Bioinspired organic‐inorganic composite materials prepared by an alternate soaking process as a tissue reconstitution matrix

The Effect of Mesoporous Bioactive Glass Nanoparticles/Graphene Oxide Composites on the Differentiation and Mineralization of Human Dental Pulp Stem Cells

The purpose of this study was to investigate the effects of mesoporous bioactive glass nanoparticle (MBN)/graphene oxide (GO) composites on the mineralization ability and differentiation potential of human dental pulp stem cells (hDPSCs). MBN/GO composites were synthesized using the sol-gel method and colloidal processing to enhance the bioactivity and mechanical properties of MBN. Characterization using FESEM, XRD, FTIR, and Raman spectrometry showed that the composites were successfully synthesized. hDPSCs were then cultured directly on the MBN/GO (40:1 and 20:1) composites in vitro. MBN/GO promoted the proliferation and alkaline phosphatase (ALP) activity of hDPSCs. In addition, qRT-PCR showed that MBN/GO regulated the mRNA levels of odontogenic markers (dentin sialophosphoprotein (DSPP), dentine matrix protein 1 (DMP-1), ALP, matrix extracellular phosphoglycoprotein (MEPE), bone morphogenetic protein 2 (BMP-2), and runt-related transcription factor 2 (RUNX-2)). The mRNA levels of DSPP and DMP-1, two odontogenesis-specific markers, were considerably upregulated in hDPSCs in response to growth on the MBN/GO composites. Western blot analysis revealed similar results. Alizarin red S staining was subsequently performed to further investigate MBN/GO-induced mineralization of hDPSCs. It was revealed that MBN/GO composites promote odontogenic differentiation via the Wnt/β-catenin signaling pathway. Collectively, the results of the present study suggest that MBN/GO composites may promote the differentiation of hDPSCs into odontoblast-like cells, and potentially induce dentin formation.

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.

Alternate Calcification in Microcapillaries for the Fabrication of Hydroxyapatite Films without Light Exposure, Calcination, or Applied Voltage

We discuss an alternate method for calcification in microcapillaries for fabricating calcium phosphate films using silicone molds and calcifying solutions. Calcium phosphate films with a line/space of 5-50 µm were fabricated by controlling the concentrations of calcium chloride and sodium phosphate solutions. Plate-type crystals of hydroxyapatite were grown when the calcium phosphate films were immersed in hydroxyapatite precursors. In the initial stage of hydroxyapatite crystal growth, the c-plane of the crystals was grown parallel to the substrates, and subsequently the growth followed with the c-plane growing perpendicular to the substrates. In narrow capillaries, dendritic structures were formed with a tendency to grow in a direction parallel to the direction of the microcapillaries. This technique is useful in the micropatterning of biocompatible ceramics with a minimized material consumption and a short fabrication time.

Poly(acrylic acid)-regulated Synthesis of Rod-Like Calcium Carbonate Nanoparticles for Inducing the Osteogenic Differentiation of MC3T3-E1 Cells

Calcium carbonate, especially with nanostructure, has been considered as a good candidate material for bone regeneration due to its excellent biodegradability and osteoconductivity. In this study, rod-like calcium carbonate nanoparticles (Rod-CC NPs) with desired water dispersibility were achieved with the regulation of poly (acrylic acid). Characterization results revealed that the Rod-CC NPs had an average length of 240 nm, a width of 90 nm with an average aspect ratio of 2.60 and a negative ζ-potential of −22.25 ± 0.35 mV. The degradation study illustrated the nanoparticles degraded 23% at pH 7.4 and 45% at pH 5.6 in phosphate-buffered saline (PBS) solution within three months. When cultured with MC3T3-E1 cells, the Rod-CC NPs exhibited a positive effect on the proliferation of osteoblast cells. Alkaline phosphatase (ALP) activity assays together with the osteocalcin (OCN) and bone sialoprotein (BSP) expression observations demonstrated the nanoparticles could induce the differentiation of MC3T3-E1 cells. Our study developed well-dispersed rod-like calcium carbonate nanoparticles which have great potential to be used in bone regeneration.

Cellulose Nanofibrils and Mechanism of their Mineralization in Biomimetic Synthesis of Hydroxyapatite/Native Bacterial Cellulose Nanocomposites: Molecular Dynamics Simulations

Molecular dynamics (MD) simulation of a nanofibril of native bacterial cellulose (BC) in solutions of mineral ions is presented. The supersaturated calcium-phosphate (CP) solution with the ionic composition of hydroxyapatite and CaCl2 solutions with the concentrations below, equal to, and above the solubility limits are simulated. The influence of solvation models (TIP3P and TIP4P-ew water models) on structural characteristics of the simulated nanofibril and on the crystal nucleation process is assessed. The structural characteristics of cellulose nanofibrils (in particular, of the surface layer) are found to be nearly independent of the solvation models used in the simulation and on the presence of ions in the solutions. It is shown that ionic clusters are formed in the solution rather than on the fibril surface. The cluster sizes are slightly different for the two water models. The effect of the ion-ion interaction parameters on the results is discussed. The main conclusion is that the activity of hydroxyl groups on the BC fibril surface is not high enough to cause adsorption of Ca(2+) ions from the solution. Therefore, the nucleation of CP crystals takes place initially in solution, and then the crystallites formed can be adsorbed on BC nanofibril surfaces.

