Page 392 - IJB-9-4

International Journal of Bioprinting Impingement shear stress during microvalve-based bioprinting

of process terminology, Lee et al. proposed four order to minimize the nozzle wall shear stress [17,25] . To
[11]
categories of bioprinting technologies: material jetting, vat elaborate the dispensing dynamics and to calculate the wall
photopolymerization, material extrusion, and free-form shear stress, as well as the shear stress during impingement
spatial printing. In the case of nozzle-based bioprinting of the cell-laden droplet at the building platform, we used
methods, the mechanical stimuli become extreme by the a finite volume method-based simulation model using
increase of required resolution. In inkjet bioprinting, the commercial software Ansys Fluent 2021 R2. In addition,
droplet ejection relies on a pressure pulse distributed in we verified and validated in vitro the findings from the
the bioink chamber to form and transfer a droplet to the in silico simulation by performing cell viability analysis
platform. Derby showed that for the drop ejection of a after dispensing cell-laden alginate from several nozzle-
[12]
Newtonian fluid, materials can be benchmarked based on to-platform distances. Two functionally diverse cell types
their rheological properties in accordance with the values (HaCaT epithelial cell line and primary human umbilical
of three dimensionless parameters: Reynolds (Re = ρvd ⁄ μ), vein endothelial cells [HUVECs]) were considered.
Weber (We = ρv d ⁄ σ), and Ohnesorge (Oh = √We ⁄ Re)
2
numbers. Yet, most bioinks exhibit viscoelastic behavior 2. Materials and methods
that makes it difficult to directly benchmark them into
printable and not printable material based on the study 2.1. Numerical simulation
cited. Gudapati et al. [13,14] showed that collagen, fibrinogen, Ansys Fluent 2021 R2 was used for the numerical
and thrombin can be printed dropwise only at a certain simulations. For this purpose, a multi-phase laminar
protein concentration and using certain nozzle sizes. model based on volume of fluid method considering
Moreover, they showed that the addition of a non-ionic continuum surface force, as for phase interaction, was
surfactant prevents the formation of the interfacial layer, chosen. The fluids were assumed to be incompressible,
which consequently helps to resolve the inconsistencies in isothermal condition was imposed, and the maximum root
-3
rheological measurements and bioprinting. Xu et al. used mean square (RMS) residual error of 10 was chosen. We
[15]
alginate solution and identified four break-up types based set a Pressure-Implicit with Splitting of Operators (PISO)
on ligament pinch-off position depending on the alginate scheme for pressure-velocity coupling. Least squares cell-
concentration. These studies focused on the dispensing based, body force weighted, second-order upwind, and
dynamics of the nozzle of piezoelectric microvalves, while compressive methods were used for spatial discretization
leaving the question of its effect on and interaction with the of gradients, pressure, momentum, and volume fraction
living cells unexplored. terms, respectively. For transient terms, first-order implicit
method was used. As for the geometry, the nozzle size of
In principle, the method-immanent mechanical mechanical microvalves (SMLD 300G, Fritz Gyger AG,
stimuli on the processed cells are determinant factors Gwatt, Switzerland; nozzle diameters 150 µm and 300 µm)
for a successful bioprinting [16,17] . It has been shown that was considered for the simulations. This valve consists of a
bioprinting-associated shear stress has an adverse effect stationary piston, a moving piston, and a spring that ensures
on cell viability [18,19] during the printing process that microvalve closure. By imposing a high upstream pressure,
eventually causes limited integrity and functionality the cell suspension is placed under pressure, and as soon
of the cells post-printing [20,21] . Some studies [16,17,22] have as the valve opens, a drop is ejected. For the numerical
used numerical simulation to determine the shear stress simulation, a non-uniform mesh with semi-structured
involved in microextrusion bioprinting and discussed grids grown from wall surfaces (boundary layer meshes)
solutions for minimizing this since it is considered a was generated. A grid study was performed on at least three
detrimental effect associated to the bioprinting. In droplet- cases of fine, medium, and coarse meshes considering the
based techniques, another source of mechanical stress on maximum wall shear stress as the sensitive parameter.
cells is the impingement shear stress that has been studied Transient simulation was performed considering a valve
partly in laser-induced forward transfer bioprinting [23,24] . opening time of 400 µs. More information regarding the
However, there is less known about the mechanical effect geometry, boundary conditions, meshing, and numerical
on the cells due to impingement onto the building platform model is presented in Supplementary File.
during microvalve bioprinting. We hypothesized that the
impingement effect can be even more critical for the cells 2.2. Experimental setup
due to higher shear stresses compared to the wall shear To capture the droplet ejection dynamic, a custom-
stresses occurring during droplet ejection inside the nozzle made experimental setup was built. This setup was based
tip. To test this hypothesis, we considered alginate 1.5% w/v on a drop-on-demand (DoD) bioprinter previously
as an optimal bioink for our study because it exhibits the custom-made in our lab. In this setup, the position of
shear-thinning behavior recommended for bioinks in the solenoid microvalve (SMLD 300G, Fritz Gyger AG,

Volume 9 Issue 4 (2023) 384 https://doi.org/10.18063/ijb.743