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Vat photopolymerization of biomimetic bone scaffolds based on Mg, Sr, Zn-substituted hydroxyapatite: Effect of sintering temperature

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Abstract

In response to the urgent demand for innovative bone regeneration solutions, the focus of this study is to develop and characterize Mg, Sr, Zn-substituted calcium phosphate scaffolds that replicate the trabecular architecture of cancellous bone. Ion substitution represents a promising approach to improve the biological effectiveness of calcium phosphates and composite materials used in bone tissue engineering applications. Porous scaffolds mimicking the natural bone structure were additively manufactured from the photosensitive ceramic suspensions for vat photopolymerization using digital light processing. The impact of the selected trace elements (0, 1 and 5 mol.% substitution) and the sintering temperature (900, 1000, 1100, 1200, and 1300 °C) was investigated in relation to the obtained crystalline phase content, microstructure, elemental distribution, thermal stability, and mechanical properties. After sintering, in addition to hydroxyapatite, β-tricalcium phosphate was detected as a result of the added trace elements in the calcium-deficient hydroxyapatite used as a starting powder. The obtained scaffolds exhibited uniform distribution of the trace elements, and they feature 3D-designed porosity predominantly ranged from 10 to 900 μm in diameter, with an average pore size of 546.25 ± 10.95 μm. The total porosity of scaffolds was 76.24 ± 1.32 vol% and an average wall thickness of 217.03 ± 8.98 μm, closely resembling the morphology of cancellous bone tissue. The mechanical properties of the scaffolds sintered at 1100 °C, 1200 °C, and 1300 °C were in line with those typically observed in trabecular bone. The study demonstrates the feasibility of using custom made bioactive hydroxyapatite powders together with vat photopolymerization to design the porosity and properties of the bone scaffolds on demand, based on the requirements of individual bone defects.

Original languageEnglish
Pages (from-to)27403-27415
JournalCeramics International
Volume50
Issue number15
DOIs
Publication statusPublished - 2024
Publication typeA1 Journal article-refereed

Funding

This project has received funding from the European Union\u2019s Horizon Europe research and innovation program under the Marie Sk\u0142odowska-Curie grant agreement No. 101062225. This research was conducted in part with resources from Ceramic 3D Printing co-innovation project (No. 6333/31/2021) funded by Business Finland. In our previous studies [27,28], we investigated the phase content of multi-phase CaP powder systems obtained from biogenic source which were mono-substituted with Sr2+, Mg2+, and Zn2+ ions, both in their as-precipitated state and after heat treatment (1200 \u00B0C). In the current study, different phase compositions of obtained HAp powders after heat treatment are expected due to the modified synthesis and multi-substitution with selected ions (0, 1, and 5 mol.%). Fig. 2a depicts the XRD patterns of the as-prepared powders (HAp, HAp_1MIX, HAp_5MIX) and heat-treated powders at 800 \u00B0C (HAp_800, HAp_1MIX_800, HAp_5MIX_800). The XRD peaks of the as-prepared powders closely match the line patterns for HAp (JCPDS No. 09\u20130431), as indicated by the reference patterns below the experimental data. After heat treatment at 800 \u00B0C, HAp_800 powders exhibited a strong resemblance to the HAp pattern. However, in the case of HAp_1MIX_800 and HAp_5MIX_800 powders, additional peaks corresponding to \u03B2-tricalcium phosphate (\u03B2-TCP, JCPDS No. 09\u20130169) were observed, with higher peak intensity in the powder containing a higher substitution level of Sr2+, Mg2+, and Zn2+ ions. No additional peaks characteristic for strontium, magnesium, or zinc compounds were observed. EDS analysis was conducted to confirm the atomic composition of the powders heat-treated at 800 \u00B0C. The EDS elemental mapping (Fig. 2b) confirmed the presence and even distribution of calcium, phosphorus and magnesium in the HAp_800 powder, as well as the presence of calcium, phosphorus, strontium, zinc, and magnesium in HAp_1MIX_800 and HAp_5MIX_800 powders. EDS elemental mapping confirmed the presence of magnesium in the HAp_800 sample, despite the powder not being nominally substituted with Mg2+ ions. In our previous studies [28,32], when synthetic sources of Ca2+ ions were used, HAp powders obtained from calcium carbonate or calcium oxide were substituted with 0.86 and 0.40 mol.% of Mg2+ ions, respectively. Synthetic sources contain ionic impurities depending on their origin, which may contribute to the presence of these elements in the resulting HAp powders. EDS spectrum of the HAp_800, HAp_1MIX_800 and HAp_5MIX_800 powders are shown in Fig. S1a of Supporting Information. Our previous studies [27,28] have shown that substitutions up to 5 mol% of Sr2+, Mg2+, and Zn2+ ions in CaPs are noncytotoxic and promote cell proliferation. Single-substituted HAp bone grafts have exhibited improved biological efficacy in vitro and in vivo. However, they frequently fall short in meeting the multifaceted demands of clinical applications. Hence, it is reasonable to hypothesize that utilizing multi-substituted HAp may further augment the advantageous effects of each individual substitute [25]. Multiple ionic substitutions in CaPs have shown a collective synergistic effect on bone cell growth, collagen type I expression and the mineral formation capacity of the human mesenchymal stem cells [17].SEM micrographs depicting the structure of fabricated HAp_1300, HAp_1MIX_1300, and HAp_5MIX_1300 scaffolds are shown in Fig. 3a. As anticipated, there are no observable differences in the microstructures among scaffolds derived from HAp_800, HAp_1MIX_800, and HAp_5MIX_800 powders sintered at 1300 \u00B0C. EDS elemental mapping (Fig. 3b) further validates the uniform distribution of elements following sintering at the highest temperature. Ensuring the even distribution of substituted ions is crucial to achieve consistent biomaterial properties across the entire scaffold, essential for its optimal regenerative potential. Upon closer examination at higher magnification, the sintered grains in the HAp_1MIX_1300 and HAp_5MIX_1300 scaffolds exhibit noticeable similarities. However, in the HAp_1300 scaffold, grain boundaries remain undetectable. It is worth noting that magnesium, which was present in powders HAp_800, was not detected in the HAp_1300 scaffold. EDS spectrum of the HAp_1300, HAp_1MIX_1300 and HAp_5MIX_1300 scaffolds are shown in Fig. S1b of Supporting Information.This project has received funding from the European Union's Horizon Europe research and innovation program under the Marie Sk\u0142odowska-Curie grant agreement No. 101062225. This research was conducted in part with resources from Ceramic 3D Printing co-innovation project (No. 6333/31/2021) funded by Business Finland.

FundersFunder number
European Union’s Horizon Europe research and innovation program
Business Finland
European Union's Horizon Europe research and innovation program
H2020 Marie Skłodowska-Curie Actions6333/31/2021, 101062225

    Keywords

    • Bone tissue engineering
    • Ceramic slurry
    • Hydroxyapatite
    • Ion substitution
    • Scaffold
    • Vat photopolymerization

    Publication forum classification

    • Publication forum level 1

    ASJC Scopus subject areas

    • Electronic, Optical and Magnetic Materials
    • Ceramics and Composites
    • Process Chemistry and Technology
    • Surfaces, Coatings and Films
    • Materials Chemistry

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