Multiphase intrafibrillar mineralization of collagen

Tubular Nanoclay-Enhanced Calcium Phosphate Mineralization and Assembly to Impart High Stiffness and Antimicrobial Properties

Achieving superior mechanical properties of composite materials in artificially engineered materials is a great challenge due to technical bottlenecks in the size and morphological modulation of inorganic nanominerals. Hence, a "bioprocess-inspired fabrication" is proposed to create multilayered organic-inorganic columnar structures. The sequential assembly of halloysite nanotubes (HNTs), polyelectrolytes (PAAs), and calcium phosphates (CaPs) results in organic-inorganic structures. PAA plays a crucial role in controlling the formation of CaP, guiding it into amorphous particles with smaller nanosizes. The introduction of HNT induces the assembly and maturation of CaP-PAA, leading to the formation of a highly crystalline hydroxyapatite. Poly(vinyl alcohol) was then woven into HNT-encapsulated hydroxyapatite nanorods, resulting in composite materials with basic hierarchical structures across multiple scales. The fabricated composite exhibits exceptional hardness (4.27 ± 0.33 GPa) and flexural strength (101.25 ± 1.72 MPa), surpassing those of most previously developed biological hard tissue materials. Additionally, the composite demonstrates effective antibacterial properties and corrosion resistance, attributed to the dense crystalline phase of CaP. This innovative approach showcases the potential of clay minerals, particularly HNT, in the advancement of biomaterial design. The outstanding mechanical and antimicrobial properties of clay-based composites make them a promising candidate for applications in hard tissue repair, offering versatility in biomedicine and engineering.

Research progress of biomimetic materials in oral medicine

Biomimetic materials are able to mimic the structure and functional properties of native tissues especially natural oral tissues. They have attracted growing attention for their potential to achieve configurable and functional reconstruction in oral medicine. Though tremendous progress has been made regarding biomimetic materials, significant challenges still remain in terms of controversy on the mechanism of tooth tissue regeneration, lack of options for manufacturing such materials and insufficiency of in vivo experimental tests in related fields. In this review, the biomimetic materials used in oral medicine are summarized systematically, including tooth defect, tooth loss, periodontal diseases and maxillofacial bone defect. Various theoretical foundations of biomimetic materials research are reviewed, introducing the current and pertinent results. The benefits and limitations of these materials are summed up at the same time. Finally, challenges and potential of this field are discussed. This review provides the framework and support for further research in addition to giving a generally novel and fundamental basis for the utilization of biomimetic materials in the future.

Collagen piezoelectricity in osteogenesis imperfecta and its role in intrafibrillar mineralization

Intrafibrillar mineralization plays a critical role in attaining desired mechanical properties of bone. It is well known that amorphous calcium phosphate (ACP) infiltrates into the collagen through the gap regions, but its underlying driving force is not understood. Based on the authors’ previous observations that a collagen fibril has higher piezoelectricity at gap regions, it was hypothesized that the piezoelectric heterogeneity of collagen helps ACP infiltration through the gap. To further examine this hypothesis, the collagen piezoelectricity of osteogenesis imperfecta (OI), known as brittle bone disease, is characterized by employing Piezoresponse Force Microscopy (PFM). The OI collagen reveals similar piezoelectricity between gap and overlap regions, implying that losing piezoelectric heterogeneity in OI collagen results in abnormal intrafibrillar mineralization and, accordingly, losing the benefit of mechanical heterogeneity from the fibrillar level. This finding suggests a perspective to explain the ACP infiltration, highlighting the physiological role of collagen piezoelectricity in intrafibrillar mineralization.

