Fractal-like hierarchical organization of bone begins at the nanoscale

Harnessing multifunctional collagen mimetic peptides to create bioinspired stimuli responsive hydrogels for controlled cell culture

The demand for synthetic soft materials with bioinspired structures continues to grow. Material applications range from in vitro and in vivo tissue mimics to therapeutic delivery systems, where well-defined synthetic building blocks offer precise and reproducible property control. This work examines a synthetic assembling peptide, specifically a multifunctional collagen mimetic peptide (mfCMP) either alone or with reactive macromers, for the creation of responsive hydrogels that capture aspects of soft collagen-rich tissues. We first explored how buffer choice impacts mfCMP hierarchical assembly, in particular, peptide melting temperature, fibril morphology, and ability to form physical hydrogels. Assembly in physiologically relevant buffer resulted in collagen-like fibrillar structures and physically assembled hydrogels with shear-thinning (as indicated through strain-yielding) and self-healing properties. Further, we aimed to create fully synthetic, composite peptide-polymer hydrogels with dynamic responses to various stimuli, inspired by the extracellular matrix (ECM). Specifically, we established mfCMP-poly(ethylene glycol) (PEG) hydrogel compositions that demonstrate increasing non-linear viscoelasticity in response to applied strain as the amount of assembled mfCMP content increases. Furthermore, the thermal responsiveness of mfCMP physical crosslinks was harnessed to manipulate the composite hydrogel mechanical properties in response to changes in temperature. Finally, cells relevant in wound healing, human lung fibroblasts, were encapsulated within these peptide-polymer hydrogels to explore the impact of increased mfCMP, and the resulting changes in viscoelasticity, on cell response. This work establishes mfCMP building blocks as versatile tools for creating hybrid and adaptable systems with applications ranging from injectable shear-thinning materials to responsive interfaces and synthetic ECMs for tissue engineering.

Neuro-evolutionary evidence for a universal fractal primate brain shape

The cerebral cortex displays a bewildering diversity of shapes and sizes across and within species. Despite this diversity, we present a universal multi-scale description of primate cortices. We show that all cortical shapes can be described as a set of nested folds of different sizes. As neighbouring folds are gradually merged, the cortices of 11 primate species follow a common scale-free morphometric trajectory, that also overlaps with over 70 other mammalian species. Our results indicate that all cerebral cortices are approximations of the same archetypal fractal shape with a fractal dimension of df = 2.5. Importantly, this new understanding enables a more precise quantification of brain morphology as a function of scale. To demonstrate the importance of this new understanding, we show a scale-dependent effect of ageing on brain morphology. We observe a more than fourfold increase in effect size (from two standard deviations to eight standard deviations) at a spatial scale of approximately 2 mm compared to standard morphological analyses. Our new understanding may, therefore, generate superior biomarkers for a range of conditions in the future.

Characterization of human trabecular bone across multiple length scales using a correlative approach combining X-ray tomography with LaserFIB and plasma FIB-SEM

Three-dimensional correlative multimodal and multiscale imaging is an emerging method for investigating the complex hierarchical structure of biological materials such as bone. This approach synthesizes images acquired across multiple length scales, for the same region of interest, to provide a comprehensive view of the material structure of a sample. Here, we develop a workflow for the structural analysis of human trabecular bone using a femtosecond laser to produce a precise grid to facilitate correlation between imaging modalities and identification of structures of interest, in this case, a single trabecula within a volume of trabecular bone. Through such image registration, high resolution X-ray microscopy imaging revealed fine architectural details, including the cement sheath and bone cell lacunae of the selected bone trabecula. The selected bone volume was exposed with a combination of manual polishing and site-specific femtosecond laser ablation and then examined with plasma focused ion beam-scanning electron microscopy. This reliable and versatile correlation approach has the potential to be applied to a variety of biological tissues and traditional engineered materials. The proposed workflow has the enhanced capability for generating highly resolved and broadly contextualized structural data for a better understanding of the architectural features of a material spanning its macroscopic to nanoscopic levels.

Seriation of Enzyme-Functionalized Multilayers for the Design of Scalable Biomimetic Mineralized Structures

Biomimetic hydroxyapatites are widely explored for their potential applications in the repair of mineralized tissues, particularly dental enamel, which is acellular and, thus, not naturally reformed after damage. Enamel is formed with a highly-controlled hierarchical structure, which is difficult to replicate up to the macroscale. A biomimetic approach is thus warranted, based on the same principles that drive biomineralization in vivo. Herein, a strategy for the design of enamel-like architectures is described, utilizing enzymes embedded in polyelectrolyte multilayers to generate inorganic phosphate locally, and provide a favorable chemical environment for the nucleation and growth of minerals. Moreover, a method is proposed to build up seriated mineral layers with scalable thicknesses, continuous mineral growth, and tunable morphology. Results show the outstanding growth of cohesive mineral layers, yielding macroscopic standalone fluoride and/or carbonate-substituted hydroxyapatite materials with comparable crystal structure and composition to native human mineralized tissues. This strategy presents a promising path forward for the biomimetic design of biomineral materials, particularly relevant for restorative applications, with an exquisite level of synthetic control over multiple orders of magnitude.

Multimodal analysis and comparison of stoichiometric and structural characteristics of parosteal and conventional osteosarcoma with massive sclerosis in human bone

Osteosarcoma (OS) is the most common malignant primary bone tumor in humans and occurs in various subtypes. Tumor formation happens through malignant osteoblasts producing immature bone. In the present paper we studied two different subtypes of osteosarcoma, from one individual with conventional OS with massive sclerosis and one individual with parosteal OS, based on a multimodal approach including small angle x-ray scattering (SAXS), wide angle x-ray diffraction (WAXS), backscattered electron imaging (BEI) and Raman spectroscopy. It was found that both tumors showed reduced mineral particle sizes and degree of orientation of the collagen-mineral composite in the affected areas, alongside with a decreased crystallinity. Distinct differences between the tumor material from the two individuals were found in the degree of mineralization. Further differences were observed in the carbonate to phosphate ratio, which is related to the degree of carbonate substitution in bone mineral and indicative of the turnover rate. The contraction of the c-axis of the bone mineral crystals proved to be a further, very sensitive parameter, potentially indicative of malignancy.

Crossing length scales: X-ray approaches to studying the structure of biological materials

Biological materials have outstanding properties. With ease, challenging mechanical, optical or electrical properties are realised from comparatively `humble' building blocks. The key strategy to realise these properties is through extensive hierarchical structuring of the material from the millimetre to the nanometre scale in 3D. Though hierarchical structuring in biological materials has long been recognized, the 3D characterization of such structures remains a challenge. To understand the behaviour of materials, multimodal and multi-scale characterization approaches are needed. In this review, we outline current X-ray analysis approaches using the structures of bone and shells as examples. We show how recent advances have aided our understanding of hierarchical structures and their functions, and how these could be exploited for future research directions. We also discuss current roadblocks including radiation damage, data quantity and sample preparation, as well as strategies to address them.

Abalone shells bioenhanced carboxymethyl chitosan/collagen/PLGA bionic hybrid scaffolds achieving biomineralization and osteogenesis for bone regeneration

Inspired by the formation of natural abalone shells (AS) similar to calcium salt deposition in human orthodontics, AS is used as an emulsifier in the scaffold to solve the problem of coexistence of natural and synthetic polymers and promote new bone formation. In this study, AS-stabilized and reinforced carboxymethyl chitosan/collagen/PLGA porous bionic composite scaffolds (AS/CMCS/Col/PLGA) were fabricated through the emulsion polymerization and bionic hybrid technology. As the addition of AS increased from 0.75 to 3.0 wt%, homogeneous distribution of flower-like particles could be observed on the inner surface of the scaffold, and its mechanical properties were improved. Particularly, 3.0 wt% AS-doped scaffolds (S3 and C + S3) exhibited excellent inorganic mineral deposition and osteoblast proliferation and differentiation abilities in vitro. In a SD rat calvarial defect model, they effectively promoted new bone formation in the defect and accelerated expression of osteogenic-angiogenic related proteins (COLI, OCN, VEGF). By virtue of its combined merits including good mechanical properties, inducing mineralization crystallization and facilitating osteogenesis, the 3.0 wt% AS-doped scaffold promises to be employed as a novel bone repair material for bone tissue regeneration.

