The osteogenic response of mesenchymal stromal cells to strontium-substituted bioactive glasses

Pore graded borosilicate bioactive glass scaffolds: in vitro dissolution and cytocompatibility

3D borosilicate bioactive glass (1393B20 and B12.5MgSr) scaffolds were prepared by robocasting, with and without a dense layer at the top. Pore graded scaffolds are promising as they allow for membrane deposition and could limit the risk of soft tissue infiltration. In vitro dissolution was studied in tris(hydroxymethyl)aminomethane (TRIS) and Simulated Body Fluid (SBF). 1393B20 scaffolds dissolved faster than B12.5MgSr in TRIS whereas they dissolved slower in SBF. The difference in dissolution profiles, as a function of the medium used, is assigned to the different rates of precipitation of hydroxyapatite (HA). While the precipitation of calcium phosphate (CaP) in the form of HA, first sign of bioactivity, was confirmed by ICP, FTIR-ATR and SEM-EDX analysis for both compositions, 1393B20 was found to precipitate HA at a faster rate. The presence of a dense top layer did not significantly impact the dissolution rate and CaP precipitation. In vitro cell culture was performed using human adipose-derived stem cells (hADSCs). Prior to cell plating, a preincubation of 3 days was found optimum to prevent burst ion release. In direct contact, cells proliferate and spread on the scaffolds while maintaining characteristic spindle morphology. Cell plated on 1393B20 scaffolds showed increased viability when compared to cell plated on B12.5MgSr. The lower cell viability, when testing B12.5MgSr, was assigned to the depletion of Ca2+ ions from culture medium and higher pH. Static cell culture leads to believe that the scaffold produced from the 1393B20 glass composition are promising in bone regeneration applications.

Silicon-containing nanomedicine and biomaterials: materials chemistry, multi-dimensional design, and biomedical application

The invention of silica-based bioactive glass in the late 1960s has sparked significant interest in exploring a wide range of silicon-containing biomaterials from the macroscale to the nanoscale. Over the past few decades, these biomaterials have been extensively explored for their potential in diverse biomedical applications, considering their remarkable bioactivity, excellent biocompatibility, facile surface functionalization, controllable synthesis, etc. However, to expedite the clinical translation and the unexpected utilization of silicon-composed nanomedicine and biomaterials, it is highly desirable to achieve a thorough comprehension of their characteristics and biological effects from an overall perspective. In this review, we provide a comprehensive discussion on the state-of-the-art progress of silicon-composed biomaterials, including their classification, characteristics, fabrication methods, and versatile biomedical applications. Additionally, we highlight the multi-dimensional design of both pure and hybrid silicon-composed nanomedicine and biomaterials and their intrinsic biological effects and interactions with biological systems. Their extensive biomedical applications span from drug delivery and bioimaging to therapeutic interventions and regenerative medicine, showcasing the significance of their rational design and fabrication to meet specific requirements and optimize their theranostic performance. Additionally, we offer insights into the future prospects and potential challenges regarding silicon-composed nanomedicine and biomaterials. By shedding light on these exciting research advances, we aspire to foster further progress in the biomedical field and drive the development of innovative silicon-composed nanomedicine and biomaterials with transformative applications in biomedicine.

Influence of Strontium on the Biological Behavior of Bioactive Glasses for Bone Regeneration

Bioactive glasses (BGs) can potentially be applied in biomedicine, mainly for bone repair and replacement, given their unique ability to connect to natural bone tissue and stimulate bone regeneration. Since their discovery, several glass compositions have been developed to improve the properties and clinical abilities of traditional bioactive glass. Different inorganic ions, such as strontium (Sr2+), have been incorporated in BG due to their ability to perform therapeutic functions. Sr2+ has been gaining prominence due to its ability to stimulate osteogenesis, providing an appropriate environment to improve bone regeneration, in addition to its antibacterial potential. However, as there are still points in the literature that are not well consolidated, such as the influence of ionic concentrations and the BG production technique, this review aims to collect information on the state of the art of the biological behavior of BGs containing Sr2+. It also aims to gather data on different types of BGs doped with different concentrations of Sr2+, and to highlight the manufacturing techniques used in order to analyze the influence of the incorporation of this ion for bone regeneration purposes.

Electrospinning Inorganic Nanomaterials to Fabricate Bionanocomposites for Soft and Hard Tissue Repair

Tissue engineering (TE) has attracted the widespread attention of the research community as a method of producing patient-specific tissue constructs for the repair and replacement of injured tissues. To date, different types of scaffold materials have been developed for various tissues and organs. The choice of scaffold material should take into consideration whether the mechanical properties, biodegradability, biocompatibility, and bioresorbability meet the physiological properties of the tissues. Owing to their broad range of physico-chemical properties, inorganic materials can induce a series of biological responses as scaffold fillers, which render them a good alternative to scaffold materials for tissue engineering (TE). While it is of worth to further explore mechanistic insight into the use of inorganic nanomaterials for tissue repair, in this review, we mainly focused on the utilization forms and strategies for fabricating electrospun membranes containing inorganic components based on electrospinning technology. A particular emphasis has been placed on the biological advantages of incorporating inorganic materials along with organic materials as scaffold constituents for tissue repair. As well as widely exploited natural and synthetic polymers, inorganic nanomaterials offer an enticing platform to further modulate the properties of composite scaffolds, which may help further broaden the application prospect of scaffolds for TE.

Effect of chitosan/inorganic nanomaterial scaffolds on bone regeneration and related influencing factors in animal models: A systematic review

Bone tissue engineering (BTE) provides a promising alternative for transplanting. Due to biocompatibility and biodegradability, chitosan-based scaffolds have been extensively studied. In recent years, many inorganic nanomaterials have been utilized to modify the performance of chitosan-based materials. In order to ascertain the impact of chitosan/inorganic nanomaterial scaffolds on bone regeneration and related key factors, this study presents a systematic comparison of various scaffolds in the calvarial critical-sized defect (CSD) model. A total of four electronic databases were searched without publication date or language restrictions up to April 2022. The Animal Research Reporting of In Vivo Experiments 2.0 guidelines (ARRIVE 2.0) were used to assess the quality of the included studies. Moreover, the risk of bias (RoB) was evaluated via the Systematic Review Center for Laboratory Animal Experimentation (SYRCLE) tool. After the screening, 22 studies were selected. None of these studies achieved high quality or had a low RoB. In the available studies, scaffolds reconstructed bone defects in radically different extensions. Several significant factors were identified, including baseline characteristics, physicochemical properties of scaffolds, surgery details, and scanning or reconstruction parameters of micro-computed tomography (micro-CT). Further studies should focus on not only improving the osteogenic performance of the scaffolds but also increasing the credibility of studies through rigorous experimental design and normative reports.

Strontium doped bioglass incorporated hydrogel-based scaffold for amplified bone tissue regeneration

Repairing of large bone injuries is an important problem in bone regeneration field. Thus, developing new therapeutic approaches such as tissue engineering using 3D scaffolds is necessary. Incorporation of some bioactive materials and trace elements can improve scaffold properties. We made chitosan/alginate/strontium-doped bioglass composite scaffolds with optimized properties for bone tissue engineering. Bioglass (BG) and Sr-doped bioglasses (Sr-BG) were synthesized using Sol–Gel method. Alginate-Chitosan (Alg/Cs) scaffold and scaffolds containing different ratio (10%, 20% and 30%) of BG (Alg/Cs/BG10, 20, 30) or Sr-BG (Alg/Cs/Sr-BG10, 20, 30) were fabricated using freeze drying method. Characterization of bioglasses/scaffolds was done using zeta sizer, FTIR, XRD, (FE) SEM and EDS. Also, mechanical strength, antibacterial effect degradation and swelling profile of scaffolds were evaluated. Bone differentiation efficiency and viability of MSCs on scaffolds were determined by Alizarin Red, ALP and MTT methods. Cell toxicity and antibacterial effect of bioglasses were determined using MTT, MIC and MBC methods. Incorporation of BG into Alg/Cs scaffolds amplified biomineralization and mechanical properties along with improved swelling ratio, degradation profile and cell differentiation. Mechanical strength and cell differentiation efficiency of Alg/Cs/BG20 scaffold was considerably higher than scaffolds with lower or higher BG concentrations. Alg/Cs/Sr-BG scaffolds had higher mechanical stability and more differentiation efficiency in comparison with Alg/Cs and Alg/Cs/BG scaffolds. Also, Mechanical strength and cell differentiation efficiency of Alg/Cs/Sr-BG20 scaffold was considerably higher than scaffolds with various Sr-BG concentrations. Biomineralization of Alg/Cs/BG scaffolds slightly was higher than Alg/Cs/Sr-BG scaffolds. Overall, we concluded that Alg/Cs/Sr-BG20 scaffolds are more suitable for repairing bone major injuries.