A Bioinspired Ultratough Composite Produced by Integration of Inorganic Ionic Oligomers within Polymer Networks

The nacre-inspired laminates are promising materials for their excellent mechanics. However, the interfacial defects between organic-inorganic phases commonly lead to the crack propagation and fracture failure of these materials under stress. A natural biomineral, bone, has much higher bending toughness than the nacre. The small size of inorganic building units in bone improves the organic-inorganic interaction, which optimizes the material toughness. Inspired by these biological structures, here, an ultratough nanocomposite laminate is prepared by the integration of ultrasmall calcium phosphate oligomers (CPO, 1 nm in diameter) within poly(vinyl alcohol) (PVA) and sodium alginate (Alg) networks through a simple three-step strategy. Owing to the small size of inorganic building units, strong multiple molecular interactions within integrated organic-inorganic hierarchical structure are built. The resulting laminates exhibit ultrahigh bending strain (>50% without fracture) and toughness (21.5-31.0 MJ m^-3), which surpass natural nacre and almost all of the synthetic laminate materials that have been reported so far. Moreover, the mechanics of this laminate is tunable by changing the water content within the bulk structure. This work provides a way for the development of organic-inorganic nanocomposites with ultrahigh bending toughness by using inorganic ionic oligomers, which can be useful in the fields of tough protective materials and energy absorbing materials.

A Flexible and Degradable Hybrid Mineral as a Plastic Substitute

The development of environmentally friendly plastics is critical to ensure sustainable development. In contrast to polymer plastics derived from petrochemicals, inorganic minerals, which are the most abundant matter in Earth's crust, are environmentally friendly. However, the brittleness of these minerals limits their applications as plastics. Here, because of the advantages of both biomineralization and inorganic ionic polymerization, the calcium phosphate (CaP, a typical geological and biological mineral) oligomers are used for biomimetic mineralization under the regulation of polyvinyl alcohol and sodium alginate, resulting in flexible CaP nanofibers with periodic structural defects. The assembly of CaP nanofibers produces a hierarchically structured bulk hybrid mineral (HM), which overcomes the intrinsic brittleness of minerals and exhibits plasticity characteristics. HM exhibits better hardness and thermostability than classical polymer plastics due to its dominant mineral composition. Notably, HM is environmentally friendly and degradable in nature, as it can potentially participate in geological cycles, indicating that this material is an optimal plastic substitute. The construction of periodic structural defects within flexible minerals expands the current understanding of materials science.

Advances in biomineralization-inspired materials for hard tissue repair

Biomineralization is the process by which organisms form mineralized tissues with hierarchical structures and excellent properties, including the bones and teeth in vertebrates. The underlying mechanisms and pathways of biomineralization provide inspiration for designing and constructing materials to repair hard tissues. In particular, the formation processes of minerals can be partly replicated by utilizing bioinspired artificial materials to mimic the functions of biomolecules or stabilize intermediate mineral phases involved in biomineralization. Here, we review recent advances in biomineralization-inspired materials developed for hard tissue repair. Biomineralization-inspired materials are categorized into different types based on their specific applications, which include bone repair, dentin remineralization, and enamel remineralization. Finally, the advantages and limitations of these materials are summarized, and several perspectives on future directions are discussed.

The effects of washing a collagen sample prior to TEM examination

Transmission electron microscopy (TEM) is an important analysis technique to visualize (bio)macromolecules and their assemblies, including collagen fibers. Many protocols for TEM sample preparation of collagen involve one or more washing steps to remove excess salts from the dispersion that could hamper analysis when dried on a TEM grid. Such protocols are not standardized and washing times as well as washing solvents vary from procedure to procedure, with each research group typically having their own protocol. Here, we investigate the influence of washing with water, ethanol, but also methanol and 2‐propanol, for both mineralized and unmineralized collagen samples via a protocol based on centrifugation. Washing with water maintains the hydrated collagen structure and the characteristic banding pattern can be clearly observed. Conversely, washing with ethanol results in dehydration of the fibrils, often leading to aggregation of the fibers and a less obvious banding pattern, already within 1 min of ethanol exposure. As we show, this process is fully reversible. Similar observations were made for methanol and propanol. Based on these results, a standardized washing protocol for collagenous samples is proposed.

