Biomechanics and mechanobiology of the bone matrix

Development of bone surrogates by material extrusion-based additive manufacturing to mimic flexural mechanical behaviour and fracture prediction via phase-field approach

BACKGROUND AND OBJECTIVE

The limited availability of human bone samples for investigation leads to the demand for alternatives. Bone surrogates are crucial in promoting research on the intricate mechanics of osseous tissue. However, solutions are restricted to commercial brands, which frequently fail to faithfully replicate the mechanical response of bone, or oversimplified customised simulants designed for a specific application. The manufacturing and assessment of reliable bone surrogates made of polylactic acid via material extrusion-based additive manufacturing are presented in this work.

METHODS

An experimental and numerical study with 3D-printed dog-bone and prismatic specimens was carried out to characterise the polymeric feedstock and analyse the influence of process parameters under three-point bending and quasi-static conditions. Besides, three porcine rib samples were considered as a reference for the development of the artificial bones. Bone surrogates were manufactured from the 3D-scanned real bone geometries. In order to reproduce the trabecular and cortical bone, a lattice structure for the infill and a compact shell surrounding the core were employed. Infill density and shell thickness were evaluated through different printing configurations. Additionally, a computational analysis based on the phase-field approach was conducted to simulate the experimental tests and predict fracture. The modelling considered homogenisation of the infill material.

RESULTS

Outcomes demonstrated the potential of the presented methodology. Maximum force and flexural stiffness were compared to real bone properties to find the optimal printing configuration, replicating the flexural mechanical behaviour of bone tissue. Certain configurations accurately reproduce the studied properties. Regarding the numerical model, strength and stiffness prediction was validated with experimental results.

CONCLUSIONS

The presented methodology enables the manufacturing of artificial bones with accurate geometries and tailored mechanical properties. Furthermore, the described modelling strategy offers a powerful tool for designing bone surrogates.

Assembled collagen films modified using polyacrylic acid with improved mechanical properties via mineralization

The imperative task of enforcing collagen materials holds paramount significance in the field of hard tissue repair. We hereby present mineralized collagen fiber films via mineralization with improved mechanical properties. Self-extracted collagen was assembled into an array with an aligned fibrous pattern and then modified with polyacrylic acid (PAA) followed by mineralization in cationic polyacrylamide (CPAM)-SBF. Biomineralization occurred at the inner and outer surface of the assembled collagen fiber films. A tensile strength of up to 40.38 ± 3.08 MPa of mineralized collagen was obtained, for the first time, which may be attributed to the synergistic effect of polyanion and polycation on the mineralization process of assembled intrafibrillar collagen fibers. It was argued that PAA may facilitate the intra-fiber interaction of collagen, which extends the elongation at break of collagen fibers. This study introduces a pioneering approach for the preparation of mineralized collagen materials with superior mechanical properties, which would be beneficial for hard tissue repair.

Advancing dentin remineralization: Exploring amorphous calcium phosphate and its stabilizers in biomimetic approaches

OBJECTIVE

This review elucidates the mechanisms underpinning intrafibrillar mineralization, examines various amorphous calcium phosphate (ACP) stabilizers employed in dentin's intrafibrillar mineralization, and addresses the challenges encountered in clinical applications of ACP-based bioactive materials.

METHODS

The literature search for this review was conducted using three electronic databases: PubMed, Web of Science, and Google Scholar, with specific keywords. Articles were selected based on inclusion and exclusion criteria, allowing for a detailed examination and summary of current research on dentin remineralization facilitated by ACP under the influence of various types of stabilizers.

RESULTS

This review underscores the latest advancements in the role of ACP in promoting dentin remineralization, particularly intrafibrillar mineralization, under the regulation of various stabilizers. These stabilizers predominantly comprise non-collagenous proteins, their analogs, and polymers. Despite the diversity of stabilizers, the mechanisms they employ to enhance intrafibrillar remineralization are found to be interrelated, indicating multiple driving forces behind this process. However, challenges remain in effectively designing clinically viable products using stabilized ACP and maximizing intrafibrillar mineralization with limited materials in practical applications.

