Angiogenic and immunomodulation role of ions for initial stages of bone tissue regeneration

It is widely known that bone has intrinsic capacity to self-regenerate after injury. However, the physiological regeneration process can be impaired when there is an extensive damage. One of the main reasons is due to the inability to establish a new vascular network that ensures oxygen and nutrient diffusion, leading to a necrotic core and non-junction of bone. Initially, bone tissue engineering (BTE) emerged to use inert biomaterials to just fill bone defects, but it eventually evolved to mimic bone extracellular matrix and even stimulate bone physiological regeneration process. In this regard, the stimulation of osteogenesis has gained a lot of attention especially in the proper stimulation of angiogenesis, being critical to achieve a successful osteogenesis for bone regeneration. Besides, the immunomodulation of a pro-inflammatory environment towards an anti-inflammatory one upon scaffold implantation has been considered another key process for a proper tissue restoration. To stimulate these phases, growth factors and cytokines have been extensively used. Nonetheless, they present some drawbacks such as low stability and safety concerns. Alternatively, the use of inorganic ions has attracted higher attention due to their higher stability and therapeutic effects with low side effects. This review will first focus in giving fundamental aspects of initial bone regeneration phases, focusing mainly on inflammatory and angiogenic ones. Then, it will describe the role of different inorganic ions in modulating the immune response upon biomaterial implantation towards a restorative environment and their ability to stimulate angiogenic response for a proper scaffold vascularization and successful bone tissue restoration. STATEMENT OF SIGNIFICANCE: The impairment of bone tissue regeneration when there is excessive damage has led to different tissue engineered strategies to promote bone healing. Significant importance has been given in the immunomodulation towards an anti-inflammatory environment together with proper angiogenesis stimulation in order to achieve successful bone regeneration rather than stimulating only the osteogenic differentiation. Ions have been considered potential candidates to stimulate these events due to their high stability and therapeutic effects with low side effects compared to growth factors. However, up to now, no review has been published assembling all this information together, describing individual effects of ions on immunomodulation and angiogenic stimulation, as well as their multifunctionality or synergistic effects when combined together.

Cobalt-Doped Bioactive Glasses for Biomedical Applications: A Review

Improving angiogenesis is the key to the success of most regenerative medicine approaches. However, how and to which extent this may be performed is still a challenge. In this regard, cobalt (Co)-doped bioactive glasses show promise being able to combine the traditional bioactivity of these materials (especially bone-bonding and osteo-stimulatory properties) with the pro-angiogenic effect associated with the release of cobalt. Although the use and local delivery of Co²⁺ ions into the body have raised some concerns about the possible toxic effects on living cells and tissues, important biological improvements have been highlighted both in vitro and in vivo. This review aims at providing a comprehensive overview of Co-releasing glasses, which find biomedical applications as various products, including micro- and nanoparticles, composites in combination with biocompatible polymers, fibers and porous scaffolds. Therapeutic applications in the field of bone repair, wound healing and cancer treatment are discussed in the light of existing experimental evidence along with the open issues ahead.

The structural role and coordination environment of cobalt in 45P2O5-CaO-Na2O phosphate glasses: thermal properties and Raman, UV-vis-NIR, and EPR spectroscopy

Cobalt-containing materials are of interest for a wide range of applications, from biomaterials to solid-state lasers in optics. For instance, Co²⁺ is known to trigger the formation of new blood vessels, i.e. angiogenesis. Here, the use of phosphate glasses as a vehicle for local release of Co²⁺ ions is an attractive strategy to overcome the vascularisation limitation in tissue engineering. This study aimed to establish structure-property correlations as a function of the coordination environment of cobalt in 45P₂O₅-(30 - x)CaO-25Na₂O-xCoO (x: 0.01 to 10 mol%) glasses. Constant polymerization and O/P ratio, resulting ultimately in constant basicity, were shown by ICP-OES and Raman spectroscopy. The latter, combined with EPR analysis, indicated that Co²⁺ was the predominant oxidation state and the presence of Co³⁺ can be excluded. UV-vis-NIR absorption spectra showed that the ratio between Co²⁺ in four- and six-fold coordination remained constant throughout the glass series. Their thermal properties measured by DSC and heating microscopy did not change much in the substitution range studied here. The steady trend in T_g values suggests a compensation between two opposite effects caused by the presence of four and six-fold coordinated Co²⁺, both being present at a constant ratio throughout the glasses. Accordingly, the higher field strength of Co²⁺ compared to that of Ca²⁺ is expected to strengthen the glass network. In contrast, four-fold coordinated cobalt is expected to weaken the network by connecting fewer fragments of the phosphate glass network than six-fold coordinated cobalt. These results indicate that the structural properties of the glasses with constant basicity are influenced by the coordination number of Co²⁺.

