4.6.1 Correlation of shear stress and cell vitality
To solve the question of whether it is reasonable to improve printing
resolution by increasing the shear stress, evaluating the impact of
shear stress on living cells is essential. For this purpose, we
investigated the short-term and possible long-term effects of
printing-induced shear stress on primary porcine hip arterial
endothelial cells (pHAE). We printed pHAE from five independent donors
(n = 5) at three different shear stress levels (20, 30, and 40 kPa) and
recorded the resultant cell viability (Figure 10a–j). With increasing
shear stress, as shown in Figure 12, the average cell viability in the
kc composite was significantly decreased from 56.2% (20 kPa) to 42.1%
(40 kPa). Nonetheless, the cell viability in kc-s and kc-c seemed
unaffected (with >93% average cell viability). Moreover,
the viability of the cells in kc-sa-c increased with the printing
pressure increasing from 94.7% (20 kPa) to 95.8% (30 kPa). Our results
indicated that a higher shear stress may stimulate cell proliferation,
as has been reported[42-45]. To understand the effect of shear
stress on quantificational cell viability, the distribution of dead
cells was assayed using the equal area method. The cross-section of the
gel filament was divided into three concentric annulus areas (Figures
12a,1, 2, 3) from the core to the shell. Meanwhile, several dead cells
in each area were calculated and averaged. As shown in Figure 12k–m),
from the centre (area 1) to the outer (area 3) area, the average cell
viability in kc significantly decreased from 60.7% (20 kPa) to 56.3%
(20 kPa).A similar conclusion was reached at 30 and 40 kPa (Figure 12k).
The average cell viability in kc-sa (Figure 10l) and kc-c (Figure 10m)
showed a similar gradient, and the difference in each area was
approximately 5%.Due to the gradient distribution of shear stress the
cells in the centre layer of the gel filament received the least
shearing force, such that the number of dead cells was the least.
Secondly, the cells in the middle layer received moderate shearing
force. The cells in the outermost layer received the largest shearing
force, showing the maximum number of dead cells. The growth of cells
exposed to different shear stress levels differed with the printing
process. However, to elucidate this phenomenon, further
mechanobiological studies with a focus on the intracellular mechanisms
in response to shear stress should be conducted.