Strontium- and peptide-modified silicate nanostructures for dual osteogenic and antimicrobial activity

Developing multifunctional nanostructures that promote bone repair while fighting infection is highly desirable in bone regenerative therapies. Previous efforts have focused on achieving one property or another by altering the chemical makeup of nanostructures or using growth factors or antibiotics. We present nanostructures with several simultaneous functional attributes including positive effects of strontium on bone formation and prevention of osteoclast differentiation along with incorporation of antimicrobial peptides (AMP) to prevent infection. To form these multifunctional nanostructures, mesoporous calcium silicate (CaMSN) was modified with high levels of strontium. For this, CaMSNs were either partially substituted (20 wt% Ca) or completely replaced with strontium (Sr) to form Sr-CaMSN or SrMSN. The mesoporous nature of these bioactive silicate nanostructures rendered a configuration for substantial AMP loading as well as their effective delivery. The physico-chemical and structural characterization of synthesized MSNs confirmed the mesoporous nature of the synthesized MSNs and their total surface area, pore size, pore volume and SBF-mediated bioactivity remained unaltered with the incorporation of Sr. However, biological evaluation confirmed that synthesized SrMSN upregulated osteogenic differentiation of mesenchymal stromal cells and significantly downregulated osteoclast differentiation. Also, the AMP-loaded MSNs prevented formation and growth of methicillin resistant Staphylococcus aureus (MRSA) biofilms. Thus, high Sr-containing AMP-loaded SrMSNs may combat MRSA-associated infection while promoting bone regeneration. The controlled availability of therapeutic Sr and AMP release as SrMSN degrade enables its potential application in bone tissue regeneration.

The Effect of Polybutylcyanoacrylate Nanoparticles as a Protos Delivery Vehicle on Dental Bone Formation

BACKGROUND

Dental implants are commonly used for missing teeth, for which success depends heavily on the quality of the alveolar bone. The creation of an ideal implant site is a key component in shortening the treatment time, which remains clinically challenging. Strontium ranelate (Protos) is an anti-osteoporotic agent which has previously been used to promote bone formation, however the systemic use of Protos has been linked to serious cardiovascular and venous thromboembolic events, thus local delivery strategies may be better suited for this purpose. In this study, a biodegradable, and biocompatible nanocarrier "polybutylcyanoacrylate" (PBCA) loaded with strontium was constructed and its ability to promote bone formation was assessed.

METHODOLOGY

PBCA nanoparticles loaded with strontium (PBCA-Sr NPs) were synthesized using the emulsion polymerization method, and their physical properties (zeta potential, size and shape) and entrapment efficiency were characterized. Committed MSCs (osteoblasts) were derived from the differentiation of cultured rat mesenchymal stem cells (MSC), which were tested with the PBCA-Sr NPs for cytotoxicity, inflammatory response, bone formation and mineralization. Scanning electron microscopy was performed following a 7-day treatment of PBCA-Sr NPs on decellularized procaine mandibular bone blocks grafted with osteoblasts.

RESULTS

Spherical PBCA-Sr NPs of 166.7 ± 2.3 nm, zeta potential of -1.15 ± 0.28 mV with a strontium loading efficiency of 90.04 ± 3.27% were constructed. The presence of strontium was confirmed by energy-dispersive X-ray spectroscopy. Rat committed MSCs incubated in PBCA-Sr NPs for 24 hrs showed viabilities in excess of 90% for concentrations of up to 250 ug/mL, the cellular expression of osteocalcin and alkaline phosphatase were 1.4 and 1.3 times higher than the untreated control, and significantly higher than those treated with strontium alone. Bone formation was evident following osteoblast engraftment on the decellularized procaine mandibular bone block with PBCA-Sr NPs, which appeared superior to those treated with strontium alone.

CONCLUSION

Treatment of committed MSCs with PBCA-Sr NPs showed higher expression of markers of bone formation when compared with strontium alone and which corresponded to greater degree of bone formation observed on the 3-dimensinal decellularized procaine mandibular bone block. Further quantitative analysis on the extent of new bone formation is warranted.

A nano approach towards the creation of a biointerface as stimulator of osteogenic differentiation

There is a great need for tissue engineering constructs with the ability to modulate stem cell behavior. The initial adhesion, growth and differentiation of stem cell are a key strategy in bone tissue engineering and it can be controlled through biomaterial-cell interface. Here we engineered a polycaprolactone/gelatin/bioactive glass (PCL/GT/BG) nanocomposite scaffold coated with Fibronectin (FN) as a potential candidate to aid the bone regeneration process by giving cells a temporary template to grow into. For this purpose, initially BG nanoparticles (nBG) of 70 ± 15 nm were synthesized, characterized and then impregnated into PCL/GT matrix to create a nanocomposite fibrous mesh. An optimized structure was selected based on fiber uniformity, diameter, and the mechanical properties. Cell adhesion, growth, and the expression of osteogenic-related genes as a result of FN tethering, through specific surface interactions, was evaluated. Furthermore, the potential of optimized nanofiberous structure as a drug delivery vehicle for the local release of therapeutic agents was studied by using amoxicillin as a model drug. The release profile revealed that around 70% of drug was released in an hour for non-crosslinked fibers (burst release) followed by a gradual release up to 72 h. The release profile was steadier for crosslinked fibers. The scaffold also showed an antibacterial effect against ubiquitous gram-positive Staphylococcus aureus. The current study provides an insight for future researchers who aim to create nanocomposite materials as multifunctional scaffolds for bone tissue engineering applications.

Biological Roles and Delivery Strategies for Ions to Promote Osteogenic Induction

Bone is the most studied tissue in the field of tissue regeneration. Even though it has intrinsic capability to regenerate upon injury, several pathologies and injuries could hamper the highly orchestrated bone formation and resorption process. Bone tissue engineering seeks to mimic the extracellular matrix of the tissue and the different biochemical pathways that lead to successful regeneration. For many years, the use of extrinsic factors (i.e., growth factors and drugs) to modulate these biological processes have been the preferred choice in the field. Even though it has been successful in some instances, this approach presents several drawbacks, such as safety-concerns, short release profile and half-time life of the compounds. On the other hand, the use of inorganic ions has attracted significant attention due to their therapeutic effects, stability and lower biological risks. Biomaterials play a key role in such strategies where they serve as a substrate for the incorporation and release of the ions. In this review, the methodologies used to incorporate ions in biomaterials is presented, highlighting the osteogenic properties of such ions and the roles of biomaterials in controlling their release.