Characterisation of osteogenic and vascular responses of hMSCs to Ti-Co doped phosphate glass microspheres using a microfluidic perfusion platform

Using microspherical scaffolds as building blocks to repair bone defects of specific size and shape has been proposed as a tissue engineering strategy. Here, phosphate glass (PG) microcarriers doped with 5 mol % TiO₂ and either 0 mol % CoO (CoO 0%) or 2 mol % CoO (CoO 2%) were investigated for their ability to support osteogenic and vascular responses of human mesenchymal stem cells (hMSCs). Together with standard culture techniques, cell-material interactions were studied using a novel perfusion microfluidic bioreactor that enabled cell culture on microspheres, along with automated processing and screening of culture variables. While titanium doping was found to support hMSCs expansion and differentiation, as well as endothelial cell-derived vessel formation, additional doping with cobalt did not improve the functionality of the microspheres. Furthermore, the microfluidic bioreactor enabled screening of culture parameters for cell culture on microspheres that could be potentially translated to a scaled-up system for tissue-engineered bone manufacturing.

Bone forming ability of recombinant human collagen peptide granules applied with β-tricalcium phosphate fine particles

Recombinant human collagen peptide, developed based on human collagen type I, contains an arginyl-glycyl-aspartic acid (RGD)-rich motif to enhance cell behavior and is anticipated as a xeno-free polymer material for use in tissue engineering. We fabricated granules containing recombinant human collagen peptide (RCP) applied with beta-tricalcium phosphate fine particles (RCP/β-TCP) as bone filling scaffold material and assessed the bone forming ability of RCP/β-TCP. Recombinant peptide was thermal crosslinked and freeze-dried to prepare RCP. An aqueous dispersion of β-TCP fine particles was added to RCP to obtain RCP/β-TCP. Subsequently, RCP/β-TCP were characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), and cell culture assessments. Furthermore, RCP/β-TCP were implanted into rat cranial bone defects for radiographic and histological evaluations. In SEM and EDX analyses of RCP/β-TCP, β-TCP particles dose-dependently covered the surface of RCP. Cell culture tests showed that RCP/β-TCP remarkably promoted proliferation and mRNA expression of various genes, such as integrin β1 and osteogenic markers, of osteoblastic MC3T3-E1 cells. Histomorphometric assessment at 4 weeks showed that RCP/β-TCP significantly promoted new skull bone formation compared to RCP (p < 0.05) and control (no application) (p < 0.01). Accordingly, these findings suggest RCP/β-TCP possess bone forming capability and would be beneficial for bone tissue engineering therapy.

Titanium alloys: in vitro biological analyzes on biofilm formation, biocompatibility, cell differentiation to induce bone formation, and immunological response

Biological effects of titanium (Ti) alloys were analyzed on biofilms of Candida albicans, Enterococcus faecalis, Pseudomonas aeruginosa, Staphylococcus aureus, Streptococcus mutans, and Streptococcus sanguinis, as well as on osteoblast-like cells (MG63) and murine macrophages (RAW 264.7). Standard samples composed of aluminum and vanadium (Ti-6Al-4V), and sample containing niobium (Ti-35Nb) and zirconium (Ti-13Nb-13Zr) were analyzed. Monomicrobial biofilms were formed on the Ti alloys. MG63 cells were grown with the alloys and the biocompatibility (MTT), total protein (TP) level, alkaline phosphatase (ALP) activity, and mineralization nodules (MN) formation were verified. Levels of interleukins (IL-1β and IL-17), tumor necrosis factor alpha (TNF-α), and oxide nitric (NO) were checked, from RAW 264.7 cells supernatants. Data were statically analyzed by one-way analysis of variance (ANOVA) and Tukey's test, or T-test (P ≤ 0.05). Concerning the biofilm formation, Ti-13Nb-13Zr alloy showed the best inhibitory effect on E. faecalis, P. aeruginosa, and S. aureus. And, it also acted similarly to the Ti-6Al-4V alloy on C. albicans and Streptococcus spp. Both alloys were biocompatible and similar to the Ti-6Al-4V alloy. Additionally, Ti-13Nb-13Zr alloy was more effective for cell differentiation, as observed in the assays of ALP and MN. Regarding the stimulation for release of IL-1β and TNF-α, Ti-35Nb and Ti-13Nb-13Zr alloys inhibited similarly the synthesis of these molecules. However, both alloys stimulated the production of IL-17. Additionally, all Ti alloys showed the same effect for NO generation. Thus, Ti-13Nb-13Zr alloy was the most effective for inhibition of biofilm formation, cell differentiation, and stimulation for release of immune mediators.

