Surface properties and ion release from fluoride-containing bioactive glasses promote osteoblast differentiation and mineralization in vitro

Sintered fluorapatite scaffolds as an autograft-like engineered bone graft

Hydroxyapatite (HA)-based materials are widely used as bone substitutes due to their inherent biocompatibility, osteoconductivity, and bio-absorption properties. However, HA scaffolds lack compressive strength when compared to autograft bone. It has been shown that the fluoridated form of HA, fluorapatite (FA), can be sintered to obtain this desired strength as well as slower degradation properties. Also, FA surfaces have been previously shown to promote stem cell differentiation toward an osteogenic lineage. Thus, it was hypothesized that FA, with and without stromal vascular fraction (SVF), would guide bone healing to an equal or better extent than the clinical gold standard. The regenerative potentials of these scaffolds were tested in 32 Lewis rats in a femoral condylar defect model with untreated (negative), isograft (positive), and commercial HA as controls. Animals were survived for 12 weeks post-implantation. A semi-quantitative micro-CT analysis was developed to quantify the percent new bone formation within the defects. Our model showed significantly higher (p < .05) new bone depositions in all apatite groups compared to the autograft group. Overall, the FA group had the most significant new bone deposition, while the differences between HA, FA, and FA + SVF were insignificant (p > .05). Histological observations supported the micro-CT findings and highlighted the presence of healthy bone tissues without interposing capsules or intense immune responses for FA groups. Most importantly, the regenerating bone tissue within the FA + SVF scaffolds resembled the architecture of the surrounding trabecular bone, showing intertrabecular spaces, while the FA group presented a denser cortical bone-like architecture. Also, a lower density of cells was observed near FA granules compared to HA surfaces, suggesting a reduced immune response. This first in vivo rat study supported the tested hypothesis, illustrating the utility of FA as a bone scaffold material.

Comparing the Healing Abilities of Fluorapatite and Hydroxyapatite Ceramics in Regenerating Bone Tissue: An In Vivo Study

Some reports in the literature show the advantages of fluoride-containing apatite ceramics over hydroxyapatite (HAP), at least in some aspects. While HAP has been used extensively in the treatment of bone defects, fluoridated apatite has hardly been tested in vivo. In order to verify the biological properties of fluoride-doped apatite and to assess its therapeutic potential, we synthesized fluorapatite (FAP) and applied it as a filling in bone defects of experimental animals (rabbits). The treatment effects were evaluated on extracted bones after 3 and 6 months from implantation using peripheral quantitative computed tomography (pQCT), dual-energy X-ray absorptiometry (DXA), radiography (X-ray) and histological staining. The study proved the integration between FAP and the bone tissue, thus indicating its stimulating effect on new bone formation and mineralization. The results achieved after 3 months of treatment were difficult to interpret unequivocally and suggested the transient delay in FAP integration of bone in comparison with HAP. The reasons for this phenomenon are unclear. Most likely, these differences between FAP and HAP resulted mainly from the different porosities, densities and ionic reactivity of the ceramics, which in our opinion affected their solubility, integration and degree of bone tissue resorption. However, it was shown that 6 months after implantation, similar level of bone defect regeneration was achieved for both FAP and HAP. In this article, we present our hypothesis concerning the basis of this phenomenon.

Analysis of In Vitro Leukocyte Responses to Biomaterials in the Presence of Antimicrobial Porcine Neutrophil Extract (AMPNE)

Implant insertion can evoke excessive inflammation which disrupts the healing process and potentially leads to complications such as implant rejection. Neutrophils and macrophages play a vital role in the early inflammatory phase of tissue repair, necessitating the study of cellular responses in host-implant interactions. In order to deepen the knowledge about these interactions, the response of neutrophils and macrophages to contact with selected biomaterials was examined in vitro on the basis of secretory response as well as reactive oxygen species/reactive nitrogen species (ROS/RNS) generation. Porcine neutrophils exposed to hydroxyapatite (HA) released more enzymes and generated higher levels of ROS/RNS compared to the control group. The addition of AMPNE diminished these responses. Although the results from porcine cells can provide valuable preliminary data, further validation using human cells or clinical studies would be necessary to fully extrapolate the findings to human medicine. Our study revealed that human neutrophils after contact of with HA increased the production of nitric oxide (NO) (10.00 ± 0.08 vs. control group 3.0 ± 0.11 µM, p < 0.05), while HAP or FAP did not elicit a significant response. Human macrophages cultured with HA produced more superoxide and NO, while HAP or FAP had a minimal effect, and curdlan reduced ROS/RNS generation. The addition of AMPNE to cultures with all biomaterials, except curdlan, reduced neutrophil activity, regardless of the peptides' origin. These results highlight the potential of antimicrobial peptides in modulating excessive biomaterial/host cell reactions involving neutrophils and macrophages, enhancing our understanding of immune reactions, and suggesting that AMPNE could regulate leukocyte response during implantation.

The Role of Trace Elements and Minerals in Osteoporosis: A Review of Epidemiological and Laboratory Findings

The objective of the present study was to review recent epidemiological and clinical data on the association between selected minerals and trace elements and osteoporosis, as well as to discuss the molecular mechanisms underlying these associations. We have performed a search in the PubMed-Medline and Google Scholar databases using the MeSH terms "osteoporosis", "osteogenesis", "osteoblast", "osteoclast", and "osteocyte" in association with the names of particular trace elements and minerals through 21 March 2023. The data demonstrate that physiological and nutritional levels of trace elements and minerals promote osteogenic differentiation through the up-regulation of BMP-2 and Wnt/β-catenin signaling, as well as other pathways. miRNA and epigenetic effects were also involved in the regulation of the osteogenic effects of trace minerals. The antiresorptive effect of trace elements and minerals was associated with the inhibition of osteoclastogenesis. At the same time, the effect of trace elements and minerals on bone health appeared to be dose-dependent with low doses promoting an osteogenic effect, whereas high doses exerted opposite effects which promoted bone resorption and impaired bone formation. Concomitant with the results of the laboratory studies, several clinical trials and epidemiological studies demonstrated that supplementation with Zn, Mg, F, and Sr may improve bone quality, thus inducing antiosteoporotic effects.