SIGNIFICANCE

The role of ACP in remineralization has gained significant attention in dental research, with substantial progress made in the study of dentin biomimetic mineralization. Given ACP's instability without additives, the presence of ACP stabilizers is crucial for achieving in vitro intrafibrillar mineralization. However, there is a lack of comprehensive and exhaustive reviews on ACP bioactive materials under the regulation of stabilizers. A detailed summary of these stabilizers is also instrumental in better understanding the complex process of intrafibrillar mineralization. Compared to traditional remineralization methods, bioactive materials capable of regulating ACP stability and controlling release demonstrate immense potential in enhancing clinical treatment standards.

Recent Advances in 3D Printing of Smart Scaffolds for Bone Tissue Engineering and Regeneration

The repair and functional reconstruction of bone defects resulting from severe trauma, surgical resection, degenerative disease, and congenital malformation pose significant clinical challenges. Bone tissue engineering (BTE) holds immense potential in treating these severe bone defects, without incurring prevalent complications associated with conventional autologous or allogeneic bone grafts. 3D printing technology enables control over architectural structures at multiple length scales and has been extensively employed to process biomimetic scaffolds for BTE. In contrast to inert and functional bone grafts, next-generation smart scaffolds possess a remarkable ability to mimic the dynamic nature of native extracellular matrix (ECM), thereby facilitating bone repair and regeneration. Additionally, they can generate tailored and controllable therapeutic effects, such as antibacterial or antitumor properties, in response to exogenous and/or endogenous stimuli. This review provides a comprehensive assessment of the progress of 3D-printed smart scaffolds for BTE applications. It begins with an introduction to bone physiology, followed by an overview of 3D printing technologies utilized for smart scaffolds. Notable advances in various stimuli-responsive strategies, therapeutic efficacy, and applications of 3D-printed smart scaffolds are discussed. Finally, the review highlights the existing challenges in the development and clinical implementation of smart scaffolds, as well as emerging technologies in this field.

Skeletal abnormalities in mice with Dnmt3a missense mutations

Overgrowth and intellectual disability disorders in humans are typified by length/height and/or head circumference ≥ 2 standard deviations above the mean as well as intellectual disability and behavioral comorbidities, including autism and anxiety. Tatton-Brown-Rahman Syndrome is one type of overgrowth and intellectual disability disorder caused by heterozygous missense mutations in the DNA methyltransferase 3A (DNMT3A) gene. Numerous DNMT3A mutations have been identified in Tatton-Brown-Rahman Syndrome patients and may be associated with varying phenotype severities of clinical presentation. Two such mutations are the R882H and P904L mutations which result in severe and mild phenotypes, respectively. Mice with paralogous mutations (Dnmt3aP900L/+ and Dnmt3aR878H/+) exhibit overgrowth in their long bones (e.g., femur, humerus), but the mechanisms responsible for their skeletal overgrowth remain unknown. The goal of this study is to characterize skeletal phenotypes in mouse models of Tatton-Brown-Rahman Syndrome and identify potential cellular mechanisms involved in the skeletal overgrowth phenotype. We report that mature mice with the Dnmt3aP900L/+ or Dnmt3aR878H/+ mutation exhibit tibial overgrowth, cortical bone thinning, and weakened bone mechanical properties. Dnmt3aR878H/+ mutants also contain larger bone marrow adipocytes while Dnmt3aP900L/+ mutants show no adipocyte phenotype compared to control animals. To understand the potential cellular mechanisms regulating these phenotypes, growth plate chondrocytes, osteoblasts, and osteoclasts were assessed in juvenile mutant mice using quantitative static histomorphometry and dynamic histomorphometry. Tibial growth plates appeared thicker in mutant juvenile mice, but no changes were observed in osteoblast activity or osteoclast number in the femoral mid-diaphysis. These studies reveal new skeletal phenotypes associated with Tatton-Brown-Rahman Syndrome in mice and provide a rationale to extend clinical assessments of patients with this condition to include bone density and quality testing. These findings may be also informative for skeletal characterization of other mouse models presenting with overgrowth and intellectual disability phenotypes.

Recent trends in bone tissue engineering: a review of materials, methods, and structures

Bone tissue engineering (BTE) provides the treatment possibility for segmental long bone defects that are currently an orthopedic dilemma. This review explains different strategies, from biological, material, and preparation points of view, such as using different stem cells, ceramics, and metals, and their corresponding properties for BTE applications. In addition, factors such as porosity, surface chemistry, hydrophilicity and degradation behavior that affect scaffold success are introduced. Besides, the most widely used production methods that result in porous materials are discussed. Gene delivery and secretome-based therapies are also introduced as a new generation of therapies. This review outlines the positive results and important limitations remaining in the clinical application of novel BTE materials and methods for segmental defects.