Calcium Phosphates: A Key to Next-Generation In Vitro Bone Modeling

The replication of bone physiology under laboratory conditions is a prime target behind the development of in vitro bone models. The model should be robust enough to elicit an unbiased response when stimulated experimentally, giving reproducible outcomes. In vitro bone tissue generation majorly requires the availability of cellular components, the presence of factors promoting cellular proliferation and differentiation, efficient nutrient supply, and a supporting matrix for the cells to anchor - gaining predefined topology. Calcium phosphates (CaP) are difficult to ignore while considering the above requirements of a bone model. Therefore, the current review focuses on the role of CaP in developing an in vitro bone model addressing the prerequisites of bone tissue generation. Special emphasis is given to the physico-chemical properties of CaP that promote osteogenesis, angiogenesis and provide sufficient mechanical strength for load-bearing applications. Finally, the future course of action is discussed to ensure efficient utilization of CaP in the in vitro bone model development field.

Selective Chiral Interactions between Hydrophilic/Hydrophobic Amino Acids and Growing Gypsum Crystals

In nature, selective interactions between chiral amino acids and crystals are important for the formation of chiral biominerals and provide insight into the mysterious origin of homochirality. Here, we show that chiral amino acids with different hydrophilicities/hydrophobicities exhibit different chiral selectivity preferences in the dynamically growing gypsum [001] steps. Hydrophilic amino acids show a chiral selectivity preference for their d-isomers, whereas hydrophobic amino acids prefer their l-isomers. These differences in chiral recognition can be attributed to the different stereochemical matching between the hydrophilic and hydrophobic amino acids on the [001] steps of growing gypsum. These different chiral selectivities resulting from the amino acid hydrophilicity/hydrophobicity are confirmed by the experimental crystallization investigations from nano regulation on dynamic steps, to microscopic modification of gypsum morphology, and to macroscopic precipitation. Furthermore, as the hydrophilicity of amino acids increases, the disparity in chiral selection rises; conversely, the increase in the hydrophobicity of amino acids results in a decline in chiral selection. These insights improve our understanding of the interaction mechanism between amino acids and crystals and provide insights into the formation process of chiral biominerals and the origin of homochirality in nature.

The role of collagen and crystallinity in the physicochemical properties of naturally derived bone grafts

Xenografts are commonly used for bone regeneration in dental and orthopaedic domains to repair bone voids and other defects. The first-generation xenografts were made through sintering, which deproteinizes them and alters their crystallinity, while later xenografts are produced using cold-temperature chemical treatments to maintain the structural collagen phase. However, the impact of collagen and the crystalline phase on physicochemical properties have not been elucidated. We hypothesized that understanding these factors could explain why the latter provides improved bone regeneration clinically. In this study, we compared two types of xenografts, one prepared through a low-temperature chemical process (Treated) and another subsequently sintered at 1100°C (Sintered) using advanced microscopy, spectroscopy, X-ray analysis and compressive testing. Our investigation showed that the Treated bone graft was free of residual blood, lipids or cell debris, mitigating the risk of pathogen transmission. Meanwhile, the sintering process removed collagen and the carbonate phase of the Sintered graft, leaving only calcium phosphate and increased mineral crystallinity. Microcomputed tomography revealed that the Treated graft exhibited an increased high porosity (81%) and pore size compared to untreated bone, whereas the Sintered graft exhibited shrinkage, which reduced the porosity (72%), pore size and strut size. Additionally, scanning electron microscopy displayed crack formation around the pores of the Sintered graft. The Treated graft displayed median mechanical properties comparable to native cancellous bone and clinically available solutions, with an apparent modulus of 166 MPa, yield stress of 5.5 MPa and yield strain of 4.9%. In contrast, the Sintered graft exhibited a lower median apparent modulus of 57 MPa. It failed in a brittle manner at a median stress of 1.7 MPa and strain level of 2.9%, demonstrating the structural importance of the collagen phase. This indicates why bone grafts prepared through cold-temperature processes are clinically favourable.

Heterogeneous Structure and Dynamics of Water in a Hydrated Collagen Microfibril

Collagen type I is well-known for its outstanding mechanical properties which it inherits from its hierarchical structure. Collagen type I fibrils may be viewed as a heterogeneous material made of protein, macromolecules (such as glycosaminoglycans and proteoglycans) and water. Water content modulates the properties of these fibrils. Yet, the properties of water and the fine interactions of water with the protein constituent of these heterofibrils have only received limited attention. Here, we propose to model collagen type I fibrils as a hydrated structure made of tropocollagen molecules assembled in a microfibril crystal. We perform large-scale all-atom molecular dynamics simulations of the hydration of collagen fibrils beyond the onset of disassembly. We found that the structural and dynamic properties of water vary strongly with the level of hydration of the microfibril. More importantly, we found that the properties vary spatially within the 67 nm D-spacing periodic structure. Alteration of the structural and dynamical properties of the collagen microfibril occur first in the gap region. Overall, we identify that the change in the role of water molecules from glue to lubricant between tropocollagen molecules arises around 100% hydration while the microfibril begins to disassemble beyond 130% water content. Our findings are supported by a decrease in hydrogen bonding, recovery of bulk water properties and amorphization of the tropocollagen molecules packing. Our simulations reveal the structure and dynamics of hydrated collagen fibrils with unprecedented spatial resolution from physiological conditions to disassembly. Beyond the process of self-assembly and the emergence of mechanical properties of collagen type I fibrils, our results may also provide new insights into mineralization of collagen fibrils.

Micron-resolution fiber mapping in histology independent of sample preparation

Detailed knowledge of the brain's nerve fiber network is crucial for understanding its function in health and disease. However, mapping fibers with high resolution remains prohibitive in most histological sections because state-of-the-art techniques are incompatible with their preparation. Here, we present a micron-resolution light-scattering-based technique that reveals intricate fiber networks independent of sample preparation for extended fields of view. We uncover fiber structures in both label-free and stained, paraffin-embedded and deparaffinized, newly-prepared and archived, animal and human brain tissues - including whole-brain sections from the BigBrain atlas. We identify altered microstructures in demyelination and hippocampal neurodegeneration, and show key advantages over diffusion magnetic resonance imaging, polarization microscopy, and structure tensor analysis. We also reveal structures in non-brain tissues - including muscle, bone, and blood vessels. Our cost-effective, versatile technique enables studies of intricate fiber networks in any type of histological tissue section, offering a new dimension to neuroscientific and biomedical research.

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.

ATR-FTIR spectroscopy imaging of bone repair in mandibular laser-osteotomy

The aim of this study was to verify the effectiveness of attenuated total reflectance-fourier transform infrared (ATR-FTIR) spectroscopy in the characterization of bone repair in mandibular osteotomy using erbium, chromium-doped yttrium, scandium, gallium and garnet (Er,Cr:YSGG) laser and multilaminate drill on each side. Two mandible bone fragments were removed from 30 rabbits, and the process of bone repair was studied immediately, 3, 7, 15, 21, and 28 days after the surgery. The histological analysis allowed detecting differences in the early stages of tissue repair after bone cutting performed with the Er,Cr:YSGG laser or multilaminate drill. The ATR-FTIR spectroscopy technique was sensitive to changes in the organic content of bone tissue repair process.

