Biomimetic mineralization of woven bone-like nanocomposites: role of collagen cross-links

Tunable polymer-peptide hybrids for dentin tissue repair

OBJECTIVES

This study targets to assess the remineralization capability of conditioned dentin infiltrated with polymeric nanoparticles (NPs) doped with tideglusib (TDg) (TDg-NPs).

METHODS

Dentin conditioned surfaces were infiltrated with NPs and TDg-NPs. Bonded interfaces were created, stored for 24 h and submitted to mechanical and thermal challenging. Resin-dentin interfaces were evaluated through nanohardness, Masson's trichrome staining microscopy, and Raman analysis.

RESULTS

Dentin surfaces treated with TDg-NPs and load cycled produced higher nanohardness than the rest of the groups at the hybrid layer. At the bottom of the hybrid layer, all samples treated with TDg-NPs showed higher nanohardness than the rest of the groups. Active remineralization underneath the hybrid layer was detected in all groups after TDg application and load cycling, inducting new dentinal tubuli formation. After thermocycling, remineralization at the hybrid layer was not evidenced in the absence of NPs. Raman analysis showed increase mineralization, enriched carbonate apatite formation, and improved crosslinking and scaffolding of the collagen.

CONCLUSIONS

Mechanical loading on the specimens obtained after TDg-NPs dentin infiltration inducts an increase of mineralization at the resin/dentin interface, indicating remineralization of peritubular and intertubular dentin with augmented crystallographic maturity in crystals. Enriched collagen quality was produced, generating an adequate matrix organization to promote apatite nucleation, after tideglusib infiltration.

CLINICAL SIGNIFICANCE

At the present research, it has been proved the creation of reparative dentin, at the resin-dentin interface, after tideglusib dentin infiltration. Chemical stability, to favor integrity of the resin-dentin interface, is warranted in the presence of the TDg-NPs in the demineralized dentin collagen.

Calcium phosphate nanoclusters modify periodontium remodeling and minimize orthodontic relapse

Orthodontic relapse is one of the most prevalent concerns of orthodontic therapy. Relapse results in patients' teeth reverting towards their pretreatment positions, which increases the susceptibility to functional problems, dental disease, and substantially increases the financial burden for retreatment. This phenomenon is thought to be induced by rapid remodeling of the periodontal ligament (PDL) in the early stages and poor bone quality in the later stages. Current therapies, including fixed or removable retainers and fiberotomies, have limitations with patient compliance and invasiveness. Approaches using biocompatible biomaterials, such as calcium phosphate polymer-induced liquid precursors (PILP), is an ideal translational approach for minimizing orthodontic relapse. Here, post-orthodontic relapse is reduced after a single injection of high concentration PILP (HC-PILP) nanoclusters by altering PDL remodeling in the early stage of relapse and improving trabecular bone quality in the later phase. HC-PILP nanoclusters are achieved by using high molecular weight poly aspartic acid (PASP, 14 kDa) and poly acrylic acid (PAA, 450 kDa), which resulted in a stable solution of high calcium and phosphate concentrations without premature precipitation. In vitro results show that HC-PILP nanoclusters prevented collagen type-I mineralization, which is essential for the tooth-periodontal ligament (PDL)-bone interphase. In vivo experiments show that the PILP nanoclusters minimize relapse and improve the trabecular bone quality in the late stages of relapse. Interestingly, PILP nanoclusters also altered the remodeling of the PDL collagen during the early stages of relapse. Further in vitro experiments showed that PILP nanoclusters alter the fibrillogenesis of collagen type-I by impacting the protein secondary structure. These findings propose a novel approach for treating orthodontic relapse and provide additional insight into the PILP nanocluster's structure and properties on collagenous structure repair.

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

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

Active Osteoblasts or Quiescent Bone Lining Cells? Preosteoblasts Fate Orchestrated by Curvature and Stiffness of an In Vitro 2.5D Biomimetic Culture System

Biomimetic cell culture systems are required to provide more physiologically relevant microenvironments for bone cells. Here, a simple 2.5D culture platform is proposed, combining adjustable stiffness and surface features that mimic bone topography by using sandpaper grits as master molds with two stiffness formulations of polydimethylsiloxane (PDMS). The subsequent replicas perfectly conform the grits and reproduce the corresponding negative relief with cavities separated by convex edges. Biomimicry is also provided by an extracellular matrix (ECM)-like thin film coating, using the layer-by-layer (LbL) method. The topographical features, alternating concave, and convex structures drive preosteoblasts organization and morphology. Strikingly, curvature orchestrates the commitment of preosteoblasts, with i) maturation to active osteoblasts able to produce a dense collagenous matrix that ultimately mineralizes in the cavities, and ii) edges hosting quiescent cells that synthetize a very thin immature collagen layer with no mineralization. In summary, the present in vitro culture system model offers a cell-instructive 2.5D microenvironment that controls preosteoblasts fate, leading to two coexisting subpopulations: mature osteoblasts and bone lining cells (BLC). This promising culture system opens new avenues to advanced tissue-engineered modeling and can be applied to precellularized bone biomaterials.

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

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

A variable mineralization time and solution concentration intervene in the microstructure of biomimetic mineralized collagen and potential osteogenic microenvironment

The absence of a conducive bone formation microenvironment between fractured ends poses a significant challenge in repairing large bone defects. A promising solution is to construct a bone formation microenvironment that mimics natural bone tissue. Biomimetic mineralized collagen possesses a chemical composition and microstructure highly similar to the natural bone matrix, making it an ideal biomimetic bone substitute material. The microstructure of biomimetic mineralized collagen is influenced by various factors, and its biomineralization and microstructure, in turn, affect its physicochemical properties and biological activity. We aimed to utilize mineralization time and solution concentration as variables and employed the polymer-induced liquid precursor strategy to fabricate mineralized collagen with diverse microstructures, to shed light on how mineralization parameters impact the material microstructure and physicochemical properties. We also investigated the influence of microstructure and physicochemical properties on cell biocompatibility and the bone-forming microenvironment. Through comprehensive characterization, we examined the physical and chemical properties of I-EMC under various mineralization conditions and assessed the in vitro and in vivo biocompatibility and osteogenic performance. By investigating the relationship between mineralization parameters, material physicochemical properties, and osteogenic performance, we revealed how microstructures influence cellular behaviors like biocompatibility and osteogenic microenvironment. Encouragingly, mineralization solutions with varying concentrations, stabilized by polyacrylic acid, successfully produced intrafibrillar and extrafibrillar mineralized collagen. Compared to non-mineralized collagen, all mineralized samples demonstrated improved bone-forming performance. Notably, samples prepared with a 1× mineralization solution exhibited relatively smooth surfaces with even mineralization. Extending the mineralization time enhanced the degree of mineralization and osteogenic performance. Conversely, samples prepared with a 2× mineralization solution had rough surfaces with large calcium phosphate particles, indicating non-uniform mineralization. Overall, our research advances the potential for commercial production of mineralized collagen protein products, characterized by dual biomimetic properties, and their application in treating various types of bone defects.

