Fractal-like hierarchical organization of bone begins at the nanoscale

Interspecies comparison of temporomandibular joint condylar cartilage extracellular matrix from macro to microscopy

Interspecies comparisons of the extracellular matrix of temporomandibular joint (TMJ) condylar cartilage are necessary to elucidate the mechanisms underlying its superior mechanical properties, to guide the construction of animal models of TMJ-related diseases, and to establish standards for the engineering of TMJ condylar cartilage. Here we characterize and compare TMJ condylar cartilage from six different species from a materials science perspective, including structure, composition and mechanical properties from the macroscopic to the microscopic level. The gross morphology showed obvious interspecies differences in size and shape, which may be related to the different joint motion patterns. Although the condylar cartilage of all species can be divided histologically into a superficial fibrous layer and a deep hyaline layer, there are significant interspecies differences in the microstructure of the fibrils in the two layers, mainly in the diameter of the fibrils. Compositionally, there were no significant differences in collagen composition between species, but the content of glycosaminoglycans (GAGs) decreased progressively with increasing body size, with the same results obtained by Safranin O staining and biochemical analysis. Mechanically, the elastic modulus of mouse condylar cartilage was significantly higher than that of the other species and tended to decrease with increasing body size. This study shows that the TMJ condylar cartilage of different species has its own specific structure-composition-mechanics matching characteristics for their unique masticatory stress dissipation, and differences in fibril diameter and GAGs content may be the two ultimate factors influencing the differences in cartilage mechanical properties between species, while the condylar cartilage of pigs is most similar to that of humans, suggesting that pigs may be a suitable animal model for TMJ studies.

Osteogenesis and osteoclastogenesis on a chip: Engineering a self-assembling 3D coculture

Healthy bone is maintained by the process of bone remodeling. An unbalance in this process can lead to pathologies such as osteoporosis which are often studied with animal models. However, data from animals have limited power in predicting the results that will be obtained in human clinical trials. In search for alternatives to animal models, human in vitro models are emerging as they address the principle of reduction, refinement, and replacement of animal experiments (3Rs). At the moment, no complete in vitro model for bone-remodeling exists. Microfluidic chips offer great possibilities, particularly because of the dynamic culture options, which are crucial for in vitro bone formation. In this study, a scaffold free, fully human, 3D microfluidic coculture model of bone remodeling is presented. A bone-on-a-chip coculture system was developed in which human mesenchymal stromal cells differentiated into the osteoblastic lineage and self-assembled into scaffold free bone-like tissues with the shape and dimensions of human trabeculae. Human monocytes were able to attach to these tissues and to fuse into multinucleated osteoclast-like cells, establishing the coculture. Computational modeling was used to determine the fluid flow induced shear stress and strain in the formed tissue. Furthermore, a set-up was developed allowing for long-term (35 days) on-chip cell culture with benefits including continuous fluid-flow, low bubble formation risk, easy culture medium exchange inside the incubator and live cell imaging options. This on-chip coculture is a crucial advance towards developing in vitro bone remodeling models to facilitate drug testing.

1 H-13 C cross-polarization kinetics to probe hydration-dependent organic components of bone extracellular matrix

Bone is a living tissue made up of organic proteins, inorganic minerals, and water. The organic component of bone (mainly made up of Type-I collagen) provides flexibility and tensile strength. Solid-state nuclear magnetic resonance (ssNMR) is one of the few techniques that can provide atomic-level structural insights of such biomaterials in their native state. In the present article, we employed the variable contact time cross-polarization (¹ H-¹³ C CP) kinetics experiments to study the hydration-dependent atomic-level structural changes in the bone extracellular matrix (ECM). The natural abundant ¹³ C CP intensity of the bone ECM is measured by varying CP contact time and best fitted to the nonclassical kinetic model. Different relaxation parameters were measured by the best-fit equation corresponding to the different hydration conditions of the bone ECM. The associated changes in the measured parameters due to varying levels of hydration observed at different sites of collagen protein have provided its structural arrangements and interaction with water molecules in bone ECM. Overall, the present study reveals a better understanding of the kinetics of the organic part inside the bone ECM that will help in comprehending the disease-associated pathways.

Cementum is key to periodontal tissue regeneration: A review on apatite microstructures for creation of novel cementum-based dental implants

The cementum is the outermost layer of hard tissue covering the dentin within the root portion of the teeth. It is the only hard tissue with a specialized structure and function that forms a part of both the teeth and periodontal tissue. As such, cementum is believed to be critical for periodontal tissue regeneration. In this review, we discuss the function and histological structure of the cementum to promote crystal engineering with a biochemical approach in cementum regenerative medicine. We review the microstructure of enamel and bone while discussing the mechanism underlying apatite crystal formation to infer the morphology of cementum apatite crystals and their complex structure with collagen fibers. Finally, the limitations of the current dental implant treatments in clinical practice are explored from the perspective of periodontal tissue regeneration. We anticipate the possibility of advancing periodontal tissue regenerative medicine via cementum regeneration using a combination of material science and biochemical methods.

Nanogels for bone tissue engineering - from synthesis to application

Nanogels are cross-linked hydrogel nanoparticles with a three-dimensional, tunable porous structure that merges the best features of hydrogels and nanoparticles, including the ability to retain their hydrated nature and to swell and shrink in response to environmental changes. Nanogels have attracted increasing attention for use in bone tissue engineering as scaffolds for growth factor transport and cell adhesion. Their three-dimensional structures allow the encapsulation of a wide range of hydrophobic and hydrophilic drugs, enhance their half-life, and impede their enzymatic breakdown in vivo. Nanogel-based scaffolds are a viable treatment modality for enhanced bone regeneration. They act as carriers for cells and active ingredients capable of controlled release, enhanced mechanical support, and osteogenesis for enhanced bone tissue regeneration. However, the development of such nanogel constructs might involve combinations of several biomaterials to fabricate active ingredients that can control release, enhance mechanical support, and facilitate osteogenesis for more effective bone tissue regeneration. Hence, this review aims to highlight the potential of nanogel-based scaffolds to address the needs of bone tissue engineering.

Application of Nanocellulose-Based Aerogels in Bone Tissue Engineering: Current Trends and Outlooks

The complex or compromised bone defects caused by osteomyelitis, malignant tumors, metastatic tumors, skeletal abnormalities, and systemic diseases are difficult to be self-repaired, leading to a non-union fracture. With the increasing demands of bone transplantation, more and more attention has been paid to artificial bone substitutes. As biopolymer-based aerogel materials, nanocellulose aerogels have been widely utilized in bone tissue engineering. More importantly, nanocellulose aerogels not only mimic the structure of the extracellular matrix but could also deliver drugs and bioactive molecules to promote tissue healing and growth. Here, we reviewed the most recent literature about nanocellulose-based aerogels, summarized the preparation, modification, composite fabrication, and applications of nanocellulose-based aerogels in bone tissue engineering, as well as giving special focus to the current limitations and future opportunities of nanocellulose aerogels for bone tissue engineering.