Kinetically Controlled and Nonequilibrium Assembly of Block Copolymers in Solution

Covalent polymers are versatile macromolecules that have found widespread use in society. Contemporary methods of polymerization have made it possible to construct sequence polymers, including block copolymers, with high precision. Such copolymers assemble in solution when the blocks have differing solubilities. This produces nano- and microparticles of various shapes and sizes. While it is straightforward to draw an analogy between such amphiphilic block copolymers and phospholipids, these two classes of molecules show quite different assembly characteristics. In particular, block copolymers often assemble under kinetic control, thus producing nonequilibrium structures. This leads to a rich variety of behaviors being observed in block copolymer assembly, such as pathway dependence (e.g., thermal history), nonergodicity and responsiveness. The dynamics of polymer assemblies can be readily controlled using changes in environmental conditions and/or integrating functional groups situated on polymers with external chemical reactions. This perspective highlights that kinetic control is both pervasive and a useful attribute in the mechanics of block copolymer assembly. Recent examples are highlighted in order to show that toggling between static and dynamic behavior can be used to generate, manipulate and dismantle nonequilibrium states. New methods to control the kinetics of block copolymer assembly will provide endless unanticipated applications in materials science, biomimicry and medicine.

Harnessing cerium-based biomaterials for the treatment of bone diseases

Bone, an actively metabolic organ, undergoes constant remodeling throughout life. Disturbances in the bone microenvironment can be responsible for pathologically bone diseases such as periodontitis, osteoarthritis, rheumatoid arthritis and osteoporosis. Conventional bone tissue biomaterials are not adequately adapted to complex bone microenvironment. Therefore, there is an urgent clinical need to find an effective strategy to improve the status quo. In recent years, nanotechnology has caused a revolution in biomedicine. Cerium(III, IV) oxide, as an important member of metal oxide nanomaterials, has dual redox properties through reversible binding with oxygen atoms, which continuously cycle between Ce(III) and Ce(IV). Due to its special physicochemical properties, cerium(III, IV) oxide has received widespread attention as a versatile nanomaterial, especially in bone diseases. This review describes the characteristics of bone microenvironment. The enzyme-like properties and biosafety of cerium(III, IV) oxide are also emphasized. Meanwhile, we summarizes controllable synthesis of cerium(III, IV) oxide with different nanostructural morphologies. Following resolution of synthetic principles of cerium(III, IV) oxide, a variety of tailored cerium-based biomaterials have been widely developed, including bioactive glasses, scaffolds, nanomembranes, coatings, and nanocomposites. Furthermore, we highlight the latest advances in cerium-based biomaterials for inflammatory and metabolic bone diseases and bone-related tumors. Tailored cerium-based biomaterials have already demonstrated their value in disease prevention, diagnosis (imaging and biosensors) and treatment. Therefore, it is important to assist in bone disease management by clarifying tailored properties of cerium(III, IV) oxide in order to promote the use of cerium-based biomaterials in the future clinical setting. STATEMENT OF SIGNIFICANCE: In this review, we focused on the promising of cerium-based biomaterials for bone diseases. We reviewed the key role of bone microenvironment in bone diseases and the main biological activities of cerium(III, IV) oxide. By setting different synthesis conditions, cerium(III, IV) oxide nanostructures with different morphologies can be controlled. Meanwhile, tailored cerium-based biomaterials can serve as a versatile toolbox (e.g., bioactive glasses, scaffolds, nanofibrous membranes, coatings, and nanocomposites). Then, the latest research advances based on cerium-based biomaterials for the treatment of bone diseases were also highlighted. Most importantly, we analyzed the perspectives and challenges of cerium-based biomaterials. In future perspectives, this insight has given rise to a cascade of cerium-based biomaterial strategies, including disease prevention, diagnosis (imaging and biosensors) and treatment.

Effect of collagen fibril orientation on the anisotropic properties of peri-implant bone

In orthopedic and dental surgery, the implantation of biomaterials within the bone to restore the integrity of the treated organ has become a standard procedure. Their long-term stability relies on the osseointegration phenomena, where bone grows onto and around metallic implants, creating a bone-implant interface. Bone is a highly hierarchical material that evolves spatially and temporally during this healing phase. A deeper understanding of its biomechanical characteristics is needed, as they are determinants for surgical success. In this context, we propose a multiscale homogenization model to evaluate the effective elastic properties of bone as a function of the distance from the implant, based on the tissue's structure and composition at lower scales. The model considers three scales: hydroxyapatite foam (nanoscale), ultrastructure (microscale), and tissue (mesoscale). The elastic properties and the volume fraction of the elementary constituents of bone matrix (mineral, collagen, and water), the orientation of the collagen fibril relative to the implant surface, and the mesoscale porosity constitute the input data of the model. The effect of a spatiotemporal variation in the collagen fibrils' orientation on the bone anisotropic properties in the proximity of the implant was investigated. The findings revealed a strong variation of the components of the effective elasticity tensor of the bone as a function of the distance from the implant. The effective elasticity appears to be primarily sensitive to the porosity (mesoscale) rather than to the collagen fibrils' orientation (sub-micro scale). However, the orientation of the fibrils has a significant influence on the isotropy of the bone. When analyzing the symmetry properties of the effective elasticity tensor, the ratio between the isotropic and hexagonal components is determined by a combination of the porosity and the fibrils' orientation. A decrease in porosity leads to a decrease in bone isotropy and, in turn, an increase in the impact of the fibrils' orientation. These results demonstrate that the collagen fibril orientation should be taken into account to properly describe the effective elastic anisotropy of bone at the organ scale.

Nanoscale osseointegration of zirconia evaluated from the interfacial structure between ceria-stabilized tetragonal zirconia and cell-induced hydroxyapatite

BACKGROUND

The osseointegration of zirconia implants has been evaluated based on their implant fixture bonding with the alveolar bone at the optical microscopic level. Achieving nano-level bonding between zirconia and bone apatite is crucial for superior osseointegration; however, only a few studies have investigated nanoscale bonding. This review outlines zirconia osseointegration, including surface modification, and presents an evaluation of nanoscale zirconia-apatite bonding and its structure.

HIGHLIGHT

Assuming osseointegration, the cells produced calcium salts on a ceria-stabilized zirconia substrate. We analyzed the interface between calcium salts and zirconia substrates using transmission electron microscopy and found that 1) the cell-induced calcium salts were bone-like apatite and 2) direct nanoscale bonding was observed between the bone-like apatite and zirconia crystals without any special modifications of the zirconia surface.

CONCLUSION

Structural affinity exists between bone apatite and zirconia crystals. Apatite formation can be induced by the zirconia surface. Zirconia bonds directly with apatite, indicating superior osseointegration in vivo.

Hydrogel Coatings of Implants for Pathological Bone Repair

Hydrogels are well-suited for biomedical applications due to their numerous advantages, such as excellent bioactivity, versatile physical and chemical properties, and effective drug delivery capabilities. Recently, hydrogel coatings have developed to functionalize bone implants which are biologically inert and cannot withstand the complex bone tissue repair microenvironment. These coatings have shown promise in addressing unique and pressing medical needs. This review begins with the major functionalized performance and interfacial bonding strategy of hydrogel coatings, with a focus on the novel external field response properties of the hydrogel. Recent advances in the fabrication strategies of hydrogel coatings and their use in the treatment of pathologic bone regeneration are highlighted. Finally, challenges and emerging trends in the evolution and application of physiological environment-responsive and external electric field-responsive hydrogel coatings for bone implants are discussed.

Guidelines derived from biomineralized tissues for design and construction of high-performance biomimetic materials: from weak to strong

Living organisms in nature have undergone continuous evolution over billions of years, resulting in the formation of high-performance fracture-resistant biomineralized tissues such as bones and teeth to fulfill mechanical and biological functions, despite the fact that most inorganic biominerals that constitute biomineralized tissues are weak and brittle. During the long-period evolution process, nature has evolved a number of highly effective and smart strategies to design chemical compositions and structures of biomineralized tissues to enable superior properties and to adapt to surrounding environments. Most biomineralized tissues have hierarchically ordered structures consisting of very small building blocks on the nanometer scale (nanoparticles, nanofibers or nanoflakes) to reduce the inherent weaknesses and brittleness of corresponding inorganic biominerals, to prevent crack initiation and propagation, and to allow high defect tolerance. The bioinspired principles derived from biomineralized tissues are indispensable for designing and constructing high-performance biomimetic materials. In recent years, a large number of high-performance biomimetic materials have been prepared based on these bioinspired principles with a large volume of literature covering this topic. Therefore, a timely and comprehensive review on this hot topic is highly important and contributes to the future development of this rapidly evolving research field. This review article aims to be comprehensive, authoritative, and critical with wide general interest to the science community, summarizing recent advances in revealing the formation processes, composition, and structures of biomineralized tissues, providing in-depth insights into guidelines derived from biomineralized tissues for the design and construction of high-performance biomimetic materials, and discussing recent progress, current research trends, key problems, future main research directions and challenges, and future perspectives in this exciting and rapidly evolving research field.

