Comparative studies on ectopic bone formation in porous hydroxyapatite scaffolds with complementary pore structures

Enhancing osteoblast proliferation and bone regeneration by poly (amino acid)/selenium-doped hydroxyapatite

Among various biomaterials employed for bone repair, composites with good biocompatibility and osteogenic ability had received increasing attention from biomedical applications. In this study, we doped selenium (Se) into hydroxyapatite (Se-HA) by the precipitation method, and prepared different amounts of Se-HA-loaded poly (amino acid)/Se-HA (PAA/Se-HA) composites (0, 10 wt%, 20 wt%, 30 wt%) byin-situmelting polycondensation. The physical and chemical properties of PAA/Se-HA composites were characterized by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and their mechanical properties. XRD and FT-IR results showed that PAA/Se-HA composites contained characteristic peaks of PAA and Se-HA with amide linkage and HA structures. DSC and TGA results specified the PAA/Se-HA30 composite crystallization, melting, and maximum weight loss temperatures at 203.33 °C, 162.54 °C, and 468.92 °C, respectively, which implied good thermal stability. SEM results showed that Se-HA was uniformly dispersed in PAA. The mechanical properties of PAA/Se-HA30 composites included bending, compressive, and yield strengths at 83.07 ± 0.57, 106.56 ± 0.46, and 99.17 ± 1.11 MPa, respectively. The cellular responses of PAA/Se-HA compositesin vitrowere studied using bone marrow mesenchymal stem cells (BMSCs) by cell counting kit-8 assay, and results showed that PAA/Se-HA30 composites significantly promoted the proliferation of BMSCs at the concentration of 2 mg ml-1. The alkaline phosphatase activity (ALP) and alizarin red staining results showed that the introduction of Se-HA into PAA enhanced ALP activity and formation of calcium nodule. Western blotting and Real-time polymerase chain reaction results showed that the introduction of Se-HA into PAA could promoted the expression of osteogenic-related proteins and mRNA (integrin-binding sialoprotein, osteopontin, runt-related transcription factor 2 and Osterix) in BMSCs. A muscle defect at the back and a bone defect at the femoral condyle of New Zealand white rabbits were introduced for evaluating the enhancement of bone regeneration of PAA and PAA/Se-HA30 composites. The implantation of muscle tissue revealed good biocompatibility of PAA and PAA/Se-HA30 composites. The implantation of bone defect showed that PAA/Se-HA30 composites enhanced bone formation at the defect site (8 weeks), exhibiting good bone conductivity. Therefore, the PAA-based composite was a promising candidate material for bone tissue regeneration.

Mechanistic insights into the spontaneous induction of bone formation

The grand discovery of morphogens, or "form-generating substances", revealed that tissue morphogenesis is initiated by soluble molecular signals or morphogens primarily belonging to the transforming growth factor-β (TGF-β) supergene family. The regenerative potential of bone rests on its extracellular matrix, which is the repository of several morphogens that tightly control cellular differentiating pathways, cellular matrix deposition and remodeling. Alluringly, the matrix also contains specific factors transferred from the heterotopic implanted bone matrices initiating "Tissue Induction", as provocatively described in Nature in 1945. Later, it was found that selected genes and gene products of the TGF-β supergene family singly, synchronously, and synergistically mastermind the induction of bone formation. This review describes the phenomenon of the spontaneous and/or intrinsic osteoinductivity of calcium phosphate-based biomaterials and titanium' constructs without the applications of soluble osteogenetic molecular signals. The review shows the spontaneous induction of bone formation initiated by Ca++ activating stem cell differentiation and up-regulation of bone morphogenetic proteins genes. Expressed gene products are embedded into the concavities of the calcium phosphate-based substrata, initiating bone formation as a secondary response. Pure titanium's substrata do not initiate the spontaneous induction of bone formation. The induction of bone is solely dependent on acid, alkali and heat treatments to form apatite layers on the treated titanium surfaces. The induction of bone formation is achieved exclusively by apatite-based biomaterial surfaces. The hydroxyapatite, in its various forms and geometric configurations, finely tunes the induction of bone formation in heterotopic sites. Cellular differentiation by fine-tuning of the cellular molecular machinery is initiated by specific geometric modularity of the hydroxyapatite substrata that push cellular buttons that start the ripple-like cascade of "Tissue Induction", generating newly formed ossicles with bone marrow in heterotopic extraskeletal sites. The highlighted mechanistic insights into the spontaneous induction of bone formation are a research platform invocating selected molecular elements to construct the induction of bone formation.

Osteoinduction and osteoimmunology: Emerging concepts

The recognition and importance of immune cells during bone regeneration, including around bone biomaterials, has led to the development of an entire field termed "osteoimmunology," which focuses on the connection and interplay between the skeletal system and immune cells. Most studies have focused on the "osteogenic" capacity of various types of bone biomaterials, and much less focus has been placed on immune cells despite being the first cell type in contact with implantable devices. Thus, the amount of literature generated to date on this topic makes it challenging to extract needed information. This review article serves as a guide highlighting advancements made in the field of osteoimmunology emphasizing the role of the osteoimmunomodulatory properties of biomaterials and their impact on osteoinduction. First, the various immune cell types involved in bone biomaterial integration are discussed, including the prominent role of osteal macrophages (OsteoMacs) during bone regeneration. Thereafter, key biomaterial properties, including topography, wettability, surface charge, and adsorption of cytokines, growth factors, ions, and other bioactive molecules, are discussed in terms of their impact on immune responses. These findings highlight and recognize the importance of the immune system and osteoimmunology, leading to a shift in the traditional models used to understand and evaluate biomaterials for bone regeneration.

Comprehensive evaluation and advanced modification of polymethylmethacrylate cement in bone tumor treatment

Bone tumors are invasive diseases with a tendency toward recurrence, disability, and high mortality rates due to their grievous complications. As a commercial polymeric biomaterial, polymethylmethacrylate (PMMA) cement possesses remarkable mechanical properties, injectability, and plasticity and is, therefore, frequently applied in bone tissue engineering. Numerous positive effects in bone tumor treatment have been demonstrated, including biomechanical stabilization, analgesic effects, and tumor recurrence prevention. However, to our knowledge, a comprehensive evaluation of the application of the PMMA cement in bone tumor treatment has not yet been reported. This review comprehensively evaluates the efficiency and complications of the PMMA cement in bone tumor treatment, for the first time, and introduces advanced modification strategies, providing an objective and reliable reference for the application of the PMMA cement in treating bone tumors. We have also summarized the current research on modifications to enhance the anti-tumor efficacy of the PMMA cement, such as drug carriers and magnetic hyperthermia.

