Melt-electrospun polycaprolactone strontium-substituted bioactive glass scaffolds for bone regeneration

Effects of aligned PLGA/SrCSH composite scaffolds on in vitro growth and osteogenic differentiation of human mesenchymal stem cells

Strontium (Sr) has important functions in bone remodeling. Incorporating strontium-doped α-calcium sulfate hemihydrate (SrCSH) into poly(lactic-co-glycolic acid) (PLGA) fibrous scaffolds were expected to increase its bio-activity and provide a potential material for bone tissue engineering. In the present study, Sr-containing aligned PLGA/SrCSH fibrous scaffolds similar to the architecture of natural bone were prepared via wet spinning. CCK-8 assay revealed that Sr-containing scaffolds possessed better bioactivity and supported favorable cell growth effectively. The aligned PLGA/SrCSH fibers exerted a contact effect on cell attachment, inducing regular cell alignment and influencing a series of cell behaviors. Releasing of high concentration Sr from a-PLGA/SrCSH scaffolds could induce high expression levels of BMP-2, increase ALP activity and upregulate RUNX-2 expression, and further promote the expressions of COL-I and OCN and the maximum mineralization. This study demonstrated that Sr and ordered structure in a-PLGA/SrCSH fibrous scaffolds could synergistically enhance the osteogenic differentiation of umbilical cord mesenchymal stem cells (UCMSCs) by regulating cell arrangement and expressions of osteogenic genes.

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

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

Composite Graded Melt Electrowritten Scaffolds for Regeneration of the Periodontal Ligament-to-Bone Interface

Periodontitis is a ubiquitous chronic inflammatory, bacteria-triggered oral disease affecting the adult population. If left untreated, periodontitis can lead to severe tissue destruction, eventually resulting in tooth loss. Despite previous efforts in clinically managing the disease, therapeutic strategies are still lacking. Herein, melt electrowriting (MEW) is utilized to develop a compositionally and structurally tailored graded scaffold for regeneration of the periodontal ligament-to-bone interface. The composite scaffolds, consisting of fibers of polycaprolactone (PCL) and fibers of PCL-containing magnesium phosphate (MgP) were fabricated using MEW. To maximize the bond between bone (MgP) and ligament (PCL) regions, we evaluated two different fiber architectures in the interface area. These were a crosshatch pattern at a 0/90° angle and a random pattern. MgP fibrous scaffolds were able to promote in vitro bone formation even in culture media devoid of osteogenic supplements. Mechanical properties after MgP incorporation resulted in an increase of the elastic modulus and yield stress of the scaffolds, and fiber orientation in the interfacial zone affected the interfacial toughness. Composite graded MEW scaffolds enhanced bone fill when they were implanted in an in vivo periodontal fenestration defect model in rats. The presence of an interfacial zone allows coordinated regeneration of multitissues, as indicated by higher expression of bone, ligament, and cementoblastic markers compared to empty defects. Collectively, MEW-fabricated scaffolds having compositionally and structurally tailored zones exhibit a good mimicry of the periodontal complex, with excellent regenerative capacity and great potential as a defect-specific treatment strategy.

Strontium Functionalization of Biomaterials for Bone Tissue Engineering Purposes: A Biological Point of View

Strontium (Sr) is a trace element taken with nutrition and found in bone in close connection to native hydroxyapatite. Sr is involved in a dual mechanism of coupling the stimulation of bone formation with the inhibition of bone resorption, as reported in the literature. Interest in studying Sr has increased in the last decades due to the development of strontium ranelate (SrRan), an orally active agent acting as an anti-osteoporosis drug. However, the use of SrRan was subjected to some limitations starting from 2014 due to its negative side effects on the cardiac safety of patients. In this scenario, an interesting perspective for the administration of Sr is the introduction of Sr ions in biomaterials for bone tissue engineering (BTE) applications. This strategy has attracted attention thanks to its positive effects on bone formation, alongside the reduction of osteoclast activity, proven by in vitro and in vivo studies. The purpose of this review is to go through the classes of biomaterials most commonly used in BTE and functionalized with Sr, i.e., calcium phosphate ceramics, bioactive glasses, metal-based materials, and polymers. The works discussed in this review were selected as representative for each type of the above-mentioned categories, and the biological evaluation in vitro and/or in vivo was the main criterion for selection. The encouraging results collected from the in vitro and in vivo biological evaluations are outlined to highlight the potential applications of materials’ functionalization with Sr as an osteopromoting dopant in BTE.

