Enzymatic Reaction Generates Biomimic Nanominerals with Superior Bioactivity

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.

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.

In Situ Enzymatic Reaction Generates Magnesium-Based Mineralized Microspheres with Superior Bioactivity for Enhanced Bone Regeneration

Bone is a naturally mineralized tissue with a remarkable hierarchical structure, and the treatment of bone defects remains challenging. Microspheres with facile features of controllable size, diverse morphologies, and specific functions display amazing potentials for bone regeneration. Herein, inspired by natural biomineralization, a novel enzyme-catalyzed reaction is reported to prepare magnesium-based mineralized microspheres. First, silk fibroin methacryloyl (SilMA) microspheres are prepared using a combination of microfluidics and photo-crosslinking. Then, the alkaline phosphatase (ALP)-catalyzed hydrolysis of adenosine triphosphate (ATP) is successfully used to induce the formation of spherical magnesium phosphate (MgP) in the SilMA microspheres. These SilMA@MgP microspheres display uniform size, rough surface structure, good degradability, and sustained Mg2+ release properties. Moreover, the in vitro studies demonstrate the high bioactivities of SilMA@MgP microspehres in promoting the proliferation, migration, and osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). Transcriptomic analysis shows that the osteoinductivity of SilMA@MgP microspheres may be related to the activation of the PI3K/Akt signaling pathway. Finally, the bone regeneration enhancement units (BREUs) are designed and constructed by inoculating BMSCs onto SilMA@MgP microspheres. In summary, this study demonstrates a new biomineralization strategy for designing biomimetic bone repair materials with defined structures and combination functions.

Calcium Phosphate-Based Nanomaterials: Preparation, Multifunction, and Application for Bone Tissue Engineering

Calcium phosphate is the main inorganic component of bone. Calcium phosphate-based biomaterials have demonstrated great potential in bone tissue engineering due to their superior biocompatibility, pH-responsive degradability, excellent osteoinductivity, and similar components to bone. Calcium phosphate nanomaterials have gained more and more attention for their enhanced bioactivity and better integration with host tissues. Additionally, they can also be easily functionalized with metal ions, bioactive molecules/proteins, as well as therapeutic drugs; thus, calcium phosphate-based biomaterials have been widely used in many other fields, such as drug delivery, cancer therapy, and as nanoprobes in bioimaging. Thus, the preparation methods of calcium phosphate nanomaterials were systematically reviewed, and the multifunction strategies of calcium phosphate-based biomaterials have also been comprehensively summarized. Finally, the applications and perspectives of functionalized calcium phosphate biomaterials in bone tissue engineering, including bone defect repair, bone regeneration, and drug delivery, were illustrated and discussed by presenting typical examples.

The synergetic effect of a gold nanocluster-calcium phosphate composite: enhanced photoluminescence intensity and superior bioactivity

Gold nanoclusters (AuNCs) are a unique class of materials that exhibit visible luminescence. Amorphous calcium phosphate (ACP) is a widely used biomaterial for a variety of purposes, such as drug delivery, bone cementing, and implant coatings. In this study, a nanocomposite of AuNCs and ACP is prepared by biomimetic mineralization in a Dulbecco's modified Eagle's medium (DMEM). The strong interaction between AuNCs and Ca²⁺ ions effectively induces aggregation of AuNCs. The as-formed nanocomposite, AuNCs@ACP, emits significantly enhanced luminescence compared to AuNCs alone. The luminescence enhancement mechanism is investigated using synchrotron X-ray absorption fine structure spectroscopy. In addition, the presence of AuNCs stabilizes ACP and also enhances the biocompatibility of ACP in promoting cell proliferation, and the nanocomposites are promising as nanoprobes for cancer therapy and/or bone tissue engineering.