3D printed pore morphology mediates bone marrow stem cell behaviors via RhoA/ROCK2 signaling pathway for accelerating bone regeneration

Bone bionics and structural engineering have sparked a broad interest in optimizing artificial scaffolds for better bone regeneration. However, the mechanism behind scaffold pore morphology-regulated bone regeneration remains unclear, making the structure design of scaffolds for bone repair challenging. To address this issue, we have carefully assessed diverse cell behaviors of bone mesenchymal stem cells (BMSCs) on the β-tricalcium phosphate (β-TCP) scaffolds with three representative pore morphologies (i.e., cross column, diamond, and gyroid pore unit, respectively). Among the scaffolds, BMSCs on the β-TCP scaffold with diamond pore unit (designated as D-scaffold) demonstrated enhanced cytoskeletal forces, elongated nucleus, faster cell mobility, and better osteogenic differentiation potential (for example, the alkaline phosphatase expression level in D-scaffold were 1.5-2 times higher than other groups). RNA-sequencing analysis and signaling pathway intervention revealed that Ras homolog gene family A (RhoA)/Rho-associated kinase-2 (ROCK2) has in-depth participated in the pore morphology-mediated BMSCs behaviors, indicating an important role of mechanical signaling transduction in scaffold-cell interactions. Finally, femoral condyle defect repair results showed that D-scaffold could effectively promote endogenous bone regeneration, of which the osteogenesis rate was 1.2-1.8 times higher than the other groups. Overall, this work provides insights into pore morphology-mediated bone regeneration mechanisms for developing novel bioadaptive scaffold designs.

Study on the influence of scaffold morphology and structure on osteogenic performance

The number of patients with bone defects caused by various bone diseases is increasing yearly in the aging population, and people are paying increasing attention to bone tissue engineering research. Currently, the application of bone tissue engineering mainly focuses on promoting fracture healing by carrying cytokines. However, cytokines implanted into the body easily cause an immune response, and the cost is high; therefore, the clinical treatment effect is not outstanding. In recent years, some scholars have proposed the concept of tissue-induced biomaterials that can induce bone regeneration through a scaffold structure without adding cytokines. By optimizing the scaffold structure, the performance of tissue-engineered bone scaffolds is improved and the osteogenesis effect is promoted, which provides ideas for the design and improvement of tissue-engineered bones in the future. In this study, the current understanding of the bone tissue structure is summarized through the discussion of current bone tissue engineering, and the current research on micro-nano bionic structure scaffolds and their osteogenesis mechanism is analyzed and discussed.

A grooved porous hydroxyapatite scaffold induces osteogenic differentiation via regulation of PKA activity by upregulating miR-129-5p expression

BACKGROUND AND OBJECTIVE

Hydroxyapatite scaffolds with different morphologies have been widely used in bone tissue engineering. Moreover, microRNAs (miRNAs) have been proven to be extensively involved in regulating bone regeneration. We developed grooved porous hydroxyapatite (HAG) scaffolds with good osteogenic efficiency. However, little is known about the role of miRNAs in HAG scaffold-mediated promotion of bone regeneration. The objective of this study was to reveal the mechanism from the perspective of differential miRNA expression.

METHODS

Scanning electron microscopy (SEM) was used to perform the coculture of cells and scaffolds. The miRNA profiles were generated by a microarray assay. A synthetic miR-129-5p mimic and inhibitor were used for overexpression or inhibition. The expression of osteogenic marker mRNAs and proteins was detected by quantitative real-time PCR (qRT-PCR), Western blotting, and immunofluorescence. An ALP activity kit and alizarin red staining (ARS) were used to measure ALP activity and mineral deposition formation. Cell migration ability was examined by wound healing and transwell assays. Protein kinase A (PKA) activity was measured by enzyme-linked immunosorbent assay (ELISA) after miR-129-5p transfection. Target genes were identified by a dual-luciferase reporter assay. H89 preculture evaluated the cross talk between miR-129-5p and PKA activity. Heterotopic implantation models, hematoxylin-eosin (HE), immunohistochemistry staining, and micro-CT were used to evaluate miR-129-5p osteogenesis in vivo.

RESULTS

miRNAs were differentially expressed during osteogenic differentiation induced by HAG in vitro and in vivo. miR-129-5p was the only highly expressed miRNA both in vitro and in vivo. miR-129-5p overexpression promoted osteoblast differentiation and cell migration, while its inhibition weakened the effect of HAG. Moreover, miR-129-5p activated PKA to regulate the phosphorylation of β-catenin and cAMP-response element binding protein (CREB) by inhibiting cAMP-dependent protein kinase inhibitor alpha (Pkia). H89 prevented the effects of miR-129-5p on osteogenic differentiation and cell migration. HE, immunohistochemistry staining and micro-CT results showed that miR-129-5p promoted in vivo osteogenesis of the HAG scaffold.

CONCLUSION

The HAG scaffold activates Pka by upregulating miR-129-5p and inhibiting Pkia, resulting in CREB-dependent transcriptional activation and accumulation of β-catenin and promoting osteogenic marker expression.

Macropore Regulation of Hydroxyapatite Osteoinduction via Microfluidic Pathway

Macroporous characteristics have been shown to play a key role in the osteoinductivity of hydroxyapatite ceramics, but the physics underlying the new bone formation and distribution in such scaffolds still remain elusive. The work here has emphasized the osteoinductive capacity of porous hydroxyapatite scaffolds containing different macroporous sizes (200–400 μm, 1200–1500 μm) and geometries (star shape, spherical shape). The assumption is that both the size and shape of a macropore structure may affect the microfluidic pathways in the scaffolds, which results in the different bone formations and distribution. Herein, a mathematical model and an animal experiment were proposed to support this hypothesis. The results showed that the porous scaffolds with the spherical macropores and large pore sizes (1200–1500 μm) had higher new bone production and more uniform new bone distribution than others. A finite element analysis suggested that the macropore shape affected the distribution of the medium–high velocity flow field, while the macropore size effected microfluid speed and the value of the shear stress in the scaffolds. Additionally, the result of scaffolds implanted into the dorsal muscle having a higher new bone mass than the abdominal cavity suggested that the mechanical load of the host tissue could play a key role in the microfluidic pathway mechanism. All these findings suggested that the osteoinduction of these scaffolds depends on both the microfluid velocity and shear stress generated by the macropore size and shape. This study, therefore, provides new insights into the inherent osteoinductive mechanisms of bioceramics, and may offer clues toward a rational design of bioceramic scaffolds with improved osteoinductivity.