Development of 3D Printed Biodegradable Mesh with Antimicrobial Properties for Pelvic Organ Prolapse

To address the increasing demand for safe and effective treatment options for pelvic organ prolapse (POP) due to the worldwide ban of the traditional polypropylene meshes, this study introduced degradable polycaprolactone (PCL)/polyethylene glycol (PEG) composite meshes fabricated with melt-electrowriting (MEW). Two PCL/PEG mesh groups: 90:10 and 75:25 (PCL:PEG, wt%) were fabricated and characterized for their degradation rate and mechanical properties, with PCL meshes used as a control. The PCL/PEG composites showed controllable degradation rates by adjusting the PEG content and produced mechanical properties, such as maximal forces, that were higher than PCL alone. The antibacterial properties of the meshes were elicited by coating them with a commonly used antibiotic: azithromycin. Two dosage levels were used for the coating: 0.5 mg and 1 mg per mesh, and both dosage levels were found to be effective in suppressing the growth of S. aureus bacteria. The biocompatibility of the meshes was assessed using human immortalized adipose derived mesenchymal stem cells (hMSC). In vitro assays were used to assess the cell viability (LIVE/DEAD assay), cell metabolic activity (alamarBlue assay) and cell morphology on the meshes (fluorescent and electron microscopy). The cell attachment was found to decrease with increased PEG content. The freshly drug-coated meshes showed signs of cytotoxicity during the cell study process. However, when pre-released for 14 days in phosphate buffered saline, the initial delay in cell attachment on the drug-coated mesh groups showed full recovery at the 14-day cell culture time point. These results indicated that the PCL/PEG meshes with antibiotics coating will be an effective anti-infectious device when first implanted into the patients, and, after about 2 weeks of drug release, the mesh will be supporting cell attachment and proliferation. These meshes demonstrated a potential effective treatment option for POP that may circumvent the issues related to the traditional polypropylene meshes.

Mesoporous Bioglasses Enriched with Bioactive Agents for Bone Repair, with a Special Highlight of María Vallet-Regí’s Contribution

Throughout her impressive scientific career, Prof. María Vallet-Regí opened various research lines aimed at designing new bioceramics, including mesoporous bioactive glasses for bone tissue engineering applications. These bioactive glasses can be considered a spin-off of silica mesoporous materials because they are designed with a similar technical approach. Mesoporous glasses in addition to SiO₂ contain significant amounts of other oxides, particularly CaO and P₂O₅ and therefore, they exhibit quite different properties and clinical applications than mesoporous silica compounds. Both materials exhibit ordered mesoporous structures with a very narrow pore size distribution that are achieved by using surfactants during their synthesis. The characteristics of mesoporous glasses made them suitable to be enriched with various osteogenic agents, namely inorganic ions and biopeptides as well as mesenchymal cells. In the present review, we summarize the evolution of mesoporous bioactive glasses research for bone repair, with a special highlight on the impact of Prof. María Vallet-Regí´s contribution to the field.

Modern approaches on stem cells and scaffolding technology for osteogenic differentiation and regeneration

The process of bone repair has always been a natural mystery. Although bones do repair themselves, supplemental treatment is required for the initiation of the self-regeneration process. Predominantly, surgical procedures are employed for bone regeneration. Recently, cell-based therapy for bone regeneration has proven to be more effective than traditional methods, as it eliminates the immune risk and painful surgeries. In clinical trials, various stem cells, especially mesenchymal stem cells, have shown to be more efficient for the treatment of several bone-related diseases, such as non-union fracture, osteogenesis imperfecta, osteosarcoma, and osteoporosis. Furthermore, the stem cells grown in a suitable three-dimensional scaffold support were found to be more efficient for osteogenesis. It has been shown that the three-dimensional bioscaffolds support and simulate an in vivo environment, which helps in differentiation of stem cells into bone cells. Bone regeneration in patients with bone disorders can be improved through modification of stem cells with several osteogenic factors or using stem cells as carriers for osteogenic factors. In this review, we focused on the various types of stem cells and scaffolds that are being used for bone regeneration. In addition, the molecular mechanisms of various transcription factors, signaling pathways that support bone regeneration and the senescence of the stem cells, which limits bone regeneration, have been discussed.

Osteogenic Capability of Vaterite-Coated Nonwoven Polycaprolactone Scaffolds for In Vivo Bone Tissue Regeneration

In current orthopedic practice, bone implants used to-date often exhibit poor osteointegration, impaired osteogenesis, and, eventually, implant failure. Actively pursued strategies for tissue engineering could overcome these shortcomings by developing new hybrid materials with bioinspired structure and enhanced regenerative potential. In this study, the osteogenic and therapeutic potential of bioactive vaterite is investigated as a functional component of a fibrous polymeric scaffold for bone regeneration. Hybrid two-layered polycaprolactone scaffolds coated with vaterite (PCL/CaCO₃ ) are studied during their 28-days implantation period in a rat femur defect. After this period, the study of tissue formation in the defected area is performed by the histological study of femur cross-sections. Immobilization of alkaline phosphatase (ALP) into PCL/CaCO₃ scaffolds accelerates new bone tissue formation and defect repair. PCL/CaCO₃ and PCL/CaCO₃ /ALP scaffolds reveal 37.3% and 62.9% areas, respectively, filled with newly formed bone tissue in cross-sections compared to unmineralized PCL scaffold (17.5%). Bone turnover markers are monitored on the 7th and 28th days after implantation and reveal an increase of osteocalcin level for both PCL/CaCO₃ and PCL/CaCO₃ /ALP compared with PCL indicating the activation of osteogenesis. These findings indicate that vaterite, as an osteoconductive component of polymeric scaffolds, promotes osteogenesis, supports angiogenesis, and facilitates bone defect repair.