The mechanism of biomineralization: Progress in mineralization from intracellular generation to extracellular deposition

Biomineralization is a highly regulated process that results in the deposition of minerals in a precise manner, ultimately producing skeletal and dental hard tissues. Recent studies have highlighted the crucial role played by intracellular processes in initiating biomineralization. These processes involve various organelles, such as the endoplasmic reticulum(ER), mitochondria, and lysosomes, in the formation, accumulation, maturation, and secretion of calcium phosphate (CaP) particles. Particularly, the recent in-depth study of the dynamic process of the formation of amorphous calcium phosphate(ACP) precursors among organelles has made great progress in the development of the integrity of the biomineralization chain. However, the precise mechanisms underlying these intracellular processes remain unclear, and they cannot be fully integrated with the extracellular mineralization mechanism and the physicochemical structure development of the mineralization particles. In this review, we aim to focus on the recent progress made in understanding intracellular mineralization organelles' processes and their relationship with the physicochemical structure development of CaP and extracellular deposition of CaP particles.

NDGA enhances the physicochemical and anti-biodegradation performance of dentin collagen

OBJECTIVES

Collagen fibrils from carious dentin matrix are prone to enzymatic degradation. This study investigates the feasibility and mechanism of nordihydroguaiaretic acid (NDGA), as a collagen crosslinker, to bio-modify the demineralized dentin matrix.

METHODS

The physicochemical properties of the crosslinked dentin matrix were characterized by swelling ratio, ninhydrin assay, Fourier Transform Infrared spectroscopy, and atomic force microscopy. The collagenase degradation resistance was evaluated by measuring loss of dry mass, hydroproline release, loss of elasticity, and micro-nano structure integrity. The cytotoxicity of NDGA-crosslinked dentin collagen was evaluated by flow cytometry.

RESULTS

NDGA crosslinked dentin matrix without destroying the integrity of collagen. Mechanistically, NDGA formed bisquinone bond between two adjacent o-quinone groups, resulting in NDGA polymeric matrix in which collagen fibrils were embedded. NDGA modification could significantly enhance the stiffness of dentin matrix at macro-nano scale. The NDGA-crosslinked dentin matrix exhibited remarkably low collagen degradation and sustained bulk elasticity after collagenase challenge, which were attributed to decreased water content, physical masking of collagenase bind sites on collagen, and improved stiffness of collagen fibrils. Notably, NDGA-crosslinked dentin matrix exhibited excellent biocompatibility.

CONCLUSION

NDGA, as a biocompatible collagen crosslinker, improves the mechanical properties and biodegradation resistance of demineralized dentin matrix.

An engineered lamellar bone mimicking full-scale hierarchical architecture for bone regeneration

Lamellar bone, compactly and ingeniously organized in the hierarchical pattern with 6 ordered scales, is the structural motif of mature bone. Each hierarchical scale exerts an essential role in determining physiological behavior and osteogenic bioactivity of bone. Engineering lamellar bone with full-scale hierarchy remains a longstanding challenge. Herein, using bioskiving and mineralization, we attempt to engineer compact constructs resembling full-scale hierarchy of lamellar bone. Through systematically investigating the effect of mineralization on physicochemical properties and bioactivities of multi-sheeted collagen matrix fabricated by bioskiving, the hierarchical mimicry and hierarchy-property relationship are elucidated. With prolongation of mineralization, hierarchical mimicry and osteogenic bioactivity of constructs are performed in a bidirectional manner, i.e. first rising and then descending, which is supposed to be related with transformation of mineralization mechanism from nonclassical to classical crystallization. Construct mineralized 9 days can accurately mimic each hierarchical scale and efficiently promote osteogenesis. Bioinformatic analysis further reveals that this construct potently activates integrin α5-PI3K/AKT signaling pathway through mechanical and biophysical cues, and thereby repairing critical-sized bone defect. The present study provides a bioinspired strategy for completely resembling complex hierarchy of compact mineralized tissue, and offers a critical research model for in-depth studying the structure-function relationship of bone.

Bone-on-a-Chip: Biomimetic Models Based on Microfluidic Technologies for Biomedical Applications

With the increasing importance of preclinical evaluation of newly developed drugs or treatments, in vitro organ or disease models are necessary. Although various organ-specific on-chip (organ-on-a-chip, or OOC) systems have been developed as emerging in vitro models, bone-on-a-chip (BOC) systems that recapitulate the bone microenvironment have been less developed or reviewed compared with other OOCs. The bone is one of the most dynamic organs and undergoes continuous remodeling throughout its lifetime. The aging population is growing worldwide, and healthcare costs are rising rapidly. Since in vitro BOC models that recapitulate native bone niches and pathological features can be important for studying the underlying mechanism of orthopedic diseases and predicting drug responses in preclinical trials instead of in animals, the development of biomimetic BOCs with high efficiency and fidelity will be accelerated further. Here, we review recently engineered BOCs developed using various microfluidic technologies and investigate their use to model the bone microenvironment. We have also explored various biomimetic strategies based on biological, geometrical, and biomechanical cues for biomedical applications of BOCs. Finally, we addressed the limitations and challenging issues of current BOCs that should be overcome to obtain more acceptable BOCs in the biomedical and pharmaceutical industries.

Influence of Crosslinking Methods on Biomimetically Mineralized Collagen Matrices for Bone-like Biomaterials

To assist in bone defect repair, ideal bone regeneration scaffolds should exhibit good osteoconductivity and osteoinductivity, but for load-bearing applications, they should also have mechanical properties that emulate those of native bone. The use of biomimetic processing methods for the mineralization of collagen fibrils has resulted in interpenetrating composites that mimic the nanostructure of native bone; however, closely matching the mechanical properties of bone on a larger scale is something that is still yet to be achieved. In this study, four different collagen crosslinking methods (EDC-NHS, quercetin, methacrylated collagen, and riboflavin) are compared and combined with biomimetic mineralization via the polymer-induced liquid-precursor (PILP) process, to obtain bone-like collagen-hydroxyapatite composites. Densified fibrillar collagen scaffolds were fabricated, crosslinked, and biomimetically mineralized using the PILP process, and the effect of each crosslinking method on the degree of mineralization, tensile strength, and modulus of the mineralized scaffolds were analyzed and compared. Improved modulus and tensile strength values were obtained using EDC-NHS and riboflavin crosslinking methods, while quercetin and methacrylated collagen resulted in little to no increase in mechanical properties. Decreased mineral contents appear to be necessary for retaining tensile strength, suggesting that mineral content should be kept below a percolation threshold to optimize properties of these interpenetrating nanocomposites. This work supports the premise that a combination of collagen crosslinking and biomimetic mineralization methods may provide solutions for fabricating robust bone-like composites on a larger scale.