Scaffold microarchitecture regulates angiogenesis and the regeneration of large bone defects

Emerging 3D printing technologies can provide exquisite control over the external shape and internal architecture of scaffolds and tissue engineered constructs, enabling systematic studies to explore how geometric design features influence the regenerative process. Here we used fused deposition modelling (FDM) and melt electrowriting (MEW) to investigate how scaffold microarchitecture influences the healing of large bone defects. FDM was used to fabricate scaffolds with relatively large fibre diameters and low porosities, while MEW was used to fabricate scaffolds with smaller fibre diameters and higher porosities, with both scaffolds being designed to have comparable surface areas. Scaffold microarchitecture significantly influenced the healing response following implantation into critically sized femoral defects in rats, with the FDM scaffolds supporting the formation of larger bone spicules through its pores, while the MEW scaffolds supported the formation of a more round bone front during healing. After 12 weeks in vivo, both MEW and FDM scaffolds supported significantly higher levels of defect vascularisation compared to empty controls, while the MEW scaffolds supported higher levels of new bone formation. Somewhat surprisingly, this superior healing in the MEW group did not correlate with higher levels of angiogenesis, with the FDM scaffold supporting greater total vessel formation and the formation of larger vessels, while the MEW scaffold promoted the formation of a dense microvasculature with minimal evidence of larger vessels infiltrating the defect region. To conclude, the small fibre diameter, high porosity and high specific surface area of the MEW scaffold proved beneficial for osteogenesis and bone regeneration, demonstrating that changes in scaffold architecture enabled by this additive manufacturing technique can dramatically modulate angiogenesis and tissue regeneration without the need for complex exogenous growth factors. These results provide a valuable insight into the importance of 3D printed scaffold architecture when developing new bone tissue engineering strategies.

Sustained local ionic homeostatic imbalance caused by calcification modulates inflammation to trigger heterotopic ossification

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. STATEMENT OF SIGNIFICANCE: The ability to regenerate bone in a spatially controlled and reproducible manner is an essential prerequisite for the treatment of large bone defects. As such, understanding the mechanism leading to heterotopic ossification (HO), a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues, would be very useful. Unfortunately, the mechanism(s) behind HO is(are) poorly understood. The present study reviews the literature on HO and based on it, proposes that HO can be caused by a combination of inflammation and calcification. This mechanism helps to better understand current strategies to prevent and treat HO. It also shows new opportunities to improve the treatment of bone defects in orthopedic and dental procedures.

Enhanced bone regenerative properties of calcium phosphate ceramic granules in rabbit posterolateral spinal fusion through a reduction of grain size

Osteoinductivity is a crucial factor to determine the success and efficiency of posterolateral spinal fusion (PLF) by employing calcium phosphate (Ca-P) bioceramics. In this study, three kinds of Ca-P ceramics with microscale to nanoscale gain size (BCP-control, BCP-micro and BCP-nano) were prepared and their physicochemical properties were characterized. BCP-nano had the spherical shape and nanoscale gain size, BCP-micro had the spherical shape and microscale gain size, and BCP-control (BAM®) had the irregular shape and microscale gain size. The obtained BCP-nano with specific nanotopography could well regulate in vitro protein adsorption and osteogenic differentiation of MC3T3 cells. In vivo rabbit PLF procedures further confirmed that nanotopography of BCP-nano might be responsible for the stronger bone regenerative ability comparing with BCP-micro and BCP-control. Collectedly, due to nanocrystal similarity with natural bone apatite, BCP-nano has excellent efficacy in guiding bone regeneration of PLF, and holds great potentials to become an alternative to standard bone grafts for future clinical applications.

•

The nanocrystal of porous BCP ceramic spheres is similar to natural bone apatite.

•

BCP nanoceramics is conducive to protein adsorption and osteogenic differentiation of MC3T3 cells.

•

Osteoindutivity of BCP ceramics is a crucial factor to determine the sucess and efficiency of PLF.

•

BCP ceramic spheres with nanotopography hold great potential in clinical PLF applications.

Effect of Temperature on Electrochemically Assisted Deposition and Bioactivity of CaP Coatings on CpTi Grade 4

Calcium phosphate (CaP) coatings are able to improve the osseointegration process due to their chemical composition similar to that of bone tissues. Among the methods of producing CaP coatings, the electrochemically assisted deposition (ECAD) is particularly important due to high repeatability and the possibility of deposition at room temperature and neutral pH, which allows for the co-deposition of inorganic and organic components. In this work, the ECAD of CaP coatings from an acetate bath with a Ca:P ratio of 1.67, was developed. The effect of the ECAD conditions on CaP coatings deposited on commercially pure titanium grade 4 (CpTi G4) subjected to sandblasting and autoclaving was presented. The physicochemical characteristics of the ECAD-derived coatings was carried out using SEM, EDS, FTIR, 2D roughness profiles, and amplitude sensitive eddy current method. It was showed that amorphous calcium phosphate (ACP) coatings can be obtained at a potential −1.5 to −10 V for 10 to 60 min at 20 to 70 °C. The thickness and surface roughness of the ACP coatings were an increasing function of potential, time, and temperature. The obtained ACP coatings are a precursor in the process of apatite formation in a simulated body fluid. The optimal ACP coating for use in dentistry was deposited at a potential of −3 V for 30 min at 20 °C.

The effect of synthetic bone graft substitutes on bone formation in rabbit calvarial defects

The aim of this study was to evaluate the influence of the intensity of the biomimetic hydroxyapatite (HA) coating of α-tricalcium phosphate (α-TCP) on biomaterial degradation and bone formation. Twenty-four female NZW rabbits of approximately 12 weeks of age were used. Critical size defects were randomly treated with 3%:97% HA:α-TCP (BBCP1), 12%:88% HA:α-TCP (BBCP2), and 23%:77% HA:α-TCP (BBCP3), respectively or sham. All defects were covered with a resorbable collagen membrane. Animals were euthanized after 3 and 12 weeks of healing and samples were investigated by micro-CT and histologic analysis. Ingrowth of newly formed woven bone from the original bone at 3-week healing period was observed in all samples. At the 12-week healing period, the new bone in the peripheral area was mainly lamellar and in the central region composed of both woven and lamellar bone. New bony tissue was found on the surface of all three types of granules and at the interior of the BBCP1 granules. Samples with 3% HA showed significantly less residual biomaterial in comparison to the other two groups. Furthermore, BBCP1 significantly promoted new bone area as compared to other three groups and more bone volume as compared to the control. Within its limitations, this study indicated the highest degradation rate in case of BBCP1 concomitant with the highest rate of bone formation. Hence, formation of new bone can be affected by the level of biomimetic HA coating of α-TCP.