3D-Printed Polycaprolactone-Based Containing Calcium Zirconium Silicate: Bioactive Scaffold for Accelerating Bone Regeneration

There is an essential clinical need to develop rapid process scaffolds to repair bone defects. The current research presented the development of calcium zirconium silicate/polycaprolactone for bone tissue engineering utilising melt extrusion-based 3D printing. Calcium zirconium silicate (CZS) nanoparticles were added to polycaprolactone (PCL) porous scaffolds to enhance their biological and mechanical properties, while the resulting properties were studied extensively. No significant difference was found in the melting point of the samples, while the crystallisation temperature points of the samples containing bioceramic increased from 36.1 to 40.2 °C. Thermal degradation commenced around 350 °C for all materials. According to our results, increasing the CZS content from 0 to 40 wt.% (PC40) in porous scaffolds (porosity about 55-62%) improved the compressive strength from 2.8 to 10.9 MPa. Furthermore, apatite formation ability in SBF solution increased significantly by enhancing the CZS percentage. According to MTT test results, the viability of MG63 cells improved remarkably (~29%) in PC40 compared to pure PCL. These findings suggest that a 3D-printed PCL/CZS composite scaffold can be fabricated successfully and shows great potential as an implantable material for bone tissue engineering applications.

Engineering Large-Scale Self-Mineralizing Bone Organoids with Bone Matrix-Inspired Hydroxyapatite Hybrid Bioinks

Addressing large bone defects remains a significant challenge owing to the inherent limitations in self-healing capabilities, resulting in prolonged recovery and suboptimal regeneration. Although current clinical solutions are available, they have notable shortcomings, necessitating more efficacious approaches to bone regeneration. Organoids derived from stem cells show great potential in this field; however, the development of bone organoids has been hindered by specific demands, including the need for robust mechanical support provided by scaffolds and hybrid extracellular matrices (ECM). In this context, bioprinting technologies have emerged as powerful means of replicating the complex architecture of bone tissue. The research focused on the fabrication of a highly intricate bone ECM analog using a novel bioink composed of gelatin methacrylate/alginate methacrylate/hydroxyapatite (GelMA/AlgMA/HAP). Bioprinted scaffolds facilitate the long-term cultivation and progressive maturation of extensive bioprinted bone organoids, foster multicellular differentiation, and offer valuable insights into the initial stages of bone formation. The intrinsic self-mineralizing quality of the bioink closely emulates the properties of natural bone, empowering organoids with enhanced bone repair for both in vitro and in vivo applications. This trailblazing investigation propels the field of bone tissue engineering and holds significant promise for its translation into practical applications.

Periodic static compression of micro-strain pattern regulates endochondral bone formation

Introduction: Developmental engineering based on endochondral ossification has been proposed as a potential strategy for repairing of critical bone defects. Bone development is driven by growth plate-mediated endochondral ossification. Under physiological conditions, growth plate chondrocytes undergo compressive forces characterized by micro-mechanics, but the regulatory effect of micro-mechanical loading on endochondral bone formation has not been investigated. Methods: In this study, a periodic static compression (PSC) model characterized by micro-strain (with 0.5% strain) was designed to clarify the effects of biochemical/mechanical cues on endochondral bone formation. Hydrogel scaffolds loaded with bone marrow mesenchymal stem cells (BMSCs) were incubated in proliferation medium or chondrogenic medium, and PSC was performed continuously for 14 or 28 days. Subsequently, the scaffold pretreated for 28 days was implanted into rat femoral muscle pouches and femoral condylar defect sites. The chondrogenesis and bone defect repair were evaluated 4 or 10 weeks post-operation. Results: The results showed that PSC stimulation for 14 days significantly increased the number of COL II positive cells in proliferation medium. However, the chondrogenic efficiency of BMSCs was significantly improved in chondrogenic medium, with or without PSC application. The induced chondrocytes (ichondrocytes) spontaneously underwent hypertrophy and maturation, but long-term mechanical stimulation (loading for 28 days) significantly inhibited hypertrophy and mineralization in ichondrocytes. In the heterotopic ossification model, no chondrocytes were found and no significant difference in terms of mineral deposition in each group; However, 4 weeks after implantation into the femoral defect site, all scaffolds that were subjected to biochemical/mechanical cues, either solely or synergistically, showed typical chondrocytes and endochondral bone formation. In addition, simultaneous biochemical induction/mechanical loading significantly accelerated the bone regeneration. Discussion: Our findings suggest that microstrain mechanics, biochemical cues, and in vivo microenvironment synergistically regulate the differentiation fate of BMSCs. Meanwhile, this study shows the potential of micro-strain mechanics in the treatment of critical bone defects.