Mineralized Collagen Fibrils: An Essential Component in Determining the Mechanical Behavior of Cortical Bone

Bone comprises mechanically different materials in a specific hierarchical structure. Mineralized collagen fibrils (MCFs), represented by tropocollagen molecules and hydroxyapatite nanocrystals, are the fundamental unit of bone. The mechanical characterization of MCFs provides the unique adaptive mechanical competence to bone to withstand mechanical load. The structural and mechanical role of MCFs is critical in the deformation mechanisms of bone and the marvelous strength and toughness possessed by bone. However, the role of MCFs in the mechanical behavior of bone across multiple length scales is not fully understood. In the present study, we shed light upon the latest progress regarding bone deformation at multiple hierarchical levels and emphasize the role of MCFs during bone deformation. We propose the concept of hierarchical deformation of bone to describe the interconnected deformation process across multiple length scales of bone under mechanical loading. Furthermore, how the deterioration of bone caused by aging and diseases impairs the hierarchical deformation process of the cortical bone is discussed. The present work expects to provide insights on the characterization of MCFs in the mechanical properties of bone and lays the framework for the understanding of the multiscale deformation mechanics of bone.

Biomechanical Characteristics and Analysis Approaches of Bone and Bone Substitute Materials

Bone has a special structure that is both stiff and elastic, and the composition of bone confers it with an exceptional mechanical property. However, bone substitute materials that are made of the same hydroxyapatite (HA) and collagen do not offer the same mechanical properties. It is important for bionic bone preparation to understand the structure of bone and the mineralization process and factors. In this paper, the research on the mineralization of collagen is reviewed in terms of the mechanical properties in recent years. Firstly, the structure and mechanical properties of bone are analyzed, and the differences of bone in different parts are described. Then, different scaffolds for bone repair are suggested considering bone repair sites. Mineralized collagen seems to be a better option for new composite scaffolds. Last, the paper introduces the most common method to prepare mineralized collagen and summarizes the factors influencing collagen mineralization and methods to analyze its mechanical properties. In conclusion, mineralized collagen is thought to be an ideal bone substitute material because it promotes faster development. Among the factors that promote collagen mineralization, more attention should be given to the mechanical loading factors of bone.

Mineralization of Bone Extracellular Matrix-like Scaffolds Fabricated as Silk Sericin-Functionalized Dense Collagen–Fibrin Hybrid Hydrogels

The design of hydrogels that combine both the biochemical cues needed to direct seeded cellular functions and mineralization to provide the structural and mechanical properties approaching those of mineralized native bone extracellular matrix (ECM) represents a significant challenge in bone tissue engineering. While fibrous hydrogels constituting of collagen or fibrin (and their hybrids) can be considered as scaffolds that mimic to some degree native bone ECM, their insufficient mechanical properties limit their application. In the present study, an automated gel aspiration–ejection (automated GAE) method was used to generate collagen–fibrin hybrid gel scaffolds with micro-architectures and mechanical properties approaching those of native bone ECM. Moreover, the functionalization of these hybrid scaffolds with negatively charged silk sericin accelerated their mineralization under acellular conditions in simulated body fluid and modulated the proliferation and osteoblastic differentiation of seeded MC3T3-E1 pre-osteoblastic cells. In the latter case, alkaline phosphatase activity measurements indicated that the hybrid gel scaffolds with seeded cells showed accelerated osteoblastic differentiation, which in turn led to increased matrix mineralization. In summary, the design of dense collagen–fibrin hybrid gels through an automated GAE process can provide a route to tailoring specific biochemical and mechanical properties to different types of bone ECM-like scaffolds, and can provide a model to better understand cell–matrix interactions in vitro for bioengineering purposes.

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.

Osteoregenerative Potential of 3D-Printed Poly ε-Caprolactone Tissue Scaffolds In Vitro Using Minimally Manipulative Expansion of Primary Human Bone Marrow Stem Cells

The repair of orthopedic and maxillofacial defects in modern medicine currently relies heavily on the use of autograft, allograft, void fillers, or other structural material composites. This study examines the in vitro osteo regenerative potential of polycaprolactone (PCL) tissue scaffolding, fabricated via a three-dimensional (3D) additive manufacturing technology, i.e., a pneumatic micro extrusion (PME) process. The objectives of this study were: (i) To examine the innate osteoinductive and osteoconductive potential of 3D-printed PCL tissue scaffolding and (ii) To perform a direct in vitro comparison of 3D-printed PCL scaffolding with allograft Allowash® cancellous bone cubes with regards to cell-scaffold interactions and biocompatibility with three primary human bone marrow (hBM) stem cell lines. This study specifically examined cell survival, cell integration, intra-scaffold cell proliferation, and differentiation of progenitor cells to investigate the potential of 3D-printed PCL scaffolds as an alternative to allograft bone material for the repair of orthopedic injuries. We found that mechanically robust PCL bone scaffolds can be fabricated via the PME process and the resulting material did not elicit detectable cytotoxicity. When the widely used osteogenic model SAOS-2 was cultured in PCL extract medium, no detectable effect was observed on cell viability or proliferation with multiple test groups showing viability ranges of 92.2% to 100% relative to a control group with a standard deviation of ±10%. In addition, we found that the honeycomb infill pattern of the 3D-printed PCL scaffold allowed for superior mesenchymal stem-cell integration, proliferation, and biomass increase. When healthy and active primary hBM cell lines, having documented in vitro growth rates with doubling times of 23.9, 24.67, and 30.94 h, were cultured directly into 3D-printed PCL scaffolds, impressive biomass increase values were observed. It was found that the PCL scaffolding material allowed for biomass increase values of 17.17%, 17.14%, and 18.18%, compared to values of 4.29% for allograph material cultured under identical parameters. It was also found that the honeycomb scaffold infill pattern was superior to the cubic and rectangular matrix structures, and provided a superior microenvironment for osteogenic and hematopoietic progenitor cell activity and auto-differentiation of primary hBM stem cells. Histological and immunohistochemical studies performed in this work confirmed the regenerative potential of PCL matrices in the orthopedic setting by displaying the integration, self-organization, and auto-differentiation of hBM progenitor cells within the matrix. Differentiation products including mineralization, self-organizing "proto-osteon" structures, and in vitro erythropoiesis were observed in conjunction with the documented expression of expected bone marrow differentiative markers including CD-99 (>70%), CD-71 (>60%), and CD-61 (>5%). All of the studies were conducted without the addition of any exogenous chemical or hormonal stimulation and exclusively utilized the abiotic and inert material polycaprolactone; setting this work apart from the vast majority of contemporary investigations into synthetic bone scaffold fabrication In summary, this study demonstrates the unique clinical potential of 3D-printed PCL scaffolds for stem cell expansion and incorporation into advanced microstructures created via PME manufacturing to generate a physiologically inert temporary bony defect graft with significant autograft features for enhanced end-stage healing.

Bone mineral properties and 3D orientation of human lamellar bone around cement lines and the Haversian system

Bone is a complex, biological tissue made up primarily of collagen fibrils and biomineral nanoparticles. The importance of hierarchical organization in bone was realized early on, but the actual interplay between structural features and the properties on the nanostructural and crystallographic level is still a matter of intense discussion. Bone is the only mineralized tissue that can be remodeled and, at the start of the formation of new bone during this process, a structure called a cement line is formed on which regular bone grows. Here, the orientational relationship of nanostructural and crystallographic constituents as well as the structural properties of both nanostructural and crystallographic constituents around cement lines and the Haversian system in human lamellar bone are investigated. A combination of small- and wide-angle X-ray scattering tensor tomography is employed together with diffraction tomography and synchrotron computed tomography to generate a multi-modal image of the sample. This work shows that the mineral properties vary as a function of the distance to the Haversian canal and, importantly, shows that the cement line has differing mineral properties from the surrounding lamellar bone, in particular with respect to crystallite size and degree of orientation. Cement lines make up a significant portion of the bone matrix despite their small size, hence the reported findings on an altered mineral structure, together with the spatial modulation around the Haversian canal, have implications for the formation and mechanics of bone.