In Vitro and In Vivo Evaluation of Carboxymethyl Cellulose Scaffolds for Bone Tissue Engineering Applications

The present study involves the development of citric acid-cross-linked carboxymethyl cellulose (C3CA) scaffolds by a freeze-drying process. Scaffolds were fabricated at different freezing temperatures of -20, -40, or -80 °C to investigate the influence of scaffold pore size on bone regeneration. All three scaffolds were porous in structure, and the pore size was measured to be 74 ± 4, 55 ± 6, and 46 ± 5 μm for -20, -40, and -80 °C scaffolds. The pores were larger in scaffolds processed at -20 °C compared to -40 and -80 °C, indicating the reduction in pore size of the scaffolds with a decrease in freezing temperature. The cytocompatibility, cell proliferation, and differentiation in C3CA scaffolds were assessed with the Saos-2 osteoblast cell line. These scaffolds supported the proliferation and differentiation of Saos-2 cells with significant matrix mineralization in scaffolds processed at -40 °C. Subcutaneous implantation of C3CA scaffolds in the rat model was investigated for its ability of vascularization and new matrix tissue formation. The matrix formation was observed at the earliest of 14 days in the scaffolds when processed at -40 °C while it was observed only after 28 days of implantation with the scaffolds processed at -20 and -80 °C. These results suggest that the citric acid-cross-linked CMC scaffolds processed at -40 °C can be promising for bone tissue engineering application.

In-situ analysis of the hydration ability of bone graft material using a synchrotron radiation X-ray micro-CT

The conventional micro-computed tomography (μCT) is a non-destructive imaging technique used for obtaining 2D and 3D information for scaffolds. The main composition and internal structure are important in mimicking and designing the characteristics of natural bone. This study was three-dimensional evaluating the external or internal structures and the hydration effects of bone graft materials by using the in-situ image technique. Synchrotron radiation micro-computed tomography (SR-μCT) was used to extract information on the geometry of two biphasic calcium phosphates (BCP) with identical chemicals and different micro-macro porosity, pore size distribution, and pore interconnection pathways. Volume analysis by hydration was used to measure the two bone graft materials at 0, 5, and 10-min intervals. The SR-μCT image was achieved with information regarding the internal pore structure and hydration effects evaluated under 3D visualization. Both types of bone graft materials showed structures suitable for tissue engineering applications. The SR-μCT in-situ techniques with 3D information provided a detailed view of the structures. Thus, SR-μCT could be an available, unbiased 3D alternative to in-situ analysis.

3D-Printed Ceramic Bone Scaffolds with Variable Pore Architectures

This study evaluated the mechanical properties and bone regeneration ability of 3D-printed pure hydroxyapatite (HA)/tricalcium phosphate (TCP) pure ceramic scaffolds with variable pore architectures. A digital light processing (DLP) 3D printer was used to construct block-type scaffolds containing only HA and TCP after the polymer binder was completely removed by heat treatment. The compressive strength and porosity of the blocks with various structures were measured; scaffolds with different pore sizes were implanted in rabbit calvarial models. The animals were observed for eight weeks, and six animals were euthanized in the fourth and eighth weeks. Then, the specimens were evaluated using radiological and histological analyses. Larger scaffold pore sizes resulted in enhanced bone formation after four weeks (p < 0.05). However, in the eighth week, a correlation between pore size and bone formation was not observed (p > 0.05). The findings showed that various pore architectures of HA/TCP scaffolds can be achieved using DLP 3D printing, which can be a valuable tool for optimizing bone-scaffold properties for specific clinical treatments. As the pore size only influenced bone regeneration in the initial stage, further studies are required for pore-size optimization to balance the initial bone regeneration and mechanical strength of the scaffold.

Grooved hydroxyapatite scaffold modulates mitochondria homeostasis and thus promotes osteogenesis in bone mesenchymal stromal cells

Hydroxyapatite scaffolds (HASs) are widely studied as suitable materials for bone replacement scaffolds due to their chemical similarities to organic materials. In our previous study, a novel HAS with a 25–30-µm groove structure (HAS-G) exhibited enhanced osteogenesis of bone mesenchymal stromal cells (BMSCs) compared with HAS, potentially by modulating the macrophage-induced immune microenvironment. However, the exact effects of different surface patterns on the physiological processes of attached cells is not known. The present study aimed to determine the effects of HAS-G on the osteogenesis and physiological processes in BMSCs. Cell counting kit-8 assays and propidium iodide staining followed by flow cytometry were performed, and the results demonstrated that both in normal medium and differentiating medium, HAS-G promoted cell proliferation by decreasing the proportion of G1/G0 cells and decreased reactive oxygen species (ROS) accumulation in BMSCs compared with HAS. Detection markers of osteogenesis revealed that compared with HAS, HAS-G increased runt-related transcription factor 2, osteocalcin and osteopontin protein levels and promoted osteogenesis, which was further confirmed by Alizarin Red S staining. Following JC-1 staining, it was observed that HAS-G maintained the mitochondrial membrane potential, similar to that achieved by N-acetylcysteine pretreatment. In addition, compared with those of HAS, HAS-G decreased mitochondrial ROS levels, which potentially contributed to the promotion of osteogenesis. The results also demonstrated that HAS-G inhibited mitophagy induced by ROS accumulation and ATP synthesis compared with HAS. In conclusion, HAS-G decreased ROS accumulation and mitophagy and thus promoted osteogenesis of BMSCs, indicating that ROS modulation of HAS-G may serve a key role in osteogenesis.

Enhanced osteogenesis of hydroxyapatite scaffolds by coating with BMP-2-loaded short polylactide nanofiber: a new drug loading method for porous scaffolds

Porous hydroxyapatite (HA) is widely used in porous forms to assist bone defect healing. However, further improvements in biological functions are desired for meeting complex clinical situations such as impaired bone regeneration in poor bone stock. The extracellular matrix (ECM) of human tissues is characterized by nanofibrous structures and a variety of signal molecules. Emulating these characteristics are expected to create a favorable microenvironment for cells and simultaneously allow release of osteogenic molecules. In this study, short polylactide fibers containing BMP-2 were prepared by electrospinning and coated on porous HA scaffolds. The coating did not affect porosity or pore interconnectivity of the scaffold but improved its compressive strength markedly. This fiber coating produced burst BMP-2 release in 1 day followed by a linear release for 24 days. The coating had a significantly lower rat calvarial osteoblasts (RCOBs) adhesion (vs. uncoated scaffold) but allowed normal proliferation subsequently. Bone marrow stem cells (MSCs) on the coated scaffolds expressed a significantly increased alkaline phosphatase activity relative to the uncoated ones. After implantation in canine dorsal muscles, the coated scaffolds formed significantly more new bone at Weeks 4 and 12, and more blood vessels at Week 12. This method offers a new option for drug delivery systems.