GelMA-catechol coated FeHAp nanorods functionalized nanofibrous reinforced bio-instructive and mechanically robust composite hydrogel scaffold for bone tissue engineering

Critical bone defects complicate tissue graft-based surgeries, raising healthcare expenditures and underscoring scaffold-based tissue-engineering strategies to support bone reconstruction. Our study highlighted that the phase-compatible combination of inorganic nanorods, nanofibers, and hydrogels is promising for developing biomimetic and cell-instructive scaffolds since the bone matrix is a porous organic/inorganic composite. In brief, methacrylated gelatin (GelMA) was reacted with dopamine to form catechol-modified GeLMA (GelMA-C). The GelMA-C was nanocoated onto an iron-doped hydroxyapatite (FeHAp) nanorod via metal-catechol network coordination. The modified nanorod (FeHAp@GelMA-C) was loaded onto GelMA-based nanofibers. The nanorods loaded pre-fibers were electrospun onto GelMA solution and photochemically crosslinked to fabricate a fiber-reinforced hydrogel. The structural, mechanical, physicochemical, biocompatibility, swelling properties, osteogenic potential, and bone remodelling potential (using rat femoral defect model) of modified nanorods, simple hydrogel, and nanorod-loaded fiber-reinforced hydrogel were studied. The results supported that the interface interaction between GelMA-C/nanorods, nanorods/nanofibers, nanorods/hydrogels, and nanofiber/hydrogels significantly improved the microstructural and mechanical properties of the scaffold. Compared to pristine hydrogel, the nanorod-loaded fiber-reinforced scaffold better supported cellular responses, osteogenic differentiation, matrix mineralization, and accelerated bone regeneration. The nanorod-loaded fiber-reinforced hydrogel proved more biomimetic and cell-instructive for guided bone reconstruction.

The actin-bundling protein, PLS3, is part of the mechanoresponsive machinery that regulates osteoblast mineralization

Plastin-3 (PLS3) is a calcium-sensitive actin-bundling protein that has recently been linked to the development of childhood-onset osteoporosis. Clinical data suggest that PLS3 mutations lead to a defect in osteoblast function, however the underlying mechanism remains elusive. To investigate the role of PLS3 in bone mineralization, we generated MC3T3-E1 preosteoblast cells that are stably depleted of PLS3. Analysis of osteogenic differentiation of control and PLS3 knockdown (PLS3 KD) cells showed that depletion of PLS3 does not alter the first stage of osteoblast mineralization in which a collagen matrix is deposited, but severely affects the subsequent mineralization of that matrix. During this phase, osteoblasts heavily rely on mechanosensitive signaling pathways to sustain mineral deposition in response to increasing stiffness of the extracellular matrix (ECM). PLS3 prominently localizes to focal adhesions (FAs), which are intricately linked to mechanosensation. In line with this, we observed that depletion of PLS3 rendered osteoblasts unresponsive to changes in ECM stiffness and showed the same cell size, FA lengths and number of FAs when plated on soft (6 kPa) versus stiff (100 kPa) substrates in contrast to control cells, which showed an increased in each of these parameters when plated on 100 kPa substrates. Defective cell spreading of PLS3 KD cells on stiff substrates could be rescued by expression of wildtype PLS3, but not by expression of three PLS3 mutations that were identified in patients with early onset osteoporosis and that have aberrant actin-bundling activity. Altogether, our results show that actin-bundling by PLS3 is part of the mechanosensitive mechanism that promotes osteoblast mineralization and thus begins to elucidate how PLS3 contributes to the development of bone defects such as osteoporosis.