Feasible and pure P2O5-CaO nanoglasses: An in-depth NMR study of synthesis for the modulation of the bioactive ion release

The use of bioactive glasses (e.g. silicates, phosphates, borates) has demonstrated to be an effective therapy for the restoration of bone fractures, wound healing and vascularization. Their partial dissolution towards the surrounding tissue has shown to trigger positive bioactive responses, without the necessity of using growth factors or cell therapy, which reduces money-costs, side effects and increases their translation to the clinics. However, bioactive glasses often need from stabilizers (e.g. SiO₄^4-, Ti⁴⁺, Co²⁺, etc.) that are not highly abundant in the body and which metabolization is not fully understood. In this study, we were focused on synthesizing pure calcium phosphate glasses without the presence of such stabilizers. We combined a mixture of ethylphosphate and calcium 2-methoxyethoxide to synthesize nanoparticles with different compositions and degradability. Synthesis was followed by an in-depth nuclear magnetic resonance characterization, complemented with other techniques that helped us to correlate the chemical structure of the glasses with their physiochemical properties and reaction mechanism. After synthesis, the organically modified xerogel (i.e. calcium monoethylphosphate) was treated at 200 or 350 °C and its solubility was maintained and controlled due to the elimination of organics, increase of phosphate-calcium interactions and phosphate polycondensation. To the best of our knowledge, we are reporting the first sol-gel synthesis of binary (P₂O₅-CaO) calcium phosphate glass nanoparticles in terms of continuous polycondensated phosphate chains structure without the addition of extra ions. The main goal is to straightforward the synthesis, to get a safer metabolization and to modulate the bioactive ion release. Additionally, we shed light on the chemical structure, reaction mechanism and properties of calcium phosphate glasses with high calcium contents, which nowadays are poorly understood. STATEMENT OF SIGNIFICANCE: The use of bioactive inorganic materials (i.e. bioactive ceramics, glass-ceramics and glasses) for biomedical applications is attractive due to their good integration with the host tissue without the necessity of adding exogenous cells or growth factors. In particular, degradable calcium phosphate glasses are completely resorbable, avoiding the retention in the body of the highly stable silica network of silicate glasses, and inducing a more controllable degradability than bioactive ceramics. However, most calcium phosphate glasses include the presence of stabilizers (e.g. Ti⁴⁺, Na⁺, Co²⁺), which metabolization is not fully understood and complicates their synthesis. The development of binary calcium phosphate glasses with controlled degradability reduces these limitations, offering a simple and completely metabolizable material with higher transfer to the clinics.

Optimized Bioactive Glass: the Quest for the Bony Graft

Technological advances have provided surgeons with a wide range of biomaterials. Yet improvements are still to be made, especially for large bone defect treatment. Biomaterial scaffolds represent a promising alternative to autologous bone grafts but in spite of the numerous studies carried out on this subject, no biomaterial scaffold is yet completely satisfying. Bioactive glass (BAG) presents many qualifying characteristics but they are brittle and their combination with a plastic polymer appears essential to overcome this drawback. Recent advances have allowed the synthesis of organic-inorganic hybrid scaffolds combining the osteogenic properties of BAG and the plastic characteristics of polymers. Such biomaterials can now be obtained at room temperature allowing organic doping of the glass/polymer network for a homogeneous delivery of the doping agent. Despite these new avenues, further studies are required to highlight the biological properties of these materials and particularly their behavior once implanted in vivo. This review focuses on BAG with a particular interest in their combination with polymers to form organic-inorganic hybrids for the design of innovative graft strategies.

