Biomimetic Apatite Research Articles

Pickering emulsion as template for porous bioceramics in the perspective of bone regeneration

Calcium phosphate (CaP) based bioceramics are widely used as bone substitutes. The most encountered CaP ceramics are obtained from high temperature phases. However, their bioactivity and their association with biomolecules are limited, as well as their bioresorption in-vivo. The aim of this work is to develop biomimetic low temperature apatites ceramics with tunable porosity via biocompatible high internal phase Pickering emulsions. The biocompatible emulsions developed were stabilized by stoichiometric hydroxyapatite (HA) particles. Several parameters (mass of HA particles, oil/water weight ratio, electrolytes concentration in the aqueous phase) were investigated to define the optimized formulation conditions leading to a kinetically stable monodisperse emulsion with a minimum drop diameter of 200 µm and drops enough percolated to induce interconnected porosity. Two types of porous bioceramics were produced by low temperature processes with controlled composition and porosity, evidenced by X-ray microtomography: calcium phosphate monoliths from an apatitic gel, and silica-HA monoliths via a sol-gel process. These low temperature processes should provide bioceramics able to perform bioactivity and bio-resorption in-vivo, and could prefigure a drug or other therapeutic ions-delivery disposals for filling bone defects in maxillofacial or orthopedic surgery.

Biomimetic Citrate-Coated Luminescent Apatite Nanoplatforms for Diclofenac Delivery in Inflammatory Environments.

Luminescent nanoparticles are innovative tools for medicine, allowing the imaging of cells and tissues, and, at the same time, carrying and releasing different types of molecules. We explored and compared the loading/release ability of diclofenac (COX-2 antagonist), in both undoped- and luminescent Terbium3+ (Tb3+)-doped citrate-coated carbonated apatite nanoparticles at different temperatures (25, 37, 40 °C) and pHs (7.4, 5.2). The cytocompatibility was evaluated on two osteosarcoma cell lines and primary human osteoblasts. Biological effects of diclofenac-loaded-nanoparticles were monitored in an in vitro osteoblast’s cytokine–induced inflammation model by evaluating COX-2 mRNA expression and production of PGE2. Adsorption isotherms fitted the multilayer Langmuir-Freundlich model. The maximum adsorbed amounts at 37 °C were higher than at 25 °C, and particularly when using the Tb3+ -doped particles. Diclofenac-release efficiencies were higher at pH 5.2, a condition simulating a local inflammation. The luminescence properties of diclofenac-loaded Tb3+ -doped particles were affected by pH, being the relative luminescence intensity higher at pH 5.2 and the luminescence lifetime higher at pH 7.4, but not influenced either by the temperature or by the diclofenac-loaded amount. Both undoped and Tb3+-doped nanoparticles were cytocompatible. In addition, diclofenac release increased COX-2 mRNA expression and decreased PGE2 production in an in vitro inflammation model. These findings evidence the potential of these nanoparticles for osteo-localized delivery of anti-inflammatory drugs and the possibility to localize the inflammation, characterized by a decrease in pH, by changes in luminescence.

3D printed concentrated alginate/GelMA hollow-fibers-packed scaffolds with nano apatite coatings for bone tissue engineering

Three-dimensional grafts/scaffolds with hierarchically biomimetic features from nano to macro scale and high porosity are required for bone tissue engineering. In this study, biomimetic organic/inorganic composite scaffolds with high porosity (78.7 ± 3.2%) and features from nano (nano apatite coatings) to macro (macro pores and hollow channels) scale were fabricated based on highly concentrated alginate/GelMA bioinks via co-axial 3D printing and in situ mineralization under mild conditions. Nano apatites were coated on both inner and outer surfaces of the hollow fiber scaffolds, homogeneously. Proteins were directly loaded in the bioinks achieving sustained release from the scaffolds over 28 days. The in vitro cell experiments showed that the scaffolds with good biocompatibility could support cells adhesion and proliferation. The nano apatite coatings presented remarkable osteogenic capability. The in vivo study indicated that the hollow fiber scaffolds with biomimetic nano apatite coatings showed the capability to enhance bone formation after 12 weeks of implantation. In conclusion, the prepared biomimetic organic/inorganic scaffolds with homogeneous nano apatite coatings and hollow channels structures might be potential candidates for bone tissue engineering.