Advanced Hydrogel-Based Strategies for Enhanced Bone and Cartilage Regeneration: A Comprehensive Review

Bone and cartilage tissue play multiple roles in the organism, including kinematic support, protection of organs, and hematopoiesis. Bone and, above all, cartilaginous tissues present an inherently limited capacity for self-regeneration. The increasing prevalence of disorders affecting these crucial tissues, such as bone fractures, bone metastases, osteoporosis, or osteoarthritis, underscores the urgent imperative to investigate therapeutic strategies capable of effectively addressing the challenges associated with their degeneration and damage. In this context, the emerging field of tissue engineering and regenerative medicine (TERM) has made important contributions through the development of advanced hydrogels. These crosslinked three-dimensional networks can retain substantial amounts of water, thus mimicking the natural extracellular matrix (ECM). Hydrogels exhibit exceptional biocompatibility, customizable mechanical properties, and the ability to encapsulate bioactive molecules and cells. In addition, they can be meticulously tailored to the specific needs of each patient, providing a promising alternative to conventional surgical procedures and reducing the risk of subsequent adverse reactions. However, some issues need to be addressed, such as lack of mechanical strength, inconsistent properties, and low-cell viability. This review describes the structure and regeneration of bone and cartilage tissue. Then, we present an overview of hydrogels, including their classification, synthesis, and biomedical applications. Following this, we review the most relevant and recent advanced hydrogels in TERM for bone and cartilage tissue regeneration.

Poly(l-lactic acid)-based double-layer composite scaffold for bone tissue repair

Bone defect is a serious threat to human health. Osteopractic total flavone (OTF) extracted from Rhizoma Drynariae has the effects of promoting bone formation. Panax notoginseng saponin (PNS) has the function of activating blood circulation and removing blood stasis. Therefore, combining OTF and PNS with poly(l-lactic acid) (PLLA) to prepare scaffolds containing PNS in the outer layer and OTF in the inner layer is a feasible solution to rapidly remove blood stasis and continue to promote bone formation. In addition, degradation rate of the scaffold can affect the release time of two drugs. Adding Mg particles in outer layer can control the degradation rate of the scaffold and the drug release. Therefore, a double-layer drug-loaded PLLA scaffold containing OTF in the inner layer, PNS and Mg particles in the outer layer was prepared and characterized to verify its feasibility. The experimental results showed that the scaffold can realize the rapid release of PNS and the continuous release of OTF. With the increase of Mg content, the drug release rate became faster. Animal experiments showed that the scaffold containing 5% Mg particles could effectively promote the formation of new bone in the bone defect of male New Zealand white rabbits, and the area and density of new bone formed were much better than those in the control group. These results demonstrated that the double-layer drug-loaded scaffold had good ability to promote bone repair.

Orthopedic Scaffolds: Evaluation of Structural Strength and Permeability of Fluid Flow via an Open Cell Neovius Structure for Bone Tissue Engineering

The ability of bone to regenerate itself through mechanobiological responses is its dynamic property. Mechanical cues from a neighboring environment produce the structural strain to promote blood flow and bone marrow mobility that in turn aids the bone regeneration process. Occurrences of these phenomena are crucial for the success of metallic scaffolds implanted in the host bone tissue. Thus, permeability and fluid flow-induced wall shear stress (WSS) are two parameters that directly influence cell bioactivities inside a scaffold and are crucial for effective bone tissue regeneration. Given that the scaffolds shall be implanted in the body, permeability assessment was carried out using non-Newtonian fluid. In this work, the triply periodic minimal surface scaffolds with Neovius architectures were fabricated by using selective laser melting technology. The estimation of fluid flow was carried out using computational fluid dynamics (CFD) analysis with a non-Newtonian blood fluid model. Further, the structural strength of various open cell Neovius lattices was evaluated using a static compression test, and in vitro cell culture using Alamar blue assay was evaluated. Results revealed that the values of intrinsic blood flow permeability of the three-dimensional (3D)-printed open cell porous scaffold with Neovius architecture were of the same order of magnitude as those of human bone, ranging from 0.0025 × 10-9 to 0.0152 × 10-9 m2. The structural elastic modulus and compressive strength of NOCL40, NOCL50, and NOCL60 lattices range from 3.27 to 3.71 GPa and 194 to 205 MPa, respectively. All of the values are comparable to the human bone, thus making these lattices a suitable alternative for orthopedic applications.

