Growth and characterization of hydroxyapatite crystals by hydrothermal method

Synthesis and Investigation of Physicochemical Properties and Biocompatibility of Phosphate-Vanadate Hydroxyapatite Co-Doped with Tb3+ and Sr2+ Ions

Searching for biocompatible materials with proper luminescent properties is of fundamental importance, as they can be applied in fluorescent labeling and regenerative medicine. In this study, we obtained new phosphate-vanadate hydroxyapatites (abbr. HVps) co-doped with Sr²⁺ and Tb³⁺ ions via the hydrothermal method. We focused on examining the effect of various annealing temperatures (500, 600 and 700 °C) on the spectroscopic properties and morphology of the obtained HVps. To characterize their morphology, XRPD (X-ray powder diffraction), SEM-EDS (scanning electron microscopy-energy-dispersive spectrometry), FT-IR (Fourier transform infrared) spectroscopy and ICP-OES (inductively coupled plasma-optical emission spectrometry) techniques were used. A further study of luminescent properties and cytocompatibility showed that the obtained HVps co-doped with Sr²⁺ and Tb³⁺ ions are highly biocompatible and able to enhance the proliferation process and can therefore be potentially used as fluorescent probes or in regenerative medicine.

Effect of the Structural and Morphological Properties of Surfactant-Assisted Hydroxyapatite on Dermal Irritation and Antibacterial Activity

Hydroxyapatite (HAp) nanoparticles with a homogeneous rod morphology were successfully synthesized using the hydrothermal method. The powders were characterized using Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. The antibacterial and dermal irritation analyses of the samples were performed and discussed. The use of cationic and anionic surfactants, namely, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), respectively, at a low concentration (2.5 mol%) modified the length/diameter (L/D) ratio of the HAp rods. Structural characterizations of hydroxyapatite synthesized without surfactant (HA), with 2.5 and 5 mol% of SDS (SDS− and SDS+, respectively), and with 2.5 and 5 mol% of CTAB (CTAB− and CTAB+, respectively) revealed well-crystallized samples in the hexagonal phase. The CTAB− sample presented antibacterial activity against Pseudomonas aeruginosa, Escherichia coli, Streptococcus anginosus, Staphylococcus aureus, Micrococcus luteus, and Klebsiella pneumoniae, suggesting that antimicrobial susceptibility was promoted by the bacterial nature and the use of the surfactant. Dermal irritation showed no clinical signs of disease in rabbits during the study, where there was neither erythema nor necrosis at the inoculation sites.

Biocompatibility Studies of Nanoengineered Polycaprolactone and Nanohydroxyapatite Scaffold for Craniomaxillofacial Bone Regeneration

Currently there is an increased demand for synthetic bone substitute materials (SBSMs) due to avoidance of donor-site surgery and morbidity. Attempts are done to create SBSM mimicking the bone microarchitecture for enhanced healing. In this study, the authors nanoengineered polycaprolactone (PCL) and nanohydroxyapatite (nHAp) composite scaffold by electrospinning. The nHAp is synthesized via hydrothermal process followed by microwave irradiation. In vitro biocompatibility evaluation with MG63 osteoblastic cell line showed enhanced cell proliferation in the PCL-nHAp scaffold than plain PCL by MTT assay and fluorescence microscopy. Increased osteogenesis in the PCL-nHAp scaffold was shown by the increased calcium load, alkaline phosphatase activity, and expression of osteogenic biomarkers namely osteocalcin, osteonectin, and osteopontin. In vivo studies conducted in rabbit femur bone defects showed increased bone regeneration in PCL-nHAp implanted defects. The results show that PCL-nHAp electrospun scaffold is biomimetic and highly osteogenic and thus a potential SBSM for critical size craniomaxillofacial bone defect applications.

Interaction of Folic Acid with Nanocrystalline Apatites and Extension to Methotrexate (Antifolate) in View of Anticancer Applications