Bone marrow stromal cells stimulated by strontium-substituted calcium silicate ceramics: release of exosomal miR-146a regulates osteogenesis and angiogenesis

Dual-functional regulation for angiogenesis and osteogenesis is crucial for desired bone regeneration especially in large-sized bone defects. Exosomes have been demonstrated to facilitate bone regeneration through enhanced osteogenesis and angiogenesis. Moreover, functional stimulation to mesenchymal stromal cells (MSCs) was reported to further boost the pro-angiogenic ability of exosomes secreted. However, whether the stimulation by bioactive trace elements of biomaterials could enhance pro-angiogenic capability of bone marrow stromal cells (BMSCs)-derived exosomes and consequently promote in vivo vascularized bone regeneration has not been investigated. In this study, strontium-substituted calcium silicate (Sr-CS) was chosen and the biological function of BMSCs-derived exosomes after Sr-CS stimulation (Sr-CS-Exo) was systemically investigated. The results showed that Sr-CS-Exo could significantly promote in vitro angiogenesis of human umbilical vein endothelial cells (HUVECs), which might be attributed to elevated pro-angiogenic miR-146a cargos and inhibition of Smad4 and NF2 proteins. Moreover, the in vivo study confirmed that Sr-CS-Exo possessed superior pro-angiogenic ability, which contributed to the accelerated developmental vascularization in zebrafish along with the neovascularization and bone regeneration in rat distal femur defects. Our findings may provide new insights into the mechanisms underlying Sr-containing biomaterials-induced angiogenesis, and for the first time, proposed that Sr-CS-Exo may serve as the candidate engineered-exosomes with dual-functional regulation for angiogenesis and osteogenesis in vascularized bone regeneration.

Bioactive and Topographically-Modified Electrospun Membranes for the Creation of New Bone Regeneration Models

Bone injuries that arise from trauma, cancer treatment, or infection are a major and growing global challenge. An increasingly ageing population plays a key role in this, since a growing number of fractures are due to diseases such as osteoporosis, which place a burden on healthcare systems. Current reparative strategies do not sufficiently consider cell-substrate interactions that are found in healthy tissues; therefore, the need for more complex models is clear. The creation of in vitro defined 3D microenvironments is an emerging topographically-orientated approach that provides opportunities to apply knowledge of cell migration and differentiation mechanisms to the creation of new cell substrates. Moreover, introducing biofunctional agents within in vitro models for bone regeneration has allowed, to a certain degree, the control of cell fate towards osteogenic pathways. In this research, we applied three methods for functionalizing spatially-confined electrospun artificial microenvironments that presented relevant components of the native bone stem cell niche. The biological and osteogenic behaviors of mesenchymal stromal cells (MSCs) were investigated on electrospun micro-fabricated scaffolds functionalized with extracellular matrix (ECM) proteins (collagen I), glycosaminoglycans (heparin), and ceramic-based materials (bioglass). Collagen, heparin, and bioglass (BG) were successfully included in the models without modifying the fibrous structures offered by the polycaprolactone (PCL) scaffolds. Mesenchymal stromal cells (MSCs) were successfully seeded in all the biofunctional scaffolds and they showed an increase in alkaline phosphatase production when exposed to PCL/BG composites. This research demonstrates the feasibility of manufacturing smart and hierarchical artificial microenvironments for studying stem cell behavior and ultimately the potential of incorporating these artificial microenvironments into multifunctional membranes for bone tissue regeneration

Electrospun Scaffolds Containing Silver-Doped Hydroxyapatite with Antimicrobial Properties for Applications in Orthopedic and Dental Bone Surgery

Preventing the development of osteomyelitis while enhancing bone regeneration is challenging, with relatively little progress to date in translating promising technologies to the clinic. Nanoscale hydroxyapatite (nHA) has been employed as a bone graft substitute, and recent work has shown that it may be modified with silver to introduce antimicrobial activity against known pathogens. The aim of this study was to incorporate silver-doped nHA into electrospun scaffolds for applications in bone repair. Silver-doped nHA was produced using a modified, rapid mixing, wet precipitation method at 2, 5, 10 mol.% silver. The silver-doped nHA was added at 20 wt.% to a polycaprolactone solution for electrospinning. Bacteria studies demonstrated reduced bacterial presence, with Escherichia coli and Staphylococcus aureus undetectable after 96 h of exposure. Mesenchymal stem cells (MSCs) were used to study both toxicity and osteogenicity of the scaffolds using PrestoBlue^® and alkaline phosphatase (ALP) assays. Innovative silver nHA scaffolds significantly reduced E. coli and S. aureus bacterial populations while maintaining cytocompatibility with mammalian cells and enhancing the differentiation of MSCs into osteoblasts. It was concluded that silver-doped nHA containing scaffolds have the potential to act as an antimicrobial device while supporting bone tissue healing for applications in orthopedic and dental bone surgery.

Composite Fiber Networks Based on Polycaprolactone and Bioactive Glass-Ceramics for Tissue Engineering Applications

In this work, composite fibers connected in three-dimensional porous scaffolds were fabricated by electrospinning, starting from polycaprolactone and inorganic powders synthesized by the sol-gel method. The aim was to obtain materials dedicated to the field of bone regeneration, with controllable properties of bioresorbability and bioactivity. The employed powders were nanometric and of a glass-ceramic type, a fact that constitutes the premise of a potential attachment to living tissue in the physiological environment. The morphological characterization performed on the composite materials validated both the fibrous character and oxide powder distribution within the polymer matrix. Regarding the biological evaluation, the period of immersion in simulated body fluid led to the initiation of polymer degradation and a slight mineralization of the embedded particles, while the osteoblast cells cultured in the presence of these scaffolds revealed a spatial distribution at different depths and a primary networking tendency, based on the composites’ geometrical and dimensional features.

Osteogenic potential of sol-gel bioactive glasses containing manganese

Bioactive glasses (BGs) are widely used for bone regeneration, and allow the incorporation of different ions with therapeutic properties into the glass network. Amongst the different ions with therapeutic benefits, manganese (Mn) has been shown to influence bone metabolism and activate human osteoblasts integrins, improving cell adhesion, proliferation and spreading. Mn has also been incorporated into bioceramics as a therapeutic ion for improved osteogenesis. Here, up to 4.4 mol% MnO was substituted for CaO in the 58S composition (60 mol% SiO₂, 36 mol% CaO, 4 mol% P₂O₅) and its effects on the glass properties and capability to influence the osteogenic differentiation were evaluated. Mn-containing BGs with amorphous structure, high specific surface area and nanoporosity were obtained. The presence of Mn²⁺ species was confirmed by X-ray photoelectron spectroscopy (XPS). Mn-containing BGs presented no cytotoxic effect on human mesenchymal stem cells (hMSCs) and enabled sustained ion release in culture medium. hMSCs osteogenic differentiation stimulation and influence on the mineralisation process was also confirmed through the alkaline phosphatase (ALP) activity, and expression of osteogenic differentiation markers, such as collagen type I, osteopontin and osteocalcin, which presented higher expression in the presence of Mn-containing samples compared to control. Results show that the release of manganese ions from bioactive glass provoked human mesenchymal stem cell (hMSC) differentiation down a bone pathway, whereas hMSCs exposed to the Mn-free glass did not differentiate. Mn incorporation offers great promise for obtaining glasses with superior properties for bone tissue regeneration.

Multiple and Promising Applications of Strontium (Sr)-Containing Bioactive Glasses in Bone Tissue Engineering

Improving and accelerating bone repair still are partially unmet needs in bone regenerative therapies. In this regard, strontium (Sr)-containing bioactive glasses (BGs) are highly-promising materials to tackle this challenge. The positive impacts of Sr on the osteogenesis makes it routinely used in the form of strontium ranelate (SR) in the clinical setting, especially for patients suffering from osteoporosis. Therefore, a large number of silicate-, borate-, and phosphate-based BGs doped with Sr and produced in different shapes have been developed and characterized, in order to be used in the most advanced therapeutic strategies designed for the management of bone defects and injuries. Although the influence of Sr incorporation in the glass is debated regarding the obtained physicochemical and mechanical properties, the biological improvements have been found to be substantial both in vitro and in vivo. In the present study, we provide a comprehensive overview of Sr-containing glasses along with the current state of their clinical use. For this purpose, different types of Sr-doped BG systems are described, including composites, coatings and porous scaffolds, and their applications are discussed in the light of existing experimental data along with the significant challenges ahead.