Biomineral/Agarose Composite Gels Enhance Proliferation of Mesenchymal Stem Cells with Osteogenic Capability

Hydroxyapatite (HA) or calcium carbonate (CaCO₃) formed on an organic polymer of agarose gel is a biomaterial that can be used for bone tissue regeneration. However, in critical bone defects, the regeneration capability of these materials is limited. Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into bone forming osteoblasts. In this study, we loaded MSCs on HA- or CaCO₃-formed agarose gel and cultured them with dexamethasone, which triggers the osteogenic differentiation of MSCs. High alkaline phosphatase activity was detected on both the HA- and CaCO₃-formed agarose gels; however, basal activity was only detected on bare agarose gel. Bone-specific osteocalcin content was detected on CaCO₃-formed agarose gel on Day 14 of culture, and levels subsequently increased over time. Similar osteocalcin content was detected on HA-formed agarose on Day 21 and levels increased on Day 28. In contrast, only small amounts of osteocalcin were found on bare agarose gel. Consequently, osteogenic capability of MSCs was enhanced on CaCO₃-formed agarose at an early stage, and both HA- and CaCO₃-formed agarose gels well supported the capability at a later stage. Therefore, MSCs loaded on either HA- or CaCO₃-formed agarose could potentially be employed for the repair of critical bone defects.

Urease-induced calcification of segmented polymer hydrogels - a step towards artificial biomineralization

Natural organic/inorganic composites, such as nacre, bones and teeth, are perfectly designed materials with exceptional mechanical properties. Numerous approaches have been taken to synthetically prepare such composites. The presented work describes a new way of mineralizing bulk materials on a large scale following the approach of bioinduced mineralization. To this end, a series of polymer conetworks with entrapped urease were prepared. After polymerization, the entrapped urease shows high enzymatic activity. The bioactive polymer conetworks were then treated with an aqueous mixture of urea and CaCl2. The urease-induced calcification indeed allows formation of carbonate crystals exclusively within the hydrogel even at room temperature. The influence of network composition, degree of cross-linking, immobilized urease concentration and temperature of calcification were investigated. By varying these parameters, spherical, monolithic clusters, as well as bar-like nanocrystals with different aspect ratios in spherical or dendritic arrays, are formed. The grown nanocrystals improve the stiffness of the starting material by up to 700-fold, provided that the microstructure shows a dense construction without pores and strong interaction between crystals and network. The process has the potential to generate a new class of hybrid materials that would be available on the macroscopic scale for use in lightweight design and medicine.

Enhanced mechanical strength and biocompatibility of electrospun polycaprolactone-gelatin scaffold with surface deposited nano-hydroxyapatite

In this study for the first time, we compared physico-chemical and biological properties of polycaprolactone-gelatin-hydroxyapatite scaffolds of two types: one in which the nano-hydroxyapatite (n-HA) was deposited on the surface of electrospun polycaprolactone-gelatin (PCG) fibers via alternate soaking process (PCG-HAAS) and other in which hydroxyapatite (HA) powders were blended in electrospinning solution of PCG (PCG-HAB). The microstructure of fibers was examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) which showed n-HA particles on the surface of the PCG-HAAS scaffold and embedded HA particles in the interior of the PCG-HAB fibers. PCG-HAAS fibers exhibited the better Young's moduli and tensile strength as compared to PCG-HAB fibers. Biological properties such as cell proliferation, cell attachment and alkaline phosphatase activity (ALP) were determined by growing human osteosarcoma cells (MG-63) over the scaffolds. Cell proliferation and confocal results clearly indicated that the presence of hydroxyapatite on the surface of the PCG-HAAS scaffold promoted better cellular adhesion and proliferation as compared to PCG-HAB scaffold. ALP activity was also observed better in alternate soaked PCG scaffold as compared to PCG-HAB scaffold. Mechanical strength and biological properties clearly demonstrate that surface deposited HA scaffold prepared by alternate soaking method may find application in bone tissue engineering.

Siloxane-poly(lactic acid)-vaterite composites with 3D cotton-like structure

Trace amounts of ionic calcium and silicon species have been reported to stimulate the proliferation, differentiation, and mineralization of bone-forming cells. Composite materials comprising siloxane-doped calcium carbonate (vaterite) particles and poly(L-lactic acid) have been developed [siloxane-poly(lactic acid)-vaterite hybrid-composite, SiPVH] so far; they were designed such that calcium and silicate ions are gradually released from SiPVH and they show the chronic effects of ions on cellular activities. In the present work, SiPVH with a 3D cotton-like structure was prepared by electrospinning to obtain the major advantages of excellent bioactivity and ease of handling for bone filling surgery. The diameter of the fibrous skeletons that form structure of the cotton-like SiPVH was controlled to ~10 μm to achieve cellular migration into the spaces between fibers. The resulting cotton-like SiPVH showed good flexibility. The fiber surface was coated rapidly with numerous particles of several hundred nanometers in size by alternate soaking in CaCl(2) and Na(2)HPO(4). The treated cotton-like material, which released calcium and silicate ions gradually, showed good cellular migration behavior into the 3D structure in cell culture tests using murine osteoblast-like MC3T3-E1 cells.