Poly-ε-Caprolactone/Fibrin-Alginate Scaffold: A New Pro-Angiogenic Composite Biomaterial for the Treatment of Bone Defects

We hypothesized that a composite of 3D porous melt-electrowritten poly-ɛ-caprolactone (PCL) coated throughout with a porous and slowly biodegradable fibrin/alginate (FA) matrix would accelerate bone repair due to its angiogenic potential. Scanning electron microscopy showed that the open pore structure of the FA matrix was maintained in the PCL/FA composites. Fourier transform infrared spectroscopy and differential scanning calorimetry showed complete coverage of the PCL fibres by FA, and the PCL/FA crystallinity was decreased compared with PCL. In vitro cell work with osteoprogenitor cells showed that they preferentially bound to the FA component and proliferated on all scaffolds over 28 days. A chorioallantoic membrane assay showed more blood vessel infiltration into FA and PCL/FA compared with PCL, and a significantly higher number of bifurcation points for PCL/FA compared with both FA and PCL. Implantation into a rat cranial defect model followed by microcomputed tomography, histology, and immunohistochemistry after 4- and 12-weeks post operation showed fast early bone formation at week 4, with significantly higher bone formation for FA and PCL/FA compared with PCL. However, this phenomenon was not extrapolated to week 12. Therefore, for long-term bone regeneration, tuning of FA degradation to ensure syncing with new bone formation is likely necessary.

Cytocompatibility of Bone Substitute Materials and Membranes

BACKGROUND/AIM

With the demographic change and associated chronic bone loss, the need for cytocompatible bone replacement materials arise in modern medicine. The aim of this in vitro study was to investigate the cytocompatibility of eleven different bone substitute materials and membranes.

MATERIALS AND METHODS

Seven bone substitute materials and four membranes were assessed in vitro. The specimens were tested based on their interaction with MC3T3 pre-osteoblasts, through the utilization of viability, proliferation, and cytotoxicity assays. Cell vitality was evaluated using live-dead staining.

RESULTS

Although we found minor differences in cytocompatibility among the assessed materials, all tested materials can be considered as cytocompatible with a viability of more than 70% of the negative control, which indicates the non-toxic range as defined in current, international standards (DIN EN ISO 10993-5:2009, German Institute for Standardization, Berlin, Germany). Direct live-dead staining assays confirmed satisfactory cytocompatibility of all tested membranes.

CONCLUSION

All examined bone substitute materials and membranes were found to be cytocompatible. In order to assess whether the observed minor differences can impact regenerative processes, further in vivo studies need to be conducted.

Inflammation and biomaterials: role of the immune response in bone regeneration by inorganic scaffolds

The regulatory role of the immune system in maintaining bone homeostasis and restoring its functionality, when disturbed due to trauma or injury, has become evident in recent years. The polarization of macrophages, one of the main constituents of the immune system, into the pro-inflammatory or anti-inflammatory phenotype has great repercussions for cellular crosstalk and the subsequent processes needed for proper bone regeneration such as angiogenesis and osteogenesis. In certain scenarios, the damaged osseous tissue requires the placement of synthetic bone grafts to facilitate the healing process. Inorganic biomaterials such as bioceramics or bioactive glasses are the most widely used due to their resemblance to the mineral phase of bone and superior osteogenic properties. The immune response of the host to the inorganic biomaterial, which is of an exogenous nature, might determine its fate, leading either to active bone regeneration or its failure. Therefore, various strategies have been employed, like the modification of structural/chemical features or the incorporation of bioactive molecules, to tune the interplay with the immune cells. Understanding how these particular modifications impact the polarization of macrophages and further osteogenic and osteoclastogenic events is of great interest in view of designing a new generation of osteoimmunomodulatory materials that support the regeneration of osseous tissue during all stages of bone healing.

Bioactive glasses and electrospun composites that release cobalt to stimulate the HIF pathway for wound healing applications

BACKGROUND

Bioactive glasses are traditionally associated with bonding to bone through a hydroxycarbonate apatite (HCA) surface layer but the release of active ions is more important for bone regeneration. They are now being used to deliver ions for soft tissue applications, particularly wound healing. Cobalt is known to simulate hypoxia and provoke angiogenesis. The aim here was to develop new bioactive glass compositions designed to be scaffold materials to locally deliver pro-angiogenic cobalt ions, at a controlled rate, without forming an HCA layer, for wound healing applications.