Biomaterial based treatment of osteoclastic/osteoblastic cell imbalance - Gelatin-modified calcium/strontium phosphates

Osteoporotic bone represents - particularly in case of fractures - difficult conditions for its regeneration. In the present study, the focus was put on a degradable bone substitute material of gelatin-modified calcium and strontium phosphates facing the special demands of osteoporotic bone. The release of strontium ions from the material ought to stimulate osteoblastogenesis either direct by ion release or indirect after material resorption by increased presence and activity of osteoclasts, which subsequently stimulate osteoblasts. A new porous material was produced from calcium phosphate, strontium phosphate and a mixed phase of calcium/strontium phosphate precipitated in presence of gelatin. Initially, ion release was analyzed in standard‑calcium containing (2.0 mM) and low-calcium (0.4 mM) minimum essential medium. The cultivation of human peripheral blood mononuclear cells next to the material led to formation of osteoclast-like cells, able to migrate, fuse, and differentiate. Especially, the mixed gelatin-modified calcium/strontium phosphate allowed osteoclastogenesis as proven morphologically and by real-time quantitative polymerase chain reaction (RT-qPCR). It was precisely this material that led to the best osteoblastic reaction of human bone marrow stromal cells cultured on the material. The investigations of the bone substitute material indicate active involvement in the balance of cells of the bone morphogenetic unit.

Human mesenchymal stem cells differentiate into an osteogenic lineage in presence of strontium containing bioactive glass nanoparticles

While bioactive glass and ions released during its dissolution are known to stimulate osteoblast cells, the effect bioactive glass has on human stem cells is not clear. Here, we show that spherical monodispersed strontium containing bioactive nanoparticles (Sr-BGNPs) of composition 90.6 mol% SiO₂, 5.0 mol% CaO, 4.4% mol% SrO (4.4%Sr-BGNPs) and 88.8 mol% SiO₂, 1.8 mol% CaO, and 9.4 mol% SrO (9.4%Sr-BGNPs) stimulate bone marrow derived human stem cell (hMSC) differentiation down an osteogenic pathway without osteogenic supplements. The particles were synthesised using a modified Stӧber process and had diameters of 90 ± 10 nm. Previous work on similar particles that did not contain Sr (80 mol% SiO₂, 20 mol% CaO) showed stem cells did not differentiate when exposed to the particles. Here, both compositions of the Sr-BGNPs (up to concentration of 250 μg/mL) stimulated the early-, mid-, and late-stage markers of osteogenic differentiation and accelerated mineralisation in the absence of osteogenic supplements. Sr ions play a key role in osteogenic stem cell differentiation. Sr-BGNP dissolution products did not adversely affect hMSC viability and no significant differences in viability were measured between each particle composition. Confocal and transmission electron microscopy (TEM) demonstrated that monodispersed Sr-BGNPs were internalised and localised within vesicles in the cytoplasm of hMSCs. Degradation of particles inside the cells was observed, whilst maintaining effective cations (Ca and Sr) in their silica network after 24 h in culture. The uptake of Sr-BGNPs by hMSCs was reduced by inhibitors of specific routes of endocytosis, indicating that the Sr-BGNPs uptake by hMSCs was probably via mixed endocytosis mechanisms. Sr-BGNPs have potential as injectable therapeutic devices for bone regeneration or treatment of conditions such as osteoporosis, because of their ability deliver a sustained release of osteogenic inorganic cations, e.g. calcium (Ca) or and strontium (Sr), through particle degradation locally to cells. STATEMENT OF SIGNIFICANCE: Here, we show that 90 nm spherical strontium containing bioactive nanoparticles of stimulate bone marrow derived human stem cell (hMSC) differentiation down an osteogenic pathway without the use of osteogenic supplements. While bioactive glass and its dissolution products are known to promote excellent bone regeneration in vivo and to stimulate osteoblast cells to produce bone matrix in vitro, their effect on human stem cells is not clear. Previously our nanoparticles that contained only SiO₂ and CaO did not provoke human bone marrow or adipose derived stem cell differentiation.

Selective Contribution of Bioactive Glasses to Molecular and Cellular Pathways

Over the past few decades, biomedical scientists and surgeons have given substantial attention to bioactive glasses as promising, long-lasting biomaterials that can make chemical connections with the neighboring hard and soft tissues. Several studies have examined the cellular and molecular responses to bioactive glasses to determine if they are suitable biomaterials for tissue engineering and regenerative medicine. In this regard, different ions and additives have been used recently to induce specific characteristics for selective cellular and molecular responses. This Review briefly describes foreign-body response mechanisms and the role of adsorbed proteins as the key players in starting interactions between cells and biomaterials. It then explains the physicochemical properties of the most common bioactive glasses, which have a significant impact on their cellular and molecular responses. It is expected that, with the development of novel strategies, the physiochemical properties of bioactive glasses can be engineered to precisely control proteins' adsorption and cellular functions after implantation.

In vitro cellular testing of strontium/calcium substituted phosphate glass discs and microspheres shows potential for bone regeneration

Phosphate-based glasses (PBGs) are ideal materials for regenerative medicine strategies because their composition, degradation rates, and ion release profiles can easily be controlled. Strontium has previously been found to simultaneously affect bone resorption and deposition. Therefore, by combining the inherent properties of resorbable PBG and therapeutic activity of strontium, these glasses could be used as a delivery device of therapeutic factors for the treatment of orthopaedic diseases such as osteoporosis. This study shows the cytocompatibility and osteogenic potential of PBGs where CaO is gradually replaced by SrO in the near invert glass system 40P2 O5 ·(16-x)CaO·20Na2 O·24MgO·xSrO (x = 0, 4, 8, 12, and 16 mol%). Direct seeding of MG63 cells onto glass discs showed no significant difference in cell metabolic activity and DNA amount measurement across the different formulations studied. Cell attachment and spreading was confirmed via scanning electron microscopy (SEM) imaging at Days 3 and 14. Alkaline phosphatase (ALP) activity was similarly maintained across the glass compositions. Follow-on studies explored the effect of each glass composition in microsphere conformation (size: 63-125 μm) on human mesenchymal stem cells (hMSCs) in 3D cultures, and analysis of cell metabolic activity and ALP activity showed no significant differences at Day 14 over the compositional range investigated, in line with the observations from MG63 cell culture studies. Environmental SEM and live cell imaging at Day 14 of hMSCs seeded on the microspheres showed cell attachment and colonisation of the microsphere surfaces, confirming these formulations as promising candidates for regenerative medicine strategies addressing compromised musculoskeletal/orthopaedic diseases.

Bibliographic review on the state of the art of strontium and zinc based regenerative therapies. Recent developments and clinical applications

This review brings up to date the state of the art of strontium and zinc based regenerative therapies, both having a promoting effect on tissue formation and a role inhibiting resorption in musculoskeletal disorders.

Bioactive Glasses and Glass-Ceramics for Healthcare Applications in Bone Regeneration and Tissue Engineering

The discovery of bioactive glasses (BGs) in the late 1960s by Larry Hench et al. was driven by the need for implant materials with an ability to bond to living tissues, which were intended to replace inert metal and plastic implants that were not well tolerated by the body. Among a number of tested compositions, the one that later became designated by the well-known trademark of 45S5 Bioglass® excelled in its ability to bond to bone and soft tissues. Bonding to living tissues was mediated through the formation of an interfacial bone-like hydroxyapatite layer when the bioglass was put in contact with biological fluids in vivo. This feature represented a remarkable milestone, and has inspired many other investigations aiming at further exploring the in vitro and in vivo performances of this and other related BG compositions. This paradigmatic example of a target-oriented research is certainly one of the most valuable contributions that one can learn from Larry Hench. Such a goal-oriented approach needs to be continuously stimulated, aiming at finding out better performing materials to overcome the limitations of the existing ones, including the 45S5 Bioglass®. Its well-known that its main limitations include: (i) the high pH environment that is created by its high sodium content could turn it cytotoxic; (ii) and the poor sintering ability makes the fabrication of porous three-dimensional (3D) scaffolds difficult. All of these relevant features strongly depend on a number of interrelated factors that need to be well compromised. The selected chemical composition strongly determines the glass structure, the biocompatibility, the degradation rate, and the ease of processing (scaffolds fabrication and sintering). This manuscript presents a first general appraisal of the scientific output in the interrelated areas of bioactive glasses and glass-ceramics, scaffolds, implant coatings, and tissue engineering. Then, it gives an overview of the critical issues that need to be considered when developing bioactive glasses for healthcare applications. The aim is to provide knowledge-based tools towards guiding young researchers in the design of new bioactive glass compositions, taking into account the desired functional properties.

