Publications

Objective: Remineralising composites with antibacterial properties may seal the cavity and prevent secondary caries. This study aimed at developing experimental flowable composites containing different concentrations of fluoride-doped calcium phosphate fillers and evaluating their remineralising and antibacterial properties. Methods: Experimental resin-based composites containing different concentrations (0–20 %) of fluoride-doped calcium phosphate fillers (VS10/VS20) were formulated. The release of calcium (Ca), phosphate (PO) and fluoride (F) ions was assessed for 30 days. Remineralisation properties were evaluated through ATR-FTIR and SEM/EDX after storage in simulated body fluid (SBF). The metabolic activity and viability of Streptococcus gordonii was also evaluated through ATP, CFU and live/dead confocal microscopy. The evaluation of specific monomer elution from the experimental composites was conducted using high-performance liquid chromatography (HPLC). Results: The composites containing VS10 showed the highest release of Ca, those containing VS20 released more F over time (p < 0.05), while there was no significant difference in terms of PO ions release between the groups (p > 0.05). A quick 7-day mineral precipitation was observed in the tested composites containing VS10 or VS20 at 10 wt%; these materials also showed the greatest antibacterial activity (p < 0.05). Moreover, the tested composites containing VS10 presented the lowest elution of monomers (p < 0.05). Conclusions: Innovative composites were developed with low monomers elution, evident antibacterial activity against S. gordonii and important remineralisation properties due to specific ions release. Clinical significance: Novel composites containing fluoride-doped calcium phosphates may be promising to modulate bacteria growth, promote remineralisation and reduce the risk of cytotoxicity related to monomers’ elution. © 2024 The Author(s)

JTD Keywords: Antibacterial, Apatite, Calcium phosphate, Ion-release, Remineralisation, Resin composite

The application of 3D printing to calcium phosphates has opened unprecedented possibilities for the fabrication of personalized bone grafts. However, their biocompatibility and bioactivity are counterbalanced by their high brittleness. In this work we aim at overcoming this problem by developing a self-hardening ink containing reactive ceramic particles in a polycaprolactone solution instead of the traditional approach that use hydrogels as binders. The presence of polycaprolactone preserved the printability of the ink and was compatible with the hydrolysis-based hardening process, despite the absence of water in the ink and its hydrophobicity. The microstructure evolved from a continuous polymeric phase with loose ceramic particles to a continuous network of hydroxyapatite nanocrystals intertwined with the polymer, in a configuration radically different from the polymer/ceramic composites obtained by fused deposition modelling. This resulted in the evolution from a ductile behavior, dominated by the polymer, to a stiffer behavior as the ceramic phase reacted. The polycaprolactone binder provides two highly relevant benefits compared to hydrogel-based inks. First, the handleability and elasticity of the as-printed scaffolds, together with the proven possibility of eliminating the solvent, opens the door to implanting the scaffolds freshly printed once lyophilized, while in a ductile state, and the hardening process to take place inside the body, as in the case of calcium phosphate cements. Second, even with a hydroxyapatite content of more than 92 wt.%, the flexural strength and toughness of the scaffolds after hardening are twice and five times those of the all-ceramic scaffolds obtained with the hydrogel-based inks, respectively. Statement of significance: Overcoming the brittleness of ceramic scaffolds would extend the applicability of synthetic bone grafts to high load-bearing situations. In this work we developed a 3D printing ink by replacing the conventional hydrogel binder with a water-free polycaprolactone solution. The presence of polycaprolactone not only enhanced significantly the strength and toughness of the scaffolds while keeping the proportion of bioactive ceramic phase larger than 90 wt.%, but it also conferred flexibility and manipulability to the as-printed scaffolds. Since they are able to harden upon contact with water under physiological conditions, this opens up the possibility of implanting them immediately after printing, while they are still in a ductile state, with clear advantages for fixation and press-fit in the bone defect. © 2024 The Authors

JTD Keywords: 3-d printing, 3d printing, 3d-printing, Binders, Biocompatibility, Biomimetic hydroxyapatites, Biomimetics, Bone cement, Bone scaffolds, Brittleness, Calcium phosphate, Ceramic phase, Ceramic scaffolds, Ceramics particles, Fracture mechanics, Hardening, Hardening process, Hydrogels, Hydroxyapatite, Mechanical properties, Plasticity, Polycaprolactone, Scaffolds, Scaffolds (biology), Self hardening, Strength and toughness

Human mesenchymal stromal cells (hMSCs) seeded on calcium phosphate (CaP) bioceramics are extensively explored in bone tissue engineering and have recently shown effective clinical outcomes. In previous pre-clinical studies, hMSCs-CaP-mediated bone formation was preceded by osteoclastogenesis at the implantation site. The current study evaluates to what extent phase composition of CaPs affects the osteoclast response and ultimately influence bone formation. To this end, four different CaP bioceramics were used, hydroxyapatite (HA), beta-tricalcium phosphate (beta-TCP) and two biphasic composites of HA/beta- TCP ratios of 60/40 and 20/80 respectively, for in vitro osteoclast differentiation and correlation with in vivo osteoclastogenesis and bone formation. All ceramics allowed osteoclast formation in vitro from mouse and human precursors, except for pure HA, which significantly impaired their maturation. Ectopic implantation alongside hMSCs in subcutis sites of nude mice revealed new bone formation at 8 weeks in all conditions with relative amounts for beta-TCP > biphasic CaPs > HA. Surprisingly, while hMSCs were essential for osteoinduction, their survival did not correlate with bone formation. By contrast, the degree of early osteoclastogenesis (2 weeks) seemed to define the extent of subsequent bone formation. Together, our findings suggest that the osteoclastic response could be used as a predictive marker in hMSC-CaPbased bone regeneration and strengthens the need to understand the underlying mechanisms for future biomaterial development. Statement of significance The combination of mesenchymal stromal cells (MSCs) and calcium phosphate (CaP) materials has demonstrated its safety and efficacy for bone regeneration in clinical trials, despite our insufficient understanding of the underlying biological mechanisms. Osteoclasts were previously suggested as key mediators between the early inflammatory phase following biomaterial implantation and the subsequent bone formation. Here we compared the affinity of osteoclasts for various CaP materials with different ratios of hydroxyapatite to beta-tricalcium phosphate. We found that osteoclast formation, both in vitro and at early stages in vivo, correlates with bone formation when the materials were implanted alongside MSCs in mice. Surprisingly, MSC survival did not correlate with bone formation, suggesting that the number or phenotype of osteoclasts formed was more important. (c) 2024 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

JTD Keywords: Acid phosphatase tartrate resistant isoenzyme, Animal, Animal cell, Animal experiment, Animal tissue, Animals, Article, Beta-tricalcium phosphate, Bioceramics, Biocompatible materials, Biomaterial, Bone, Bone development, Bone formation, Bone regeneration, Calcium phosphate, Calcium phosphate materials, Calcium phosphates, Cd14 antigen, Cell differentiation, Cell engineering, Cell maturation, Cell survival, Ceramics, Chemical composition, Controlled study, Correlation analysis, Correlation coefficient, Data correlation, Durapatite, Engraftment, Flowcharting, Human, Human cell, Human mesenchymal stromal cell, Human mesenchymal stromal cells, Humans, Hydroxyapatite, Hydroxyapatites, In vitro study, In vivo study, In-vitro, In-vivo, Mammals, Marrow stromal cells, Material composition, Material compositions, Mesenchymal stroma cell, Mesenchymal stromal cells, Mice, Mice, nude, Monocyte, Mouse, Nonhuman, Nude mouse, Ossification, Osteoclast, Osteoclastogenesis, Osteoclasts, Osteogenesis, Osteoinduction, Phase composition, Regeneration strategies, Resorption, Scaffolds, Stem-cells, Subcutaneous tissue, Tissue engineering, Transmission control protocol, Tri-calcium phosphates, Vimentin

The present study investigated early interactions between three alloplastic materials (calcium phosphate (CaP), titanium alloy (Ti), and polyetheretherketone (PEEK) with human whole blood using an established in vitro slide chamber model. After 60 min of contact with blood, coagulation (thrombin-antithrombin complexes, TAT) was initiated on all test materials (Ti > PEEK > CaP), with a significant increase only for Ti. All materials showed increased contact activation, with the KK-AT complex significantly increasing for CaP (p < 0.001), Ti (p < 0.01), and PEEK (p < 0.01) while only CaP demonstrated a notable rise in KK-C1INH production (p < 0.01). The complement system had significant activation across all materials, with CaP (p < 0.0001, p < 0.0001) generating the most pronounced levels of C3a and sC5b-9, followed by Ti (p < 0.001, p < 0.001) and lastly, PEEK (p < 0.001, p < 0.01). This activation correlated with leukocyte stimulation, particularly myeloperoxidase release. Consequently, the complement system may assume a more significant role in the early stages post implantation in response to CaP materials than previously recognized. Activation of the complement system and the inevitable activation of leukocytes might provide a more favorable environment for tissue remodeling and repair than has been traditionally acknowledged. While these findings are limited to the early blood response, complement and leukocyte activation suggest improved healing outcomes, which may impact long-term clinical outcomes.

JTD Keywords: activation, biomaterial, calcium phosphate, coagulation, complement, cranioplasty, human whole blood, platelets, receptors, surface, titanium, Biomaterials, Calcium phosphate, Coagulation, Complement, Human whole blood, Peroxidase enzymes

JTD Keywords: alpha-tricalcium phosphate, cement, crystallinity, hydrolysis, hydroxyapatite, particle-size, powders, relative humidity, setting reaction, stability, tcp, vapor sorption, Calcium-phosphate, Moisture

The in vitro biological characterization of biomaterials is largely based on static cell cultures. However, for highly reactive biomaterials such as calcium-deficient hydroxyapatite (CDHA), this static environment has limitations. Drastic alterations in the ionic composition of the cell culture medium can negatively affect cell behavior, which can lead to misleading results or data that is difficult to interpret. This challenge could be addressed by a microfluidics-based approach (i.e. on-chip), which offers the opportunity to provide a continuous flow of cell culture medium and a potentially more physiologically relevant microenvironment. The aim of this work was to explore microfluidic technology for its potential to characterize CDHA, particularly in the context of inflammation. Two different CDHA substrates (chemically identical, but varying in microstructure) were integrated on-chip and subsequently evaluated. We demonstrated that the on-chip environment can avoid drastic ionic alterations and increase protein sorption, which was reflected in cell studies with RAW 264.7 macrophages. The cells grown on-chip showed a high cell viability and enhanced proliferation compared to cells maintained under static conditions. Whereas no clear differences in the secretion of tumor necrosis factor alpha (TNF-α) were found, variations in cell morphology suggested a more anti-inflammatory environment on-chip. In the second part of this study, the CDHA substrates were loaded with the drug Trolox. We showed that it is possible to characterize drug release on-chip and moreover demonstrated that Trolox affects the TNF-α secretion and morphology of RAW 264.7 ​cells. Overall, these results highlight the potential of microfluidics to evaluate (bioactive) biomaterials, both in pristine form and when drug-loaded. This is of particular interest for the latter case, as it allows the biological characterization and assessment of drug release to take place under the same dynamic in vitro environment.© 2022 The Authors.