An overview of translational research in bone graft biomaterials

Natural bone healing is often inadequate to treat fractures with critical size bone defects and massive bone loss. Immediate surgical interventions through bone grafts have been found to be essential on such occasions. Naturally harvested bone grafts, although are the preferred choice of the surgeons; they suffer from serious clinical limitations, including disease transmission, donor site morbidity, limited supply of graft etc. Synthetic bone grafts, on the other hand, offer a more clinically appealing approach to decode the pathways of bone repair through use of tissue engineered biomaterials. This article critically retrospects the translational research on various engineered biomaterials towards bringing transformative changes in orthopaedic healthcare. The first section of the article discusses about composition and ultrastructure of bone along with the global perspectives on statistical escalation of bone fracture surgeries requiring use of bone grafts. The next section reviews the types, benefits and challenges of various natural and synthetic bone grafts. An overview of clinically relevant biomaterials from traditionally used metallic, bioceramic, and biopolymeric biomaterials to new generation composites have been summarised. Finally, this narrative review concludes with the discussion on the emerging trends and future perspectives of the promising bone grafts.

Stage-specific and location-specific cartilage calcification in osteoarthritis development

OBJECTIVES

This study investigated the stage-specific and location-specific deposition and characteristics of minerals in human osteoarthritis (OA) cartilages via multiple nano-analytical technologies.

METHODS

Normal and OA cartilages were serially sectioned for micro-CT, scanning electron microscopy with energy dispersive X-ray spectroscopy, micro-Raman spectroscopy, focused ion beam scanning electron microscopy, high-resolution electron energy loss spectrometry with transmission electron microscopy, nanoindentation and atomic force microscopy to analyse the structural, compositional and mechanical properties of cartilage in OA progression.

RESULTS

We found that OA progressed by both top-down calcification at the joint surface and bottom-up calcification at the osteochondral interface. The top-down calcification process started with spherical mineral particle formation in the joint surface during early-stage OA (OA-E), followed by fibre formation and densely packed material transformation deep into the cartilage during advanced-stage OA (OA-A). The bottom-up calcification in OA-E started when an excessive layer of calcified tissue formed above the original calcified cartilage, exhibiting a calcified sandwich structure. Over time, the original and upper layers of calcified cartilage fused, which thickened the calcified cartilage region and disrupted the cartilage structure. During OA-E, the calcified cartilage was hypermineralised, containing stiffer carbonated hydroxyapatite (HAp). During OA-A, it was hypomineralised and contained softer HAp. This discrepancy may be attributed to matrix vesicle nucleation during OA-E and carbonate cores during OA-A.

CONCLUSIONS

This work refines our current understanding of the mechanism underlying OA progression and provides the foothold for potential therapeutic targeting strategies once the location-specific cartilage calcification features in OA are established.

Bone Apatite Nanocrystal: Crystalline Structure, Chemical Composition, and Architecture

The biological and mechanical functions of bone rely critically on the inorganic constituent, which can be termed as bone apatite nanocrystal. It features a hydroxylapatite-like crystalline structure, complex chemical compositions (e.g., carbonate-containing and calcium- and hydroxyl-deficient), and fine geometries and properties. The long research with vast literature across broad spectra of disciplines and fields from chemistry, crystallography, and mineralogy, to biology, medical sciences, materials sciences, mechanics, and engineering has produced a wealth of knowledge on the bone apatite nanocrystal. This has generated significant impacts on bioengineering and industrial engineering, e.g., in developing new biomaterials with superior osteo-inductivities and in inspiring novel strong and tough composites, respectively. Meanwhile, confusing and inconsistent understandings on the bone mineral constituent should be addressed to facilitate further multidisciplinary progress. In this review, we present a mineralogical account of the bone-related ideal apatite mineral and then a brief historical overview of bone mineral research. These pave the road to understanding the bone apatite nanocrystal via a material approach encompassing crystalline structure, diverse chemical formulae, and interesting architecture and properties, from which several intriguing research questions emerge for further explorations. Through providing the classical and latest findings with decent clearness and adequate breadth, this review endeavors to promote research advances in a variety of related science and engineering fields.

Lamellar thickness measurements in control and osteogenesis imperfecta human bone, with development of a method of automated thickness averaging to simplify quantitation

Lamellar bone that forms in moderate and severe osteogenesis imperfecta (OI) is composed of structurally irregular lamellae compared to those in control bone. OI and control cortical bone fragments were prepared for light microscopy in standardized fashion: decalcified, embedded in plastic, sectioned and stained with toluidine blue. Polarization light microscopy (PLM) was used to demonstrate and quantify bright and dark lamellar thicknesses in cortical bone fragments from 5 patients with moderate to severe OI in whom type I collagen structural/molecular defects were detected and in control bone from 5 patients. Rigid selection criteria identified lamellar regions for quantification. Thicknesses of bright and dark lamellae were measured manually at 20X magnification using a histomorphometric image analysis system. A method of automated thickness averaging was developed to determine lamellar thicknesses from PLM images to make measurement faster. Our study demonstrates, for the first time, that in OI bone from patients with type I collagen structural/molecular defects mean lamellar thickness measurements (along with the bright and dark lamellar thicknesses) were less than those in control bone by statistically highly significant differences. The mean value for bright lamellae was less than that for dark lamellae in both control and OI bone. The ratio of mean values for bright/dark lamellar thicknesses was the same in control and OI bone. The automated method obtained similar results to the manual method. Lamellar bone in moderate and severe OI with type I collagen defects is composed of thinner and less structurally regular lamellae than those in control bone. This finding indicates that lamellar thickness measurements can be helpful in assessing the effect of specific collagen and collagen-related mutations on OI bone synthesis and warrant inclusion in research and clinical histomorphometric assessments.

Periodontal bone regeneration with a degradable thermoplastic HA/PLCL bone graft

Strategic bone grafts are required to regenerate periodontal bone defects owing to limited self-healing. Current bioceramic particle or deproteinized bovine bone (DBB) products are not able to ideally meet clinical requirements, such as insufficient operability and slow degradation rates. Herein, a strong-interacted bone graft was designed and synthesized by modifying hydroxyapatite (HA) with a lactide-caprolactone copolymer (PLCL) to improve component homogeneity and mechanical properties. The physical-chemical analysis indicated that HA particles were homogenously distributed in HA/PLCL bone grafts, possessed outstanding thermoplasticity, and facilitated clinic operability and initial mechanical support. The in vitro study suggested that HA/PLCL bone graft degraded in a spatiotemporal model. Micropores were formed on the non-porous surface at the beginning, and interconnected porous structures were gradually generated. Furthermore, HA/PLCL bone grafts exhibited excellent biocompatibility and osteogenic ability as revealed in vitro cell culture and in vivo animal experiments. When applied to rat periodontal bone defects, the HA/PLCL bone graft showed a non-inferior bone regeneration compared to the commercial DBB. This study proposes a potential bone graft for periodontal bone repair with thermoplastic, spatiotemporal degraded, and osteogenic characteristics.

