BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 156

This method has been successfully used to produce constructs that mimic native vasculature as well
as in the printing of heterogeneous cell populations to encourage the vascularization of the printed bio-
material. Upon implantation into a mouse model, a group has demonstrated the ability of a printed
material laden with multiple cell types to encourage vasculogenesis in vivo. As such, this method of
producing tissue-engineered blood vessels has been steadily making progress from in vitro studies to in
vivo with the end goal of translating to clinical applications.
7.1.8 EXTRUSION BIOPRINTING
Mechanical extruders provide another method of generating 3D scaffolds with embedded cells. In
this approach, there are advantages due to the fact that each deposited bead can be thought of as a tis-
sue fragment. However, this method is currently expensive due to the upfront cost of the printers and
the technology required. The basic concept behind this printing method is that droplets (multicellular
particles + desired scaffold material) are deposited on the template at desired locations based upon a
computer-generated design.
The building blocks used in this method, much like those used for the other printing techniques, can
contain either a single cell type or a heterogeneous population. For the generation of vascular tissue,
however, it is important that individual cell types are used to ensure proper layering and spatial ar-
rangement in the final printed scaffold. Another consideration that is important for the success of these
implants is that the final printed construct must be as similar as possible, structurally and functionally,
to the native tissue.
As such, materials such as collagen hydrogels have been successfully used as scaffolds to encapsu-
late the cells during the printing process. This has not been without several drawbacks. For example,
the system must place the hydrogels under high pressure during the printing process. Based upon the
viscosity of the printed hydrogel, the shear force exerted on the cells may be of large enough magnitude
to cause cell death. Additionally this method of printing is currently restricted to a very specific set of
materials, thus limiting the number of overall applications. However, by careful selection of materials
and engineering, it has been possible to produce 3D printed tissues that mimic the native vasculature in
terms of distinct functional layers with the ability to apply flow to the inner channel.
Figure 7.6 is an illustration of how extrusion bioprinting can be applied to the production of
tissue-engineered vasculature. The layers are built in a step-wise fashion where the outermost layers are
deposited with cells embedded. The inner layer, which will have the applied flow once the construct is
complete, needs to be printed with an additional material that can be removed in the final printed product
without compromising the seeded cell viability. One example of a secondary material that can be used for
this process is agarose. The printed agarose rods fill in the void spaces during the printing process, which
allow for the deposition of layers above the desired void space and prevent cell invasion and remodeling
( Jakab et al., 2010 ). After completion of the printing process and the final gelation of the printed scaffold,
the agarose rods could be removed from the center of the print. Thus, the printed scaffold has defined
layers and an inner void that allows for the flow of nutrients/media as well as separate functional layers.
For vascular applications, this dual print method is especially important due to the weak mechanical
properties typically associated with the hydrogels compatible with this printing process.
An additional application of this printing method for vascular regeneration was in the fabrication
of more complex structures such as the printing of an aortic valve. In this work, researches success-
fully fabricated a 3D scaffold laden with different cell types and maintained viability for one week