Structural and Dielectrical Properties of Ag- and Ba-Substituted Hydroxyapatites

The Effect of Doping on the Electrical and Dielectric Properties of Hydroxyapatite for Medical Applications: From Powders to Thin Films

Recently, the favorable electrical properties of biomaterials have been acknowledged as crucial for various medical applications, including both bone healing and growth processes. This review will specifically concentrate on calcium phosphate (CaP)-based bioceramics, with a notable emphasis on hydroxyapatite (HA), among the diverse range of synthetic biomaterials. HA is currently the subject of extensive research in the medical field, particularly in dentistry and orthopedics. The existing literature encompasses numerous studies exploring the physical-chemical, mechanical, and biological properties of HA-based materials produced in various forms (i.e., powders, pellets, and/or thin films) using various physical and chemical vapor deposition techniques. In comparison, there is a relative scarcity of research on the electrical and dielectric properties of HA, which have been demonstrated to be essential for understanding dipole polarization and surface charge. It is noteworthy that these electrical and dielectric properties also offer valuable insights into the structure and functioning of biological tissues and cells. In this respect, electrical impedance studies on living tissues have been performed to assess the condition of cell membranes and estimate cell shape and size. The need to fill the gap and correlate the physical-chemical, mechanical, and biological characteristics with the electrical and dielectric properties could represent a step forward in providing new avenues for the development of the next-generation of high-performance HA-doped biomaterials for future top medical applications. Therefore, this review focuses on the electrical and dielectric properties of HA-based biomaterials, covering a range from powders and pellets to thin films, with a particular emphasis on the impact of the various dopants used. Therefore, it will be revealed that each dopant possesses unique properties capable of enhancing the overall characteristics of the produced structures. Considering that the electrical and dielectric properties of HA-based biomaterials have not been extensively explored thus far, the aim of this review is to compile and thoroughly discuss the latest research findings in the field, with special attention given to biomedical applications.

Bioinorganic Preparation of Hydroxyapatite and Rare Earth Substituted Hydroxyapatite for Biomaterials Applications

Rare Earth elements in the lanthanide series are regarded as one of the finest options for the cationic substitution of calcium ions in hydroxyapatite (HA) because of their favorable impact on the biological characteristics of substituted HA. Neodymium and cerium were used to substitute 5% of calcium ions in HA, prepared via the wet precipitation method. Characterization tests for pure and substituted HA were conducted using XRD, FTIR, EDS, and FESEM. The results showed that changing part from calcium ions in hydroxyapatite to Nd and Ce ions altered its structure, composition, and morphology. Regarding the biological tests, the cytotoxicity test revealed a change in IC₅₀ for both normal and cancer cell lines, where substitution part of the Ca ions with rare Earth elements led to increasing antitumor activity in comparison with HA without substitution; in addition, antibacterial and fungicide activity was evident for both HA and Nd-Ce/HA, with a modest increase in antibacterial activity of Nd-Ce/HA against S. epidermidis and E. coli in comparison with HA. These findings may shed light on the process by which Nd and Ce ions improve the biological characteristics of pure HA and the increased potential of these bioceramics.

Generating bioactive and antiseptic interfaces with nano-silver hydroxyapatite-based coatings by pulsed electrochemical deposition for long-term efficient cervical soft tissue sealing

Infections related to osseointegrated implants have sparked the interest in studying titanium modification for long-term effective soft tissue sealing. Constructing a silver (Ag)-hydroxyapatite (HA) coating is regarded as an effective strategy for integrating antibiosis with osteanagenesis; however, the outcome for long-term cervical soft tissue sealing in vivo is compromised. It is challenging to construct an Ag-HA coating for long-term efficient soft tissue integration that instills a maximum antibacterial effect while retaining favorable bioactivity to normal gingival mesenchymal cells in vivo. In this study, we employed gradient concentrations of Ag/CaP by pulsed electrochemical deposition to fabricate optimal Ag-HA nanocoatings. By physicochemical analyses, these uniform coatings were mainly formed with spherical metallic and hydroxyapatite nanoparticles, which facilitated good hydrophilicity, moderate rough surfaces and corrosion protection. Furthermore, the nanocoating of the 1.5Ag/CaP group exhibited superior performances in dental follicle cells' proliferation, osteogenic differentiation and antibacterial properties mainly through direct contact inhibition and partially through sustained silver ion release, which resulted in functional cervical soft tissue sealing in beagles lasting for one year. Our investigations provide a feasible strategy to balance the long-term antibacterial demand and bioactive induction around osseointegrated implants for long-term efficient cervical soft tissue sealing.