Hydroxyapatite or Fluorapatite—Which Bioceramic Is Better as a Base for the Production of Bone Scaffold?—A Comprehensive Comparative Study

Hydroxyapatite (HAP) is the most common calcium phosphate ceramic that is used in biomedical applications, e.g., as an inorganic component of bone scaffolds. Nevertheless, fluorapatite (FAP) has gained great attention in the area of bone tissue engineering in recent times. The aim of this study was a comprehensive comparative evaluation of the biomedical potential of fabricated HAP- and FAP-based bone scaffolds, to assess which bioceramic is better for regenerative medicine applications. It was demonstrated that both biomaterials had a macroporous microstructure, with interconnected porosity, and were prone to slow and gradual degradation in a physiological environment and in acidified conditions mimicking the osteoclast-mediated bone resorption process. Surprisingly, FAP-based biomaterial revealed a significantly higher degree of biodegradation than biomaterial containing HAP, which indicated its higher bioabsorbability. Importantly, the biomaterials showed a similar level of biocompatibility and osteoconductivity regardless of the bioceramic type. Both scaffolds had the ability to induce apatite formation on their surfaces, proving their bioactive property, that is crucial for good implant osseointegration. In turn, performed biological experiments showed that tested bone scaffolds were non-toxic and their surfaces promoted cell proliferation and osteogenic differentiation. Moreover, the biomaterials did not exert a stimulatory effect on immune cells, since they did not generate excessive amounts of reactive oxygen species (ROS) and reactive nitrogen species (RNS), indicating a low risk of inflammatory response after implantation. In conclusion, based on the obtained results, both FAP- and HAP-based scaffolds have an appropriate microstructure and high biocompatibility, being promising biomaterials for bone regeneration applications. However, FAP-based biomaterial has higher bioabsorbability than the HAP-based scaffold, which is a very important property from the clinical point of view, because it enables a progressive replacement of the bone scaffold with newly formed bone tissue.

Biodegradable Poly(D-L-lactide-co-glycolide) (PLGA)-Infiltrated Bioactive Glass (CAR12N) Scaffolds Maintain Mesenchymal Stem Cell Chondrogenesis for Cartilage Tissue Engineering

Regeneration of articular cartilage remains challenging. The aim of this study was to increase the stability of pure bioactive glass (BG) scaffolds by means of solvent phase polymer infiltration and to maintain cell adherence on the glass struts. Therefore, BG scaffolds either pure or enhanced with three different amounts of poly(D-L-lactide-co-glycolide) (PLGA) were characterized in detail. Scaffolds were seeded with primary porcine articular chondrocytes (pACs) and human mesenchymal stem cells (hMSCs) in a dynamic long-term culture (35 days). Light microscopy evaluations showed that PLGA was detectable in every region of the scaffold. Porosity was greater than 70%. The biomechanical stability was increased by polymer infiltration. PLGA infiltration did not result in a decrease in viability of both cell types, but increased DNA and sulfated glycosaminoglycan (sGAG) contents of hMSCs-colonized scaffolds. Successful chondrogenesis of hMSC-colonized scaffolds was demonstrated by immunocytochemical staining of collagen type II, cartilage proteoglycans and the transcription factor SOX9. PLGA-infiltrated scaffolds showed a higher relative expression of cartilage related genes not only of pAC-, but also of hMSC-colonized scaffolds in comparison to the pure BG. Based on the novel data, our recommendation is BG scaffolds with single infiltrated PLGA for cartilage tissue engineering.

Nano-imaging confirms improved apatite precipitation for high phosphate/silicate ratio bioactive glasses

Bioactive glasses convert to a biomimetic apatite when in contact with physiological solutions; however, the number and type of phases precipitating depends on glass composition and reactivity. This process is typically followed by X-ray diffraction and infrared spectroscopy. Here, we visualise surface mineralisation in a series of sodium-free bioactive glasses, using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) and X-ray nano-computed tomography (nano-CT). In the glasses, the phosphate content was increased while adding stoichiometric amounts of calcium to maintain phosphate in an orthophosphate environment in the glass. Calcium fluoride was added to keep the melting temperature low. TEM brought to light the presence of phosphate clustering and nearly crystalline calcium fluoride environments in the glasses. A combination of analytical methods, including solid-state NMR, shows how with increasing phosphate content in the glass, precipitation of calcium fluoride during immersion is superseded by fluorapatite precipitation. Nano-CT gives insight into bioactive glass particle morphology after immersion, while TEM illustrates how compositional changes in the glass affect microstructure at a sub-micron to nanometre-level.

Highly Porous Fluorapatite/β-1,3-Glucan Composite for Bone Tissue Regeneration: Characterization and In-Vitro Assessment of Biomedical Potential

A novel fluorapatite/glucan composite (“FAP/glucan”) was developed for the treatment of bone defects. Due to the presence of polysaccharide polymer (β-1,3-glucan), the composite is highly flexible and thus very convenient for surgery. Its physicochemical and microstructural properties were evaluated using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), mercury intrusion, mechanical testing and compared with the reference material, which was a hydroxyapatite/glucan composite (“HAP/glucan”) with hydroxyapatite granules (HAP) instead of FAP. It was found that FAP/glucan has a higher density and lower porosity than the reference material. The correlation between the Young’s modulus and the compressive strength between the materials is different in a dry and wet state. Bioactivity assessment showed a lower ability to form apatite and lower uptake of apatite-forming ions from the simulated body fluid by FAP/glucan material in comparison to the reference material. Moreover, FAP/glucan was determined to be of optimal fluoride release capacity for osteoblasts growth requirements. The results of cell culture experiments showed that fluoride-containing biomaterial was non-toxic, enhanced the synthesis of osteocalcin and stimulated the adhesion of osteogenic cells.