JTD Keywords: alpha-tocopherol, antioxidant, biomaterials, calcium phosphate cement, culture, delivery, drug release, in vitro, in-vitro, ion, macrophage, on-chip, release, tool, Biomaterial, Calcium phosphate cement, Calcium-phosphate cements, Drug release, In vitro, Macrophage, On-chip

Heterotopic ossification (HO) is a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues. Despite being a frequent complication of orthopedic and trauma surgery, brain and spinal injury, the etiology of HO is poorly understood. The aim of this study is to evaluate the hypothesis that a sustained local ionic homeostatic imbalance (SLIHI) created by mineral formation during tissue calcification modulates inflammation to trigger HO. This evaluation also considers the role SLIHI could play for the design of cell-free, drug-free osteoinductive bone graft substitutes. The evaluation contains five main sections. The first section defines relevant concepts in the context of HO and provides a summary of proposed causes of HO. The second section starts with a detailed analysis of the occurrence and involvement of calcification in HO. It is followed by an explanation of the causes of calcification and its consequences. This allows to speculate on the potential chemical modulators of inflammation and triggers of HO. The end of this second section is devoted to in vitro mineralization tests used to predict the ectopic potential of materials. The third section reviews the biological cascade of events occurring during pathological and material-induced HO, and attempts to propose a quantitative timeline of HO formation. The fourth section looks at potential ways to control HO formation, either acting on SLIHI or on inflammation. Chemical, physical, and drug-based approaches are considered. Finally, the evaluation finishes with a critical assessment of the definition of osteoinduction.

JTD Keywords: apatite, beta-tricalcium phosphate, bone, bone graft, bone morphogenetic protein, demineralized bone-matrix, experimental myositis-ossificans, extracellular calcium, heterotopic ossification, in-vitro, inflammation, multinucleated giant-cells, osteoinduction, spinal-cord-injury, total hip-arthroplasty, traumatic brain-injury, Apatite, Calcium-sensing receptor, Osteoinduction

Concave surfaces have shown to promote bone regeneration in vivo. However, bone scaffolds obtained by direct ink writing, one of the most promising approaches for the fabrication of personalized bone grafts, consist mostly of convex surfaces, since they are obtained by microextrusion of cylindrical strands. By modifying the geometry of the nozzle, it is possible to print 3D structures composed of non-cylindrical strands and favor the presence of concave surfaces. In this work, we compare the in vivo performance of 3D-printed calcium phosphate scaffolds with either conventional cylindrical strands or star-shaped strands, in a rabbit femoral condyle model. Mono cortical defects, drilled in contralateral positions, are randomly grafted with the two scaffold configurations, with identical composition. The samples are explanted eight weeks post-surgery and assessed by ??-CT and resin embedded histological observations. The results reveal that the scaffolds containing star-shaped strands have better osteoconductive properties, guiding the newly formed bone faster towards the core of the scaffolds, and enhance bone regeneration, although the increase is not statistically significant (p > 0.05). This new approach represents a turning point towards the optimization of pore shape in 3D-printed bone grafts, further boosting the possibilities that direct ink writing technology offers for patient-specific applications.

JTD Keywords: 3d printing, biomimetic calcium phosphate, bone regeneration, in vivo, pore architecture, 3d printing, Architecture, Biomimetic calcium phosphate, Bone regeneration, Calcium-phosphate scaffolds, Geometry, Growth, Implants, In vivo, Induction, Microporosity, Osteoinduction, Pore architecture, Scaffold, Surfaces, Tissue

Since the discovery that nanostructured surfaces were able to kill bacteria, many works have been published focusing on the design of nanopatterned surfaces with antimicrobial properties. Synthetic bone grafts, based on calcium phosphate (CaP) formulations, can greatly benefit from this discovery if adequate nanotopographies can be developed. However, CaP are reactive materials and experience ionic exchanges when placed into aqueous solutions which may in turn affect cell behaviour and complicate the interpretation of the bactericidal results. The present study explores the bactericidal potential of two nanopillared CaP prepared by hydrolysis of two different sizes of alpha-tricalcium phosphate (alpha-TCP) powders under biomimetic or hydrothermal conditions. A more lethal bactericidal response toward Pseudomonas aeruginosa (similar to 75% killing efficiency of adhered bacteria) was obtained from the hydrothermally treated CaP which consisted in a more irregular topography in terms of pillar size (radius: 20-60 nm), interpillar distances (100-1500 nm) and pillar distribution (pillar groups forming bouquets) than the biomimetically treated one (radius: 20-40 nm and interpillar distances: 50-200 nm with a homogeneous pillar distribution). The material reactivity was greatly influenced by the type of medium (nutrient-rich versus nutrient-free) and the presence or not of bacteria. A lower reactivity and superior bacterial attachment were observed in the nutrient-free medium while a lower attachment was observed for the nutrient rich medium which was explained by a superior reactivity of the material paired with the lower tendency of planktonic bacteria to adhere on surfaces in the presence of nutrients. Importantly, the ionic exchanges produced by the presence of materials were not toxic to planktonic cells. Thus, we can conclude that topography was the main contributor to mortality in the bacterial adhesion tests.

JTD Keywords: bactericidal, calcium deficient hydroxyapatite, calcium phosphates, nanopillars, pseudomonas aeruginosa, reactivity, Adhesion, Antibacterial, Bactericidal, Biomaterials, Calcium deficient hydroxyapatite, Calcium phosphates, Hydroxyapatite, In-vitro, Infections, Nanopillars, Pseudomonas aeruginosa, Pseudomonas-aeruginosa, Reactivity, Recent progress, Silver, Topography, Transmission

The design of catalysts with controlled selectivity at will, also known as catalytic plasticity, is a very attractive approach for the recycling of carbon dioxide (CO2). In this work, we study how catalytically active hydroxyapatite (HAp) and brushite (Bru) interact synergistically, allowing the production of formic acid or acetic acid depending on the HAp/Bru ratio in the catalyst. Raman, wide angle X-ray scattering, X-ray photoelectron spectroscopy, scanning electron microscopy and electrochemical impedance spectroscopy studies, combined with an exhaustive revision of the crystalline structure of the catalyst at the atomic level, allowed to discern how the Bru phase can be generated and stabilized at high temperatures. Results clearly indicate that the presence of OH– groups to maintain the crystalline structural integrity in conjunction with Ca2+ ions less bonded to the lattice fixate carbon into C1, C2 and C3 molecules from CO2 and allow the evolution from formic to acetic acid and acetone. In this way, the plasticity of the HAp-Bru system is demonstrated, representing a promising green alternative to the conventional metal-based electrocatalysts used for CO2 fixation. Thus, the fact that no electric voltage is necessary for the CO2 reduction has a very favorable impact in the final energetic net balance of the carbon fixation reaction. © 2021

JTD Keywords:

ethanol production & nbsp, brushite, co2 reduction, conversion, electrocatalytic reduction, electrode, formate, heterogeneous catalysis & nbsp, hydrogen evolution, insights, monetite, polarized hydroxyapatite,

Visual impairments are a critical medical hurdle to be addressed in modern society. Müller glia (MG) have regenerative potential in the retina in lower vertebrates, but not in mammals. However, in mice, in vivo cell fusion between MG and adult stem cells forms hybrids that can partially regenerate ablated neurons.We used organotypic cultures of human retina and preparations of dissociated cells to test the hypothesis that cell fusion between human MG and adult stem cells can induce neuronal regeneration in human systems. Moreover, we established a microinjection system for transplanting human retinal organoids to demonstrate hybrid differentiation.We first found that cell fusion occurs between MG and adult stem cells, in organotypic cultures of human retina as well as in cell cultures. Next, we showed that the resulting hybrids can differentiate and acquire a proto-neural electrophysiology profile when the Wnt/beta-catenin pathway is activated in the adult stem cells prior fusion. Finally, we demonstrated the engraftment and differentiation of these hybrids into human retinal organoids.We show fusion between human MG and adult stem cells, and demonstrate that the resulting hybrid cells can differentiate towards neural fate in human model systems. Our results suggest that cell fusion-mediated therapy is a potential regenerative approach for treating human retinal dystrophies.This work was supported by La Caixa Health (HR17-00231), Velux Stiftung (976a) and the Ministerio de Ciencia e Innovación, (BFU2017-86760-P) (AEI/FEDER, UE), AGAUR (2017 SGR 689, 2017 SGR 926).Published by Elsevier B.V.

JTD Keywords: cell fusion, expression, fusion, ganglion-cells, in-vitro, mouse, müller glia, neural differentiation, organoids, regeneration, retina regeneration, stem cells, stromal cells, transplantation, 4',6 diamidino 2 phenylindole, 5' nucleotidase, Agarose, Alcohol, Arpe-19 cell line, Article, Beta catenin, Beta tubulin, Bone-marrow-cells, Bromophenol blue, Buffer, Calcium cell level, Calcium phosphate, Calretinin, Canonical wnt signaling, Cd34 antigen, Cell culture, Cell fusion, Cell viability, Coculture, Complementary dna, Confocal microscopy, Cornea transplantation, Cryopreservation, Cryoprotection, Crystal structure, Current clamp technique, Dimethyl sulfoxide, Dodecyl sulfate sodium, Edetic acid, Electrophysiology, Endoglin, Fetal bovine serum, Fibroblast growth factor 2, Flow cytometry, Fluorescence activated cell sorting, Fluorescence intensity, Glyceraldehyde 3 phosphate dehydrogenase, Glycerol, Glycine, Hoe 33342, Immunofluorescence, Immunohistochemistry, Incubation time, Interleukin 1beta, Lentivirus vector, Matrigel, Mercaptoethanol, Microinjection, Mueller cell, Müller glia, N methyl dextro aspartic acid, Nerve cell differentiation, Neural differentiation, Nitrogen, Nonhuman, Organoids, Paraffin, Paraffin embedding, Paraformaldehyde, Patch clamp technique, Penicillin derivative, Phenolsulfonphthalein, Phenotype, Phosphate buffered saline, Phosphoprotein phosphatase inhibitor, Polyacrylamide gel electrophoresis, Potassium chloride, Povidone iodine, Promoter region, Proteinase inhibitor, Real time polymerase chain reaction, Receptor type tyrosine protein phosphatase c, Restriction endonuclease, Retina, Retina dystrophy, Retina regeneration, Retinol, Rhodopsin, Rna extraction, Stem cell, Stem cells, Subcutaneous fat, Tunel assay, Visual impairment, Western blotting

JTD Keywords: augmentation, calcium-phosphate, expansion, fabrication, hydroxyapatite, made artificial bones, osteogenesis, reconstruction, regeneration, 3-d printing, 3d printers, 3d printing, 3d-printing, Abstracting, Additives, Biomaterial, Bone, Bone regeneration, Bone substitution, Bone tissue engineering, Ceramic, Ceramics, Clinic, Controlled drug delivery, Personalized medicines, Surgical planning, Targeted drug delivery, Three-dimensional-printing, Tricalcium phosphate scaffolds

In contrast to sintered calcium phosphates (CaPs) commonly employed as scaffolds to deliver mesenchymal stromal cells (MSCs) targeting bone repair, low temperature setting conditions of calcium deficient hydroxyapatite (CDHA) yield biomimetic topology with high specific surface area. In this study, the healing capacity of CDHA administering MSCs to bone defects is evaluated for the first time and compared with sintered beta-tricalcium phosphate (β-TCP) constructs sharing the same interconnected macroporosity. Xeno-free expanded human bone marrow MSCs attached to the surface of the hydrophobic β-TCP constructs, while infiltrating the pores of the hydrophilic CDHA. Implantation of MSCs on CaPs for 8 weeks in calvaria defects of nude mice exhibited complete healing, with bone formation aligned along the periphery of β-TCP, and conversely distributed within the pores of CDHA. Human monocyte-osteoclast differentiation was inhibited in vitro by direct culture on CDHA compared to β-TCP biomaterials and indirectly by administration of MSC-conditioned media generated on CDHA, while MSCs increased osteoclastogenesis in both CaPs in vivo. MSC engraftment was significantly higher in CDHA constructs, and also correlated positively with bone in-growth in scaffolds. These findings demonstrate that biomimetic CDHA are favorable carriers for MSC therapies and should be explored further towards clinical bone regeneration strategies. Statement of significance: Delivery of mesenchymal stromal cells (MSCs) on calcium phosphate (CaP) biomaterials enhances reconstruction of bone defects. Traditional CaPs are produced at high temperature, but calcium deficient hydroxyapatite (CDHA) prepared at room temperature yields a surface structure more similar to native bone mineral. The objective of this study was to compare the capacity of biomimetic CDHA scaffolds with sintered β-TCP scaffolds for bone repair mediated by MSCs for the first time. In vitro, greater cell infiltration occurred in CDHA scaffolds and following 8 weeks in vivo, MSC engraftment was higher in CDHA compared to β-TCP, as was bone in-growth. These findings demonstrate the impact of material features such as surface structure, and highlight that CDHA should be explored towards clinical bone regeneration strategies.