Calcium Phosphate Delivery Systems for Regeneration and Biomineralization of Mineralized Tissues of the Craniofacial Complex

Calcium phosphate (CaP)-based materials have been extensively used for mineralized tissues in the craniofacial complex. Owing to their excellent biocompatibility, biodegradability, and inherent osteoconductive nature, their use as delivery systems for drugs and bioactive factors has several advantages. Of the three mineralized tissues in the craniofacial complex (bone, dentin, and enamel), only bone and dentin have some regenerative properties that can diminish due to disease and severe injuries. Therefore, targeting these regenerative tissues with CaP delivery systems carrying relevant drugs, morphogenic factors, and ions is imperative to improve tissue health in the mineralized tissue engineering field. In this review, the use of CaP-based microparticles, nanoparticles, and polymer-induced liquid precursor (PILPs) amorphous CaP nanodroplets for delivery to craniofacial bone and dentin are discussed. The use of these various form factors to obtain either a high local concentration of cargo at the macroscale and/or to deliver cargos precisely to nanoscale structures is also described. Finally, perspectives on the field using these CaP materials and next steps for the future delivery to the craniofacial complex are presented.

3D printing of gear-inspired biomaterials: Immunomodulation and bone regeneration

It is of significance to construct the immunomodulatory and osteogenic microenvironment for three dimension (3D) regeneration of bone tissues. 3D scaffolds, with various chemical composition, macroporous structure and surface characteristics offer a beneficial microenvironment for bone tissue regeneration. However, there is a gap between the well-ordered surface microstructure of bioceramic scaffolds and immune microenvironment for bone regeneration. In this study, a gear-inspired 3D scaffold with well-ordered surface microstructure was successfully prepared through a modified extrusion-based 3D printing strategy for immunomodulation and bone regeneration. The prepared gear-inspired scaffolds could induce M2 phenotype polarization of macrophages and further promoted osteogenic differentiation of bone mesenchymal stem cells in vitro. The subsequent in vivo study demonstrated that the gear-inspired scaffolds were able to attenuate inflammation and further promote new bone formation. The study develops a facile strategy to construct well-ordered surface microstructure which plays a key role in 3D immunomodulatory and osteogenic microenvironment for bone tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE.

The Role of the Extrafibrillar Volume on the Mechanical Properties of Molecular Models of Mineralized Bone Microfibrils

Bones are responsible for body support, structure, motion, and several other functions that enable and facilitate life for many different animal species. They exhibit a complex network of distinct physical structures and mechanical properties, which ultimately depend on the fraction of their primary constituents at the molecular scale. However, the relationship between structure and mechanical properties in bones are still not fully understood. Here, we investigate structural and mechanical properties of all-atom bone molecular models composed of type-I collagen, hydroxyapatite (HA), and water by means of fully atomistic molecular dynamics simulations. Our models encompass an extrafibrillar volume (EFV) and consider mineral content in both the EFV and intrafibrillar volume (IFV), consistent with experimental observations. We investigate solvation structures and elastic properties of bone microfibril models with different degrees of mineralization, ranging from highly mineralized to weakly mineralized and nonmineralized models. We find that the local tetrahedral order of water is lost in similar ways in the EFV and IFV regions for all HA containing models, as calcium and phosphate ions are strongly coordinated with water molecules. We also subject our models to tensile loads and analyze the spatial stress distribution over the nanostructure of the material. Our results show that both mineral and water contents accumulate significantly higher stress levels, most notably in the EFV, thus revealing that this region, which has been only recently incorporated in all-atom molecular models, is fundamental for studying the mechanical properties of bones at the nanoscale. Furthermore, our results corroborate the well-established finding that high mineral content makes bone stiffer.

Advances in biomimetic collagen mineralisation and future approaches to bone tissue engineering

With an ageing world population and ~20% of adults in Europe being affected by bone diseases, there is an urgent need to develop advanced regenerative approaches and biomaterials capable to facilitate tissue regeneration while providing an adequate microenvironment for cells to thrive. As the main components of bone are collagen and apatite mineral, scientists in the tissue engineering field have attempted in combining these materials by using different biomimetic approaches to favour bone repair. Still, an ideal bone analogue capable of mimicking the distinct properties (i.e., mechanical properties, degradation rate, porosity, etc.) of cancellous bone is to be developed. This review seeks to sum up the current understanding of bone tissue mineralisation and structure while providing a critical outlook on the existing biomimetic strategies of mineralising collagen for bone tissue engineering applications, highlighting where gaps in knowledge exist.

From mesenchymal niches to engineered in vitro model systems: Exploring and exploiting biomechanical regulation of vertebrate hedgehog signalling

Tissue patterning is the result of complex interactions between transcriptional programs and various mechanical cues that modulate cell behaviour and drive morphogenesis. Vertebrate Hedgehog signalling plays key roles in embryogenesis and adult tissue homeostasis, and is central to skeletal development and the osteogenic differentiation of mesenchymal stem cells. The expression of several components of the Hedgehog signalling pathway have been reported to be mechanically regulated in mesodermal tissue patterning and osteogenic differentiation in response to external stimulation. Since a number of bone developmental defects and skeletal diseases, such as osteoporosis, are directly linked to aberrant Hedgehog signalling, a better knowledge of the regulation of Hedgehog signalling in the mechanosensitive bone marrow-residing mesenchymal stromal cells will present novel avenues for modelling these diseases and uncover novel opportunities for extracellular matrix-targeted therapies. In this review, we present a brief overview of the key molecular players involved in Hedgehog signalling and the basic concepts of mechanobiology, with a focus on bone development and regeneration. We also highlight the correlation between the activation of the Hedgehog signalling pathway in response to mechanical cues and osteogenesis in bone marrow-derived mesenchymal stromal cells. Finally, we propose different tissue engineering strategies to apply the expanding knowledge of 3D material-cell interactions in the modulation of Hedgehog signalling in vitro for fundamental and translational research applications.

Nanotopographical cues for regulation of macrophages and osteoclasts: emerging opportunities for osseointegration

Nanotopographical cues of bone implant surface has direct influences on various cell types during the establishment of osseointegration, a prerequisite of implant bear-loading. Given the important roles of monocyte/macrophage lineage cells in bone regeneration and remodeling, the regulation of nanotopographies on macrophages and osteoclasts has arisen considerable attentions recently. However, compared to osteoblastic cells, how nanotopographies regulate macrophages and osteoclasts has not been properly summarized. In this review, the roles and interactions of macrophages, osteoclasts and osteoblasts at different stages of bone healing is firstly presented. Then, the diversity and preparation methods of nanotopographies are summarized. Special attentions are paid to the regulation characterizations of nanotopographies on macrophages polarization and osteoclast differentiation, as well as the focal adhesion-cytoskeleton mediated mechanism. Finally, an outlook is indicated of coordinating nanotopographies, macrophages and osteoclasts to achieve better osseointegration. These comprehensive discussions may not only help to guide the optimization of bone implant surface nanostructures, but also provide an enlightenment to the osteoimmune response to external implant.

Polymeric multimaterials by photochemical patterning of crystallinity

An organized combination of stiff and elastic domains within a single material can synergistically tailor bulk mechanical properties. However, synthetic methods to achieve such sophisticated architectures remain elusive. We report a rapid, facile, and environmentally benign method to pattern strong and stiff semicrystalline phases within soft and elastic matrices using stereo-controlled ring-opening metathesis polymerization of an industrial monomer, cis-cyclooctene. Dual polymerization catalysis dictates polyolefin backbone chemistry, which enables patterning of compositionally uniform materials with seamless stiff and elastic interfaces. Visible light-induced activation of a metathesis catalyst results in the formation of semicrystalline trans polyoctenamer rubber, outcompeting the formation of cis polyoctenamer rubber, which occurs at room temperature. This bottom-up approach provides a method for manufacturing polymeric materials with promising applications in soft optoelectronics and robotics.