Calcium Phosphate-Based Biomaterials for Bone Repair

Traumatic, tumoral, and infectious bone defects are common in clinics, and create a big burden on patient's families and society. Calcium phosphate (CaP)-based biomaterials have superior properties and have been widely used for bone defect repair, due to their similarities to the inorganic components of human bones. The biological performance of CaPs, as a determining factor for their applications, are dependent on their physicochemical properties. Hydroxyapatite (HAP) as the most thermally stable crystalline phase of CaP is mostly used in the form of ceramics or composites scaffolds with polymers. Nanostructured CaPs with large surface areas are suitable for drug/gene delivery systems. Additionally, CaP scaffolds with hierarchical nano-/microstructures have demonstrated excellent ability in promoting bone regeneration. This review focuses on the relationships and interactions between the physicochemical/biological properties of CaP biomaterials and their species, sizes, and morphologies in bone regeneration, including synthesis strategies, structure control, biological behavior, and the mechanisms of CaP in promoting osteogenesis. This review will be helpful for scientists and engineers to further understand CaP-based biomaterials (CaPs), and be useful in developing new high-performance biomaterials for bone repair.

In-situ mineralized homogeneous collagen-based scaffolds for potential guided bone regeneration

For guided bone regeneration (GBR) in clinical orthopedics, the importance of a suitable scaffold which can provide the space needed for bone regeneration and simultaneously promotes the new bone formation cannot be overemphasized. Due to its excellent biocompatibility, mechanical strength, and similarity in structure and composition to natural bone, the mineralized collagen-based scaffolds have been increasingly considered as promising GBR scaffolds. Herein, we propose a novel method to fabricate an in-situ mineralized homogeneous collagen-based scaffold (IMHCS) with excellent osteogenic capability for GBR by electrospinning the collagen solution in combination with essential mineral ions. The IMHCS exhibited homogeneous distribution of apatite crystals in electrospun fibers, which helped to achieve a significantly higher tensile strength than the pure collagen scaffold (CS) and the scaffold with directly added nano-hydroxyapatite particles (HAS). Furthermore, the IMHCS had significantly better cell compatibility, cell migration ratio, and osteogenic differentiation property than the HAS and CS. Therefore, the IMHCS not only retains traditional function of inhibiting fibroblast invasion, but also possesses excellent osteogenic differentiation property, indicating a robust alternative for GBR applications.

Mineralized Enzyme-Based Biomaterials with Superior Bioactivities for Bone Regeneration

The formation and metabolic balance of bone tissue is a controllable process of biomineralization, which is regulated by various cells, biomolecules, and ions. Enzyme molecules play an important role in this process, and alkaline phosphatase (ALP) is one of the most critical factors. In this study, inspired by the process of bone biomineralization, a biomimetic strategy is achieved for the preparation of mineralized ALP nanoparticles (MALPNs), by taking advantages of the unique reaction between ALP and calcium ions in Dulbecco's modified Eagle's medium. Benefiting from the mild biomineralization reaction, the MALPN system highly maintains the activity of ALP. Furthermore, the in vitro studies show that the MALPN system significantly enhances the proliferation of bone marrow mesenchymal stem cells and upregulates their osteogenic differentiation. When evaluated as synthetic graft materials for bone regeneration, the MALPN-incorporated gelatin methacryloyl graft shows excellent mechanical properties, a sustained release profile of ALP, and high biocompatibility and efficacy in guiding bone regeneration and vascularization for critical-sized rat calvarial defect. Moreover, we also demonstrate that the biomimetic mineralization strategy can be adopted for other proteins such as acid phosphatase, bovine serum albumin, fibrinogen, and gelatin, suggesting its universality for constructing mineralized protein-/enzyme-based bioactive materials for the application of tissue regeneration.