Effect of plate orientation on apparent thickness of mineral plates by transmission electron microscopy

PURPOSE

Transmission electron microscopy (TEM) is widely used to study the ultrastructure of bone. The mineral of bone occurs as polycrystalline mineral plates about 3 to 6 nm in thickness. A problem in using TEM to make quantitative analyses of bone is that the orientation of the plates with respect to the plane of the section being imaged is expected to affect their apparent thickness. The purpose of this study was to test if this was true, if the apparent thickness of plates changed substantially as a result of tilt of the section.

METHODS

We prepared TEM sections of samples of cortical human bone by ion beam milling, orienting one section parallel to the collagen fibril axes and one perpendicular to them. We obtained TEM bright field and HAADF images of these sections, tilting the sections up to ± 20° at 2° intervals and measuring the apparent thickness of individual mineral platelets at each angle of tilt.

RESULTS

Thickness appears to double as section is tilted ± 20°. True thickness of plates is determined by tilting the section along an axis parallel to the plate orientation and determining the minimum apparent thickness. However, as plates are tilted away from minimum-thickness orientation, they become less well-resolved, disappearing when tilted more than 20°. We therefore also measured apparent thickness of only the darkest (most electron scattering) plate images in an untilted section and obtained the same average thickness as that obtained by tilting.

CONCLUSION

We conclude that tilting of the section is not necessary to obtain an accurate measurement of the thickness of mineral plates.

Assembled/Disassembled Modular Scaffolds for Multicellular Tissue Engineering

The behavior of tissue resident cells can be influenced by the spatial arrangement of cellular interactions. Therefore, it is of significance to precisely control the spatial organization of various cells within multicellular constructs. It remains challenging to construct a versatile multicellular scaffold with ordered spatial organization of multiple cell types. Herein, a modular multicellular tissue engineering scaffold with ordered spatial distribution of different cell types is constructed by assembling varying cell-laden modules. Interestingly, the modular scaffolds can be disassembled into individual modules to evaluate the specific contribution of each cell type in the system. Through assembling cell-laden modules, the macrophage-mesenchymal stem cell (MSC), endothelial cell-MSC, and chondrocyte-MSC co-culture models are successfully established. The in vitro results indicate that the intercellular cross-talk can promote the proliferation and differentiation of each cell type in the system. Moreover, MSCs in the modular scaffolds may regulate the behavior of chondrocytes through the nuclear factor of activated T-Cells (NFAT) signaling pathway. Furthermore, the modular scaffolds loaded with co-cultured chondrocyte-MSC exhibit enhanced regeneration ability of osteochondral tissue, compared with other groups. Overall, this work offers a promising strategy to construct a multicellular tissue engineering scaffold for the systematic investigation of intercellular cross-talk and complex tissue engineering.

Towards bone regeneration: Understanding the nucleating ability of proline-rich peptides in biomineralisation

Obtaining rapid mineralisation is a challenge in current bone graft materials, which has been attributed to the difficulty of guiding the biological processes towards osteogenesis. Amelogenin, a key protein in enamel formation, inspired the design of two intrinsically disordered peptides (P2 and P6) that enhance in vivo bone formation, but the process is not fully understood. In this study, we have elucidated the mechanism by which these peptides induce improved mineralisation. Our molecular dynamics analysis demonstrated that in an aqueous environment, P2 and P6 fold to interact with the surrounding Ca2+, PO43- and OH- ions, which can lead to apatite nucleation. Although P2 has a less stable backbone, it folds to a stable structure that allows for the nucleation of larger calcium phosphate aggregates than P6. These results were validated experimentally in a concentrated simulated body fluid solution, where the peptide solutions accelerated the mineralisation process compared to the control and yielded mineral structures mimicking the amorphous calcium phosphate crystals that can be found in lamella bone. A pH drop for the peptide groups suggests depletion of calcium and phosphate, a prerequisite for intrinsic osteoinduction, while S/TEM and SEM suggested that the peptide regulated the mineral nucleation into lamella flakes. Evidently, the peptides accelerate and guide mineral formation, elucidating the mechanism for how these peptides can improve the efficacy of P2 or P6 containing devices for bone regeneration. The work also demonstrates how experimental mineralisation study coupled with molecular dynamics is a valid method for understanding and predicting in vivo performance prior to animal trials.

Biomaterial design for regenerating aged bone: materiobiological advances and paradigmatic shifts

China's aging demographic poses a challenge for treating prevalent bone diseases impacting life quality. As bone regeneration capacity diminishes with age due to cellular dysfunction and inflammation, advanced biomaterials-based approaches offer hope for aged bone regeneration. This review synthesizes materiobiology principles, focusing on biomaterials that target specific biological functions to restore tissue integrity. It covers strategies for stem cell manipulation, regulation of the inflammatory microenvironment, blood vessel regeneration, intervention in bone anabolism and catabolism, and nerve regulation. The review also explores molecular and cellular mechanisms underlying aged bone regeneration and proposes a database-driven design process for future biomaterial development. These insights may also guide therapies for other age-related conditions, contributing to the pursuit of 'healthy aging'.

Inorganic Pyrophosphate at Serum Concentration May Not Be Able to Inhibit Mineralization: A Study in Aqueous Solutions and Serum

The constituent ions of calcium phosphate in body fluids are in the supersaturated state and tend to form minerals physiologically or pathologically. Inorganic pyrophosphate (PPi) has been considered as one of the most important inhibitors against the formation of calcium phosphate minerals. However, serum PPi concentrations in humans are maintained at a level of several μmol/L, and its effectiveness and mechanism for mineralization inhibition remain ambiguous. Therefore, this work studied the mineralization process in an aqueous solution, explored the effective inhibitory concentration of PPi by titration, and characterized the species during the reactions. We find that PPi at a normal serum concentration does not inhibit mineralization significantly. Such a conclusion was further confirmed in the PPi-added serum. This work indicates that PPi may not be a major direct inhibitor of mineralization in serum and possibly functions via alternative mechanisms.

Nanocrystal residual strains and density layers enhance failure resistance in the cleithrum bone of evolutionary advanced pike fish

Failure-resistant designs are particularly crucial for bones subjected to rapid loading, as is the case for the ambush-hunting northern pike (Esox lucius). These fish have slim and low-density osteocyte-lacking bones. As part of the swallowing mechanism, the cleithrum bone opens and closes the jaw. The cleithrum needs sufficient strength and damage tolerance, to withstand years of repetitive rapid gape-and-suck cycles of feeding. The thin wing-shaped bone comprises anisotropic layers of mineralized collagen fibers that exhibit periodic variations in mineral density on the mm and micrometer length scales. Wavy collagen fibrils interconnect these layers yielding a highly anisotropic structure. Hydrated cleithra exhibit Young's moduli spanning 3-9 GPa where the yield stress of ∼40 MPa increases markedly to exceed ∼180 MPa upon drying. This 5x observation of increased strength corresponds to a change to brittle fracture patterns. It matches the emergence of compressive residual strains of ∼0.15% within the mineral crystals due to forces from shrinking collagen layers. Compressive stresses on the nanoscale, combined with the layered anisotropic microstructure on the mm length scale, jointly confer structural stability in the slender and lightweight bones. By employing a range of X-ray, electron and optical imaging and mechanical characterization techniques, we reveal the structure and properties that make the cleithra impressively damage resistant composites. STATEMENT OF SIGNIFICANCE: By combining structural and mechanical characterization techniques spanning the mm to the sub-nanometer length scales, this work provides insights into the structural organization and properties of a resilient bone found in pike fish. Our observations show how the anosteocytic bone within the pectoral gridle of these fish, lacking any biological (remodeling) repair mechanisms, is adapted to sustain natural repeated loading cycles of abrupt jaw-gaping and swallowing. We find residual strains within the mineral apatite nanocrystals that contribute to forming a remarkably resilient composite material. Such information gleaned from bony structures that are different from the usual bones of mammals showcases how nature incorporates smart features that induce damage tolerance in bone material, an adaptation acquired through natural evolutionary processes.