Sumecton reinforced gelatin-based scaffolds for cell-free bone regeneration

Bone tissue engineering has risen to tackle the challenges of the current clinical need concerning bone fractures that is already considered a healthcare system problem. Scaffold systems for the repair of this tissue have yielded different combinations including biomaterials with nanotechnology or biological agents. Herein, three-dimensional porous hydrogels were engineered based on gelatin as a natural biomaterial and reinforced with synthetic saponite nanoclays. Scaffolds were biocompatible and shown to enhance the inherent properties of pristine ones, in particular, proved to withstand pressures similar to load-bearing tissues. Studies with murine mesenchymal stem cells found that scaffolds had the potential to proliferate and promote cell differentiation. In vivo experiments were conducted to gain insight about the ability of these cell-free scaffolds to regenerate bone, as well as to determine the role that these nanoparticles in the scaffold could play as a drug delivery system. SDF-1 loaded scaffolds showed the highest percentage of bone formation, which was corroborated by osteogenic markers and new blood vessels. Albeit a first attempt in the field of synthetic nanosilicates, these results suggest that the designed constructs may serve as delivery platforms for biomimetic agents to mend bony defects, circumventing high doses of therapeutics and cell-loading systems.

Development and Fabrication of Biocompatible Ti-Based Bulk Metallic Glass Matrix Composites for Additive Manufacturing

Ti-based metallic glasses have a high potential for implant applications. The feasibility of a new biocompatible Ti-based bulk metallic glass composite for selective laser melting (SLM) had been examined. Therefore, it is necessary to design a high-glass-forming-ability Ti-based metallic glass (∆Tx = 81 K, γ = 0.427, γm = 0.763), to fabricate a partial glass-formable spherical powder (the volume fraction of the amorphous phase in the atomized Ti-based powders being 73% [size < 25 μm], 61% [25-37 μm], and 50% [37-44 μm]), and establish an SLM parameter (a scan rate of 600 mm/s, a power of 120 W, and an overlap of 10%). The Ti42Zr35Si5Co12.5Sn2.5Ta3 bulk metallic glass composite was successfully fabricated through SLM. This study demonstrates that the TiZrSiCoSnTa system constitutes a promising basis for the additive manufacturing process in terms of preparing biocompatible metallic glass composites into complicated graded foam shapes.

Direct Integration of 3D Printing and Cryogel Scaffolds for Bone Tissue Engineering

Cryogels, known for their biocompatibility and porous structure, lack mechanical strength, while 3D-printed scaffolds have excellent mechanical properties but limited porosity resolution. By combining a 3D-printed plastic gyroid lattice scaffold with a chitosan-gelatin cryogel scaffold, a scaffold can be created that balances the advantages of both fabrication methods. This study compared the pore diameter, swelling potential, mechanical characteristics, and cellular infiltration capability of combined scaffolds and control cryogels. The incorporation of the 3D-printed lattice demonstrated patient-specific geometry capabilities and significantly improved mechanical strength compared to the control cryogel. The combined scaffolds exhibited similar porosity and relative swelling ratio to the control cryogels. However, they had reduced elasticity, reduced absolute swelling capacity, and are potentially cytotoxic, which may affect their performance. This paper presents a novel approach to combine two scaffold types to retain the advantages of each scaffold type while mitigating their shortcomings.

Design of Ti64/Ta Hybrid Materials by Powder Metallurgy Mimicking Bone Structure

This work reports on the fabrication of a novel two-layer material composed of a porous tantalum core and a dense Ti6Al4V (Ti64) shell by powder metallurgy. The porous core was obtained by mixing Ta particles and salt space-holders to create large pores, the green compact was obtained by pressing. The sintering behavior of the two-layer sample was studied by dilatometry. The interface bonding between the Ti64 and Ta layers was analyzed by SEM, and the pore characteristics were analyzed by computed microtomography. Images showed that two distinct layers were obtained with a bonding achieved by the solid-state diffusion of Ta particles into Ti64 during sintering. The formation of β-Ti and α' martensitic phases confirmed the diffusion of Ta. The pore size distribution was in the size range of 80 to 500 µm, and a permeability value of 6 × 10^-10 m² was close to the trabecular bones one. The mechanical properties of the component were dominated mainly by the porous layer, and Young's modulus of 16 GPa was in the range of bones. Additionally, the density of this material (6 g/cm³) was much lower than the one of pure Ta, which helps to reduce the weight for the desired applications. These results indicate that structurally hybridized materials, also known as composites, with specific property profiles can improve the response to osseointegration for bone implant applications.