Формирование биомиметического апатита на кальцийфосфатной пенокерамике в концентрированной модельной среде

By firing polyurethane foam templates (“STR” brand, 12 pores per cm, China) with a porosity of ~ 65 % at 1200 °С, an open-pore calcium phosphate foam ceramics was obtained using a highly concentrated suspension based on synthetic hydroxyapatite, heat-treated at 800 °С, monocalcium phosphate monohydrate and 0.8 % polyvinyl alcohol. The resulting calcium phosphate foam ceramics after modification in the SBF (Simulated Body Fluid) solution concentrated 5 times (SBF×5) consisted of β-tricalcium phosphate, β-calcium pyrophosphate and biomimetic apatite, had a porosity of 53 – 59 % and a static strength of ~ 0.05 MPa. The formed biomimetic apatite, consisting of amorphous calcium phosphate Ca9(PO4)6 and apatite tricalcium phosphate Ca9HPO4(PO4)5OH, crystallizes into β-tricalcium phosphate at 1200 °С. Calcium phosphate foam ceramics modified with biomimetic apatite, after soaking in 5 % hydroxyapatite gel and SBF×5, which simulating a bone defect in vitro, in parallel with the formation of biomimetic apatite, is partially destroyed, which confirmed its high bioactivity and degradation.

Surface characteristics and in-vitro studies of TiO2 coatings by plasma electrolytic oxidation in potassium-phosphate electrolyte

Plasma electrolytic oxidation (PEO) was used to produce titanium oxide (TiO2) coatings on Ti surface in potassium – phosphate electrolyte. The morphology, wettability, phase, and chemical compositions were studied as a function of processing parameters. The bioactivity of the coating was assessed by the ability to form biomimetic apatite in-vitro using cell culture medium. In-vitro studies using human mesenchymal stem cells were also conducted to evaluate cells' proliferation and viability of the treated Ti. The results revealed that the produced TiO2 coatings comprised pore features with the pore size increasing with applied current density and treatment duration due to high energy discharge channels at higher potential. The PEO treated Ti exhibited superhydrophilic characteristics with a contact angle <1°. The findings indicated that the large actual surface area produced by the PEO treatment and the presence of negatively charge PO43− are the key factors for the superhydrophilic behavior. The in-vitro studies revealed that the PEO treated groups had higher amount of biomimetic apatite formation compared to the as-polished Ti. The PEO treatment significantly enhanced the cells’ adhesion and growth after 24 and 72 h compared to the untreated Ti. A significant difference in the bioactivity was not observed between anatase and rutile.

Activated Carbon Fiber Cloth/Biomimetic Apatite: A Dual Drug Delivery System

A biomaterial that is both bioactive and capable of controlled drug release is highly attractive for bone regeneration. In previous works, we demonstrated the possibility of combining activated carbon fiber cloth (ACC) and biomimetic apatite (such as calcium-deficient hydroxyapatite (CDA)) to develop an efficient material for bone regeneration. The aim to use the adsorption properties of an activated carbon/biomimetic apatite composite to synthetize a biomaterial to be used as a controlled drug release system after implantation. The adsorption and desorption of tetracycline and aspirin were first investigated in the ACC and CDA components and then on ACC/CDA composite. The results showed that drug adsorption and release are dependent on the adsorbent material and the drug polarity/hydrophilicity, leading to two distinct modes of drug adsorption and release. Consequently, a double adsorption approach was successfully performed, leading to a multifunctional and innovative ACC-aspirin/CDA-tetracycline implantable biomaterial. In a second step, in vitro tests emphasized a better affinity of the drug (tetracycline or aspirin)-loaded ACC/CDA materials towards human primary osteoblast viability and proliferation. Then, in vivo experiments on a large cortical bone defect in rats was carried out to test biocompatibility and bone regeneration ability. Data clearly highlighted a significant acceleration of bone reconstruction in the presence of the ACC/CDA patch. The ability of the aspirin-loaded ACC/CDA material to release the drug in situ for improving bone healing was also underlined, as a proof of concept. This work highlights the possibility of bone patches with controlled (multi)drug release features being used for bone tissue repair.

