Amelogenin-inspired peptide, calcium phosphate solution, fluoride and their synergistic effect on enamel biomimetic remineralization: an in vitro pH-cycling model

BACKGROUND

Several methods were introduced for enamel biomimetic remineralization that utilize a biomimetic analogue to interact and absorb bioavailable calcium and phosphate ions and induce crystal nucleation on demineralized enamel. Amelogenin is the most predominant enamel matrix protein that is involved in enamel biomineralization. It plays a major role in developing the enamel's hierarchical microstructure. Therefore, this study was conducted to evaluate the ability of an amelogenin-inspired peptide to promote the remineralization potential of fluoride and a supersaturated calcium phosphate solution in treating artificially induced enamel carious lesions under pH-cycling regimen.

METHODS

Fifty enamel slices were prepared with a window (4*4 mm2 ) on the surface. Five samples were set as control healthy enamel and 45 samples were subjected to demineralization for 3 days. Another 5 samples were set as control demineralized enamel and 40 enamel samples were assigned into 8 experimental groups (n=5) (P/I, P/II, P/III, P/AS, NP/I, NP/II, NP/III and NP/AS) according to peptide treatment (peptide P or non-peptide NP) and remineralizing solution used (I; calcium phosphate solution, II; calcium phosphate fluoride solution, III; fluoride solution and AS; artificial saliva). Samples were then subjected to demineralization/remineralization cycles for 9 days. Samples in all experimental groups were evaluated using Raman spectroscopy for mineral content recovery percentage, microhardness and nanoindentation as healthy, demineralized enamel and after pH-cycling. Data were statistically analysed using two-way repeated measures Anova followed by Bonferroni-corrected post hoc test for pairwise multiple comparisons between groups. Statistical significance was set at p= 0.05. Additionally, XRD, FESEM and EDXS were used for crystal orientation, surface morphology and elemental analysis after pH-cycling.

RESULTS

Nanocrystals clumped in a directional manner were detected in peptide-treated groups. P/II showed the highest significant mean values in mineral content recovery (63.31%), microhardness (268.81±6.52 VHN), elastic modulus (88.74±2.71 GPa), nanohardness (3.08±0.59 GPa) and the best crystal orientation with I002/I300 (1.87±0.08).

CONCLUSION

Despite pH changes, the tested peptide was capable of remineralizing enamel with ordered crystals. Moreover, the supplementary use of calcium phosphate fluoride solution with peptide granted an enhancement in enamel mechanical properties after remineralization.

Performance and mechanism of microbial fuel cell coupled with anaerobic membrane bioreactor system for fouling control

To remove membrane fouling, a bio-electrochemical system that can generate a micro-electric field and micro-current was constructed. After 11 days of operation, the trans-membrane pressure difference of membrane modules in the open- and closed-circuit groups increased by 35.8 kPa and 6.2 kPa, respectively. The concentrations of total polysaccharide and protein in the open-circuit group were 1.8 and 1.1 times higher than those in the closed-circuit group, respectively. In addition, X-ray photoelectron spectroscopy and thermogravimetric analysis showed that inorganic crystals such as calcium carbonate were present on the membrane surface, and the concentration of calcium ion in the control group was 14.7 times that of the experimental group. High-throughput sequencing demonstrated that the enrichment of some electroactive bacteria and other microorganisms has a positive effect on the control of membrane fouling. Therefore, this system can effectively alleviate membrane fouling of a bioreactor, by targeting the membrane foulants.

Copper-doped zeolitic imidazolate frameworks-8/hydroxyapatite composite coating endows magnesium alloy with excellent corrosion resistance, antibacterial ability and biocompatibility

Magnesium (Mg) and its alloys exhibit an excellent prospect for orthopedic clinical application due to their outstanding biodegradability and mechanical adaptability. However, the rapid corrosion rate/latent device-associated infections may lead to a failed internal fixation of Mg-based implants. Herein, a novel composite coating consisted of outer copper-doped zeolitic imidazolate frameworks-8 and inner hydroxyapatite (Cu@ZIF-8/HA) was in situ constructed on AZ31B Mg alloy via a two-step approach of hydrothermal treatment and seeded solvothermal method. The results verified that the electrochemical impedance of the obtained Cu45@ZIF-8/HA composite coating increased by two orders of magnitude to 6.6013 × 104 Ω·cm2 compared to that of bare Mg alloy. This was attributed to the reduced particle size of ZIF-8 nanoparticles due to the doped copper ions, which could be effectively grown in situ on the micro-nano flower-like structure of the HA-coated Mg alloy. Meanwhile, the Cu@ZIF-8/HA coating exhibited excellent antibacterial properties due to the release of copper ions and zinc ions from Cu@ZIF-8 dissolved in bacterial culture solution. The ICP results unraveled that the released concentration of copper and zinc ions could enhance the activity of alkaline phosphatase in the appropriate range during MC3T3-E1 cell culture in vitro for 7 days. This research revealed that the preparation of multifunctional metal-organic frameworks coating doped with antimicrobial metal ions via the seed layer solvothermal method was significant for studying the antimicrobial properties, osteogenic performance and corrosion resistance of Mg-based bioactive coatings.

