Biomimetic Inspired Hydrogels for Regenerative Vertebral Body Stenting

PURPOSE OF REVIEW

This review aims to explore the potential of biomimetic hydrogels as an alternative to bone cement in vertebral body stenting (VBS), a minimally invasive treatment for vertebral compression fractures.

RECENT FINDINGS

The use of bone cement in VBS procedures can lead to complications such as incomplete fracture reduction and cement leakage. Biomimetic hydrogels have gained significant attention as potential biomaterial alternatives for VBS due to their unique properties, including tuneable therapeutic and mechanical properties. Over the past decade, there has been significant advancements in the development of biomimetic hydrogels for bone regeneration, employing a wide range of approaches to enhance the structural and functional properties of hydrogels. Biomimetic hydrogels hold significant promise as safer and reparative alternatives to bone cement for VBS procedures. However, further research and development in this field are necessary to explore the full potential of hydrogel-based systems for vertebral bone repair.

Alkaline earth metals for osteogenic scaffolds: From mechanisms to applications

Regeneration of bone defects is a significant challenge today. As alternative approaches to the autologous bone, scaffold materials have remarkable features in treating bone defects; however, the various properties of current scaffold materials still fall short of expectations. Due to the osteogenic capability of alkaline earth metals, their application in scaffold materials has become an effective approach to improving their properties. Furthermore, numerous studies have shown that combining alkaline earth metals leads to better osteogenic properties than applying them alone. In this review, the physicochemical and physiological characteristics of alkaline earth metals are introduced, mainly focusing on their mechanisms and applications in osteogenesis, especially magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Furthermore, this review highlights the possible cross-talk between pathways when alkaline earth metals are combined. Finally, some of the current drawbacks of scaffold materials are enumerated, such as the high corrosion rate of Mg scaffolds and defects in the mechanical properties of Ca scaffolds. Moreover, a brief perspective is also provided regarding future directions in this field. It is worth exploring that whether the levels of alkaline earth metals in newly regenerated bone differs from those in normal bone. The ideal ratio of each element in the bone tissue engineering scaffolds or the optimal concentration of each elemental ion in the created osteogenic environment still needs further exploration. The review not only summarizes the research developments in osteogenesis but also offers a direction for developing new scaffold materials.

Differentiation of committed osteoblast progenitors by octacalcium phosphate compared to calcium-deficient hydroxyapatite in Lepr-cre/Tomato mouse tibia

This study aimed to investigate the accumulation and differentiation of mesenchymal stem cells (MSCs) around octacalcium phosphate (OCP) compared with those around calcium-deficient hydroxyapatite (CDHA), a material obtained through hydrolysis of the original OCP. Leptin receptor (Lepr)-expressing bone marrow-derived MSCs around the OCP and CDHA were pursued utilizing genetically modified Lepr-cre/Tomato mice. OCP and CDHA granules were implanted into the tibia defect of the mice for 10 weeks and subjected to histomorphometric and immunohistochemical analyses. The structural properties of OCP and CDHA after inoculation into mouse subcutaneous tissue (until 4 weeks) or culture mediums (14 days) were analyzed using physicochemical techniques. In vitro osteoblastic differentiation of primary MSCs was examined with the materials for 14 days. While Lepr-cre/Tomato positive cells (red) accumulated around both OCP and CDHA, Lepr and osteocalcin double-positive osteoblastic cells (yellow) were significantly more abundant around OCP than around CDHA in the early implantation period. OCP enhanced the osteoblastic differentiation of MSCs more than CDHA in vitro. Physicochemical and structual analyses provided evidence that OCP tended to convert to the apatitic phase in the tested physiological environments. The higher osteoconductivity of OCP originated from a capacity-enhancing osteoblastic differentiation of committed osteoblast progenitors in bone marrow accompanied by OCP hydrolysis. STATEMENT OF SIGNIFICANCE: MSCs play a key role in bone regeneration through osteoblastic differentiation. Calcium phosphates have been widely applied as bone substitute materials, and OCP has a better ability to promote osteoblast differentiation of MSCs than that of HA in vitro. However, it is not clear how MSCs accumulate in the bone marrow and differentiate into osteoblasts during bone regeneration in vivo. In this study, we focused on the leptin receptor, a marker of bone marrow-derived MSCs. Using genetically modified mice labeled with the red fluorescent protein Tomato, we observed the accumulation of MSCs around calcium phosphates implanted in tibia bone defects and their differentiation into osteoblasts.