METHODS

New melt-derived bioactive glass compositions were designed that had the same network connectivity (mean number of bridging covalent bonds between silica tetrahedra), and therefore similar biodegradation rate, as the original 45S5 Bioglass. The amount of magnesium and cobalt in the glass was varied, with the aim of reducing or removing calcium and phosphate from the compositions. Electrospun poly(ε-caprolactone)/bioactive glass composites were also produced. Glasses were tested for ion release in dissolution studies and their influence on Hypoxia-Inducible Factor 1-alpha (HIF-1α) and expression of Vascular Endothelial Growth Factor (VEGF) from fibroblast cells was investigated.

RESULTS

Dissolution tests showed the silica rich layer differed depending on the amount of MgO in the glass, which influenced the delivery of cobalt. The electrospun composites delivered a more sustained ion release relative to glass particles alone. Exposing fibroblasts to conditioned media from these composites did not cause a detrimental effect on metabolic activity but glasses containing cobalt did stabilise HIF-1α and provoked a significantly higher expression of VEGF (not seen in Co-free controls).

CONCLUSIONS

The composite fibres containing new bioactive glass compositions delivered cobalt ions at a sustained rate, which could be mediated by the magnesium content of the glass. The dissolution products stabilised HIF-1α and provoked a significantly higher expression of VEGF, suggesting the composites activated the HIF pathway to stimulate angiogenesis.

Development of Mechanically Enhanced Polycaprolactone Composites by a Functionalized Titanate Nanofiller for Melt Electrowriting in 3D Printing

Three-dimensional (3D) printing technologies are widely applied in various industries and research fields and are currently the subject of intensive investigation and development. However, high-performance materials that are suitable for 3D printing are still in short supply, which is a major limitation for 3D printing, particularly for biomedical applications. The physicochemical properties of single constituent materials may not be sufficient to meet the needs of modern biotechnology development and production. To enhance the materials' performance and broaden their applications, this work designed and tested a series of titanate nanofiller (nanowire and nanotube)-enhanced polycaprolactone (PCL) composites that were 3D-printable and provided superior mechanical properties. By grafting two different functional groups (phenyl- and thiol-terminated ligands), the nanofiller surface showed improved hydrophobicity, which significantly improved their dispersion in the PCL matrix. After characterizing the surface modification, we evaluated the significance of the homogeneity of the ceramic nanofiller in terms of printability, formability, and mechanical strength. Melt electrowriting additive manufacturing was used to fabricate microfibers of PCL and PCL/nanofiller composites. Improved nanofiller dispersion enabled intact and uniform sample morphology, and in contrast, nanofiller aggregation greatly varied the viscosity during the printing process, which could result in poorly printed structures. Importantly, the modified ceramic/PCL composite delivered enhanced and stable mechanical properties, where its Young's modulus was measured to be 1.67 GPa, which is more than 7 times higher compared to that of pristine PCL (0.22 GPa). Retaining the cell safety properties (comparable to PCL), the concept of enhancing biocompatible polymers with modified nanofillers shows great potential in the field of customized 3D printing for biomedicine.

Formation of PA12 fibres via melt electrospinning process: parameter analysis and optimisation

Melt electrospinning is a fast-emerging technique for fibre formation. While the process is similar to solution electrospinning, the absence of solvents broadens the applications, avoiding the potential toxicity of solvent residues and enables the usage of non-dissolvable polymers. In this article, the influence of selected melt electrospinning process parameters (tip-to-collector distance, voltage, and melt temperature) on fibre diameter and diameter distribution was investigated. The screening experiments indicated that the lowest fibre diameter median was 2.19 μm. Based on the dependencies between each process parameter and median fibre diameter, the authors used response-surface plots to determine the optimal conditions to produce fibres with the desired fibre diameters. The lowest fibre diameters were obtained with the following process parameter input values: temperature, 348°C; voltage, 19 kV; and tip-to-collector distance, 3 cm. The obtained fibres indicated that the average value of fibre diameter medians decreased in comparison to the screening experiment and the median fibre diameter for the sample “Optim.” was 1.27 μm.

Fabrication and Characteristics of PCL Membranes Containing Strontium-Substituted Hydroxyapatite Nanofibers for Guided Bone Regeneration

Poly(ε-caprolactone) (PCL) membranes have been widely used in guided tissue regeneration (GTR) and guided bone regeneration (GBR). In addition, hydroxyapatite is the major inorganic component and an essential composition of hard bone and teeth. Recently, numerous studies have demonstrated that strontium-substituted hydroxyapatite (SrHA) not only enhances osteogenesis but also inhibits adipogenesis of mesenchymal stem cells. Therefore, SrHA incorporated into PCL could be an alternative material for GBR. In this study, strontium-substituted hydroxyapatite nanofibers (SrHANFs) were fabricated by a sol-gel route followed by electrospinning. We then fabricated PCL-SrHANF membranes as cell culture substrates and assessed the cellular behavior of osteoblast-like cells. Based on the observations of alkaline phosphatase (ALP) activity, bone sialoprotein (BSP) and osteocalcin (OCN) immunofluorescence staining, and Alizarin Red-S staining of cells cultured on the PCL-SrHANF and PCL membranes, we concluded that SrHANFs can promote the differentiation and mineralization of osteoblast-like cells and that PCL-SrHANF membranes have potential for GBR applications.