Nano-imaging confirms improved apatite precipitation for high phosphate/silicate ratio bioactive glasses

Bioactive glasses convert to a biomimetic apatite when in contact with physiological solutions; however, the number and type of phases precipitating depends on glass composition and reactivity. This process is typically followed by X-ray diffraction and infrared spectroscopy. Here, we visualise surface mineralisation in a series of sodium-free bioactive glasses, using transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy (EDXS) and X-ray nano-computed tomography (nano-CT). In the glasses, the phosphate content was increased while adding stoichiometric amounts of calcium to maintain phosphate in an orthophosphate environment in the glass. Calcium fluoride was added to keep the melting temperature low. TEM brought to light the presence of phosphate clustering and nearly crystalline calcium fluoride environments in the glasses. A combination of analytical methods, including solid-state NMR, shows how with increasing phosphate content in the glass, precipitation of calcium fluoride during immersion is superseded by fluorapatite precipitation. Nano-CT gives insight into bioactive glass particle morphology after immersion, while TEM illustrates how compositional changes in the glass affect microstructure at a sub-micron to nanometre-level.

Biofunctionalization of electrospun fiber membranes by LbL-collagen/chondroitin sulfate nanocoating followed by mineralization for bone regeneration

It is of great significance to develop osteoinductive artificial scaffold for bone repair and regeneration. We constructed a biomimetic apatite interface on electrospun polycaprolactone fibers by combining layer-by-layer (LbL) nanocoating with mineralization to fabricate an osteoinductive artificial scaffold. After polydopamine modification, cationic type-І collagen and anionic chondroitin sulfate were sequentially adsorbed on the fiber surface. The fibers coated with the multilayer components served as the precursor matrix to induce apatite deposition. By adjusting the number of the layers and duration of mineralization, the nanoscale morphology of composite fibers was optimized. When ten bilayers of the collagen and chondroitin sulfate were deposited onto the fibers followed by one day-mineralization, the obtained polycaprolactone–apatite composite scaffolds significantly promoted the adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 cells. In a subcutaneous implantation in mice, this composite fiber membrane enhanced in vivo ectopic osteogenesis. Our nano-architectural scaffolds were able to mimic the composition and structure of the bone matrix to a certain extent, holding great potential for bone repair and regeneration.

Mesoporous Silica-Bioglass Composite Pellets as Bone Drug Delivery System with Mineralization Potential.

For decades, local bone drug delivery systems have been investigated in terms of their application in regenerative medicine. Among them, inorganic polymers based on amorphous silica have been widely explored. In this work, we combined two types of amorphous silica: bioglass and doxycycline-loaded mesoporous silica MCM-41 into the form of spherical granules (pellets) as a bifunctional bone drug delivery system. Both types of silica were obtained in a sol-gel method. The drug adsorption onto the MCM-41 was performed via adsorption from concentrated doxycycline hydrochloride solution. Pellets were obtained on a laboratory scale using the wet granulation-extrusion-spheronization method and investigated in terms of physical properties, drug release, antimicrobial activity against Staphylococcus aureus, mineralization properties in simulated body fluid, and cytotoxicity towards human osteoblasts. The obtained pellets were characterized by satisfactory mechanical properties which eliminated the risk of pellets cracking during further investigations. The biphasic drug release from pellets was observed: burst stage (44% of adsorbed drug released within the first day) followed by prolonged release with zero-order kinetics (estimated time of complete drug release was 19 days) with maintained antimicrobial activity. The progressive biomimetic apatite formation on the surface of the pellets was observed. No cytotoxic effect of pellets towards human osteoblasts was noticed.