Involvement of distant octacalcium phosphate scaffolds in enhancing early differentiation of osteocytes during bone regeneration

This study hypothesized that distant octacalcium phosphate (OCP) scaffolds may enhance osteocyte differentiation in newly formed bone matrices. The results obtained were compared with those of Ca-deficient hydroxyapatite (OCP hydrolyzate, referred to as HL hereafter). Granular OCP and HL, 300-500 µm in diameter, were implanted in critical-sized rat calvarial defects for eight weeks and subjected to histology, immunohistochemistry, histomorphometry, and transmission electron microscopy (TEM). Early osteocyte differentiation from an osteoblastic cell line (IDG-SW3) was examined using materials without contacting the surfaces for 10 days. The material properties and the medium composition were analyzed through selected area electron diffraction (SAED) using TEM observation and curve fitting of Fourier transform infrared (FT-IR) spectroscopy. The number of positive cells of an osteocyte earlier differentiation marker podoplanin (PDPN) in bone matrices, along the direction of bone formation, was significantly higher in OCP than that in HL. The ultrastructure around the OCP surfaces observed by TEM showed the infiltration of some cells, including osteocytes adjacent to the OCP surface layers. The OCP structure remained unchanged by SAED analysis. Nanoparticle deposition and hydrolysis on OCP surfaces were detected by TEM and FT-IR, respectively, during early osteocyte differentiation in vitro. The medium saturation degree varied in accord with ionic dissolution, resulting in possible hydroxyapatite formation on OCP but not on HL. These results suggested that OCP stimulates early osteocyte differentiation in the bone matrix from a distance through its metastable chemical properties. STATEMENT OF SIGNIFICANCE: This study demonstrated that octacalcium phosphate (OCP) implanted in critical-sized rat calvaria bone defects is capable of enhancing the early differentiation of osteocytes embedded in newly formed bone matrices, even when the surface OCP is separated from the osteocytes. This prominent bioactive property of OCP was demonstrated by comparing the in vivo and in vitro performances with a control material, Ca-deficient hydroxyapatite (OCP hydrolyzate). The findings were elucidated by histomorphometry, which analyzed the differentiation of osteocytes along the parallel direction of new bone growth by osteoblasts. Therefore, OCP should stimulate osteocyte differentiation through ionic dissolution even in vivo owing to its metastable chemical properties, as previously reported in an in vitro study (Acta Biomater 69:362, 2018).

Enhanced Proliferation and Differentiation of Human Mesenchymal Stem Cell-laden Recycled Fish Gelatin/Strontium Substitution Calcium Silicate 3D Scaffolds

Cell-encapsulated bioscaffold is a promising and novel method to allow fabrication of live functional organs for tissue engineering and regenerative medicine. However, traditional fabrication methods of 3D scaffolds and cell-laden hydrogels still face many difficulties and challenges. This study uses a newer 3D fabrication technique and the concept of recycling of an unutilized resource to fabricate a novel scaffold for bone tissue engineering. In this study, fish-extracted gelatin was incorporated with bioactive ceramic for bone tissue engineering, and with this we successfully fabricated a novel fish gelatin methacrylate (FG) polymer hydrogel mixed with strontium-doped calcium silicate powder (FGSr) 3D scaffold via photo-crosslinking. Our results indicated that the tensile strength of FGSr was almost 2.5-fold higher as compared to FG thus making it a better candidate for future clinical applications. The in-vitro assays illustrated that the FGSr scaffolds showed good biocompatibility with human Wharton jelly-derived mesenchymal stem cells (WJMSC), as well as enhancing the osteogenesis differentiation of WJMSC. The WJMSC-laden FGSr 3D scaffolds expressed a higher degree of alkaline phosphatase activity than those on cell-laden FG 3D scaffolds and this result was further proven with the subsequent calcium deposition results. Therefore, these results showed that 3D-printed cell-laden FGSr scaffolds had enhanced mechanical property and osteogenic-related behavior that made for a more suitable candidate for future clinical applications.