Biomimetic Mineralization of Fibrillar Collagen with Strontium-doped Hydroxyapatite

Fibrillar collagen structures mineralized with hydroxyapatite using the polymer-induced liquid precursor (PILP) process have been explored as synthetic models for studying biomineralization of human hard tissues and have also been applied in the fabrication of scaffolds for hard tissue regeneration. Strontium has important biological functions in bone and has been used as a therapeutic agent for treating diseases that result in bone defects, such as osteoporosis. Here, we developed a strategy to mineralize collagen with Sr-doped hydroxyapatite (HA) using the PILP process. Doping with Sr altered the crystal lattice of HA and inhibited the degree of mineralization in a concentration-dependent manner, but did not affect the unique formation of intrafibrillar minerals using the PILP. The Sr-doped HA nanocrystals were aligned in the [001] direction but did not recapitulate the parallel alignment of the c-axis of pure Ca HA in relation to the collagen fiber long axis. The mimicry of doping Sr in PILP-mineralized collagen can help understand the doping of Sr in natural hard tissues and during treatment. The fibrillary mineralized collagen with Sr-doped HA will be explored in future work as biomimetic and bioactive scaffolds for regeneration of bone and tooth dentin.

Injectable, Hierarchically Degraded Bioactive Scaffold for Bone Regeneration

Bioactive materials play vital roles in the repair of critical bone defects. However, bone tissue engineering and regenerative medicine are still challenged by the need to repair bone defects evenly and completely. In this study, we functionally simulated the natural creeping substitution process of autologous bone repair by constructing an injectable, hierarchically degradable bioactive scaffold with a composite hydrogel, decalcified bone matrix (DBM) particles, and bone morphogenetic protein 2. This composite scaffold exhibited superior mechanical properties. The scaffold promoted cell proliferation and osteogenic differentiation through multiple signaling pathways. The hierarchical degradation rates of the crosslinked hydrogel and DBM particles accelerated tissue ingrowth and bone formation with a naturally woven bone-like structure in vivo. In the rat calvarial critical defect repair model, the composite scaffold provided even and complete repair of the entire defect area while also integrating the new and host bone effectively. Our results indicate that this injectable, hierarchically degradable bioactive scaffold promotes bone regeneration and provides a promising strategy for evenly and completely repairing the bone defects.

Dexamethasone-doped nanoparticles improve mineralization, crystallinity and collagen structure of human dentin

OBJECTIVES

Bioactive materials have been used for functionalization of adhesives to promote dentin remineralization. This study aims to evaluate bonding ability and both mechanical and chemical behavior of demineralized dentin infiltrated with polymeric nanoparticles doped with dexamethasone (Dex-NPs).

METHODS

Dentin conditioned surfaces were infiltrated with NPs, Dex-NPs or Dex-Zn-NPs. Bonded interfaces were also created and stored for 24 h or 21d, and then submitted to microtensile bond strength testing. Dentin remineralization was analyzed by Nanohardness, Young's modulus and Raman analysis.

RESULTS

At 21d of storage, dentin treated with undoped-NPs attained the lowest nanohardness and Young's modulus. Dex-NPs and Zn-Dex-NPs increased dentin nanohardness and Young's modulus after 21d Raman analysis showed high remineralization, crystallinity, crosslinking and better structure of collagen when functionalized Dex-NPs were present at the dentin interface.

CONCLUSIONS

Infiltration of dentin with Dex-NPs promoted functional remineralization as proved by nanomechanical and morpho-chemical evaluation tests. Dexamethasone in dentin facilitated crystallographic maturity, crystallinity and improved maturity and secondary structure of dentin collagen.

CLINICAL SIGNIFICANCE

Using dexamethasone-functionalized NPs before resin infiltration is a clear option to obtain dentin remineralization, as these NPs produce the reinforcement of the dentin structure, which will lead to the improvement of the longevity of resin restorations.

Poly(acrylic acid)-Assisted Intrafibrillar Mineralization of Type I Collagen: A Review

The mineralization of type I collagen is a biological process occurring in vertebrates by which some hard tissues such as bone and dentin are constructed. Due to the extensive clinical needs for bone defect repair and remineralization of mineral-depleted dentin, biomimetic mineralization of collagen is attracting more and more interests. Synthetic analogs of noncollagenous proteins are necessary for directing the in vitro mineralization. In this paper, the function and mechanism of poly(acrylic acid) (PAA) in regulating the mineralization, especially intrafibrillar mineralization (IM) of collagen are reviewed. As two mineralization patterns (extrafibrillar and intrafibrillar) co-exist in natural hard tissues, differences between them in terms of microstructure, biodegradation, cytocompatibility, osteoinduction in vitro, and performance in vivo are systematically compared. Then the roles of PAA in biomimetic collagen IM within one-analog and two-analog systems are discussed, respectively. Moreover, mineralization of some self-mineralizable collagen matrices is described. Due to the interactions between collagen and PAA play a crucial role in the processes of collagen mineralization, some reference researches are also provided involving the collagen/PAA interactions in some other fields. Finally, this review is ended with an outlook for future potential improvements based on the collection of existing bottlenecks in this field.

Advances in osseointegration of biomimetic mineralized collagen and inorganic metal elements of natural bone for bone repair

At this stage, bone defects caused by trauma, infection, tumor, or congenital diseases are generally filled with autologous bone or allogeneic bone transplantation, but this treatment method has limited sources, potential disease transmission and other problems. Ideal bone-graft materials remain continuously explored, and bone defect reconstruction remains a significant challenge. Mineralized collagen prepared by bionic mineralization combining organic polymer collagen with inorganic mineral calcium phosphate can effectively imitate the composition and hierarchical structure of natural bone and has good application value in bone repair materials. Magnesium, strontium, zinc and other inorganic components not only can activate relevant signaling pathways to induce differentiation of osteogenic precursor cells but also stimulate other core biological processes of bone tissue growth and play an important role in natural bone growth, and bone repair and reconstruction. This study reviewed the advances in hydroxyapatite/collagen composite scaffolds and osseointegration with natural bone inorganic components, such as magnesium, strontium and zinc.

Engineering biomaterials to 3D-print scaffolds for bone regeneration: practical and theoretical consideration

There are more than 2 million bone grafting procedures performed annually in the US alone. Despite significant efforts, the repair of large segmental bone defects is a substantial clinical challenge which requires bone substitute materials or a bone graft. The available biomaterials lack the adequate mechanical strength to withstand the static and dynamic loads while maintaining sufficient porosity to facilitate cell in-growth and vascularization during bone tissue regeneration. A wide range of advanced biomaterials are being currently designed to mimic the physical as well as the chemical composition of a bone by forming polymer blends, polymer-ceramic and polymer-degradable metal composites. Transforming these novel biomaterials into porous and load-bearing structures via three-dimensional printing (3DP) has emerged as a popular manufacturing technique to develop engineered bone grafts. 3DP has been adopted as a versatile tool to design and develop bone grafts that satisfy porosity and mechanical requirements while having the ability to form grafts of varied shapes and sizes to meet the physiological requirements. In addition to providing surfaces for cell attachment and eventual bone formation, these bone grafts also have to provide physical support during the repair process. Hence, the mechanical competence of the 3D-printed scaffold plays a key role in the success of the implant. In this review, we present various recent strategies that have been utilized to design and develop robust biomaterials that can be deployed for 3D-printing bone substitutes. The article also reviews some of the practical, theoretical and biological considerations adopted in the 3D-structure design and development for bone tissue engineering.

