Nanoscale confinement controls the crystallization of calcium phosphate: relevance to bone formation

Enzyme-Activated Biomimetic Vesicles Confining Mineralization for Bone Maturation

Inspired by the crucial role of matrix vesicles (MVs), a series of biomimetic vesicles (BVs) fabricated by calcium glycerophosphate (CaGP) modified polyurethane were designed to mediate the mineralization through in situ enzyme activation for bone therapy. In this study, alkaline phosphatase (ALP) was harbored in the porous BVs by adsorption (Ad-BVs) or entrapment (En-BVs). High encapsulation of ALP on En-BVs was effectively self-activating by calcium ions of CaGP-modified PU that specifically hydrolyzed the organophosphorus (CaGP) to inorganic phosphate, thus promoting the formation of the highly oriented bone-like apatite in vitro. Enzyme-catalyzed kinetics confirms the regulation of apatite crystallization by the synergistic action of self-activated ALP and the confined microcompartments of BVs. This leads to a supersaturated microenvironment, with the En-BVs group exhibiting inorganic phosphate (Pi) levels 4.19 times higher and Ca2+ levels 3.67 times higher than those of simulated body fluid (SBF). Of note, the En-BVs group exhibited excellent osteo-inducing differentiation of BMSCs in vitro and the highest maturity with reduced bone loss in rat femoral defect in vivo. This innovative strategy of biomimetic vesicles is expected to provide valuable insights into the enzyme-activated field of bone therapy.

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.

Electron transparent nanotubes reveal crystallization pathways in confinement

The cylindrical pores of track-etched membranes offer excellent environments for studying the effects of confinement on crystallization as the pore diameter is readily varied and the anisotropic morphologies can direct crystal orientation. However, the inability to image individual crystals in situ within the pores in this system has prevented many of the underlying mechanisms from being characterized. Here, we study the crystallization of calcium sulfate within track-etched membranes and reveal that oriented gypsum forms in 200 nm diameter pores, bassanite in 25-100 nm pores and anhydrite in 10 nm pores. The crystallization pathways are then studied by coating the membranes with an amorphous titania layer prior to mineralization to create electron transparent nanotubes that protect fragile precursor materials. By visualizing the evolutionary pathways of the crystals within the pores we show that the product single crystals derive from multiple nucleation events and that orientation is determined at early reaction times. Finally, the transformation of bassanite to gypsum within the membrane pores is studied using experiment and potential mean force calculations and is shown to proceed by localized dissolution/reprecipitation. This work provides insight into the effects of confinement on crystallization processes, which is relevant to mineral formation in many real-world environments.

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

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

Intrafibrillar Mineralization and Immunomodulatory for Synergetic Enhancement of Bone Regeneration via Calcium Phosphate Nanocluster Scaffold

Inspired by the bionic mineralization theory, organic-inorganic composites with hydroxyapatite nanorods orderly arranged along collagen fibrils have attracted extensive attention. Planted with an ideal bone scaffold will contribute greatly to the osteogenic microenvironment; however, it remains challenging to develop a biomimetic scaffold with the ability to promote intrafibrillar mineralization and simultaneous regulation of immune microenvironment in situ. To overcome these challenges, a scaffold containing ultra-small particle size calcium phosphate nanocluster (UsCCP) is prepared, which can enhance bone regeneration through the synergetic effect of intrafibrillar mineralization and immunomodulatory. By efficient infiltration into collagen fibrils, the UsCCP released from the scaffold achieves intrafibrillar mineralization. It also promotes the M2-type polarization of macrophages, leading to an immune microenvironment with both osteogenic and angiogenic potential. The results confirm that the UsCCP scaffold has both intrafibrillar mineralization and immunomodulatory effects, making it a promising candidate for bone regeneration.

Cooperative Calcium Phosphate Deposition on Collagen-Inspired Short Peptide Nanofibers for Application in Bone Tissue Engineering

In recent years, immense attention has been devoted over the production of osteoinductive materials. To this direction, collagen has a dominant role in developing hard tissues and plays a crucial role in the biomineralization of these tissues. Here, we demonstrated for the first time the potential of the shortest molecular pentapeptide domain inspired from collagen toward mineralizing hydroxyapatite on peptide fibers to develop bone-filling material. Our simplistic approach adapted the easy and facile route of introducing the metal ions onto the peptide nanofibers, displaying adsorbed glutamate onto the surface. This negatively charged surface further induces the nucleation of the crystalline growth of hydroxyapatite. Interestingly, nucleation and growth of the hydroxyapatite crystals lead to the formation of a self-supporting hydrogel to construct a suitable interface for cellular interactions. Furthermore, microscopic and spectroscopic investigations revealed the crystalline growth of the hydroxyapatite onto peptide fibers. The physical properties were also influenced by this crystalline deposition, as evident from the hierarchical organization leading to hydrogels with enhanced mechanical stiffness and improved thermal stability of the scaffold. Furthermore, the mineralized peptide fibers were highly compatible with osteoblast cells and showed increased cellular biomarkers production, which further reinforced the potential application toward effectively fabricating the grafts for bone tissue engineering.

Effects of an orthodontic primer containing amorphous fluorinated calcium phosphate nanoparticles on enamel white spot lesions

OBJECTIVES

The study investigated the effects of an orthodontic primer containing amorphous fluorinated calcium phosphate (AFCP) nanoparticles on enamel white spot lesions (WSLs).