Functionalization of biomimetic mineralized collagen for bone tissue engineering

Mineralized collagen (MC) is the basic unit of bone structure and function and is the main component of the extracellular matrix (ECM) in bone tissue. In the biomimetic method, MC with different nanostructures of neo-bone have been constructed. Among these, extra-fibrous MC has been approved by regulatory agencies and applied in clinical practice to play an active role in bone defect repair. However, in the complex microenvironment of bone defects, such as in blood supply disorders and infections, MC is unable to effectively perform its pro-osteogenic activities and needs to be functionalized to include osteogenesis and the enhancement of angiogenesis, anti-infection, and immunomodulation. This article aimed to discuss the preparation and biological performance of MC with different nanostructures in detail, and summarize its functionalization strategy. Then we describe the recent advances in the osteo-inductive properties and multifunctional coordination of MC. Finally, the latest research progress of functionalized biomimetic MC, along with the development challenges and future trends, are discussed. This paper provides a theoretical basis and advanced design philosophy for bone tissue engineering in different bone microenvironments.

Longitudinal alterations in bone morphometry, mechanical integrity and composition in Type-2 diabetes in a Zucker diabetic fatty (ZDF) rat

Individuals with Type-2 Diabetes (T2D) have an increased risk of bone fracture, without a reduction in bone mineral density. It is hypothesised that the hyperglycaemic state caused by T2D forms an excess of Advanced Glycated End-products (AGEs) in the organic matrix of bone, which are thought to stiffen the collagen network and lead to impaired mechanical properties. However, the mechanisms are not well understood. This study aimed to investigate the geometrical, structural and material properties of diabetic cortical bone during the development and progression of T2D in ZDF (fa/fa) rats at 12-, 26- and 46-weeks of age. Longitudinal bone growth was impaired as early as 12-weeks of age and by 46-weeks bone size was significantly reduced in ZDF (fa/fa) rats versus controls (fa/+). Diabetic rats had significant structural deficits, such as bending rigidity, ultimate moment and energy-to-failure measured via three-point bend testing. Tissue material properties, measured by taking bone geometry into account, were altered as the disease progressed, with significant reductions in yield and ultimate strength for ZDF (fa/fa) rats at 46-weeks. FTIR analysis on cortical bone powder demonstrated that the tissue material deficits coincided with changes in tissue composition, in ZDF (fa/fa) rats with long-term diabetes having a reduced carbonate:phosphate ratio and increased acid phosphate content when compared to age-matched controls, indicative of an altered bone turnover process. AGE accumulation, measured via fluorescent assays, was higher in the skin of ZDF (fa/fa) rats with long-term T2D, bone AGEs did not differ between strains and neither AGEs correlated with bone strength. In conclusion, bone fragility in the diabetic ZDF (fa/fa) rats likely occurs through a multifactorial mechanism influenced initially by impaired bone growth and development and proceeding to an altered bone turnover process that reduces bone quality and impairs biomechanical properties as the disease progresses.

Osteocyte β3 integrin promotes bone mass accrual and force-induced bone formation in mice

Background

Cell culture studies demonstrate the importance of β3 integrin in osteocyte mechanotransduction. However, the in vivo roles of osteocyte β3 integrin in the regulation of bone homeostasis and mechanotransduction are poorly defined.

Materials and methods

To study the in vivo role of osteocyte β3 integrin in bone, we utilized the 10-kb Dmp1 (dentin matrix acidic phosphoprotein 1)-Cre to delete β3 integrin expression in osteocyte in mice. Micro-computerized tomography (μCT), bone histomorphometry and in vitro cell culture experiments were performed to determine the effects of osteocyte β3 integrin loss on bone mass accrual and biomechanical properties. In addition, in vivo tibial loading model was applied to study the possible involvement of osteocyte β3 integrin in the mediation of bone mechanotransduction.

Results

Deletion of β3 integrin in osteocytes resulted in a low bone mass and impaired biomechanical properties in load-bearing long bones in adult mice. The loss of β3 integrin led to abnormal cell morphology with reduced number and length of dentritic processes in osteocytes. Furthermore, osteocyte β3 integrin loss did not impact the osteoclast formation, but significantly reduced the osteoblast-mediated bone formation rate and reduced the osteogenic differentiation of the bone marrow stromal cells in the bone microenvironment. In addition, mechanical loading failed to accelerate the anabolic bone formation in mutant mice.