Cobalt-containing bioactive glass mimics vascular endothelial growth factor A and hypoxia inducible factor 1 function

Bioactive glasses (BGs) have shown great potential for tissue regeneration and their composition flexibility allows the incorporation of different ions with physiological activities and therapeutic properties in the glass network. Among the many ions that could be incorporated, cobalt (Co) is a significant one, as it mimics hypoxia, triggering the formation of new blood vessels by the vascular endothelial growth factor A (VEGFA), due to the stabilizing effect on the hypoxia inducible factor 1 subunit alpha (HIF1A), an activator of angiogenesis-related genes, and is therefore of great interest for tissue engineering applications. However, despite its promising properties, the effects of glasses incorporated with Co on angiogenesis, through human umbilical cord vein endothelial cells (HUVECs) studies, need to be further investigated. Therefore, this work aimed to evaluate the biocompatibility and angiogenic potential of a new sol-gel BG, derived from the SiO2 -CaO-P2 O5 -CoO system. The structural evaluation showed the predominance of an amorphous glass structure, and the homogeneous presence of cobalt in the samples was confirmed. in vitro experiments showed that Co-containing glasses did not affect the viability of HUVECs, stimulated the formation of tubes and the gene expression of HIF1A and VEGFA. in vivo experiments showed that Co-containing glasses stimulated VEGFA and HIF1A expression in blood vessels and cell nuclei, respectively, in the deep dermis layer of the dorsal region of rats, featuring considerable local stimulation of the angiogenesis process due to Co-release. Co-containing glasses showed therapeutic effect, and Co incorporation is a promising strategy for obtaining materials with superior angiogenesis properties for tissue engineering applications.

Buffering capacity and antibacterial properties among bioactive glass-containing orthodontic adhesives

This study was to evaluate the acid-buffering capacity and antibacterial properties of orthodontic adhesives containing bioactive glasses (BAGs) (45S5, 45S5F, S53P4), Hydroxyapatite, beta-tricalcium phosphate, and Canasite. Fillers comprising 15 wt% bioactive glasses, HAp, β-TCP, and Canasite incorporated with 55 wt% silanated glass were added to a mixture of UDMA/TEGDMA. Acid-buffering capacity was tested by exposing disc-shaped samples of each adhesive to medium of bacteria-produced acids, and pH changes were recorded at 24 and 48 h. Antibacterial properties were assessed by indirect testing by exposing polymerized adhesive samples to a medium and direct testing by immersing the specimens in solutions containing S. mutans and S. sanguinis. A significant buffering capacity was shown by the 45S5, 45S5F and S53P4 BAG adhesives. The antibacterial properties were not significant in all experimental adhesives. Therefore, the experimental orthodontic adhesives containing BAGs demonstrated a significant buffering capacity but did not show significant antibacterial properties against S. mutans and S. sanguinis.

Ultrafast fluorescent probe with near-infrared analytical wavelength for fluoride ion detection in real samples

The first ultrafast fluorescence probe with response time in seconds (10 s) for fluoride ions (F^-) has been proposed by conjugating dimethylthiophosphoryl group as a recognition unit with the near-infrared fluorophore of hemicyanine. The response mechanism is the F^--induced cleavage of the dimethylthiophosphoryl group, along with the liberation of the fluorophore, which results in a distinctly enhanced fluorescence intensity at 730 nm (λ_ex = 680 nm). The fluorescence enhancement of the probe is directly proportional to the F^- concentration in the range of 10-300 µM with the detection limit of 4.28 µM. The probe has been successfully used to determine F^- concentration in real water and toothpaste samples as well as image F^- in living cells. The simplicity and quick response of this probe endow it with the ability of detecting F^- rapidly in real samples.

A simple hydrogel scaffold with injectability, adhesivity and osteogenic activity for bone regeneration

A simple hydrogel scaffold with injectability, adhesivity and osteogenic activity is facilely prepared by directly mixing strontium chloride and Alg-DA aqueous solutions.

Sustained Calcium(II)-Release to Impart Bioactivity in Hybrid Glass Scaffolds for Bone Tissue Engineering

In this study, we integrated different calcium sources into sol-gel hybrid glass scaffolds with the aim of producing implants with long-lasting calcium release while maintaining mechanical strength of the implant. Calcium(II)-release was used to introduce bioactivity to the material and eventually support implant integration into a bone tissue defect. Tetraethyl orthosilicate (TEOS) derived silica sols were cross-linked with an ethoxysilylated 4-armed macromer, pentaerythritol ethoxylate and processed into macroporous scaffolds with defined pore structure by indirect rapid prototyping. Triethyl phosphate (TEP) was shown to function as silica sol solvent. In a first approach, we investigated the integration of 1 to 10% CaCl2 in order to test the hypothesis that small CaCl2 amounts can be physically entrapped and slowly released from hybrid glass scaffolds. With 5 and 10% CaCl2 we observed an extensive burst release, whereas slightly improved release profiles were found for lower Calcium(II) contents. In contrast, introduction of melt-derived bioactive 45S5 glass microparticles (BG-MP) into the hybrid glass scaffolds as another Calcium(II) source led to an approximately linear release of Calcium(II) in Tris(hydroxymethyl)aminomethane (TRIS) buffer over 12 weeks. pH increase caused by BG-MP could be controlled by their amount integrated into the scaffolds. Compression strength remained unchanged compared to scaffolds without BG-MP. In cell culture medium as well as in simulated body fluid, we observed a rapid formation of a carbonated hydroxyapatite layer on BG-MP containing scaffolds. However, this mineral layer consumed the released Calcium(II) ions and prevented an additional increase in Calcium(II) concentration in the cell culture medium. Cell culture studies on the different scaffolds with osteoblast-like SaOS-2 cells as well as bone marrow derived mesenchymal stem cells (hMSC) did not show any advantages concerning osteogenic differentiation due to the integration of BG-MP into the scaffolds. Nonetheless, via the formation of a hydroxyapatite layer and the ability to control the pH increase, we speculate that implant integration in vivo and bone regeneration may benefit from this concept.

Influence of low amounts of zinc or magnesium substitution on ion release and apatite formation of Bioglass 45S5

Magnesium and zinc ions play various key roles in the human body, being involved, among others, in skeletal development and wound healing. Zinc is also known to have antimicrobial properties. While low concentrations can stimulate cells in vitro, high concentrations of magnesium or zinc introduced into bioactive glasses significantly reduce glass degradation and ion release and inhibit apatite precipitation. On the other hand, magnesium and zinc ions improve the high temperature processing of bioactive glasses, even when present at low concentrations only. Results here show that by substituting small amounts of Mg or Zn for Ca, ion release remains high enough to allow for apatite precipitation. In addition, magnesium and zinc containing bioactive glasses are shown to be very susceptible to changes in particle size and relative surface area. For a given magnesium or zinc content in the glass, ion release and apatite formation can be enhanced dramatically by reducing the particle size, reaching comparable levels as Bioglass 45S5 of the same particle size range. Taken together, these findings suggest that when introducing these ions into bioactive glasses, ideally low Mg or Zn for Ca substitution as well as small particle sizes are used. This way, bioactive glasses combining good high temperature processing with fast ion release and apatite precipitation can be obtained, providing the potential additional benefit of releasing magnesium or zinc ions in therapeutic concentrations.

