Page 195 - IJB-10-2

International Journal of Bioprinting Optimizing inkjet bioprinting

the resistor surface. In TIJ kogation, the resistor pyrolyzes stress (t), time in shear (t), critical shear stress (t ),and
c
a small portion of the ink on the resistor surface. This several fitting parameters has been proposed; it can be
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process degrades the heat transfer between the resistor’s expressed as: 48-50
surface and the liquid above it, affecting surface roughness, p k ( ) ab (VII)
t
bubble nucleation, and ultimately vapor bubble generation. s c
When cells lyse in the print chamber, the resulting sticky Here, the critical shear stress represents the maximum
debris, believed to be nucleic acid material, adheres to the stress that cells can endure without showing a noticeable
chamber walls and promotes the adhesion of other cells decrease in viability over an extended period. This model,
to the walls. The presence of debris on the chamber walls with laminar shear of k = 1, a = -1, b = - 0.5, has shown
also alters the wetting properties of the walls and affects the good agreement with experimental data for erythrocytes
rate of refill, potentially changing the achievable dispense (red blood cells). It is important to note that erythrocytes
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frequencies. Notably, debris-induced cell adhesion can have unique properties, such as their discoid shape and
lead to the formation of cell clumps, preventing chamber the absence of nuclei, which may make them not perfectly
refill and resulting in the failure of the print chamber. representative of other mammalian cells. Nonetheless,
Piezo inkjet printing chamber typically consists of an there are numerous advanced models available to assess
ink supply inlet, a piezo element, and a nozzle, through the viability of erythrocytes under shear stress due to the
which the ink exits the chamber (Figure 3). The ink is significant interest in modeling and developing devices like
expelled from the printing chamber through mechanical cardiac assist systems and other blood-contacting devices.
A recent paper has also provided a detailed review of more
deformation caused by a piezo element. The displacement advanced models of hemolysis, which could beneficial for
of the print chamber by the piezo element is usually much numerical simulations and modeling purposes.
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smaller than the deformation of the liquid caused by vapor
bubble. To counteract this, the print chamber can be There is a scarcity of data regarding the probability of cell
designed with a larger area. Typically, the print chamber survival as a function of shear stress and exposure time for
has a length ranging from 5 to 20 mm and a cross-sectional various mammalian cells. To address this knowledge gap,
2
area of 10,000 to 50,000 µm . The channel material is often additional data points have been provided to offer insights
made of silicon, brass, or graphite, and the nozzle typically into the conditions that different cells can endure without
has a diameter in the range of 18 to 50 µm. experiencing a loss of viability or a change in phenotype
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(Table 1). Embryonic stem cells were found to be sensitive
During standard operation, a 40 V pulse (10 µs) is to shear stress, as exposure to a low shear stress of 1 Pa for
applied to the piezo element. Initially, this pulse causes 24 h led to cell differentiation. Similarly, hybridoma cells
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the piezo element to contract, increasing the height of the experienced an increase in lactate dehydrogenase release
print chamber by ~20 nm. This action draws liquid into the into the medium when subjected to a shear stress as low
chamber and generates a negative pressure wave of around as 0.16 Pa for up to 15 h. The response to shear stress is
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100 kPa. This negative pressure wave propagates upstream highly dependent on the cell type, as well as the level of shear
into the ink reservoir. As the reservoir is significantly larger stress and the duration of exposure. The leukocytes and
than the channel, the pressure wave reflects toward the nozzle, erythrocytes typically experience shear stresses of 1.5 Pa in
transforming into a positive pressure wave. Simultaneously, arterial circulation and 0.1–0.6 Pa in venous circulation. 54,55
the piezo element returns to its original size, creating a In some cases, these cells can be exposed to shear stresses of
positive pressure pulse. In an acoustically tuned channel, up to 300 Pa for short periods near the walls of large blood
these two pressure pulses combine, working in tandem to vessels and in the heart. While exposing erythrocytes to
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eject the ink out of the nozzle for the printing process. shear stresses of 450–560 Pa for milliseconds can induce
3.2. Effect of shear on printed cells hemolysis, 56,57 some studies have shown that these cells can
The impact of mechanical shear stress on biological cells withstand shear stresses as high as 4000 Pa for 10 µs. 58,59 It was
over an extended duration can result in changes to their found that the response of baby hamster kidney cells (BHK-
phenotype and viability, and in severe cases, lead to cell 21/C13) to shear stress varies with the duration of exposure.
lysis. When it comes to typical cell handling procedures, These cells experienced a decrease in viability after 2 h of
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such as pipetting, cells are subjected to shear stress levels exposure to a shear stress on the order of 10 Pa. However,
ranging from 1 to 10 Pa for durations of 1 to 10 ms. the cells withstood shear stresses as high as 80 Pa without
Conversely, different stages of inkjet bioprinting expose significant loss of viability when exposed to shear stress for
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cells to much higher shear stress levels, reaching as high under 1 h, although there were changes in cell morphology.
as 500 Pa but only for a very brief period of ~ 100 µs. A In a more detailed study, the viability of different cell
probability model for cell survival as a function of shear types, including primary cells, immortalized cell lines,

Volume 10 Issue 2 (2024) 187 doi: 10.36922/ijb.2135