Impact of copper on in-vitro biomineralization, drug release efficacy and antimicrobial properties of bioactive glasses

This study highlights the incorporation of copper in the bioactive glasses (BAG) network that greatly influences the morphological, structural and biological properties. By increasing the copper incorporation in BAG, increment in cell volume was obtained from XRD patterns, and concomitantly, dominant phosphate bands and latent silica bands were observed by FT-IR and Raman spectroscopic results. The Cu addition also affected particle appearance to vary from spherical to cluster-like cubes in 1.5% and 2.5% copper-doped BAG. Due to the mesoporous network 1.5% and 2.5% copper-doped BAG showed enhanced release of anti-inflammatory drugs such as Acetaminophen (ACE) and Ibuprofen (IBU) in which, the drug release profiles showed best fit with kinetic models of First order, Korsmeyar-Peppas and Higuchi. Copper doping influences the lattice of BAG, as a result morphology and porosity varied, which regulates the ionic dissolution, hence, prompting bioactivity was perceived from 1.5% and 2.5% copper-doped bioactive glasses (Cu-BGs). Moreover, 2.5% Cu-BG and 1.5% Cu-BG showed highest rate of ROS detection, as well as improved antimicrobial activity. This study established that up to certain proportion of copper incorporation in BAG network, potentially enhances the biomineralization and turns the morphology towards minimal size with mesoporous nature. Due to the abundance in oral microbial exposure, copper amplifies the superior antimicrobial properties, and Cu-BGs act as a drug carrier to load ACE and IBU, which potentially up-regulate the healing properties in dental application.

Biological evaluation of porous nanocomposite scaffolds based on strontium substituted β-TCP and bioactive glass: An in vitro and in vivo study

In the current study, in vitro analysis of the osteogenic potential of different scaffolds based on strontium-substituted β-TCP (Sr-TCP) and bioactive glass (BG) ceramics was conducted using rabbit bone marrow-derived mesenchymal stem cells (rBMSCs) and the osteogenic ability of the prepared Sr-TCP and BG scaffold was evaluated through alkaline phosphatase activity, mineral deposition by Alizarin red staining, and osteoblastic gene expression experiments. The obtained in vitro results revealed that among experimental Sr-TCP/BG nanocomposite scaffold samples with the composition of Sr-TCP/BG: 100/0, 50/50, 75/25, and 25/75, the 50Sr-TCP/50BG sample presented better osteoinductive properties. Therefore, the optimized 50Sr-TCP/50BG nanocomposite scaffold was chosen for further in vivo experiments. In vivo implantation of 50Sr-TCP/50BG scaffold and hydroxyapatite (HA)/TCP granules in a rabbit calvarial defect model showed slow degradation of 50Sr-TCP/50BG scaffold and high resorption rate of HA/TCP granules at 5 months' post-surgery. However, the 50Sr-TCP/50BG scaffolds loaded by mesenchymal stem cells (MSCs) were mainly replaced with new bone even at 2 months post-implantation. Based on the obtained engineering and biological results, 50Sr-TCP/50BG nanocomposite scaffold containing MSCs could be considered as a promising alternative substitute even for load-bearing bone tissue engineering applications.

Modulatory effect of simultaneously released magnesium, strontium, and silicon ions on injectable silk hydrogels for bone regeneration