Gradient matters via filament diameter-adjustable 3D printing

Gradient matters with hierarchical structures endow the natural world with excellent integrity and diversity. Currently, direct ink writing 3D printing is attracting tremendous interest, and has been used to explore the fabrication of 1D and 2D hierarchical structures by adjusting the diameter, spacing, and angle between filaments. However, it is difficult to generate complex 3D gradient matters owing to the inherent limitations of existing methods in terms of available gradient dimension, gradient resolution, and shape fidelity. Here, we report a filament diameter-adjustable 3D printing strategy that enables conventional extrusion 3D printers to produce 1D, 2D, and 3D gradient matters with tunable heterogeneous structures by continuously varying the volume of deposited ink on the printing trajectory. In detail, we develop diameter-programmable filaments by customizing the printing velocity and height. To achieve high shape fidelity, we specially add supporting layers at needed locations. Finally, we showcase multi-disciplinary applications of our strategy in creating horizontal, radial, and axial gradient structures, letter-embedded structures, metastructures, tissue-mimicking scaffolds, flexible electronics, and time-driven devices. By showing the potential of this strategy, we anticipate that it could be easily extended to a variety of filament-based additive manufacturing technologies and facilitate the development of functionally graded structures.

Twisted-plywood-like tissue formation in vitro. Does curvature do the twist?

Little is known about the contribution of 3D surface geometry to the development of multilayered tissues containing fibrous extracellular matrix components, such as those found in bone. In this study, we elucidate the role of curvature in the formation of chiral, twisted-plywood-like structures. Tissues consisting of murine preosteoblast cells (MC3T3-E1) were grown on 3D scaffolds with constant-mean curvature and negative Gaussian curvature for up to 32 days. Using 3D fluorescence microscopy, the influence of surface curvature on actin stress-fiber alignment and chirality was investigated. To gain mechanistic insights, we did experiments with MC3T3-E1 cells deficient in nuclear A-type lamins or treated with drugs targeting cytoskeleton proteins. We find that wild-type cells form a thick tissue with fibers predominantly aligned along directions of negative curvature, but exhibiting a twist in orientation with respect to older tissues. Fiber orientation is conserved below the tissue surface, thus creating a twisted-plywood-like material. We further show that this alignment pattern strongly depends on the structural components of the cells (A-type lamins, actin, and myosin), showing a role of mechanosensing on tissue organization. Our data indicate the importance of substrate curvature in the formation of 3D tissues and provide insights into the emergence of chirality.

Biomimetic fabrication of sr-silk fibroin co-assembly hydroxyapatite based microspheres with angiogenic and osteogenic properties for bone tissue engineering

Bone defects caused by trauma, tumor resection, or developmental abnormalities are important issues in clinical practice. The vigorous development of tissue engineering technology provides new ideas and directions for regenerating bone defects. Hydroxyapatite (HAp), a bioactive ceramic, is extensively used in bone tissue engineering because of its excellent osteoinductive performance. However, its application is challenged by its single function and conventional environment-unfriendly synthesis methods. In this study, we successfully "green" synthesized sr-silk fibroin co-assembly hydroxyapatite nanoparticles (Sr-SF-HA) using silk fibroin (SF) as a biomineralized template, thus enabling it to have angiogenic activity and achieving the combination of organic and inorganic substances. Then, the rough composite microspheres loaded with Sr-SF-HA (CS/Sr-SF-HA) through electrostatic spraying technology and freeze-drying method were prepared. The CCK-8 test and live/dead cell staining showed excellent biocompatibility of CS/Sr-SF-HA. Alkaline phosphatase (ALP) staining, alizarin red staining (ARS), immunofluorescence, western blotting, and qRT-PCR test showed that CS/Sr-SF-HA activated the expression of related genes and proteins, thus inducing the osteogenic differentiation of rBMSCs. Moreover, tube formation experiments, scratch experiments, immunofluorescence, and qRT-PCR detection indicated that CS/Sr-SF-HA have good angiogenic activity. Furthermore, in vivo studies showed that the CS/Sr-SF-HA possesses excellent biocompatibility, vascular activity, as well as ectopic osteogenic ability in the subcutaneous pocket of rats. This study indicates that the construction of CS/Sr-SF-HA with angiogenic and osteogenic properties has great potential for bone tissue engineering.

Bioinspired Macroporous Materials of MXene Nanosheets: Ice-Templated Assembly and Multifunctional Applications

Biological macroporous materials, such as stems of the plants and bone of the animals, possess outstanding properties for powerful guarantee of creatures' survival through the well-aligned architecture constructed from limited components. Transition metal carbides or nitrides (MXenes), as novel 2D assemblies, have attracted numerous attentions in various applications due to their unique properties. Therefore, mimicking the bioinspired architecture with MXenes will boost the development of human-made materials with unparalleled properties. Freeze casting has been widely applied to fabricate bioinspired MXene-based materials and achieve the assembly of MXene nanosheets into 3D forms. This process solves the inherent restacking problems of MXenes, simultaneously preserving the unique properties of MXenes with a physical process. Here, the ice-templated assembly of MXene in terms of the freezing processes and their potential mechanisms is summarized. In addition, applications of MXene-based materials in electromagnetic interference shielding and absorption, energy storage and conversion, as well as piezoresistive pressure sensors are also reviewed. Finally, the current challenges and bottlenecks of ice-templated assembly of MXene are further discussed to guide the development of bioinspired MXene-based materials.

Tolerant and Rapid Endochondral Bone Regeneration Using Framework-Enhanced 3D Biomineralized Matrix Hydrogels

Tissue-engineered bone has emerged as a promising alternative for bone defect repair due to the advantages of regenerative bone healing and physiological functional reconstruction. However, there is very limited breakthrough in achieving favorable bone regeneration due to the harsh osteogenic microenvironment after bone injury, especially the avascular and hypoxic conditions. Inspired by the bone developmental mode of endochondral ossification, a novel strategy is proposed for tolerant and rapid endochondral bone regeneration using framework-enhanced 3D biomineralized matrix hydrogels. First, it is meticulously designed 3D biomimetic hydrogels with both hypoxic and osteoinductive microenvironment, and then integrated 3D-printed polycaprolactone framework to improve their mechanical strength and structural fidelity. The inherent hypoxic 3D matrix microenvironment effectively activates bone marrow mesenchymal stem cells self-regulation for early-stage chondrogenesis via TGFβ/Smad signaling pathway due to the obstacle of aerobic respiration. Meanwhile, the strong biomineralized microenvironment, created by a hybrid formulation of native-constitute osteogenic inorganic salts, can synergistically regulate both bone mineralization and osteoclastic differentiation, and thus accelerate the late-stage bone maturation. Furthermore, both in vivo ectopic osteogenesis and in situ skull defect repair successfully verified the high efficiency and mechanical maintenance of endochondral bone regeneration mode, which offers a promising treatment for craniofacial bone defect repair.