Bioinspired macromolecular templates for crystallographic orientation control of ZnO thin films through zinc hydroxide carbonate

The biomineralization-inspired preparation of inorganic hybrid materials has attracted attention. Here, we report a new approach to the orientation control of zinc oxide (ZnO) thin-film crystals through the preparation of zinc hydroxide carbonate (ZHC) by the macromolecular templates of poly(2-hydroxyethyl methacrylate) (PHEMA) and poly(vinyl alcohol) (PVA). Using 100-nm-thick PHEMA templates, ZHC thin films with the c-axis oriented parallel to the substrate were obtained, while ZHC thin films prepared by 100-nm-thick PVA templates showed perpendicular orientation. After the thermal treatment of ZHC, the crystal orientations of the ZnO thin films were maintained. The effects of the thickness and annealing time for the polymer templates on the morphologies of the ZnO thin films were examined.

Preparation of citric acid-modified poly(vinyl alcohol) films for effectively precipitating calcium phosphate particles

Development of a technology for effectively controlling the precipitation of CP particles on PVA films via a biomimetic process was achieved using the CA-modification technique.

A thermostability perspective on enhancing physicochemical and cytological characteristics of octacalcium phosphate by doping iron and strontium

Investigation of thermostability will lead the groundbreaking of unraveling the mechanism of influence of ion-doping on the properties of calcium phosphates. In this work, octacalcium phosphate (OCP), a metastable precursor of biological apatite, was used as a stability model for doping ions (Fe³⁺ and Sr²⁺) with different ionic charges and radii. After treated under hot air at different temperatures (110–200 °C), the phase, morphology, structure, physicochemical properties, protein affinity, ions release, and cytological responses of the ion-doped OCPs were investigated comparatively. The results showed that the collapse of OCP crystals gradually occurred, accompanying with the dehydration of hydrated layers and the disintegration of plate-like crystals as the temperature increased. The collapsed crystals still retained the typical properties of OCP and the potential of conversion into hydroxyapatite. Compared to the undoped OCP, Fe-OCP, and Sr-OCP had lower and higher thermostability respectively, leading to different material surface properties and ions release. The adjusted thermostability of Fe-OCP and Sr-OCP significantly enhanced the adsorption of proteins (BSA and LSZ) and the cytological behavior (adhesion, spreading, proliferation, and osteogenic differentiation) of bone marrow mesenchymal stem cells to a varying extent under the synergistic effects of corresponding surface characteristics and early active ions release. This work paves the way for understanding the modification mechanism of calcium phosphates utilizing ion doping strategy and developing bioactive OCP-based materials for tissue repair.

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OCP was used as a stability model for doping ions with different charges and radii.

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Collapse of OCP crystals occurred with structural dehydration after heat treatment.

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Fe and Sr doping altered the thermostability of OCP crystals in an opposite way.

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The thermostable difference affected the surface properties and ion release of OCP.

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Active surface and ion release of OCP synergistically mediated its biocompatibility.

Shear-induced liquid-crystalline phase transition behaviour of colloidal solutions of hydroxyapatite nanorod composites

Liquid-crystalline (LC) bio-inspired materials based on colloidal nanoparticles with anisotropic morphologies such as sheets, plates, rods and fibers were used as functional materials. They show stimuli-responsive behaviour under mechanical force and in electric and magnetic fields. Understanding the effects of external stimuli on the structures of anisotropic colloidal particles is important for the development of highly ordered structures. Recently, we have developed stimuli-responsive hydroxyapatite (HAP)-based colloidal LC nanorods that are environmentally-friendly functional materials. In the present study, the ordering behaviour of HAP nanorod dispersions, which show LC states, has been examined using in situ small-angle neutron scattering and rheological measurements (Rheo-SANS) under shearing force. The structural analyses and dynamic viscosity observations provided detailed information about the effects of shear force on the structural changes of HAP nanorods in D₂O dispersion. The present Rheo-SANS measurements unraveled three kinds of main effects of the shear force: the enhancement of interactions between the HAP nanorods, the alignment of HAP nanorods to the shear flow direction, and the formation and disruption of HAP nanorod assemblies. Simultaneous analyses of dynamic viscosity and structural changes revealed that the HAP nanorod dispersions exhibited distinctive rheological properties accompanied by their ordered structural changes.