A Dopamine Acrylamide Molecule for Promoting Collagen Biomimetic Mineralization and Regulating Crystal Growth Direction

The reconstruction of the intra/interfibrillar mineralized collagen microstructure is extremely important in biomaterial science and regeneration medicine. However, certain problems, such as low efficiency and long period of mineralization, are apparent, and the mechanism of interfibrillar mineralization is often neglected in the present literature. Thus, we propose a novel model of biomimetic collagen mineralization that uses molecules with the dual function of cross-linking collagen and regulating collagen mineralization to construct the intrafibrillar and interfibrillar collagen mineralization of the structure of mineralized collagen hard tissues. In the present study completed in vitro, N-2-(3,4-dihydroxyphenyl) acrylamide (DAA) is used to bind and cross-link collagen molecules and further stabilize the self-assembled collagen fibers. The DAA-collagen complex provides more affinity with calcium and phosphate ions, which can reduce the calcium phosphate/collagen interfacial energy to promote hydroxyapatite (HA) nucleation and accelerate the rate of collagen fiber mineralization. Besides inducing intrafibrillar mineralization, the DAA-collagen complex mineralization template can realize interfibrillar mineralization with the c-axis of the HA crystal on the surface of collagen fibers and between fibers that are parallel to the long axis of collagen fibers. The DAA-collagen complex, as a new type of mineralization template, may provide a new collagen mineralization strategy to produce a mineralized scaffold material for tissue engineering or develop bone-like materials.

Rapid collagen-directed mineralization of calcium fluoride nanocrystals with periodically patterned nanostructures

Collagen fibrils present periodic structures, which provide space for intrafibrillar growth of oriented hydroxyapatite nanocrystals in bone and contribute to the good mechanical properties of bone. However, there are not many reports focused on bioprocess-inspired synthesis of non-native inorganic materials inside collagen fibrils and detailed forming processes of crystals inside collagen fibrils remain poorly understood. Herein, the rapid intrafibrillar mineralization of calcium fluoride nanocrystals with a periodically patterned nanostructure is demonstrated. The negatively charged calcium fluoride precursor phase infiltrates collagen fibrils through the gap zones creating an intricate periodic mineralization pattern. Later, the nanocrystals initially filling the gap zones only expand gradually into the remaining space within the collagen fibrils. Mineralized tendons with organized calcium fluoride nanocrystals acquire mechanical properties (indentation elastic modulus ∼25.1 GPa and hardness ∼1.5 GPa) comparable or even superior to those of native human dentin and lamellar bone. Understanding the mineral growth processes in collagen may facilitate the development of tissue engineering and repairing.

Matrix stiffening by self-mineralizable guided bone regeneration

Collagen membranes produced in vitro with different degrees of intrafibrillar mineralization are potentially useful for guided bone regeneration (GBR). However, highly-mineralized collagen membranes are brittle and difficult for clinical manipulation. The present study aimed at developing an intrafibrillar self-mineralization strategy for GBR membrane by covalently conjugating high-molecular weight polyacrylic acid (HPAA) on Bio-Gide® membranes (BG). The properties of the self-mineralizable membranes (HBG) and their potential to induce bone regeneration were investigated. The HBG underwent the progressive intrafibrillar mineralization as well as the increase in stiffness after immersed in supersaturated calcium phosphate solution, osteogenic medium, or after being implanted into a murine calvarial bone defect. The HBG promoted in-situ bone regeneration via stimulating osteogenic differentiation of mesenchymal stromal cells (MSCs). Hippo signaling was inhibited when MSCs were cultured on the self-mineralized HBG, and in HBG-promoted MSC osteogenesis during in-situ bone regeneration. This resulted in translocation of the transcription co-activators Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ) into the nucleus to induce transcription of genes promoting osteogenic differentiation of MSCs. Taken together, these findings indicated that HBG possessed the ability to self-mineralize in situ via intrafibrillar mineralization. The increase in stiffness of the extracellular matrix expedited in-situ bone regeneration by inactivating the Hippo-YAP/TAZ signaling cascade. STATEMENT OF SIGNIFICANCE: Guided bone regeneration (GBR) membranes made of naturally derived collagen have been widely used in the bone defect restoration. However, application of collagen GBR membranes run into the bottleneck with the challenges like insufficient stress strength, relatively poor dimensional stability and unsatisfactory osteoinductivity. This study develops a modified GBR membrane that can undergo progressive self-mineralization and matrix stiffening in situ. Increase in extracellular matrix stiffness provides the mechanical cues required for MSCs differentiation and expedites in-situ bone regeneration by inactivating the Hippo-YAP/TAZ signaling cascade.