Comparison of Synthetic vs. Biogenic Polymeric Process-Directing Agents for Intrafibrillar Mineralization of Collagen

With the aging population, there is a growing need for mineralized tissue restoration and synthetic bone substitutes. Previous studies have shown that a polymer-induced liquid-precursor (PILP) process can successfully mineralize collagen substrates to achieve compositions found in native bone and dentin. This process also leads to intrafibrillar apatitic crystals with their [001] axes aligned roughly parallel to the long axis of the collagen fibril, emulating the nanostructural organization found in native bone and dentin. When demineralized bovine bone was remineralized via the PILP process using osteopontin (OPN), the samples were able to activate mouse marrow-derived osteoclasts to similar levels to those of native bone, suggesting a means for fabricating bioactive bone substitutes that could trigger remodeling through the native bone multicellular unit (BMU). In order to determine if OPN derived from bovine milk could be a cost-effective process-directing agent, the mineralization of type I collagen scaffolds using this protein was compared to the benchmark polypeptide of polyaspartic acid (sodium salt; pAsp). In this set of experiments, we found that OPN led to much faster and more uniform mineralization when compared with pAsp, making it a cheaper and commercially attractive alternative for mineralized tissue restorations.

Liquid-Liquid Phase Separation in Nucleation Process of Biomineralization

Biomineralization is a typical interdisciplinary subject attracting biologists, chemists, and geologists to figure out its potential mechanism. A mounting number of studies have revealed that the classical nucleation theory is not suitable for all nucleation process of biominerals, and phase-separated structures such as polymer-induced liquid precursors (PILPs) play essential roles in the non-classical nucleation processes. These structures are able to play diverse roles biologically or pathologically, and could also give inspiring clues to bionic applications. However, a lot of confusion and dispute occurred due to the intricacy and interdisciplinary nature of liquid precursors. Researchers in different fields may have different opinions because the terminology and current state of understanding is not common knowledge. As a result, our team reviewed the most recent articles focusing on the nucleation processes of various biominerals to clarify the state-of-the-art understanding of some essential concepts and guide the newcomers to enter this intricate but charming field.

Diffusion-Controlled Crystallization of Calcium Phosphate in a Hydrogel toward a Homogeneous Octacalcium Phosphate/Agarose Composite

Diffusion-controlled crystallization in a hydrogel has been investigated to synthesize organic/inorganic hybrid composites and obtain a fundamental understanding of the detailed mechanism of biomineralization. Although calcium phosphate/hydrogel composites have been intensively studied and developed for the application of bone substitutes, the synthesis of homogeneous and integrated composites remains challenging. In this work, diffusion-controlled systems were optimized by manipulating the calcium ion flux at the interface, concentration gradient, and diffusion coefficient to synthesize homogeneous octacalcium phosphate/hydrogel composites with respect to the crystal morphology and density. The ion flux and local pH play an important role in determining the morphology, density, and phase of the crystals. This study suggests a model system that can reveal the relation between local conditions and the resulting crystal phase in diffusion-limited systems and provides a synthetic method for homogeneously organized organic/inorganic composites.

Human osteoclast formation and resorptive function on biomineralized collagen

Biomineralized collagen composite materials pose an intriguing alternative to current synthetic bone graft substitutes by offering a biomimetic composition that closely resembles native bone. We hypothesize that this composite can undergo cellular resorption and remodeling similar to natural bone. We investigate the formation and activity of human osteoclasts cultured on biomineralized collagen and pure collagen membranes in comparison to cortical bone slices. Human monocytes/macrophages from peripheral blood differentiate into multinucleated, tartrate-resistant alkaline phosphatase (TRAP)-positive osteoclast-like cells on all substrates. These cells form clear actin rings on cortical bone, but not on biomineralized collagen or pure collagen membranes. Osteoclasts form resorption pits in cortical bone, resulting in higher calcium ion concentration in cell culture medium; however, osteoclast resorption of biomineralized collagen and collagen membranes does not measurably occur. Activity of osteoclast enzymes – TRAP, carbonic anhydrase II (CA-II), and cathepsin-K (CTS-K) – is similar on all substrates, despite phenotypic differences in actin ring formation and resorption. The mesh-like structure, relatively low stiffness, and lack of RGD-containing binding domains are likely the factors responsible for preventing formation of stable actin rings on and resorption of (biomineralized) collagen membranes. This insight helps to guide further research toward the optimized design of biomineralized collagen composites as a more biomimetic bone-graft substitute.

•

Human osteoclasts differentiated on biomineralized collagen, but no resorption occured.

•

Porous surface of fibrillary collagen and lack of RGD ligands to the vitronectin receptor likely prevented osteoclast resorption.

•

Quantification of resorption on uneven surfaces in comparison with bone was possible with 3D laser microscopy.

Shaping collagen for engineering hard tissues: Towards a printomics approach

Hard tissue engineering has evolved over the past decades, with multiple approaches being explored and developed. Despite the rapid development and success of advanced 3D cell culture, 3D printing technologies and material developments, a gold standard approach to engineering and regenerating hard tissue substitutes such as bone, dentin and cementum, has not yet been realised. One such strategy that differs from conventional regenerative medicine approach of other tissues, is the in vitro mineralisation of collagen templates in the absence of cells. Collagen is the most abundant protein within the human body and forms the basis of all hard tissues. Once mineralised, collagen provides important support and protection to humans, for example in the case of bone tissue. Multiple in vitro fabrication strategies and mineralisation approaches have been developed and their success in facilitating mineral deposition on collagen to achieve bone-like scaffolds evaluated. Critical to the success of such fabrication and biomineralisation approaches is the collagen template, and its chemical composition, organisation, and density. The key factors that influence such properties are the collagen processing and fabrication techniques utilised to create the template, and the mineralisation strategy employed to deposit mineral on and throughout the templates. However, despite its importance, relatively little attention has been placed on these two critical factors. Here, we critically examine the processing, fabrication and mineralisation strategies that have been used to mineralise collagen templates, and offer insights and perspectives on the most promising strategies for creating mineralised collagen scaffolds. STATEMENT OF SIGNIFICANCE: In this review, we highlight the critical need to fabricate collagen templates with advanced processing techniques, in a manner that achieves biomimicry of the hierarchical collagen structure, prior to utilising in vitro mineralisation strategies. To this end, we focus on the initial collagen that is selected, the extraction techniques used and the native fibril forming potential retained to create reconstituted collagen scaffolds. This review synthesises current best practises in material sourcing, processing, mineralisation strategies and fabrication techniques, and offers insights into how these can best be exploited in future studies to successfully mineralise collagen templates.

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.

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.

Cobalt-containing calcium phosphate induces resorption of biomineralized collagen by human osteoclasts

BACKGROUND

Biomineralized collagen, consisting of fibrillar type-I collagen with embedded hydroxyapatite mineral, is a bone-mimicking material with potential application as a bone graft substitute. Despite the chemical and structural similarity with bone extracellular matrix, no evidence exists so far that biomineralized collagen can be resorbed by osteoclasts. The aim of the current study was to induce resorption of biomineralized collagen by osteoclasts by a two-fold modification: increasing the calcium phosphate content and introducing cobalt ions (Co2+), which have been previously shown to stimulate resorptive activity of osteoclasts.