Role of Phosphorus-Containing Molecules on the Formation of Nano-Sized Calcium Phosphate for Bone Therapy

Calcium phosphate (CaP) is the principal inorganic constituent of bone and teeth in vertebrates and has various applications in biomedical areas. Among various types of CaPs, amorphous calcium phosphate (ACP) is considered to have superior bioactivity and biodegradability. With regard to the instability of ACP, the phosphorus-containing molecules are usually adopted to solve this issue, but the specific roles of the molecules in the formation of nano-sized CaP have not been clearly clarified yet. Herein, alendronate, cyclophosphamide, zoledronate, and foscarnet are selected as the model molecules, and theoretical calculations were performed to elucidate the interaction between calcium ions and different model molecules. Subsequently, CaPs were prepared with the addition of the phosphorus-containing molecules. It is found that cyclophosphamide has limited influence on the generation of CaPs due to their weak interaction. During the co-precipitation process of Ca2+ and PO43-, the competitive relation among alendronate, zoledronate, and foscarnet plays critical roles in the produced inorganic-organic complex. Moreover, the biocompatibility of CaPs was also systematically evaluated. The DFT calculation provides a convincing strategy for predicting the structure of CaPs with various additives. This work is promising for designing CaP-based multifunctional drug delivery systems and tissue engineering materials.

The Osteoinductivity of Calcium Phosphate-Based Biomaterials: A Tight Interaction With Bone Healing

Calcium phosphate (CaP)-based bioceramics are the most widely used synthetic biomaterials for reconstructing damaged bone. Accompanied by bone healing process, implanted materials are gradually degraded while bone ultimately returns to its original geometry and function. In this progress report, we reviewed the complex and tight relationship between the bone healing response and CaP-based biomaterials, with the emphasis on the in vivo degradation mechanisms of such material and their osteoinductive properties mediated by immune responses, osteoclastogenesis and osteoblasts. A deep understanding of the interaction between biological healing process and biomaterials will optimize the design of CaP-based biomaterials, and further translate into effective strategies for biomaterials customization.

Overview of Liquid Crystal Biosensors: From Basic Theory to Advanced Applications

Liquid crystals (LCs), as the remarkable optical materials possessing stimuli-responsive property and optical modulation property simultaneously, have been utilized to fabricate a wide variety of optical devices. Integrating the LCs and receptors together, LC biosensors aimed at detecting various biomolecules have been extensively explored. Compared with the traditional biosensing technologies, the LC biosensors are simple, visualized, and efficient. Owning to the irreplaceable superiorities, the research enthusiasm for the LC biosensors is rapidly rising. As a result, it is necessary to overview the development of the LC biosensors to guide future work. This article reviews the basic theory and advanced applications of LC biosensors. We first discuss different mesophases and geometries employed to fabricate LC biosensors, after which we introduce various detecting mechanisms involved in biomolecular detection. We then focus on diverse detection targets such as proteins, enzymes, nucleic acids, glucose, cholesterol, bile acids, and lipopolysaccharides. For each of these targets, the development history and state-of-the-art work are exhibited in detail. Finally, the current challenges and potential development directions of the LC biosensors are introduced briefly.

Platelet-Activating Biominerals Enhanced Injectable Hydrogels With Superior Bioactivity for Bone Regeneration

Refractory bone fracture, which is difficult to be treated, is a common clinical disease. Taking inspiration from the natural process of bone regeneration, we provide a biomimetic strategy to develop a new injectable biomaterial for repairing bone defects, which is mainly composed of platelets, fibrins, and biominerals. Biomineral nanoparticles (EACPNs) with an amorphous phase are prepared by an enzyme-catalyzed route and display a platelet-activating property. The composite hydrogel (EPH) of EACPNs, fibrins, and platelets is injectable, and has similar chemical properties to natural materials in bone regeneration. The dried EPH samples display a highly porous structure, which would be favorable for cell attachment and growth. The results from in vitro studies indicate that EPH has high biocompatibility and superior bioactivity in promoting the osteogenic differentiation of rat bone marrow stem cells (rBMSCs). Furthermore, the results from in vivo studies clearly indicate that EPH can induce the formation of new collagen and vessels in the defect area, thus leading to faster regeneration of bone defects at 2 weeks. Our study provides a strategy for designing new biomimetic materials, which may be favorable in the treatment of refractory bone fracture.