Application of a direct pulp capping cement containing S-PRG filler

OBJECTIVES

To evaluate new pulp capping cements containing surface pre-reacted glass ionomer (S-PRG) filler and to investigate ion release kinetics and pH shift of eluates from the cement.

MATERIALS AND METHODS

Molars of Wistar rats were directly pulp capped using three kinds of cement containing S-PRG filler and mineral tri-oxide aggregate (MTA) was used as a control. After 1, 2, or 4 weeks, histological evaluation was performed and differences of tertiary dentin formation were analyzed. Release of Sr²⁺, BO₃^3-, SiO₃^2-, Na⁺, and Al³⁺ ions was determined by inductively coupled plasma-atomic emission spectrometry, and F^- ion release was measured using a fluoride ion selective electrode. The pH of the eluate from each cement after mixing was measured with a pH electrode.

RESULTS

One of S-PRG cements promoted tertiary dentin formation to the same extent as the control (p > 0.05) and it showed a tendency of less inflammatory response. This cement released more BO₃^3- and SiO₃^2-, but less Sr²⁺, Na⁺, and F^- than other S-PRG specimens. Each cement recovered nearly neutral compared with glass ionomer cement.

CONCLUSIONS

S-PRG cement induced tertiary dentin formation based on multiple ion releases, suggesting that it is suitable as a pulp capping material.

CLINICAL RELEVANCE

This new material can be an alternative pulp capping agent to MTA.

Strontium-Doped Bioactive Glass Nanoparticles in Osteogenic Commitment

The present work has explored bioactive glass nanoparticles (BGNPs) and developed strontium-doped nanoparticles (BGNPsSr), envisioning orthopedic strategies compatible with vascularization. The nanoparticles were synthesized by the sol-gel method, achieving a diameter of 55 nm for BGNPs and 75 nm for BGNPsSr, and the inclusion of strontium caused no structural alteration. The nanoparticles exhibited high cytocompatibility for human umbilical vein endothelial cells (HUVECs) and SaOS-2. Additionally, the incorporation of strontium emphasized the tubule networking behavior of HUVECs. Our results demonstrate that the nanoparticle dissolution products encouraged the osteogenic differentiation of human adipose stem cells as it favored the expression of key genes and proteins associated with osteogenic lineage. This effect was markedly enhanced for BGNPsSr, which could prompt stem cell osteogenic differentiation without the typical osteogenic inducers. This study not only supports the hypothesis that BGNPs might play a significant role in osteogenic commitment but also highlights that the designed BGNPsSr is a valuable tool for stem cell "tune-up" in bone tissue engineering applications.

Strontium-releasing fluorapatite glass-ceramics: Crystallization behavior, microstructure, and solubility

The purpose of this work was to investigate the effect of strontium partial replacement for calcium on the crystallization behavior, microstructure and solubility of fluorapatite glass-ceramics. Four glass compositions were prepared with increasing amounts of strontium partially replacing calcium. The crystallization behavior was analyzed by differential scanning calorimetry and X-ray diffraction (XRD). The microstructure was investigated by scanning electron microscopy. The chemical solubility was quantified according to ISO standard 10993-14. The amount of strontium released in solution after incubation in TRIS-HCl or citric acid buffer was measured by atomic absorption spectroscopy. XRD analyses revealed that partially substituted strontium-fluorapatite and strontium-åkermanite crystallized after strontium additions. The lattice cell volume of both phases increased linearly with the amount of strontium in the composition. Strontium additions led to a reduction in crystal size and an increase in crystal number density. The chemical solubility and amount of strontium released in solution increased linearly with the amount of strontium present in the composition in both TRIS-HCl and citric acid buffers. Total amounts of strontium released reached a maximum of 547 ± 80 ppm in TRIS-HCl and 1252 ± 290 ppm in citric acid buffer for the glass composition with the highest amount of strontium. For all strontium-containing compositions, the amount released in TRIS-HCl continued to increase between 70 and 120 h, indicating sustained release rather than burst release. © 2017 Wiley Periodicals, Inc. J Biomater Res Part B: 106B: 1421-1430, 2018.

A comparative in vivo study of strontium-functionalized and SLActive™ implant surfaces in early bone healing

Purpose

Studies have shown that strontium-doped medical applications benefit bone metabolism leading to improved bone healing and osseointegration. Based on this knowledge, the aim of the study was to evaluate the performance of an implant surface, functionalized by a physical vapor deposition (PVD) coating (Ti-Sr-O), designed to yield predictable release of strontium. The Ti-Sr-O functionalized surface is compared to a routinely used, commercially available surface (SLActive™) with respect to bone-to-implant contact (BIC%) and new bone formation (BF%) in two defined regions of interest (ROI-I and ROI-II, respectively).

Materials and methods:

Ti-Sr-O functionalized, SLActive, and Grade 4 titanium implants were inserted in the femoral condyle of adult male New Zealand White rabbits. The PVD magnetron-sputtered Ti-Sr-O surface coating was characterized using scanning electron microscopy (SEM) for morphology and coating thickness. Strontium release and mechanical stability of the coating, under simulated insertion conditions, were evaluated. Furthermore, histomorphometrical BIC and BF were carried out 2 weeks after insertion.

Results

Histomorphometry revealed increased bone formation of Ti-Sr-O with significant differences compared to SLActive and Grade 4 titanium in both regions of interest, ROI-I and ROI-II, at 0–250 µm and 250–500 µm distance from the implant surfaces. Analogous results of bone-to-implant contact were observed for the two modified surfaces.

Conclusion

The results show that a nanopatterned Ti-Sr-O functionalized titanium surface, with sustained release of strontium, increases peri-implant bone volume and could potentially contribute to enhancement of bone anchorage of osseointegrated implants.

In vitro osteogenesis by intracellular uptake of strontium containing bioactive glass nanoparticles

Monodispersed strontium containing bioactive glass nanoparticles (Sr-BGNPs) with two compositions were synthesised, through a modified sol-gel Stöber process, wherein silica nanoparticles (SiO₂-NPs) were formed prior to incorporation of calcium and strontium, with diameters of 90 ± 10 nm. The osteogenic response of a murine preosteoblast cell line, MC3T3-E1, was investigated in vitro for a nanoparticle concentration of 250 µg/mL with compositions of 87 mol% SiO₂, 7 mol% CaO, 6 mol% SrO and 83 mol% SiO₂, 3 mol% CaO, 14 mol% SrO. Dissolution studies in minimum essential media (α-MEM) at pH 7.4 and artificial lysosomal fluid (ALF) at pH 4.5 showed that the particles dissolved and that Sr²⁺ ions were released from Sr-BGNPs in both environments. Both particle compositions and their ionic dissolution products enhanced the alkaline phosphatase (ALP) activity of the cells and calcium deposition. Immunohistochemistry (IHC) staining of Col1a1, osteocalcin (OSC) and osteopontin (OSP) showed that these proteins were expressed in the MC3T3-E1 cells following three weeks of culture. In the basal condition, the late osteogenic differentiation markers, OSC and OSP, were more overtly expressed by cells cultured with Sr-BGNPs with 14 mol% SrO and their ionic release products than in the control condition. Col1a1 expression was only slightly enhanced in the basal condition, but was enhanced further by the osteogenic supplements. These data demonstrate that Sr-BGNPs accelerate mineralisation without osteogenic supplements. Sr-BGNPs were internalised into MC3T3-E1 cells by endocytosis and stimulated osteogenic differentiation of the pre-osteoblast cell line. Sr-BGNPs are likely to be beneficial for bone regeneration and the observed osteogenic effects of these particles can be attributed to their ionic release products.