JTD Keywords: beta-tricalcium phosphate, bone regeneration, calcium deficient hydroxyapatite, differentiation, engraftment, human bone marrow mesenchymal stromal cells, hydroxyapatite scaffolds, in-vitro, inhibition, osteogenesis, osteoinduction, stem-cells, surface-topography, tissue, Beta-tricalcium phosphate, Bone regeneration, Calcium deficient hydroxyapatite, Engraftment, Human bone marrow mesenchymal stromal cells

Hydrothermal (H) processes accelerate the hydrolysis reaction of α-tricalcium phosphate (α-TCP) compared to the long-established biomimetic (B) treatments. They are of special interest for patient-specific 3D-printed bone graft substitutes, where the manufacturing time represents a critical constraint. Altering the reaction conditions has implications for the physicochemical properties of the reaction product. However, the impact of the changes produced by the hydrothermal reaction on the in vivo performance was hitherto unknown. The present study compares the bone regeneration potential of 3D-printed α-TCP scaffolds hardened using these two treatments in rabbit condyle monocortical defects. Although both consolidation processes resulted in biocompatible scaffolds with osseointegrative and osteoconductive properties, the amount of newly formed bone increased by one third in the hydrothermal vs the biomimetic samples. B and H scaffolds consisted mostly of high specific surface area calcium-deficient hydroxyapatite (38 and 27 m2 g-1, respectively), with H samples containing also 10 wt.% β-tricalcium phosphate (β-TCP). The shrinkage produced during the consolidation process was shown to be very small in both cases, below 3%, and smaller for H than for B samples. The differences in the in vivo performance were mainly attributed to the distinct crystallisation nanostructures, which proved to have a major impact on permeability and protein adsorption capacity, using BSA as a model protein, with B samples being highly impermeable. Given the crucial role that soluble proteins play in osteogenesis, this is proposed to be a relevant factor behind the distinct in vivo performances observed for the two materials. Statement of significance: The possibility to accelerate the consolidation of self-setting calcium phosphate inks through hydrothermal treatments has aroused great interest due to the associated advantages for the development of 3D-printed personalised bone scaffolds. Understanding the implications of this approach on the in vivo performance of the scaffolds is of paramount importance. This study compares, for the first time, this treatment to the long-established biomimetic setting strategy in terms of osteogenic potential in vivo in a rabbit model, and relates the results obtained to the physicochemical properties of the 3D-printed scaffolds (composition, crystallinity, nanostructure, nanoporosity) and their interaction with soluble proteins.

JTD Keywords: 3d printing, behavior, biomimetic, bone scaffolds, calcium phosphate, deficient hydroxyapatite, design, graft, hydrothermal, in vivo, morbidity, osteoinduction, porosity, standard, tricalcium phosphate, 3d printing, Biomimetic, Bone scaffolds, Calcium phosphate, Fibula free-flap, Hydrothermal, In vivo

Conventional direct ink writing uses circular nozzles and, therefore, results in cylindrical strands. 3D printing with non-circular nozzles adds new degrees of freedom to this versatile technology, and allows obtaining structures with higher specific surface area or even introducing concave surfaces in the printed architecture. This is an enticing prospect for countless applications, including tissue engineering, chemical reaction catalysts, water evaporators and electrochemical energy storage devices. Despite this, it has been hardly explored by the 3D-printing community. Herein, we develop for the first time 3D printed structures with complex filament section morphologies using a custom-made modular nozzle and a self-setting ceramic ink. The fast elastic recovery of the ink allows obtaining good shape fidelity in the printed filaments, permitting the creation of intricate surfaces with up to 30% concavity and increasing up to 2.5 times the specific surface area compared to cylindrical strands. The use of non-circular nozzles introduces some specific constraints in the printing process. The geometry of the nozzle determines the stable printing directions, and nozzle orientation becomes a critical parameter to achieve a stable printing. Strand torsion, a phenomenon that remains unnoticed with circular nozzles, may result in relevant changes in the geometrical features of the printed structures.

JTD Keywords: calcium phosphate, ceramic, ceramics, flow, geometry, microextrusion, robocasting, Calcium phosphate, Ceramic, Direct ink writing, Microextrusion, Robocasting, Scaffolds

Biomimetic calcium-deficient hydroxyapatite (CDHA) as a bioactive material exhibits exceptional intrinsic osteoinductive and osteogenic properties because of its nanostructure and composition, which promote a favorable microenvironment. Its high reactivity has been hypothesized to play a relevant role in the in vivo performance, mediated by the interaction with the biological fluids, which is amplified by its high specific surface area. Paradoxically, this high reactivity is also behind the in vitro cytotoxicity of this material, especially pro-nounced in static conditions. The present work explores the structural and physicochemical changes that CDHA undergoes in contact with physiological fluids and to investigate its interaction with proteins. Calcium-deficient hydroxyapatite discs with different micro/nanostructures, coarse (C) and fine (F), were exposed to cell-free complete culture medium over extended periods of time: 1, 7, 14, 21, 28, and 50 days. Precipitate formation was not observed in any of the materials in contact with the physiological fluid, which would indicate that the ionic exchanges were linked to incorporation into the crystal structure of CDHA or in the hydrated layer. In fact, CDHA experienced a maturation process, with a progressive increase in crystallinity and the Ca/P ratio, accompanied by an uptake of Mg and a B-type carbonation process, with a gradual propagation into the core of the samples. However, the reactivity of biomimetic hydroxyapatite was highly dependent on the specific surface area and was amplified in nanosized needle-like crystal structures (F), whereas in coarse specimens the ionic exchanges were restricted to the surface, with low penetration in the material bulk. In addition to showing a higher protein adsorption on F substrates, the proteomics study revealed the existence of protein selectivity to-ward F or C microstructures, as well as the capability of CDHA, and more remarkably of F-CDHA, to concentrate specific proteins from the culture medium. Finally, a substantial improvement in the material's ability to support cell proliferation was observed after the CDHA maturation process.

JTD Keywords: calcium phosphates, ion exchange, nanostructure, protein adsorption, Biological-systems, Biomaterials, Biomimetic hydroxyapatites, Biomimetics, Bone-formation, Calcium deficient hydroxyapatite, Calcium phosphate, Calcium phosphates, Cell proliferation, Crystal structure, Crystallinity, Crystals structures, Culture medium, Growth, High reactivity, Hydroxyapatite, In-vitro, Ion exchange, Ionic exchange, Molecular biology, Nanocrystalline apatites, Nanostructure, Nanostructures, Octacalcium phosphate, Physicochemical studies, Physiological fluids, Physiology, Protein adsorption, Proteins, Proteomic studies, Raman spectroscopy, Serum-albumin, Specific surface area

Implants for orthopedic applications need to be biocompatible and bioactive, with mechanical properties similar to those of surrounding natural bone. Given this scenario titanium (Ti) scaffolds obtained by Direct Ink Writing technique offer the opportunity to manufacture customized structures with controlled porosity and mechanical properties. Considering that 3D Ti scaffolds have a significant surface area, it is necessary to develop strategies against the initial bacterial adhesion in order to prevent infection in the early stages of the implantation, while promoting cell adhesion to the scaffold. The challenge is not only achieving a balance between antibacterial activity and osseointegration, it is also to develop a homogeneous coating on the inner and outer surface of the scaffold. The purpose of this work was the development of a single-step electrodeposition process in order to uniformly cover Ti scaffolds with a layer of calcium phosphate (CaP) loaded with chlorhexidine digluconate (CHX). Scaffold characterization was assessed by scanning electron microscopy, Energy dispersive X-ray spectroscopy, X-ray diffraction, micro-Raman microscopy and compressive strength tests. Results determined that the surface of scaffolds was covered by plate-like and whisker-like calcium phosphate crystals, which main phases were octacalcium phosphate and brushite. Biological tests showed that the as-coated scaffolds reduced bacteria adhesion (73 +/- 3% for Staphylococcus aureus and 70 +/- 2% for Escherichia coli). In vitro cell studies and confocal analysis revealed the adhesion and spreading of osteoblast-like SaOS-2 on coated surfaces. Therefore, the proposed strategy can be a potential candidate in bone replacing surgeries.

JTD Keywords: Antibacterial, Bacterial, Behavior, Biocompatibility, Calcium phosphate coating, Chlorhexidine, Chlorhexidine digluconate, Deposition, Electrodeposition, Hydroxyapatite coatings, Implants, One-step pulse electrodeposition, Plasma-spray, Release, Surface, Titanium scaffolds

Nowadays, the main limitation for clinical application of scaffolds is considered to be an insufficient vascularization of the implanted platforms and healing tissues. In our studies, we proposed a novel PLA-based hybrid platform with aligned and random fibrous internal structure and incorporated calcium phosphate (CaP) ormoglass nanoparticles (0, 10, 20 and 30 wt%) as an off-the-shelf method for obtaining scaffolds with pro-angiogenic properties. Complex morphological and physicochemical evaluation of PLA–CaP ormoglass composites was performed before and after in vitro degradation test in SBF solution to assess their biological potential. The degradation process of PLA–CaP ormoglass composites was accompanied by numerous CaP-based precipitations with extended topography and cauliflower-like shape which may enhance bonding of the material with the bone tissue and accelerate the regenerative process. Random fiber orientation was preferable for CaP compounds deposition upon in vitro degradation. CaP compounds precipitated firstly for randomly oriented composite nonwovens with 20 and 30 wt% addition of ormoglass. Moreover, the preliminary bioactivity test has shown that BSA adsorbed to PLA–CaP ormoglass composites (both aligned and randomly oriented) with 20 and 30 wt% of ormoglass nanoparticles which was not observed for pure PLA scaffolds.

JTD Keywords: Calcium phosphate ormoglass, Composites, Degradation, Electrospinning, PLA

Bone apatite consists of carbonated calcium-deficient hydroxyapatite (CDHA) nanocrystals. Biomimetic routes allow fabricating synthetic bone grafts that mimic biological apatite. In this work, we explored the role of two distinctive features of biomimetic apatites, namely, nanocrystal morphology (plate vs needle-like crystals) and carbonate content, on the bone regeneration potential of CDHA scaffolds in an in vivo canine model. Both ectopic bone formation and scaffold degradation were drastically affected by the nanocrystal morphology after intramuscular implantation. Fine-CDHA foams with needle-like nanocrystals, comparable in size to bone mineral, showed a markedly higher osteoinductive potential and a superior degradation than chemically identical coarse-CDHA foams with larger plate-shaped crystals. These findings correlated well with the superior bone-healing capacity showed by the fine-CDHA scaffolds when implanted intraosseously. Moreover, carbonate doping of CDHA, which resulted in small plate-shaped nanocrystals, accelerated both the intrinsic osteoinduction and the bone healing capacity, and significantly increased the cell-mediated resorption. These results suggest that tuning the chemical composition and the nanostructural features may allow the material to enter the physiological bone remodeling cycle, promoting a tight synchronization between scaffold degradation and bone formation.