MATERIALS AND METHODS

The AFCP nanoparticles were prepared and incorporated into Transbond XT Primer. Thirty-two human enamel slices were highly polished and randomly divided into four groups: no part covered (control), half covered with a primer containing 0 wt%, 25 wt%, and 35 wt% AFCP. Subsequently, samples were challenged by a modified pH-cycling and characterized by color measurement, micro-computed tomography, and scanning electron microscope (SEM). The bonding properties of the primers containing AFCP were assessed using shear bond strength test, and the mouse fibroblasts (L929) were employed to evaluate the cytotoxicity.

RESULTS

When the enamel was challenged by pH cycling, 25 wt% and 35 wt% AFCP groups exhibited less color change (ΔE) and less mineral loss than the control and 0 wt% AFCP groups. The SEM images showed that the original microstructural integrity and mineral deposition rate of the enamel surface were better in the 25 wt% and 35 wt% AFCP groups. In particular, the 35 wt% AFCP group exhibited the best performance after 3 weeks of pH cycling. The shear bond strength and cell viability revealed no significant difference among the tested groups (P > 0.05).

CONCLUSION

Using the primer containing 35 wt% AFCP might be a promising strategy for preventing the occurrence and development of WSLs during orthodontic treatment.

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

Heterotopic ossification (HO) is a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues. Despite being a frequent complication of orthopedic and trauma surgery, brain and spinal injury, the etiology of HO is poorly understood. The aim of this study is to evaluate the hypothesis that a sustained local ionic homeostatic imbalance (SLIHI) created by mineral formation during tissue calcification modulates inflammation to trigger HO. This evaluation also considers the role SLIHI could play for the design of cell-free, drug-free osteoinductive bone graft substitutes. The evaluation contains five main sections. The first section defines relevant concepts in the context of HO and provides a summary of proposed causes of HO. The second section starts with a detailed analysis of the occurrence and involvement of calcification in HO. It is followed by an explanation of the causes of calcification and its consequences. This allows to speculate on the potential chemical modulators of inflammation and triggers of HO. The end of this second section is devoted to in vitro mineralization tests used to predict the ectopic potential of materials. The third section reviews the biological cascade of events occurring during pathological and material-induced HO, and attempts to propose a quantitative timeline of HO formation. The fourth section looks at potential ways to control HO formation, either acting on SLIHI or on inflammation. Chemical, physical, and drug-based approaches are considered. Finally, the evaluation finishes with a critical assessment of the definition of osteoinduction. STATEMENT OF SIGNIFICANCE: The ability to regenerate bone in a spatially controlled and reproducible manner is an essential prerequisite for the treatment of large bone defects. As such, understanding the mechanism leading to heterotopic ossification (HO), a condition triggered by an injury leading to the formation of mature lamellar bone in extraskeletal soft tissues, would be very useful. Unfortunately, the mechanism(s) behind HO is(are) poorly understood. The present study reviews the literature on HO and based on it, proposes that HO can be caused by a combination of inflammation and calcification. This mechanism helps to better understand current strategies to prevent and treat HO. It also shows new opportunities to improve the treatment of bone defects in orthopedic and dental procedures.

Polyethylene terephthalate textile heart valve: How poly(ethylene glycol) grafting limits fibrosis

Transcatheter aortic valve replacement (TAVR) is an alternative technique to surgical valve replacement for over 300,000 patients worldwide. The valve material used in the TAVR is made of biological tissues, whose durability remains unknown. The success of the TAVR favors the research toward synthetic valve leaflet materials as an alternative to biological tissues. In particular, polyethylene terephthalate (PET) textile valves have recently proven durability over a 6-month period in animal sheep models. Excessive fibrotic tissue formation remains, however, a critical issue to be addressed. The aim of this work was therefore to investigate the potential of PET textiles covalently conjugated with polyethylene glycol (PEG), known for its antifouling properties, to modulate the fibrosis formation both in vitro and in vivo. For this purpose, the surfaces of heart valves made of PET textiles were functionalized with an atmospheric pressure plasma, leading to the formation of carboxylic acid (COOH) groups, further used for PEG-NH₂ conjugation. Surface modification efficiency was assessed by X-ray photoelectron spectroscopy and water contact angle measurements. The biological behavior of the as-modified surfaces was evaluated by in vitro assays, using rat cardiac fibroblast cells. The results show that PEG treated substrates restrained the fibroblasts adhesion and proliferation. The PEG treated valve, implanted in a juvenile sheep model, showed a significant fibrosis reduction. The explant also revealed calcification issues that need to be addressed.

Natural bone-mimicking nanopore-incorporated hydroxyapatite scaffolds for enhanced bone tissue regeneration

Background

A considerable number of studies has been carried out to develop alloplastic bone graft materials such as hydroxyapatite (HAP) that mimic the hierarchical structure of natural bones with multiple levels of pores: macro-, micro-, and nanopores. Although nanopores are known to play many essential roles in natural bones, only a few studies have focused on HAPs containing them; none of those studies investigated the functions of nanopores in biological systems.

Method

We developed a simple yet powerful method to introduce nanopores into alloplastic HAP bone graft materials in large quantities by simply pressing HAP nanoparticles and sintering them at a low temperature.

Results

The size of nanopores in HAP scaffolds can be controlled between 16.5 and 30.2 nm by changing the sintering temperature. When nanopores with a size of ~ 30.2 nm, similar to that of nanopores in natural bones, are introduced into HAP scaffolds, the mechanical strength and cell proliferation and differentiation rates are significantly increased. The developed HAP scaffolds containing nanopores (SNPs) are biocompatible, with negligible erythema and inflammatory reactions. In addition, they enhance the bone regeneration when are implanted into a rabbit model. Furthermore, the bone regeneration efficiency of the HAP-based SNP is better than that of a commercially available bone graft material.