An analytical and numerical approach to the determination of thermal necrosis in cortical bone drilling

In the process of repairing fractures in the bone region with orthopedic injuries, the application of supporting and strengthening the bone tissue with screws, wires, rods, and plates is a widely preferred internal fixation method. In this treatment process, it is necessary to drill the bone tissue to fix the screws. Due to the heat generated in the drilling process, mechanical and thermal damage occurs in the bone tissue. In this study, it is focused that the effect of different cutting conditions on the temperature distribution and necrosis zones in the drilling of human cortical bone. In this context, by selecting variable drill geometry (diameter, point angle, and helix angle) and variable cutting parameters (cutting speed and feed rate), temperature distribution and necrosis zones were investigated with finite element analyses and analytical calculations. When the findings were evaluated, it was understood that drill diameter and cutting speed did not have a significant effect on temperatures and necrosis zone at low cutting speeds. At high cutting speeds, it was observed that the feed rate and drill point angle had an indeterminate effect on the temperatures. The lowest temperature values were obtained at cutting speed of 750 rpm and a feed rate of 0.1 mm/rev for low cutting speeds, and cutting speed of 1500 rpm, helix angle of 10° and drill bit diameter of 2 mm for high cutting speeds. The narrowest necrosis zones were obtained at cutting speed of 250 rpm and feed rate of 0.1 mm/rev for both drill diameters. As a result, the effects of different drill geometry and cutting parameters were determined in order to obtain low temperature distribution and narrow necrosis zone in cortical bone drilling.

Matrix-Directed Mineralization for Bulk Structural Materials

Mineral-based bulk structural materials (MBSMs) are known for their long history and extensive range of usage. The inherent brittleness of minerals poses a major problem to the performance of MBSMs. To overcome this problem, design principles have been extracted from natural biominerals, in which the extraordinary mechanical performance is achieved via the hierarchical organization of minerals and organics. Nevertheless, precise and efficient fabrication of MBSMs with bioinspired hierarchical structures under mild conditions has long been a big challenge. This Perspective provides a panoramic view of an emerging fabrication strategy, matrix-directed mineralization, which imitates the in vivo growth of some biominerals. The advantages of the strategy are revealed by comparatively analyzing the conventional fabrication techniques of artificial hierarchically structured MBSMs and the biomineral growth processes. By introducing recent advances, we demonstrate that this strategy can be used to fabricate artificial MBSMs with hierarchical structures. Particular attention is paid to the mass transport and the precursors that are involved in the mineralization process. We hope this Perspective can provide some inspiring viewpoints on the importance of biomimetic mineralization in material fabrication and thereby spur the biomimetic fabrication of high-performance MBSMs.

Imaging Orientation of a Single Molecular Hierarchical Self-Assembled Sheet: The Combined Power of a Vibrational Sum Frequency Generation Microscopy and Neural Network

In this work, we determined the tilt angles of molecular units in hierarchical self-assembled materials on a single-sheet level, which were not available previously. This was achieved by developing a fast line-scanning vibrational sum frequency generation (VSFG) hyperspectral imaging technique in combination with neural network analysis. Rapid VSFG imaging enabled polarization resolved images on a single sheet level to be measured quickly, circumventing technical challenges due to long-term optical instability. The polarization resolved hyperspectral images were then used to extract the supramolecular tilt angle of a self-assembly through a set of spectra-tilt angle relationships which were solved through neural network analysis. This unique combination of both novel techniques offers a new pathway to resolve molecular level structural information on self-assembled materials. Understanding these properties can further drive self-assembly design from a bottom-up approach for applications in biomimetic and drug delivery research.

Aging exacerbates the morphological and mechanical response of mineralized collagen fibrils in murine cortical bone to disuse

Mineralized collagen fibrils (MCFs) are the fundamental building blocks of bone tissue and contribute significantly to the mechanical behavior of bone. However, it is still largely unknown how the collagen network in bone responds to aging and the disuse normally accompanying it. Utilizing atomic force microscopy, nanoindentation and Raman spectroscopy, age-related alterations in the microstructure and mechanical properties of murine cortical tibia at multiple scales were investigated in this study. The potential difference in the responses of bone to disuse at different ages was studied. The results indicated that the age- and disuse-related alterations in bone initiate from MCFs in the bone matrix. The D-periodic spacing, radial elastic modulus of a single MCF and the mineral-to-matrix ratio on the cortical bone surface were larger in aged mice than in adult mice. Disuse, on the other hand, mainly has a major influence on aged mice, particularly on the morphology and mechanical properties of MCFs, but it only has modest effects on adult bone. These findings revealed insights into the morphological and mechanical adaptation of mineralized collagen fibrils in murine cortical bone to aging and disuse. STATEMENT OF SIGNIFICANCE: Bone is a complex structured composite material consisting of an interwoven framework of collagen fibrils reinforced by mineral particles and embedded in an extrafibrillar mineralized matrix. Utilizing atomic force microscopy, nanoindentation and Raman spectroscopy, this study suggests that the effects of aging, as well as the accompanying disuse, on the morphology and mechanical properties of bone initiate from the mineralized collagen fibril level. More interestingly, the MCF in the bone of aged mice seems to be more sensitive to disuse than that in adult mice. These findings significantly further the current understanding of the adaptation process of bone to aging at the mineralized collagen fibril level and provide direct insights into the physiological response of bone to aging and the abnormal mechanical environment.

Biomineralization of bone tissue: calcium phosphate-based inorganics in collagen fibrillar organic matrices

BACKGROUND

Bone regeneration research is currently ongoing in the scientific community. Materials approved for clinical use, and applied to patients, have been developed and produced. However, rather than directly affecting bone regeneration, these materials support bone induction, which regenerates bone. Therefore, the research community is still researching bone tissue regeneration. In the papers published so far, it is hard to find an improvement in the theory of bone regeneration. This review discusses the relationship between the existing theories on hard tissue growth and regeneration and the biomaterials developed so far for this purpose and future research directions.

MAINBODY

Highly complex nucleation and crystallization in hard tissue involves the coordinated action of ions and/or molecules that can produce different organic and inorganic composite biomaterials. In addition, the healing of bone defects is also affected by the dynamic conditions of ions and nutrients in the bone regeneration process. Inorganics in the human body, especially calcium- and/or phosphorus-based materials, play an important role in hard tissues. Inorganic crystal growth is important for treating or remodeling the bone matrix. Biomaterials used in bone tissue regeneration require expertise in various fields of the scientific community. Chemical knowledge is indispensable for interpreting the relationship between biological factors and their formation. In addition, sources of energy for the nucleation and crystallization processes of such chemical bonds and minerals that make up the bone tissue must be considered. However, the exact mechanism for this process has not yet been elucidated. Therefore, a convergence of broader scientific fields such as chemistry, materials, and biology is urgently needed to induce a distinct bone tissue regeneration mechanism.

CONCLUSION

This review provides an overview of calcium- and/or phosphorus-based inorganic properties and processes combined with organics that can be regarded as matrices of these minerals, namely collagen molecules and collagen fibrils. Furthermore, we discuss how this strategy can be applied to future bone tissue regenerative medicine in combination with other academic perspectives.

