Page 197 - IJB-10-2

International Journal of Bioprinting Optimizing inkjet bioprinting

time coefficient of ~ 0.5 as that for erythrocytes, minimal and drop size), and the characteristics of the substrate
damage to mammalian cells with inkjet dispensing is surface (advancing contact angle, surface roughness,
expected. For TIJ-based printers, little change in viability and temperature, stiffness). In the context of bioprinting,
or functionality has been experimentally confirmed, precise cell placement is crucial, and different droplet
for example, for neural cells dispensed by HP 51626a impact regimes have varying degrees of suitability for this
cartridge and more recently for variety of mammalian purpose. There are six primary regimes of droplet impact
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cells by specialized HP cell dispense cartridges. For PIJ on dry surfaces: deposition, prompt splash, corona splash,
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printers, minimal changes to RNA expression post dispense receding breakup, partial rebound, and rebound (Figure
were observed for mouse embryonic stem cells. These 4). 67,68 In the deposition regime, the droplet deforms upon
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findings highlight the complex and cell type-dependent impact but remains on the surface throughout the entire
nature of cellular responses to shear stress. Understanding process without breaking apart. This is the most favorable
these responses is crucial for bioprinting, where cells are regime for bioprinting applications as it allows for high
exposed to shear stress during dispensing, to ensure cell cell placement accuracy. Additionally, the lower surface-
viability and phenotype preservation. to-volume ratio results in a reduced rate of evaporation
and minimal changes in media concentration. In the
4. Droplet formation prompt splash regime, droplets are generated directly at
the contact line during the initial spreading phase when
The presence of particles, such as cells, in bio-inks the lamella has a high radial (outward) velocity. The
can affect the jet break-up and droplet formation. For characteristics of the surface, particularly its roughness,
mammalian cell sizes of ~16 µm and typical bio-ink cell influence the formation of droplets. In the corona splash
concentrations of 10 –10 cells/mL, the particle volume regime, droplets are formed around the rim of a bowl-
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fractions, φ, range between 0.02% and 2.15%. Jet break-up like shape (corona), often at a distance from the impact
for low φ suspensions ( 10 %) is similar to that of simple surface. This phenomenon is common for droplet
Newtonian bio-inks, with the exception that individual impacting liquid films and is a significant consideration
particles can be captured in the droplet tail, which leads for bioprinting applications. The receding breakup
to new modes of satellite drop formation. For higher φ regime is primarily controlled by surface wetting,
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suspensions, presence of large number of particles in the especially the dynamic contact angle. As the liquid
droplet tail results in thick cone-like structures. These retracts from the maximum spreading radius, droplets
structures are similar to beads-on-string structures found are left behind on the surface when the dynamic contact
in the jets of dilute polymer-based bio-inks. angle decreases to zero. In the partial rebound regime,
part of the initial droplet remains attached on the surface
The presence of particles suppresses the formation of while the remaining part rebounds off the surface. In the
satellite droplets, and the satellite droplets that do form are complete rebound regime, the entire droplet rebounds
usually larger and fewer in number. This is advantageous from the surface. Both rebound regimes occur only when
for bioprinting as this leads to better cell placement and the droplet recedes after spreading. The droplet receding
accuracy as well as to reduction in evaporation and thus process is influenced by the maximum spreading diameter
change in cell media composition. Thinning of the droplet and the receding contact angle. Partial rebound typically
tail slows down as particle concentration increases, and occurs for low values of the dynamic receding contact
this can be attributed to the increasing effective viscosity angle, while complete rebound occurs for high values of
as a function particle concentration. The length of the the dynamic receding contact angle.
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droplet tail at rupture decreases with increasing particle
concentration due to the action of discrete particles. In bioprinting process, droplets of bio-inks are
Specifically, the particles trapped in the tail induce another typically dispensed in a layer-by-layer manner on
axis of curvature perpendicular to the original axis of the top of other cells in media. Hence, it is important to
column as the tail narrows to the size comparable to the consider droplet impact on a liquid surface, which can
particle diameter and thus encourages the breakup of the be modeled as a small pool of liquid. Two regimes of
liquid column. droplet impact on liquid surfaces have been reported:
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splashing regime, and a combined regime of bouncing,
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5. Droplet impact floating, and coalescing. In the splashing regime, the
droplet collides with the surface of the target liquid, and
Droplet impact behavior is influenced by a variety of a flared film of liquid is thrown upward and outward
factors, including fluid properties (density, viscosity, and from the periphery of the colliding region. This regime
surface tension), impact conditions (impact velocity, has a similar morphology to the corona splash regime

Volume 10 Issue 2 (2024) 189 doi: 10.36922/ijb.2135