Conclusion

Nanopores of HAP scaffolds are very important for improving the bone regeneration efficiency and may be one of the key factors to consider in designing highly efficient next-generation alloplastic bone graft materials.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40824-022-00253-x.

Classical and Nonclassical Nucleation and Growth Mechanisms for Nanoparticle Formation

All solid materials are created via nucleation. In this evolutionary process, nuclei form in solution or at interfaces and expand by monomeric growth, oriented attachment, and phase transformation. Nucleation determines the location and size of nuclei, whereas growth controls the size, shape, and aggregation of newly formed nanoparticles. These physical properties of nanoparticles can determine their functionalities, reactivities, and porosities, as well as their fate and transport. Recent advances in nanoscale analytical technologies allow in situ real-time observations, enabling us to uncover the molecular nature of nuclei and the critical controlling factors for nucleation and growth. Although a single theory cannot yet fully explain such evolving processes, we have started to better understand how both classical and nonclassical theories can work together, and we have begun to recognize the importance of connecting these theories. This review discusses the recent convergence of knowledge about the nucleation and the growth of nanoparticles. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Oriented Crystallization of Hydroxyapatite in Self-Assembled Peptide Fibrils as a Bonelike Material

Controlling oriented crystallization is key to producing bonelike composite materials with a well-organized structure. However, producing this type of composite material using synthetic biopolymers as scaffolds is challenging. Inspired by the molecular structure of collagen-I, a collagenlike peptide─(Pro-Hyp-Gly)₁₀ (POG10)─was designed to produce self-assembled fibrils that resemble the structure of collagen-I fibrils. In addition, the oriented mineralization of HAP crystals is formed in the fibrils that reproduces a bonelike material similar to collagen-I fibril mineralization. Unlike collagen-I fibrils, POG10 fibrils do not contain gap spaces. The molecular simulation results indicate that in addition to space confinement, the molecular field generated by POG10 can also confine the orientation of HAP, enriching our understanding of physical confinement and shedding light on the design of synthetic biopolymer scaffolds for bonelike material fabrication.

Enzymatic Approach in Calcium Phosphate Biomineralization: A Contribution to Reconcile the Physicochemical with the Physiological View

Biomineralization is the process by which organisms produce hard inorganic matter from soft tissues with outstanding control of mineral deposition in time and space. For this purpose, organisms deploy a sophisticated "toolkit" that has resulted in significant evolutionary innovations, for which calcium phosphate (CaP) is the biomineral selected for the skeleton of vertebrates. While CaP mineral formation in aqueous media can be investigated by studying thermodynamics and kinetics of phase transitions in supersaturated solutions, biogenic mineralization requires coping with the inherent complexity of biological systems. This mainly includes compartmentalization and homeostatic processes used by organisms to regulate key physiological factors, including temperature, pH and ion concentration. A detailed analysis of the literature shows the emergence of two main views describing the mechanism of CaP biomineralization. The first one, more dedicated to the study of in vivo systems and supported by researchers in physiology, often involves matrix vesicles (MVs). The second one, more investigated by the physicochemistry community, involves collagen intrafibrillar mineralization particularly through in vitro acellular models. Herein, we show that there is an obvious need in the biological systems to control both where and when the mineral forms through an in-depth survey of the mechanism of CaP mineralization. This necessity could gather both communities of physiologists and physicochemists under a common interest for an enzymatic approach to better describe CaP biomineralization. Both homogeneous and heterogeneous enzymatic catalyses are conceivable for these systems, and a few preliminary promising results on CaP mineralization for both types of enzymatic catalysis are reported in this work. Through them, we aim to describe the relevance of our point of view and the likely findings that could be obtained when adding an enzymatic approach to the already rich and creative research field dealing with CaP mineralization. This complementary approach could lead to a better understanding of the biomineralization mechanism and inspire the biomimetic design of new materials.

A Hydroxypropyl Methylcellulose Film Loaded with AFCP Nanoparticles for Inhibiting Formation of Enamel White Spot Lesions

Objective

This study investigated the effects of mineralizing film consisting of hydroxypropyl methylcellulose (HPMC) and amorphous fluorinated calcium phosphate (AFCP) nanoparticles on enamel white spot lesions (WSLs).

Material and Methods

The AFCP nanoparticles and mineralizing film were prepared via nanoprecipitation and solvent evaporation, respectively. They were characterized with Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma atomic emission spectrometry (ICP-AES), and fluoride ion selective electrode. Thirty-two human enamel slices (4 mm × 4 mm × 1.5 mm) were highly polished and randomly assigned to four groups: negative control (no treatment); pure HPMC film; mineralizing film; GC Tooth Mousse Plus^® (contains 10% CPP-ACP and 0.2% NaF). Subsequently, samples were challenged by a modified pH-cycling and characterized by color measurement, Micro-CT, SEM/EDX, and nanoindentation.

Results

The mineralizing film could sustain release of Ca, P and F ions over 24 h and maintain AFCP nanoparticles in metastable state over 8~12 h. During 4 weeks of pH cycling, the mineralizing film group exhibited least color change (∆E), mineral loss and lesion depth (120 ± 10 µm) among four groups (p < 0.05). SEM findings revealed that the porosities among enamel crystals increased in negative control and pure HPMC film groups after pH cycling, whereas in mineralizing film group, the original microstructure of enamel was well conserved and mineral deposits were detected between enamel prisms. Mineralizing film group demonstrated a least reduction of nanomechanical properties such as elastic modulus of 77.02 ± 6.84 GPa and hardness of 3.62 ± 0.57 GPa (p < 0.05).