Fluorapatite ceramics for bone tissue regeneration: Synthesis, characterization and assessment of biomedical potential

Calcium phosphates, due to their similarity to the inorganic fraction of mineralized tissues, are of great importance in treatment of bone defects. In order to improve the biological activity of hydroxyapatite (HAP), its fluoride-substituted modification (FAP) was synthesized using the sol-gel method and calcined at three different temperatures in the range of 800-1200 °C. Physicochemical and biological properties were evaluated to indicate which material would support bone regeneration the best. X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry (EDS), and Fourier transform infrared spectroscopy (FTIR) revealed that fluoride ions were incorporated into the apatite lattice structure. In studies it was found that fluorapatite sintered at the highest temperature had the lowest porosity, no internal pores and the highest density. In vitro ion reactivity assessments showed that during the 28-day immersion of the samples in the simulated body fluid, the uptake of calcium and phosphorus ions was inversely correlated to the calcination temperature. All tested materials were non-toxic since the cytotoxicity MTT assay demonstrated that the viability of preosteoblast cells incubated with sample extracts was high. Fluorapatite sintered at 800 °C was determined to be of optimal porosity and fluoride release capacity and then used in cell proliferation studies. The results showed that it significantly shortened the doubling time and thus enhanced the proliferation of osteogenic cells, as compared to the fluoride solutions and control group. Therefore, this material is proposed for the use in orthopedic applications and bone tissue engineering.

Characterization of the bioactivity of two commercial composites

The aim of this study was to characterize the ion release, pH changes and apatite formation ability of two potentially bioactive composites Cention N (CN) and Activa (ACT). Ion release and apatite formation was investigated in three different immersion media: Tris buffer pH 7.3 (TB), Artificial Saliva pH 4 (AS4) and Artificial Saliva pH 7 (AS7) in order to mimic the conditions present in the mouth. Fluoride release was followed using an ion selective electrode, whilst all other ions were determined by inductively coupled plasma optical emission spectroscopy. Apatite formation was followed by FTIR and XRD. SEM was used to follow glass degradation and apatite formation on both polished cross-sections and surfaces of the composites. ACT released very few ions including fluoride upon immersion in TB and AS7, but released more ions including significant quantities of Al in AS4. This would suggest the glasses in ACT are acid degradable fluoro-alumino-silicate glasses similar to the glasses used in glass ionomer cements. There was no evidence of any apatite formation with ACT. CN released more ions in TB and AS7 than ACT and formed an apatite like phase in AS7. The calcium fluoro-silicate glass in CN was observed to degrade significantly in AS4. CN has bioactive properties that may explain the low incidence of secondary caries found clinically with this composite.

Calcium carbonate: Adored and ignored in bioactivity assessment

The title of this article could sound a bit curious to some readers since a layer of apatite - and not calcium carbonate - is well-known to form on the surface of bioactive glasses upon immersion in simulated body fluids. However, calcium carbonate (commonly reported as calcite crystals) can form on the surface of bioactive glasses as well, instead of or in competition with hydroxyapatite, during in vitro tests. Major factors that govern calcium carbonate formation are a high concentration of Ca²⁺ ions in the testing solution - and, in this regard, glass composition/texture and type of medium play key roles - along with the volume of solution used during in vitro tests. To date, this phenomenon has received relatively little attention and is still partly unexplored. This article provides a critical overview of the available literature on this topic in order to stimulate constructive discussion among biomaterials scientists and further research for better understanding the mechanisms involved in glass bioactivity. STATEMENT OF SIGNIFICANCE: A literature search indicates that a layer of apatite - and not calcium carbonate - is well known to form on the surface of biomaterials during the bioactivity assessment. However, calcium carbonate can form on the surface as well, instead of or in competition with apatite. To date, this phenomenon has received relatively little attention and is still partly unexplored. This review provides a critical overview of the available literature on this topic in order to stimulate constructive discussions that can be further useful for clinical success.

A comparison of lithium-substituted phosphate and borate bioactive glasses for mineralised tissue repair

OBJECTIVES

Wnt/β-catenin signalling plays important roles in regeneration, particularly in hard tissues such as bone and teeth, and can be regulated by small molecule antagonists of glycogen synthase kinase 3 (GSK3); however, small molecules can be difficult to deliver clinically. Lithium (Li) is also a GSK3 antagonist and can be incorporated into bioactive glasses (BG), which can be used clinically in dental and bone repair applications and tuned to quickly release their constituent ions.

METHODS

Here, we created phosphate (P)- and borate (B)-based BG that also contained Li (LiPBG and LiBBG) and examined their ion release kinetics and the toxicity of their dissolution ions on mouse 17IA4 dental pulp cells.

RESULTS

We found that although LiPBG and LiBBG can both quickly release Li at concentrations known to regulate Wnt/β-catenin signalling, the P and B ions they concomitantly release are highly toxic to cells. Only when relatively low concentrations of LiPBG and LiBBG were placed in cell culture medium were their dissolution products non-toxic. However, at these concentrations, LiPBG and LiBBG's ability to regulate Wnt/β-catenin signalling was limited.

SIGNIFICANCE

These data suggest that identifying a BG composition that can both quickly deliver high concentrations of Li and is non-toxic remains a challenge.

Fluorine-contained hydroxyapatite suppresses bone resorption through inhibiting osteoclasts differentiation and function in vitro and in vivo

Objectives

Fluorine, an organic trace element, has been shown to unfavourably effect osteoclasts function at a low dose. Use of hydroxyapatite (HA) has been effective in exploring its roles in promoting bone repair. In this study, we used HA modified with fluorine to investigate whether it could influence osteoclastic activity in vitro and ovariectomy‐induced osteoclasts hyperfunction in vivo.

