, both period of two weeks with constant perfusion. In addition,
markers of endothelium cells. Also, HUVECs were the vascular channels were able to support and maintain
shown to attach and spread on the construct surface 1 the viability of adjacent cells with barrier effect against
day after culturing and by day 7, HUVECs had fused plasma proteins and high molecular weight molecules.
and covered the construct with extracellular matrix. 2.2 Self-assembly Approach
In a related procedure but using a naturally derived
polysaccharide agarose as the sacrificial ink and GelMA An alternative approach to generating vascular
as the main biomaterial. In this study, the authors constructs lies in endothelial cells' abilities to self-
first printed the vascular channels with agarose gel organize into blood vessels. The main difference
before casting GelMA to fully cover the agarose fibers, between self-assembly approach and tissue engineering
followed by exposing the whole scaffoldto UV light for approach is that for self-assembly approach, cells
photo-crosslinking. GelMA undergo cross-linking via are often left and cultivated to form tubular channels
free-radical photo-polymerization of acrylate groups, whilst for tissue engineering approach, tubular channels
thus were unable to form covalent bonds with the are instantaneously available after bioprinting and
agarose fibers. Because of this, agarose fibers could be modification. Therefore, the main problem in self-
easily removed by pulling them out of the scaffolds, assembly approach is the direct control over distribution

8 International Journal of Bioprinting (2018)–Volume 4, Issue 2