Conclusion

The mineralizing film might be a promising strategy for prevention and management of WSLs via inhibiting enamel demineralization and promoting enamel remineralization.

Self-assembling systems comprising intrinsically disordered protein polymers like elastin-like recombinamers

Despite lacking cooperatively folded structures under native conditions, numerous intrinsically disordered proteins (IDPs) nevertheless have great functional importance. These IDPs are hybrids containing both ordered and intrinsically disordered protein regions (IDPRs), the structure of which is highly flexible in this unfolded state. The conformational flexibility of these disordered systems favors transitions between disordered and ordered states triggered by intrinsic and extrinsic factors, folding into different dynamic molecular assemblies to enable proper protein functions. Indeed, prokaryotic enzymes present less disorder than eukaryotic enzymes, thus showing that this disorder is related to functional and structural complexity. Protein-based polymers that mimic these IDPs include the so-called elastin-like polypeptides (ELPs), which are inspired by the composition of natural elastin. Elastin-like recombinamers (ELRs) are ELPs produced using recombinant techniques and which can therefore be tailored for a specific application. One of the most widely used and studied characteristic structures in this field is the pentapeptide (VPGXG)_n . The structural disorder in ELRs probably arises due to the high content of proline and glycine in the ELR backbone, because both these amino acids help to keep the polypeptide structure of elastomers disordered and hydrated. Moreover, the recombinant nature of these systems means that different sequences can be designed, including bioactive domains, to obtain specific structures for each application. Some of these structures, along with their applications as IDPs that self-assemble into functional vesicles or micelles from diblock copolymer ELRs, will be studied in the following sections. The incorporation of additional order- and disorder-promoting peptide/protein domains, such as α-helical coils or β-strands, in the ELR sequence, and their influence on self-assembly, will also be reviewed. In addition, chemically cross-linked systems with controllable order-disorder balance, and their role in biomineralization, will be discussed. Finally, we will review different multivalent IDPs-based coatings and films for different biomedical applications, such as spatially controlled cell adhesion, osseointegration, or biomaterial-associated infection (BAI).

Intraosseous Injection of Calcium Phosphate Polymer-Induced Liquid Precursor Increases Bone Density and Improves Early Implant Osseointegration in Ovariectomized Rats

PURPOSE

Osteoporosis, due to bone loss and structural deterioration, is a risk factor for dental implant failure, as it impedes initial stability and osseointegration. We aim to assess the effects of calcium phosphate polymer-induced liquid precursor (CaP-PILP) treatment, which significantly increases bone density and improves early implant osseointegration in ovariectomized rats.

METHODS

In this study, CaP-PILP was synthesized and characterized through TEM, FTIR and XRD. A rat model of osteoporosis was generated by ovariectomy. CaP-PILP or hydroxyapatite (HAP, negative control) was injected into the tibia, and the resulting changes in bone quality were determined. Further, implants were installed in the treated tibias, and implantation characteristics were assessed after 4 weeks.

RESULTS

The CaP-PILP group had superior bone repair. Importantly, CaP-PILP had excellent properties, similar to those of normal bone, in terms of implant osseointegration. In vivo experiment displayed that CaP-PILP group had better bone contact rate (65.97±3.176) than HAP and OVX groups. Meanwhile, a mound of mature and continuous new bone formed. Moreover, the values of BIC and BA showed no significant difference between the CaP-PILP group and the sham group.

CONCLUSION

In summary, CaP-PILP is a promising material for application in poor-quality bones to improve implant success rates in patients with osteoporosis. This research provides new perspectives on the application of nano-apatite materials in bone repair.

Embracing Mechanobiology in Next Generation Organ-On-A-Chip Models of Bone Metastasis

Bone metastasis in breast cancer is associated with high mortality. Biomechanical cues presented by the extracellular matrix play a vital role in driving cancer metastasis. The lack of in vitro models that recapitulate the mechanical aspects of the in vivo microenvironment hinders the development of novel targeted therapies. Organ-on-a-chip (OOAC) platforms have recently emerged as a new generation of in vitro models that can mimic cell-cell interactions, enable control over fluid flow and allow the introduction of mechanical cues. Biomaterials used within OOAC platforms can determine the physical microenvironment that cells reside in and affect their behavior, adhesion, and localization. Refining the design of OOAC platforms to recreate microenvironmental regulation of metastasis and probe cell-matrix interactions will advance our understanding of breast cancer metastasis and support the development of next-generation metastasis-on-a-chip platforms. In this mini-review, we discuss the role of mechanobiology on the behavior of breast cancer and bone-residing cells, summarize the current capabilities of OOAC platforms for modeling breast cancer metastasis to bone, and highlight design opportunities offered by the incorporation of mechanobiological cues in these platforms.

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.

Citrate Stabilizes Hydroxylapatite Precursors: Implications for Bone Mineralization

Mineralization of hydroxylapatite (HAp), the main inorganic phase in bone, follows nonclassical crystallization routes involving metastable precursors and is strongly influenced by organic macromolecules. However, the effect of small organic molecules such as citrate on the formation of HAp is not well constrained. Using potentiometric titration experiments and titration calorimetry, in combination with a multianalytical approach, we show that citrate stabilizes prenucleation species as well as a liquid-like calcium phosphate precursor formed before any solid phase nucleates in the system. The stabilization of a liquid-like precursor phase could facilitate infiltration into the cavities of the collagen fibrils during bone mineralization, explaining the enhancement of collagen-mediated mineralization by citrate reported in previous studies. Hence, citrate can influence bone mineralization way before any solid phase (amorphous or crystalline) is formed. We also show that HAp formation after amorphous calcium phosphate (ACP) in the absence and presence of citrate results in nanoplates of about 5-12 nm thick, elongated along the c axis. Such nanoplates are made up of HAp nanocrystallites with a preferred c axis orientation and with interspersed ACP. The nanoplatelet morphology, size, and preferred crystallographic orientation, remarkably similar to those of bone HAp nanocrystals, appear to be an intrinsic feature of HAp formed from an amorphous precursor. Our results challenge current models for HAp mineralization in bone and the role of citrate, offering new clues to help answer the long-standing question as to why natural evolution favored HAp as the mineral phase in bone.