[Application of biomimetic restoration in oral-maxillofacial hard tissue repair]

Oral-maxillofacial hard tissue is the support of maxillofacial structure and appearance, and lays the foundation for functions of oral and maxillofacial system. Once the defect occurs, it will not only affect the physiological functions such as chewing and pronunciation, but also have a significant impact on the psychological and social life of patients. However, the self-repairing capability of the oral-maxillofacial hard tissue is pretty limited, in which case, substitute materials are required for tissue repair. A huge gap exists between the physical, chemical, structural characteristics of conventional substitute materials and those of human hard tissues, resulting in poor repair effect. Based on this, scholars simulated the process of biomineralization in the development of hard tissues, to improve the structure and function of materials through biomimetic mineralization technology and enhance the repair performance of materials. The current understanding of biomineralization theory and the construction of biomimetic repair technology is still in the stage of rapid development. In recent years, a mass of innovative studies are keeping emerging. In this review, the representative advances in the repair of oral-maxillofacial hard tissues of the past five years are reviewed.

In Vitro Mineralization of Collagen

Collagen mineralization is a biological process in many skeletal elements in the animal kingdom. Examples of these collagen-based skeletons are the siliceous spicules of glass sponges or the intrafibrillar hydroxyapatite platelets in vertebrates. The mineralization of collagen in vitro has gained interest for two reasons: understanding the processes behind bone formation and the synthesis of scaffolds for tissue engineering. In this paper, the efforts toward collagen mineralization in vitro are reviewed. First, general introduction toward collagen type I, the main component of the extracellular matrix in animals, is provided, followed by a brief overview of collagenous skeletons. Then, the in vitro mineralization of collagen is critically reviewed. Due to their biological abundance, hydroxyapatite and silica are the focus of this review. To a much lesser extent, also some efforts with other minerals are outlined. Combining all minerals and the suggested mechanisms for each mineral, a general mechanism for the intrafibrillar mineralization of collagen is proposed. This review concludes with an outlook for further improvement of collagen-based tissue engineering scaffolds.

Highly Mineralized Biomimetic Polysaccharide Nanofiber Materials Using Enzymatic Mineralization

Many biological high-performance composites, such as bone, antler, and crustacean cuticles, are composed of densely mineralized and ordered nanofiber materials. The mimicry of even simplistic bioinspired structures, i.e., of densely and homogeneously mineralized nanofibrillar materials with controllable mechanical performance, continues to be a grand challenge. Here, using alkaline phosphatase as an enzymatic catalyst, we demonstrate the dense, homogeneous, and spatially controlled mineralization of calcium phosphate nanostructures within networks of anionically charged cellulose nanofibrils (CNFs) and cationically charged chitin nanofibrils (ChNFs)-both emerging biobased nanoscale building blocks for sustainable high-performance materials design. Our study reveals that anionic CNFs lead to a more homogeneous nanoscale mineralization with very high mineral contents up to ca. 70 wt % with a transition from amorphous to crystalline deposits, while cationic ChNFs yield rod-like crystalline morphologies. The bone-inspired CNF bulk films exhibit a significantly increased stiffness, maintain good flexibility and translucency, and have a significant gain in wet state mechanical properties. The mechanical properties can be tuned both by the enzyme concentration and the mineralization time. Moreover, we also show a spatial control of the mineralization using kinetically controlled substrate uptake in a dialysis reactor, and by spatially selectively incorporating the enzyme into 2D printed filament patterns. The strategy highlights possibilities for spatial encoding of enzymes in tailored structures and patterns and programmed mineralization processes, promoting the potential application of mineralized CNF biomaterials with complex gradients for bone substitutes and tissue regeneration in general.