Biomineralization-inspired synthesis of amorphous manganese phosphates for GLUT5-targeted drug-free catalytic therapy of osteosarcoma

Osteosarcoma, occurring most frequently in children, teens, and young adults, is a lethal bone cancer with a high incidence of distant metastases and drug resistance. Developing a therapeutic platform that integrates targeting, curing and imaging is highly desirable for enhanced osteosarcoma therapy, yet quite challenging. In this work, we demonstrate a novel biomineralization-inspired strategy for the synthesis of a fructose incorporated manganese phosphate (Fru-MnP) nanoplatform for tumour targeting, drug-free therapy, and MRI imaging. Benefitting from the glucose transporter 5 (GLUT5)-mediated endocytosis, our Fru-MnP nanoplatform produces a high level of reactive oxygen species (ROS) via the Mn²⁺-driven Fenton reaction within osteosarcoma cells, leading to efficient cancer cell killing due to caspase-mediated apoptosis. By virtue of the T1 signal enhancement of Mn²⁺, our Fru-MnP nanoplatform also acts as an effective tumour-specific MRI contrast agent, realizing the MRI-monitored chemodynamic therapy. The proposed synergistic therapeutic platform opens new possibilities for high efficacy therapy for osteosarcoma.

Amorphous calcium magnesium phosphate nanocomposites with superior osteogenic activity for bone regeneration

The seek of bioactive materials for promoting bone regeneration is a challenging and long-term task. Functionalization with inorganic metal ions or drug molecules is considered effective strategies to improve the bioactivity of various existing biomaterials. Herein, amorphous calcium magnesium phosphate (ACMP) nanoparticles and simvastatin (SIM)-loaded ACMP (ACMP/SIM) nanocomposites were developed via a simple co-precipitation strategy. The physiochemical property of ACMP/SIM was explored using transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction (XRD) and high-performance liquid chromatograph (HPLC), and the role of Mg²⁺ in the formation of ACMP/SIM was revealed using X-ray absorption near-edge structure (XANES). After that, the transformation process of ACMP/SIM in simulated body fluid (SBF) was also tracked to simulate and explore the in vivo mineralization performance of materials. We find that ACMP/SIM releases ions of Ca²⁺, Mg²⁺ and PO43−, when it is immersed in SBF at 37°C, and a phase transformation occurred during which the initially amorphous ACMP turns into self-assembled hydroxyapatite (HAP). Furthermore, ACMP/SIM displays high cytocompatibility and promotes the proliferation and osteogenic differentiation of MC3T3-E1 cells. For the in vivo studies, lamellar ACMP/SIM/Collagen scaffolds with aligned pore structures were prepared and used to repair a rat defect model in calvaria. ACMP/SIM/Collagen scaffolds show a positive effect in promoting the regeneration of calvaria defect after 12 weeks. The bioactive ACMP/SIM nanocomposites are promising as bone repair materials. Considering the facile preparation process and superior in vitro/vivo bioactivity, the as-prepared ACMP/SIM would be a potential candidate for bone related biomedical applications.

Biomimetic inorganic-organic hybrid nanoparticles from magnesium-substituted amorphous calcium phosphate clusters and polyacrylic acid molecules

Amorphous calcium phosphate (ACP) has been widely found during bone and tooth biomineralization, but the meta-stability and labile nature limit further biomedical applications. The present study found that the chelation of polyacrylic acid (PAA) molecules with Ca²⁺ ions in Mg-ACP clusters (~2.1 ± 0.5 nm) using a biomineralization strategy produced inorganic-organic Mg-ACP/PAA hybrid nanoparticles with better thermal stability. Mg-ACP/PAA hybrid nanoparticles (~24.0 ± 4.8 nm) were pH-responsive and could be efficiently digested under weak acidic conditions (pH 5.0–5.5). The internalization of assembled Mg-ACP/PAA nanoparticles by MC3T3-E1 cells occurred through endocytosis, indicated by laser scanning confocal microscopy and cryo-soft X-ray tomography. Our results showed that cellular lipid membranes remained intact without pore formation after Mg-ACP/PAA particle penetration. The assembled Mg-ACP/PAA particles could be digested in cell lysosomes within 24 h under weak acidic conditions, thereby indicating the potential to efficiently deliver encapsulated functional molecules. Both the in vitro and in vivo results preliminarily demonstrated good biosafety of the inorganic-organic Mg-ACP/PAA hybrid nanoparticles, which may have potential for biomedical applications.