An Ammonia-Induced Calcium Phosphate Nanostructure: A Potential Assay for Studying Osteoporosis and Bone Metastasis

Osteoclastic resorption of bones plays a central role in both osteoporosis and bone metastasis. A reliable in vitro assay that simulates osteoclastic resorption in vivo would significantly speed up the process of developing effective therapeutic solutions for those diseases. Here, we reported the development of a novel and robust nanostructured calcium phosphate coating with unique functions on the track-etched porous membrane by using an ammonia-induced mineralization (AiM) technique. The calcium phosphate coating uniformly covers one side of the PET membrane, enabling testing for osteoclastic resorption. The track-etched pores in the PET membrane allow calcium phosphate mineral pins to grow inside, which, on the one hand, enhances coating integration with a membrane substrate and, on the other hand, provides diffusion channels for delivering drugs from the lower chamber of a double-chamber cell culture system. The applications of the processed calcium phosphate coating were first demonstrated as a drug screening device by using alendronate, a widely used drug for osteoporosis. It was confirmed that the delivery of alendronate significantly decreased both the number of monocyte-differentiated osteoclasts and coating resorption. To demonstrate the application in studying bone metastasis, we delivered a PC3 prostate cancer-conditioned medium and confirmed that both the differentiation of monocytes into osteoclasts and the osteoclastic resorption of the calcium phosphate coating were significantly enhanced. This novel assay thus provides a new platform for studying osteoclastic activities and assessing drug efficacy in vitro.

Electronic effects of nano-confinement in functional organic and inorganic materials for optoelectronics

This review provides a comprehensive overview of the electronic effects of nano-confinement (from 1D to 3D geometries) on optoelectronic materials and their applications.

Intermolecular channels direct crystal orientation in mineralized collagen

The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO₃ and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

Cell Scaffolds for Bone Tissue Engineering

Currently, well-known surgical procedures for bone defects are classified into four types: (1) autogenous bone graft transplantation, (2) allogeneic bone graft transplantation, (3) xenogeneic bone graft transplantation, and (4) artificial bone graft transplantation. However, they are often risky procedures and related to postoperative complications. As an alternative, tissue engineering to regenerate new bone often involves the use of mesenchymal stem cells (MSCs), derived from bone marrow, adipose tissues, and so on, which are cultured into three-dimensional (3D) scaffolds to regenerate bone tissue by osteoinductive signaling. In this manuscript, we provide an overview of recent treatment of bone defects and the studies on the creation of cell scaffolds for bone regeneration. Bone regeneration from bone marrow-derived mesenchymal stem cells using silica nonwoven fabric by the authors' group were provided. Potential application and future direction of the present systems were also described.

Biological stenciling of mineralization in the skeleton: Local enzymatic removal of inhibitors in the extracellular matrix

Biomineralization is remarkably diverse and provides myriad functions across many organismal systems. Biomineralization processes typically produce hardened, hierarchically organized structures usually having nanostructured mineral assemblies that are formed through inorganic-organic (usually protein) interactions. Calcium‑carbonate biomineral predominates in structures of small invertebrate organisms abundant in marine environments, particularly in shells (remarkably it is also found in the inner ear otoconia of vertebrates), whereas calcium-phosphate biomineral predominates in the skeletons and dentitions of both marine and terrestrial vertebrates, including humans. Reconciliation of the interplay between organic moieties and inorganic crystals in bones and teeth is a cornerstone of biomineralization research. Key molecular determinants of skeletal and dental mineralization have been identified in health and disease, and in pathologic ectopic calcification, ranging from small molecules such as pyrophosphate, to small membrane-bounded matrix vesicles shed from cells, and to noncollagenous extracellular matrix proteins such as osteopontin and their derived bioactive peptides. Beyond partly knowing the regulatory role of the direct actions of inhibitors on vertebrate mineralization, more recently the importance of their enzymatic removal from the extracellular matrix has become increasingly understood. Great progress has been made in deciphering the relationship between mineralization inhibitors and the enzymes that degrade them, and how adverse changes in this physiologic pathway (such as gene mutations causing disease) result in mineralization defects. Two examples of this are rare skeletal diseases having osteomalacia/odontomalacia (soft bones and teeth) - namely hypophosphatasia (HPP) and X-linked hypophosphatemia (XLH) - where inactivating mutations occur in the gene for the enzymes tissue-nonspecific alkaline phosphatase (TNAP, TNSALP, ALPL) and phosphate-regulating endopeptidase homolog X-linked (PHEX), respectively. Here, we review and provide a concept for how existing and new information now comes together to describe the dual nature of regulation of mineralization - through systemic mineral ion homeostasis involving circulating factors, coupled with molecular determinants operating at the local level in the extracellular matrix. For the local mineralization events in the extracellular matrix, we present a focused concept in skeletal mineralization biology called the Stenciling Principle - a principle (building upon seminal work by Neuman and Fleisch) describing how the action of enzymes to remove tissue-resident inhibitors defines with precision the location and progression of mineralization.