JTD Keywords: Biomimetic, Calcium phosphate, Carbonated apatite, Foaming, Nanostructure, Osteogenesis, Osteoinduction

Calcium phosphate (CaP) substrates are successfully used as bone grafts due to their osteogenic properties. However, the influence of the physicochemical features of CaPs in angiogenesis is frequently neglected despite it being a crucial process for bone regeneration. The present work focuses on analyzing the effects of textural parameters of biomimetic calcium deficient hydroxyapatite (CDHA) and sintered beta-tricalcium phosphate (β-TCP), such as specific surface area, surface roughness, and microstructure, on the behavior of rat endothelial progenitor cells (rEPCs) and their crosstalk with rat mesenchymal stem cells (rMSCs). The higher reactivity of CDHA results in low proliferation rates in monocultured and cocultured systems. This effect is especially pronounced for rMSCs alone, and for CDHA with a fine microstructure. In terms of angiogenic and osteogenic gene expressions, the upregulation of particular genes is especially enhanced for needle-like CDHA compared to plate-like CDHA and β-TCP, suggesting the importance not only of the chemistry of the substrate, but also of its textural features. Moreover, the coculture of rEPCs and rMSCs on needle-like CDHA results in early upregulation of osteogenic modulator, i.e., protein deglycase 1 might be a possible cause of overexpression of osteogenic-related genes on the same substrate.

JTD Keywords: Angiogenesis, Calcium phosphates, Cocultures, Osteogenesis

Biomaterial implantation triggers inflammatory reactions. Understanding the effect of physicochemical features of biomaterials on the release of inflammatory cytokines from immune cells would be of great interest in view of designing bone graft materials to enhance the healing of bone defects. The present work investigated the interactions of two chemically and texturally different calcium phosphate (CaPs) substrates with macrophages, one of the main innate immune cells, and its further impact on osteogenic differentiation of bone forming cells. The behaviour of macrophages seeded on biomimetic calcium deficient hydroxyapatite (CDHA) and sintered β-tricalcium phosphate (β-TCP) was assessed in terms of the release of inflammatory cytokines and osteoclastogenic factors. The osteogenic differentiation of bone progenitor cells (bone marrow stromal cells (BMSCs) and osteoblastic cell line (SaOS-2)) were subsequently studied by incubating with the conditioned medium induced by macrophage-CaPs interaction in order to reveal the effect of immune cell reaction to CaPs on osteogenic differentiation. It was found that the incubation of macrophages with CaPs substrates caused a decrease of pro-inflammatory cytokines, more pronounced for β-TCP compared with CDHA showing significantly decreased IL-6, TNF-a, and iNOS. However, the macrophage-CDHA interaction resulted in a more favourable environment for osteogenic differentiation of osteoblasts with more collagen type I production and osteogenic genes (Runx2, BSP) expression, suggesting that osteogenic differentiation of bone cells is not only determined by the nature of biomaterials, but also significantly influenced by the inflammatory environment generated by the interaction of immune cells and biomaterials. Statement of Significance: The field of osteoimmunology highlights the importance of the cross-talk between immune and bone cells for effective bone regeneration. This tight interaction opens the door to new strategies that encompass the development of smart cell-instructive biomaterials which performance covers the events from early inflammation to osteogenesis. The present work links the anti-inflammatory and osteoimmunomodulatory features of synthetic bone grafts to their chemistry and texture, focussing on the cross-talk between macrophages and two major orchestrators of bone healing, namely primary mesenchymal stem cells and osteoblasts. The results emphasize the importance of the microenvironment created through the interaction between the substrate and the immune cells as it can stimulate osteogenic events and subsequently foster bone healing.

JTD Keywords: Calcium phosphates, Immunomodulation, Inflammation, Osteogenesis, Osteoimmunomodulation

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. SiO44−, 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.

JTD Keywords: Calcium phosphate glasses, Sol-gel process, NMR spectroscopy, Ion release, Biomaterials

The purpose of the present work was to evaluate in vivo different antimicrobial therapies to eradicate osteomyelitis created in the femoral head of New Zealand rabbits. Five phosphate-based cements were evaluated: calcium phosphate cements (CPC) and calcium phosphate foams (CPF), both in their pristine form and loaded with doxycycline hyclate, and an intrinsic antimicrobial magnesium phosphate cement (MPC; not loaded with an antibiotic). The cements were implanted in a bone previously infected with Staphylococcus aureus to discern the effects of the type of antibiotic administration (systemic vs. local), porosity (microporosity, i.e. <5 μm vs. macroporosity, i.e. >5 μm) and type of antimicrobial mechanism (release of antibiotic vs. intrinsic antimicrobial activity) on the improvement of the health state of the infected animals. A new method was developed, with a more comprehensive composite score that integrates 5 parameters of bone infection, 4 parameters of bone structural integrity and 4 parameters of bone regeneration. This method was used to evaluate the health state of the infected animals, both before and after osteomyelitis treatment. The results showed that the composite score allows to discern statistically significant differences between treatments that individual evaluations were not able to identify. Despite none of the therapies completely eradicated the infection, it was observed that macroporous materials (CPF and CPFd, the latter loaded with doxycycline hyclate) and intrinsic antimicrobial MPC allowed a better containment of the osteomyelitis. This study provides novel insights to understand the effect of different antimicrobial therapies in vivo, and a promising comprehensive methodology to evaluate the health state of the animals was developed. We expect that the implementation of such methodology could improve the criteria to select a proper antimicrobial therapy.

JTD Keywords: Calcium phosphate cements, Calcium phosphate foams, Drug delivery, In vivo, Magnesium phosphate cements, Osteomyelitis

Metallic implants have some limitations related to bioactivity and bacteria colonization leading to infections. In this regard, calcium phosphate coatings can be used as carrier for drug delivery in order to improve the mentioned drawbacks. The present work proposes the introduction of an antibacterial agent in the course of a pulsed and reverse pulsed electrodeposition. Calcium phosphate coatings were prepared in 30 min using different pulse waveforms (unipolar-bipolar), current densities (2–5 mA/cm2) and temperatures (40–60 °C). Mechanical stability of the as-coated surfaces was studied in order to select the optimal electrodeposition conditions. Subsequently, selected coatings were loaded with an antiseptic agent, chlorhexidine digluconate (CHX), via a single-step co-deposition procedure. CHX concentration added to the electrolyte was adjusted to 3 mM based on the antibacterial efficacy of the loaded coatings evaluated in vitro with Staphylococcus aureus and Escherichia coli bacteria strains. Whereas the same chlorhexidine concentration was added to the electrolyte, results showed that the amount of CHX loaded was different for each condition while release kinetics was maintained. The results of this work demonstrate that a pulsed co-deposition strategy has great potential to modulate local delivery of antibacterial agents such as chlorhexidine digluconate, which may prevent early phase infections of metallic implants after insertion.

JTD Keywords: Antibacterial agent, Calcium phosphate, Characterization, Coating, Pulse electrodeposition, Titanium

Immune cells are sensitive to the microstructural and textural properties of materials. Tuning the structural features of synthetic bone grafts could be a valuable strategy to regulate the specific response of the immune system, which in turn modulates the activity of bone cells. The aim of this study was to analyse the effect of the structural characteristics of biomimetic calcium deficient hydroxyapatite (CDHA) on the innate immune response of macrophages and the subsequent impact on osteogenesis and osteoclastogenesis. Murine RAW 264.7 cells were cultured, under standard and inflammatory conditions, on chemically identical CDHA substrates that varied in microstructure and porosity. The impact on osteogenesis was evaluated by incubating osteoblastic cells (SaOS-2) with RAW-CDHA conditioned extracts. The results showed that macrophages were sensitive to different textural and structural properties of CDHA. Under standard conditions, the impact of inflammatory cytokine production by RAW cells cultured on CDHA played a significant role in the degradation of substrates, suggesting the impact of resorptive behaviour of RAW cells on biomimetic surfaces. Osteoblast differentiation was stimulated by the conditioned media collected from RAW cells cultured on needle-like nanostructured CDHA. The results demonstrated that needle-like nanostructured CDHA was able to generate a favourable osteoimmune environment to regulate osteoblast differentiation and osteogenesis. Under inflammatory conditions, the incubation of RAW cells with less porous CDHA resulted in a decreased gene expression and release of pro-inflammatory cytokines.

JTD Keywords: Calcium phosphates, Biomimetic hydroxyapatite, Osteoimmunomodulation, Inflammation, Osteogenesis, Osteoclastogesis

Direct ink writing (DIW) techniques open up new possibilities for the fabrication of patient-specific bone grafts. Self-setting calcium phosphate inks, which harden at low temperature, allow obtaining nanostructured scaffolds with biomimetic properties and enhanced bioactivity. However, the slow hardening kinetics hampers the translation to the clinics. Different hydrothermal treatments for the consolidation of DIW scaffolds fabricated with an α-tricalcium phosphate /pluronic F127 ink were explored, comparing them with a biomimetic treatment. Three different scaffold architectures were analysed. The hardening process, associated to the conversion of α-tricalcium phosphate to hydroxyapatite was drastically accelerated by the hydrothermal treatments, reducing the time for complete reaction from 7 days to 30 minutes, while preserving the scaffold architectural integrity and retaining the nanostructured features. β-tricalcium phosphate was formed as a secondary phase, and a change of morphology from plate-like to needle-like crystals in the hydroxyapatite phase was observed. The binder was largely released during the treatment. The hydrothermal treatment resulted in a 30% reduction of the compressive strength, associated to the residual presence of β-tricalcium phosphate. Biomimetic and hydrothermally treated scaffolds supported the adhesion and proliferation of rat mesenchymal stem cells, indicating a good suitability for bone tissue engineering applications. Statement of Significance: 3D plotting has opened up new perspectives in the bone regeneration field allowing the customisation of synthetic bone grafts able to fit patient-specific bone defects. Moreover, this technique allows the control of the scaffolds’ architecture and porosity. The present work introduces a new method to harden biomimetic hydroxyapatite 3D-plotted scaffolds which avoids high-temperature sintering. It has two main advantages: i) it is fast and simple, reducing the whole fabrication process from the several days required for the biomimetic processing to a few hours; and ii) it retains the nanostructured character of biomimetic hydroxyapatite and allows controlling the porosity from the nano- to the macroscale. Moreover, the good in vitro cytocompatibility results support its suitability for cell-based bone regeneration therapies.