Materials and methods

Fluorohydroxyapatite (FHA) was obtained and characterized by scanning electron microscope (SEM). Osteoclasts proliferation and apoptosis treated with FHA were assessed by MTT and TUNEL assay. SEM, F‐actin, TRAP activity and bone resorption experiment were performed to determine the influence of FHA on osteoclasts differentiation and function. Moreover, HA and FHA were implanted into ovariectomized osteoporotic and sham surgery rats. Histology and Micro‐CT were examined for further verification.

Results

Fluorine released from FHA slowly and sustainably. FHA hampered osteoclasts proliferation, promoted osteoclasts apoptosis, suppressed osteoclasts differentiation and function. Experiments in vivo validated that FHA participation brought about an inhibitory effect on osteoclasts hyperfunction and less bone absorption.

Conclusion

The results indicated that FHA served as an efficient regulator to attenuate osteoclasts formation and function and was proposed as a candidature for bone tissue engineering applications.

Effect of Fluoride Doping in Laponite Nanoplatelets on Osteogenic Differentiation of Human Dental Follicle Stem Cells (hDFSCs)

Bioactive nanosilicates are emerging prominent next generation biomaterials due to their intrinsic functional properties such as advanced biochemical and biophysical cues. Recent studies show interesting dose-dependent effect of fluoride ions on the stem cells. Despite of interesting properties of fluoride ions as well as nanosilicate, there is no reported literature on the effect of fluoride-doped nanosilicates on stem cells. We have systematically evaluated the interaction of fluoride nanosilicate platelets (NS + F) with human dental follicle stem cells (hDFSCs) to probe the cytotoxicity, cellular transport (internalization) and osteogenic differentiation capabilities in comparison with already reported nanosilicate platelets without fluoride (NS − F). To understand the osteoinductive and osteoconductive properties of the nanosilicate system, nanosilicate treated hDFSCs are cultured in three different medium namely normal growth medium, osteoconductive medium, and osteoinductive medium up to 21 d. NS + F treated stem cells show higher ALP activity, osteopontin levels and significant alizarin red staining compared to NS − F treated cells. This study highlights that the particles having fluoride additives (NS + F) aid in enhancing the osteogenic differentiation capabilities of hDFSCs thus potential nanobiomaterial for periodontal bone tissue regeneration.

F-doped TiO2 microporous coating on titanium with enhanced antibacterial and osteogenic activities

To enhance bacterial resistance and osteogenesis of titanium (Ti) -based implants, TiO2/calcium-phosphate coatings (TiCP) doped with various amounts of fluorine (F) (designated as TiCP-F1, TiCP-F6, and TiCP-F9) were prepared on Ti by micro-arc oxidation. The F doped TiCP coatings possess a microporous structure (pore size of 3-4 μm in average diameter) which is evenly covered by nano-grains of 30-60 nm in size. Successful F incorporation into TiCP was determined by X-ray photoelectron spectroscopy, and it shows weak influence on the microstructure, phase compositions, surface roughness and wettability of TiCP. All the coatings bonded firmly to the Ti substrates and showed enduring high adhesion strength in biological circumstances. The bacterial resistance and osteogenesis of the coatings were evaluated by implanting testing materials in vitro and in an infected rabbit model caused by bacteria. Both the in vitro and in vivo results indicated that TiCP and TiCP-F1 were of much higher osteogenic activity compared with Ti but lacking of bacterial resistance, whereas TiCP with high F addition (TiCP-F6 and TiCP-F9) exhibited both dramatically improved bacterial resistance and osteogenesis. In summary, TiCP-F6 possessed the best antibacterial and osteogenic activities, especially exhibited excellent osseointegration efficacy in the infected rabbit model.

Marine Collagen/Apatite Composite Scaffolds Envisaging Hard Tissue Applications

The high prevalence of bone defects has become a worldwide problem. Despite the significant amount of research on the subject, the available therapeutic solutions lack efficiency. Autografts, the most commonly used approaches to treat bone defects, have limitations such as donor site morbidity, pain and lack of donor site. Marine resources emerge as an attractive alternative to extract bioactive compounds for further use in bone tissue-engineering approaches. On one hand they can be isolated from by-products, at low cost, creating value from products that are considered waste for the fish transformation industry. One the other hand, religious constraints will be avoided. We isolated two marine origin materials, collagen from shark skin (Prionace glauca) and calcium phosphates from the teeth of two different shark species (Prionace glauca and Isurus oxyrinchus), and further proposed to mix them to produce 3D composite structures for hard tissue applications. Two crosslinking agents, 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride/N-Hydroxysuccinimide (EDC/NHS) and hexamethylene diisocyanate (HMDI), were tested to enhance the scaffolds' properties, with EDC/NHS resulting in better properties. The characterization of the structures showed that the developed composites could support attachment and proliferation of osteoblast-like cells. A promising scaffold for the engineering of bone tissue is thus proposed, based on a strategy of marine by-products valorisation.

High chloride content calcium silicate glasses

Chloride is known to volatilize from silicate glass melts and until now, only a limited number of studies on oxychloride silicate glasses have been reported. In this paper we have synthesized silicate glasses that retain large amounts of CaCl₂. The CaCl₂ has been added to the calcium metasilicate composition (CaO·SiO₂). Glasses were produced via a melt quench route and an average of 70% of the chloride was retained after melting. Up to 31.6 mol% CaCl₂ has been successfully incorporated into these silicate glasses without the occurrence of crystallization. ²⁹Si MAS-NMR spectra showed the silicon being present mainly as a Q² silicate species. This suggests that chloride formed Cl-Ca(n) species, rather than Si-Cl bonds. Upon increasing the CaCl₂ content, the T_g reduced markedly from 782 °C to 370 °C. Glass density and glass crystallization temperature decreased linearly with an increase in the CaCl₂ content. However, both linear regressions revealed a breakpoint at a CaCl₂ content just below 20 mol%. This might be attributed to a significant change in the structure and is also correlated with the nature of the crystallizing phases formed upon heat treatment. The glasses with less than 19.2 mol% CaCl₂ crystallized to wollastonite, whilst the compositions with CaCl₂ content equal to or greater than 19.2 mol% are thought to crystallize to CaCl₂. In practice, the crystallization of CaCl₂ could not occur until the crystallization temperature fell below the melting point of CaCl₂. The implications of the results along with the high chloride retention are discussed.