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

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

Electrospun Nanofibers for Tissue Engineering with Drug Loading and Release

Electrospinning technologies have been applied in the field of tissue engineering as materials, with nanoscale-structures and high porosity, can be easily prepared via this method to bio-mimic the natural extracellular matrix (ECM). Tissue engineering aims to fabricate functional biomaterials for the repairment and regeneration of defective tissue. In addition to the structural simulation for accelerating the repair process and achieving a high-quality regeneration, the combination of biomaterials and bioactive molecules is required for an ideal tissue-engineering scaffold. Due to the diversity in materials and method selection for electrospinning, a great flexibility in drug delivery systems can be achieved. Various drugs including antibiotic agents, vitamins, peptides, and proteins can be incorporated into electrospun scaffolds using different electrospinning techniques and drug-loading methods. This is a review of recent research on electrospun nanofibrous scaffolds for tissue-engineering applications, the development of preparation methods, and the delivery of various bioactive molecules. These studies are based on the fabrication of electrospun biomaterials for the repair of blood vessels, nerve tissues, cartilage, bone defects, and the treatment of aneurysms and skin wounds, as well as their applications related to oral mucosa and dental fields. In these studies, due to the optimal selection of drugs and loading methods based on electrospinning, in vitro and in vivo experiments demonstrated that these scaffolds exhibited desirable effects for the repair and treatment of damaged tissue and, thus, have excellent potential for clinical application.

Medical-grade polycaprolactone scaffolds made by melt electrospinning writing for oral bone regeneration - a pilot study in vitro

BACKGROUND

The spectrum of indications for the use of membranes and scaffolds in the field of oral and maxillofacial surgery includes, amongst others, guided bone regeneration (GBR). Currently available membrane systems face certain disadvantages such as difficult clinical handling, inconsistent degradation, undirected cell growth and a lack of stability that often complicate their application. Therefore, new membranes which can overcome these issues are of great interest in this field.

METHODS

In this pilot study, we investigated polycaprolactone (PCL) scaffolds intended to enhance oral wound healing by means of melt electrospinning writing (MEW), which allowed for three-dimensional (3D) printing of micron scale fibers and very exact fiber placement. A singular set of box-shaped scaffolds of different sizes consisting of medical-grade PCL was examined and the scaffolds' morphology was evaluated via scanning electron microscopy (SEM). Each prototype sample with box sizes of 225 μm, 300 μm, 375 μm, 450 μm and 500 μm was assessed for cytotoxicity and cell growth by seeding each scaffold with human osteoblast-like cell line MG63.

RESULTS

All scaffolds demonstrated good cytocompatibility according to cell viability, protein concentration, and cell number. SEM analysis revealed an exact fiber placement of the MEW scaffolds and the growth of viable MG63 cells on them. For the examined box-shaped scaffolds with pore sizes between 225 μm and 500 μm, a preferred box size for initial osteoblast attachment could not be found.

CONCLUSIONS

These well-defined 3D scaffolds consisting of medical-grade materials optimized for cell attachment and cell growth hold the key to a promising new approach in GBR in oral and maxillofacial surgery.

A comprehensive review of biodegradable synthetic polymer-ceramic composites and their manufacture for biomedical applications

The application of various materials in biomedical procedures has recently experienced rapid growth. One area that is currently receiving significant attention from the scientific community is the treatment of a number of different types of bone-related diseases and disorders by using biodegradable polymer-ceramic composites. Biomaterials, the most common materials used to repair or replace damaged parts of the human body, can be categorized into three major groups: metals, ceramics, and polymers. Composites can be manufactured by combining two or more materials to achieve enhanced biocompatibility and biomechanical properties for specific applications. Biomaterials must display suitable properties for their applications, about strength, durability, and biological influence. Metals and their alloys such as titanium, stainless steel, and cobalt-based alloys have been widely investigated for implant-device applications because of their excellent mechanical properties. However, these materials may also manifest biological issues such as toxicity, poor tissue adhesion and stress shielding effect due to their high elastic modulus. To mitigate these issues, hydroxyapatite (HA) coatings have been used on metals because their chemical composition is similar to that of bone and teeth. Recently, a wide range of synthetic polymers such as poly (l-lactic acid) and poly (l-lactide-co-glycolide) have been studied for different biomedical applications, owing to their promising biocompatibility and biodegradability. This article gives an overview of synthetic polymer-ceramic composites with a particular emphasis on calcium phosphate group and their potential applications in tissue engineering. It is hoped that synthetic polymer-ceramic composites such as PLLA/HA and PCL/HA will provide advantages such as eliminating the stress shielding effect and the consequent need for revision surgery.