A parameterised mathematical model to elucidate osteoblast cell growth in a phosphate-glass microcarrier culture

Tissue engineering has the potential to augment bone grafting. Employing microcarriers as cell-expansion vehicles is a promising bottom-up bone tissue engineering strategy. Here we propose a collaborative approach between experimental work and mathematical modelling to develop protocols for growing microcarrier-based engineered constructs of clinically relevant size. Experiments in 96-well plates characterise cell growth with the model human cell line MG-63 using four phosphate glass microcarrier materials. Three of the materials are doped with 5 mol% TiO₂ and contain 0%, 2% or 5% CoO, and the fourth material is doped only with 7% TiO₂ (0% CoO). A mathematical model of cell growth is parameterised by finding material-specific growth coefficients through data-fitting against these experiments. The parameterised mathematical model offers more insight into the material performance by comparing culture outcome against clinically relevant criteria: maximising final cell number starting with the lowest cell number in the shortest time frame. Based on this analysis, material 7% TiO₂ is identified as the most promising.

Dual therapeutic cobalt-incorporated bioceramics accelerate bone tissue regeneration

Bone grafting on defects caused by trauma or tumor stimulates bone regeneration, a complex process requiring highly orchestrated cell-signal interactions. Bone vascular growth is coupled with osteogenesis, but less is known about the interplay between angiogenesis and osteogenesis. Understanding this relationship is relevant to improved bone regeneration. Here, tricalcium phosphate (TCP) scaffolds doped with varying concentration of cobalt (Co-TCP) were designed to investigate the dosage effect of vascularization on bone formation. The surface structure, phase composition, mechanical features, and chemical composition were investigated. Co doping improved the mechanical properties of TCP. Co-TCP, particularly 2% and 5% Co-TCP, boosted cell viability of bone marrow stromal cells (BMSCs). The 2% Co-TCP promoted alkaline phosphatase activity, matrix mineralization, and expression of osteogenic genes in BMSCs in vitro. However, excessive Co doping decreased TCP-induced osteogenesis. Meanwhile, Co-TCP dose-dependently favored the growth and migration of human umbilical vein endothelial cells (HUVECs), and the expression of vascular endothelial growth factor (VEGF). The 2% Co-TCP significantly shrank the defect area in rat alveolar bone compared with TCP. Smaller bone volume and more abundant blood vessels were observed for 5% Co-TCP compared with 2% Co-TCP. The CD31 immunostaining in the 5% Co-TCP group was more intense than the other two groups, indicating of the increment of endothelium cells. Besides, 5% Co-TCP led to mild inflammatory response in bone defect area. Overall, TCP doped appropriately with Co has positive effect on osteogenesis, while excessive Co suppressed osteoblast differentiation and bone formation. These data indicate that vascularization within a proper range promotes osteogenesis, which may be a design consideration for bone grafts.

The Antimicrobial Efficacy of Hypoxia Mimicking Cobalt Oxide Doped Phosphate-Based Glasses against Clinically Relevant Gram Positive, Gram Negative Bacteria and a Fungal Strain

Bioactive phosphate glasses are of considerable interest for a range of soft and hard tissue engineering applications. The glasses are degradable and can release biologically important ions in a controlled manner. The glasses can also potentially be used as an antimicrobial delivery system. In the given study, novel cobalt-doped phosphate-based glasses, (P₂O₅)₅₀(Na₂O)₂₀(CaO)_30-x(CoO)_x where 0 ≤ x (mol %) ≤ 10, were manufactured and characterized. As the cobalt oxide concentration increased, the rate of dissolution was observed to decrease. The antimicrobial potential of the glasses was studied using direct and indirect contact methods against both Escherichia coli (NCTC 10538) Staphylococcus aureus (ATCC 6538) and Candida albicans (ATCC 76615). The results showed strong, time dependent, and strain specific antimicrobial activity of the glasses against microorganisms when in direct contact. Antimicrobial activity (R) ≥ 2 was observed within 2 h against Escherichia coli, whereas a similar effect was achieved in 6 h against Staphylococcus aureus and Candida albicans. However, when in indirect contact, the dissolution products from the bioactive glasses failed to show an antimicrobial effect. Following direct exposure to the glasses for 7 days, osteoblast-like SAOS-2 cells showed a 5-fold increase in VEGF mRNA while THP-1 monocytic cells showed a 4-fold increase in VEGF mRNA expression when exposed to 10% CoO-doped glass compared with the cobalt free control glass. Endothelial cells stimulated with conditioned medium taken from cell cultures of THP-1 monocytes exposed to 10% CoO doped glass showed clear tubelike structure (blood vessel) formation after 4 h.