Crystallization in Confinement

Many crystallization processes of great importance, including frost heave, biomineralization, the synthesis of nanomaterials, and scale formation, occur in small volumes rather than bulk solution. Here, the influence of confinement on crystallization processes is described, drawing together information from fields as diverse as bioinspired mineralization, templating, pharmaceuticals, colloidal crystallization, and geochemistry. Experiments are principally conducted within confining systems that offer well-defined environments, varying from droplets in microfluidic devices, to cylindrical pores in filtration membranes, to nanoporous glasses and carbon nanotubes. Dramatic effects are observed, including a stabilization of metastable polymorphs, a depression of freezing points, and the formation of crystals with preferred orientations, modified morphologies, and even structures not seen in bulk. Confinement is also shown to influence crystallization processes over length scales ranging from the atomic to hundreds of micrometers, and to originate from a wide range of mechanisms. The development of an enhanced understanding of the influence of confinement on crystal nucleation and growth will not only provide superior insight into crystallization processes in many real-world environments, but will also enable this phenomenon to be used to control crystallization in applications including nanomaterial synthesis, heavy metal remediation, and the prevention of weathering.

Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures

Materials science has been informed by nonclassical pathways to crystallization, based on biological processes, about the fabrication of damage-tolerant composite materials. Various biomineralizing taxa, such as stony corals, deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further assemble and transform into higher-order mineral structures. Here, we examine a similar process in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of high-resolution imaging and in situ solid-state nuclear magnetic resonance spectroscopy, we reveal the underlying mechanism of the solid-state phase transformation of these amorphous nanoparticles into crystals under aqueous conditions. These amorphous nanoparticles are covered by a hydration shell of bound water molecules. Fast chemical exchanges occur: the hydrogens present within the nanoparticles exchange with the hydrogens from the surface-bound H₂O molecules which, in turn, exchange with the hydrogens of the free H₂O molecule of the surrounding aqueous medium. This cascade of chemical exchanges is associated with an enhanced mobility of the ions/molecules that compose the nanoparticles which, in turn, allow for their rearrangement into crystalline domains via solid-state transformation. Concurrently, the starting amorphous nanoparticles aggregate and form ordered mineral structures through crystal growth by particle attachment. Sphere-like aggregates and spindle-shaped structures were, respectively, formed from relatively high or low weights per volume of the same starting amorphous nanoparticles. These results offer promising prospects for exerting control over such a nonclassical pathway to crystallization to design mineral structures that could not be achieved through classical ion-by-ion growth.

Enzyme-assisted mineralization of calcium phosphate: exploring confinement for the design of highly crystalline nano-objects

In hard tissues of vertebrates, calcium phosphate (CaP) biomineralization is a fascinating process that combines specific physicochemical and biochemical reactions, resulting in the formation of extracellular matrices with elegant nanoarchitectures. Although several "biomimetic" strategies have been developed for the design of mineralized nanostructured biointerfaces, the control of the crystallization process remains complex. Herein, we report an innovative approach to overcome this challenge by generating, in situ, CaP precursors in a confined medium. For this purpose, we explore a combination of (i) the layer-by-layer assembly, (ii) the template-based method and (iii) the heterogeneous enzymatic catalysis. We show the possibility of embedding active alkaline phosphatase in a nanostructured multilayered film and inducing the nucleation and growth of CaP compounds under different conditions. Importantly, we demonstrate that the modulation of the crystal phase from spheroid-shaped amorphous CaP to crystalline platelet-shaped hydroxyapatite depends on the degree of confinement of active enzymes. This leads to the synthesis of highly anisotropic mineralized nanostructures that are mechanically stable and with controlled dimensions, composition and crystal phase. The present study provides a straightforward, yet powerful, way to design anisotropic nanostructured materials, including a self-supported framework, which may be used in broad biomedical applications.

Tuning the c-Axis Orientation of Calcium Phosphate Hybrid Thin Films Using Polymer Templates

The orientation of the c-axis in octacalcium phosphate (OCP) nanocrystals that were incorporated into hybrid thin films was successfully tuned using poly(vinyl alcohol) (PVA) thin-film templates of varying thicknesses. This approach was inspired by biomineralization. Thicker PVA templates enhanced the c-axis orientation of the OCP crystals perpendicular to the substrate. Using this approach with a 900 nm thick PVA template, OCP/PVA hybrid thin films (1.8 μm thick) with a c-axis orientation perpendicular to the substrate were formed. Hydroxyapatite (HAP) hybrid thin films that also exhibited a perpendicular c-axis orientation were obtained through the topotactic transformation of the OCP/PVA hybrid thin films in aqueous solution. The thickness change of the polymer templates had a significant effect on the structure of the OCP nanocrystals in the hybrid thin films. The structural control of the OCP hybrid thin films that were formed through the biomineralization-inspired approach allowed the formation of HAP hybrid thin films with controlled structures.

Composite Materials Design: Biomineralization Proteins and the Guided Assembly and Organization of Biomineral Nanoparticles

There has been much discussion of the role of proteins in the calcium carbonate biomineralization process, particularly with regard to nucleation, amorphous stabilization/transformation, and polymorph selection. However, there has been little if any discussion of the potential role that proteins might play in another important process: the guided assembly and organization of mineral nanoparticles into higher-ordered structures such as mesocrystals. This review discusses particle attachment theory and recent evidence of mineral-associated proteins forming hydrogels that assemble and organize mineral clusters into crystalline phase. From this discussion we postulate a mechanism by which biomineralization protein hydrogel aggregation assists in mineral nanoparticle assembly and organization within calcium carbonate skeletal elements and discuss potentials ways for harnessing this process in materials design.