The response of host blood vessels to graded distribution of macro-pores size in the process of ectopic osteogenesis

Angiogenesis is of great importance to bone regeneration, but it remains a significant challenge to induce sufficient angiogenesis and osteogenesis within bone grafts for large bone defect healing. The aim of this study is to investigate the effects of hydroxyapatite (HA) scaffold via a novel graded pore distribution approach on vascularization and osteoinduction. Two types of graded porous scaffolds were fabricated by sugar templates-leaching techniques: (1) one with large pores of 1100-1250 μm in the center and small pores of 500-650 μm at the periphery (HALS); (2) the other with small pores of 500-650 μm in the center and large pores of 1100-1250 μm at the periphery (HASL). In vivo data showed different pore size distribution had a remarkable impact on blood vessel formation during bone formation, which led to distinct localization of new bone within the defects. After one month of implantation, the diameters of the blood vessels infiltrated on the periphery of HASL were substantially larger than those in the center though the host blood vessels were successful in infiltrating throughout the whole scaffold. In contrast, vascularization within HALS appeared to be poor with very few blood vessels formed in the center, indicating heterogeneous vascularization in the scaffolds. After 3 months of implantation, we found that HASL induced more homogeneous bone formation in the whole bone graft but new bone was only found at the periphery of HALS. This study suggests that the pores size distribution in graded scaffolds cannot only affected early stage vascularization, but also influence late stage bone formation and remodeling. The architecture of larger pores at the periphery of graded scaffold may be capable of enhancing angiogenesis and osteogenesis during large size bone defect healing.

The role of calcium phosphate surface structure in osteogenesis and the mechanisms involved

Calcium phosphate (CaP) ceramics have been widely used for bone regeneration because of their ability to induce osteogenesis. Surface properties, including chemical composition and surface structure, are known to play a crucial role in osteoconduction and osteoinduction. This review systematically analyzes the effects of surface properties, in particular the surface structure, of CaP scaffolds on cell behavior and new bone formation. We also summarize the possible signaling pathways involved in the osteogenic differentiation of bone-related cells when cultured on surfaces with various structures in vitro. The significant immune response initiated by surface structure involved in osteogenic differentiation of cells is also discussed in this review. Taken together, the new biological principle for advanced biomaterials is not only to directly stimulate osteogenic differentiation of bone-related cells but also to modulate the immune response in vivo. Although the reaction mechanism responsible for bone formation induced by CaP surface structure is not clear yet, the insights on surface structure-mediated osteogenic differentiation and osteoimmunomodulation could aid the optimization of CaP-based biomaterials for bone regeneration. STATEMENT OF SIGNIFICANCE: CaP ceramics have similar inorganic composition with natural bone, which have been widely used for bone tissue scaffolds. CaP themselves are not osteoinductive; however, osteoinductive properties could be introduced to CaP materials by surface engineering. This paper systematically summarizes the effects of surface properties, especially surface structure, of CaP scaffolds on bone formation. Additionally, increasing evidence has proved that the bone healing process is not only affected by the osteogenic differentiation of bone-related cells, but also relevant to the the cooperation of immune system. Thus, we further review the possible signaling pathways involved in the osteogenic differentiation and immune response of cells cultured on scaffold surface. These insights into surface structure-mediated osteogenic differentiation and osteoimmunomodulated-based strategy could aid the optimization of CaP-based biomaterials.

In vitro measurement of the chemical changes occurring within β-tricalcium phosphate bone graft substitutes

Several mechanisms proposed to explain the osteoinductive potential of calcium phosphates involve surface mineralization ("bioactivity") and mention the occurrence of concentration gradients between the inner and the outer part of the implanted material. Determining the evolution of the local chemical environment occurring inside the pores of an implanted bone graft substitute (BGS) is therefore highly relevant. A quantitative and fast method was developed to measure the chemical changes occurring within the pores of β-Tricalcium Phosphate (β-TCP) granules incubated in a simulated body fluid. A factorial design of experiment was used to test the effect of particle size, specific surface area, microporosity, and purity of the β-TCP granules. Large pH, calcium and phosphate concentration changes were observed inside the BGS and lasted for several days. The kinetics and magnitude of these changes (up to 2 pH units) largely depended on the processing and properties of the granules. Interestingly, processing parameters that increased the kinetics and magnitude of the local chemical changes are parameters considered to favor calcium phosphate osteoinduction, suggesting that the model might be useful to predict the osteoinductive potential of BGSs. STATEMENT OF SIGNIFICANCE: Recent results suggest that in situ mineralization of biomaterials (polymers, ceramics, metals) might be key in their ability to trigger ectopic bone formation. This is the reason why the effect on in situ mineralization of various synthesis parameters of β-tricalcium phosphate granules was studied (size, microporosity, specific surface area, and Ca/P molar ratio). To the best of our knowledge, this is the first article devoted to the chemical changes occurring within the pores of a bone graft substitute. We believe that the manuscript will prove to be highly important in the design and mechanistic understanding of drug-free osteoinductive biomaterials.

Bone regeneration using β-tricalcium phosphate (β-TCP) block with interconnected pores made by setting reaction of β-TCP granules

We fabricated an interconnected dual porous β-tricalcium phosphate (β-TCP) block via a setting reaction of β-TCP granules. This β-TCP block was unique because it exhibits a fully interconnected macroporous structure with micropores in the walls surrounding macropores and a roughened surface. The porosity and diametral tensile strength of the resulting product were 58.1 ± 1.7% and 1.4 ± 0.2 MPa, respectively. Rabbit distal femur bone defects were reconstructed using the porous β-TCP block and the efficacy of the porous β-TCP block as an artificial bone substitute was evaluated histomorphometrically. For a dense β-TCP control, 4 weeks following implantation, only 0.2 ± 0.1% of the β-TCP was resorbed, and the amount of newly formed bone was limited (0.1 ± 0.1%), whereas when the defect was reconstructed with porous β-TCP, 9.2 ± 3.1% was resorbed, and the amount of new bone was 18.9 ± 5.5%. This represents an approximately 50-fold enhancement in resorption and a 200-fold increase in bone formation for our porous β-TCP block. Therefore, interconnected dual porous β-TCP made via β-TCP granule setting has good potential as an artificial bone substitute.