Alginate-Assisted Mineralization of Collagen by Collagen Reconstitution and Calcium Phosphate Formation

The understanding of the mineralization of collagen for bone formation is a current key theme in bone tissue engineering and is of great relevance to the fabrication of novel biomimetic bone grafting materials. The noncollagenous proteins (NCPs) play a vital role in bone formation and are considered to be responsible for regulating intrafibrillar penetration of minerals into collagen fibrils by means of their abundant polyanionic domains. In this study, alginate, as a NCPs analogue, was introduced in the mineralization of collagen to mediate the collagen self-assembly with simultaneous hydroxyapatite (HA) synthesis. The biomimetic systems were based upon the self-assembly of collagen (Col) or collagen-alginate (CA) in the absence or presence of a varying content of HA. The alginate-mediated effects were found to include the lateral aggregation of small fibrils into the extremely large bundles and the assisted deposition of HA for a larger mineralized fibril. This alginate-assisted mineralization of collagen gave rise to an exquisite 3D mineralized architecture with enhanced mechanical property. The cell viability experiments showed the excellent proliferation and spreading morphologies of rat bone mesenchymal stem cells (MSCs) on the assembled products, and a higher expression of osteogenic differentiation related transcription factor was obtained in the alginate-assisted mineralization of collagen. This study indicated that the selection of an appropriate substance, e.g., alginate as an anionic polyelectrolyte with Ca-capturing property, could be a convenient, simple solution to achieve a mineralized collagen scaffold with the reinforced mechanical property for potential applications in bone regeneration.

Bioenergetic-active materials enhance tissue regeneration by modulating cellular metabolic state

Cellular bioenergetics (CBE) plays a critical role in tissue regeneration. Physiologically, an enhanced metabolic state facilitates anabolic biosynthesis and mitosis to accelerate regeneration. However, the development of approaches to reprogram CBE, toward the treatment of substantial tissue injuries, has been limited thus far. Here, we show that induced repair in a rabbit model of weight-bearing bone defects is greatly enhanced using a bioenergetic-active material (BAM) scaffold compared to commercialized poly(lactic acid) and calcium phosphate ceramic scaffolds. This material was composed of energy-active units that can be released in a sustained degradation-mediated fashion once implanted. By establishing an intramitochondrial metabolic bypass, the internalized energy-active units significantly elevate mitochondrial membrane potential (ΔΨm) to supply increased bioenergetic levels and accelerate bone formation. The ready-to-use material developed here represents a highly efficient and easy-to-implement therapeutic approach toward tissue regeneration, with promise for bench-to-bedside translation.

Calcium Phosphate Nanocluster-Loaded Injectable Hydrogel for Bone Regeneration

Bone is a hierarchical tissue in which the extracellular matrix consists of hydroxyapatite (HAP) crystals embedded in the collagen matrix. Artificial bone regeneration remains a great challenge due to the difficulty of balancing the chemical composition, biological compatibility, and mechanical performance of the implant. Biomineralization starts from the formation of a hydrogel-like biomacromolecule matrix in many cases, while the mineralization of HAP often builds from amorphous calcium phosphate (ACP) nanoclusters. Inspired by these discoveries, here we use a hydrogel loaded with ∼1 nm sized polymer-stabilized ACP nanoclusters (cluster-loaded hydrogel) as an injectable bone regeneration material. The hydrogel is biocompatible and stabilizes the ACP clusters such that they could efficiently infiltrate into collagen fibrils leading to intrafibrillar mineralization of HAP nanocrystals. Ex vivo results reveal that the cluster-loaded hydrogel has an excellent bone affinity as well as provides a suitable environment for the proliferation and differentiation of bone cells. In vivo experiments with rat bone show that the cluster-loaded hydrogel can generate HAP-based fillings within bone defects with perfect bonding to the surrounding tissue and a mechanical performance comparable with native bone. The fluidity of the hydrogel is further beneficial by providing a feasible minimally invasive bone healing procedure via syringe injection. The discovery and utilization of the cluster-loaded hydrogel described here provide a promising bioinspired approach for bone tissue regeneration.