Organization of Bone Mineral: The Role of Mineral–Water Interactions

The mechanism (s) that drive the organization of bone mineral throughout the bone extracellular matrix remain unclear. The long-standing theory implicates the organic matrix, namely specific non-collagenous proteins and/or collagen fibrils, while a recent theory proposes a self-assembly mechanism. Applying a combination of spectroscopic and microscopic techniques in wet and dry conditions to bone-like hydroxyapatite nanoparticles that were used as a proxy for bone mineral, we confirm that mature bone mineral particles have the capacity to self-assemble into organized structures. A large quantity of water is present at the surface of bone mineral due to the presence of a hydrophilic, amorphous surface layer that coats bone mineral nanoparticles. These water molecules must not only be strongly bound to the surface of bone mineral in the form of a rigid hydration shell, but they must also be trapped within the amorphous surface layer. Cohesive forces between these water molecules present at the mineral–mineral interface not only hold the mature bone mineral particles together, but also promote their oriented stacking. This intrinsic ability of mature bone mineral particles to organize themselves without recourse to the organic matrix forms the foundation for the development of the next generation of orthopedic biomaterials.

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.

Destabilised human transthyretin shapes the morphology of calcium carbonate crystals

Human transthyretin (TTR) is a homotetramer that transports thyroid hormones and retinol in the serum and cerebrospinal fluid. TTR is also an intracellular protein found in tissues such as those in the brain, eye and pancreas. TTR is a nutrition marker, reflecting the health of the organism, and TTR levels are linked to the normal and diseased states of the body. The switch from a protective to a pathological role is attributed to the destabilisation of the TTR structure, which leads to tetramer dissociation and amyloid formation. Native and destabilised TTR have been associated with osteoarthritis and bone density in humans. Moreover, TTR is present in eggshell mammillary cones; therefore, we verified the putative TTR engagement in the process of mineral formation. Using an in vitro assay, we found that TTR affected calcium carbonate crystal growth and morphology, producing asymmetric crystals with a complex nanocrystalline composition. The crystals possessed rounded edges and corners and irregular etch pits, suggesting the selective inhibition of crystal growth and/or dissolution imposed by TTR. The occurrence of many porosities, fibrillary inclusions and amorphous precipitates suggested that destabilisation of the TTR structure is an important factor involved in the mineralisation process. Crystals grown in the presence of TTR exhibited the characteristic features of crystals controlled by biomineralisation-active proteins, suggesting novel functions of TTR in the mineral formation process.

Bioinspired Structural Hierarchy within Macroscopic Volumes of Synthetic Composites

A key challenge in developing bioinspired composites is the fabrication of well-defined 3D hierarchical structures ranging from nano to the macroscale. Herein, the development of a synthetic polymer-apatite composite realized by integrating bottom-up self-assembly and additive manufacturing (AM) is described. The resulting composite exhibits a bioinspired hierarchical structure over its 3D macroscopic volume. The composite is assembled in a bottom-up manner, where periodic nanoscale assemblies of organic micellar fibrils and inorganic apatite nanocrystals are organized as bundles of mineralized microstructures. These microstructural bundles are preferentially oriented throughout the macroscopic volume of the material via extrusion based AM. The obtained structural hierarchy is investigated in 3D using electron microscopy and small angle X-ray scattering tensor tomography and correlated to the structural hierarchy and anisotropy observed in biological tissues such as bone and the bone-cartilage interface. This work demonstrates the possibility to form polymer-apatite composites with a well-defined hierarchical structure throughout its macroscopic volume, which is crucial for the development of mechanically optimized materials for applications such as bone and osteochondral implants.

Cavity Formation in Confined Growing Crystals

Growing crystals form a cavity when placed against a wall. The birth of the cavity is observed both by optical microscopy of sodium chlorate crystals (NaClO_{3}) growing in the vicinity of a glass surface, and in simulations with a thin film model. The cavity appears when growth cannot be maintained in the center of the contact region due to an insufficient supply of growth units through the liquid film between the crystal and the wall. We obtain a nonequilibrium morphology diagram characterizing the conditions under which a cavity appears. Cavity formation is a generic phenomenon at the origin of the formation of growth rims observed in many experiments, and is a source of complexity for the morphology of growing crystals in natural environments. Our results also provide restrictions for the conditions under which compact crystals can grow in confinement.

Confinement generates single-crystal aragonite rods at room temperature

The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph.

Effects of Molecular Weight and Concentration of Poly(Acrylic Acid) on Biomimetic Mineralization of Collagen

Inspired by nature, poly(acrylic acid) (PAA) and other polyelectrolytes have been used as noncollagenous proteins (NCPs) surrogates for biomimetic intrafibrillar mineralization of collagen fibrils and thus, to model the ultrastructure of bone, to study the mechanism of bone mineralization and, more scarcely to fabricate scaffolds for hard tissue engineering. The objective of this study was to systematically investigate the effect of the molecular weight (MW) and the concentration of PAA on the rate and pattern of biomineralization of collagen matrices. Densified type I collagen films were mineralized in supersaturated PAA-stabilized amorphous calcium-phosphate (PAA-ACP) solutions containing increasing MW (2 kDa, 50 kDA, 450 kDa) and concentrations (10, 25, 50 mg/L) of PAA up to 7 days. The stability and physical properties of collagen-free PAA-ACP solutions were also investigated. In our system, lowering PAA MW and increasing PAA concentration resulted in solutions with increasing stability. Over stable PAA-ACP solutions that fully inhibited mineralization of the collagen matrices were achieved using PAA 2k-50. Conversely, unstable solutions were obtained using high PAA MW at low concentrations. Nucleation and growth of significant amount of extrafibrillar minerals on the collagen fibrils was obtained using these solutions. In a wide range of combined MW and concentration of PAA we obtained intrafibrillar mineralization of collagen with hydroxyapatite crystals aligned parallel to the collagen fibril as in natural tissues. Intrafibrillar mineralization was correlated with PAA-ACP stability and growth of the PAA-ACP particles in solution. Our results support using PAA to surrogate NCPs function as selective inhibitors or promoters of biological mineralization and provide parameters to manufacture new biomimetic scaffolds and constructs for bone and dentin tissue engineering.