Acidic Osteoid Templates the Plywood Structure of Bone Tissue

Bone is created by osteoblasts that secrete osteoid after which an ordered texture emerges, followed by mineralization. Plywood geometries are a hallmark of many trabecular and cortical bones, yet the origin of this texturing in vivo has never been shown. Nevertheless, extensive in vitro work revealed how plywood textures of fibrils can emerge from acidic molecular cholesteric collagen mesophases. This study demonstrates in sheep, which is the preferred model for skeletal orthopaedic research, that the deeper non-fibrillar osteoid is organized in a liquid-crystal cholesteric geometry. This basophilic domain, rich in acidic glycosaminoglycans, exhibits low pH which presumably fosters mesoscale collagen molecule ordering in vivo. The results suggest that the collagen fibril motif of twisted plywood matures slowly through self-assembly thermodynamically driven processes as proposed by the Bouligand theory of biological analogues of liquid crystals. Understanding the steps of collagen patterning in osteoid-maturation processes may shed new light on bone pathologies that emerge from collagen physico-chemical maturation imbalances.

Type 1 collagen: Synthesis, structure and key functions in bone mineralization

Collagen is a highly abundant protein in the extracellular matrix of humans and mammals, and it plays a critical role in maintaining the body's structural integrity. Type I collagen is the most prevalent collagen type and is essential for the structural integrity of various tissues. It is present in nearly all connective tissues and is the main constituent of the interstitial matrix. Mutations that affect collagen fiber formation, structure, and function can result in various bone pathologies, underscoring the significance of collagen in sustaining healthy bone tissue. Studies on type 1 collagen have revealed that mutations in its encoding gene can lead to diverse bone diseases, such as osteogenesis imperfecta, a disorder characterized by fragile bones that are susceptible to fractures. Knowledge of collagen's molecular structure, synthesis, assembly, and breakdown is vital for comprehending embryonic and foetal development and several aspects of human physiology. In this review, we summarize the structure, molecular biology of type 1 collagen, its biomineralization and pathologies affecting bone.

Advanced Synthetic Scaffolds Based on 1D Inorganic Micro-/Nanomaterials for Bone Regeneration

Inorganic nanoparticulate biomaterials, such as calcium phosphate and bioglass particles, with chemical compositions similar to that of the inorganic component of natural bone, and hence having excellent biocompatibility and bioactivity, are widely used for the fabrication of synthetic bone graft substitutes. Growing evidence suggests that structurally anisotropic, or 1D inorganic micro-/nanobiomaterials are superior to inorganic nanoparticulate biomaterials in the context of mechanical reinforcement and construction of self-supporting 3D network structures. Therefore, in the past decades, efforts have been devoted to developing advanced synthetic scaffolds for bone regeneration using 1D micro-/nanobiomaterials as building blocks. These scaffolds feature extraordinary physical and biological properties, such as enhanced mechanical properties, super elasticity, multiscale hierarchical architecture, extracellular matrix-like fibrous microstructure, and desirable biocompatibility and bioactivity, etc. In this review, an overview of recent progress in the development of advanced scaffolds for bone regeneration is provided based on 1D inorganic micro-/nanobiomaterials with a focus on their structural design, mechanical properties, and bioactivity. The promising perspectives for future research directions are also highlighted.

COLLAGEN MINERALIZATION DECREASES NK CELL-MEDIATED CYTOTOXICITY OF BREAST CANCER CELLS VIA INCREASED GLYCOCALYX THICKNESS

Skeletal metastasis is common in patients with advanced breast cancer, and often caused by immune evasion of disseminated tumor cells (DTCs). In the skeleton, tumor cells not only disseminate to the bone marrow, but also to osteogenic niches in which they interact with newly mineralizing bone extracellular matrix (ECM). However, it remains unclear how mineralization of collagen type I, the primary component of bone ECM, regulates tumor-immune cell interactions. Here, we have utilized a combination of synthetic bone matrix models with controlled mineral content, nanoscale optical imaging, and flow cytometry to evaluate how collagen type I mineralization affects the biochemical and biophysical properties of the tumor cell glycocalyx, a dense layer of glycosylated proteins and lipids decorating their cell surface. Our results suggest that collagen mineralization upregulates mucin-type O-glycosylation and sialylation by tumor cells, which increased their glycocalyx thickness while enhancing resistance to attack by Natural Killer (NK) cells. These changes were functionally linked as treatment with a sialylation inhibitor decreased mineralization-dependent glycocalyx thickness and made tumor cells more susceptible to NK cell attack. Together, our results suggest that interference with glycocalyx sialylation may represent a therapeutic strategy to enhance cancer immunotherapies targeting bone-metastatic breast cancer.

A fresh perspective on infrared spectroscopy as a prescreening method for molecular and stable isotopes analyses on ancient human bones

Following the development of modern genome sequencing technologies, the investigation of museum osteological finds is increasingly informative and popular. Viable protocols to help preserve these collections from exceedingly invasive analyses, would allow greater access to the specimens for scientific research. The main aim of this work is to survey skeletal tissues, specifically petrous bones and roots of teeth, using infrared spectroscopy as a prescreening method to assess the bone quality for molecular analyses. This approach could overcome the major problem of identifying useful genetic material in archaeological bone collections without resorting to demanding, time consuming and expensive laboratory studies. A minimally invasive sampling of archaeological bones was developed and bone structural and compositional changes were examined, linking isotopic and genetic data to infrared spectra. The predictive model based on Infrared parameters is effective in determining the occurrence of ancient DNA (aDNA); however, the quality/quantity of aDNA cannot be determined because of the influence of environmental and local factors experienced by the examined bones during the burial period.

Advances in 3D Printing of Highly Bioadaptive Bone Tissue Engineering Scaffolds

The number of patients with bone defects caused by trauma, bone tumors, and osteoporosis has increased considerably. The repair of irregular, recurring, and large bone defects poses a great challenge to clinicians. Bone tissue engineering is emerging as an appropriate strategy to replace autologous bone grafting in the repair of critically sized bone defects. However, the suitability of bone tissue engineering scaffolds in terms of structure, mechanics, degradation, and the microenvironment is inadequate. Three-dimensional (3D) printing is an advanced additive-manufacturing technology widely used for bone repair. 3D printing constructs personalized structurally adapted scaffolds based on 3D models reconstructed from CT images. The contradiction between the mechanics and degradation is resolved by altering the stacking structure. The local microenvironment of the implant is improved by designing an internal pore structure and a spatiotemporal factor release system. Therefore, there has been a boom in the 3D printing of personalized bone repair scaffolds. In this review, successful research on the preparation of highly bioadaptive bone tissue engineering scaffolds using 3D printing is presented. The mechanisms of structural, mechanical, degradation, and microenvironmental adaptations of bone prostheses and their interactions were elucidated to provide a feasible strategy for constructing highly bioadaptive bone tissue engineering scaffolds.

An osteoporosis bone defect regeneration strategy via three-dimension short fibers loaded with alendronate modified hydroxyapatite

Osteoporotic bone defect has become clinic challenge due to its morbid bone microenvironment. Overactive bone resorption and limited bone formation lead to unstable combination between bone tissue and scaffolds. Electrospinning has been widely used in guide tissue membrane, but its barrier property results in limited application. In order to optimize the structure and add anti-bone resorption function of electrospinning fibers, we exploited the application of short fibers generated by homogenization at osteoporotic tibial bone defect. The modified nano-hydroxyapatite (m-HA) was loaded with alendronate. It overcame the problem that hydrophilic drugs were difficult to distribute uniformly in hydrophobic fibers. We confirmed that m-HA was loaded into polycaprolactone (PCL) short fibers. PCL short fibers with m-HA (PCL/m-HA) continuously released ALN, provided stable structure and showed good cytocompatibility. In vitro, PCL/m-HA increased the activity of alkaline phosphatase (ALP), promoted extracellular matrix mineralization and upregulated the expression of osteogenesis-related genes, Col 1, Alp, osteopontin (Opn) and runt-related transcription factor 2 (Runx2). In vivo, PCL/m-HA short fibers accelerated the new bone formation, inhibited the bone resorption and rebalanced the bone microenvironment through regulating osteoprotegerin (OPG) /receptor activator of NF-kB (RANKL) ratio. The above results confirmed that the PCL/m-HA short fibers achieved the application of three-dimension osteoporotic bone defect and had potential prospects in bone tissue scaffolds.