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Mg-ACP/PAA hybrid nanoparticles have been synthesized following a biomineralization strategy.

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The chelation of PAA molecules in synergy with Mg²⁺ substitution improves thermal stability of Mg-ACP/PAA nanoparticles.

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The Mg-ACP/PAA nanoparticles are pH sensitive and can be digested in cell lysosomes within 24 h.

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.

Ratiometric Fluorescent Strategy for Localizing Alkaline Phosphatase Activity in Mitochondria Based on the ESIPT Process

Fluorescent probes are powerful tools for detecting and mapping the species of interest in vitro and in vivo. Although the probes always show high selectivity and sensitivity, they are usually affected by some factors, such as detecting conditions and the probe concentrations. Ratiometric fluorescent strategies, possessing advantage of low background noise, would solve the problem effectively and lead to a higher sensing performance. Thus, an ESIPT-based ratiometric probe (HBTP-mito) was developed on the basis of a phosphorylated 2-(2'-hydroxyphenyl)-benzothiazole derivative for the determination of ALP activity. HBTP-mito is water soluble and emits green fluorescence in TBS buffer due to the blockage of ESIPT. Upon the introduction of ALP, the phosphate ester of HBTP-mito was hydrolyzed and the ESIPT process was restored. Accordingly, the fluorescence at 514 nm decreases, while emission at 650 nm shows a "turn-on" response. The ratio of intensity (I_514nm/I_650nm) decreases linearly with ALP activity increasing from 0 to 60 mU/mL, obtained an LOD of 0.072 mU/mL. The favorable performance of the probe enables its application not only in the detection of ALP activity in biological samples, but also in the localization of the ALP levels in living cells and in vivo.

Biomolecule-assisted green synthesis of nanostructured calcium phosphates and their biomedical applications

Calcium phosphates (CaPs) are ubiquitous in nature and vertebrate bones and teeth, and have high biocompatibility and promising applications in various biomedical fields. Nanostructured calcium phosphates (NCaPs) are recognized as promising nanocarriers for drug/gene/protein delivery owing to their high specific surface area, pH-responsive degradability, high drug/gene/protein loading capacity and sustained release performance. In order to control the structure and surface properties of NCaPs, various biomolecules with high biocompatibility such as nucleic acids, proteins, peptides, liposomes and phosphorus-containing biomolecules are used in the synthesis of NCaPs. Moreover, biomolecules play important roles in the synthesis processes, resulting in the formation of various NCaPs with different sizes and morphologies. At room temperature, biomolecules can play the following roles: (1) acting as a biocompatible organic phase to form biomolecule/CaP hybrid nanostructured materials; (2) serving as a biotemplate for the biomimetic mineralization of NCaPs; (3) acting as a biocompatible modifier to coat the surface of NCaPs, preventing their aggregation and increasing their colloidal stability. Under heating conditions, biomolecules can (1) control the crystallization process of NCaPs by forming biomolecule/CaP nanocomposites before heating; (2) prevent the rapid and disordered growth of NCaPs by chelating with Ca2+ ions to form precursors; (3) provide the phosphorus source for the controlled synthesis of NCaPs by using phosphorus-containing biomolecules. This review focuses on the important roles of biomolecules in the synthesis of NCaPs, which are expected to guide the design and controlled synthesis of NCaPs. Moreover, we will also summarize the biomedical applications of NCaPs in nanomedicine and tissue engineering, and discuss their current research trends and future prospects.