The role of confined collagen geometry in decreasing nucleation energy barriers to intrafibrillar mineralization

Mineralization of collagen is critical for the mechanical functions of bones and teeth. Calcium phosphate nucleation in collagenous structures follows distinctly different patterns in highly confined gap regions (nanoscale confinement) than in less confined extrafibrillar spaces (microscale confinement). Although the mechanism(s) driving these differences are still largely unknown, differences in the free energy for nucleation may explain these two mineralization behaviors. Here, we report on experimentally obtained nucleation energy barriers to intra- and extrafibrillar mineralization, using in situ X-ray scattering observations and classical nucleation theory. Polyaspartic acid, an extrafibrillar nucleation inhibitor, increases interfacial energies between nuclei and mineralization fluids. In contrast, the confined gap spaces inside collagen fibrils lower the energy barrier by reducing the reactive surface area of nuclei, decreasing the surface energy penalty. The confined gap geometry, therefore, guides the two-dimensional morphology and structure of bioapatite and changes the nucleation pathway by reducing the total energy barrier.

Passive Picoinjection Enables Controlled Crystallization in a Droplet Microfluidic Device

Segmented flow microfluidic devices offer an attractive means of studying crystallization processes. However, while they are widely employed for protein crystallization, there are few examples of their use for sparingly soluble compounds due to problems with rapid device fouling and irreproducibility over longer run-times. This article presents a microfluidic device which overcomes these issues, as this is constructed around a novel design of "picoinjector" that facilitates direct injection into flowing droplets. Exploiting a Venturi junction to reduce the pressure within the droplet, it is shown that passive injection of solution from a side-capillary can be achieved in the absence of an applied electric field. The operation of this device is demonstrated for calcium carbonate, where highly reproducible results are obtained over long run-times at high supersaturations. This compares with conventional devices that use a Y-junction to achieve solution loading, where in-channel precipitation of calcium carbonate occurs even at low supersaturations. This work not only opens the door to the use of microfluidics to study the crystallization of low solubility compounds, but the simple design of a passive picoinjector will find wide utility in areas including multistep reactions and investigation of reaction dynamics.

Synchrotron FTIR mapping of mineralization in a microfluidic device

Fourier transform infrared micro-spectroscopy provides an effective means of performing rapid, non-destructive, and label-free analysis of specimens according to their vibrational modes. However, as water absorbs very strongly in the infrared region, analysis of aqueous solutions in transmission mode can suffer from problems with signal saturation. We here describe the fabrication of a novel microfluidic device that overcomes this problem. Devices with channel depths of just 3 μm were constructed from calcium fluoride using photolithography and hot embossing bonding, where calcium fluoride was selected due to its transparency in the IR region. The utility of this device was then demonstrated by employing it to follow the precipitation pathways of calcium sulfate and calcium carbonate using synchrotron FTIR micro-spectroscopy. Importantly, due to the high brightness provided by synchrotron radiation, and the fact that the reacting ions (HCO₃^-, CO₃^2- and SO₄^2-) and the different mineral polymorphs all have finger print spectra in the measured IR range, this method can be used to acquire time-resolved, hyperspectral maps of the mineral particles formed within the sample cell, and then study the interaction and evolution of particles. The data provide new insight into the formation pathway of a population of crystals in confined volumes, and demonstrate that this in situ, real-time detection system provides a powerful tool for studying crystallization processes.

Intrafibrillar Mineralization of Self-Assembled Elastin-Like Recombinamer Fibrils

Biomineralization of bone, a controlled process where hydroxyapatite nanocrystals preferentially deposit in collagen fibrils, is achieved by the interplay of the collagen matrix and noncollagenous proteins. Mimicking intrafibrillar mineralization in synthetic systems is highly attractive for the development of advanced hybrid materials with elaborated morphologies and outstanding mechanical properties, as well as understanding the mechanisms of biomineralization. Inspired by nature, intrafibrillar mineralization of collagen fibrils has been successfully replicated in vitro via biomimetic systems, where acidic polymeric additives are used as analogue of noncollagenous proteins in mediating mineralization. The development of synthetic templates that mimic the structure and functions of collagenous matrix in mineralization has yet to be explored. In this study, we demonstrated that self-assembled fibrils of elastin-like recombinamers (ELRs) can induce intrafibrillar mineralization. The ELRs displayed a disordered structure at low temperature but self-assembled into nanofibrils above its inverse transition temperature. In the presence of the self-assembled ELR fibrils, polyaspartate-stabilized amorphous calcium phosphates preferentially infiltrated into the fibrils and then crystallized into hydroxyapatite nanocrystals with their [001] axes aligned parallel to the long axis of the ELR fibril. As the recombinant technology enables designing and producing well-defined ELRs, their molecular and structural properties can be fine-tuned. By examining the ultrastructure of the self-assembled ELRs fibrils as well as their mineralization, we concluded that the spatial confinement formed by a continuum β-spiral structure in an unperturbed fibrillar structure rather than electrostatic interactions or bioactive sequences in the recombinamer composition played the crucial role in inducing intrafibrillar mineralization.