Synthesis and physicochemical characteristics of Ag-doped hydroxyapatite nanoparticles, and their potential biomedical applications

In this present scenario, hydroxyapatite (HAp) nanostructures were synthesized through green routes for biomedical applications, particularly remediation towards human pathogens and cancer cells. The present study aims at forming non-toxic and eco-friendly silver (Ag⁺) doped HAp using Polyethylene glycol (PEG), Cetyl Trimethyl ammonium bromide (CTAB) and curcumin. Ag⁺ doped HAp nanoparticles (NPs) were prepared by the sol-gel method with a cube and rod-like morphology. Ag-HApNPs showed a sharp and well-defined diffraction peak, which possesses the hexagonal crystalline structure with space group P6₃/m. The Fourier-transform infrared spectroscopy and Raman spectra confirmed the formation of Ag-HApNPs, and the bandgap values were obtained using UV-DRS analysis. The Ag-HApNPs with PEG, CTAB and curcumin might be fabricated materials were examined against antibacterial, antifungal, antioxidant, and cytotoxic activities, which provided exemplary biomedical applications. Overall, Ag-HApNPs can be used as potential drug delivery and perspectives to control multidrug-resistant pathogens.

The Physico-Chemical Properties and Exploratory Real-Time Cell Analysis of Hydroxyapatite Nanopowders Substituted with Ce, Mg, Sr, and Zn (0.5-5 at.%)

Cation-substituted hydroxyapatite (HA), standalone or as a composite (blended with polymers or metals), is currently regarded as a noteworthy candidate material for bone repair/regeneration either in the form of powders, porous scaffolds or coatings for endo-osseous dental and orthopaedic implants. As a response to the numerous contradictions reported in literature, this work presents, in one study, the physico-chemical properties and the cytocompatibility response of single cation-doped (Ce, Mg, Sr or Zn) HA nanopowders in a wide concentration range (0.5–5 at.%). The modification of composition, morphology, and structure was multiparametrically monitored via energy dispersive X-ray, X-ray photoelectron, Fourier-transform infrared and micro-Raman spectroscopy methods, as well as by transmission electron microscopy and X-ray diffraction. From a compositional point of view, Ce and Sr were well-incorporated in HA, while slight and pronounced deviations were observed for Mg and Zn, respectively. The change of the lattice parameters, crystallite size, and substituting cation occupation factors either in the Ca(I) or Ca(II) sites were further determined. Sr produced the most important HA structural changes. The in vitro biological performance was evaluated by the (i) determination of leached therapeutic cations (by inductively coupled plasma mass spectrometry) and (ii) assessment of cell behaviour by both conventional assays (e.g., proliferation—3-(4,5-dimethyl thiazol-2-yl) 5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium assay; cytotoxicity—lactate dehydrogenase release assay) and, for the first time, real-time cell analysis (RTCA). Three cell lines were employed: fibroblast, osteoblast, and endothelial. When monophasic, the substituted HA supported the cells’ viability and proliferation without signs of toxicity. The RTCA results indicate the excellent adherence of cells. The study strived to offer a perspective on the behaviour of Ce-, Mg-, Sr-, or Zn-substituted HAs and to deliver a well-encompassing viewpoint on their effects. This can be highly important for the future development of such bioceramics, paving the road toward the identification of candidates with highly promising therapeutic effects.

Enhanced antibacterial and corrosion resistance properties of Ag substituted hydroxyapatite/functionalized multiwall carbon nanotube nanocomposite coating on 316L stainless steel for biomedical application

The present study reports the fabrication of silver substituted hydroxyapatite/functionalized multiwall carbon nanotube (Ag-HA/f-MWCNT) on 316L stainless steel (SS) implant by spray pyrolysis technique. XRD results show an enhanced crystallinity and crystallite sizes with increasing concentration of silver in HA/f-MWCNT. The vibrational spectral analysis revealed the presence of P-O stretching vibration of phosphate group (PO₄^3-) in all the samples. The morphology of Ag substituted HA/f-MWCNT coatings revealed regular rod-like particles arranged in the form of sheet exhibiting slight variation in the size of the particle with increasing the Ag concentration. All the samples indicate the presence of calcium, phosphor, carbon, silver and oxygen constituents in the coating surface. The minimum inhibitory concentration of the nanocomposite decreased from 0.25 mg to 0.125 mg with the increase of Ag concentration, and AO/EB results confirmed the mode of cell distraction. The 1 and 3 wt% Ag-HA/f-MWCNT nanocomposite revealed less toxic effect to the normal human osteoblast cells. The corrosion efficiencies of the fabricated films in the stimulated body fluid reveal the increase in polarization resistance with a decrease in current density (i_corr) from 3.9 to 3.5 μA due to the increase of Ag concentration. The estimated hemolysis rate for 1 and 3 wt% Ag substituted HA/f-MWCNT was less than 10%. Therefore, it can be concluded that 3 wt% Ag substituted HA/f-MWCNT coating on passivated 316L SS is nonhemolytic and most suited as a novel alternative to dental and orthopaedic implants.