JTD Keywords: Calcium phosphate, Hydroxyapatite, Biomimetic, Bone regeneration, 3D plotting, Direct ink writing, Bone graft

Biomaterials can interact with cells directly, that is, by direct contact of the cells with the material surface, or indirectly, through soluble species that can be released to or uptaken from the surrounding fluids. However, it is difficult to characterise the relevance of this fluid-mediated interaction separately from the topography and composition of the substrate, because they are coupled variables. These fluid-mediated interactions are amplified in the case of highly reactive calcium phosphates (CaPs) such as biomimetic calcium deficient hydroxyapatite (CDHA), particularly in static in vitro cultures. The present work proposes a strategy to decouple the effect of ion exchange from topographical features by adjusting the volume ratio between the cell culture medium and biomaterial (VCM/VB). Increasing this ratio allowed mitigating the drastic ionic exchanges associated to the compositional changes experienced by the material exposed to the cell culture medium. This strategy was validated using rat mesenchymal stem cells (rMSCs) cultured on CDHA and beta-tricalcium phosphate (β-TCP) discs using different VCM/VB ratios. Whereas in the case of β-TCP the cell response was not affected by this ratio, a significant effect on cell adhesion and proliferation was found for the more reactive CDHA. The ionic exchange, produced by CDHA at low VCM/VB, altered cell adhesion due to the reduced number of focal adhesions, caused cell shrinkage and further rMCSs apoptosis. This was mitigated when using a high VCM/VB, which attenuated the changes of calcium and phosphate concentrations in the cell culture medium, resulting in rMSCs spreading and a viability over time. Moreover, rMSCs showed an earlier expression of osteogenic genes on CDHA compared to sintered β-TCP when extracellular calcium fluctuations were reduced. Statement of Significance: Fluid mediated interactions play a significant role in the bioactivity of calcium phosphates. Ionic exchange is amplified in the case of biomimetic hydroxyapatite, which makes the in vitro characterisation of cell-material interactions especially challenging. The present work proposes a novel and simple strategy to explore the mechanisms of interaction of biomimetic and sintered calcium phosphates with mesenchymal stem cells. The effects of topography and ion exchange are analysed separately by modifying the volume ratio between cell culture medium and biomaterial. High ionic fluctuations interfered in the maturation of focal adhesions, hampering cell adhesion and leading to increased apoptosis and reduced proliferation rate.

JTD Keywords: Calcium phosphates, Mesenchymal stem cells, Intracellular calcium, Cell adhesion

There is an urgent need of synthetic bone grafts with enhanced osteogenic capacity. This can be achieved by combining biomaterials with exogenous growth factors, which however can have numerous undesired side effects, but also by tuning the intrinsic biomaterial properties. In a previous study, we showed the synergistic effect of nanostructure and pore architecture of biomimetic calcium deficient hydroxyapatite (CDHA) scaffolds in enhancing osteoinduction, i.e. fostering the differentiation of mesenchymal stem cells to bone forming cells. This was demonstrated by assessing bone formation after implanting the scaffolds intramuscularly. The present study goes one step forward, since it analyzes the effect of the geometrical features of the same CDHA scaffolds, obtained either by 3D-printing or by foaming, on the osteogenic potential and resorption behaviour in a bony environment. After 6 and 12 weeks of intraosseous implantation, both bone formation and material degradation had been drastically affected by the macropore architecture of the scaffolds. Whereas nanostructured CDHA was shown to be highly osteoconductive both in the robocast and foamed scaffolds, a superior osteogenic capacity was observed in the foamed scaffolds, which was associated with their higher intrinsic osteoinductive potential. Moreover, they showed a significantly higher cell-mediated degradation than the robocast constructs, with a simultaneous and progressive replacement of the scaffold by new bone. In conclusion, these results demonstrate that the control of macropore architecture is a crucial parameter in the design of synthetic bone grafts, which allows fostering both material degradation and new bone formation. Statement of Significance: 3D-printing technologies open new perspectives for the design of patient-specific bone grafts, since they allow customizing the external shape together with the internal architecture of implants. In this respect, it is important to design the appropriate pore geometry to maximize the bone healing capacity of these implants. The present study analyses the effect of pore architecture of nanostructured hydroxyapatite scaffolds, obtained either by 3D-printing or foaming, on the osteogenic potential and scaffold resorption in an in vivo model. While nanostructured hydroxyapatite showed excellent osteoconductive properties irrespective of pore geometry, we demonstrated that the spherical, concave macropores of foamed scaffolds significantly promoted both material resorption and bone regeneration compared to the 3D-printed scaffolds with orthogonal-patterned struts and therefore prismatic, convex macropores.

JTD Keywords: Osteogenesis, Pore architecture, 3D-printing, Foaming, Calcium phosphate

Immune cells play a vital role in regulating bone dynamics. This has boosted the interest in developing biomaterials that can modulate both the immune and skeletal systems. In this study, calcium phosphates discs (i.e., beta-tricalcium phosphate, β-TCP) are functionalized with heparin to investigate the effects on immune and stem cell responses. The results show that the functionalized surfaces downregulate the release of hydrogen peroxide and proinflammatory cytokines (tumor necrosis factor alpha and interleukin 1 beta) from human monocytes and neutrophils, compared to nonfunctionalized discs. The macrophages show both elongated and round shapes on the two ceramic substrates, but the morphology of cells on heparinized β-TCP tends toward a higher elongation after 72 h. The heparinized substrates support rat mesenchymal stem cell (MSC) adhesion and proliferation, and anticipate the differentiation toward the osteoblastic lineage as compared to β-TCP and control. The coupling between the inflammatory response and osteogenesis is assessed by culturing MSCs with the macrophage supernatants. The downregulation of inflammation in contact with the heparinized substrates induces higher expression of bone-related markers by MSCs.

JTD Keywords: Calcium phosphates, Heparinization, Inflammation, Osteogenesis

One of the treatments for recurrent or complicated osteomyelitis is by local antibiotherapy mediated by suitable bone grafts. β–Tricalcium Phosphate (β–TCP) bioceramic is a resorbable bone graft. Its microporosity allows for incorporation of drugs, but a too fast release is often obtained. Complex strategies have been explored to obtain controlled drug release. In this work, plasma polymerization of a biocompatible polymer was investigated on β-TCP. Polyethyleneglycol (PEG)-like polymer coatings of different thickness were deposited on microporous β-TCP loaded with antibiotics. A highly hydrophobic surface was obtained despite the hydrophilicity of the PEG-like layer produced, which was associated to the roughness of the β-TCP substrate. The bioceramics nevertheless retained their suitable biological behavior with regard to human osteoblast cells. The microbiological activity of the antibiotics was preserved, and the coatings reduced the total amount of drug released as a function of the increasing plasma treatment time.

JTD Keywords: Plasma polymerization, β–Tricalcium phosphate, PEG-like polymer, Antibiotics, Drug release, Biocompatibility

Abstract The aim of the present study was the in vitro and in vivo analysis of a bi-layered 3D-printed scaffold combining a PLA layer and a biphasic PLA/bioglass G5 layer for regeneration of osteochondral defects in vivo Focus of the in vitro analysis was on the (molecular) weight loss and the morphological and mechanical variations after immersion in SBF. The in vivo study focused on analysis of the tissue reactions and differences in the implant bed vascularization using an established subcutaneous implantation model in CD-1 mice and established histological and histomorphometrical methods. Both scaffold parts kept their structural integrity, while changes in morphology were observed, especially for the PLA/G5 scaffold. Mechanical properties decreased with progressive degradation, while the PLA/G5 scaffolds presented higher compressive modulus than PLA scaffolds. The tissue reaction to PLA included low numbers of BMGCs and minimal vascularization of its implant beds, while the addition of G5 lead to higher numbers of BMGCs and a higher implant bed vascularization. Analysis revealed that the use of a bi-layered scaffold shows the ability to observe distinct in vivo response despite the physical proximity of PLA and PLA/G5 layers. Altogether, the results showed that the addition of G5 enables to reduce scaffold weight loss and to increase mechanical strength. Furthermore, the addition of G5 lead to a higher vascularization of the implant bed required as basis for bone tissue regeneration mediated by higher numbers of BMGCs, while within the PLA parts a significantly lower vascularization was found optimally for chondral regeneration. Thus, this data show that the analyzed bi-layered scaffold may serve as an ideal basis for the regeneration of osteochondral tissue defects. Additionally, the results show that it might be able to reduce the number of experimental animals required as it may be possible to analyze the tissue response to more than one implant in one experimental animal.

JTD Keywords: Bioactive glass, Polylactic acid (PLA), Bi-layer scaffold, Multinucleated giant cells, Bone substitute, Vascularization, Calcium phosphate glass

Some biomaterials are osteoinductive, that is, they are able to trigger the osteogenic process by inducing the differentiation of mesenchymal stem cells to the osteogenic lineage. Although the underlying mechanism is still unclear, microporosity and specific surface area (SSA) have been identified as critical factors in material-associated osteoinduction. However, only sintered ceramics, which have a limited range of porosities and SSA, have been analyzed so far. In this work, we were able to extend these ranges to the nanoscale, through the foaming and 3D-printing of biomimetic calcium phosphates, thereby obtaining scaffolds with controlled micro- and nanoporosity and with tailored macropore architectures. Calcium-deficient hydroxyapatite (CDHA) scaffolds were evaluated after 6 and 12 weeks in an ectopic-implantation canine model and compared with two sintered ceramics, biphasic calcium phosphate and β-tricalcium phosphate. Only foams with spherical, concave macropores and not 3D-printed scaffolds with convex, prismatic macropores induced significant ectopic bone formation. Among them, biomimetic nanostructured CDHA produced the highest incidence of ectopic bone and accelerated bone formation when compared with conventional microstructured sintered calcium phosphates with the same macropore architecture. Moreover, they exhibited different bone formation patterns; in CDHA foams, the new ectopic bone progressively replaced the scaffold, whereas in sintered biphasic calcium phosphate scaffolds, bone was deposited on the surface of the material, progressively filling the pore space. In conclusion, this study demonstrates that the high reactivity of nanostructured biomimetic CDHA combined with a spherical, concave macroporosity allows the pushing of the osteoinduction potential beyond the limits of microstructured calcium phosphate ceramics.

JTD Keywords: 3D-printing, Calcium phosphate, Foaming, Nanostructure, Osteoinduction

Calcium phosphate cements (CPC) have seen clinical success in many dental and orthopaedic applications in recent years. The properties of CPC essential for clinical success are reviewed in this article, which includes properties of the set cement (e.g. bioresorbability, biocompatibility, porosity and mechanical properties) and unset cement (e.g. setting time, cohesion, flow properties and ease of delivery to the surgical site). Emphasis is on the delivery of calcium phosphate (CaP) pastes and CPC, in particular the occurrence of separation of the liquid and solid components of the pastes and cements during injection; and established methods to reduce this phase separation. In addition a review of phase separation mechanisms observed during the extrusion of other biphasic paste systems and the theoretical models used to describe these mechanisms are discussed. Statement of Significance Occurrence of phase separation of calcium phosphate pastes and cements during injection limits their full exploitation as a bone substitute in minimally invasive surgical applications. Due to lack of theoretical understanding of the phase separation mechanism(s), optimisation of an injectable CPC that satisfies clinical requirements has proven difficult. However, phase separation of pastes during delivery has been the focus across several research fields. Therefore in addition to a review of methods to reduce phase separation of CPC and the associated constraints, a review of phase separation mechanisms observed during extrusion of other pastes and the theoretical models used to describe these mechanisms is presented. It is anticipated this review will benefit future attempts to develop injectable calcium phosphate based systems.

JTD Keywords: Bone cements, Calcium phosphates, Injectability, Material properties, Phase separation

The capacity of calcium phosphates to be replaced by bone is tightly linked to their resorbability. However, the relative importance of some textural parameters on their degradation behavior is still unclear. The present study aims to quantify the effect of composition, specific surface area (SSA), and porosity at various length scales (nano-, micro- and macroporosity) on the in vitro degradation of different calcium phosphates. Degradation studies were performed in an acidic medium to mimic the osteoclastic environment. Small degradations were found in samples with interconnected nano- and micropores with sizes below 3 µm although they were highly porous (35–65%), with maximum weight loss of 8 wt%. Biomimetic calcium deficient hydroxyapatite, with high SSA and low crystallinity, presented the highest degradation rates exceeding even the more soluble β-TCP. A dependence of degradation on SSA was indisputable when porosity and pore sizes were increased. The introduction of additional macroporosity with pore interconnections above 20 µm significantly impacted degradation, more markedly in the substrates with high SSA (>15 m2/g), whereas in sintered substrates with low SSA (<1 m2/g) it resulted just in a linear increase of degradation. Up to 30 % of degradation was registered in biomimetic substrates, compared to 15 % in β-TCP or 8 % in sintered hydroxyapatite. The incorporation of carbonate in calcium deficient hydroxyapatite did not increase its degradation rate. Overall, the study highlights the importance of textural properties, which can modulate or even outweigh the effect of other features such as the solubility of the compounds. Statement of Significance The physicochemical features of calcium phosphates are crucial to tune biological events like resorption during bone remodeling. Understanding in vitro resorption can help to predict the in vivo behavior. Besides chemical composition, other parameters such as porosity and specific surface area have a strong influence on resorption. The complexity of isolating the contribution of each parameter lies in the close interrelation between them. In this work, a multiscale study was proposed to discern the extent to which each parameter influences degradation in a variety of calcium phosphates, using an acidic medium to resemble the osteoclastic environment. The results emphasize the importance of textural properties, which can modulate or even outweigh the effect of the intrinsic solubility of the compounds.