Conclusions

Our studies demonstrate the essential roles of osteocyte β3 integrin in regulating bone mass and mechanotransduction.

Bioengineering of bone tissues using bioreactors for modulation of mechano-sensitivity in bone

Since the last decade, significant developments have been made in the area of bone tissue engineering associated with the emergence of novel biomaterials as well as techniques of scaffold fabrication. Despite all these developments, the translation from research findings to clinical applications is still very limited. Manufacturing the designed tissue constructs in a scalable manner remains the most challenging aspect. This bottleneck could be overcome by using bioreactors for the manufacture of these tissue constructs. In this review, a current scenario of bone injuries/defects and the cause of the translational gap between laboratory research and clinical use has been emphasized. Furthermore, various bioreactors being used in the area of bone tissue regeneration in recent studies have been highlighted along with their advantages and limitations. A vivid literature survey on the ideal attributes of bioreactors has been accounted, viz. dynamic, versatile, automated, reproducible and commercialization aspects. Additionally, the illustration of computational approaches that should be combined with bone tissue engineering experiments using bioreactors to simulate and optimize cellular growth in bone tissue constructs has also been done extensively.

Significance of mechanical loading in bone fracture healing, bone regeneration, and vascularization

In 1892, J.L. Wolff proposed that bone could respond to mechanical and biophysical stimuli as a dynamic organ. This theory presents a unique opportunity for investigations on bone and its potential to aid in tissue repair. Routine activities such as exercise or machinery application can exert mechanical loads on bone. Previous research has demonstrated that mechanical loading can affect the differentiation and development of mesenchymal tissue. However, the extent to which mechanical stimulation can help repair or generate bone tissue and the related mechanisms remain unclear. Four key cell types in bone tissue, including osteoblasts, osteoclasts, bone lining cells, and osteocytes, play critical roles in responding to mechanical stimuli, while other cell lineages such as myocytes, platelets, fibroblasts, endothelial cells, and chondrocytes also exhibit mechanosensitivity. Mechanical loading can regulate the biological functions of bone tissue through the mechanosensor of bone cells intraosseously, making it a potential target for fracture healing and bone regeneration. This review aims to clarify these issues and explain bone remodeling, structure dynamics, and mechano-transduction processes in response to mechanical loading. Loading of different magnitudes, frequencies, and types, such as dynamic versus static loads, are analyzed to determine the effects of mechanical stimulation on bone tissue structure and cellular function. Finally, the importance of vascularization in nutrient supply for bone healing and regeneration was further discussed.

Fluid Shear Stress Regulates Osteogenic Differentiation via AnnexinA6-Mediated Autophagy in MC3T3-E1 Cells

Fluid shear stress (FSS) facilitates bone remodeling by regulating osteogenic differentiation, and extracellular matrix maturation and mineralization. However, the underlying molecular mechanisms of how mechanical stimuli from FSS are converted into osteogenesis remain largely unexplored. Here, we exposed MC3T3-E1 cells to FSS with different intensities (1 h FSS with 0, 5, 10, and 20 dyn/cm² intensities) and treatment durations (10 dyn/cm² FSS with 0, 0.5, 1, 2 and 4 h treatment). The results demonstrate that the 1 h of 10 dyn/cm² FSS treatment greatly upregulated the expression of osteogenic markers (Runx2, ALP, Col I), accompanied by AnxA6 activation. The genetic ablation of AnxA6 suppressed the autophagic process, demonstrating lowered autophagy markers (Beclin1, ATG5, ATG7, LC3) and decreased autophagosome formation, and strongly reduced osteogenic differentiation induced by FSS. Furthermore, the addition of autophagic activator rapamycin to AnxA6 knockdown cells stimulated autophagy process, and coincided with more expressions of osteogenic proteins ALP and Col I under both static and FSS conditions. In conclusion, the findings in this study reveal a hitherto unidentified relationship between FSS-induced osteogenic differentiation and autophagy, and point to AnxA6 as a key mediator of autophagy in response to FSS, which may provide a new target for the treatment of osteoporosis and other diseases.