Beyond dissolution: Xerostomia rinses affect composition and structure of biomimetic dental mineral in vitro.

Xerostomia, known as dry mouth, is caused by decreased salivary flow. Treatment with lubricating oral rinses provides temporary relief of dry mouth discomfort; however, it remains unclear how their composition affects mineralized dental tissues. Therefore, the objective of this study was to analyze the effects of common components in xerostomia oral rinses on biomimetic apatite with varying carbonate contents. Carbonated apatite was synthesized and exposed to one of the following solutions for 72 hours at varying pHs: water-based, phosphorus-containing (PBS), mucin-like containing (MLC), or fluoride-containing (FC) solutions. Post-exposure results indicated that apatite mass decreased irrespective of pH and solution composition, while solution buffering was pH dependent. Raman and X-ray diffraction analysis showed that the addition of phosphorus, mucin-like molecules, and fluoride in solution decreases mineral carbonate levels and changed the lattice spacing and crystallinity of bioapatite, indicative of dissolution/recrystallization processes. The mineral recrystallized into a less-carbonated apatite in the PBS and MLC solutions, and into fluorapatite in FC. Tap water did not affect the apatite lattice structure suggesting formation of a labile carbonate surface layer on apatite. These results reveal that solution composition can have varied and complex effects on dental mineral beyond dissolution, which can have long term consequences on mineral solubility and mechanics. Therefore, clinicians should consider these factors when advising treatments for xerostomia patients.

Carbonate substitution significantly affects the structure and mechanics of carbonated apatites

Bone mineral comprises nanoparticles of carbonate-substituted bioapatite similar to hydroxylapatite. Yet mechanical values of macroscopic-sized geological hydroxylapatite are often used to model bone properties due to a lack of experimental data for bioapatite. Here, we investigated the effects of carbonate substitution and hydration on biomimetic apatite response to load using in situ hydrostatic pressure loading and synchrotron x-ray diffraction. We find that increasing carbonate levels reduced the bulk modulus and elastic strain ratio. Elastic constants, determined using computational optimization techniques, revealed that compliance values and elastic moduli decreased with increasing carbonate content, likely a result of decreased bond strength due to CO32− substitution and Ca2+ loss. Hydration environment had no clear effects on the elastic properties likely due to dissolution and reprecipitation processes modifying the crystal structure organization. These results reinforce the need to consider carbonate composition when selecting mechanical properties and provide robust data for carbonate-substituted apatite stiffness.

Osteopontin regulates biomimetic calcium phosphate crystallization from disordered mineral layers covering apatite crystallites

Details of apatite formation and development in bone below the nanometer scale remain enigmatic. Regulation of mineralization was shown to be governed by the activity of non-collagenous proteins with many bone diseases stemming from improper activity of these proteins. Apatite crystal growth inhibition or enhancement is thought to involve direct interaction of these proteins with exposed faces of apatite crystals. However, experimental evidence of the molecular binding events that occur and that allow these proteins to exert their functions are lacking. Moreover, recent high-resolution measurements of apatite crystallites in bone have shown that individual crystallites are covered by a persistent layer of amorphous calcium phosphate. It is therefore unclear whether non-collagenous proteins can interact with the faces of the mineral crystallites directly and what are the consequences of the presence of a disordered mineral layer to their functionality. In this work, the regulatory effect of recombinant osteopontin on biomimetic apatite is shown to produce platelet-shaped apatite crystallites with disordered layers coating them. The protein is also shown to regulate the content and properties of the disordered mineral phase (and sublayers within it). Through solid-state NMR atomic carbon-phosphorous distance measurements, the protein is shown to be located in the disordered phases, reaching out to interact with the surfaces of the crystals only through very few sidechains. These observations suggest that non-phosphorylated osteopontin acts as regulator of the coating mineral layers and exerts its effect on apatite crystal growth processes mostly from afar with a limited number of contact points with the crystal.