JTD Keywords: Calcium phosphates, Degradation, Porosity, Textural properties

Insufficient angiogenesis remains a major hurdle in current bone tissue engineering strategies. An extensive body of work has focused on the use of angiogenic factors or endothelial progenitor cells. However, these approaches are inherently complex, in terms of regulatory and methodologic implementation, and present a high cost. We have recently demonstrate the potential of electrospun poly(lactic acid) (PLA) fiber-based membranes, containing calcium phosphate (CaP) ormoglass particles, to elicit angiogenesis in vivo, in a subcutaneous model in mice. Here we have devised an injectable composite, containing CaP glass-ceramic particles, dispersed within a (Hydroxypropyl)methyl cellulose (HPMC) matrix, with the capacity to release calcium in a more sustained fashion. We show that by tuning the release of calcium in vivo, in a rat bone defect model, we could improve both bone formation and increase angiogenesis. The bone regeneration kinetics was dependent on the Ca2+ release rate, with the faster Ca2+ release composite gel showing improved bone repair at 3 weeks, in relation to control. In the same line, improved angiogenesis could be observed for the same gel formulation at 6 weeks post implantation. This methodology allows to integrate two fundamental processes for bone tissue regeneration while using a simple, cost effective, and safe approach. Statement of Significance In current bone tissue engineering approaches the achievement of sufficient angiogenesis, during tissue regeneration, is a major limitation in order to attain full tissue functionality. Recently, we have shown that calcium ions, released by the degradation of calcium phosphate ormoglasses (CaP), are effective angiogenic promoters, in both in vitro and in a subcutaneous implantation model. Here, we devised an injectable composite, containing CaP glass-ceramic particles, dispersed within a HPMC matrix, enabling the release of calcium in a more sustained fashion. We show that by tuning the release of calcium in vivo, in a rat bone defect model, we could improve both bone formation and increase angiogenesis. This simple and cost effective approach holds great promise to translate to the clinics.

JTD Keywords: Angiogenesis, Bone regeneration, Calcium phosphate ormoglasses

Although calcium phosphate cements (CPCs) are used for bone regeneration in a wide range of clinical applications, various physicochemical phenomena are known to hinder their potential use in minimally invasive surgery or in highly vascularized surgical sites, mainly because of their lack of injectability or their low washout resistance. The present work shows that the combination of CPCs with an inverse-thermoresponsive hydrogel is a good strategy for finely tuning the cohesive and rheological properties of CPCs to achieve clinical acceptable injectability to prevent phase separation during implantation and cohesion to avoid washout of the paste. The thermoresponsive CPC developed combines alpha-tricalcium phosphate with an aqueous solution of pluronic F127, which exhibits an inverse thermoresponsive behaviour, with a gelling transformation at around body temperature. These novel CPCs exhibited temperature-dependent properties. Addition of the polymer enhanced the injectability of the paste, even at a low liquid-to-powder ratio, and allowed the rheological properties of the cement to be tuned, with the injection force decreasing with the temperature of the paste. Moreover, the cohesion of the paste was also temperature-dependent and increased as the temperature of the host medium increased due to gelling induced in the paste. The thermoresponsive cement exhibited excellent cohesion and clinically acceptable setting times at 37 °C, irrespective of the initial temperature of the paste. The addition of pluronic F127 slightly delayed the setting reaction in the early stages but did not hinder the full transformation to calcium-deficient hydroxyapatite. Moreover, the frozen storage of premixed thermoresponsive cement pastes was explored, the main physicochemical properties of the cements being maintained upon thawing, even after 18 months of frozen storage. This avoids the need to mix the cement in the operating theatre and allows its use off-the-shelf. The reverse thermoresponsive cements studied herein open up new perspectives in the surgical field, where the sequential gelling/hardening of these novel cements could allow for a better and safer clinical application. Statement of Significance: Calcium phosphate cements are attractive bone substitutes due to their similarity to the bone mineral phase. Although they can be injectable, cohesion and stability of the paste are crucial in terms of performance and safety. A common strategy is the combination with hydrogels. However, this often results in a decrease of viscosity with increasing temperature, which can lead to extravasation and particle leakage from the bone defect. The preferred evolution would be the opposite: a low viscosity would enhance mixing and injection, and an instantaneous increase of viscosity after injection would ensure washout resistance to the blood flow. Here we develop for the first time a calcium phosphate cement exhibiting reverse thermoresponsive properties using a poloxamer featuring inverse thermal gelling.

JTD Keywords: Calcium phosphate cement, Cohesion, Hydroxyapatite, Injectability, Pluronic, Thermoresponsive

The fabrication of calcium phosphates using biomimetic routes, namely, precipitation processes at body temperature, results in distinct features compared to conventional sintered calcium phosphate ceramics, such as a high specific surface area (SSA) and micro-or nanometric crystal size. The aim of this article is to analyze the effects of these parameters on cell response, focusing on two bone cell types: rat mesenchymal stem cells (rMSCs) and human osteoblastic cells (SaOS-2). Biomimetic calcium-deficient hydroxyapatite (CDHA) was obtained by a low temperature setting reaction, and α-Tricalcium phosphate (α-TCP) and β-Tricalcium phosphate were subsequently obtained by sintering CDHA either at 1400°C or 1100°C. Sintered stoichiometric hydroxyapatite (HA) was also prepared using ceramic routes. The materials were characterized in terms of SSA, skeletal density, porosity, and pore size distribution. SaOS-2 cells and rMSCs were seeded either directly on the surfaces of the materials or on glass coverslips subsequently placed on top of the materials to expose the cells to the CaP-induced ionic changes in the culture medium, while avoiding any topography-related effects. CDHA produced higher ionic fluctuations in both cell culture media than sintered ceramics, with a strong decrease of calcium and a release of phosphate. Indirect contact cell cultures revealed that both cell types were sensitive to these ionic modifications, resulting in a decrease in proliferation rate, more marked for CDHA, this effect being more pronounced for rMSCs. In direct contact cultures, good cell adhesion was found on all materials, but, while cells were able to proliferate on the sintered calcium phosphates, cell number was significantly reduced with time on biomimetic CDHA, which was associated to a higher percentage of apoptotic cells. Direct contact of the cells with biomimetic CDHA resulted also in a higher alkaline phosphatase activity for both cell types compared to sintered CaPs, indicating a promotion of the osteoblastic phenotype.

JTD Keywords: Biomimetic hydroxyapatite, Calcium phosphate, Mesenchymal stem cell, Osteoblast

This article presents the application of dual focused ion beam/scanning electron microscopy (FIB-SEM) imaging for preclinical testing of calcium phosphates with osteoclast precursor cells and how this high-resolution imaging technique is able to reveal microstructural changes at a level of detail previously not possible. Calcium phosphate substrates, having similar compositions but different microstructures, were produced using low-and high-Temperature processes (biomimetic calcium-deficient hydroxyapatite [CDHA] and stoichiometric sintered hydroxyapatite, respectively). Human osteoclast precursor cells were cultured for 21 days before evaluating their resorptive potential on varying microstructural features. Alternative to classical morphological evaluation of osteoclasts (OC), FIB-SEM was used to observe the subjacent microstructure by transversally sectioning cells and observing both the cells and the substrates. Resorption pits, indicating OC activity, were visible on the smoother surface of high-Temperature sintered hydroxyapatite. FIB-SEM analysis revealed signs of acidic degradation on the grain surface under the cells, as well as intergranular dissolution. No resorption pits were evident on the surface of the rough CDHA substrates. However, whereas no degradation was detected by FIB sections in the material underlying some of the cells, early stages of OC-mediated acidic degradation were observed under cells with more spread morphology. Collectively, these results highlight the potential of FIB to evaluate the resorptive activity of OC, even in rough, irregular, or coarse surfaces where degradation pits are otherwise difficult to visualize.

JTD Keywords: Bone Regeneration, Calcium Phosphate, Focus Ion Beam, Osteoclast, Resorption, Scanning Electron Microscopy

Abstract In current bone tissue engineering strategies the achievement of sufficient angiogenesis during tissue regeneration is still a major limitation in order to attain full functionality. Several strategies have been described to tackle this problem, mainly by the use of angiogenic factors or endothelial progenitor cells. However, when facing a clinical scenario these approaches are inherently complex and present a high cost. As such, more cost effective alternatives are awaited. Here, we demonstrate the potential of electrospun poly(lactic acid) (PLA) fiber-based membranes, containing calcium phosphate ormoglass (CaP) particles, to elicit angiogenesis in vivo, in a subcutaneous model in mice. We show that the current approach elicited the local expression of angiogenic factors, associated to a chemotactic effect on macrophages, and sustained angiogenesis into the biomaterial. As both PLA and CaP are currently accepted for clinical application these off-the-shelf novel membranes have great potential for guided bone regeneration applications. Statement of significance In current bone tissue engineering approaches the achievement of sufficient angiogenesis, during tissue regeneration, is a major limitation in order to attain full tissue functionality. Recently, our group has found that calcium ions released by the degradation of calcium phosphate ormoglasses (CaP) are effective angiogenic promoters. Based on this, in this work we successfully produced hybrid fibrous mats with different contents of CaP nanoparticles and thus with different calcium ion release rates, using an ormoglass – poly(lactic acid) blend approach. We show that these matrices, upon implantation in a subcutaneous site, could elicit the local expression of angiogenic factors, associated to a chemotactic effect on macrophages, and sustained angiogenesis into the biomaterial, in a CaP dose dependent manner. This off-the-shelf cost effective approach presents great potential to translate to the clinics.

JTD Keywords: Angiogenesis, Bone regeneration, Calcium phosphate ormoglass

Major limitations of calcium phosphate cements (CPCs) are their relatively slow degradation rate and the lack of macropores allowing the ingrowth of bone tissue. The development of self-setting cement foams has been proposed as a suitable strategy to overcome these limitations. In previous work we developed a gelatine-based hydroxyapatite foam (G-foam), which exhibited good injectability and cohesion, interconnected porosity and good biocompatibility in vitro. In the present study we evaluated the in vivo performance of the G-foam. Furthermore, we investigated whether enrichment of the foam with soybean extract (SG-foam) increased its bioactivity. G-foam, SG-foam and non-foamed CPC were implanted in a critical-size bone defect in the distal femoral condyle of New Zealand white rabbits. Bone formation and degradation of the materials were investigated after 4, 12 and 20 weeks using histological and biomechanical methods. The foams maintained their macroporosity after injection and setting in vivo. Compared to non-foamed CPC, cellular degradation of the foams was considerably increased and accompanied by new bone formation. The additional functionalization with soybean extract in the SG-foam slightly reduced the degradation rate and positively influenced bone formation in the defect. Furthermore, both foams exhibited excellent biocompatibility, implying that these novel materials may be promising for clinical application in non-loaded bone defects.

