BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 154

However, for vascular engineering, the conditions for extruding these materials are not conducive
for cell growth and, as a result, approaches using this method generally use 3D printing to deposit a
scaffold that will ultimately be removed. The sacrificial scaffolds are extruded according to a 3D design
and the rate of extrusion as well as temperature are tightly controlled to ensure that a proper filament
diameter is being deposited as the scaffold is being printed. Since a motorized stage is being used to
control the deposition in the x - y directions, the resolution in these directions is typically limited to 100
m m. This is also dependent upon the material being used so the resolution can be improved depending
on the application. In the case of generating scaffolding for vasculature and microvasculature, the scaf-
folding material can be a carbohydrate glass that is capable of being dissolved postprinting without the
use of harsh solvents ( Miller et al., 2012 ). These glasses can be extruded at a controlled rate and result
in filament fibers ranging from 150 m m in diameter up to the size of the filament.
This method allows for the unique ability to generate scaffolds that have both vasculature as well
as microvasculature. After the fabrication of the sacrificial scaffold, the scaffold is then coated with a
protective layer that prevents the scaffold from degrading by the deposited matrix. This matrix can be
a hydrogel with embedded cells. This hydrogel can be a wide range of gels such as alginate, fibrin, or
Matrigel ® . Importantly, the hydrogel can be chosen based upon the desired cell type that will be en-
capsulated and vascularized once the scaffolding is removed. The hydrogel can be cross-linked either
chemically, through the use of a photo-initiator (if the scaffold is sufficiently optically clear), or by
modulating the temperature of the hydrogel plus scaffold. After the hydrogel is set with the desired
cell type encapsulated, the sacrificial scaffold is then removed by perfusing water or media through
the scaffold. Upon the removal of the scaffold, ECs could then be seeded onto the inner surface of the
leftover porous network. This vasculature could then be placed under flow allowing for nutrient transfer
from the cell culture media to the inside of the hydrogel, where typically cells would not survive due to
diffusion limits and nutrient exchange.
Other applications based upon this technology have been rapidly advancing ( Sodian et al., 2002;
Sodian et al., 2005 ), and prove that cell-free methods still have the potential of providing vascularization
to tissue-engineered implants. Thus, allowing for the formation of larger tissue-engineered implants
while at the same time allowing for proper nutrient exchange ensures viability of the implanted
vascularized construct.
7.1.6.1 Cell-Based Scaffolds
Due to the ever-changing market for commercial and research-based 3D printing systems, it is very dif-
ficult to discuss the merits of individual systems for their application to a specific research problem. As
such, this section will focus mainly on the technologies and concepts behind a few cell-based printing
techniques that have been directly applied to the problem of vascularization and angiogenesis. This sec-
tion aims to give the reader an understanding of the principles behind the different printing processes as
well as a working knowledge of how the individual technologies currently compare. 3D printing of cells
or cell-laden tissue-engineered implants can be broken down into a few main technologies, many of which
have been around for several decades and can be further divided based upon components utilized: (1)
Ink-jet printing, (2) extrusion-based printing (fused deposition modeling), and (3) laser-assisted printing.
7.1.7 INK-JET PRINTING
As one of the main, current methods for generating 3D scaffolds for tissue engineering, ink-jet printing
has several advantages ( Figure 7.5 ) ( Roth et al., 2004; Boland et al., 2006 ). Due to the fact that ink-jet
printing technologies have been around for several decades, the cost of production of these systems is