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International Journal of Bioprinting Optimizing inkjet bioprinting

becomes thicker. The expanding cavity can displace cells cells onto cell layers saturated with isosmotic media, or by
in the target fluid, reducing accuracy of cell placement. co-dispensing hypoosmotic media (from a separate print
The cavity stops its downward penetration into the fluid, chamber) along with the cells to maintain the correct
and subsequently, the capillary waves from the crown osmolality. Similarly, it is also important to compensate for
spread into the cavity and distort its shape. At this point, the change in concentration of other reagents in the bio-
the cavity begins to close from the bottom and decrease ink, such as signaling molecules.
its penetration into the fluid while its radial dimension
(width) does not change significantly. A central jet then 5.1. Polymer-based droplet impact on media
emerges and grows upward. It then breaks up with a The addition of polymers to bio-inks can improve
droplet at its very tip, and the remnants of the jet fall bioprinting performance, decreasing splashing, improving
toward the target surface. cell placement accuracy, and increasing cell viability.
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Polymer-based solutions can be modeled either as a
It is somewhat debated if the combined regime of power-law fluid, a yield-stress fluid, or a viscoelastic fluid.
bouncing, floating, and coalescing should be classified With these non-Newtonian fluids, it is difficult to predict
as separate regimes or as separate stages of a single droplet impact behavior, as the fluid viscosity will vary
regime. 69-72 In this regime, the target liquid and the cells both spatially and temporally as a function of the local
deposited in it are substantially less disturbed. The most shear rate. For a power-law fluid, the effective viscosity
striking characteristic of this regime is droplet bouncing,
n1
which unfortunately reduces the cell placement accuracy. Ku( y / ) , where K is the flow consistency index
eff
As a droplet approaches the target liquid surface, a and n is the flow behavior index; n < 1 for shear-thinning
thin film of gas prevents the droplet from touching the fluids; and n > 1 for shear-thickening fluids. Shear thinning
surface and the droplet bounces upward. In the floating is typically attributed to breakdown of structure formed
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regime, the droplet is also prevented from touching by interacting particles in the fluid (e.g., cells, polymer
the target liquid by a thin gas film. However, instead of chains), and shear-thickening behavior is attributed to
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bouncing upward, it travels laterally and skates on the gas jamming of these particles induced by the flow. It was
film over the target fluid. In the coalescing regime, the observed that increasing the flow behavior index n for a
droplet penetrates the gas film, touches the target liquid, shear-thinning fluid (0.125–0.75 wt.% xanthan gum-based
and coalesces with it. The transition between bouncing, solutions) increases the maximum spreading diameter.
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floating to the coalescing regime is dependent on the This is counterintuitive as lower n leads to lower viscosity
relative importance of inertia to surface tension (as the for high velocity gradients (as can be seen in the beginning
droplet needs to have enough inertia to break through of the impact) and thus lower viscous dissipation of
the gas film), gas mean-free path, and gas viscosity. The the initial kinetic energy, and so should lead to a larger
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bouncing, floating, and partial coalescing can coexist as spreading diameter. It may be that the average apparent
one regime and can proceed as a cascade whereby each viscosity, which is governed by the flow consistency index
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step generates a smaller droplet. In this case, the droplet K, has a more significant effect on the maximum spreading
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first bounces, then floats after losing some momentum, diameter. Furthermore, the dimensionless retraction rate
and lastly coalesces partially with the target fluid after decreases with the flow consistency index K. 76
losing more momentum. However, the resulting daughter Low concentration polyethylene oxide (PEO) solutions
droplet has enough momentum to bounce and repeat the (in the order of 100 ppm) form a class of viscoelastic bio-inks.
cascade. These solutions have a reduced tendency for drop rebound
Splashing and formation of thin lamella, as found in the on impact with hydrophobic surfaces, thus improving cell
many droplet impact regimes, dramatically increases the placement accuracy. 77,78 As these solutions have a similar
surface to volume ratio of the liquid, as well as produce high surface tension and shear viscosity as the base solvent, it
liquid velocity relative to the surrounding air, and so can is speculated that the reduction of rebound was due to the
dramatically increase evaporation rates. This accelerated increase in elongational viscosity of the solution due to the
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evaporation leads to increases in concentration of reagents presence of PEO. It appears that the droplet behavior is
in the bio-ink which, if not accounted for, can adversely due to the energy dissipation caused by the stretching of
affect the resulting cell viability. Most dramatically, at the polymer molecules by a combination of hydrodynamic
low dispense flow rates, the cell media may completely and surface forces during the droplet retraction phase.
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evaporate, resulting in high vitality loss. Typically, bio-inks However, suppression of prompt splash is observed in
are isosmotic prior to dispense, and due to evaporation, dilute (0.01 wt.%) 18 MDa polyacrylamide viscoelastic
they will become hyperosmotic post dispense, lowering cell solutions, and this is attributed to the elastic forces of the
viability. This can be mitigated, for example, by dispensing fluid. The mechanism behind this phenomena appears to
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Volume 10 Issue 2 (2024) 191 doi: 10.36922/ijb.2135