Osteoclasts at Bone Remodeling: Order from Order

Osteoclasts are multinucleated bone-resorbing cells derived from the monocyte/macrophage lineage. The macrophage colony-stimulating factor/receptor activator of nuclear factor κB ligand (M-CSF/RANKL) signaling network governs the differentiation of precursor cells into fusion-competent mononucleated cells. Repetitive fusion of fusion-competent cells produces multinucleated osteoclasts. Osteoclasts are believed to die via apoptosis after bone resorption. However, recent studies have found that osteoclastogenesis in vivo proceeds by replacing the old nucleus of existing osteoclasts with a single newly differentiated mononucleated cell. Thus, the formation of new osteoclasts is minimal. Furthermore, the sizes of osteoclasts can change via cell fusion and fission in response to external conditions. On the other hand, osteoclastogenesis in vitro involves various levels of heterogeneity, including osteoclast precursors, mode of fusion, and properties of the differentiated osteoclasts. To better understand the origin of these heterogeneities and the plasticity of osteoclasts, we examine several processes of osteoclastogenesis in this review. Candidate mechanisms that create heterogeneity involve asymmetric cell division, osteoclast niche, self-organization, and mode of fusion and fission. Elucidation of the plasticity or fluctuation of the M-CSF/RANKL network should be an important topic for future researches.

Journey of Mineral Precursors in Bone Mineralization: Evolution and Inspiration for Biomimetic Design

Bone mineralization is a ubiquitous process among vertebrates that involves a dynamic physical/chemical interplay between the organic and inorganic components of bone tissues. It is now well documented that carbonated apatite, an inorganic component of bone, is proceeded through transient amorphous mineral precursors that transforms into the crystalline mineral phase. Here, the evolution on mineral precursors from their sources to the terminus in the bone mineralization process is reviewed. How organisms tightly control each step of mineralization to drive the formation, stabilization, and phase transformation of amorphous mineral precursors in the right place, at the right time, and rate are highlighted. The paradigm shifts in biomineralization and biomaterial design strategies are intertwined, which promotes breakthroughs in biomineralization-inspired material. The design principles and implementation methods of mineral precursor-based biomaterials in bone graft materials such as implant coatings, bone cements, hydrogels, and nanoparticles are detailed in the present manuscript. The biologically controlled mineralization mechanisms will hold promise for overcoming the barriers to the application of biomineralization-inspired biomaterials.

Integrating Melt Electrowriting and Fused Deposition Modeling to Fabricate Hybrid Scaffolds Supportive of Accelerated Bone Regeneration

Emerging additive manufacturing (AM) strategies can enable the engineering of hierarchal scaffold structures for guiding tissue regeneration. Here, the advantages of two AM approaches, melt electrowriting (MEW) and fused deposition modelling (FDM), are leveraged and integrated to fabricate hybrid scaffolds for large bone defect healing. MEW is used to fabricate a microfibrous core to guide bone healing, while FDM is used to fabricate a stiff outer shell for mechanical support, with constructs being coated with pro-osteogenic calcium phosphate (CaP) nano-needles. Compared to MEW scaffolds alone, hybrid scaffolds prevent soft tissue collapse into the defect region and support increased vascularization and higher levels of new bone formation 12 weeks post-implantation. In an additional group, hybrid scaffolds are also functionalized with BMP2 via binding to the CaP coating, which further accelerates healing and facilitates the complete bridging of defects after 12 weeks. Histological analyses demonstrate that such scaffolds support the formation of well-defined annular bone, with an open medullary cavity, smooth periosteal surface, and no evidence of abnormal ectopic bone formation. These results demonstrate the potential of integrating different AM approaches for the development of regenerative biomaterials, and in particular, demonstrate the enhanced bone healing outcomes possible with hybrid MEW-FDM constructs.

Engineered pre-dentin with well-aligned hierarchical mineralized collagen fibril bundles promote bio-root regeneration

Stem cell-mediated bio-root regeneration is an alternative tooth replacement strategy; however, physiologically functional bio-root regeneration with distinctive dentin structure remains challenging. In this study, the distinct arrangements of collagen fibril bundles were identified that account for hierarchical structural differences between dentin, cementum, and alveolar bone. Thus, an "engineered pre-dentin" was fabricated, which was a dentin hierarchical structure mimicking collagen (MC) scaffold, with well-aligned hierarchical mineralized collagen fibril bundles. The results revealed that it has a stronger effect on promoting biological root regeneration in nude mice and miniature pigs with dental pulp stem cell (DPSC) and periodontal ligament stem cell (PDLSC) sheets compared to hydroxyapatite tricalcium phosphate (HA/TCP). The success rate in the MC group was also higher than that in the HA/TCP group (67% and 33%, respectively). In conclusion, the hierarchical dentin-mimicking scaffold can enhance the regeneration of bio-roots, which provides a promising strategy for tooth regeneration.

Ultrastructure and Nanoporosity of Human Bone Shown with Correlative On-Axis Electron and Spectroscopic Tomographies

Mineralized collagen fibrils are the building block units of bone at the nanoscale. While it is known that collagen fibrils are mineralized both inside their gap zones (intra-fibrillar mineralization) and on their outer surfaces (extra-fibrillar mineralization), a clear visualization of this architecture in three dimensions (3D), combining structural and compositional information over large volumes, but without compromising the resolution, remains challenging. In this study, we demonstrate the use of on-axis Z-contrast electron tomography (ET) with correlative energy-dispersive X-ray spectroscopy (EDX) tomography to examine rod-shaped samples with diameters up to 700 nm prepared from individual osteonal lamellae in the human femur. Our work mainly focuses on two aspects: (i) low-contrast nanosized circular spaces ("holes") observed in sections of bone oriented perpendicular to the long axis of a long bone, and (ii) extra-fibrillar mineral, especially in terms of morphology and spatial relationship with respect to intra-fibrillar mineral and collagen fibrils. From our analyses, it emerges quite clearly that most "holes" are cross-sectional views of collagen fibrils. While this had been postulated before, our 3D reconstructions and reslicing along meaningful two-dimensional (2D) cross-sections provide a direct visual confirmation. Extra-fibrillar mineral appears to be composed of thin plates that are interconnected and span over several collagen fibrils, confirming that mineralization is cross-fibrillar, at least for the extra-fibrillar phase. EDX tomography shows mineral signatures (Ca and P) within the gap zones, but the signal appears weaker than that associated with the extra-fibrillar mineral, pointing toward the existence of dissimilarities between the two types of mineralization.

Functional and analytical recapitulation of osteoclast biology on demineralized bone paper

Osteoclasts are the primary target for osteoporosis drug development. Recent animal studies revealed the crucial roles of osteoblasts in regulating osteoclastogenesis and the longer lifespans of osteoclasts than previously thought with fission and recycling. However, existing culture platforms are limited to replicating these newly identified cellular processes. We report a demineralized bone paper (DBP)-based osteoblast culture and osteoclast assay platform that replicates osteoclast fusion, fission, resorption, and apoptosis with high fidelity and analytical power. An osteoid-inspired DBP supports rapid and structural mineral deposition by osteoblasts. Coculture osteoblasts and bone marrow monocytes under biochemical stimulation recapitulate osteoclast differentiation and function. The DBP-based bone model allows longitudinal quantitative fluorescent monitoring of osteoclast responses to bisphosphonate drug, substantiating significantly reducing their number and lifespan. Finally, we demonstrate the feasibility of humanizing the bone model. The DBP-based osteo assay platforms are expected to advance bone remodeling-targeting drug development with improved prediction of clinical outcomes.

