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International Journal of Bioprinting Sr-doped printed scaffolds for bone repair

effect on bone repair, some studies have combined MBG growth in the bone defect area. The bone repair effect was
with PCL to prepare MBG/PCL scaffolds that possess then microscopically evaluated using quantitative indexes,
better immune-regulating and bone-repair capabilities. such as bone mineral density and bone volume fraction.
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Another study prepared SrBG by doping Sr into MBG, The micro-CT 3D-reconstructed images highlighted that
2+
leading to a significantly enhanced immunomodulatory the PSBP scaffolds—as bone defect-filling materials—
capacity by promoting M2 MP polarization. The PSBP exhibited significant new bone growth (Figure 11A).
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scaffolds effectively upregulated the expression of M2 MP Likewise, the bone volume fraction and bone density
polarization genes (CD206 and ARG) while suppressing analysis results were consistent with the micro-CT results
the expression of M1 MP polarization genes (TNF-α and (Figure 11B and C), further validating that PSBP scaffolds
IL1β) (Figure 8). This may be related to the release of have better bone repair effects compared to the other
various ions from SrBG, including Sr² , Si⁴ , Ca² , and P⁵ . scaffolds. In addition, HE staining, a commonly used
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These ions enhanced the scaffold’s immunomodulatory method to observe tissue growth, was performed on rat
properties by inhibiting M1 MP polarization and cranial specimens from all scaffold groups to observe bone
promoting M2 MP polarization. 68,69 Results from the Sr² + tissue structure. The HE staining results revealed that at
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release experiments demonstrated that Sr² from the SBP postoperative months 1, 2, and 3, the PSBP scaffold group
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and PSBP scaffolds was continuously released over time had more bone tissue formation than the SBP, P, and blank
(Figure 4D). The overall Sr² release concentration in scaffold groups (Figure 12). Masson staining results also
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the PSBP scaffold group was lower than that in the SBP indicated that the PSBP scaffold group exhibited better
scaffold group. ELISA experiments indicated that PSBP osteogenesis (Figure 13).
scaffolds inhibited the expression of the pro-inflammatory The immunomodulatory capacity of implanted
factor IL-12 and increased the expression of the anti- scaffolds is important in promoting osteogenic
inflammatory factor IL-10 at a greater extent compared differentiation and angiogenesis. To further assess the
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to SBP scaffolds (Figure 8B1 and B2), suggesting that the immunomodulation and bone repair effects of each
relatively low Sr² concentration in the PSBP scaffolds scaffold group, immunofluorescence staining was used
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facilitates better immune microenvironment coordination to analyze cranial defect repair in rats. Generally, iNOS
to promote osteogenesis. Römer et al. found that adding and CD163 are used as polarization markers for M1 and
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3 mM (approximately 263 ppm) of Sr² to the culture M2 MPs, respectively. 78,79 It has been reported that PDA
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medium significantly inhibited the expression of the reduces the expression of M1 MP markers by scavenging
pro-inflammatory factor IL-6 in human periodontal cells ROS 80,81 and that Sr² activates the PI3K/AKT/mTOR
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compared to a control group. Buache et al. reported pathway to promote M2 MP polarization. In this study, the
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that 10 µM Sr-doped bidirectional calcium phosphate immunofluorescence staining results demonstrated that
significantly inhibited the expression of TNF-α and IL-6 the PSBP scaffolds significantly upregulated the expression
in human primary monocytes. Both SrBG extracts (Sr² of the M2 MP marker CD163 and downregulated the
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concentration: 6.227 ppm and 10 µM [approximately 0.88 expression of the M1 MP marker iNOS (Figure 14). This
ppm]), which are obtained after immersing SrBG in cell suggests that the PSBP scaffolds promoted MP polarization
culture medium and processed, significantly inhibited of M1 MPs to M2 MPs, resulting in more M2 MPs in the
the expression of TNF-α, IL1β, and IL-6 and elevated the immune microenvironment. This polarization attenuated
expression of anti-inflammatory factors IL-1ra, IL-10, the inflammatory response in the early stage of bone repair
and Arg-1. Lower Sr² concentrations correlated with a and promoted bone defect repair. 82,83 In addition, more M2
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lower expression of pro-inflammatory factors and a higher MPs contributed to osteogenesis on the scaffold surface,
expression of anti-inflammatory factors. This is consistent ultimately leading to good osseointegration. Therefore,
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with the results in the present study, validating that PSBP PSBP scaffolds have good immunomodulatory properties,
scaffolds have excellent immunomodulatory capabilities. and PDA, together with Sr² , promotes MP polarization
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Micro-computed tomography (micro-CT) is a non- toward the M2 phenotype. In promoting bone repair, BMP-
invasive, high-resolution imaging technique that enables 2, a key osteoinductive growth factor, induces targeted
clear visualization of the internal microstructure of bone differentiation of undifferentiated BMSCs into osteoblasts
tissue, especially for detecting new bone tissue in small and promotes osteoblast differentiation and maturation,
animals. 60,72 To evaluate the in vivo bone repair effects of thereby accelerating bone defect repair. 85,86 In this study,
the three scaffold groups, this study established a bilateral the PSBP scaffolds expressed higher levels of BMP-2 at the
cranial bone defect model in SD rats. SD rats were chosen defect site, suggesting a strong osteogenic induction ability.
for their low cost and size, which is suitable for surgical Additionally, vascularization is a key part of the bone repair
manipulation. 73,74 Micro-CT was used to observe new bone process and directly affects the repair and remodeling

Volume 11 Issue 4 (2025) 370 doi: 10.36922/IJB025210211