Phosphatidylserine controls calcium phosphate nucleation and growth on lipid monolayers: A physicochemical understanding of matrix vesicle-driven biomineralization

Bone biomineralization is an exquisite process by which a hierarchically organized mineral matrix is formed. Growing evidence has uncovered the involvement of one class of extracellular vesicles, named matrix vesicles (MVs), in the formation and delivery of the first mineral nuclei to direct collagen mineralization. MVs are released by mineralization-competent cells equipped with a specific biochemical machinery to initiate mineral formation. However, little is known about the mechanisms by which MVs can trigger this process. Here, we present a combination of in situ investigations and ex vivo analysis of MVs extracted from growing-femurs of chicken embryos to investigate the role played by phosphatidylserine (PS) in the formation of mineral nuclei. By using self-assembled Langmuir monolayers, we reconstructed the nucleation core - a PS-enriched motif thought to trigger mineral formation in the lumen of MVs. In situ infrared spectroscopy of Langmuir monolayers and ex situ analysis by transmission electron microscopy evidenced that mineralization was achieved in supersaturated solutions only when PS was present. PS nucleated amorphous calcium phosphate that converted into biomimetic apatite. By using monolayers containing lipids extracted from native MVs, mineral formation was also evidenced in a manner that resembles the artificial PS-enriched monolayers. PS-enrichment in lipid monolayers creates nanodomains for local increase of supersaturation, leading to the nucleation of ACP at the interface through a multistep process. We posited that PS-mediated nucleation could be a predominant mechanism to produce the very first mineral nuclei during MV-driven bone/cartilage biomineralization.

Structure of biomimetic apatite grown on hydroxyapatite (HA)

Nanocrystalline hydroxyapatite (HA) is the bone mineral phase in vertebrate animals. When bones suffer illness or injuries, natural hydroxyapatite reconstructs and remodels its structure. It has been demonstrated that synthetic hydroxyapatite materials emulate bone properties and enhance bone-bonding ability through a biomimetic apatite coating, obtained when bioactive materials are immersed in a physiological-like media. In this research, the biomimetic apatite structure deposited in three synthetic hydroxyapatites (HAs: HAMS, HAUB, and HAUT), after being immersed in a simulated body fluid (SBF), is studied. The synthetic HAs are obtained by co-precipitation, using three different stirring methods: magnetic stirring (HAMS), ultrasonic bath (HAUB), and ultrasonic tip (HAUT). The synthetic HAs are characterized by X-Ray Diffraction (X-RD), High-Resolution Scanning Electron Microscopy (HR-SEM), Energy Dispersive X-ray (EDX) microanalysis and Transmission Electron Microscopy (TEM) before and after SBF assays. The crystal sizes of synthetic HAs are 12.567 nm, 15.188 nm, and 17.903 nm for HAMS, HAUB, and HAUT, respectively. Additionally, HAs have values between 0.940 and 0.942 nm for a-cell parameter, and values between 0.688 and 0.689 nm for c-cell parameter. After 28 days of immersion in Simulated Body Fluid (SBF), the materials show poor crystalline-fine films of biomimetic apatite covering their surfaces. These biomimetic apatite films have values between 0.946 and 0.969 nm for the a-cell parameter and values between 0.698 and 0.710 nm for the c-cell parameter. Hence, crystallinity, crystal size, and morphology of synthetic HAs are dependent on the stirring method used during the synthesis process, which affects the structural characteristics of the biomimetic apatite layers deposited on the synthetic HAs surfaces.

Characterization of the in Vitro Osteogenic Response to Submicron TiO2 Particles of Varying Structure and Crystallinity.