Nanocrystalline apatites mimicking bone mineral represent a versatile platform for biomedical applications thanks to their similarity to bone apatite and the possibility to (multi)functionalize them so as to provide "à la carte" properties. One relevant domain is in particular oncology, where drug-loaded biomaterials and engineered nanosystems may be used for diagnosis, therapy, or both. In a previous contribution, we investigated the adsorption of doxorubicin onto two nanocrystalline apatite substrates, denoted HA and FeHA (superparamagnetic apatite doped with iron ions), and explored these drug-loaded systems against tumor cells. To widen their applicability in the oncology field, here we examine the interaction between the same two substrates and two other molecules: folic acid (FA), often used as cell targeting agent, and the anticancer drug methotrexate (MTX), an antifolate analogue. In a first stage, we investigated the adsorptive behavior of FA (or MTX) on both substrates, evidencing their specificities. At low concentration, typically under 100 mmol/L, adsorption onto HA was best described using the Sips isotherm model, while the formation of a calcium folate secondary salt was evidenced at high concentration by Raman spectroscopy. Adsorption onto FeHA was instead fitted to the Langmuir model. A larger adsorptive affinity was found for the FeHA substrate compared to HA; accordingly, a faster release was noticed from HA. In vitro tests carried out on human osteosarcoma cell line (SAOS-2) allowed us to evaluate the potential of these compounds in oncology. Finally, in vivo (subcutaneous) implantations in the mouse were run to ascertain the biocompatibility of the two substrates. These results should allow a better understanding of the interactions between FA/MTX and bioinspired nanocrystalline apatites in view of applications in the field of cancer.

Salivary acquired pellicle-inspired DpSpSEEKC peptide for the restoration of demineralized tooth enamel

Salivary acquired pellicle (SAP) is a layer of proteins and glycoproteins of salivary origin that tightly coat the tooth surface. Statherin is an important part of the SAP. The initial six-peptide sequence DpSpSEEK (where pS denotes phosphorylated serine) of the N-terminus of statherin can be immobilized on a hydroxyapatite (HAP) surface and the negatively charged domains of the DpSpSEEK side chain can catch free Ca²⁺ in saliva due to the charge adsorption effect. In order to prepare more functional materials based on DpSpSEEK, we designed a cysteine-labeled peptide sequence DpSpSEEKC, which could conjugate other macromolecules by forming a sulfur-based linkage. In this work, we measured the adsorption of DpSpSEEKC to HAP by various methods. We also coated DpSpSEEKC on a demineralized tooth enamel surface to evaluate its biomineralization capacity. The DpSpSEEKC-coated samples were characterized after immersion in artificial saliva for 2 weeks. The results showed that DpSpSEEKC has a strong adsorption capacity to HAP and could induce remineralization on the demineralized tooth enamel surface due to its carboxyl and phosphate groups. Compared with the control samples, the mechanical properties of the DpSpSEEKC-coated samples were obviously improved. In conclusion, DpSpSEEKC can provide a potential method for restoring demineralized tooth enamel.

Amelogenin in Enamel Tissue Engineering

In this chapter the basic premises, the recent findings and the future challenges in the use of amelogenin for enamel tissue engineering are being discoursed on. Results emerging from the experiments performed to assess the fundamental physicochemical mechanisms of the interaction of amelogenin, the main protein of the enamel matrix, and the growing crystals of apatite, are mentioned, alongside a moderately comprehensive literature review of the subject at hand. The clinical importance of understanding this protein/mineral interaction at the nanoscale are highlighted as well as the potential for tooth enamel to act as an excellent model system for studying some of the essential aspects of biomineralization processes in general. The dominant paradigm stating that amelogenin directs the uniaxial growth of apatite crystals in enamel by slowing down the growth of (hk0) faces on which it adheres is being questioned based on the results demonstrating the ability of amelogenin to promote the nucleation and crystal growth of apatite under constant titration conditions designed to mimic those present in the developing enamel matrix. The role of numerous minor components of the enamel matrix is being highlighted as essential and impossible to compensate for by utilizing its more abundant ingredients only. It is concluded that the three major aspects of amelogenesis outlined hereby--(1) the assembly of amelogenin and other enamel matrix proteins, (2) the proteolytic activity, and (3) crystallization--need to be in precise synergy with each other in order for the grounds for the proper imitation of amelogenesis in the lab to be created.

Droplet-fused microreactors for room temperature synthesis of nanoscale needle-like hydroxyapatite

A microfluidic device using droplet-fused microreactors is introduced for room temperature synthesis of nanoscale needle-shaped hydroxyapatite (HAp, Ca10(PO4)6(OH)2). The device is integrated with multifunctional units, e.g., T-junctions for droplet generation and fusion, winding channels for rapid mixing, and a delay line for simple visualization of the HAp formation process. The necessary conditions such as surfactant and fluid flow rate for an aqueous stream to merge with water-in-oil droplets are investigated. The nanoscale morphologies of the HAp produced by this method are also compared with HAp prepared by conventional bulk mixing. This paper shows that further reaction could be initiated by flowing additional reagent streams directly into the droplets of the initial reaction mixture, which is a novel approach for synthesizing a needle-like morphology of the HAp with a high aspect ratio under room temperature.