Mineral Crystal Thickness in Calcified Cartilage and Subchondral Bone in Healthy and Osteoarthritic Human Knees

Osteoarthritis (OA) is the most common joint disease, where articular cartilage degradation is often accompanied with sclerosis of the subchondral bone. However, the association between OA and tissue mineralization at the nanostructural level is currently not understood. In particular, it is technically challenging to study calcified cartilage, where relevant but poorly understood pathological processes such as tidemark multiplication and advancement occur. Here, we used state-of-the-art microfocus small-angle X-ray scattering with a 5-μm spatial resolution to determine the size and organization of the mineral crystals at the nanostructural level in human subchondral bone and calcified cartilage. Specimens with a wide spectrum of OA severities were acquired from both medial and lateral compartments of medial compartment knee OA patients (n = 15) and cadaver knees (n = 10). Opposing the common notion, we found that calcified cartilage has thicker and more mutually aligned mineral crystals than adjoining bone. In addition, we, for the first time, identified a well-defined layer of calcified cartilage associated with pathological tidemark multiplication, containing 0.32 nm thicker crystals compared to the rest of calcified cartilage. Finally, we found 0.2 nm thicker mineral crystals in both tissues of the lateral compartment in OA compared with healthy knees, indicating a loading-related disease process because the lateral compartment is typically less loaded in medial compartment knee OA. In summary, we report novel changes in mineral crystal thickness during OA. Our data suggest that unloading in the knee might be involved with the growth of mineral crystals, which is especially evident in the calcified cartilage. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Low-Temperature Printed Hierarchically Porous Induced-Biomineralization Polyaryletherketone Scaffold for Bone Tissue Engineering

Polyetheretherketone (PEEK) as a popular orthopaedic implant is usually fabricated into a hierarchically porous structure for improving osteogenic activity. However, the applications are limited due to the excessively high processing temperature and uncontrollably tedious modification routes. Here, an amorphous polyaryletherketone with carboxyl groups (PAEK-COOH) is synthesized and fabricated to the hierarchically controllable porous scaffolds via a low-temperature 3D-printing process. The prepared PAEK-COOH scaffolds present controllable porous structures ranging from nano- to micro-scale, and their mechanical strengths are comparable to that of trabecular bone. More importantly, the in vitro experiments show that the nanoporous surface is conducive to promoting cellular adhesion, and carboxyl groups can induce hydroxyapatite mineralization via electrostatic interaction. The in vivo experiments demonstrate that the PAEK-COOH scaffolds offer much better osseointegration without additional active ingredients, compared to that of PEEK. Therefore, this work will not only develop a promising candidate for bone tissue engineering, but provide a viable method to design PAEK biomaterials.

Biomechanics and mechanobiology of the bone matrix

The bone matrix plays an indispensable role in the human body, and its unique biomechanical and mechanobiological properties have received much attention. The bone matrix has unique mechanical anisotropy and exhibits both strong toughness and high strength. These mechanical properties are closely associated with human life activities and correspond to the function of bone in the human body. None of the mechanical properties exhibited by the bone matrix is independent of its composition and structure. Studies on the biomechanics of the bone matrix can provide a reference for the preparation of more applicable bone substitute implants, bone biomimetic materials and scaffolds for bone tissue repair in humans, as well as for biomimetic applications in other fields. In providing mechanical support to the human body, bone is constantly exposed to mechanical stimuli. Through the study of the mechanobiology of the bone matrix, the response mechanism of the bone matrix to its surrounding mechanical environment can be elucidated and used for the health maintenance of bone tissue and defect regeneration. This paper summarizes the biomechanical properties of the bone matrix and their biological significance, discusses the compositional and structural basis by which the bone matrix is capable of exhibiting these mechanical properties, and studies the effects of mechanical stimuli, especially fluid shear stress, on the components of the bone matrix, cells and their interactions. The problems that occur with regard to the biomechanics and mechanobiology of the bone matrix and the corresponding challenges that may need to be faced in the future are also described.

Microwave-Assisted Hydrothermal Rapid Synthesis of Ultralong Hydroxyapatite Nanowires Using Adenosine 5'-Triphosphate

Ultralong hydroxyapatite (HAP) nanowires are promising for various biomedical applications owing to their chemical similarity to the inorganic constituent of bone, high biocompatibility, good flexibility, excellent mechanical properties, etc. However, it is still challenging to control the formation of ultralong HAP nanowires because of the presence of free PO₄³⁻ ions in the reaction system containing the inorganic phosphate source. In addition, it takes a long period of time (usually tens of hours) for the synthetic process of ultralong HAP nanowires. Herein, for the first time, we have developed an eco-friendly calcium oleate precursor microwave hydrothermal method using biocompatible adenosine 5′-triphosphate (ATP) as a bio-phosphorus source and water as the only solvent for the rapid synthesis of ultralong HAP nanowires. The controllable hydrolysis of ATP can avoid the premature formation of calcium phosphate nuclei and uncontrollable crystal growth. Microwave heating can significantly shorten the synthetic time from tens of hours required by the traditional heating to 1 h, thus achieving high efficiency, energy saving and low cost. The as-prepared ultralong HAP nanowires with high flexibility have lengths of several hundred micrometers and diameters of 10~20 nm, and they usually self-assemble into nanowire bundles along their longitudinal direction. The as-prepared ultralong HAP nanowire/chitosan porous scaffold has excellent bioactivity, good biodegradation and cytocompatibility owing to the bioactive adenosine adsorbed on the surface of ultralong HAP nanowires. It is expected that ultralong HAP nanowires will be promising for various applications in the biomedical fields, such as bone defect repair, skin wound healing, and as a drug nanocarrier.

Molecular simulations of the interfacial properties in silk-hydroxyapatite composites

Biomineralization is a common strategy used in Nature to improve the mechanical strength and toughness of biological materials. This strategy, applied in materials like bone or nacre, serves as inspiration for materials scientists and engineers to design new materials for applications in healthcare, soft robotics or the environment. In this regard, composites consisting of silk and hydroxyapatite have been extensively researched for bone regeneration applications, due to their reported cytocompatibility and osteoinduction capacity that supports bone formation in vivo. Thus, it becomes relevant to understand how silk and hydroxyapatite interact at their interface, and how this affects the overall mechanical properties of these composites. This theoretical-experimental work investigates the interfacial dynamic and structural properties of silk in contact with hydroxyapatite, combining molecular dynamics simulations with analytical characterization. Our data indicate that hydroxyapatite decreases the β-sheets in silk, which are a key load-bearing element of silk. The β-sheets content can usually be increased in silk biomaterials via post-processing methods, such as water vapor annealing. However, the presence of hydroxyapatite appears to reduce also for the formation of β-sheets via water vapor annealing. This work sheds light into the interfacial properties of silk-hydroxyapatite composite and their relevance for the design of composite biomaterials for bone regeneration.

Promotion of collagen mineralization and dentin repair by succinates

Biomineralization of collagen fibers is regulated by non-collagenous proteins and small biomolecules, which are essential in bone and teeth formation. In particular, small biomolecules such as succinic acid (SA) exist at a high level in hard tissues, but their role is yet unclear. Here, our work demonstrated that SA could significantly promote intrafibrillar mineralization in two- and three-dimensional collagen models, where the relative mineralization rate was 16 times faster than the control group. Furthermore, the FTIR spectra and isothermal experimental results showed that collagen molecules could interact with SA via a hydrogen bond and that the interaction energy was about 4.35 kJ mol^-1. As expected, the SA-pretreated demineralized dentin obtained full remineralization within two days, whereas it took more than four days in the control group, and their mechanical properties were considerably enhanced compared with those of the demineralized one. The possible mechanism of the promotion effect of SA was ultimately illustrated, with SA modification strengthening the capacity of the collagen matrix to attract more calcium ions, which might create a higher local concentration that could accelerate the mineralization of collagen fibers. These findings not only advance the understanding of the vital role of small biomolecules in collagen biomineralization but also facilitate the development of an effective strategy to repair hard tissues.