STATEMENT OF SIGNIFICANCE

We report, for the first time, that monodispersed bioactive glass nanoparticles (∼90 nm) are internalised into preosteoblast cells by endocytosis but by unspecific mechanisms. The bioactive nanoparticles and their dissolution products (without the particles present) stimulated the expression of osteogenic markers from preosteoblast cells without the addition of other osteogenic supplements. Incorporating Sr into the bioactive glass nanoparticle composition, in addition to Ca, increased the total cation content (and therefore dissolution rate) of the nanoparticles, even though nominal total cation addition was constant, without changing size or morphology. Increasing Sr content in the nanoparticles and in their dissolution products enhanced osteogenesis in vitro. The particles therefore have great potential as an injectable therapeutic for bone regeneration, particularly in patients with osteoporosis, for which Sr is known to be therapeutic agent.

Effect of titanium implants with strontium incorporation on bone apposition in animal models: A systematic review and meta-analysis

This systematic review aims to assess the efficacy of titanium (Ti) implant surfaces with or without strontium (Sr) incorporation on osseointegration in animal experimental studies. An electronic search was conducted using databases of PubMed and EMBASE up to November 2016 to identify studies focusing on osseointegration of strontium-modified titanium implants following PRISMA criteria. The primary outcome was the percentage of bone-to-implant contact (BIC) around the implants with or without strontium-modified surface. Of the 1320 studies, 17 studies fulfilling the inclusion criteria were finally included. A random effect meta-analysis was conducted based on BIC in 17 studies, and the results demonstrated considerable heterogeneity (I² = 79%). A sensitivity analysis found that three studies using the same surface modification method were the major source of the heterogeneity. Therefore, exploratory subgroup analysis was performed. Subgroup one including 14 studies showed a standard mean differences (SMD) of 1.42 (95% CI, 1.13-1.71) with no heterogeneity (I² = 0.0%), while subgroup two including the other three studies showed a SMD of 9.49.95% CI, 7.51-11.47) with low heterogeneity (I² = 0.1%). Sr-modified implants in both subgroups showed significantly higher BIC than unmodified implants (P < 0.01). The results showed a statistically significant effect of Sr-modified titanium implant surfaces on osseointegration and bone apposition in animal models.

The synergistic effects of Sr and Si bioactive ions on osteogenesis, osteoclastogenesis and angiogenesis for osteoporotic bone regeneration

Bioactive ions released from bioceramics play important roles in bone regeneration; however, it is unclear how each ionic composition in complex bioceramics exerts its specific effect on bone regeneration. The aim of this study is to elucidate the functional effects of Sr and Si ions in bioceramics on the regeneration of osteoporotic bone. A model bioceramic with Sr- and Si-containing components (SMS) was successfully fabricated and the effects of ionic products from SMS bioceramics on the osteogenic, osteoclastic and angiogenic differentiation of rBMSCs-OVX and RANKL-induced osteoclasts were investigated. The results showed that SMS bioceramics could enhance ALP activity and expression of Col 1, OCN, Runx2, and angiogenic factors including VEGF and Ang-1. SMS bioceramics not only rebalanced the OPG/RANKL ratio of rBMSCs-OVX at early stage, but also repressed RANKL-induced osteoclast formation and expression of TRAP, DC-STAMP, V-ATPase a3, and NFATc1. The synergistic effects of Sr and Si ions were further investigated as compared with those of similar concentrations of Sr and Si ions alone. Sr and Si ions possessed synergistic effects on osteogenesis, osteoclastogenesis, and angiogenesis, attributed to the dominant effects of Sr ions on enhancing angiogenesis and repressing osteoclastogenesis, and the dominant effects of Si ions on stimulating osteogenesis. The in vivo study using critical-size mandibular defects of OVX rat models showed that SMS bioceramics could significantly enhance bone formation and mineralization compared with β-TCP bioceramics. Our results are the first to elucidate the specific effect of each ion from bioceramics on osteogenesis, osteoclastogenesis and angiogenesis for osteoporotic bone regeneration, paving the way for the design of functional biomaterials with complex compositions for tissue engineering and regenerative medicine.

STATEMENT OF SIGNIFICANCE

Bioactive ions released from bioceramics play important roles for bone regeneration; however, it is unclear how each of ionic compositions in complex bioceramics exerts its specific effect on bone regeneration. The aim of present study is to elucidate the functional effects of Sr and Si ions in complex bioceramics on the regeneration of osteoporotic bone. A model bioceramic with Sr and Si-containing components (SMS) was successfully fabricated and the effects of ionic products from SMS bioceramics on the osteogenic, osteoclastic and angiogenic differentiation of rBMSCs-OVX and RANKL-induced osteoclasts were investigated. The results showed that SMS bioceramics could enhance ALP activity and expression of Col 1, OCN, Runx2 and angiogenic factors including VEGF and Ang-1. SMS bioceramics not only rebalanced the ratio of OPG/RANKL of OVX-BMSCs at early stage, but also repressed RANKL-induced osteoclast formation and expression of TRAP, DC-STAMP, V-ATPase a3, and NFATc1. The synergistic effects of Sr and Si ions were further investigated as compared with the similar concentration of Sr and Si ions alone. It was found that Sr and Si ions possessed synergistic effects on osteogenesis, osteoclastogenesis and angiogenesis, attributed to the dominant effects of Sr ions on enhancing angiogenesis and repressing osteoclastogenesis, and the dominant effects of Si ions on stimulating osteogenesis. The in vivo study using critical-size mandibular defects of OVX rat models showed that SMS bioceramics could significantly enhance bone formation and mineralization as compared with β-TCP bioceramics. It is suggested that SMS bioceramics may be a promising biomaterial for osteoporotic bone regeneration. To our knowledge, this is the first time to elucidate the specific effect of each ion from bioceramics on osteogenesis, osteoclastogenesis and angiogenesis for osteoporotic bone regeneration, paving the way to design functional biomaterials with complex compositions for tissue engineering and regenerative medicine.

Multifunctional bioactive glass and glass-ceramic biomaterials with antibacterial properties for repair and regeneration of bone tissue

Bioactive glasses (BGs) and related glass-ceramic biomaterials have been used in bone tissue repair for over 30years. Previous work in this field was comprehensively reviewed including by their inventor Larry Hench, and the key features and properties of BGs are well understood. More recently, attention has focused on their modification to further enhance the osteogenic behaviour, or further compositional changes that may introduce additional properties, such as antimicrobial activity. Evidence is emerging that BGs and related glass-ceramics may be modified in such a way as to simultaneously introduce more than one desirable property. The aim of this review is therefore to consider the evidence that these more recent inorganic modifications to glass and glass-ceramic biomaterials are effective, and whether or not these new compositions represent sufficiently versatile systems to underpin the development of a new generation of truly multifunctional biomaterials to address pressing clinical needs in orthopaedic and dental surgery. Indeed, a number of classical glass compositions exhibited antimicrobial activity, however the structural design and the addition of specific ions, i.e. Ag⁺, Cu⁺, and Sr²⁺, are able to impart a multifunctional character to these systems, through the combination of, for example, bioactivity with bactericidal activity.

STATEMENT OF SIGNIFICANCE

In this review we demonstrate the multifunctional potential of bioactive glasses and related glass-ceramics as biomaterials for orthopaedic and craniofacial/dental applications. Therefore, it considers the evidence that the more recent inorganic modifications to glass and glass-ceramic biomaterials are able to impart antimicrobial properties alongside the more classical bone bonding and osteoconduction. These properties are attracting a special attention nowadays that bacterial infections are an increasing challenge in orthopaedics. We also focus the manuscript on the versatility of these systems as a basis to underpin the development of a new generation of truly multifunctional biomaterials to address pressing clinical needs in orthopaedic, craniofacial and dental surgery.

