Page 244 - IJB-10-6

International Journal of Bioprinting DIW of concave hydroxyapatite scaffolds

The strain energy density (SED) displayed in Figure 6c3 3.5. In vitro study
indicates the toughness of the scaffolds, i.e., the energy The results of SaOs-2 cell adhesion on the different scaffolds
required for scaffold breakage normalized by the external are presented in Figure 7a. The seeding efficiency for all
scaffold volume. The OP scaffold reported the highest SED scaffolds is low, especially for the OP scaffold, which is
(0.62 ± 0.13 MJ/m ), followed by the S geometry (0.37 ± 0.20 typical of porous scaffolds. 65,66 In addition, CDHA is known
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MJ/m ), with no statistically significant difference between to promote ionic exchange due to its high bioactivity,
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them. The OP scaffold was broken upon column splitting which reduces cell adhesion in static conditions. Among
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(Figure 6b), consistent with the critical fracture points the different TPMS-based scaffolds, significant differences
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identified from a finite element simulation study, and the were observed for G and D geometries compared to OP
breakage was more abrupt than the progressive compaction (Figure 7a). A similar trend was observed after a day of
observed for the S scaffolds. The SED for the G and D scaffolds culture, and similar cell proliferation was reported for all
was significantly lower at 0.23 ± 0.05 and 0.13 ± 0.05 MJ/m , the scaffolds on days 1, 7, and 14 (Figure 7b). Statistically
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respectively, similar to the trend observed for compressive significant differences were observed only on day 21, where
strength, though no significant differences were observed cell proliferation was higher in the TPMS-based scaffolds
between them. Moreover, their breaking mechanism was compared to OP scaffolds.
more similar to the S structure than to the OP, i.e., without
any abrupt breakage. Finally, the Weibull modulus was not Regarding cell differentiation, higher ALP activity
affected by pore architecture (Figure 6c (iv)). was observed on day 1 for all TPMS-based structures
compared to OP; the ALP activity for all TPMS-based
Comparing our results with those of other studies structures was relatively similar to OP structure on day 7;
is challenging due to the high dependence on material, the ALP activity for G and D structures was higher than
porosity, and manufacturing process. For instance, the OP structure on day 14 (Figure 7c). At the end of the
utilizing a sintering process to harden the green body in culture period (day 21), all TPMS-based scaffolds reported
CaP scaffolds results in significantly enhanced mechanical similar ALP levels, which were higher than the OP scaffold
properties compared to scaffolds produced via a biomimetic but lower than the control.
route (similar to this study) due to microstructural
differences. 37,64 However, two studies focusing on TPMS Mineralization was measured via AR quantification
primarily composed of CaP materials can be compared after 14 days (Figure 7d). Similarly, mineralization was
to the present study. Sintered hydroxyapatite (HA) G highest for the G structure, followed by the D structure.
structures with 10% åkermanite were manufactured by Shi No differences were observed between the S and OP
et al. through SL technology, resulting in a slightly lower structures, both of which were slightly higher than the
compressive strength (≈2 MPa) and elastic modulus (≈0.3 control. After 21 days, G and D structures displayed
GPa) than the biomimetic G structures in the present similar values, with the S structure exhibiting a significant
study, despite having twice the porosity. In addition, a increase from its value on day 14. Notably, the high ALP
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twofold improvement in the mechanical properties was levels in the control samples did not result in increased
achieved by introducing radial porosity gradients from mineralization. The natural tendency of high ALP levels
50% to 80%, closely mimicking the natural anisotropy of promoting mineralization may be affected by other factors.
bone. Likewise, Zhang et al. produced six different TPMS Topographical or compositional aspects could play a role,
structures (including G, D, and S structures) of pure HA but further studies are warranted to clarify their effects on
using SL technology coupled with a high-temperature cell maturation. CaP is known to be more bioactive and
sintering process. Between the G, D, and S structures, cell-stimulating substrates compared to inert cover glass,
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the D structure exhibited the highest compressive strength providing biochemical and physical cues, including ionic
(≈110 MPa), followed by the S (≈90 MPa), and G (≈25 fluctuations, absent in the glass substrate. For example,
MPa) structures. However, the results are inconclusive the measured intracellular ALP values do not directly
due to differing porosities, which resulted from unequal translate to ALP release, as the concentrations of calcium
shrinkage during sintering, i.e., ≈35% for G, ≈45% for D, and phosphate in the medium, mediated by a CaP scaffold,
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and ≈55% for S structures. Notably, the absolute values affect ALP release from SaOS-2 cells. In contrast,
between these studies and our results cannot be directly differentiation in the control samples is confluent-induced,
compared as the layer height and resolution are also i.e., depending on the cell density, which can be more
significantly different between SL and DIW printing. variable and slower. Regardless of the presence of ALP
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Nonetheless, the biomimetic approach adopted in the protein, subsequent mineralization is assumed to be
present study avoided scaffold shrinkage, which mitigated triggered by enzymatic hydrolysis, especially in in vitro
subsequent dimensional variations. environments. Nevertheless, the presence of concavities
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Volume 10 Issue 6 (2024) 236 doi: 10.36922/ijb.3805