3D-Printed Hydrogel Composites for Predictive Temporal (4D) Cellular Organizations and Patterned Biogenic Mineralization

Materials chemistries for hydrogel scaffolds that are capable of programming temporal (4D) attributes of cellular decision-making in supported 3D microcultures are described. The scaffolds are fabricated using direct-ink writing (DIW)-a 3D-printing technique using extrusion to pattern scaffolds at biologically relevant diameters (≤ 100 µm). Herein, DIW is exploited to variously incorporate a rheological nanoclay, Laponite XLG (LAP), into 2-hydroxyethyl methacrylate (HEMA)-based hydrogels-printing the LAP-HEMA (LH) composites as functional modifiers within otherwise unmodified 2D and 3D HEMA microstructures. The nanoclay-modified domains, when tested as thin films, require no activating (e.g., protein) treatments to promote robust growth compliances that direct the spatial attachment of fibroblast (3T3) and preosteoblast (E1) cells, fostering for the latter a capacity to direct long-term osteodifferentiation. Cell-to-gel interfacial morphologies and cellular motility are analyzed with spatial light interference microscopy (SLIM). Through combination of HEMA and LH gels, high-resolution DIW of a nanocomposite ink (UniH) that translates organizationally dynamic attributes seen with 2D gels into dentition-mimetic 3D scaffolds is demonstrated. These analyses confirm that the underlying materials chemistry and geometry of hydrogel nanocomposites are capable of directing cellular attachment and temporal development within 3D microcultures-a useful material system for the 4D patterning of hydrogel scaffolds.

Biomineralization-Inspired Material Design for Bone Regeneration

Synthetic substitutes of bone grafts, such as calcium phosphate-based ceramics, have shown some good clinical successes in the regeneration of large bone defects and are currently extensively used. In the past decade, the field of biomineralization has delivered important new fundamental knowledge and techniques to better understand this fascinating phenomenon. This knowledge is also applied in the field of biomaterials, with the aim of bringing the composition and structure, and hence the performance, of synthetic bone graft substitutes even closer to those of the extracellular matrix of bone. The purpose of this progress report is to critically review advances in mimicking the extracellular matrix of bone as a strategy for development of new materials for bone regeneration. Lab-made biomimicking or bioinspired materials are discussed against the background of the natural extracellular matrix, starting from basic organic and inorganic components, and progressing into the building block of bone, the mineralized collagen fibril, and finally larger, 2D and 3D constructs. Moreover, bioactivity studies on state-of-the-art biomimicking materials are discussed. By addressing these different topics, an overview is given of how far the field has advanced toward a true bone-mimicking material, and some suggestions are offered for bridging current knowledge and technical gaps.