Long term effects of bioactive glass particulates on dental pulp stem cells in vitro

Bioactive glasses (BG) are known for their ability to induce bone formation by the action of their dissolution products. Glasses can deliver active ions at a sustained rate, determined by their composition and surface area. Nanoporous sol-gel derived BGs can biodegrade rapidly, which can lead to a detrimental burst release of ions and a pHrise. The addition of phosphate into the glass can buffer the pH during dissolution. Here, dissolution of BG with composition 60 mol% SiO

Regenerating bone with bioactive glass scaffolds: A review of in vivo studies in bone defect models

Large bone defects resulting from fractures and disease are a medical concern, being often unable to heal spontaneously by the body's repair mechanisms. Bone tissue engineering (BTE) is a promising approach for treating bone defects through providing a template to guide osseous regeneration. 3D scaffolds with microstructure mimicking host bone are necessary in common BTE strategies. Bioactive glasses (BGs) attract researchers' attention as BTE scaffolds as they are osteoconductive and osteoinductive in certain formulations. In vivo animal models allow understanding and evaluation of materials' performance in the complex physiological environment, being an inevitable step before clinical trials. The aim of this paper is to review for the first time published research investigating the in vivo osseous regenerative capacity of 3D BG scaffolds in bone defect animal models, to better understand and evaluate the progress and future outlook of the use of such scaffolds in BTE. The literature analysis reveals that the regenerative capacity of BG scaffolds depends on several factors; including BG composition, fabrication method, scaffold microstructure and pore characteristics, in addition to scaffold pretreatment and whether or not the scaffolds are loaded with growth factors. In addition, animal species selected, defect size and implantation time affect the scaffold in vivo behavior and outcomes. The review of the literature also makes clear the difficulty encountered to compare different types of bioactive glass scaffolds in their bone forming ability. Even considering such limitations of the current state-of-the-art, results generated from animal bone defect models provide an essential source of information to guide the design of BG scaffolds in future.

STATEMENT OF SIGNIFICANCE

Bioactive glasses are at the centre of increasing research efforts in bone tissue engineering as the number of research groups around the world carrying out research on this type of biomaterials continues to increase. However, there are no previous reviews in literature which specifically cover investigations of the performance of bioactive glass scaffolds in bone defect animal models. This is the topic of the present review, in which we have analysed comprehensively all available literature in the field. The review thus fills a gap in the biomaterials literature providing a broad platform of information for researchers interested in bioactive glasses in general and specifically in the outcomes of in vivo models. Bioactive glass scaffolds of different compositions tested in relevant bone defect models are covered.

Optimisation of lithium-substituted bioactive glasses to tailor cell response for hard tissue repair

Bioactive glasses (BG) are used clinically because they can both bond to hard tissue and release therapeutic ions that can stimulate nearby cells. Lithium has been shown to regulate the Wnt/β-catenin cell signalling pathway, which plays important roles in the formation and repair of bone and teeth. Lithium-releasing BG, therefore, have the potential to locally regulate hard tissue formation; however, their design must be tailored to induce an appropriate biological response. Here, we optimised the release of lithium from lithium-substituted BG by varying BG composition, particle size and concentration to minimise toxicity and maximise upregulation of the Wnt target gene Axin2 in in vitro cell cultures. Our results show that we can tailor lithium release from BG over a wide therapeutic and non-toxic range. Increasing the concentration of BG in cell culture medium can induce toxicity, likely due to modulations in pH. Nevertheless, at sub-toxic concentrations, lithium released from BG can upregulate the Wnt pathway in 17IA4 cells, similarly to treatment with LiCl. Taken together, these data demonstrate that ion release from lithium-substituted BG can be tailored to maximise biological response. These data may be important in the design of BG that can regulate the Wnt/β-catenin pathway to promote hard tissue repair or regeneration.

Structural analysis of fluorine-containing bioactive glass nanoparticles synthesized by sol-gel route assisted by ultrasound energy

In the last decades, studies about the specific effects of bioactive glass on remineralization of dentin were the focus of attention, due to their excellent regenerative properties in mineralized tissues. The incorporation of Fluorine in bioactive glass nanoparticles may result in the formation of fluorapatite (FAP), which is chemically more stable than hydroxyapatite or carbonated hydroxyapatite, and therefore is of interest for dental applications. The aim of this study was to synthesize and characterize a new system of Fluorine-containing bioactive glass nanoparticles (BGNPF). A sol-gel route assisted by ultrasound was used for the synthesis of BGNPF. The particles obtained were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD), dynamic light scattering (DLS), nitrogen adsorption, and X-ray photoelectron spectroscopy (XPS). SEM micrographs showed that the particles are quite uniform spherical nanostructures, occurring agglomeration or partial sinterization of the particulate system after heat treatment. XRD and XPS analysis results suggest the formation of fluorapatite crystals embedded within the matrix of the bioactive glass nanoparticles. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 360-366, 2018.

Mimicking physiological flow conditions to study alterations of bioactive glass surfaces in vitro

Bioactive glasses form a strong bond with surrounding tissue and slowly degrade when implanted in vivo, stimulating the host bone to regenerate itself. We investigated the behaviour of microstructured bioactive glass surfaces (13-93) in an SBF reactor, which mimics physiological flow conditions. The structures were developed to potentially influence cell-biological long term processes such as osteogenic differentiation. It is therefore important that the structures withstand a certain time in SBF or body fluids. The experiments revealed that these structures were preserved up to 30 days. Although macroscopically stable, mass loss under flowing conditions was 2-2.5%, in contrast to <1% under static conditions. Polished samples in flowing medium lost 2.7% up to day 7 and then regained mass, resulting in overall 0.5% mass loss after 30 days. Thicker calcium phosphate rich layers for the samples in flowing medium were detected, demonstrating better bone bonding capacity than predicted conventionally. The hydroxyapatite conversion in the reactor was comparable to published in vivo data. We conclude that surface alterations that occur in vivo can be better mimicked by using the proposed flow bioreactor than by the established SBF method in static medium. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 228-236, 2018.