Titanium oxide (TiO2) nano-/microparticles have been widely used in orthopedic and dental sciences because of their excellent mechanical properties, chemical stability, and ability to promote the osseointegration of implants. However, how the structure and crystallinity of TiO2 particles may affect their osteogenic activity remains elusive. Herein, we evaluated the osteogenic response to submicron amorphous, anatase, and rutile TiO2 particles with controlled size and morphology. First, the ability of TiO2 particles to precipitate apatite was assessed in an acellular medium by using a simulated body fluid (SBF). Three days after the addition to SBF, anatase and rutile TiO2 particles induced the precipitation of aggregates of nanoparticles with a platelike morphology, typical for biomimetic apatite. Conversely, amorphous TiO2 particles induced the precipitation of particles with poor Ca/P atomic ratio only after 14 days of exposure to SBF. Next, the osteogenic response to TiO2 particles was assessed in vitro by incubating MC3T3-E1 preosteoblasts with the particles. The viability and mineralization efficiency of osteoblastic cells were maintained in the presence of all the tested TiO2 particles despite the differences in the induction of apatite precipitation in SBF by TiO2 particles with different structures. Analysis of the particles’ surface charge and of the proteins adsorbed onto the particles from the culture media suggested that all the tested TiO2 particles acquired a similar biological identity in the culture media. We posited that this phenomenon attenuated potential differences in osteoblast response to amorphous, anatase, and rutile particles. Our study provides an important insight into the complex relationship between the physicochemical properties and function of TiO2 particles and sheds light on their safe use in medicine.

On the amorphous layer in bone mineral and biomimetic apatite: A combined small- and wide-angle X-ray scattering analysis

The occurrence of an amorphous calcium phosphate layer covering the crystalline apatite core has been suggested to be an intrinsic feature of both bone mineral and synthetic biomimetic analogs. However, an exahustive quantitative picture of the amorphous-crystalline relationship in these materials is still missing. Here, we present a multiple scale modelling that combines small-angle X-ray scattering (SAXS) and synchrotron wide-angle X-ray total scattering (WAXTS) analyses to investigate the amorphous-crystalline spatial interplay in bone sample and biomimetic carbonated nano-apatites. SAXS analysis indicates the presence of a single morphology consisting of tiny nanoplates (NPLs) and provides a measure of their thickness (falling in the 3–5 nm range). WAXTS analysis was performed by developing atomistic models of apatite NPLs incorporating lattice strain, mostly attributed to the carbonate content, and calculating the X-ray patterns using the Debye Scattering Equation. Upon model optimization, the size and strain parameters of the crystalline platelets were derived and the amorphous component, co-existing with the crystalline one, separated and quantified (in the 23–33 wt% range). Notably, the thickness of the apatite core was found to exhibit nearly null (bone) or minor (< 0.5 nm, biomimetic samples) deviations from that of the entire NPLs, suggesting that the amorphous material remains predominantly distributed along the lateral sides of the NPLs, in a core-crown-like arrangement. The lattice strain analysis indicates a significant stiffness along the c axis, which is comparable in bone and synthetic samples, and larger deformations in the other directions. Statement of SignificanceCurrent models of bone mineral and biomimetic nanoapatites suggest the occurrence of an amorphous layer covering the apatitic crystalline nanoplates in a core-shell arrangement. By combining X-ray scattering techniques in the small and wide angle regions, we propose a joint atomic-to-nanometre scale modelling to investigate the amorphous-crystalline interplay within the nanoplates. Estimates are extracted for the thickness of the entire nanoplates and the crystalline core, together with the quantification of the amorphous fraction and apatite lattice strain. Based on the thickness matching, the location of the amorphous material mostly along the edges of the nanoplates is inferred, with a vanishing or very thin layer in the thickness direction, suggesting a core-crown-like arrangement, with possible implications on the mineral surface reactivity.