A novel fluorescent adhesive-assisted biomimetic mineralization

We propose a novel fluorescent adhesive-assisted biomimetic mineralization strategy, based on which 1 wt% of sodium fluorescein and 25 wt% of polyacrylic acid stabilized amorphous calcium phosphate (PAA-ACP) nanoparticles were incorporated into a mild self-etch adhesive (Clearfil S3 Bond) as a fluorescent mineralizing adhesive. The characterization of the PAA-ACP nanoparticles indicates that they were spherical particles clustered together, each particle with a diameter of approximately 20-50 nm, in a metastable phase with two characteristic absorption peaks (1050 cm-1 and 580 cm-1). Our results suggest that the fluorescent mineralizing adhesive was non-cytotoxic with minimal esthetic interference and its fluorescence intensity did not significantly decrease within 6 months. Our data reveal that the fluorescent mineralizing adhesive could induce the extra- and intra-fibrillar remineralization of the reconstituted type I collagen, the demineralized enamel and dentin substrate. Our data demonstrate that a novel fluorescent adhesive-assisted biomimetic mineralization strategy will pave the way to design and produce anti-carious materials for the prevention of dental caries.

Intrafibrillar, bone-mimetic collagen mineralization regulates breast cancer cell adhesion and migration

Bone metastasis is a leading cause of death in patients with breast cancer, but the underlying mechanisms are poorly understood. While much work focuses on the molecular and cellular events that drive breast cancer bone metastasis, it is mostly unclear what role bone extracellular matrix (ECM) properties play in this process. Bone ECM primarily consists of mineralized collagen fibrils, which are composed of non-stoichiometric carbonated apatite (HA) and collagen type I. Reduced bone mineral content is epidemiologically linked with increased risk of bone metastasis. Yet elucidating the potential functional impact of collagen mineralization on breast cancer cells has remained challenging because of a lack of model systems that allow studying tumor cell behavior as a function of physiological, intrafibrillar collagen mineralization. Here, we have developed cell culture substrates composed of mineralized collagen type I fibrils using a polymer-induced liquid-precursor (PILP) process. Intrafibrillar HA decreased breast cancer cell adhesion forces and accordingly reduced collagen fiber alignment relative to cells cultured on control collagen. The resulting mineral-mediated changes in collagen network characteristics and mechanosignaling correlated with increased cell motility, but inhibited directed migration of breast cancer cells. These results suggest that physiological mineralization of collagen fibrils reduces tumor cell adhesion with potential functional consequences on skeletal homing of disseminated tumor cells in early stages of breast cancer metastasis.

Structure analysis of collagen fibril at atomic-level resolution and its implications for intra-fibrillar transport in bone biomineralization

Bone is a hierarchical biocomposite material in which a collagen fibril matrix self-assembled in a three-dimensional (3-D) pseudohexagonal array controls many important processes in mineralization such as providing the pathways by which calcium and phosphate species are delivered and a template for the earliest nucleation sites, determining the spatial distribution of the mineral and the topology for binding of associated non-collagenous proteins. However, the structural characteristics of collagen molecules in the fibril remain unclear at the atomic level. Here we performed the first large-scale molecular dynamics simulations to provide a comprehensive all-atom structural analysis of the entire fibril of Type I collagen including intra-fibrillar water distribution. We found that the ideal fibril structure is preserved in specific sites where the earliest nucleation occurs, but is severely distorted in areas that mineralize later. In detail, the ideal pseudohexagonal structure is well-preserved in the overlap zone (c1, c2 and b bands), in the a bands of the hole zone but is severely distorted at the hole/overlap transition (d and c3 bands). As a result, the expected uniform "channel," formed by connecting holes in adjacent unit cells along the b-axis, and having dimensions of 1.5 nm height along the a-axis and width of 40 nm along the c-axis is not formed. The expected uniform channel of 1.5 nm height is preserved only in the a bands in a narrow sub-channel region only 5.8 nm wide. At the hole/overlap transition, an irregular, tortuous sub-channel of widely varying dimensions (∼1.8-4.0 nm height × ∼3.0 nm width) is formed. The well-defined sub-channel in the a bands along with their preferred orientation of charged amino acid residues could facilitate faster molecular diffusion than the tortuous sub-channels and ionic interactions, thus providing the first nucleation sites. Intra-fibrillar water occupies nano-spaces and shows low density (∼0.7 g cm^-3), which should promote dehydration of ion species. These results provide the first atomic-level understanding of the structure of the collagen fibril and the properties of the aqueous compartments within the fibril, which offer a physical, chemical and steric explanation for calcium phosphate infiltration paths and for the initiation of mineralization at the a band collagen fibril. The mechanism revealed here for the observed specificity of collagen biomineralization in bone formation ultimately contributes to the biochemical and biomechanical functions of the skeleton.