JTD Keywords: Bone regeneration, Calcium phosphate cement, Gelatine, Rabbit model, Soybean

In bone regeneration, silicon-based calcium phosphate glasses (Bioglasses) have been widely used since the 1970s. However, they dissolve very slowly because of their high amount of Si (SiO2 > 45%). Recently, our group has found that calcium ions released by the degradation of glasses in which the job of silicon is done by just 5% of TiO2 are effective angiogenic promoters, because of their stimulation of a cell-membrane calcium sensing receptor (CaSR). Based on this, other focused tests on angiogenesis have found that Bioglasses also have the potential to be angiogenic promoters even with high contents of silicon (80%); however, their slow degradation is still a problem, as the levels of silicon cannot be decreased any lower than 45%. In this work, we propose a new generation of hybrid organically modified glasses, ormoglasses, that enable the levels of silicon to be reduced, therefore speeding up the degradation process. Using electrospinning as a faithful way to mimic the extracellular matrix (ECM), we successfully produced hybrid fibrous mats with three different contents of Si (40, 52, and 70%), and thus three different calcium ion release rates, using an ormoglass–polycaprolactone blend approach. These mats offered a good platform to evaluate different calcium release rates as osteogenic promoters in an in vivo subcutaneous environment. Complementary data were collected to complement Ca2+ release analysis, such as stiffness evaluation by AFM, ζ-potential, morphology evaluation by FESEM, proliferation and differentiation analysis, as well as in vivo subcutaneous implantations. Material and biological characterization suggested that compositions of organic/inorganic hybrid materials with a Si content equivalent to 40%, which were also those that released more calcium, were osteogenic. They also showed a greater ability to form blood vessels. These results suggest that Si-based ormoglasses can be considered an efficient tool for calcium release modulation, which could play a key role in the angiogenic promoting process.

JTD Keywords: Biological materials, Blood vessels, Calcium, Electrospinning, Glass, Hybrid materials, Silicon oxides, Sol-gel process, Sol-gels, Angiogenesis, Biological characterization, Calcium phosphate glass, Calcium-sensing receptors, Degradation process, Extracellular matrices, Organic/inorganic hybrid materials, ormoglasses, Silicon

The aim of this work is to shed light on the role of porosity and pore architecture in the in vivo bone regeneration capacity of biodegradable glass scaffolds. A calcium phosphate glass in the system P2O5-CaO-Na2O-TiO2 was foamed using two different porogens, namely albumen and hydrogen peroxide (H2O2); the resulting three-dimensional porous structures were characterized and implanted in New Zealand rabbits to study their in vivo behavior. Scaffolds foamed with albumen displayed a monomodal pore size distribution centered around 150 μm and a porosity of 82%, whereas scaffolds foamed with H2O2 showed lower porosity (37%), with larger elongated pores, and multimodal size distribution. After 12 weeks of implantation, histology results revealed a good osteointegration for both types of scaffolds. The quantitative morphometric analysis showed the substitution of the biomaterial by new bone in the case of glasses foamed with albumen. In contrast, bone neoformation and material resorption were significantly lower in the defects filled with the scaffolds foamed with H2O2. The results obtained in this study showed that both calcium phosphate glass scaffolds were osteoconductive, biocompatible, and biodegradable materials. However, differences in porosity, pore architecture, and microstructure led to substantially different in vivo response.

JTD Keywords: Bone substitutes, Calcium phosphate glasses, in vivo, Scaffolds, Tissue engineering

In this study gelatin (Gel) modified with calcium phosphate nanoparticles (SG5) and polycaprolactone (PCL) were used to prepare a 3D bi-layer scaffold by collecting electrospun PCL and gelatin/SG5 fibers separately in the same collector. The objective of this study was to combine the desired properties of PCL and Gel/SG5 in the same scaffold in order to enhance mineralization, thus improving the ability of the scaffold to bond to the bone tissue. The scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and the wide angle X-ray diffraction (WAXD) measurements confirmed that SG5 nanoparticles were successfully incorporated into the fibrous gelatin matrix. The composite Gel/SG5/PCL scaffold exhibited more enhanced mechanical properties than individual Gel and Gel/SG5 scaffolds. The presence of SG5 nanoparticles accelerated the nucleation and growth of apatite crystals on the surface of the composite Gel/SG5/PCL scaffold in simulated body fluid (SBF). The osteoblast response in vitro to developed electrospun scaffolds (PCL and Gel/SG5/PCL) was investigated by using normal human primary NHOst cell lines. NHOst cell culture studies showed that higher alkaline phosphatase (ALP) activity and better mineralization were obtained in the case of composite materials than in pure PCL scaffolds. The mechanically strong PCL scaffold served as a skeleton, while the Gel/SG5 fibers facilitated cell spreading and mineralization of the scaffold.

JTD Keywords: Bilayer fibrous scaffold, Ceramic nanoparticles, Electrospinning, Gelatin, Polycaprolactone, Biomechanics, Bone, Calcium phosphate, Cell culture, Electrospinning, Fourier transform infrared spectroscopy, Mechanical properties, Mineralogy, Nanoparticles, Phosphatases, Polycaprolactone, Scanning electron microscopy, X ray diffraction, Polycaprolactone, Alkaline phosphatase activity, Bone tissue engineering, Calcium phosphate nanoparticles, Ceramic nanoparticles, Fibrous scaffolds, Gelatin, Simulated body fluids, Wide-angle x-ray diffraction, Electrospuns, Scaffolds (biology), Electrospinning

Calcium phosphate cements (CPCs) were meant to produce hydroxyapatite (HA), which is the calcium phosphate that usually results when the cements are mixed with or immersed in aqueous media. The golden age of CPCs was in the late 1990s and the beginning of the 21st century, when they were presented as promising bone substitutes and drug delivery systems. The different reactions that take part in the cement self-setting process depend on many experimental factors – the composition of the cement, the stability of the different components, pH, liquid-to-powder ratio (LPR), and temperature, among others. CPCs have demonstrated fair efficiency for bone regeneration. Cements have gradually been embraced in the wider field of composites by hybridizing their compositions in order that they may adapt to the new trends.

JTD Keywords: Calcium phosphate cements (CPCs), Cements, Hydroxyapatite (HA), Liquid-to-powder ratio (LPR)

The aim of the present work was to analyze the influence of the setting reaction on the injectability of tricalcium phosphate (TCP) pastes. Even if the injection was performed early after mixing powder and liquid, powder reactivity was shown to play a significant role in the injectability of TCP pastes. Significant differences were observed between the injection behavior of non-hardening β-TCP pastes and that of self-hardening α-TCP pastes. The differences were more marked at low liquid-to-powder ratios, using fine powders and injecting through thin needles. α-TCP was, in general, less injectable than β-TCP and required higher injection loads. Moreover, clogging was identified as a mechanism hindering or even preventing injectability, different and clearly distinguishable from the filter-pressing phenomenon. α-TCP pastes presented transient clogging episodes, which were not observed in β-TCP pastes with equivalent particle size distribution. Different parameters affecting powder reactivity were also shown to affect paste injectability. Thus, whereas powder calcination resulted in an increased injectability due to lower particle reactivity, the addition of setting accelerants, such as hydroxyapatite nanoparticles, tended to reduce the injectability of the TCP pastes, especially if adjoined simultaneously with a Na2HPO4 solution. Although, as a general trend, faster-setting pastes were less injectable, some exceptions to this rule were found. For example, whereas in the absence of setting accelerants fine TCP powders were more injectable than the coarse ones, in spite of their shorter setting times, this trend was inverted when setting accelerants were added, and coarse powders were more injectable than the fine ones.

JTD Keywords: Calcium phosphate cement, Hydroxyapatite, Injectability, Setting reaction, Tricalcium phosphate

Novel hydroxyapatite (HA)-collagen microcarriers (MCs) with different micro/nanostructures were developed for bone tissue-engineering applications. The MCs were fabricated via calcium phosphate cement (CPC) emulsion in oil. Collagen incorporation in the liquid phase of the CPC resulted in higher MC sphericity. The MCs consisted of a porous network of entangled hydroxyapatite crystals, formed as a result of the CPC setting reaction. The addition of collagen to the MCs, even in an amount as small as 0.8wt%, resulted in an improved interaction with osteoblast-like Saos-2 cells. The micro/nanostructure and the surface texture of the MCs were further tailored by modifying the initial particle size of the CPC. A synergistic effect between the presence of collagen and the nanosized HA crystals was found, resulting in significantly enhanced alkaline phosphatase activity on the collagen-containing nanosized HA MCs.

JTD Keywords: Bone regeneration, Calcium phosphate cement, Cell response, Collagen, Hydroxyapatite, Microcarrier

Calcium phosphate cements are used as synthetic bone grafts, with several advantages, such as their osteoconductivity and injectability. Moreover, their low-temperature setting reaction and intrinsic porosity allow for the incorporation of drugs and active principles in the material. It is the aim of the present work to: a) provide an overview of the different approaches taken in the application of calcium phosphate cements for drug delivery in the skeletal system, and b) identify the most significant achievements. The drugs or active principles associated to calcium phosphate cements are classified in three groups, i) low molecular weight drugs; ii) high molecular weight biomolecules; and iii) ions.

JTD Keywords: Antibiotic, Bioceramic, Biomaterial, Bone regeneration, Calcium phosphate cement, Ceramic matrix, Growth factor, Hydroxyapatite, Ions, Protein

Calcium phosphate compounds can potentially influence cellular fate through ionic substitutions. However, to be able to turn such solution-mediated processes into successful directors of cellular response, a perfect understanding of the material-induced chemical reactions in situ is required. We therefore report on the application of home-made electrochemical microelectrodes, tested as pH and chloride sensors, for precise spatial and temporal characterization of different aqueous environments around calcium phosphate-based biomaterials prepared from α-tricalcium phosphate using clinically relevant liquid to powder ratios. The small size of the electrodes allowed for online measurements in traditionally inaccessible in vitro environments, such as the immediate material-liquid interface and the interior of curing bone cement. The kinetic data obtained has been compared to theoretical sorption models, confirming that the proposed setup can provide key information for improved understanding of the biochemical environment imposed by chemically reactive biomaterials.

JTD Keywords: Calcium phosphate, Hydroxyapatite, Ion sorption, Iridium oxide, Sensors, Animals, Biocompatible Materials, Bone Cements, Calcium Phosphates, Cells, Cultured, Chlorides, Electrochemical Techniques, Gold, Hydrogen-Ion Concentration, Hydroxyapatites, Iridium, Materials Testing, Microelectrodes, Powders, Silver, Silver Compounds, Water

Smart biomaterials play a key role when aiming at successful tissue repair by means of regenerative medicine approaches, and are expected to contain chemical as well as mechanical cues that will guide the regenerative process. Recent advances in the understanding of stem cell biology and mechanosensing have shed new light onto the importance of the local microenvironment in determining cell fate. Herein we report the biological properties of a bioactive, biodegradable calcium phosphate glass/polylactic acid composite biomaterial that promotes bone marrow-derived endothelial progenitor cell (EPC) mobilisation, differentiation and angiogenesis through the creation of a controlled bone healing-like microenvironment. The angiogenic response is triggered by biochemical and mechanical cues provided by the composite, which activate two synergistic cell signalling pathways: a biochemical one mediated by the calcium-sensing receptor and a mechanosensitive one regulated by non-muscle myosin II contraction. Together, these signals promote a synergistic response by activating EPCs-mediated VEGF and VEGFR-2 synthesis, which in turn promote progenitor cell homing, differentiation and tubulogenesis. These findings highlight the importance of controlling microenvironmental cues for stem/progenitor cell tissue engineering and offer exciting new therapeutical opportunities for biomaterialbased vascularisation approaches and clinical applications.

