Page 299 - IJB-10-3

International Journal of Bioprinting Design and optimization of 3DP bioscaffolds

in the middle of the bioreactor will flow slowly from the was investigated, as shown in Figure 9a. Increasing the
middle of the scaffold. The results revealed that a constant flow rate from 600 to 6000 μL/min resulted in a relatively
inflow may not be suitable for dynamic culturing because large elevation in oxygen concentration, while it was
the nutrient flow in the porous scaffold becomes slower as minor when the flow rate changed from 60 to 600 μL/min.
cell proliferation proceeds. In addition, using a material Accordingly, the average cell density in the scaffold was
with a higher degree of microporosity (softer materials) is increased with a higher flow rate, mainly after 4 days (see
also beneficial for cell proliferation. Figure 9b). However, it should be noted that the flow rate

Oxygen concentration in the scaffold is a critical cannot be increased indefinitely, as this can increase wall
indicator for cell activities and is driven by both the shear stress at the edges of the scaffold. C2C12 cells require
convective action of the fluid and its diffusion. Figure 8a–d a specific range of wall shear stress (1.6–3.3 dyn/cm²) for
20
shows the simulation results of the bioreactor’s oxygen growth stimulation, and excessive or insufficient wall
concentration field, presenting the spatial distribution shear stress can lead to cell death or failure to grow. For this
from days 1 to 7. It can be observed that the oxygen reason, the model was adopted to explore the relationship
concentration in the non-scaffold area is uniform, while between flow rate and wall shear stress to obtain the
it changes significantly inside the scaffold. The magnitude optimal flow rate range. Figure 9c displays the shear stress
of oxygen concentration reduces gradually from the variation against flow rate, demonstrating a strong linear
surface of the scaffold to the center, where the oxygen correlation. When the wall shear stress was constrained
consumed by cells is not compensated adequately via the within the range of 1.6–3.3 dyn/cm², it yielded the optimal
diffusion process due to the lengthened diffusion path. values of inlet flow rates, being 2249 to 4515 μL/min.
A similar trend was observed for cell density, as shown 4.2.2. Geometric parameters
in Figure 8e–h. At the initial stage of cultivation, the cell Figure 10 illustrates the impact of the scaffold’s diameter and
density distribution was set to be uniform with a density of thickness on the average oxygen concentration, cell density,
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3.0 × 10 cell/cm . On day 4, the cell density in the center and cell count. Changes in scaffold diameter had minimal
regions of the scaffold was lowered approximately by half impact on oxygen diffusion and cell density (see Figure
compared to the outer areas, while the discrepancy was 10a and b). The cell count was enlarged by approximately
further amplified on day 7. To enhance the transportation 4 times from day 0 to day 7 (Figure 10c). The scaffolds
of oxygen, macro channels or voids are commonly added with a larger diameter demonstrated a greater proliferation
to solid scaffolds to promote permeability. 14,15,20 However, rate primarily due to the higher degree of initial cell count
excessive channels or voids can decline the cell loading of since the initial density remained consistent, highlighting
the scaffold, resulting in adverse effects on cell proliferation. the importance of initial cell count on cell growth. In
Determining optimal geometric parameters of scaffolds is contrast, changes in thickness demonstrated a remarkable
somewhat arbitrary or based on a trial-and-error approach influence on cell proliferation behaviors (see Figure
currently. For example, structures with different geometric 10d and f). Oxygen mass transfer efficiency decreased
parameters are repeatedly used in cell culturing. This significantly as the thickness increased from 0.3 mm to
greatly increases the cost of experimentation and time, 1.5 mm, beyond which it remained stable at a particularly
but the results obtained are still uncertain as to whether low level (~30%). As the effective diffusion length of
they are optimal. It is possible to use the modeling system oxygen in the biocompatible materials is generally within
developed in this study to optimize the architecture of the a few hundred microns, 19,20 the oxygen concentration was
scaffold to maximize its cell activities (see section 4.3). lowered by only ~13% after 7 days for the 0.3 mm scaffold
4.2. Parametric study in this study; however, the reduction reached 75% when
Prior to optimizing the scaffold, it is necessary to gain an the thickness was increased to 5 mm (see Figure 10d). Cell
understanding of how material, geometrical, and culturing density exhibited a minor correlation to the thickness from
parameters impact cell growth. The parameters, including days 0 to 3, while the difference in cell density between
the inlet flow of the bioreactor, scaffold’s diameter and various scaffolds was more pronounced afterward (Figure
height, initial cell density, initial material porosity, 10e). Due to enhanced oxygen transportation for thinner
maximum oxygen uptake rate, and maximum cell growth scaffolds, the cell density was amplified by 7 times on day
rate, were varied in the model to investigate the changes in 7, while it was 4.5 times for a thickness of 3.0 mm. For
oxygen concentration and cell growth. the cases of cell count, the cell number on day 7 increased
proportionally to the increase in thickness due to the
4.2.1. Inlet flow enlarged volume of scaffolds. The results of the sensitivity
The impact of inlet flow rates (60 μL/min, 600 μL/min, study indicate that the oxygen concentration and average
and 6000 μL/min) on oxygen distribution in scaffolds cell density are more sensitive to the axial dimension that

Volume 10 Issue 3 (2024) 291 doi: 10.36922/ijb.1838