Effect of Menin Deletion in Early Osteoblast Lineage on the Mineralization of an In Vitro 3D Osteoid-like Dense Collagen Gel Matrix

Bone has a complex microenvironment formed by an extracellular matrix (ECM) composed mainly of mineralized type I collagen fibres. Bone ECM regulates signaling pathways important in the differentiation of osteoblast-lineage cells, necessary for bone mineralization and in preserving tissue architecture. Compared to conventional 2D cell cultures, 3D in vitro models may better mimic bone ECM and provide an environment to support osteoblastic differentiation. In this study, a biomimetic 3D osteoid-like dense collagen gel model was used to investigate the role of the nuclear protein menin plays in osteoblastic differentiation and matrix mineralization. Previous in vitro and in vivo studies have shown that when expressed at later stages of osteoblastic differentiation, menin modulates osteoblastogenesis and regulates bone mass in adult mice. To investigate the role of menin when expressed at earlier stages of the osteoblastic lineage, conditional knockout mice in which the Men1 gene is specifically deleted early (i.e., at the level of the pluripotent mesenchymal stem cell lineage), where generated and primary calvarial osteoblasts were cultured in plastically compressed dense collagen gels for 21 days. The proliferation, morphology and differentiation of isolated seeded primary calvarial osteoblasts from knockout (Prx1-Cre; Men1f/f) mice were compared to those isolated from wild-type (Men1f/f) mice. Primary calvarial osteoblasts from knockout and wild-type mice did not show differences in terms of proliferation. However, in comparison to wild-type cells, primary osteoblast cells derived from knockout mice demonstrated deficient mineralization capabilities and an altered gene expression profile when cultured in 3D dense collagen gels. In summary, these findings indicate that when expressed at earlier stages of osteoblast differentiation, menin is important in maintaining matrix mineralization in 3D dense collagen gel matrices, in vitro.

Improving Biological Functions of Three-Dimensional Printed Ti2448 Scaffolds by Decoration with Polydopamine and Extracellular Matrices

Extracellular matrices (ECMs) provide important cues for cell proliferation and differentiation in the complex environment, which show a significant influence on cell functions. Herein, cell-derived ECMs were deposited on the polydopamine (PDA)-decorated porous Ti-24Nb-4Zr-8Sn (Ti2448) scaffolds fabricated by the electron beam melting method in order to improve biological functions. The influence of PDA-ECM coatings on cell functions was further investigated. The results demonstrated that the PDA-ECM coating facilitated adhesion, proliferation, and migration of MC3T3-E1 cells on Ti2448 scaffolds. Moreover, Ti2448-PDA-ECM scaffolds promoted osteogenesis differentiation of cells indicated by greater alkaline phosphatase activity and further mineralization, compared to the plain Ti2448 group. Meanwhile, Ti2448-PDA-ECM scaffolds enhanced bone growth after implantation for one month in rabbit femoral bone defects. Our findings suggest that the bioinspired PDA-ECM coating can be implemented on the porous Ti2448 scaffolds, which significantly improve the biological functions of orthopedic implants.

Synergistic effects of nanoattapulgite and hydroxyapatite on vascularization and bone formation in a rabbit tibia bone defect model

Hydroxyapatite (HA) is a promising scaffold material for the treatment of bone defects. However, the lack of angiogenic properties and undesirable mechanical properties (such as fragility) limits the application of HA. Nanoattapulgite (ATP) is a nature-derived clay mineral and has been proven to be a promising bioactive material for bone regeneration due to its ability to induce osteogenesis. In this study, polyvinyl alcohol/collagen/ATP/HA (PVA/COL/ATP/HA) scaffolds were printed. Mouse bone marrow mesenchymal stem/stromal cells (BMSCs) and human umbilical vein endothelial cells (HUVECs) were used in vitro to assess the biocompatibility and the osteogenesis and vascularization induction potentials of the scaffolds. Subsequently, in vivo micro-CT and histological staining were carried out to evaluate new bone formation in a rabbit tibial defect model. The in vitro results showed that the incorporation of ATP increased the printing fidelity and mechanical properties, with values of compressive strengths up to 200% over raw PC-H scaffolds. Simultaneously, the expression levels of osteogenic-related genes and vascularization-related genes were significantly increased after the incorporation of ATP. The in vivo results showed that the PVA/COL/ATP/HA scaffolds exhibited synergistic effects on promoting vascularization and bone formation. The combination of ATP and HA provides a promising strategy for vascularized bone tissue engineering.

Rapid Flow Synthesis of a Biomimetic Carbonate Apatite as an Effective Drug Carrier

A facile synthesis of apatite nanocrystals analogous to bioapatites with increased biocompatibility and biodegradability can remedy the shortcomings of the widely applied synthetic hydroxyapatite (HAp) for bone defect treatment. Here, we propose an expeditious synthesis method to develop a biomimetic B-type carbonate apatite (CAp) with a simple capillary microfluidic device at room temperature. The process not only eliminates fluctuations with the addition of carbonate but also produces safe CAp drug carriers through simultaneous alendronate incorporation to the CAp structure. CAp displayed superior mineralization on osteoblast-like MG-63 cells when compared with HAp and HAp drug carriers that were produced using identical methods. Furthermore, alendronate-incorporated CAp drug carriers potentially displayed higher cancer cell suppression when applied to breast cancer cells attached to the bone tissue model, which signifies enhanced cancer metastasis to bone suppression due to the likelihood of increased alendronate release of CAp owing to its faster dissolution. Overall, our results may provide promising opportunities for enhanced clinical CAp application for bone defect treatment, particularly for bone loss and cancer to bone metastasis.

Bone Tissue Engineering through 3D Bioprinting of Bioceramic Scaffolds: A Review and Update

Trauma and bone loss from infections, tumors, and congenital diseases make bone repair and regeneration the greatest challenges in orthopedic, craniofacial, and plastic surgeries. The shortage of donors, intrinsic limitations, and complications in transplantation have led to more focus and interest in regenerative medicine. Structures that closely mimic bone tissue can be produced by this unique technology. The steady development of three-dimensional (3D)-printed bone tissue engineering scaffold therapy has played an important role in achieving the desired goal. Bioceramic scaffolds are widely studied and appear to be the most promising solution. In addition, 3D printing technology can simulate mechanical and biological surface properties and print with high precision complex internal and external structures to match their functional properties. Inkjet, extrusion, and light-based 3D printing are among the rapidly advancing bone bioprinting technologies. Furthermore, stem cell therapy has recently shown an important role in this field, although large tissue defects are difficult to fill by injection alone. The combination of 3D-printed bone tissue engineering scaffolds with stem cells has shown very promising results. Therefore, biocompatible artificial tissue engineering with living cells is the key element required for clinical applications where there is a high demand for bone defect repair. Furthermore, the emergence of various advanced manufacturing technologies has made the form of biomaterials and their functions, composition, and structure more diversified, and manifold. The importance of this article lies in that it aims to briefly review the main principles and characteristics of the currently available methods in orthopedic bioprinting technology to prepare bioceramic scaffolds, and finally discuss the challenges and prospects for applications in this promising and vital field.