Current state of fabrication technologies and materials for bone tissue engineering

A range of traditional and free-form fabrication technologies have been investigated and, in numerous occasions, commercialized for use in the field of regenerative tissue engineering (TE). The demand for technologies capable of treating bone defects inherently difficult to repair has been on the rise. This quest, accompanied by the advent of functionally tailored, biocompatible, and biodegradable materials, has garnered an enormous research interest in bone TE. As a result, different materials and fabrication methods have been investigated towards this end, leading to a deeper understanding of the geometrical, mechanical and biological requirements associated with bone scaffolds. As our understanding of the scaffold requirements expands, so do the capability requirements of the fabrication processes. The goal of this review is to provide a broad examination of existing scaffold fabrication processes and highlight future trends in their development. To appreciate the clinical requirements of bone scaffolds, a brief review of the biological process by which bone regenerates itself is presented first. This is followed by a summary and comparisons of commonly used implant techniques to highlight the advantages of TE-based approaches over traditional grafting methods. A detailed discussion on the clinical and mechanical requirements of bone scaffolds then follows. The remainder of the manuscript is dedicated to current scaffold fabrication methods, their unique capabilities and perceived shortcomings. The range of biomaterials employed in each fabrication method is summarized. Selected traditional and non-traditional fabrication methods are discussed with a highlight on their future potential from the authors' perspective. This study is motivated by the rapidly growing demand for effective scaffold fabrication processes capable of economically producing constructs with intricate and precisely controlled internal and external architectures. STATEMENT OF SIGNIFICANCE: The manuscript summarizes the current state of fabrication technologies and materials used for creating scaffolds in bone tissue engineering applications. A comprehensive analysis of different fabrication methods (traditional and free-form) were summarized in this review paper, with emphasis on recent developments in the field. The fabrication techniques suitable for creating scaffolds for tissue engineering was particularly targeted and their use in bone tissue engineering were articulated. Along with the fabrication techniques, we emphasized the choice of materials in these processes. Considering the limitations of each process, we highlighted the materials and the material properties critical in that particular process and provided a brief rational for the choice of the materials. The functional performance for bone tissue engineering are summarized for different fabrication processes and the choice of biomaterials. Finally, we provide a perspective on the future of the field, highlighting the knowledge gaps and promising avenues in pursuit of effective scaffolds for bone tissue engineering. This extensive review of the field will provide research community with a reference source for current approaches to scaffold preparation. We hope to encourage the researchers to generate next generation biomaterials to be used in these fabrication processes. By providing both advantages and disadvantage of each fabrication method in detail, new fabrication techniques might be devised that will overcome the limitations of the current approaches. These studies should facilitate the efforts of researchers interested in generating ideal scaffolds, and should have applications beyond the repair of bone tissue.

Multi-compartment scaffold fabricated via 3D-printing as in vitro co-culture osteogenic model

The development of in vitro 3D models to get insights into the mechanisms of bone regeneration could accelerate the translation of experimental findings to the clinic, reducing costs and duration of experiments. This work explores the design and manufacturing of multi-compartments structures in poly(ε-caprolactone) (PCL) 3D-printed by Fused Filament Fabrication technique. The construct was designed with interconnected stalls to host stem cells and endothelial cells. Cells were encapsulated within an optimised gellan gum (GG)-based hydrogel matrix, crosslinked using strontium (Sr2+) ions to exploit its bioactivity and finally, assembled within compartments with different sizes. Calcium (Ca2+)-crosslinked gels were also used as control for comparison of Sr2+ osteogenic effect. The results obtained demonstrated that Sr2+ ions were successfully diffused within the hydrogel matrix and increased the hydrogel matrix strength properties under compressive load. The in vitro co-culture of human-TERT mesenchymal stem cells (TERT- hMSCs) and human umbilical vein endothelial cells (HUVECs), encapsulated within Sr2+ ions containing GG-hydrogels and inter-connected by compartmentalised scaffolds under osteogenic conditions, enhanced cell viability and supported osteogenesis, with a significant increase of alkaline phosphatase activity, osteopontin and osteocalcin respect with the Ca2+-crosslinked GG-PCL scaffolds. These outcomes demonstrate that the design and manufacturing of compartmentalised co-culture of TERT-hMSCs and HUVEC populations enables an effective system to study and promote osteogenesis.

Electrospun F18 Bioactive Glass/PCL-Poly (ε-caprolactone)-Membrane for Guided Tissue Regeneration

Barrier membranes that are used for guided tissue regeneration (GTR) therapy usually lack bioactivity and the capability to promote new bone tissue formation. However, the incorporation of an osteogenic agent into polymeric membranes seems to be the most assertive strategy to enhance their regenerative potential. Here, the manufacturing of composite electrospun membranes made of poly (ε-caprolactone) (PCL) and particles of a novel bioactive glass composition (F18) is described. The membranes were mechanically and biologically tested with tensile strength tests and tissue culture with MG-63 osteoblast-like cell line, respectively. The PCL-F18 composite membranes demonstrated no increased cytotoxicity and an enhanced osteogenic potential when compared to pure PCL membranes. Moreover, the addition of the bioactive phase increased the membrane tensile strength. These preliminary results suggested that these new membranes can be a strong candidate for small bone injuries treatment by GTR technique.