METHODS

To this end, we produced biomineralized collagen membranes and coated them with a cobalt-containing calcium phosphate (CoCaP). Human osteoclasts, derived from CD14+ monocytes from peripheral blood, were differentiated directly on the membranes. Upon fluorescent staining of nuclei, F-actin and tartrate-resistant alkaline phosphatase, the cells were analyzed by laser confocal microscopy. Their resorption capacity was assessed by scanning electron microscopy (SEM), as well as indirectly quantified by measuring the release of calcium ions into cell culture medium.

RESULTS

The CoCaP coating increased the mineral content of the membranes by 4 wt.% and their elastic modulus from 1 to 10 MPa. The coated membranes showed a sustained Co2+ release in water of about 7 nM per 2 days. In contrast to uncoated membranes, on CoCaP-coated biomineralized collagen membranes, osteoclasts sporadically formed actin rings, and induced formation of resorption lacunae, as observed by SEM and confirmed by increase in Ca2+ concentration in cell culture medium. The effect of the CoCaP layer on osteoclast function is thought to be mainly caused by the increase of membrane stiffness, although the effect of Co2+, which was released in very low amounts, cannot be fully excluded.

CONCLUSIONS

This work shows the potential of this relatively simple approach to induce osteoclast resorption of biomineralized collagen, although the extent of osteoclast resorption was limited, and the method needs further optimization. Moreover, the coating method is suitable for incorporating bioactive ions of interest into biomineralized collagen, which is typically not possible using the common biomineralization methods, such as polymer-induced liquid precursor method.

Polymeric zinc-doped nanoparticles for high performance in restorative dentistry

OBJECTIVES

The aim was to state the different applications and the effectiveness of polymeric zinc-doped nanoparticles to achieve dentin remineralization.

DATA, SOURCES AND STUDY SELECTION

Literature search was conducted using electronic databases, such as PubMed, MEDLINE, DIMDI, Embase, Scopus and Web of Science. A narrative exploratory review was undertaken.

CONCLUSIONS

Polymeric nanospheres (NPs) were efficiently loaded with zinc. NPs sequestered calcium and phosphate in the presence of silicon, and remained effectively embedded at the hybrid layer. NPs incorporation did not alter bond strength and inhibited MMP-mediated dentin collagen degradation. Zn-loaded NPs remineralized the hybrid layer inducing a generalized low-carbonate substitute apatite precipitation, chemically crystalline with some amorphous components, and an increase in mechanical properties was also promoted. Viscoelastic analysis determined that dentin infiltrated with Zn-NPs released the stress by breaking the resin-dentin interface and creating specific mineral formations in response to the energy dissipation. Bacteria were scarcely encountered at the resin-dentin interface. The combined antibacterial and remineralizing effects, when Zn-NPs were applied, reduced biofilm formation. Zn-NPs application at both cervical and radicular dentin attained the lowest microleakage and also promoted durable sealing ability. The new zinc-based salt minerals generated covered the dentin surface totally occluding cracks, porosities and dentinal tubules.

CLINICAL SIGNIFICANCE

Zinc-doped NPs are proposed for effective dentin remineralization and tubular occlusion. This offers new strategies for regeneration of eroded cervical dentin, effective treatment of dentin hypersensitivity and in endodontically treated teeth previous to the canal filling. Zn-NPs also do reduce biofilm formation due to antibacterial properties.

Biomimetic mineralized hybrid scaffolds with antimicrobial peptides

Infection in hard tissue regeneration is a clinically-relevant challenge. Development of scaffolds with dual function for promoting bone/dental tissue growth and preventing bacterial infections is a critical need in the field. Here we fabricated hybrid scaffolds by intrafibrillar-mineralization of collagen using a biomimetic process and subsequently coating the scaffold with an antimicrobial designer peptide with cationic and amphipathic properties. The highly hydrophilic mineralized collagen scaffolds provided an ideal substrate to form a dense and stable coating of the antimicrobial peptides. The amount of hydroxyapatite in the mineralized fibers modulated the rheological behavior of the scaffolds with no influence on the amount of recruited peptides and the resulting increase in hydrophobicity. The developed scaffolds were potent by contact killing of Gram-negative Escherichia coli and Gram-positive Streptococcus gordonii as well as cytocompatible to human bone marrow-derived mesenchymal stromal cells. The process of scaffold fabrication is versatile and can be used to control mineral load and/or intrafibrillar-mineralized scaffolds made of other biopolymers.

•

A biomimetic intrafibrillar-mineralized scaffold was prepared using a non-classical pathway for mineralization.

•

The mineralized scaffold was stably coated with designer antimicrobial peptide GL13K.

•

The hybrid scaffold was cytocompatible and potent against biofilms of model Gram-positive and Gram-negative bacteria.

•

The mineral content affected the rheological properties of the scaffolds, but not the loading of antimicrobial peptides.

Collagen Structure-Function Mapping Informs Applications for Regenerative Medicine

Type I collagen, the predominant protein of vertebrates, assembles into fibrils that orchestrate the form and function of bone, tendon, skin, and other tissues. Collagen plays roles in hemostasis, wound healing, angiogenesis, and biomineralization, and its dysfunction contributes to fibrosis, atherosclerosis, cancer metastasis, and brittle bone disease. To elucidate the type I collagen structure-function relationship, we constructed a type I collagen fibril interactome, including its functional sites and disease-associated mutations. When projected onto an X-ray diffraction model of the native collagen microfibril, data revealed a matrix interaction domain that assumes structural roles including collagen assembly, crosslinking, proteoglycan (PG) binding, and mineralization, and the cell interaction domain supporting dynamic aspects of collagen biology such as hemostasis, tissue remodeling, and cell adhesion. Our type III collagen interactome corroborates this model. We propose that in quiescent tissues, the fibril projects a structural face; however, tissue injury releases blood into the collagenous stroma, triggering exposure of the fibrils' cell and ligand binding sites crucial for tissue remodeling and regeneration. Applications of our research include discovery of anti-fibrotic antibodies and elucidating their interactions with collagen, and using insights from our angiogenesis studies and collagen structure-function model to inform the design of super-angiogenic collagens and collagen mimetics.

Embedding cells within nanoscale, rapidly mineralizing hydrogels: A new paradigm to engineer cell-laden bone-like tissue

Bone mineralization is a highly specific and dynamic nanoscale process that has been studied extensively from a structural, chemical, and biological standpoint. Bone tissue, therefore, may be defined by the interplay of its intricately mineralized matrix and the cells that regulate its biological function. However, the far majority of engineered bone model systems and bone replacement materials have been unable to replicate this key characteristic of bone tissue; that is, the ability of cells to be gradually and rapidly embedded in a three-dimensional (3D) heavily calcified matrix material. Here we review the characteristics that define the bone matrix from a nanostructural perspective. We then revisit the benefits and challenges of existing model systems and engineered bone replacement materials, and discuss recent efforts to replicate the biological, cellular, mechanical, and materials characteristics of bone tissue on the nano- to microscale. We pay particular attention to a recently proposed method developed by our group, which seeks to replicate key aspects of the entrapment of bone cells within a mineralized matrix with precisions down to the level of individual nano-crystallites, inclusive of the bone vasculature, and osteogenic differentiation process. In summary, this paper discusses existing and emerging evidence pointing towards future developments bridging the gap between the fields of biomineralization, structural biology, stem cells, and tissue engineering, which we believe will hold the key to engineer truly functional bone-like tissue in the laboratory.