Architectural Design of 3D Printed Scaffolds Controls the Volume and Functionality of Newly Formed Bone

The successful regeneration of functional bone tissue in critical-size defects remains a significant clinical challenge. To address this challenge, synthetic bone scaffolds are widely developed, but remarkably few are translated to the clinic due to poor performance in vivo. Here, it is demonstrated how architectural design of 3D printed scaffolds can improve in vivo outcomes. Ceramic scaffolds with different pore sizes and permeabilities, but with similar porosity and interconnectivity, are implanted in rabbit calvaria for 12 weeks, and then the explants are harvested for microcomputed tomography evaluation of the volume and functionality of newly formed bone. The results indicate that scaffold pores should be larger than 390 µm with an upper limit of 590 µm to enhance bone formation. It is also demonstrated that a bimodal pore topology-alternating large and small pores-enhances the volume and functionality of new bone substantially. Moreover, bone formation results indicate that stiffness of new bone is highly influenced by the scaffold's permeability in the direction concerned. This study demonstrates that manipulating pore size and permeability in a 3D printed scaffold architecture provides a useful strategy for enhancing bone regeneration outcomes.

Effect of magnesium particle fraction on osteoinduction of hydroxyapatite sphere-based scaffolds

HAs-30Mg (incorporation of 30% Mg into HA sphere-based scaffolds) induced the optimum new bone formation.

Core-Shell Biphasic Microspheres with Tunable Density of Shell Micropores Providing Tailorable Bone Regeneration

IMPACT STATEMENT

We have developed the new core-shell bioceramic CSi-Sr4@CaP-px microspheres with tuning porous shell layer so that the biodegradation of both CSi-Sr4 core and CaP shell is readily adjusted synergistically. This is for the first time, to the best of our knowledge, that the bioceramic scaffolds concerning gradient distribution and microstructure-tailoring design is available for tailoring biodegradation and ion release (bioactivity) to optimizing osteogenesis. Furthermore, it is possibly helpful to develop new bioactive scaffold system for time-dependent tailoring bioactivity and microporous structure to significantly enhance bone regeneration and repair applications, especially in some non-load-bearing arbitrary 3D anatomical bone and teeth defects.

3D-Plotted Beta-Tricalcium Phosphate Scaffolds with Smaller Pore Sizes Improve In Vivo Bone Regeneration and Biomechanical Properties in a Critical-Sized Calvarial Defect Rat Model

Due to the difficulty in fabricating bioceramic scaffolds with smaller pore sizes by the current 3D printing technique, the effect of smaller pore sizes (below 400 µm) of 3D printed bioceramic scaffolds on the bone regeneration and biomechanical behavior is never studied. Herein beta-tricalcium phosphate (β-TCP) scaffolds with interconnected smaller pores of three different sizes (100, 250, and 400 µm) are fabricated by 3D plotting. The resultant scaffolds are then implanted into rat critical-sized calvarial defects without any seeded cells. A custom-designed device is developed to investigate the biomechanical properties of the scaffolds after surgical implantation for 4, 8, and 12 weeks. The scaffolds with the 100 µm pore size are found to present the highest maximum load and stiffness, comparable to those of the autogenous bone, after being implanted for 12 weeks. Micro-computed tomography (micro-CT) and histological analysis further indicate that the scaffolds with the 100 µm pore size achieve the highest percentage of new bone ingrowth, which correlates to their best in vivo biomechanical properties. This study demonstrates that tailoring the pore size of β-TCP scaffolds to a smaller range by 3D-plotting can be a facile and efficient approach to enhanced bone regeneration and biomechanical behaviors in bone repair.

A bioactive implant in situ and long-term releases combined drugs for treatment of osteoarticular tuberculosis

Anti-tuberculosis chemotherapy with a long duration and adequate dosing is the mainstay for treatment of osteoarticular tuberculosis (TB). However, it is difficult for systemic administration to reach adequate local drug concentrations and achieve effective treatment. Herein, a hydroxyapatite (HA) scaffold implant combined with a drug-releasing system was designed to achieve in situ and long-term anti-TB drug release and highly efficient therapeutic activity in vitro and in vivo. The clinical anti-TB drugs hydrophilic isoniazid (INH) and hydrophobic rifampicin (RFP) were molecularly dispersed into polyvinyl alcohol (PVA) through immersion-curing techniques and were steadily adhered onto the surfaces of HA scaffolds (HA-drug@PVA). The HA-drug@PVA scaffolds showed a long-term, sustained drug release profile and killed proliferating Mycobacterium in vitro. In vivo experimental results revealed that the HA-drug@PVA scaffolds provided over 10- and 100-fold higher concentrations in muscles and bones, respectively, as well as a much lower concentration (<0.025) in blood. Furthermore, the HA-drug@PVA scaffold implanted in an osteoarticular TB rabbit model showed obvious bone regeneration and fusion due to the inhibition of TB-associated inflammatory changes. The excellent therapeutic effects indicate that in situ implant materials combined with a long-term drug release system are promising for the treatment of osteoarticular TB and other osteoarticular infections.

Optimized Bone Regeneration in Calvarial Bone Defect Based on Biodegradation-Tailoring Dual-shell Biphasic Bioactive Ceramic Microspheres

Bioceramic particulates capable of filling bone defects have gained considerable interest over the last decade. Herein, dual-shell bioceramic microspheres (CaP@CaSi@CaP, CaSi@CaP@CaSi) with adjustable beta-tricalcium phosphate (CaP) and beta-calcium silicate (CaSi) distribution were fabricated using a co-concentric capillary system enabling bone repair via a tailorable biodegradation process. The in vitro results showed the optimal concentration (1/16 of 200 mg/ml) of extracts of dual-shell microspheres could promote bone marrow mesenchymal cell (BMSC) proliferation and enhance the level of ALP activity and Alizarin Red staining. The in vivo bone repair and microsphere biodegradation in calvarial bone defects were compared using micro-computed tomography and histological evaluations. The results indicated the pure CaP microspheres were minimally resorbed at 18 weeks post-operatively and new bone tissue was limited; however, the dual-shell microspheres were appreciably biodegraded with time in accordance with the priority from CaSi to CaP in specific layers. The CaSi@CaP@CaSi group showed a significantly higher ability to promote bone regeneration than the CaP@CaSi@CaP group. This study indicates that the biphasic microspheres with adjustable composition distribution are promising for tailoring material degradation and bone regeneration rate, and such versatile design strategy is thought to fabricate various advanced biomaterials with tailorable biological performances for bone reconstruction.

Osteoinduction by Foamed and 3D-Printed Calcium Phosphate Scaffolds: Effect of Nanostructure and Pore Architecture

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.