Bioactive Sr(II)/Chitosan/Poly(ε-caprolactone) Scaffolds for Craniofacial Tissue Regeneration. In Vitro and In Vivo Behavior

In craniofacial tissue regeneration, the current gold standard treatment is autologous bone grafting, however, it presents some disadvantages. Although new alternatives have emerged there is still an urgent demand of biodegradable scaffolds to act as extracellular matrix in the regeneration process. A potentially useful element in bone regeneration is strontium. It is known to promote stimulation of osteoblasts while inhibiting osteoclasts resorption, leading to neoformed bone. The present paper reports the preparation and characterization of strontium (Sr) containing hybrid scaffolds formed by a matrix of ionically cross-linked chitosan and microparticles of poly(ε-caprolactone) (PCL). These scaffolds of relatively facile fabrication were seeded with osteoblast-like cells (MG-63) and human bone marrow mesenchymal stem cells (hBMSCs) for application in craniofacial tissue regeneration. Membrane scaffolds were prepared using chitosan:PCL ratios of 1:2 and 1:1 and 5 wt % Sr salts. Characterization was performed addressing physico-chemical properties, swelling behavior, in vitro biological performance and in vivo biocompatibility. Overall, the composition, microstructure and swelling degree (≈245%) of scaffolds combine with the adequate dimensional stability, lack of toxicity, osteogenic activity in MG-63 cells and hBMSCs, along with the in vivo biocompatibility in rats allow considering this system as a promising biomaterial for the treatment of craniofacial tissue regeneration.

3D Near-Field Electrospinning of Biomaterial Microfibers with Potential for Blended Microfiber-Cell-Loaded Gel Composite Structures

This paper describes the development of a novel low-cost and efficient method, 3D near-field electrospinning, to fabricate high-resolution, and repeatable 3D polymeric fiber patterns on nonconductive materials with potential use in tissue engineering. This technology is based on readily available hobbyist grade 3D printers. The result is exquisite control of the deposition of single fibers in an automated manner. Additionally, the fabrication of various fiber patterns, which are subsequently translated to unique cellular patterns, is demonstrated. Finally, poly(methyl methacrylate) fibers are printed within 3D collagen gels loaded with cells to introduce anisotropic properties of polymeric fibers within the cell-loaded gels.

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

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

Polymer Bioprocessing to Fabricate 3D Scaffolds for Tissue Engineering

Traditional methods for polymer processing involve the use of hazardous organic solvents which may compromise the biological function of scaffolds in tissue engineering. Indeed, the toxic effect of them on biological microenvironment has a tremendous impact on cell fate so altering the main activities involved in in vitro tissue formation. To date, extensive researches focus on seeking newer methods for bio-safely processing polymeric biomaterials to be implanted in the human body. Here, we aim at over viewing two approaches based on solvent free or green solvent based processes in order to identify alternative solutions to fabricate bio-inspired scaffolds to be successfully used in regenerative and degenerative medicine.

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

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

STATEMENT OF SIGNIFICANCE

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

Biodegradable ceramic-polymer composites for biomedical applications: A review

The present work focuses on the state-of-the-art of biodegradable ceramic-polymer composites with particular emphasis on influence of various types of ceramic fillers on properties of the composites. First, the general needs to create composite materials for medical applications are briefly introduced. Second, various types of polymeric materials used as matrices of ceramic-containing composites and their properties are reviewed. Third, silica nanocomposites and their material as well as biological characteristics are presented. Fourth, different types of glass fillers including silicate, borate and phosphate glasses and their effect on a number of properties of the composites are described. Fifth, wollastonite as a composite modifier and its effect on composite characteristics are discussed. Sixth, composites containing calcium phosphate ceramics, namely hydroxyapatite, tricalcium phosphate and biphasic calcium phosphate are presented. Finally, general possibilities for control of properties of composite materials are highlighted.

Highly controlled coating of strontium-doped hydroxyapatite on electrospun poly(ɛ-caprolactone) fibers

Electrospun fibers show great potential as scaffolds for bone tissue engineering due to their architectural biomimicry to the extracellular matrix (ECM). Cation substitution of strontium for calcium in hydroxyapatite (HAp) positively influences the mechanism of bone remodeling including enhancing bone regeneration and reducing bone resorption. The objective of this study was to attach strontium-doped HAp (SrHAp) to electrospun poly(ɛ-caprolactone) (PCL) fibers for creation of novel composite scaffolds that can not only mimic the architecture and composition of ECM but also affect bone remodeling favorably. We demonstrated for the first time the highly controlled SrHAp coatings on electrospun PCL fibers. We showed the reproducible manufacturing of composite fiber scaffolds with controllable thickness, composition, and morphology of SrHAp coatings. We further showed that the released strontium and calcium cations from coatings could reach effective concentrations within 1 day and endure more than 28 days. Additionally, the Young's modulus of the SrHAp-coated PCL fibers was up to around six times higher than that of raw fibers dependent on the coating thickness and composition. Together, this novel class of composite fiber scaffolds may hold great promise for bone regeneration. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 753-763, 2017.