Proliferation and Osteogenic Differentiation of hMSCs on Biomineralized Collagen

Biomineralized collagen with intrafibrillar calcium phosphate mineral provides an excellent mimic of the composition and structure of the extracellular matrix of bone, from nano- to micro-scale. Scaffolds prepared from this material have the potential to become the next-generation of synthetic bone graft substitutes, as their unique properties make them closer to the native tissue than synthetic alternatives currently available to clinicians. To understand the interaction between biomineralized collagen and cells that are relevant in the context of bone regeneration, we studied the growth and osteogenic differentiation of bone marrow derived human mesenchymal stromal cells (hMSCs) cultured on biomineralized collagen membranes, and compared it to the cell behavior on collagen membranes without mineral. Cells proliferated normally on both biomimetic membranes, and were more triggered to differentiate toward the osteogenic lineage by the biomineralized collagen. This was shown by the elevated mRNA levels of RUNX2, SPP1, ENPP1, and OCN after 3 days of culture, and COL1A1 after 14 days of culture on mineralized collagen. The mRNA levels of the tested markers of osteogenesis were lower on collagen membranes without mineral, with the exception of OCN, which was more highly expressed on collagen than on biomineralized collagen membranes. Expression by hMSCs of OPG, a gene involved in inhibition of osteoclastogenesis, was higher on biomineralized collagen at day 3, while M-CSF, involved in osteoblast-osteoclast communication, was upregulated on both membranes at day 3 and 14 of culture. Alkaline phosphatase activity of hMSCs was high on both biomimetic membranes when compared with cells cultured on tissue culture plastic. Cell-induced mineralization was observed on collagen membranes, while the high mineral content of the biomineralized membranes prohibited a reliable analysis of cell-induced mineralization on these membranes. In conclusion, we have identified that both collagen and biomineralized collagen support proliferation, osteogenic differentiation and mineralization of hMSCs, with biomineralized membranes having a more pronounced positive effect. These findings support the existing evidence that biomineralized collagen is a promising material in the field of bone regeneration.

Harnessing biomolecules for bioinspired dental biomaterials

Dental clinicians have relied for centuries on traditional dental materials (polymers, ceramics, metals, and composites) to restore oral health and function to patients. Clinical outcomes for many crucial dental therapies remain poor despite many decades of intense research on these materials. Recent attention has been paid to biomolecules as a chassis for engineered preventive, restorative, and regenerative approaches in dentistry. Indeed, biomolecules represent a uniquely versatile and precise tool to enable the design and development of bioinspired multifunctional dental materials to spur advancements in dentistry. In this review, we survey the range of biomolecules that have been used across dental biomaterials. Our particular focus is on the key biological activity imparted by each biomolecule toward prevention of dental and oral diseases as well as restoration of oral health. Additional emphasis is placed on the structure-function relationships between biomolecules and their biological activity, the unique challenges of each clinical condition, limitations of conventional therapies, and the advantages of each class of biomolecule for said challenge. Biomaterials for bone regeneration are not reviewed as numerous existing reviews on the topic have been recently published. We conclude our narrative review with an outlook on the future of biomolecules in dental biomaterials and potential avenues of innovation for biomaterial-based patient oral care.

Mechanical behavior of biomimetically mineralized collagen matrix using the polymer - induced liquid precursor process

Biomimetic mineralization is a promising technique in biomedical applications. To understand the mechanical behavior of biomimetically mineralized collagen material (BMC), we examined the composition and structure of the mineral deposition in BMCs mineralized by the polymer-induced liquid precursor (PILP) process and applied wide angle x-ray scattering (WAXS) with in situ tensile testing to investigate the mineral-to-tissue co-deformation in the material. We found that the PILP process is able to achieve good biomimetic mineralization in bulk collagen matrix. Compositionally, the mineral deposition showed high crystallinity with no carbonation. However, the morphology of extrafibrillar mineral deposition and the preferential crystal orientation were different from natural bone. Further, the Young's modulus and mineral-to-tissue co-deformation ratio of the BMC were significantly lower than both natural bone and partially demineralized bone with similar mineral volume fraction. It was concluded that while biomimetic mineralization can achieve good mineral deposition volume in the BMC, the mechanical behavior of the material was different from natural bone.

The Injectable Woven Bone-Like Hydrogel to Perform Alveolar Ridge Preservation With Adapted Remodeling Performance After Tooth Extraction

Grafting bone substitute is paramount to prevent the alveolar ridge resorption after tooth extraction and facilitate the subsequent implant treatment. An ideal bone substitute should acquire the excellent osteogenic property, more importantly, possess the suitable remodeling rate in balance with bone formation and desirable clinical manageability. However, none of bone substitute is simultaneously characterized by these features, and currently, the limited remodeling property leads to the excessive waiting time before implantation. Enlightened by woven bone, the transitional tissue that is able to induce osteogenesis during bone healing could be easily remodeled within a short period and depend on the favorable injectability of hydrogel, an injectable woven bone-like hydrogel (IWBLH) was constructed in this study to address the above problems. To mimic the component and hierarchical structure of woven bone, amorphous calcium phosphate (ACP) and mineralized collagen fibril were synthesized and compounded with alginate to form IWBLHs with various ratio. Screened by physiochemical characterization and in vitro biological assays, an optimal IWBLH was selected and further explored in rat model of tooth extraction. Compared with the most widely used bone substitute, we showed that IWBLH could be easily handled to fully fill the tooth socket, perform a comparable function to prevent the alveolar bone resorption, and completely remodeled within 4 weeks. This IWBLH stands as a promising candidate for alveolar ridge preservation (ARP) in future.

The Significance and Utilisation of Biomimetic and Bioinspired Strategies in the Field of Biomedical Material Engineering: The Case of Calcium Phosphat-Protein Template Constructs

This review provides a summary of recent research on biomimetic and bioinspired strategies applied in the field of biomedical material engineering and focusing particularly on calcium phosphate-protein template constructs inspired by biomineralisation. A description of and discussion on the biomineralisation process is followed by a general summary of the application of the biomimetic and bioinspired strategies in the fields of biomedical material engineering and regenerative medicine. Particular attention is devoted to the description of individual peptides and proteins that serve as templates for the biomimetic mineralisation of calcium phosphate. Moreover, the review also presents a description of smart devices including delivery systems and constructs with specific functions. The paper concludes with a summary of and discussion on potential future developments in this field.