Blood compatibility of iron-doped nanosize hydroxyapatite and its drug release

Nanosize hydroxyapatite (nHAp) doped with varying levels of Fe(3+) (Fe-nHAp of average size 75 nm) was synthesized by hydrothermal and microwave techniques. The samples were characterized for physiochemical properties by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), inductively coupled plasma optical emission spectrometer (ICP-OES), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), mechanical and dielectric properties. The biological properties like hemocompatibility, antibacterial efficacy, in vitro bioactivity and the cell proliferation of the samples were determined. XRD pattern of the samples were of single phase hydroxyapatite. As the content of Fe(3+) increased, the crystallite size as well as crystallinity decreased along with a morphological change from spherulites to rods. The dielectric constants and Vickers hardness were enhanced on Fe(3+) doping. The VSM studies revealed that the saturation magnetization (M(s)) and retentivity (M(r)) were found to increase for Fe-nHAp. nHAp impregnated with an antibiotic as a new system for drug delivery in the treatment of chronic osteomyelitis was also attempted. The in vitro drug release with an antibiotic amoxicillin and anticancer drug 5-fluorouracil showed sustained release for the lowest concentration of Fe(3+), while with an increase in the content; there was a rapid release of the drug. The hemolytic assay of Fe(3+) doped samples revealed high blood compatibility (<5% hemolysis). The antibacterial activities of the antibiotic impregnated materials were tested against a culture of E. coli, S. epidermidis and S. aureus by agar diffusion test. The in vitro bioactivity test using simulated body fluid (SBF) showed better bone bonding ability by the formation of an apatite layer on the doped samples. The growth of the apatite layer on the samples surface has been confirmed by EDS analysis. The proliferative potential of MG63 cells by MTT assay confirmed the noncytotoxicity of the samples.

Prospects and Pits on the Path of Biomimetics: The case of tooth enamel

This review presents a discourse on challenges in understanding and imitating the process of amelogenesis in vitro on the molecular scale. In light of the analysis of imitation of the growth of dental enamel, it also impends on the prospects and potential drawbacks of the biomimetic approach in general. As the formation of enamel proceeds with the protein matrix guiding the crystal growth, while at the same time conducting its own degradation and removal, it is argued that three aspects of amelogenesis need to be induced in parallel: a) crystal growth; b) protein assembly; c) proteolytic degradation. A particular emphasis is therefore placed on ensuring conditions for proteolysis-coupled protein-guided crystallization to occur. Discussed are structural and functional properties of the protein species involved in amelogenesis, mainly amelogenin and enamelysin, the main protein and the protease of the developing enamel matrix, respectively. A model of enamel growth based on controlled delivery of constituent ions or crystalline or amorphous building blocks by means of amelogenin is proposed. The importance of high viscosity of the enamel matrix and a more intricate role that water may play in such a gelatinous medium are also touched upon. The tendency of amelogenin to self-assemble into fibrous and rod-shaped morphologies is considered as potentially important in explaining the formation of elongated apatite crystals. The idea that a preassembling protein matrix serves as a template for the uniaxial growth of apatite crystals in enamel is finally challenged with the one based on co-assembly of the protein and the mineral phases.

Kinetics of hydrothermal crystallization under saturated steam pressure and the self-healing effect by nanocrystallite for hydroxyapatite coatings

Hydroxyapatite coatings (HACs) with a low crystalline state were prepared using the plasma spraying process followed by hermetic autoclaving hydrothermal treatment at 100, 150 and 200 degrees C. Experimental evidence confirmed that the HACs became significantly crystallized and the content of amorphous calcium phosphate decreased by performing the autoclaving hydrothermal treatment in an ambient saturated steam pressure system. The obvious chemisorbed hydroxy groups (OH) peak in the X-ray photoelectron spectra detected from the hydrothermally crystallized HAC specimens means that the hydroxyl-deficient state of plasma-sprayed HACs is significantly improved by the abundant replenished OH groups. The HA nanocrystallite observed from scanning electron microscopy and transmission electron microscopy images within hydrothermally treated HACs is the result of nucleation and grain growth through the replenishment of OH groups into the hydroxyl-deficient HA crystal structure. The microstructural self-healing effect is a result of reduction in defects (pores, microcracks and lamellar boundaries) due to new-growth HA nanocrystallite. According to the systematic derivation of the Arrhenius equation, the HA crystallization is a second-order Arrhenius reaction kinetics. Besides the effects of heating temperature and an atmosphere with abundant water molecules, the saturated steam pressure is a crucial factor which significantly improves the crystallization rate constant and further reduces the activation energy for the hydrothermal HA crystallization.