Nano-layered magnesium fluoride reservoirs on biomaterial surfaces strengthen polymorphonuclear leukocyte resistance to bacterial pathogens

Biomaterial-related bacterial infections cause patient suffering, mortality and extended periods of hospitalization, imposing a substantial burden on medical systems. In this context, understanding of nanomaterials-bacteria-cells interactions is of both fundamental and clinical significance. Herein, nano-MgF₂ films were deposited on titanium substrate via magnetron sputtering. Using this platform, the antibacterial behavior and mechanism of the nano-MgF₂ films were investigated in vitro and in vivo. It was found that, for S. aureus (CA-MRSA, USA300) and S. epidermidis (RP62A), the nano-MgF₂ films possessed excellent anti-biofilm activity, but poor anti-planktonic bacteria activity in vitro. Nevertheless, both the traditional SD rat osteomyelitis model and the novel stably luminescent mouse infection model demonstrated that nano-MgF₂ films exerted superior anti-infection effect in vivo, which cannot be completely explained by the antibacterial activity of the nanomaterial itself. Further, using polymorphonuclear leukocytes (PMNs), the critical immune cells of innate immunity, a complementary investigation of MgF₂-bacteria-PMNs co-culturing revealed that the nano-MgF₂ films improved the antibacterial effect of PMNs through enhancing their phagocytosis and stability. To our knowledge, this is the first time of exploring the antimicrobial mechanism of nano-MgF₂ from the perspective of innate immunity both in vitro and in vivo. Based on the research results, a plausible mechanism is put forward for the predominant antibacterial effect of nano-MgF₂in vivo, which may originate from the indirect immune enhancement effect of nano-MgF₂ films. In summary, this study of surface antibacterial design using MgF₂ nanolayer is a meaningful attempt, which can promote the host innate immune response to bacterial pathogens. This may give us a new understanding towards the antibacterial behavior and mechanism of nano-MgF₂ films and pave the way towards their clinical applications.

Via precise interface engineering towards bioinspired composites with improved 3D printing processability and mechanical properties

Precise interface engineering in inorganic–organic hybrid materials enhances both the elastic moduli and toughness of a biodegradable composite, which is of relevance for load-bearing applications in bone tissue engineering.

Cell studies of hybridized carbon nanofibers containing bioactive glass nanoparticles using bone mesenchymal stromal cells

Bone regeneration required suitable scaffolding materials to support the proliferation and osteogenic differentiation of bone-related cells. In this study, a kind of hybridized nanofibrous scaffold material (CNF/BG) was prepared by incorporating bioactive glass (BG) nanoparticles into carbon nanofibers (CNF) via the combination of BG sol-gel and polyacrylonitrile (PAN) electrospinning, followed by carbonization. Three types (49 s, 68 s and 86 s) of BG nanoparticles were incorporated. To understand the mechanism of CNF/BG hybrids exerting osteogenic effects, bone marrow mesenchymal stromal cells (BMSCs) were cultured directly on these hybrids (contact culture) or cultured in transwell chambers in the presence of these materials (non-contact culture). The contributions of ion release and contact effect on cell proliferation and osteogenic differentiation were able to be correlated. It was found that the ionic dissolution products had limited effect on cell proliferation, while they were able to enhance osteogenic differentiation of BMSCs in comparison with pure CNF. Differently, the proliferation and osteogenic differentiation were both significantly promoted in the contact culture. In both cases, CNF/BG(68 s) showed the strongest ability in influencing cell behaviors due to its fastest release rate of soluble silicium-relating ions. The synergistic effect of CNF and BG would make CNF/BG hybrids promising substrates for bone repairing.

In vitro and in vivo evaluation of MgF2 coated AZ31 magnesium alloy porous scaffolds for bone regeneration

Porous magnesium scaffolds are attracting increasing attention because of their degradability and good mechanical property. In this work, a porous and degradable AZ31 magnesium alloy scaffold was fabricated using laser perforation technique. To enhance the corrosion resistance and cytocompatibility of the AZ31 scaffolds, a fluoride treatment was used to acquire the MgF₂ coating. Enhanced corrosion resistance was confirmed by immersion and electrochemical tests. Due to the protection provided by the MgF₂ coating, the magnesium release and pH increase resulting from the degradation of the FAZ31 scaffolds were controllable. Moreover, in vitro studies revealed that the MgF₂ coated AZ31 (FAZ31) scaffolds enhanced the proliferation and attachment of rat bone marrow stromal cells (rBMSCs) compared with the AZ31 scaffolds. In addition, our present data indicated that the extract of the FAZ31 scaffold could enhance the osteogenic differentiation of rBMSCs. To compare the in vivo bone regenerative capacity of the AZ31 and FAZ31 scaffolds, a rabbit femoral condyle defect model was used. Micro-computed tomography (micro-CT) and histological examination were performed to evaluate the degradation of the scaffolds and bone volume changes. In addition to the enhanced the corrosion resistance, the FAZ31 scaffolds were more biocompatible and induced significantly more new bone formation in vivo. Conversely, bone resorption was observed from the AZ31 scaffolds. These promising results suggest potential clinical applications of the fluoride pretreated AZ31 scaffold for bone tissue repair and regeneration.

A novel open-porous magnesium scaffold with controllable microstructures and properties for bone regeneration

The traditional production methods of porous magnesium scaffolds are difficult to accurately control the pore morphologies and simultaneously obtain appropriate mechanical properties. In this work, two open-porous magnesium scaffolds with different pore size but in the nearly same porosity are successfully fabricated with high-purity Mg ingots through the titanium wire space holder (TWSH) method. The porosity and pore size can be easily, precisely and individually controlled, as well as the mechanical properties also can be regulated to be within the range of human cancellous bone by changing the orientation of pores without sacrifice the requisite porous structures. In vitro cell tests indicate that the scaffolds have good cytocompatibility and osteoblastic differentiation properties. In vivo findings demonstrate that both scaffolds exhibit acceptable inflammatory responses and can be almost fully degraded and replaced by newly formed bone. More importantly, under the same porosity, the scaffolds with larger pore size can promote early vascularization and up-regulate collagen type 1 and OPN expression, leading to higher bone mass and more mature bone formation. In conclusion, a new method is introduced to develop an open-porous magnesium scaffold with controllable microstructures and mechanical properties, which has great potential clinical application for bone reconstruction in the future.