Influence of molecular weight and concentration of carboxymethyl chitosan on biomimetic mineralization of collagen

The objective of the present study was to systematically investigate the influence of molecular weight (MW) and concentration of carboxymethyl chitosan (CMC), which served as non-collagenous protein (NCP) surrogates, on biomimetic mineralization of type I collagen. Supersaturated CMC-stabilized amorphous calcium-phosphate (CMC-ACP) dispersions containing different MWs (20 kDa, 60 kDa, 150 kDa) and concentrations (25, 50, 100, 200, 400 μg ml⁻¹) of CMC were prepared. After mineralization in the aforementioned dispersions for 7 days, the pattern and extent of biomimetic mineralization of collagen scaffolds were investigated. Our study showed that increasing CMC concentration resulted in increasing stability and decreasing particle size of CMC-ACP dispersions. Images from scanning and transmission electron microscopy revealed that intrafibrillar mineralization of collagen was obtained with 20k-200, 60k-100, 60k-200 and 150k-200 CMC-ACP dispersions, with hydroxyapatite (HAp) formation confirmed by Fourier transform infrared spectroscopy and X-ray diffraction measurements, whereas HAp formed extrafibrillar clusters in other collagen scaffolds. Thermogravimetric analysis showed that the combined effect of MW and concentration of CMC contributed to different extents of biomimetic mineralization, and was correlated with the stability and particle size of CMC-ACP dispersions, and the size-exclusion characteristics of type I collagen. The results of this work support the effective function of CMC as NCP analogs, and provide parameters of MWs and concentrations of CMC for applications in hard tissue engineering as well as insights into intersections of mechanisms in biomimetic mineralization.

The study systematically investigated the influence of molecular weight and concentration of CMC on CMC-ACP nanoparticles and biomimetic mineralization.

Bone-Inspired Mineralization with Highly Aligned Cellulose Nanofibers as Template

Bioinspired by the aligned structure and building blocks of bone, this work mineralized the aligned bacterial cellulose (BC) through in situ mineralization using CaCl₂ and K₂HPO₄ solutions. The cellulose nanofibers were aligned by a scalable stretching process. The aligned and mineralized bacterial cellulose (AMBC) homogeneously incorporated hydroxyapatite (HAP) with a high mineral content and exhibited excellent mechanical strength. The ordered 3D structure allowed the AMBC composite to achieve a high elastic modulus and hardness and the development of a nanostructure inspired by natural bone. The AMBC composite exhibited an elastic modulus of 10.91 ± 3.26 GPa and hardness of 0.37 ± 0.18 GPa. Compared with the nonaligned mineralized bacterial cellulose (NMBC) composite with mineralized crystals of HAP randomly distributed into the BC scaffolds, the AMBC composite possessed a 210% higher elastic modulus and 95% higher hardness. The obtained AMBC composite had excellent mechanical properties by mimicking the natural structure of bone, which indicated that the organic BC aerogel with aligned nanofibers was a promising template for biomimetic mineralization.

Modulation of biomimetic mineralization of collagen by soluble ectodomain of discoidin domain receptor 2

Collagen fibrils serve as the major template for mineral deposits in both biologically derived and engineered tissues. In recent years certain non-collagenous proteins have been elucidated as important players in differentially modulating intra vs. extra-fibrillar mineralization of collagen. We and others have previously shown that the expression of the collagen receptor, discoidin domain receptor 2 (DDR2) positively correlates with matrix mineralization. The objective of this study was to examine if the ectodomain (ECD) of DDR2 modulates intra versus extra-fibrillar mineralization of collagen independent of cell-signaling. For this purpose, a decellularized collagenous substrate, namely glutaraldehyde fixed porcine pericardium (GFPP) was subjected to biomimetic mineralization protocols. GFPP was incubated in modified simulated body fluid (mSBF) or polymer-induced liquid precursor (PILP) solutions in the presence of recombinant DDR2 ECD (DDR2-Fc) to mediate extra or intra-fibrillar mineralization of collagen. Thermogravimetric analysis revealed that DDR2-Fc increased mineral content in GFPP calcified in mSBF while no significant differences were observed in PILP mediated mineralization. Electron microscopy approaches were used to evaluate the quality and quantity mineral deposits. An increase in the matrix to mineral ratio, frequency of particles and size of mineral deposits was observed in the presence of DDR2-Fc in mSBF. Von Kossa staining and immunohistochemistry analysis of adjacent sections indicated that DDR2-Fc bound to both the matrix and mineral phase of GFPP. Further, DDR2-Fc was found to bind to hydroxyapatite (HAP) particles and enhance the nucleation of mineral deposits in mSBF solutions independent of collagen. Taken together, our results elucidate DDR2 ECD as a novel player in the modulation of extra-fibrillar mineralization of collagen.

Bio-instructive materials for musculoskeletal regeneration

The prevalence and cost of disorders affecting the musculoskeletal system are predicted to rise significantly in the coming years due to the aging global population and the increase of associated risk factors. Despite being the second largest cause of disability, the clinical options for therapeutic intervention remain limited. The clinical translation of cell-based therapies for the treatment of musculoskeletal disorders faces many challenges including maintenance of cell survival in the harsh in vivo environment and the lack of control over regulating cell phenotype upon implantation. In order to address these challenges, the development of bio-instructive materials to modulate cell behavior has taken center stage as a strategy to increase the therapeutic potential of various cell populations. However, the determination of the necessary cues for a specific application and how these signals should be presented from a biomaterial remains elusive. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues. There is a particular emphasis on emerging efforts such as the engineering of immunomodulatory and antibacterial materials, as well as the incorporation of these strategies into biofabrication and organ-on-a-chip approaches. STATEMENT OF SIGNIFICANCE: Disorders affecting the musculoskeletal system affect individuals across the lifespan and have a profound effect on mobility and quality of life. While small defects in many tissues can heal successfully, larger defects are often unable to heal or instead heal with inferior quality fibrous tissue and require clinical intervention. Cell-based therapies are a promising option for clinical translation, yet challenges related to maintaining cell survival and instructing cell phenotype upon implantation have limited the success of this approach. Bio-instructive materials provide an exciting opportunity to modulate cell behavior and enhance the efficacy of cell-based approaches for musculoskeletal repair. However, the identification of critical instructive cues and how to present these stimuli is a focus of intense investigation. This review highlights recent biochemical and physical strategies used to engineer bio-instructive materials for the repair of musculoskeletal tissues, while also considering exciting progress in the engineering of immunomodulatory and antibacterial materials.

Bioinspired Mineralization with Hydroxyapatite and Hierarchical Naturally Aligned Nanofibrillar Cellulose

We used cellulose and a nonclassical mineralization process to fabricate a bioinspired nanohybrid material that exhibited structural features and properties similar to those of human hard tissues. We made a hydrogel with highly compacted and aligned cellulose nanofibers. We thoroughly mineralized the cellulose hydrogel with hydroxyapatite nanocrystals, using poly(acrylic acid) as a soluble template for precursor minerals, which infiltrated the nanocompartments of the aligned cellulose nanofiber network. The ultrastructure and mechanical properties of the mineralized gels were strikingly similar to those of bone and dentin, which supports further use of cellulose-based fibrillary materials as affordable, biocompatible scaffolds for repair and regeneration of hard tissues. The versatility of the bioinspired mineralization processes used here can broaden the applications of these cellulosic nanohybrids.