Multilayer Nanoscale Encapsulation of Biofunctional Peptides to Enhance Bone Tissue Regeneration In Vivo

Bone tissue healing is a dynamic process that is initiated by the recruitment of osteoprogenitor cells followed by their migration, proliferation, differentiation, and development of a mineralizing extracellular matrix. The work aims to manufacture a functionalized porous membrane that stimulates early events in bone healing for initiating a regenerative cascade. Layer-by-layer (LbL) assembly is proposed to modify the surface of osteoconductive electrospun meshes, based on poly(lactic-co-glycolic acid) and nanohydroxyapatite, by using poly(allylamine hydrochloride) and poly(sodium 4-styrenesulfonate) as polyelectrolytes. Molecular cues are incorporated by grafting peptide fragments into the discrete nanolayers. KRSR (lysine-arginine-serine-arginine) sequence is grafted to enhance cell adhesion and proliferation, NSPVNSKIPKACCVPTELSAI to guide bone marrow mesenchymal stem cells differentiation in osteoblasts, and FHRRIKA (phenylalanine-histidine-arginine-arginine-isoleucine-lysine-alanine) to improve mineralization matrix formation. Scanning electron microscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy demonstrate the successful surface functionalization. Furthermore, the peptide incorporation enhances cellular processes, with good viability and significant increase of alkaline phosphatase activity, osteopontin, and osteocalcin. The functionalized membrane induces a favorable in vivo response after implantation for four weeks in nonhealing rat calvarial defect model. It is concluded that the multilayer nanoencapsulation of biofunctional peptides using LbL approach has significant potential as innovative manufacturing technique to improve bone regeneration in orthopedic and craniofacial medical devices.

* Substituted Borosilicate Glasses with Improved Osteogenic Capacity for Bone Tissue Engineering

Borosilicate bioactive glasses (BBGs) have shown the capacity to promote higher formation of new bone when compared with silicate bioactive glasses. Herein, we assessed the capacity of BBGs to induce osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) as a function of their substituted divalent cations (Mg²⁺, Ca²⁺, Sr²⁺). To this purpose, we synthesized BBG particles by melt quenching. The cell viability, proliferation, and morphology (i.e., PrestoBlue^®, PicoGreen^®, and DAPI and Phalloidin stainings, respectively), as well as protein expression (i.e., alkaline phosphatase, ALP; osteopontin, OP; and osteocalcin, OC), of BM-MSCs in contact with BBGs were evaluated for 21 days. We observed an enhanced expression of bone-specific proteins (ALP, OP, and OC) and high mineralization of BM-MSCs under BBG-Mg and BBG-Sr-conditioned osteogenic media for concentrations of 20 and 50 mg/mL with low cytotoxic effects. Moreover, BBG-Sr, at a concentration of 50 mg/mL, was able to increase the mineralization and expression of the same bone-specific proteins even under basal medium conditions. These results indicated that the proposed BBGs improved osteogenic differentiation of BM-MSCs, therefore showing their potential as relevant biomaterials for bone tissue regeneration, not only by bonding to bone tissue but also by stimulating new bone formation.

Fabrication of amorphous strontium polyphosphate microparticles that induce mineralization of bone cells in vitro and in vivo

Here we describe the fabrication process of amorphous strontium-polyphosphate microparticles ("Sr-a-polyP-MP"). The effects of these particles on growth and gene expression were investigated with SaOS-2 cells as well as with human mesenchymal stem cells (MSC) and compared with those particles prepared of amorphous calcium-polyphosphate ("Ca-a-polyP-MP") and of strontium salt. The results revealed a markedly higher stimulation of growth of MSC by "Sr-a-polyP-MP" compared to "Ca-a-polyP-MP" and a significant increase in mineralization of SaOS-2 cells, as well as an enhanced upregulation of the expression of the genes encoding for alkaline phosphatase and the bone morphogenetic protein 2 (BMP-2), likewise performed with SaOS-2 cells. On the other hand, "Sr-a-polyP-MP" only slightly changes the expression of the osteocyte-specific sclerostin, a negative regulator of the canonical Wnt signaling pathway and an inhibitor of bone cell differentiation as well as of mineralization in SaOS-2 cells. In contrast, "Ca-a-polyP-MP" strongly increased the steady-state expression of the SOST (sclerostin) gene. In animal studies poly(d,l-lactide-co-glycolide (PLGA) microspheres, containing polyP particles, were implanted into critical-size calvarial defects in rats. The results show that the amorphous Sr-polyP-containing microspheres caused an increased healing/mineralization of the bone defect even after short implantation periods of 8-12weeks, if compared to the β-tri-calcium phosphate control as well as to Ca-polyP. It is proposed that "Sr-a-polyP-MP" might elicit suitable properties to be applied as a regeneratively active implant material for bone repair.

STATEMENT OF SIGNIFICANCE

In this manuscript, we fabricated amorphous strontium-polyphosphate microparticles ("Sr-a-polyP-MP") and studied their effects on bone mineral formation in vitro as well as in vivo. In vitro, those particles substantially increased the expression of the genes encoding for alkaline phosphatase, the bone morphogenetic protein 2 and the mineralization. In vivo, the "Sr-a-polyP-MP" packed into PLGA microspheres and implanted into critical-size calvarial defects in rats resulted in a speeded up of the healing/mineralization of the bone defect. Those properties qualify Sr-a-polyP as a suitable biomaterial for bone regenerative implants.

Strontium-Substituted Submicrometer Bioactive Glasses Modulate Macrophage Responses for Improved Bone Regeneration

Host immune response induced by foreign bone biomaterials plays an important role in determining their fate after implantation. Hence, it is well worth designing advanced bone substitute materials with beneficial immunomodulatory properties to modulate the host-material interactions. Bioactive glasses (BG), with excellent osteoconductivity and osteoinductivity, are regarded as important biomaterials in the field of bone regeneration. In order to explore a novel BG-based osteoimmunomodulatory implant with the capacity of potentially enhancing bone regeneration, it is a possible way to regulate the local immune microenvironment through manipulating macrophage polarization. In this study, strontium-substituted submicrometer bioactive glass (Sr-SBG) was prepared as an osteoimmunomodulatory bone repair material. To investigate whether the incorporation of Sr into SBG could synergistically improve osteogenesis by altering macrophage response, we systematically evaluated the interaction between Sr-SBG and macrophage during the process of bone regeneration by in vitro biological evaluation and in vivo histological assessment. It was found that the Sr-SBG modulates proper inflammatory status, leading to enhanced osteogenesis of mouse mesenchymal stem cells (mMSCs) and suppressed osteoclastogenesis of RAW 264.7 cells compared to SBG without strontium substitution. In vivo study confirmed that Sr-SBG initiated a less severe immune response and had an improved effect on bone regeneration than SBG, which corresponded with the in vitro evaluation. In conclusion, these findings suggested that Sr-SBG could be a promising immunomodulatory bone repair material designed for improved bone regeneration.

Reinforcement of poly-l-lactic acid electrospun membranes with strontium borosilicate bioactive glasses for bone tissue engineering

Herein, for the first time, we combined poly-l-lactic acid (PLLA) with a strontium borosilicate bioactive glass (BBG-Sr) using electrospinning to fabricate a composite bioactive PLLA membrane loaded with 10% (w/w) of BBG-Sr glass particles (PLLA-BBG-Sr). The composites were characterised by scanning electron microscopy (SEM) and microcomputer tomography (μ-CT), and the results showed that we successfully fabricated smooth and uniform fibres (1-3μm in width) with a homogeneous distribution of BBG-Sr microparticles (<45μm). Degradation studies (in phosphate buffered saline) demonstrated that the incorporation of BBG-Sr glass particles into the PLLA membranes increased their degradability and water uptake with a continuous release of cations. The addition of BBG-Sr glass particles enhanced the membrane's mechanical properties (69% higher Young modulus and 36% higher tensile strength). Furthermore, cellular in vitro evaluation using bone marrow-derived mesenchymal stem cells (BM-MSCs) demonstrated that PLLA-BBG-Sr membranes promoted the osteogenic differentiation of the cells as demonstrated by increased alkaline phosphatase activity and up-regulated osteogenic gene expression (Alpl, Sp7 and Bglap) in relation to PLLA alone. These results strongly suggest that the composite PLLA membranes reinforced with the BBG-Sr glass particles have potential as an effective biomaterial capable of promoting bone regeneration.