JTD Keywords: Calcium phosphate glass composite, Smart biomaterial, Endothelial progenitor cell, Angiogenesis, Mechanosensing, Calcium-sensing receptor

Hydroxyapatite foams are potential synthetic bone grafting materials or scaffolds for bone tissue engineering. A novel method to obtain injectable hydroxyapatite foams consists in foaming the liquid phase of a calcium phosphate cement. In this process, the cement powder is incorporated into a liquid foam, which acts as a template for macroporosity. After setting, the cement hardens maintaining the macroporous structure of the foam. In this study a low molecular weight surfactant, Polysorbate 80, and a protein, gelatine, were compared as foaming agents of a calcium phosphate cement. The foamability of Polysorbate 80 was greater than that of gelatine, resulting in higher macroporosity in the set hydroxyapatite foam and higher macropore interconnectivity. Gelatine produced less interconnected foams, especially at high concentrations, due to a higher liquid foam stability. However it increased the injectability and cohesion of the foamed paste, and enhanced osteoblastic-like cell adhesion, all of them important properties for bone grafting materials.

JTD Keywords: Hydroxyapatite, Porosity, Calcium phosphate cement, Scaffolds, Foaming, Bone regeneration

Despite their known osteoconductivity, clinical use of calcium phosphate cements is limited both by their relatively slow rate of resorption and by rheological properties incompatible with injectability. Bone in-growth and material resorption have been improved by the development of porous calcium phosphate cements. However, injectable formulations have so far only been obtained through the addition of relatively toxic surfactants. The present work describes the response of osteoblasts to a novel injectable foamed bone cement based on a composite formulation including the bioactive foaming agents soybean and gelatine. The foaming properties of both defatted soybean and gelatine gels were exploited to develop a self-hardening soy/gelatine/hydroxyapatite composite foam able to retain porosity upon injection. After setting, the foamed paste produced a calcium-deficient hydroxyapatite scaffold, showing good injectability and cohesion as well as interconnected porosity after injection. The intrinsic bioactivity of soybean and gelatine was shown to favour osteoblast adhesion and growth. These findings suggest that injectable, porous and bioactive calcium phosphate cements can be produced for bone regeneration through minimally invasive surgery.

JTD Keywords: Calcium phosphate cement, Composite, Bone tissue engineering, Cell viability, Bioactivity

Hydroxyapatite and hybrid gelatine/hydroxyapatite microspheres were obtained through a water in oil emulsion of a calcium phosphate cement (CPC). The setting reaction of the CPC, in this case the hydrolysis of alpha-tricalcium phosphate, was responsible for the consolidation of the microspheres. After the setting reaction, the microspheres consisted of an entangled network of hydroxyapatite crystals, with a high porosity and pore sizes ranging between 0.5 and 5 mu m. The size of the microspheres was tailored by controlling the viscosity of the hydrophobic phase, the rotation speed, and the initial powder size of the CPC. The incorporation of gelatin increased the sphericity of the microspheres, as well as their size and size dispersion. To assess the feasibility of using the microspheres as cell microcarriers, Saos-2 cells were cultured on the microspheres. Fluorescent staining, SEM studies, and LDH quantification showed that the microspheres were able to sustain cell growth. Cell adhesion and proliferation was significantly improved in the hybrid gelatin/hydroxyapatite microspheres as compared to the hydroxyapatite ones.

JTD Keywords: Calcium phosphate(s), Bone graft, Microspheres, Composite/hard tissue, Hydroxy(1)lapatite

Mechanical stimuli are one of the factors that influence tissue differentiation. In the development of biomaterials for bone tissue engineering, mechanical stimuli and formation of a vascular network that transport oxygen to cells within the pores of the scaffolds are essential. Angiogenesis and cell differentiation have been simulated in scaffolds of regular porosity; however, the dynamics of differentiation can be different when the porosity is not uniform. The objective of this study was to investigate the effect of the mechanical stimuli and the capillary network formation on cell differentiation within a scaffold of irregular morphology. A porous scaffold of calcium phosphate based glass was used. The pores and the solid phase were discretized using micro computed tomography images. Cell activity was simulated within the interconnected pore domain of the scaffold using a lattice modeling approach. Compressive strains of 0.5 and 1% of total deformation were applied and two cases of mesenchymal stem cells initialization (in vitro seeding and in vivo) were simulated. Similar capillary networks were formed independently of the cell initialization mode and the magnitude of the mechanical strain applied. Most of vessels grew in the pores at the periphery of the scaffolds and were blocked by the walls of the scaffold. When 0.5% of strain was applied, 70% of the pore volume was affected by mechano-regulatory stimuli corresponding to bone formation; however, because of the lack of oxygen, only 40% of the volume was filled with osteoblasts. 40% of volume was filled with chondrocytes and 3% with fibroblasts. When the mechanical strain was increased to 1%, 11% of the pore volume was filled with osteoblasts, 59% with chondrocytes, and 8% with fibroblasts. This study has shown the dynamics of the correlation between mechanical load, angiogenesis and tissue differentiation within a scaffold with irregular morphology.

JTD Keywords: Tissue engineering, Calcium phosphates, Mechanoregulation, Micro computer tomography, Finite element modeling

The application of minimally invasive surgical techniques in the field of orthopaedic surgery has created a growing need for new injectable synthetic materials that can be used for bone grafting In this work a novel fully synthetic injectable calcium phosphate foam was developed by mixing alpha-tricalcium phosphate (alpha-TCP) powder with a foamed polysorbate 80 solution Polysorbate 80 is a non-ionic surfactant approved for parenteral applications The foam was able to retain the porous structure after injection provided that the foamed paste was injected shortly after mixing (typically 2 5 min), and set through the hydrolysis of alpha-TCP to a calcium-deficient hydroxyapatite, thus producing a hydroxyapatite solid foam in situ The effect of different processing parameters on the porosity. microstructure, injectability and mechanical properties of the hydroxyapatite foams was analysed, and the ability of the pre-set foam to support osteoblastic-like cell proliferation and differentiation was assessed. Interestingly, the concentration of surfactant needed to obtain the foams was lower than that considered safe in drug formulations for parenteral administration The possibility of combining bioactivity, injectability, macroporosity and self-setting ability in a single fully synthetic material represents a step forward in the design of new materials for bone regeneration compatible with minimally invasive surgical techniques.

JTD Keywords: Calcium phosphate cement, Hydroxyapatite foam, Scaffold, Surfactant, Injectable material

The key feature of calcium phosphate cements (CPCs) lies in the setting reaction triggered by mixing one or more solid calcium phosphate salts with an aqueous solution. Upon mixture, the reaction takes place through a dissolution-precipitation process which is macroscopically observed by a gradual hardening of the cement paste. The precipitation of hydroxyapatite nanocrystals at body or room temperature, and the fact that those materials can be used as self-setting pastes, have for many years been the most attractive features of CPCs. However, the need to develop materials able to sustain bone tissue ingrowth and be capable of delivering drugs and bioactive molecules, together with the continuous requirement from surgeons to develop more easily handling cements, has pushed the development of new processing routes that can accommodate all these requirements, taking advantage of the possibility of manipulating the self-setting CPC paste. It is the goal of this paper to provide a brief overview of the new processing developments in the area of CPCs and to identify the most significant achievements.

JTD Keywords: Bone regeneration, Calcium phosphate cements, Granules, Microcarriers, Scaffolds

Surface topography is known to have an influence on osteoblast activity. However, in the case of bioactive materials, topographical changes can affect also ion exchange properties. This makes the problem more complex, since it is often difficult to separate the strictly topographical effects from the effects of ionic fluctuations in the medium. The scope of this paper is to analyze the simultaneous effect of topography and topography-mediated ion exchange on the initial cellular behavior of osteoblastic-like cells cultured on bioactive tissue engineering substrates. Two apatitic substrates with identical chemical composition but different micro/nanostructural features were obtained by low-temperature setting of a calcium phosphate cement. MG63 osteoblastic-like cells were cultured either in direct contact with the substrates or with their extracts. A strong and permanent decrease of calcium concentration in the culture medium, dependent on substrate topography, was detected. A major effect of the substrate microstructure on cell proliferation was observed, explained in part by the topography-mediated ion exchange, but not specifically by the ionic Ca(2+) fluctuations. Cell differentiation was strongly enhanced when cells were cultured on the finer substrate. This effect was not explained by the chemical modification of the medium, but rather suggested a strictly topographical effect.

JTD Keywords: Alkaline Phosphatase/metabolism, Bone Cements/pharmacology, Calcium/metabolism, Calcium Phosphates/pharmacology, Cell Adhesion/drug effects, Cell Differentiation/drug effects, Cell Proliferation/drug effects, Cell Shape/drug effects, Cells, Cultured, Culture Media, Durapatite/pharmacology, Humans, Interferometry, Ion Exchange, Materials Testing, Osteoblasts/ cytology/drug effects/enzymology/ultrastructure, Phosphorus/metabolism, Powders, Tissue Engineering, Tissue Scaffolds

The use of injectable self-setting calcium phosphate cements or soluble glass granules represent two different strategies for bone regeneration, each with distinct advantages and potential applications. This study compares the in vivo behavior of two calcium phosphate cements and two phosphate glasses with different composition, microstructure and solubility, using autologous bone as a control, in a rabbit model. The implanted materials were alpha-tricalcium phosphate cement (cement H), calcium sodium potassium phosphate cement (cement R), and two phosphate glasses in the P2O5-CaO-Na2O and P2O5-CaO-Na2O-TiO2 systems. The four materials were osteoconductive, biocompatible and biodegradable. Radiological and histological studies demonstrated correct osteointegration and substitution of the implants by new bone. The reactivity of the different materials, which depends on their solubility, porosity and specific surface area, affected the resorption rate and bone formation mainly during the early stages of implantation, although this effect was weak. Thus, at 4 weeks the degradation was slightly higher in cements than in glasses, especially for cement R. However, after 12 weeks of implantation all materials showed a similar degradation degree and promoted bone neoformation equivalent to that of the control group.

JTD Keywords: Calcium phosphates, Calcium phosphate cements, Phosphate glasses, Bone grafts, Bone regenerations

Biodegradable polymers reinforced with an inorganic phase such as calcium phosphate glasses may be a promising approach to fulfil the challenging requirements presented by 3D porous scaffolds for tissue engineering. Scaffolds' success depends mainly on their biological behaviour. This work is aimed to the in vitro study of polylactic acid (PLA)/CaP glass 3D porous constructs for bone regeneration. The scaffolds were elaborated using two different techniques, namely solvent-casting and phase-separation. The effect of scaffolds' micro and macrostructure on the biological response of these scaffolds was assayed. Cell proliferation, differentiation and morphology within the scaffolds were studied. Furthermore, polymer/glass scaffolds were seeded under dynamic conditions in a custom-made perfusion bioreactor. Results indicate that the final architecture of the solvent-cast or phase separated scaffolds have a significant effect on cells' behaviour. Solvent-cast scaffolds seem to be the best candidates for bone tissue engineering. Besides, dynamic seeding yielded a higher seeding efficiency in comparison with the static method.

JTD Keywords: Biocompatible Materials/ chemistry, Bone and Bones/ metabolism, Calcium Phosphates/ chemistry, Cell Differentiation, Cell Proliferation, Humans, Lactic Acid/ chemistry, Microscopy, Confocal, Microscopy, Electron, Scanning, Osteoblasts/metabolism, Permeability, Polymers/ chemistry, Porosity, Solvents/chemistry, Tissue Engineering/ methods