Effect of laser functionalization of titanium on bioactivity and biological response

This study proposes Direct Laser Interference Lithography as a highly suitable technique for functionalization titanium surfaces for biomedical applications. DLIL was employed to produce periodic patterns on titanium after two commonly used surface treatments of implantable devices (shot peening and acid etching). The biomedical potential of the proposed method was analyzed using immersion tests in a simulated body fluid solution and cell adhesion tests. After 48 h of immersion surface morphology, the chemical composition and phase structure of the apatite layers deposited on the modified titanium were analyzed. In order to analyze the kinetics of the apatite layer growth, X-Ray Photoelectron Spectroscopy measurements at different soaking times were performed. Cell adhesion tests were performed using human fetal osteoblastic cells (hFOB). The adhered cells were analyzed using confocal and scanning electron microscopies after 48 h of incubation. The formation of biomimetic apatite layers was accelerated on the titanium surface structures after DLIL modification. The periodic titanium patterns induced more uniform and direct cell growth. This effect is mainly connected with the surface properties of the DLIL-modified substrates. The formation mechanism of biomimetic apatite on the textured titanium samples, as well as the compounds created on the surface, are discussed.

Insights into OCP identification and quantification in the context of apatite biomineralization

Monitoring apatite formation throughin situRAMAN andex situssNMR spectroscopy.

Ag/ZnO heterostructure fabricated on AZO platform for SERS based sensitive detection of biomimetic hydroxyapatite

Versatile surface-enhanced Raman spectroscopy (SERS) active Ag/ZnO heterostructures grown on glass and conducting Al-doped ZnO (AZO) have been developed to enable detection of analytes having very low molar concentration. The SERS substrate is composed of Ag nanoparticles decorated on ZnO microrods arranged mostly in flower-like patterns. ZnO microrods are synthesized by a hydrothermal method followed by the photo-deposition of Ag nanoparticles to fabricate Ag/ZnO heterostructure. The demonstration of the SERS activity is first accomplished by probing methyl orange and rhodamine 6G with a band specific SERS substrate enhancement factor (SSEF) as high as 5.4 × 107 and analytical enhancement factor (AEF) of 1.3 × 1010, exceptionally high enhancement achieved through use of the metal-semiconductor heterostructure where charge transfer at the interface plays an important role during SERS. The nanocrystalline hydroxyapatite prepared by a biomimetic process is detectable using the SERS active substrate with considerable enhancement of the characteristic Raman signals. This work is a novel attempt to probe biomimetic hydroxyapatite using SERS which would pave the way to understand early stages of bone mineralization. The use of AZO, an indium free transparent conductor is crucial to facilitate enhanced charge transfer at the interface which accentuates Raman active modes at low concentration of biomimetic apatite.

Influence of carbonation on the low-temperature consolidation by Spark Plasma Sintering of carbonated calcium phosphate bioceramics

Calcium phosphates (CaP) such as biomimetic nanocrystalline apatite or amorphous calcium phosphate are hydrated bioactive compounds particularly suitable for bone repair applications due to their similarity with bone mineral. However, their consolidation in ceramic parts deserves special attention as they are thermodynamically metastable and can decompose into less bioactive phases upon heating. Adapted strategies are needed to obtain bulk bioceramics. Spark Plasma Sintering (SPS) has been shown to allow cold sintering of such compounds at temperatures like 150 °C while preserving the hydrated character and nanosized dimensions of the precursor powders. To this date, however, the role of the degree of carbonation of these precursors on the densification of CO3-bearing CaP compounds via SPS has not been explored despite the natural carbonation of bone. In this work, several carbonated CaP hydrated compounds were prepared and consolidated by SPS and the characteristics of the obtained ceramics was scrutinized with respect to the starting powders. Two carbonation routes were carried out: via volume carbonation during powder synthesis or via subsequent surface ion exchange. All samples tested led to apatitic compounds after SPS, including amorphous CaP. We show that the degree of carbonation negatively affects the densification rate and propose possible hypotheses explaining this behavior. Evolution in the nature of the carbonate sites (apatitic A-, B-types and labile surface carbonates) before and after SPS is also noticed and commented. The consolidation of such compounds is however proven possible, and gives rise to bone-like apatitic compounds with great potential as bioactive resorbable ceramics for bone regeneration.