Hydrogels and Bioprinting in Bone Tissue Engineering: Creating Artificial Stem-Cell Niches for In Vitro Models

Advances in bioprinting have enabled the fabrication of complex tissue constructs with high speed and resolution. However, there remains significant structural and biological complexity within tissues that bioprinting is unable to recapitulate. Bone, for example, has a hierarchical organization ranging from the molecular to whole organ level. Current bioprinting techniques and the materials employed have imposed limits on the scale, speed, and resolution that can be achieved, rendering the technique unable to reproduce the structural hierarchies and cell-matrix interactions that are observed in bone. The shift toward biomimetic approaches in bone tissue engineering, where hydrogels provide biophysical and biochemical cues to encapsulated cells, is a promising approach to enhancing the biological function and development of tissues for in vitro modeling. A major focus in bioprinting of bone tissue for in vitro modeling is creating dynamic microenvironmental niches to support, stimulate, and direct the cellular processes for bone formation and remodeling. Hydrogels are ideal materials for imitating the extracellular matrix since they can be engineered to present various cues whilst allowing bioprinting. Here, recent advances in hydrogels and 3D bioprinting toward creating a microenvironmental niche that is conducive to tissue engineering of in vitro models of bone are reviewed.

Synergistic effect of FGF-2 and TGF-β1 on the mineralization of human umbilical cord perivascular cells

OBJECTIVE

Human umbilical cord perivascular cells (HUCPVCs) are derived from the human umbilical cord perivascular tissue and are expected to replace mesenchymal stromal cells in the future. We investigated the synergistic effects of fibroblast growth factor 2 (FGF-2) and transforming growth factor-beta 1 (TGF-β1) on HUCPVC mineralization.

DESIGN

We prepared HUCPVCs with (FGF(+)HUCPVCs) or without FGF-2 (FGF(-)HUCPVCs) in the presence of activated vitamin D3, a bone morphogenic protein inhibitor, and TGF-β1. We examined the cell proliferative capacity, expression of various hard tissue-forming cell gene markers, and mineralization induction ability and identified the crystalline phases of the mineralized nodules.

RESULTS

FGF(+)HUCPVCs exhibited higher intracellular alkaline phosphatase (ALP) gene expression and ALP activity, and their cell proliferation rate was higher than that of FGF(-)HUCPVCs. The expression levels of osteoblast marker genes increased in FGF(+)HUCPVCs, whereas those of elastic fiber and muscle cell markers increased in FGF(-)HUCPVCs. The expression of genes related to matrix vesicle-mediated mineralization was increased in FGF(+)HUCPVCs. While FGF(-)HUCPVCs displayed myofibroblast-like properties and could not induce mineralization, FGF(+)HUCPVCs demonstrated the ability to produce mineralized nodules. The resulting mineralized nodules consisted of hydroxyapatite as the major phase and minor amounts of octacalcium phosphate. The mineralized nodules exhibited the morphological characteristics of bone hydroxyapatite, composed of fibrous hydroxyapatite nanorods and polycrystalline sheets.

CONCLUSION

We found that FGF-2 synergizes with TGF-β1 and is a key factor in the differentiation of HUCPVCs into osteoblast-like cells. Thus, HUCPVCs can potentially serve as a new stem cell source for future bone regeneration and dental treatments.

In Vivo Assessment of Skin Surface Pattern: Exploring Its Potential as an Indicator of Bone Biomechanical Properties

The mechanical properties of bone tissue are the result of a complex process involving collagen-crystal interactions. The mineral density of the bone tissue is correlated with bone strength, whereas the characteristics of collagen are often associated with the ductility and toughness of the bone. From a clinical perspective, bone mineral density alone does not satisfactorily explain skeletal fragility. However, reliable in vivo markers of collagen quality that can be easily used in clinical practice are not available. Hence, the objective of the present study is to examine the relationship between skin surface morphology and changes in the mechanical properties of the bone. An experimental study was conducted on healthy children (n = 11), children with osteogenesis imperfecta (n = 13), and women over 60 years of age (n = 22). For each patient, the skin characteristic length (SCL) of the forearm skin surface was measured. The SCL quantifies the geometric patterns formed by wrinkles on the skin's surface, both in terms of size and elongation. The greater the SCL, the more deficient was the organic collagen matrix. In addition, the bone volume fraction and mechanical properties of the explanted femoral head were determined for the elderly female group. The mean SCL values of the healthy children group were significantly lower than those of the elderly women and osteogenesis imperfecta groups. For the aged women group, no significant differences were indicated in the elastic mechanical parameters, whereas bone toughness and ductility decreased significantly as the SCL increased. In conclusion, in bone collagen pathology or bone aging, the SCL is significantly impaired. This in vivo skin surface parameter can be a non-invasive tool to improve the estimation of bone matrix quality and to identify subjects at high risk of bone fracture.

Liquid Transmission Electron Microscopy for Probing Collagen Biomineralization

Collagen biomineralization is fundamental to hard tissue assembly. While studied extensively, collagen mineralization processes are not fully understood, with the majority of theories derived from electron microscopy (EM) under static, dehydrated, or frozen conditions, unlike the liquid phase environment where mineralization occurs. Herein, novel liquid transmission EM (TEM) strategies are presented, in which collagen mineralization was explored in liquid for the first time via TEM. Custom thin-film enclosures were employed to visualize the mineralization of reconstituted collagen fibrils in a calcium phosphate and polyaspartic acid solution to promote intrafibrillar mineralization. TEM highlighted that at early time points precursor mineral particles attached to collagen and progressed to crystalline mineral platelets aligned with fibrils at later time points. This aligns with observations from other techniques and validates the liquid TEM approach. This work provides a new liquid imaging approach for exploring collagen biomineralization, advancing toward understanding disease pathogenesis and remineralization strategies for hard tissues.

Investigating the interfaces of the epiphyseal plate: An integrated approach of histochemistry, microtomography and SEM

We investigated the interfaces of the epiphyseal plate with over- and underlying bone segments using an integrated approach of histochemistry, microtomography and scanning electron microscopy (SEM) to overcome the inherent limitations of sections-based techniques. Microtomography was able to provide an unobstructed, frontal view of large expanses of the two bone surfaces facing the growth plate, while SEM observation after removal of the soft matrix granted an equally unhindered access with a higher resolution. The two interfaces appeared widely dissimilar. On the diaphyseal side the hypertrophic chondrocytes were arranged in tall columns packed in a sort of compact palisade; the interposed extracellular matrix was actively calcifying into a thick mineralized crust growing towards the epiphysis. Behind the mineralization front, histochemical data revealed a number of surviving cartilage islets which were being slowly remodelled into bone. In contrast, the epiphyseal side of the cartilage consisted of a relatively quiescent reserve zone whose mineralization was marginal in amount and discontinuous in extension; the epiphyseal bone consisted of a loose trabecular meshwork, with ample vascular spaces opening directly into the non-mineralized cartilage. On both sides the calcification process took place through the formation of spheroidal bodies 1-2 μm wide which gradually grew by apposition and coalesced into a solid mass, in a way distinctly different from that of bone and other calcified tissues.

Bone-matrix mineralization dampens integrin-mediated mechanosignalling and metastatic progression in breast cancer

In patients with breast cancer, lower bone mineral density increases the risk of bone metastasis. Although the relationship between bone-matrix mineralization and tumour-cell phenotype in breast cancer is not well understood, mineralization-induced rigidity is thought to drive metastatic progression via increased cell-adhesion forces. Here, by using collagen-based matrices with adjustable intrafibrillar mineralization, we show that, unexpectedly, matrix mineralization dampens integrin-mediated mechanosignalling and induces a less proliferative stem-cell-like phenotype in breast cancer cells. In mice with xenografted decellularized physiological bone matrices seeded with human breast tumour cells, the presence of bone mineral reduced tumour growth and upregulated a gene-expression signature that is associated with longer metastasis-free survival in patients with breast cancer. Our findings suggest that bone-matrix changes in osteogenic niches regulate metastatic progression in breast cancer and that in vitro models of bone metastasis should integrate organic and inorganic matrix components to mimic physiological and pathologic mineralization.