Injectable silk hydrogels are ideal carriers of therapeutic agents due to their biocompatibility and low immunogenicity. Injectable silk hydrogels for bone regeneration have been previously developed but often utilize expensive biologics. In this study, we have developed an injectable silk composite incorporated with a triphasic ceramic called MSM-10 (54 Mg₂SiO₄, 36 Si₃Sr₅ and 10 MgO (wt%)) capable of simultaneously releasing magnesium, silicon, and strontium ions into its environment. These ions have been previously reported to possess therapeutic effects for bone regeneration. MSM-10 particles were incorporated into the silk hydrogels at various weight percentages [0.1 (SMH-0.1), 0.6 (SMH-0.6), 1 (SMH-1) and 2 (SMH-2)]. The effects of the released ions on the physicochemical and biological properties of the silk hydrogel were comprehensively evaluated. Increased MSM-10 loading was found to hinder the gelation kinetics of the silk hydrogel through the reduction of beta-sheet phase formation, which in turn affected the required sonication time for gelation, compressive strength, force of injection, microstructure and in vitro degradation rate. Primary human osteoblasts seeded on SMH-0.6 demonstrated increased proliferation and early alkaline phosphatase activity, as well as enhanced osteogenic gene expression compared to pure silk hydrogel and SMH-0.1. In vivo results in subcutaneous mouse models showed both decreased fibrous capsule formation and increased number of new blood vessels around the injected SMH-0.1 and SMH-0.6 implants compared to pure silk hydrogels. The results in this study indicate that the ions released from MSM-10 is able to influence the physicochemical and biological properties of silk hydrogels, and SMH-0.6 in particular shows promising properties for bone regeneration.

Strontium-doped calcium silicate bioceramic with enhanced in vitro osteogenic properties

Gehlenite (GLN, Ca₂SiAl₂O₇) is a bioceramic that has been recently shown to possess excellent mechanical strength and in vitro osteogenic properties for bone regeneration. Substitutional incorporation of strontium in place of calcium is an effective way to further enhance biological properties of calcium-based bioceramics and glasses. However, such strategy has the potential to affect other important physicochemical parameters such as strength and degradation due to differences in the ionic radius of strontium and calcium. This study is the first to investigate the effect of a range of concentrations of strontium substitution of calcium at 1, 2, 5, 10 mol% (S1-GLN, S2-GLN, S5-GLN and S10-GLN) on the physicochemical and biological properties of GLN. We showed that up to 2 mol% strontium ion substitution retains the monophasic GLN structure when sintered at 1450 °C, whereas higher concentrations resulted in presence of calcium silicate impurities. Increased strontium incorporation resulted in changes in grain morphology and reduced densification when the ceramics were sintered at 1450 °C. Porous GLN, S1-GLN and S2-GLN scaffolds (∼80% porosity) showed compressive strengths of 2.05 ± 0.46 MPa, 1.76 ± 0.79 MPa and 1.57 ± 0.52 MPa respectively. S1-GLN and S2-GLN immersed in simulated body fluid showed increased strontium ion release but reduced calcium and silicon ion release compared to GLN without affecting overall weight loss and pH over a 21 d period. The bioactivity of the S2-GLN ceramics was significantly improved as reflected in the significant upregulation of HOB proliferation and differentiation compared to GLN. Overall, these results suggest that increased incorporation of strontium presents a trade-off between bioactivity and mechanical strength for GLN bioceramics. This is an important consideration in the development of strontium-doped bioceramics.

The performance of bone tissue engineering scaffolds in in vivo animal models: A systematic review

Bone tissue engineering is an excellent alternative for the regeneration of large bone defects caused by trauma or bone pathologies. Scaffolds, stem cells, and bioactive molecules are the three key components of bone regeneration. Although a wide range of biomaterials of various compositions and structures has been proposed in the literature, these materials are rarely used in clinical applications. Therefore, more standardized studies are required to design scaffolds that enable better bone regeneration and are suitable for clinical use. The aim of this systematic review was to compare the performance of scaffolds used in preclinical animal studies to determine which class of materials has achieved a higher rate of bone neoformation (osteoinduction and osteoconduction). The selected studies were divided into three groups according to the following experimental models: studies that used subcutaneous models, bone defects in calvaria, and bone defects in long bones. Despite the large number of parameters in the included studies, we generally concluded that biomaterials containing calcium phosphates had important osteoinductive effects and were essential for better performance of the materials. Furthermore, natural polymers generally had better performance than synthetic polymers did, especially when the materials were associated with stem cells. The combination of materials from different classes was the most promising strategy for bone tissue regeneration.