A decade of progress in liver regenerative medicine

Liver diseases can be caused by viral infection, metabolic disorder, alcohol consumption, carcinoma or injury, chronically progressing to end-stage liver disease or rapidly resulting in acute liver failure. In either situation, liver transplantation is most often sought for life saving, which is, however, significantly limited by severe shortage of organ donors. Until now, tremendous multi-disciplinary efforts have been dedicated to liver regenerative medicine, aiming at providing transplantable cells, microtissues, or bioengineered whole liver via tissue engineering, or maintaining partial liver functions via extracorporeal support. In both directions, new compatible biomaterials, stem cell sources, and bioengineering approaches have fast-forwarded liver regenerative medicine towards potential clinical applications. Another important progress in this field is the development of liver-on-a-chip technologies, which enable tissue engineering, disease modeling, and drug testing under biomimetic extracellular conditions. In this review, we aim to highlight the last decade's progress in liver regenerative medicine from liver tissue engineering, bioartificial liver devices (BAL), to liver-on-a-chip platforms, and then to present challenges ahead for further advancement.

HPLC-MS/MS: A potential method to track the in vivo degradation of zein-based biomaterial

Given the inadequacies of existing clinic tracking strategies, such as isotopic tracer techniques, one of the major thrusts in protein-based tissue engineering substitutes prior to use in clinic is to develop a safe technique that can effectively track their degradation in vivo. Keeping in view the possible application of a natural polymer, zein as a novel bone substitute, with the advantages of good bio-compatibility, bio-degradability and outstanding mechanical properties, we attempted here to construct a HPLC-MS/MS method to track the in vivo degradation of zein porous scaffold. Histological observation and immunohistochemistry analysis using the intramuscular implantation model of rats clearly indicated that zein porous scaffold has certain osteoinductive ability. More importantly, HPLC-MS/MS detected the changes of amino acids levels in plasma and different organs after the implantation of scaffolds. With these findings, it could be concluded that HPLC-MS/MS might be a potential method to track the in vivo degradation of protein-based tissue engineering substitutes. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 606-613, 2018.

Enhancing the Osteogenic Capability of Core-Shell Bilayered Bioceramic Microspheres with Adjustable Biodegradation

This study describes the fabrication and biological evaluation of core-shell bilayered bioceramic microspheres with adjustable compositional distribution via a coaxial bilayer capillary system. Beyond the homogeneous hybrid composites, varying the diameter of capillary nozzles and the composition of the bioceramic slurries makes it easy to create bilayered β-tricalcium phosphate (CaP)/β-calcium silicate (CaSi) microspheres with controllable compositional distribution in the core or shell layer. Primary investigations in vitro revealed that biodegradation could be adjusted by compositional distribution or shell thickness and that poorly soluble CaP located on the shell layer of CaP or CaSi@CaP microspheres was particularly beneficial for mesenchymal stem cell adhesion and growth in the early stage, but the ion release from the CaP@CaSi exhibited a potent stimulating effect on alkaline phosphatase expression of the cells at longer times. When the bilayered microspheres (CaSi@CaP, CaP@CaSi) and the monolayered microspheres (CaP, CaSi) were implanted into the critical-sized femoral bone defect in rabbit models, significant differences in osteogenic capacity over time were measured at 6-18 weeks post implantation. The CaP microspheres showed the lowest biodegradation rate and slow new bone regeneration, whereas the CaSi@CaP showed a fast degradation of the CaSi core through the porous CaP shell so that a significant osteogenic response was observed at 12-18 weeks. The CaP@CaSi microspheres possessed excellent surface bioactivity and osteogenic activity, whereas the CaSi microspheres group exhibited a poor bone augmentation in the later stage due to extreme biodegradation. These findings demonstrated that the bioactive response in such core-shell-structured bioceramic systems could be adjusted by compositional distribution, and this strategy can be used to fabricate a variety of bioceramic microspheres with adjustable biodegradation rates and enhanced biological response for bone regeneration applications in medicine.

Bone Response to Porous Poly(methyl methacrylate) Cement Loaded with Hydroxyapatite Particles in a Rabbit Mandibular Model

The aim of the current study was to evaluate bone formation and tissue response to porous poly(methyl methacrylate) (PMMA) cement with or without hydroxyapatite (HA) in a rabbit mandibular model. Therefore, 14 New Zealand White rabbits were randomly divided into two groups of seven according to the designed study end points of 4 and 12 weeks. For each rabbit, two decorticated defects (6 mm in height and 10 mm in width for each) were prepared at both sides of the mandible. Subsequently, the defects were filled with, respectively, porous PMMA and porous PMMA-HA cement. After reaching the designated implantation period, the rabbits were euthanized and the mandibles were retrieved for histological analysis. Results showed that both porous PMMA and porous PMMA-HA supported bone repair. Neither of the bone cements caused significant inflammation to nerve or other surrounding tissues. After implantation of 12 weeks, majority of the porosity was filled with newly formed bone for both cements, which supports the concept that a porous structure within PMMA can enhance bone ingrowth. Histomorphometrical evaluation, using histological grading scales, demonstrated that, at both implantation times, the presence of HA in the PMMA enhanced bone formation. Bone was always in direct contact with the HA particles, while intervening fibrous tissue was present at the PMMA-bone interface. On the basis of results, it was concluded that injectable porous PMMA-HA cement might be a good candidate for craniofacial bone repair, which should be further evaluated in a more clinically relevant large animal model.

Fabrication of β-tricalcium phosphate composite ceramic sphere-based scaffolds with hierarchical pore structure for bone regeneration

Polymer sphere-based scaffolds, which are prepared by bonding the adjacent spheres via sintering the randomly packed spheres, feature uniform pore structure, full three-dimensional (3D) interconnection, and considerable mechanical strength. However, bioceramic sphere-based scaffolds fabricated by this method have never been reported. Due to high melting temperature of bioceramic, only limited diffusion rate can be achieved when sintering the bioceramic spheres, which is far from enough to form robust bonding between spheres. In the present study, for the first time we fabricated 3D interconnected β-tricalcium phosphate ceramic sphere-based (PG/TCP) scaffolds by introducing phosphate-based glass (PG) as sintering additive and placing uniaxial pressure during the sintering process. The sintering mechanism of PG/TCP scaffolds was unveiled. The PG/TCP scaffolds had hierarchical pore structure, which was composed by interconnected macropores (>200 μm) among spheres, pores (20–120 μm) in the interior of spheres, and micropores (1–3 μm) among the grains. During the sintering process, partial PG reacted with β-TCP, forming β-Ca2P2O7; metal ions from PG substituted to Ca2+ sites of β-TCP. The mechanical properties (compressive strength 2.8–10.6 MPa; compressive modulus 190–620 MPa) and porosity (30%–50%) of scaffolds could be tailored by manipulating the sintering temperatures. The introduction of PG accelerated in vitro degradation of scaffolds, and the PG/TCP scaffolds showed good cytocompatibility. This work may offer a new strategy to prepare bioceramic scaffolds with satisfactory physicochemical properties for application in bone regeneration.

