BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 157

FIGURE 7.6 Extrusion Bioprinting.
(A) The basic setup of an extrusion-based 3D printer. The printer consists of a build plate and a printer head/
nozzle on a motorized axis. The printing nozzle can contain either a polymer for extrusion or a biomaterial
loaded with cells and it moves in the X , Y , and Z directions depositing each layer in the desired location. These
layers can be built off of each other much like the other 3D printing processes. However, one distinction is that
the extrusion rate and the speed of the nozzle can be used to modulate the fiber size being deposited. In the case
of printing vasculature, void spaces can either be included by not depositing any material or by the printing of a
second material that provides structural support during the printing process. (B) The desired cell-laden scaffold
is built layer-by-layer with one nozzle (black cylinders) and a second nozzle containing a secondary material
(gray cylinder) that is used to deposit a support structure to be removed after the printing process is complete.
A 3D vasculature can be fabricated in this manner with a controlled vasculature shape and inner diameter once
the secondary matrix is removed (gray cylinder) Adapted from ( Jakab et al., 2010 ).
( Ghista and Kabinejadian, 2013 ). This research demonstrates the ability to apply this technology to
the various structures that comprise the vascular system. As such, this method possesses great promise
in addressing the need for tissue-engineered vasculature and in aiding the blood vessel regeneration.
7.1.9 LASER-ASSISTED BIOPRINTING
One of the potential problems associated with the 3D printing of cells is due to the limited cell den-
sities possible based upon the printing technique chosen. Laser-assisted cell printing is one method
that can overcome the low viability and low cell density problem ( Guillotin et al., 2010; Tasoglu and
Demirci, 2013; Koch et al., 2013 ). The technique of laser printing is not new, as it has been used in
the fabrication of circuits and biosensors. However, its application to layer-by-layer cell deposition is
relatively recent and allows for easy control of cell density in the final printed product ( Figure 7.7 ).
This method overcomes limitations on cell seeding density by seeding a source gel at a high density
and ensuring the cells' viability prior to the printing process ( Barron et al., 2004; Guillotin et al., 2010;
Barron et al., 2005 ). The source gel is transferred to a target or collection slide on a spot-by-spot basis
through the excitation of the gel or a specialized substrate. There are currently two main modes of ex-
citation where either the excitation energy is directly matched by the continuous wave laser source or
a two-photon, pulsed excitation source is used, where the required excitation energy is approximately
double the laser energy.
There are many pros and cons of using two-photon excitation; however, the most important are the
ability to focus the excitation energy to very small volumes and the large upfront cost of the system
especially with a tunable excitation source. Although there has been some success with Nd:YAG
and Ti:sapphire tunable lasers, the costs of these systems can be prohibitive for the development of
a two-photon printing system for tissue engineering applications. Even though there are differences
in terms of the generation of the required excitation energy, the basic concept and procedure of using
continuous wave and pulsed lasers for laser-assisted cell printing remain basically the same. Cells prior