Silk fibroin-derived polypeptides additives to promote hydroxyapatite nucleation in dense collagen hydrogels

Silk fibroin-derived polypeptides (FDPs) are polypeptides resulting from the enzymatic separation of the hydrophobic crystalline (Cp) and hydrophilic electronegative amorphous (Cs) components of silk fibroin (SF). The role of these polypeptides in promoting the nucleation of hydroxyapatite (HA) has been previously investigated, yet is still not fully understood. Here we study the potential of HA mineralization via FDPs incorporated at 1:10, 1:2 and 1:1 in a plastically compressed (PC) and dense collagen (DC) scaffold. Scaffolds were immersed in simulated body fluid (SBF) at physiological conditions (pH = 7.4, 37°C) to promote biomineralization. The effect of Cs and Cp to promote HA nucleation was investigated at different time points, and compared to pure DC scaffolds. Characterization of Cs and Cp fragments using Liquid Chromatography-Mass Spectrometry (LCMS) showed little difference in the amino acid composition of the FDPs. Results obtained in vitro using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), Scanning Electron Microscopy (SEM) X-Ray Diffraction (XRD) and mass analysis showed little difference between scaffolds that incorporated Cs, Cp, and DC hydrogels. These results demonstrated that silk FDPs incorporation are not yet suitable to promote HA nucleation in vivo without further refining the collagen-FDP system.

The influence of osteopontin-guided collagen intrafibrillar mineralization on pericyte differentiation and vascularization of engineered bone scaffolds

Biomimetically mineralized collagen scaffolds are promising for bone regeneration, but vascularization of these materials remains to be addressed. Here, we engineered mineralized scaffolds using an osteopontin-guided polymer-induced liquid-precursor mineralization method to recapitulate bone's mineralized nanostructure. SEM images of mineralized samples confirmed the presence of collagen with intrafibrillar mineral, also EDS spectra and FTIR showed high peaks of calcium and phosphate, with a similar mineral/matrix ratio to native bone. Mineralization increased collagen compressive modulus up to 15-fold. To evaluate vasculature formation and pericyte-like differentiation, HUVECs and hMSCs were seeded in a 4:1 ratio in the scaffolds for 7 days. Moreover, we used RT-PCR to investigate the gene expression of pericyte markers ACTA2, desmin, CD13, NG2, and PDGFRβ. Confocal images showed that both nonmineralized and mineralized scaffolds enabled endothelial capillary network formation. However, vessels in the nonmineralized samples had longer vessel length, a larger number of junctions, and a higher presence of αSMA+ mural cells. RT-PCR analysis confirmed the downregulation of pericytic markers in mineralized samples. In conclusion, although both scaffolds enabled endothelial capillary network formation, mineralized scaffolds presented less pericyte-supported vessels. These observations suggest that specific scaffold characteristics may be required for efficient scaffold vascularization in future bone tissue engineering strategies. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1522-1532, 2019.

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.

Embryonic-Like Mineralized Extracellular Matrix/Stem Cell Microspheroids as a Bone Graft Substitute

Native bone extracellular matrix (ECM) secreted by mesenchymal precursors provides an optimal biological framework, comprising structural collagen proteins and a microenvironment niche, which supports cell attachment and differentiation, and bone growth. Inspired by nature, the embryonic-like mineralized ECM/stem cell microspheroids (MECS) are developed, in which self-assembly of the stem cell microspheroids (CS) and mineralization of the self-produced ECM occur simultaneously. The uniform-sized MECS exhibit a solid spherical appearance with stem cells embedded inside, recapitulating the early stage of intramembranous ossification. Compared with pure CS, MECS show enhanced Young's modulus, cell viability, intercellular communication, and osteogenic differentiation. Additionally, the capability of MECS is explored without the use of exogenous scaffolds to substitute and repair lost bone in rat critical-sized defects. It is found that the MECS can achieve excellent bone regeneration outcomes with 97.99 ± 2.28% of the defect area filled with new bony structures and blood vessels, while nearly half or one-third of the defect area is repaired by CS (52.79 ± 4.63%) or β-tricalcium phosphate (38.09 ± 7.79%), respectively. The study demonstrates that embryonic-like MECS is a novel effective bone graft substitute for bone tissue regeneration.

Hierarchical Biomineralization: from Nature's Designs to Synthetic Materials for Regenerative Medicine and Dentistry

Biomineralization is a highly dynamic, yet controlled, process that many living creatures employ to develop functional tissues such as tooth enamel, bone, and others. A major goal in materials science is to create bioinspired functional structures based on the precise organization of building blocks across multiple length scales. Therefore, learning how nature has evolved to use biomineralization could inspire new ways to design and develop synthetic hierarchical materials with enhanced functionality. Toward this goal, this review dissects the current understanding of structure-function relationships of dental enamel and bone using a materials science perspective and discusses a wide range of synthetic technologies that aim to recreate their hierarchical organization and functionality. Insights into how these strategies could be applied for regenerative medicine and dentistry are also provided.

Influence of fluoride on the mineralization of collagen via the polymer-induced liquid-precursor (PILP) process

OBJECTIVE

The polymer-induced liquid-precursor (PILP) mineralization process has been shown to remineralize artificial dentin lesions to levels consistent with those of native dentin. However, nanoindentation revealed that the moduli of those remineralized lesions were only ∼50% that of native dentin. We hypothesize that this may be due to the PILP process having been previously optimized to obtain high amounts (∼70wt%) of intrafibrillar crystals, but without sufficient interfibrillar mineral, another significant component of dentin.

METHODS

Fluoride was added to the PILP-mineralization of collagen from rat tail tendon at varying concentrations to determine if a better balance of intra- versus inter-fibrillar mineralization could be obtained, as determined by electron microscopy. Nanoindentation was used to determine if fluoridated apatite could improve the mechanical properties of the composites.

RESULTS

Fluoride was successfully incorporated into the PILP-mineralization of rat tail tendon and resulted in collagen-mineral composite systems with the mineral phase of hydroxyapatite containing various levels of fluoridation. As the fluoride concentration increased, the crystals became larger and more rod-like, with an increasing tendency to form on the fibril surfaces rather than the interior. Nanomechanical testing of the mineralized tendons revealed that fluoride addition did not increase modulus over PILP mineralization alone. This likely resulted from the separated nature of collagen fibrils that comprise tendon, which does not provide lateral reinforcement and therefore may not be suited for the compressive loads of nanoindentation.

SIGNIFICANCE

This work contributes to the development of minimally invasive approaches to caries treatment by determining if collagen can be functionally mineralized.

Current advances for bone regeneration based on tissue engineering strategies

Bone tissue engineering (BTE) is a rapidly developing strategy for repairing critical-sized bone defects to address the unmet need for bone augmentation and skeletal repair. Effective therapies for bone regeneration primarily require the coordinated combination of innovative scaffolds, seed cells, and biological factors. However, current techniques in bone tissue engineering have not yet reached valid translation into clinical applications because of several limitations, such as weaker osteogenic differentiation, inadequate vascularization of scaffolds, and inefficient growth factor delivery. Therefore, further standardized protocols and innovative measures are required to overcome these shortcomings and facilitate the clinical application of these techniques to enhance bone regeneration. Given the deficiency of comprehensive studies in the development in BTE, our review systematically introduces the new types of biomimetic and bifunctional scaffolds. We describe the cell sources, biology of seed cells, growth factors, vascular development, and the interactions of relevant molecules. Furthermore, we discuss the challenges and perspectives that may propel the direction of future clinical delivery in bone regeneration.