Page 443 - IJB-9-5

International Journal of Bioprinting Core-shell bioarchitectures

and hinder their scale-up; they are time-consuming and 2. Materials and methods
expensive to generate and maintain. Moreover, they suffer Alginate, a widely used material in bioprinting [21] , was
from a lack of reproducibility which is in part intrinsic to used as the base material for its biocompatibility and
the self-assembly process and to the fact that protocols rapid physical gelation after extrusion was used as the
vary from laboratory to laboratory [18,19] .
base material for its biocompatibility, mildness, and
The bioprinting of structured core–shell constructs fast gelation after extrusion. Other materials considered
(CSCs) using standardized cell lines can be an optimal in our models and experiments were air, water, and
solution to overcome these limitations, as it enables the Pluronic. The material properties and core–shell materials
generation of curved 3D structures which can be controlled combinations analyzed are reported (Section S1
in terms of material composition, size, and lumen/external in Supplementary File). In this study, we used two
diameter ratio . Bioprinting is a process based on 3D commercial coaxial needles (Ramé-hart Instrument Co.,
[20]
printing techniques that exploits the combination of cells, USA): needle 1 with an inner diameter (ID) = 26 Gauge
adhesion factors, and biomaterials to produce constructs (0.254 mm) and an outer diameter (OD) = 19 Gauge
for mimicking the characteristics of a tissue and includes (0.69 mm), and needle 2 with the same ID and an OD =
a wide range of techniques based on droplet or filament 16 Gauge (1.19 mm).
extrusion and deposition . Cell-laden drops or filaments
[21]
constitute simple 3D structures that are further crosslinked 2.1. In silico modeling of the CSCs fabrication
to maintain their shape, while more complex architectures In the in silico workflow, the initial shell and core extruded
*
can be obtained by layer-by-layer deposition. Several drop radii (R and R ) were estimated as a function of
c
s
fabrication strategies in the literature are almost exclusively the extrusion flow rates and material properties (surface
based on the use of alginate which undergoes rapid tension γ and dynamic viscosity μ) by numerically solving
gelation in contact with divalent cations . Techniques the surface tension–gravity–flow force balance equation
[22]
range from coaxial electro-dropping to microfluidic and (detailed in Section S2 in Supplementary File). Despite
[29]
in air-microfluidics with applications in drug delivery, the complex nature of the hydrodynamic problem , this
drug release, and therapy and tissue engineering [23–26] . simplified approach enabled the estimation of initial R and
s
*
For instance, a flow focusing microfluidic device was R , minimizing the computational cost. Surface tension
c
used to encapsulate a soft cell-collagen core in an alginate was measured with a tensiometer (Optics Theta Lite, Biolin
shell. The solutions were extruded in a continuous oil Scientific, Sweden) using the pendant drop test, while
flow containing Ca ions, allowing shell gelation and the dynamic viscosity was characterized using a viscosimeter
2+
formation of CSCs with an average diameter of 380 μm . (Brookfield DV-II+ Pro, AMETEK Brookfield, Germany)
[27]
Gelatin methacrylate (GelMA) has also been used to equipped with an LV1 spindle, at 37°C (see Section S2 and
encapsulate cells in a methyl cellulose core. The polymers Tables S1 and S2 in Supplementary File), at a shear rate of
-1
3
were extruded in oil, and the GelMA shell was crosslinked 1.3 × 10 s .
by ultraviolet (UV) radiation, obtaining core–shell The radii derived for each combination of core and
microgels with diameters of around 278 μm . shell materials were used as initial values to define the
[28]
Some of these strategies are equipment-intensive, and geometry of an axial symmetric finite element method
the majority are limited in terms of their dynamic range (FEM) model implemented in Comsol Multiphysics
and compatibility with cell encapsulation. The novelty of 6, solving the reaction–diffusion equations for the
2+
our approach is the integration of computational methods transport of diluted species (Ca ions) from an external
with coaxial bioprinting strategies for the fabrication of fluid domain to porous media domains representing the
structured luminal bioarchitectures using a variety of cell- alginate shell and core, respectively. The formation of
2+
compatible materials. This enables the a priori definition G-blocks during Ca -mediated alginate crosslinking
of a working window, thus minimizing experimental (Equation I) was implemented as a second-order
time and cost. Here, we describe the integrated workflow, reaction since it depends on both the concentration
2+
exploiting the COre-Shell MIcrobead Creator (COSMIC), of Ca ions and the concentration of non-crosslinked
which was designed for bioprinting cell-incorporated CSCs alginate [30,31] .
in a repeatable manner and with a wide range of materials (I)
(Figure 1B), resulting in a variety of structures with solid
shells and either solid, liquid, or air-filled cores, capable
of replicating different biological interfaces. As a proof Where k is the reaction rate of the gelation, c Ca 2+ is the
of concept, a core–shell multilayer barrier model with concentration of Ca and c is the initial concentration of
2+
Alg
0
alveolar epithelial cells and fibroblasts was generated . un-crosslinked alginate [30,31] . An apparent diffusion coefficient
[29]
Volume 9 Issue 5 (2023) 435 https://doi.org/10.18063/ijb.771