STATEMENT OF SIGNIFICANCE

PLLA membranes were reinforced with 10% (w/w) of strontium-bioactive borosilicate glass microparticles, and their capacity to induce the osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) was evaluated. These membranes presented an increased: degradability, water uptake, Young modulus and tensile strength. We also demonstrated that these membranes are non-cytotoxic and promote the attachment of BM-MSCs. The addition of the glass microparticles into the PLLA membranes promoted the increase of ALP activity (under osteogenic conditions), as well as the BM-MSCs osteogenic differentiation as shown by the upregulation of Alpl, Sp7 and Bglap gene expression. Overall, we demonstrated that the reinforcement of PLLA with glass microparticles results in a biomaterial with the appropriate properties for the regeneration of bone tissue.

Enhanced bioactivity, biocompatibility and mechanical behavior of strontium substituted bioactive glasses

Strontium contained biomaterials have been reported as a potential bioactive material for bone regeneration, as it reduces bone resorption and stimulates bone formation. In the present investigation, the bioactive glasses were designed to partially substitute SrO for SiO2 in Na2O-CaO-SrO-P2O5-SiO2 system. This work demonstrates that the substitution of SrO for SiO2 has got significant benefit than substitution for CaO in the bioactive glass. Bioactivity was assessed by the immersion of the samples in simulated body fluid for different intervals. The formation of hydroxy carbonate apatite layer was identified by X-ray diffractometry, scanning electron microscopy (SEM) and energy dispersive spectroscopy. The elastic modulus of the bioactive glasses was measured and found to increase with increasing SrO for SiO2. The blood compatibility of the samples was evaluated. In vitro cell culture studies of the samples were performed using human osteosarcoma U2-OS cell lines and found a significant improvement in cell viability and proliferation. The investigation showed enhancement in bioactivity, mechanical and biological properties of the strontia substituted for silica in glasses. Thus, these bioactive glasses would be highly potential for bone regeneration.

Bone regenerating effect of surface-functionalized titanium implants with sustained-release characteristics of strontium in ovariectomized rats

Since strontium (Sr) is known for its anabolic and anticatabolic effect on bone, research has been focused on its potential impact on osseointegration. The objective of this study was to investigate the performance of nanotopographic implants with a Sr-functionalized titanium (Ti) coating (Ti–Sr–O) with respect to osseointegration in osteoporotic bone. The trial was designed to examine the effect of sustained-release characteristics of Sr in poor-quality bone. Three Ti–Sr–O groups, which differed from each other in coating thickness, Sr contents, and Sr release, were examined. These were prepared by a magnetron sputtering process and compared to uncoated grade 4 Ti. Composition, morphology, and mechanical stability of the coatings were analyzed, and Sr release data were gained from in vitro washout experiments. In vivo investigation was carried out in an osteoporotic rat model and analyzed histologically, 6 weeks and 12 weeks after implantation. Median values of bone-to-implant contact and new bone formation after 6 weeks were found to be 84.7% and 54.9% (best performing Sr group) as compared to 65.2% and 23.8% (grade 4 Ti reference), respectively. The 12-week observation period revealed 84.3% and 56.5% (best performing Sr group) and 81.3% and 39.4% (grade 4 Ti reference), respectively, for the same measurements. The increase in new bone formation was found to correlate with the amount of Sr released in vitro. The results indicate that sputtered nanostructured Ti–Sr–O coatings showed sustained release of Sr and accelerate osseointegration even in poor-quality bone, and thus, may have impact on practical applications for medical implants.

Monodispersed strontium containing bioactive glass nanoparticles and MC3T3-E1 cellular response

Non-porous monodispersed strontium containing bioactive glass (Si2O-CaO-SrO) nanoparticles (Sr- BGNPs), were synthesised using a modified Stöber process. Silica nanoparticles (Si-NPs) with diameters 90 ± 10 nm were produced through hydrolysis and polycondensation reactions of the silicon alkoxide precursor, tetraethyl orthosilicate (TEOS), prior to the incorporation of cations; calcium (Ca) and strontium (Sr), into the silica networks through heat treatment (calcination). Sr was substituted for Ca on a mole basis from non- (0SrBGNPs) to fullsubstitution (100SrBGNPs) in order to increase the amount of network modifiers in the Si-NPs. The different ratios of Si: Ca; 1:1.3 and 1:8.0, presented various elemental compositions (i.e. 77–92 mol% of SiO

Guided bone regeneration using resorbable membrane and different bone substitutes: Early histological and molecular events

Bone insufficiency remains a major challenge for bone-anchored implants. The combination of guided bone regeneration (GBR) and bone augmentation is an established procedure to restore the bone. However, a proper understanding of the interactions between the bone substitute and GBR membrane materials and the bone-healing environment is lacking. This study aimed to investigate the early events of bone healing and the cellular activities in response to a combination of GBR membrane and different calcium phosphate (CaP) materials. Defects were created in the trabecular region of rat femurs, and filled with deproteinized bovine bone (DBB), hydroxyapatite (HA) or strontium-doped HA (SrHA) or left empty (sham). All the defects were covered with an extracellular matrix membrane. Defects were harvested after 12h, 3d and 6d for histology/histomorphometry, immunohistochemistry and gene expression analyses. Histology revealed new bone, at 6d, in all the defects. Larger amount of bone was observed in the SrHA-filled defect. This was in parallel with the reduced expression of osteoclastic genes (CR and CatK) and the osteoblast-osteoclast coupling gene (RANKL) in the SrHA defects. Immunohistochemistry indicated fewer osteoclasts in the SrHA defects. The observations of CD68 and periostin-expressing cells in the membrane per se indicated that the membrane may contribute to the healing process in the defect. It is concluded that the bone-promoting effects of Sr in vivo are mediated by a reduction in catabolic and osteoblast-osteoclast coupling processes. The combination of a bioactive membrane and CaP bone substitute material doped with Sr may produce early synergistic effects during GBR.

STATEMENT OF SIGNIFICANCE

The study provides novel molecular, cellular and structural evidence on the promotion of early bone regeneration in response to synthetic strontium-containing hydroxyapatite (SrHA) substitute, in combination with a resorbable, guided bone regeneration (GBR) membrane. The prevailing view, based mainly upon in vitro data, is that the beneficial effects of Sr are exerted by the stimulation of bone-forming cells (osteoblasts) and the inhibition of bone-resorbing cells (osteoclasts). In contrast, the present study demonstrates that the local effect of Sr in vivo is predominantly via the inhibition of osteoclast number and activity and the reduction of osteoblast-osteoclast coupling. This experimental data will form the basis for clinical studies, using this material as an interesting bone substitute for guided bone regeneration.

The osteogenic response of mesenchymal stromal cells to strontium-substituted bioactive glasses

Bioactive glasses are known to stimulate bone healing, and the incorporation of strontium has the potential to increase their potency. In this study, calcium oxide in the 45S5 bioactive glass composition was partially (50%, Sr50) or fully (100%, Sr100) substituted with strontium oxide on a molar basis. The effects of the substitution on bioactive glass properties were studied, including density, solubility, and in vitro cytotoxicity. Stimulation of osteogenic differentiation was investigated using mesenchymal stromal cells obtained from rat bone marrow. Strontium substitution resulted in altered physical properties including increased solubility. Statistically significant reductions in cell viability were observed with the addition of bioactive glass powders to culture medium. Specifically, addition of ≥ 13.3 mg/ml of 45S5 bioactive glass or Sr50, or ≥ 6.7 mg/ml of Sr100, resulted in significant inhibition. Real‐time PCR analyses detected the upregulation of genes associated with osteoblastic differentiation in the presence of all bioactive glass compositions. Some genes, including Alpl and Bglap, were further stimulated in the presence of Sr50 and Sr100. It was concluded that strontium‐substituted bioactive glasses promoted osteogenesis in a differentiating bone cell culture model and, therefore, have considerable potential for use as improved bioactive glasses for bone tissue regeneration. © 2015 The Authors. Journal of Tissue Engineering and Regenerative Medicine Published by John Wiley & Sons Ltd.