Page 486 - IJB-10-1

International Journal of Bioprinting Endothelial monolayer formation on scaffolds

use of immunosuppressants that are accompanied by the body reactions or surface-activating events. Therefore,
9,10
corresponding side effects. Autologous vessels, on the scaffolds should be seeded with endothelial cells before
other hand, are not always available due to factors such the respective TEVG is implanted. Physiologically,
as morbidity and age, or are of variable quality. In this endothelial cells line the inner surface of blood vessels
2
context, tissue engineering generally could offer a solution as well as lymphatic tissue and play an essential role in
by supplying limitless bio-artificial tissue replacement vascular tissue as a barrier, controlling substance flow
and counteracting shortage of tissue and organ for in and out of the blood in the surrounding tissue and
transplantation, but here a sufficient vascularization of maintaining hemostasis. Hemostasis depends upon a
these created tissues is very relevant. variety of mechanisms, including inhibition of platelet
For the creation of such macro- and micro-vessels, aggregation, vasoreactivity (e.g., through secretion of
optimal tubular scaffolds are needed to guide tissue nitric oxide), and formation of glycosaminoglycans on
11
shape, cell growth, and differentiation, e.g., of endothelial the cellular surface to prevent blood coagulation. It has
progenitor cells, as well as to offer structural support been shown that pre-endothelialization of scaffolds and
with sufficient biomechanical resistance concerning the dynamic pre-conditioning of the used endothelial cells
respective blood pressure profile. These scaffolds need to improve hemocompatibility and promote anti-thrombotic
12
be biocompatible to avoid overwhelming foreign body and anti-inflammatory properties. In detail, flow-
reactions and should slowly degrade in the host milieu so sensitive sensoring mechanisms induce the expression of
that they will be substituted by host cells and extracellular anti-coagulative proteins, such as Krüppel-like factor 2
13
matrix. Therefore, scaffolds should promote cell–cell (KLF2), which induces the vasodilatory endothelial nitric
3
communication, tissue formation, and matrix synthesis. oxide synthase 3, with anti-inflammatory properties. In
14
4
addition, thrombomodulin is also induced, which has an
Biocompatible degradable polymers, such as anti-thrombotic effect by binding thrombin. Optimal
15
polycaprolactone (PCL), offer favorable properties for scaffolds must allow a continuous endothelial cell layer,
implantation and scaffold fabrication, e.g., by three- while at the same time, a diffusible porous structure is
5
dimensional (3D) printing. 3D printing is a promising needed to allow oxygen and nutrient exchange in the
technique in tissue engineering to enable highly precise surrounding tissue for sustained cell survival even under
and reproducible scaffold structures and offers the dynamic conditions to offer long-term patency of the later
possibility to obtain patient-specific scaffold structures
when imaging techniques like CT are used to create TEVG with anti-thrombogenic properties.
3D models as a blueprint for scaffold design. While The work presented in this paper describes an approach
6
conventional 3D printing methods such as fused of 3D-printing scaffolds using FDM and MEW and of
deposition modeling (FDM) do not offer the resolution coating optimization with fibrin to achieve confluent
to print on a cellular scale, novel printing techniques such endothelial layers in vitro. MES as another technique
as melt electrowriting (MEW) enable the production of to enable microfiber scaffolds was comparably tested
microscale to nanoscale fibers of biocompatible polymers as a fabrication technique to create scaffolds suitable for
with defined patterns, and allow the fabrication of highly the formation of confluent endothelial layers. Scaffolds
porous and diffusible scaffolds. In short, MEW uses a underwent a coating optimization to promote the formation
7
high electrostatic field to draw microscopically small fibers of a continuous endothelial layer on the scaffold surface.
toward a collector (the printing bed). Definable structures Here, human-derived fibrin was used as a coating material
can be obtained by adjusting the printing parameters, to improve seeding with a cell line (human umbilical
which are printing speed, nozzle offset to the collector, venous endothelial cells [HUVECs]) on the scaffolds.
voltage between collector and nozzle, pressure for material Fibrin is especially suitable as a coating material since it
extrusion, and temperature of the printed material. contains a signal peptide motif named arginyl-glycyl-
8
Another electrohydrodynamic fabrication technology is aspartic acid (RGD), which is responsible for cell adhesion
melt electrospinning (MES), which is capable of producing to the extracellular matrix. With these printed and coated
16
microscale to nanoscale fibers. In comparison to MEW, scaffolds, the effect of fiber spacing and non-square-shaped
fiber deposition is random, and precise pore geometries or pores on the endothelial monolayer was investigated.
fiber spacing is not achievable. Nevertheless, fiber diameter Furthermore, seeded endothelial cells on a scaffold should
and pore size can be considerably reduced in such chaotic be pre-conditioned with the same mechanical stress as
MES scaffolds compared to MEW. 4 subjected to in vivo. Shear stress is the most crucial element
The absence of luminal cellularization on respective in conditioning endothelial cells. Endothelial cells typically
-2
synthetic scaffold, in particular, can cause thrombosis by experience shear stress levels of 10–20 dyn cm in arteries
stimulation of the coagulation cascade, e.g., by foreign- and 1–6 dyn cm in veins. In the case of endothelial cells,
2
17
Volume 10 Issue 1 (2024) 478 https://doi.org/10.36922/ijb.1111