Combinatorial incorporation of fluoride and cobalt ions into calcium phosphates to stimulate osteogenesis and angiogenesis

Bone healing requires two critical mechanisms, angiogenesis and osteogenesis. In order to improve bone graft substitutes, both mechanisms should be addressed simultaneously. While the individual effects of various bioinorganics have been studied, an understanding of the combinatorial effects is lacking. Cobalt and fluoride ions, in appropriate concentrations, are known to individually favor the vascularization and mineralization processes, respectively. This study investigated the potential of using a combination of fluoride and cobalt ions to simultaneously promote osteogenesis and angiogenesis in human mesenchymal stromal cells (hMSCs). Using a two-step biomimetic method, wells of tissue culture plates were coated with a calcium phosphate (CaP) layer without or with the incorporation of cobalt, fluoride, or both. In parallel, hMSCs were cultured on uncoated well plates, and cultured with cobalt and/or fluoride ions within the media. The results revealed that cobalt ions increased the expression of angiogenic markers, with the effects being stronger when the ions were added as a dissolved salt in cell medium as compared to incorporation into CaP. Cobalt ions generally suppressed the ALP activity, the expression of osteogenic genes, and the level of mineralization, regardless of delivery method. Fluoride ions, individually or in combination with cobalt, significantly increased the expression of many of the selected osteogenic markers, as well as mineral deposition. This study demonstrates an approach to simultaneously target the two essential mechanisms in bone healing: angiogenesis and osteogenesis. The incorporation of cobalt and fluoride into CaPs is a promising method to improve the biological performance of fully synthetic bone graft substitutes.

Biological behavior of bioactive glasses and their composites

This review summarizes current developments in improving the biological behavior of bioactive glasse and their composites.

Novel nanocomposite biomaterials with controlled copper/calcium release capability for bone tissue engineering multifunctional scaffolds

This work aimed to develop novel composite biomaterials for bone tissue engineering (BTE) made of bioactive glass nanoparticles (Nbg) and alginate cross-linked with Cu(2+) or Ca(2+) (AlgNbgCu, AlgNbgCa, respectively). Two-dimensional scaffolds were prepared and the nanocomposite biomaterials were characterized in terms of morphology, mechanical strength, bioactivity, biodegradability, swelling capacity, release profile of the cross-linking cations and angiogenic properties. It was found that both Cu(2+) and Ca(2+) are released in a controlled and sustained manner with no burst release observed. Finally, in vitro results indicated that the bioactive ions released from both nanocomposite biomaterials were able to stimulate the differentiation of rat bone marrow-derived mesenchymal stem cells towards the osteogenic lineage. In addition, the typical endothelial cell property of forming tubes in Matrigel was observed for human umbilical vein endothelial cells when in contact with the novel biomaterials, particularly AlgNbgCu, which indicates their angiogenic properties. Hence, novel nanocomposite biomaterials made of Nbg and alginate cross-linked with Cu(2+) or Ca(2+) were developed with potential applications for preparation of multifunctional scaffolds for BTE.

Bioactive glass ions as strong enhancers of osteogenic differentiation in human adipose stem cells

Bioactive glasses are known for their ability to induce osteogenic differentiation of stem cells. To elucidate the mechanism of the osteoinductivity in more detail, we studied whether ionic extracts prepared from a commercial glass S53P4 and from three experimental glasses (2-06, 1-06 and 3-06) are alone sufficient to induce osteogenic differentiation of human adipose stem cells. Cells were cultured using basic medium or osteogenic medium as extract basis. Our results indicate that cells stay viable in all the glass extracts for the whole culturing period, 14 days. At 14 days the mineralization in osteogenic medium extracts was excessive compared to the control. Parallel to the increased mineralization we observed a decrease in the cell amount. Raman and Laser Induced Breakdown Spectroscopy analyses confirmed that the mineral consisted of calcium phosphates. Consistently, the osteogenic medium extracts also increased osteocalcin production and collagen Type-I accumulation in the extracellular matrix at 13 days. Of the four osteogenic medium extracts, 2-06 and 3-06 induced the best responses of osteogenesis. However, regardless of the enhanced mineral formation, alkaline phosphatase activity was not promoted by the extracts. The osteogenic medium extracts could potentially provide a fast and effective way to differentiate human adipose stem cells in vitro.

Effects of fluoridation of porcine hydroxyapatite on osteoblastic activity of human MG63 cells

Biological hydroxyapatite, derived from animal bones, is the most widely used bone substitute in orthopedic and dental treatments. Fluorine is the trace element involved in bone remodeling and has been confirmed to promote osteogenesis when administered at the appropriate dose. To take advantage of this knowledge, fluorinated porcine hydroxyapatite (FPHA) incorporating increasing levels of fluoride was derived from cancellous porcine bone through straightforward chemical and thermal treatments. Physiochemical characteristics, including crystalline phases, functional groups and dissolution behavior, were investigated on this novel FPHA. Human osteoblast-like MG63 cells were cultured on the FPHA to examine cell attachment, cytoskeleton, proliferation and osteoblastic differentiation for in vitro cellular evaluation. Results suggest that fluoride ions released from the FPHA play a significant role in stimulating osteoblastic activity in vitro, and appropriate level of fluoridation (1.5 to 3.1 atomic percents of fluorine) for the FPHA could be selected with high potential for use as a bone substitute.

Bioactive glasses—structure and properties

Bioactive glasses were the first synthetic materials to show bonding to bone, and they are successfully used for bone regeneration. They can degrade in the body at a rate matching that of bone formation, and through a combination of apatite crystallization on their surface and ion release they stimulate bone cell proliferation, which results in the formation of new bone. Despite their excellent properties and although they have been in clinical use for nearly thirty years, their current range of clinical applications is still small. Latest research focuses on developing new compositions to address clinical needs, including glasses for treating osteoporosis, with antibacterial properties, or for the sintering of scaffolds with improved mechanical stability. This Review discusses how the glass structure controls the properties, and shows how a structure-based design may pave the way towards new bioactive glass implants for bone regeneration.