Capacitive behaviour of nanocrystalline octacalcium phosphate (OCP) (Ca8H2(PO4)6·5H2O) as an electrode material for supercapacitors: biosupercaps

Octacalcium phosphate (OCP) is classified as a low-temperature phase of calcium phosphate (CaPs); it is a widely used ceramic material in biomedical applications. Interestingly, this study demonstrated the capacitive behavior of OCP as an electrode material in supercapacitors, alternatively named biosupercaps, for the first time in the literature. OCP powder was synthesized by solution precipitation at pH 5.5 at 60 °C in the presence of succinic acid. X-Ray diffraction (XRD) fully confirmed the OCP phase, with a crystallite size of around 40 nm, as calculated by the Scherrer equation. The FE-SEM micrographs of the OCP powder revealed plate-like morphology with a high surface area/thickness ratio. The surface widths of these layers ranged from about 2 to 100 microns, whereas the thickness of the layers was on the nanoscale (<100 nm). Raman spectroscopy was performed to confirm the microstructural formation of the OCP powder and electrodes according to the Raman spectra. Asymmetric and symmetric capacitors were prepared by various designs using OCP powder as a potential electrode material. The electrochemical performance of each biosupercap containing OCP was analyzed by a potentiostat in terms of current-voltage (CV) curves; each sample presented a typical pseudocapacitive behaviour. The electrochemical impedance spectra (EIS) of the OPC materials confirmed their significant capacitive performance, with up to 6 mA h g^-1 specific capacity (SCp); this may be valuable for future medical electronics such as biocompatible energy storage and harvesting microdevices.

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

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

Bioinspired Environmentally Friendly Amorphous CaCO3-Based Transparent Composites Comprising Cellulose Nanofibers

Amorphous calcium carbonate (ACC) stabilized by acidic macromolecules is a useful material for the development of environmentally friendly composites. In this study, we synthesized transparent and mechanically tough ACC-based composite materials by the incorporation of water-dispersible cellulose derivatives, namely, carboxymethyl cellulose (CMC) and surface-modified crystalline cellulose nanofibers (CNFs). A solution mixing method used in the present work proved to be a powerful and efficient method for the production of mechanically tough and environmentally friendly materials. Molecular-scale interactions between carboxyl groups and Ca2+ ions induce homogeneous dispersion of CNFs in the composites, and this gives composite films with high transparency and high mechanical properties. The composite films of CMC, CNFs, and ACC at the mixture ratios of 40, 40, and 20 wt %, showed high mechanical properties of 15.8 ± 0.93 GPa for the Young's modulus and 268 ± 20 MPa for the tensile strength. These designed materials that are based on ACC may open up new opportunities in many fields in applications that require the use of environmentally friendly, biodegradable, mechanically tough, and transparent composite materials.

Stimuli-responsive hydroxyapatite liquid crystal with macroscopically controllable ordering and magneto-optical functions

Liquid crystals are mostly formed by self-assembly of organic molecules. In contrast, inorganic materials available as liquid crystals are limited. Here we report the development of liquid-crystalline (LC) hydroxyapatite (HAp), which is an environmentally friendly and biocompatible biomineral. Its alignment behavior, magneto-optical properties, and atomic-scale structures are described. We successfully induce LC properties into aqueous colloidal dispersions of rod-shaped HAp by controlling the morphology of the material using acidic macromolecules. These LC HAp nanorod materials are macroscopically oriented in response to external magnetic fields and mechanical forces. We achieve magnetic modulation of the optical transmission by dynamic control of the LC order. Atomic-scale observations using transmission electron microscopy show the self-organized inorganic/organic hybrid structures of mesogenic nanorods. HAp liquid crystals have potential as bio-friendly functional materials because of their facile preparation, the bio-friendliness of HAp, and the stimuli-responsive properties of these colloidal ordered fluids.

Periodic Surface-Ring Pattern Formation for Hydroxyapatite Thin Films Formed by Biomineralization-Inspired Processes

Surface morphology is a key factor that might significantly influence the properties of biomaterials. In this study, periodic surface-ring structures have been constructed for calcium phosphate thin films via biomineralization-inspired crystallization process. The patterned octacalcium phosphate crystals have been obtained on poly(2-hydroxyethyl methacrylate) (PHEMA) matrix in the presence of poly(acrylic acid) (PAA). The patterned surface morphologies of the crystal thin films could be tuned by the amount of PAA additives. In addition, the rapid and topotactic transformation to hydroxyapatite (HAP) thin films with surface-ring structures has also been achieved. This study may provide new strategy toward the design of functional calcium phosphate-based thin-film hybrids.

Enantioselective incorporation of dicarboxylate guests by octacalcium phosphate

Enantioselectivity by octacalcium phosphate (OCP) is revealed through the incorporation of (S)-(-)-methylsuccinic acid (MeSuc) into its crystal lattice, with hardly any (R)-(+)-MeSuc incorporated. This phenomenon clearly indicates that OCP recognizes the steric structures of guest molecules, extending chiral recognition in inorganic materials to three-dimensional crystal growth.