Bioinspired Collagen Scaffolds in Cranial Bone Regeneration: From Bedside to Bench

Calvarial defects are common reconstructive dilemmas secondary to a variety of etiologies including traumatic brain injury, cerebrovascular disease, oncologic resection, and congenital anomalies. Reconstruction of the calvarium is generally undertaken for the purposes of cerebral protection, contour restoration for psychosocial well-being, and normalization of neurological dysfunction frequently found in patients with massive cranial defects. Current methods for reconstruction using autologous grafts, allogeneic grafts, or alloplastic materials have significant drawbacks that are unique to each material. The combination of wide medical relevance and the need for a better clinical solution render defects of the cranial skeleton an ideal target for development of regenerative strategies focused on calvarial bone. With the improved understanding of the instructive properties of tissue-specific extracellular matrices and the advent of precise nanoscale modulation in materials science, strategies in regenerative medicine have shifted in paradigm. Previously considered to be simple carriers of stem cells and growth factors, increasing evidence exists for differential materials directing lineage specific differentiation of progenitor cells and tissue regeneration. In this work, we review the clinical challenges for calvarial reconstruction, the anatomy and physiology of bone, and extracellular matrix-inspired, collagen-based materials that have been tested for in vivo cranial defect healing.

Confinement controlled mineralization of calcium carbonate within collagen fibrils

The amorphous calcium carbonate infiltrates into collagen fibrils and transforms into a co-oriented crystalline phase under the function of confinement.

Biphasic silica/apatite co-mineralized collagen scaffolds stimulate osteogenesis and inhibit RANKL-mediated osteoclastogenesis

The effects of a biphasic mineralized collagen scaffold (BCS) containing intrafibrillar silica and apatite on osteogenesis of mouse mesenchymal stem cells (mMSCs) and inhibition of receptor activator of nuclear factor κB ligand (RANKL)-mediated osteoclastogenesis were investigated in the present study. mMSCs were cultured by exposing to BCS for 7 days for cell proliferation/viability examination, and stimulated to differentiate in osteogenic medium for 7-21 days for evaluation of alkaline phosphatase activity, secretion of osteogenic deposits and expression of osteoblast lineage-specific phenotypic markers. The effect of BCS-conditioned mMSCs on osteoclastogenesis of RAW 264.7 cells was evaluated by tartrate-resistant acid phosphatase staining and resorption pit analysis. The contributions of mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) signal transduction pathways to osteogenesis of mMSCs and their osteoprotegerin (OPG) and RANKL expressions were also evaluated. Compared with unmineralized, intrafibrillarly-silicified or intrafibrillarly-calcified collagen scaffolds, BCS enhanced osteogenic differentiation of mMSCs by activation of the extracellular signal regulated kinases (ERK)/MAPK and p38/MAPK signaling pathways. After mMSCs were exposed to BCS, they up-regulated OPG expression and down-regulated RANKL expression through activation of the p38/MAPK and PI3K/protein kinase B (Akt) pathways, resulting in inhibition of the differentiation of RAW 264.7 cells into multinucleated osteoclasts and reduction in osteoclast function. These observations collectively suggest that BCS has the potential to be used in bone tissue engineering when the demand for anabolic activities is higher than catabolic metabolism during the initial stage of wound rehabilitation.

Biomimetic mineralization of anionic gelatin hydrogels: effect of degree of methacrylation

The crosslinker contents of the hydrogel have a significant effect on the mineralization outcome, including crystallinity, content, and morphology of the mineral growth within the 3d gelatin methacrylate scaffold.