Page 100 - IJB-9-4

International Journal of Bioprinting 3D-Printed liver model

Table 1. Properties of the materials used to print the liver model
Material property Silicone rubber (structure) PDMS oil (filler fluid) PLA (support)
Color Translucent Red Black
Density (g/cm³) 1.01 1.00 1.24
Viscosity (Pa·s) 410 100 N/A
Shore hardness A 18 N/A Approximately D 70
Tensile strength (MPa) 1.10 N/A 60
Elongation at break (%) 400 N/A 160
Printing temperature (°C) Room temperature Room temperature 200
Shear thinning / shape holding Yes No N/A
Pot life Approximately 10 min Inf. N/A
Cure time Approximately 24 h Inf. N/A

Furthermore, preliminary experience of printing a viscous It was also expected that the liver geometry will require
fluid into an infill structure has shown to increase viscous some degree of hard overhang support during printing.
macroscopic mechanical behavior upon deformation. A common poly-lactic-acid (PLA) filament was selected
(Material4Print GmbH & co. KG, Löhne, Germany) for
Considering these limitations, a 40% gyroid infill
volume fraction was chosen for the liver model, which printing the required support structure. The properties of
would be filled with a viscous but inert filler fluid. This all used materials are summarized in Table 1.
material structure is not expected to perfectly mimic the
mechanical properties of human liver due to technological 2.3. Print settings and slicing
limitations, but to offer a more realistic—albeit still The print setup, slicing and G-code generation was
[44]
printable—alternative to printing or casting solely out of conducted in the open-source Prusa Slicer v2.4.2
silicone rubber, while getting as close to the target elastic software (Figure 2). The liver model size was linearly
modulus (100 kPa) as possible with the printer and the downscaled to 33% in all three directions, so that printing
known single-component printing materials. the whole object would be possible with a single 55-mL
cartridge of each material and within a working day. This
The material chosen to print the shell and infill downscaled STL file represented the outer shape of the
structure of the liver model was the softest available single- liver model. Due to the organic and often overhanging
component liquid silicone rubber already tested with shape, a relatively thick contour was desired to ensure
the system , namely the Elkem AMSil 20101 (Elkem sufficient sealing against filler fluid leakages, requiring
[38]
Silicones SAS, Lyon, France) . This material starts curing an approximately 2-mm thick solid shell around the infill
[41]
upon contact with air, with skin formation within 10 min structure. To achieve this, the downscaled STL of the outer
and full crosslinking within 24 h after deposition at room shape was further offset by 2 mm inwards in Meshmixer
temperature. to represent the shape of the inner structure of the liver

The rest of the internal space was filled up with a model (Figure 2A), occupied by both the infill structure
red-colored poly-dimethyl-siloxane (PDMS) oil with and the filler fluid.
100 Pa·s dynamic viscosity (Optimal Products GmbH, Bad To enable filling the infill structure with a fluid, the
Oeynhausen, Germany) . This was chosen as the highest infill structure was generated and sliced first, then the
[42]
viscosity fluid available at the supplier that could still be resulting G-code pathways were exported as an OBJ file
filled into a cartridge to feed the printhead. Preliminary from Prusa Slicer (Figure 2B). This gyroid geometry was
experience also showed that the presence of PDMS oil then smoothened in Meshmixer (Figure 2C), saved as
does not inhibit the crosslinking of the chosen silicone, as a standalone STL file and reimported into Prusa Slicer
long as they are not mixed together. Dark red color was (Figure 2D). Finally, this gyroid infill geometry was
chosen to improve visual appearance and was achieved by overlaid with the original outer shape as a multi-part (and
mixing 1 w/w% of Silc Pig “Blood” paint (Smooth-On Inc., multi-material) object, allowing the assignment of the
Macungie, PE) with PDMS oil. It was assumed that this silicone rubber nozzle to the outer shell and the gyroid
[43]
coloring additive does not have a significant effect on the pattern, and the PDMS oil nozzle to the cavities within the
overall mechanical behavior of the PDMS oil. gyroid infill (Figure 2E). Support structures were generated

Volume 9 Issue 4 (2023) 92 https://doi.org/10.18063/ijb.721