BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 159

thickness of the layers can be adjusted to mimic the relative ratios present in the native vasculature.
These concentric rings can be printed one on top of another thereby building up complex architecture
in a layer-by-layer approach.
One drawback to the utilization of this method for vascular regeneration is that hydrogels can have very
different mechanical properties from the native tissue. Even though the end product will only have the
mechanical properties of the hydrogels, which tend to be too weak on their own without additional sup-
port, the cells deposited can be precisely deposited in heterogeneous layers that very closely mimic the
native heterogeneous properties of vasculature. Furthermore, these cells will be positioned close to one
another such that appropriate cell-cell adhesions and communications can be established thus enabling
the printed scaffold to develop into a functioning implant. Also, since this method can precisely deposit
very small volumes, it may be possible to use this fabrication approach to build microvasculature that
has a very small inner diameter, and very few cells are needed to create each of the concentric layers
that comprise the vasculature.
Although laser-assisted 3D printing has been successfully used to create tissue- engineered blood
vessels, it also has been used for patterning stem cells and ECs for cardiac regeneration ( Gaebel
et al., 2011 ). Additionally, this technology has been utilized for complex tissues such as bioprinting
skin with its various ECM and cellular components ( Koch et al., 2012; Pirlo et al., 2012 ).
7.1.9.1 Comparison of the Technologies
All of these technologies presented here represent a subset of approaches for the regeneration of vascu-
lature. Depending on the technique used and the materials available, surface modification with proteins,
cytokines, and growth factors can be included in the manufacturing process. These modifications can
further enhance the cellular response to improve blood vessel regeneration. This is especially important
in order to recruit endothelial cells to the tunica intima of the implant to aid in the prevention of stenosis
( Hashi et al., 2010; Han et al., 2013; Zhang et al., 2013; He et al., 2005; Avci-Adali et al., 2013; Avci-
Adali et al., 2010 ). Additionally, the mechanical properties of the vascular implant can be modified by
the recruitment or incorporation of smooth muscle cells and appropriate ECM proteins such as elastin
( Fu et al., 2014; Cui and Boland, 2009b; Norotte et al., 2009; Patel et al., 2006; Kasalkova et al., 2014;
Greenwald and Berry, 2000; Berglund et al., 2004; Wang et al., 2013; Heydarkhan-Hagvall et al., 2006;
Nerem, 2003 ). It is critical that these competing technologies incorporate the heterogeneous nature
of each layer in the vasculature in the design and fabrication of scaffolds for therapeutic applications.
Although it has been shown that electrospinning can have control over each layer's orientation and
fiber size ( Shapira et al., 2014; Hashi et al., 2010; Wu et al., 2010; Zhang et al., 2013; He et al., 2005;
Ziabari et al., 2010; Subbiah et al., 2005 ), these implants do not have as stringent control over each layer
as rapid fabrication techniques. As the cost continues to decrease for 3D printing, the use of 3D printing
to develop scaffolds for blood vessel regeneration will only increase. As previously mentioned, cell-free
scaffolds tend to be modified in order to increase their biological activity, whereas cell-based scaffolds
are already using materials that will encourage the growth of cells in substrates specific to the desired
cell type. This is an important advantage and distinction because the cells are ultimately responsible for
the long-term viability of the implant as well as its long-term mechanical properties. However, there is
a distinct disadvantage currently with cell-based scaffolds, and that is that the base materials used have
very poor mechanical properties when compared with that of native tissue or the cell-free scaffolds.
In addition, for vascular applications there are design parameters that need to be taken into account
when choosing an appropriate fabrication method. For example, the complexity of the desired product