ORGAN PRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 327

Alternatively, others have implemented electrostatically driven ink-jet-based printer systems to deposit
cell suspensions with single-cell seeding control ( Nakamura et al., 2010 ). Others have simultaneously
ejected hydrogels and cells from an ink-jet nozzle into chemical cross-linking solutions ( Nishiyama
et al., 2009 ). While these high-resolution techniques enable cells to be spatially patterned with hetero-
geneous distribution by modulating the ejection frequency and multiple material heads, the question of
cell viability from the jetting of high-density cell suspensions through a small cartridge orifice remains
unanswered. Furthermore, on one hand, ink-jet printing cell suspensions in media at high ejection
frequencies and smaller drop feature sizes are hampered by evaporative water losses and a lack of
a material support apparatus. On the other hand, ink-jet printing cells with polymer materials place
restrictions on the types of polymers that can be processed to low-viscosity, low-strength hydrogel
materials with a viscosity ceiling of approximately 30 mPa*s ( Melchels et al., 2012 ). A corollary of
these material handling restrictions is the challenge with inkjet-based printing to achieve buildup in 3D
for organ printing.
15.1.1.3 Laser-based Printing
Laser-based printing, sometimes referred to in the literature as laser-assisted bioprinting or laser for-
ward transfer, is a nozzle-free printing technique that enables high-resolution patterning of cells ( Guil-
lemot et al., 2010 ; Gruene et al., 2011 ; Koch et al., 2010 ; Mezel et al., 2010 ). In laser-based printing
system configurations, a collector plate with tunable wetting properties is first coated with encapsu-
lating polymer droplets loaded with cells. Either UV or IR laser is then focused with a microscope
objective lens onto an absorbing thin layer film typically composed of Au, Ag, or Ti that shields the
cell-polymer droplets from direct laser interactions. Upon reaching a minimum threshold energy of
the nanosecond pulsed laser, microscale droplet deposition is achieved. Key process parameters in
laser-based printing that dictate droplet feature sizes include the thickness of the absorbing layer, dis-
tance between the absorbing layer and collector plate, wavelength or energy of the laser pulses, and the
optical properties of the polymer material. The significant advantages of laser-based printing include
the small feature sizes attainable and the ability to pattern cells without subjecting them to the mechani-
cal forces inherent in nozzle-based, ink-jet-based, and microextrusion-based printing methods. A noz-
zle-free configuration also circumvents occlusion issues that beset ink-jet-based and microextrusion-
based techniques. Akin to ink-jet-based printing, however, scale-up of laser-based printed constructs is
challenging due to physical constraints in the Z-dimension that precludes facile layer-by-layer buildup
in 3D organ printing. Laser-based printing is also restricted to the processing of low-viscosity materials
and requires cells to be immobilized within a gel.
15.1.2 CHALLENGES IN ORGAN PRINTING
Based on a delineation of the aforementioned organ printing techniques and their relative virtues and
limitations, it is apparent that significant manufacturing and biological challenges exist and must be
overcome for printed organs to be successfully and clinically translated to benefit patients. As might be ex-
pected, many of the manufacturing challenges (e.g. resolution enhancement, increasing scalability, and
widening manufacturing process windows) are in direct conflict with the biological challenges (e.g.
postprint cell viability and functionality). While the incorporation of a vascular network into organs
is a significant barrier in tissue engineering, this challenge has been extensively posed and addressed
elsewhere, and will not be discussed here.