Page 475 - IJB-10-2

International Journal of Bioprinting Bioprinting with ASCs and bioactive glass

for 2.5G) were added to modify the AG hydrogel. The ions crosslink more alginate polymer chains in the AG
dissolution rate of borate-based glasses including B3 hydrogel that causes increased stiffness. Researchers have
glass in different forms such as glass microspheres, large previously reported that human mesenchymal stem cells
particles (>200 µm), and scaffolds in DI water, PBS, and (MSCs) encapsulated in ionically crosslinked alginate
simulated body fluid (SBF) have been investigated in the hydrogels show adipogenic differentiation at moduli of
past. 38-41 These studies reported that the majority of B3 <10 kPa and show osteogenic differentiation at moduli
glass dissolution occurred during the first 24-h soak period of 11–30 kPa. 42,43 The authors also suggest that osteogenic
after which the dissolution rate slowed due to the ionic differentiation of MSCs could be enhanced in hydrogels
concentration gradient and other factors. In comparison, with faster stress relaxation. AG hydrogels modified with
the glass particles used in the current study are significantly B3 glass (2.5G and 1.25G) have elastic moduli in similar
smaller with an average particle size of 3 µm (20 µm mesh range, and the stress relaxation occurs through breakage
sieved) that could have dissolved within 24 h. Therefore, and subsequent forming of ionic crosslinks. In addition,
it could be safely assumed that the rheological, swelling, stress relaxation could also occur because of the fast
and mechanical properties of the modified AG hydrogels swelling of B3 glass modified AG hydrogels (especially,
are mostly dependent on the ionic dissolution products of 2.5G). Therefore, the addition of B3 glass to modify AG
B3 glass and not the physical presence of glass particulates. hydrogels could be useful to tune the human stem cells
Figure 8a shows the microstructure of the 2.5G hydrogel, activity.
indicating a highly porous and honeycomb-like structure An important factor to consider is the retention of
with interconnected porosity. B3 glass particles were not scaffold mechanical properties (or scaffold integrity and
detected in the microstructure, and the characteristic structure) in culture conditions with time. AG hydrogel
amorphous peaks of the glasses were also absent in the strength greatly depends on the w/v % of the alginate and
XRD analysis of the 2.5G hydrogel. gelatin used in the preparation and the ionic strength of
Figure 8b shows the dog-bone-shaped specimens the crosslinking solution. For example, Duan et al. used 6
utilized for evaluating the tensile strength of hydrogels w/v % of alginate and gelatin (compared to 3 w/v % used
immediately after crosslinking. A typical load vs. deflection in our study), and the crosslinked samples with 0.3 M
curve for all three specimens is shown in Figure 8c, which CaCl (compared to 0.1 M CaCl in our study) reported
2
2
indicates the increase in scaffold stiffness with the addition improved mechanical properties that were able to sustain
of B3 glass. The elastic modulus of AG, 1.25G, and 2.5G for up to 7 days in culture conditions. Our attempts to
14
specimens was 33 ± 17 kPa, 62 ± 7 kPa, and 73 ± 13 kPa, test specimens after soaking them in DMEM for 7 days
respectively, and the ultimate tensile strength of specimens at culture conditions were not successful as specimens
was 26 ± 5 kPa, 21 ± 4 kPa, and 34 ± 9 kPa. 2.5G specimens were broken in the grips of the Instron machine, and
had highest ultimate tensile strength and were significantly insignificant elastic modulus and strength values were
stiffer (p < 0.05) in comparison to AG specimens. The recorded. Our results indicated a rapid loss of modulus
increased stiffness was in agreement with the rheological and strength to a point where they were not suitable for
data that showed increased viscosity, decreased recovery any load-bearing applications, which is in agreement with
time, and a viscoelastic solid-like behavior for 2.5G results reported by Giuseppe et al. However, it must be
10
hydrogels. As the added B3 glass dissolves, the dissolution noted that 1.25G and 2.5G scaffolds had sufficient integrity

Figure 8. (a) SEM image of 2.5G hydrogel with ~100 µm interconnected pores shown in the magnified inlet picture; b) dog-bone specimens used for tensile
tests; (c) typical load vs. extension graphs of AG, 1.25G, and 2.5G specimens.

Volume 10 Issue 2 (2024) 467 doi. 10.36922/ijb.2057