Laser Sintering Approaches for Bone Tissue Engineering

The adoption of additive manufacturing (AM) techniques into the medical space has revolutionised tissue engineering. Depending upon the tissue type, specific AM approaches are capable of closely matching the physical and biological tissue attributes, to guide tissue regeneration. For hard tissue such as bone, powder bed fusion (PBF) techniques have significant potential, as they are capable of fabricating materials that can match the mechanical requirements necessary to maintain bone functionality and support regeneration. This review focuses on the PBF techniques that utilize laser sintering for creating scaffolds for bone tissue engineering (BTE) applications. Optimal scaffold requirements are explained, ranging from material biocompatibility and bioactivity, to generating specific architectures to recapitulate the porosity, interconnectivity, and mechanical properties of native human bone. The main objective of the review is to outline the most common materials processed using PBF in the context of BTE; initially outlining the most common polymers, including polyamide, polycaprolactone, polyethylene, and polyetheretherketone. Subsequent sections investigate the use of metals and ceramics in similar systems for BTE applications. The last section explores how composite materials can be used. Within each material section, the benefits and shortcomings are outlined, including their mechanical and biological performance, as well as associated printing parameters. The framework provided can be applied to the development of new, novel materials or laser-based approaches to ultimately generate bone tissue analogues or for guiding bone regeneration.

Techniques for Bone Assessment and Characterization: Porcine Hard Palate Case Study

The hard palate plate has an important structural function that separates the nasal cavity and the nasopharynx. Incomplete regeneration of palatal fistulae in children with a cleft palate deformity after primary palatoplasty is a relatively common complication. To date, the information about the physicochemical bone features of this region is deficient, due to the low availability of human samples. Swine and human bone share anatomical similarities. Specifically, pig bones are widely used as experimental animal models in dental, orthopedic, or surgical techniques. The aim of this study was to show different techniques to evaluate and characterize alternative properties of pig hard palate bone, compared to commercial hydroxyapatite, one of the most used biomaterials for bone tissue regeneration. Chemical analyses by Energy dispersive spectroscopy (EDS) and X-ray fluorescence (XRF) showed calcium and phosphate ions as the main constituents of bone, while magnesium, iron, sodium, potassium, and zinc ions were minor constituents. The calcium phosphate molar ratio (Ca/P) in the bone was low (1.1 ± 0.2) due to the very young specimen sample used. The FTIR spectrum shows the presence of phosphates ions (PO₄^3-) and the main characteristics of collagen type I. The XRD results showed that the hard palate bone has a mixture of calcium, octacalcium dihydrogen phosphate (OCP), and apatite, where OCP is the predominant phase. Besides, this research demonstrated that the young bone has low crystallinity and small crystal size compared with commercial hydroxyapatite (HA). The palatine process of maxilla density and porosity data reported, suggest that the palate bone is getting closer to the compact bone with a 52.78 ± 2.91% porosity and their mechanical properties depend on the preparation conditions and the area of the bone analyzed.

Bone hydration: How we can evaluate it, what can it tell us, and is it an effective therapeutic target?

Water constitutes roughly a quarter of the cortical bone by volume yet can greatly influence mechanical properties and tissue quality. There is a growing appreciation for how water can dynamically change due to age, disease, and treatment. A key emerging area related to bone mechanical and tissue properties lies in differentiating the role of water in its four different compartments, including free/pore water, water loosely bound at the collagen/mineral interfaces, water tightly bound within collagen triple helices, and structural water within the mineral. This review summarizes our current knowledge of bone water across the four functional compartments and discusses how alterations in each compartment relate to mechanical changes. It provides an overview on the advent of- and improvements to- imaging and spectroscopic techniques able to probe nano-and molecular scales of bone water. These technical advances have led to an emerging understanding of how bone water changes in various conditions, of which aging, chronic kidney disease, diabetes, osteoporosis, and osteogenesis imperfecta are reviewed. Finally, it summarizes work focused on therapeutically targeting water to improve mechanical properties.

Bone hierarchical structure: spatial variation across length scales

Bone is a complex hierarchical biomineralized material, which is special amongst biominerals because it is replete with cells, namely, osteocytes. While bone has been scrutinized for centuries, many questions remain open and new research hints that the ultrastructure of bone, encompassing both the bone matrix itself and the embedded cell network, is much more heterogeneous than hitherto realized. A number of these new findings have been made thanks to the enormous developments in X-ray imaging that have occurred in recent decades, and there is promise that they will also allow many of the remaining open questions to be addressed. X-ray absorption or phase imaging affords high three-dimensional (3D) resolution and allows traversing the length scales of bone all the way down to the fine details of the lacuno-canalicular network housing the osteocytes. Multimodal X-ray imaging provides combined information covering both the length scales defined by the size of the measured volume and tomographic resolution, as well as those probed by the signal that is measured. In X-ray diffraction computed tomography (XRD-CT), for example, diffraction signals can be reconstructed tomographically, which offers detailed information about the spatial variations in the crystallographic properties of the bone biomineral. Orientational information can be obtained by tensor tomography. The combination of both small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) tensor tomography gives information on the orientation of bone nanostructure and crystals, respectively. These new technical developments promise that great strides towards understanding bone structure can be expected in the near future. In this review, recent findings that have resulted from X-ray imaging are highlighted and speculation is given on what can be expected to follow.

Application of 3D-Printed, PLGA-Based Scaffolds in Bone Tissue Engineering

Polylactic acid–glycolic acid (PLGA) has been widely used in bone tissue engineering due to its favorable biocompatibility and adjustable biodegradation. 3D printing technology can prepare scaffolds with rich structure and function, and is one of the best methods to obtain scaffolds for bone tissue repair. This review systematically summarizes the research progress of 3D-printed, PLGA-based scaffolds. The properties of the modified components of scaffolds are introduced in detail. The influence of structure and printing method change in printing process is analyzed. The advantages and disadvantages of their applications are illustrated by several examples. Finally, we briefly discuss the limitations and future development direction of current 3D-printed, PLGA-based materials for bone tissue repair.

Cryo-Electron Microscopy: the field of 1,000+ methods

Cryo-Electron Microscopy (CryoEM) is currently a well-established method to elucidate a biological macromolecule's three-dimensional (3D) structure. Its success is due to technological and methodological advances in several fronts: sample preparation, electron optics and detection, image acquisition, image processing, and map interpretation. The first methods started in the late 1960s and, since then, new methods on all fronts have continuously been published, maturating the field as we know it now. In terms of publications, we can distinguish several periods, witnessing a substantial acceleration of methodological publications in recent years, pointing out to an increased interest in the domain. On the other hand, this accelerated increase of methods development may confuse practitioners about which method they should be using (and how) and highlight the importance of paying attention to establishing best practices for methods reporting and usage. In this paper, we analyze the trends identified in over 1,000 methodological papers. Our focus is primarily on computational image processing methods. However, our list also covers some aspects of sample preparation and image acquisition. Several interesting ideas stem out from this study: 1) Single Particle Analysis (SPA) has largely accelerated in the last decade and sample preparation methods in the last five years; 2) Electron Tomography is not yet in a rapidly growing phase, but it is foreseeable that it will soon be; 3) the work horses of SPA are 3D classification, 3D reconstruction, and 3D alignment, and there have been many papers on these topics, which are not considered to be solved yet, but ever improving; and 4) since the resolution revolution, atomic modelling has also caught on as a hot topic.