Core–shell-structured nonstoichiometric bioceramic spheres for improving osteogenic capability

Precisely controlling the composition distribution and pore-network evolution in the foreign ion doped, core–shell Ca-silicate bioceramic microspheres is favorable for tailoring osteogenicity in critical size bone defects.

Effects of bone substitute architecture and surface properties on cell response, angiogenesis, and structure of new bone

This paper presents an overview of the effect of porous biomaterial architecture on seeding efficiency, cell response, angiogenesis, and bone formation.

Immobilization of salvianolic acid B-loaded chitosan microspheres distributed three-dimensionally and homogeneously on the porous surface of hydroxyapatite scaffolds

Porous hydroxyapatite (HA) scaffolds combined with a drug delivery system have attracted much attention for bone tissue engineering. In this study, an easy and highly efficient method was developed to immobilize salvianolic acid B (Sal B)-loaded chitosan (CS) microspheres three dimensionally and homogeneously on the surface of HA scaffolds pre-coated with alginate. Porous HA scaffolds were prepared via a template-leaching process and CS microspheres (used as drug carriers) were fabricated by an emulsion method. To improve adhesion between the microspheres and HA scaffolds, alginate was used to pre-coat the porous surface of the HA scaffolds. Various concentrations of alginate were used to optimize the adhesion of Sal B-loaded CS microspheres to the scaffold surface. During the adherence process, coated HA scaffolds were immersed in an aqueous solution containing Sal B-loaded CS microspheres, followed by standing or shaking at 37 °C for a certain time. The results showed that the microspheres were solidly and homogeneously distributed on the porous surface of the alginate pre-coated HA scaffolds via electrostatic interactions. Few microspheres detached from the porous surface, even after the HA scaffolds with microspheres were treated by shaking in distilled water for as long as 7 d. Compared with the static condition, the distribution of Sal B-loaded CS microspheres on the porous surface of pre-coated HA scaffolds in the shaken condition was more homogeneous and almost unaggregated. Additionally, the compressive strength of the scaffolds coated with alginate was obviously improved. The optimal alginate coating concentration was 1% (i.e. the microstructure of the porous surfaces of the HA scaffolds was almost unchanged). The release profile of Sal B over a 30 d immersion found an initial burst release followed by a sustained release. The result of cell culture in vitro was that 1% alginate-coated scaffolds with Sal B-loaded CS microspheres obviously promoted cell proliferation after cell culture for 3 and 7 d, and cells were attached and uniformly distributed on the porous surface of the scaffolds. The strategy of incorporating drug-loaded microspheres with porous HA scaffolds could provide an excellent bone substitute for repair of bone tissue defects.

Muscle as an osteoinductive niche for local bone formation with the use of a biphasic calcium sulphate/hydroxyapatite biomaterial

Objectives

We have observed clinical cases where bone is formed in the overlaying muscle covering surgically created bone defects treated with a hydroxyapatite/calcium sulphate biomaterial. Our objective was to investigate the osteoinductive potential of the biomaterial and to determine if growth factors secreted from local bone cells induce osteoblastic differentiation of muscle cells.

Materials and Methods

We seeded mouse skeletal muscle cells C2C12 on the hydroxyapatite/calcium sulphate biomaterial and the phenotype of the cells was analysed. To mimic surgical conditions with leakage of extra cellular matrix (ECM) proteins and growth factors, we cultured rat bone cells ROS 17/2.8 in a bioreactor and harvested the secreted proteins. The secretome was added to rat muscle cells L6. The phenotype of the muscle cells after treatment with the media was assessed using immunostaining and light microscopy.

Results

C2C12 cells differentiated into osteoblast-like cells expressing prominent bone markers after seeding on the biomaterial. The conditioned media of the ROS 17/2.8 contained bone morphogenetic protein-2 (BMP-2 8.4 ng/mg, standard deviation (sd) 0.8) and BMP-7 (50.6 ng/mg, sd 2.2). In vitro, this secretome induced differentiation of skeletal muscle cells L6 towards an osteogenic lineage.

Conclusion

Extra cellular matrix proteins and growth factors leaking from a bone cavity, along with a ceramic biomaterial, can synergistically enhance the process of ectopic ossification. The overlaying muscle acts as an osteoinductive niche, and provides the required cells for bone formation.

Cite this article: D. B. Raina, A. Gupta, M. M. Petersen, W. Hettwer, M. McNally, M. Tägil, M-H. Zheng, A. Kumar, L. Lidgren. Muscle as an osteoinductive niche for local bone formation with the use of a biphasic calcium sulphate/hydroxyapatite biomaterial. Bone Joint Res 2016;5:500–511. DOI: 10.1302/2046-3758.510.BJR-2016-0133.R1.

Ectopic osteogenesis and angiogenesis regulated by porous architecture of hydroxyapatite scaffolds with similar interconnecting structure in vivo

The macro-pore sizes of porous scaffold play a key role for regulating ectopic osteogenesis and angiogenesis but many researches ignored the influence of interconnection between macro-pores with different sizes. In order to accurately reveal the relationship between ectopic osteogenesis and macro-pore sizes in dorsal muscle and abdominal cavities of dogs, hydroxyapatite (HA) scaffolds with three different macro-pore sizes of 500–650, 750–900 and 1100–1250 µm were prepared via sugar spheres-leaching process, which also had similar interconnecting structure determined by keeping the d/s ratio of interconnecting window diameter to macro-pore size constant. The permeability test showed that the seepage flow of fluid through the porous scaffolds increased with the increase of macro-pore sizes. The cell growth in three scaffolds was not affected by the macro-pore sizes. The in vivo ectopic implantation results indicated that the macro-pore sizes of HA scaffolds with the similar interconnecting structure have impact not only the speed of osteogenesis and angiogenesis but also the space distribution of newly formed bone. The scaffold with macro-pore sizes of 750–900 µm exhibited much faster angiogenesis and osteogenesis, and much more uniformly distribution of new bone than those with other macro-pore sizes. This work illustrates the importance of a suitable macro-pore sizes in HA scaffolds with the similar interconnecting structure which provides the environment for ectopic osteogenesis and angiogenesis.