Page 107 - IJB-9-4

International Journal of Bioprinting 3D-Printed liver model

Table 4. Average elastic moduli, dissipated energy ratios and Hounsfield units
Liver model and flu- Full silicone Liver tissue
id-filled specimens specimens (based on [35,36,49,50] )
(n = 1 + 3) (n = 3)
Initial elastic modulus (kPa) 260 370 Approximately 100
(E1 on Figure 7, at 0%–3% strain) (target in this study)
Final elastic modulus (kPa) 190 250 N/A
(E2 on Figure 7, at 22%–27% strain)
Dissipated energy ratio (–) of first loading cycle (0%–7.5% strain) N/A N/A Approximately 0.6 ± 0.1
Dissipated energy ratio (–) of second loading cycle (0%–15% strain) 0.140 0.118
Dissipated energy ratio (–) of third loading cycle (0%–22.5% strain) 0.167 0.093
Dissipated energy ratio (–) of fourth loading cycle (0%–30% strain) 0.183 0.081
HU of fluid-filled internal structure of liver model 225 ± 30 N/A Approximately 70 ± 30
HU of bulk outer shell of liver model 340 ± 50 N/A

PDMS oil, using PLA as support. The printed liver model would prevent a large soft object from such deformations.
underwent CT scanning, and it was found that its internal However, this would also prevent the use of any closed
structuring has brought it closer to actual liver tissue HUs internal cavities or filler fluids. In general, solving such
compared to the bulk silicone material. Meanwhile the practical limitations of the printer were not in the scope
tensile testing results show that the used internal structure of this study, since the demonstration of material property
has also brought the liver model closer to real liver tissue tuning was possible.
from a mechanical standpoint compared to bulk silicone,
decreasing elastic moduli and increasing dissipated energy 4.2. Mechanical behavior and limitations
ratios. In case of the fluid-filled structure, the lower elastic moduli
(compared to full silicone) are likely caused by having
4.1. Printing performance and limitations less silicone in the internal structure (Figure 8), while
Despite the extensive use of PLA support structures, the the increase in dissipated energy ratios (Table 4) can be
liver model presented a relatively straightforward print associated with the viscous filler fluid circulating within
run. The visible color inhomogeneities (Figure 6A and B) the silicone gyroid structure upon deformation.
were likely caused by unwanted droplets of the filler fluid
falling onto the object from the inactive nozzle. This could As stated in section 2.2 and Table 4, the desired initial
be avoided either with longer pullback settings upon tool elastic modulus was 100 kPa, after which some degree
switching, or by upgrading the printer with a tool-changing of strain-stiffening and overall viscoelastic behavior was
or lifting mechanism for inactive extruders, as described in desired. As expected with the knowledge of printing
refs. and . limitations, the fluid-filled tensile testing specimens (and
[18]
[19]
thus, the liver model) provided a mechanical behavior that
Theoretically, the liver model could also be printed
without downscaling to 33% in each direction, but a full- is closer to real liver tissue than the behavior of the full
silicone benchmark specimens, in terms of both elasticity
scale model would consume large amounts of material, and and viscosity, although not matching it perfectly with an
require frequent cartridge changes, and either a printing initial elastic modulus of 260 kPa instead of 100 kPa, and
time over a week, or a much coarser printing resolution dissipated energy ratios in the range of 0.14–0.19, instead
with larger nozzle. In such a case, some anatomical of approx. 0.5–0.7 (Table 4).
features—like larger and medium-sized vessels—could
also be represented inside the internal structure of the liver The elastic modulus could have been even lower with a
model, but at a cost of potentially compromising freedom softer material or if the specimens were printable with no
in mechanical and radiological property tuning. solid outer shell. Unfortunately, the latter is not possible by
principle, as the fluid would leak from the specimens upon
Moreover, in case of larger prints with soft materials,
the deformation of the printed object under its own weight deformation if there was no shell.
would likely require some degree of compensation, and Also, the typical strain-stiffening behavior characteristic
[35]
such a feature is not yet available in current 3D printing to liver tissue was not reflected in the results in this
software. Alternatively, printing could happen in a support study. This is not surprising, since most biological tissues
bath with the same density as the printing materials, which feature a fibrous hierarchic structure, which stiffens when

Volume 9 Issue 4 (2023) 99 https://doi.org/10.18063/ijb.721