ORGAN PRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 326

various cell types following process-induced mechanical loading. For example, the authors have
examined the effect of dispensing pressure and nozzle tip diameter on the postprint viability of
HepG2 liver cells encapsulated within alginate, demonstrating a quantifiable loss of cell viability
due to process-induced mechanical damage with some cells showing recovery over a short-duration
study period ( Chang et al., 2008a ). Others have shown with cross-validating cell damage models of
printed Schwann cells and 3T3 fibroblasts that differential postprint viability is observed in tapered
versus cylindrical needle tip geometries ( Li et al., 2011 ). While microextrusion-based postprinted
aggregate cell functional retention has been demonstrated in various cell types for narrow process
windows, more systematic study to quantify and isolate these process-induced effects in the context
of diverse material parameters (e.g. viscosity) would contribute to fundamental understanding of
these mechanical processes toward widening the permissible process windows with predictable
biological postprint outcomes. This understanding of process-induced mechanical forces in micro-
extrusion-based printing will become even more critical as (1) the requirement for high-density cell
aggregates and tissue spheroids becomes paramount in organ printing applications, and (2) stem
cells sensitive to mechanical cues become a mainstay in microextrusion-based organ printing for
regenerative medicine. The latter challenge of predicting the effect of process-induced mechani-
cal cues on stem cell differentiation also represents an opportunity to reliably guide cell fates for
the first time with bioadditive manufacturing systems. Compared with the other organ techniques
discussed in this chapter, microextrusion-based printing offers the lowest resolution on the order of
100-200 m m.
15.1.1.2 Ink-jet-based Printing
Drop-on-demand or ink-jet-based printing has been adapted from commercial desktop printers to
precisely deposit and pattern biological cells and materials with picoliter-sized volumes ( Sekula
et al., 2008 ). Relative to the microextrusion-based printing methods, ink-jet-based printers are able
to achieve feature sizes on the order of 20-100 m m ( Melchels et al., 2012 ). This approach entails re-
purposing desktop printer ink cartridges by replacing the ink in the material reservoir with bioink (i.e.
aqueous cell solutions) to print microscale cell droplets ( Xu et al., 2005 ; Roth et al., 2004 ). Since ink-
jet printing was initially aimed at fabricating two-dimensional systems (i.e. a single layer of droplets
for surface patterning applications), the feature sizes have been determined to be primarily dictated
by the size of the printed drop in flight and its contact angle on the surface upon impact ( Stringer
and Derby, 2010 ). The resultant resolution is that the smallest drops attainable through an ink-jet-based
process are ∼ 1 pl volumes with an equivalent radius of less than 10 m m. Therefore, on prescribed
hydrophilic surfaces, minimum features sizes on the order of single-cell dimensions are achievable
with this technique. In order to create a layered configuration of cells with encapsulating biopolymers,
others have designed an elevator chamber model for ink-jet-based printer in which robotic control
of a metal elevator rod is regulated using a step motor for precise positioning ( Boland et al., 2006 ).
In this study, precise volumes of chemical cross-linking solution are ejected from the ink-jet head or
nozzle onto the motorized elevator submerged within a solution of uncrosslinked hydrogel precursor
intermixed with cells homogeneously distributed. While this approach successfully circumvents the
undesired effect of cells being subjected to process-induced mechanical loading (e.g. shear stresses and
impact forces) through an ink-jet cartridge orifice, the ability to spatially pattern, using multiple ink-jet
cartridges as well as multicellular and multimaterial constructs with functionally gradient distributions,
is limited by the initial cell and prepolymer material configuration residing in the elevator chamber.