Osteoinduction of bone grafting materials for bone repair and regeneration

Regeneration of bone defects caused by trauma, infection, tumours or inherent genetic disorders is a clinical challenge that usually necessitates bone grafting materials. Autologous bone or autograft is still considered the clinical "gold standard" and the most effective method for bone regeneration. However, limited bone supply and donor site morbidity are the most important disadvantages of autografting. Improved biomaterials are needed to match the performance of autograft as this is still superior to that of synthetic bone grafts. Osteoinductive materials would be the perfect candidates for achieving this task. The aim of this article is to review the different groups of bone substitutes in terms of their most recently reported osteoinductive properties. The different factors influencing osteoinductivity by biomaterials as well as the mechanisms behind this phenomenon are also presented, showing that it is very limited compared to osteoinductivity shown by bone morphogenetic proteins (BMPs). Therefore, a new term to describe osteoinductivity by biomaterials is proposed. Different strategies for adding osteoinductivity (BMPs, stem cells) to bone substitutes are also discussed. The overall objective of this paper is to gather the current knowledge on osteoinductivity of bone grafting materials for the effective development of new graft substitutes that enhance bone regeneration.

An Assessment of Cell Culture Plate Surface Chemistry for in Vitro Studies of Tissue Engineering Scaffolds

The use of biopolymers as a three dimensional (3D) support structure for cell growth is a leading tissue engineering approach in regenerative medicine. Achieving consistent cell seeding and uniform cell distribution throughout 3D scaffold culture in vitro is an ongoing challenge. Traditionally, 3D scaffolds are cultured within tissue culture plates to enable reproducible cell seeding and ease of culture media change. In this study, we compared two different well-plates with different surface properties to assess whether seeding efficiencies and cell growth on 3D scaffolds were affected. Cell attachment and growth of murine calvarial osteoblast (MC3T3-E1) cells within a melt-electrospun poly-ε-caprolactone scaffold were assessed when cultured in either “low-adhesive” non-treated or corona discharged-treated well-plates. Increased cell adhesion was observed on the scaffold placed in the surface treated culture plates compared to the scaffold in the non-treated plates 24 h after seeding, although it was not significant. However, higher cell metabolic activity was observed on the bases of all well-plates than on the scaffold, except for day 21, well metabolic activity was higher in the scaffold contained in non-treated plate than the base. These results indicate that there is no advantage in using non-treated plates to improve initial cell seeding in 3D polymeric tissue engineering scaffolds, however non-treated plates may provide an improved metabolic environment for long-term studies.

Modulating the stem cell niche for tissue regeneration

The field of regenerative medicine holds considerable promise for treating diseases that are currently intractable. Although many researchers are adopting the strategy of cell transplantation for tissue repair, an alternative approach to therapy is to manipulate the stem cell microenvironment, or niche, to facilitate repair by endogenous stem cells. The niche is highly dynamic, with multiple opportunities for intervention. These include administration of small molecules, biologics or biomaterials that target specific aspects of the niche, such as cell-cell and cell-extracellular matrix interactions, to stimulate expansion or differentiation of stem cells, or to cause reversion of differentiated cells to stem cells. Nevertheless, there are several challenges in targeting the niche therapeutically, not least that of achieving specificity of delivery and responses. We envisage that successful treatments in regenerative medicine will involve different combinations of factors to target stem cells and niche cells, applied at different times to effect recovery according to the dynamics of stem cell-niche interactions.

Deregulation of bone forming cells in bone diseases and anabolic effects of strontium-containing agents and biomaterials

Age-related bone loss and osteoporosis are associated with bone remodeling changes that are featured with decreased trabecular and periosteal bone formation relative to bone resorption. Current anticatabolic therapies focusing on the inhibition of bone resorption may not be sufficient in the prevention or reversal of age-related bone deterioration and there is a big need in promoting osteoblastogenesis and bone formation. Enhanced understanding of the network formed by key signaling pathways and molecules regulating bone forming cells in health and diseases has therefore become highly significant. The successful development of agonist/antagonist of the PTH and Wnt signaling pathways are profits of the understanding of these key pathways. As the core component of an approved antiosteoporosis agent, strontium takes its effect on osteoblasts at multilevel through multiple pathways, representing a good example in revealing and exploring anabolic mechanisms. The recognition of strontium effects on bone has led to its expected application in a variety of biomaterial scaffolds used in tissue engineering strategies aiming at bone repairing and regeneration. While summarizing the recent progress in these respects, this review also proposes the new approaches such as systems biology in order to reveal new insights in the pathology of osteoporosis as well as possible discovery of new therapies.