Interactions binding mineral and organic phases in nanocomposites based on bacterial cellulose and calcium phosphates

The interactions responsible for the adhesion of calcium phosphate (CP) nanocrystals and bacterial cellulose (BC) nanofibrils in the composite material obtained by mixing aqueous suspensions of presynthesized CP and BC and the dependence of these interactions on the CP morphology and chemical structure have been elucidated by molecular mechanics calculations of the CP-BC interfacial structures. The interactions between the superficial CP and BC crystal layers have been simulated. Two crystalline CP structures (i.e., hydroxyapatite (HAP) and whitlockite) with two morphologies (plate-shaped and rod-shaped) were considered. Electrostatics has been found to be the major contributor to the adhesion of the CP crystallites and BC nanofibers, and the formation of interfacial hydrogen bonds makes a minor contribution to the interaction energy. It has also been found that, in general, the energy gain resulting from whitlockite-BC binding is greater than that for HAP-BC binding, and the binding of the rod-shaped crystallites of whitlockite with BC is the most profitable. The energy loss and entropy gain upon replacement of the BC-water and CP-water contacts by the BC-CP contacts have been estimated.

Biomimetic bone-like composites fabricated through an automated alternate soaking process

Hydroxyapatite-gelatin composites have been proposed as suitable scaffolds for bone and dentin tissue regeneration. There is considerable interest in producing these scaffolds using biomimetic methods due to their low energy costs and potential to create composites similar to the tissues they are intended to replace. Here an existing process used to coat a surface with hydroxyapatite under near physiological conditions, the alternate soaking process, is modified and automated using an inexpensive "off the shelf" robotics kit. The process is initially used to precipitate calcium phosphate coatings. Then, in contrast to previous utilizations of the alternate soaking process, gelatin was added directly to the solutions in order to co-precipitate hydroxyapatite-gelatin composites. Samples were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and nanoindentation. Calcium phosphate coatings formed by the alternate soaking process exhibited different calcium to phosphate ratios, with correspondingly distinct structural morphologies. The coatings demonstrated an interconnected structure with measurable mechanical properties, even though they were 95% porous. In contrast, hydroxyapatite-gelatin composite coatings over 2mm thick could be formed with little visible porosity. The hydroxyapatite-gelatin composites demonstrate a composition and mechanical properties similar to those of cortical bone.

Anisotropic hydroxyapatite formation inside agarose gels by integration of electrophoretic and alternate soaking approaches

We report here anisotropic hydroxyapatite (HAp) formation inside an agarose gel using an electrophoretic approach and an alternate soaking process. Effective ion migration is shown to be crucial to integrate hydrogel with biominerals, for example, calcium carbonate and HAp. Calcium and phosphate ions easily migrated into the gel interior from ionic solutions when an electric field was applied to both terminals of the gel. The time to reach complete biomineral formation was only 3 min. In this process, roughly 75-300 microg of HAp was formed in 1 mg of dry gel. Moreover, we carried out alternate soaking using the resulting materials and further HAp formation was observed (750 microg HAp/mg dry gel). HAp was characterized by infrared spectroscopy and X-ray diffraction, and was then assigned. Moreover, the HAp was formed in an anisotropic manner. First, (210) and (211) were preferentially formed by the electrophoretic approach. Alternatively, (002) and (211) were formed after the alternate soaking process. This result indicated that preparative conditions were shown to be crucial to regulate anisotropic crystal growth and to keep it the major component. In summary, it is considered that anisotropic ion diffusion inside the hydrogel would be the dominant factor by electrophoretic and alternate soaking approaches.

Controlled release based on the dissolution of a calcium carbonate layer deposited on hydrogels

It is possible that inorganic materials conjugated to suitable organic materials may induce unique mechanical, optical and other functional properties. Therefore, artificial conjugation of organic and inorganic components is attractive for preparing novel functional materials. Recently, we developed an alternate soaking process for calcium salt formation on/in polymer materials. In this study, a poly(vinyl alcohol) (PVA) hydrogel-calcium carbonate (CaCO(3)) composite was prepared by the aforementioned process as a controlled release support. Brilliant blue FCF (Mw = 794), FITC labeled BSA (Mw = 6.6 x 10(4)), FITC labeled dextran-10 k (Mw = 9.5 x 10(3)) and FITC labeled dextran-40 k (Mw = 4.3 x 10(4)) were loaded into the composite as model drugs. CaCO(3) dissolution and model drug release rates increased with a decrease in buffer pH. In addition, model drug release rates increased with a decrease in model drug molecular weight. These results show that CaCO(3) layers on hydrogels behave as capping layers for model drug release; the release rate of model drugs can be controlled by the dissolution rate of CaCO(3) and the molecular weight of the drug.