Silver Doping Mechanism in Bioceramics—From Ag+: Doped HAp to Ag°/BCP Nanocomposite

The results presented in this paper, based on the powder X-ray diffraction technique followed by Rietveld analyses, are devoted to the mechanism of silver incorporation in biphasic calcium phosphates. Results were confirmed by SEM observation. Samples were synthesized via the sol-gel route, followed by heat treatments. Two incorporation sites were highlighted: Ca2+ replacement by Ag+ into the calcium phosphates (HAp: hydroxyapatite and β-TCP: tricalcium phosphate), and the other as metallic silver Ag° nanoparticles (formed by autogenous reduction). The samples obtained were thus nanocomposites, written Ag°/BCP, composed of closely-mixed Ag° particles of about 100 nm at 400 °C (which became micrometric upon heating) and calcium phosphates, themselves substituted by Ag+ cations. Between 400 °C and 700 °C the cationic silver part was mainly located in the HAp phase of the composition Ca10-xAgx(PO4)6(OH)2-x (written Ag+: HAp). From 600 °C silver cations migrated to β-TCP to form the definite compound Ca10Ag(PO4)7 (written Ag+: TCP). Due to the melting point of Ag°, the doping element completely left our sample at temperatures above 1000 °C. In order to correctly understand the biological behavior of such material, which is potentially interesting for biomaterial applications, its complex doping mechanism should be taken into consideration for subsequent cytotoxic and bacteriologic studies.

Surface protolytic property characterization of hydroxyapatite and titanium dioxide nanoparticles

We provide characterization data of hydroxyapatite (nHAp) and titanium dioxide (nTiO₂) nanoparticles as potential materials for ion sorption, e.g. in targeted therapy, barrier materials for waste repositories or photovoltaics. The study is focused on the determination of the values of protonation and ion exchange constants and site densities (∑SOH, ∑X; [mol kg⁻¹]) of nTiO₂ and nHAp for further Ra kinetics and sorption experiments. These data are very important for further investigation of the materials, which can be used e.g. as drug delivery systems or in engineered barriers of deep geological repositories. The characterization was based on the evaluation of the dependence of titrating agent consumption on pH. Titration results were evaluated on the basis of several model combinations, however the combination of the Chemical Equilibrium Model (CEM) and Ion Exchange Model (IExM) fits best to the experimental titration curves. However, the differences between the two sorbents were relatively large. Due to stability in a broad pH range and available surface sites, nTiO₂ seems to have a wide application range. The applicability of nHAp is not so wide because of its dissolution under pH 5. Both sorbents are virtually able to sorb cationic species on deprotonated edge and layer sites with different capacities, which can be important for sorption and decontaminating applications.

We provide characterization data of hydroxyapatite (nHAp) and titanium dioxide (nTiO₂) nanoparticles as potential materials for ion sorption, e.g. in targeted therapy, barrier materials for waste repositories or photovoltaics.

Cationic Substitutions in Hydroxyapatite: Current Status of the Derived Biofunctional Effects and Their In Vitro Interrogation Methods

High-performance bioceramics are required for preventing failure and prolonging the life-time of bone grafting scaffolds and osseous implants. The proper identification and development of materials with extended functionalities addressing socio-economic needs and health problems constitute important and critical steps at the heart of clinical research. Recent findings in the realm of ion-substituted hydroxyapatite (HA) could pave the road towards significant developments in biomedicine, with an emphasis on a new generation of orthopaedic and dentistry applications, since such bioceramics are able to mimic the structural, compositional and mechanical properties of the bone mineral phase. In fact, the fascinating ability of the HA crystalline lattice to allow for the substitution of calcium ions with a plethora of cationic species has been widely explored in the recent period, with consequent modifications of its physical and chemical features, as well as its functional mechanical and in vitro and in vivo biological performance. A comprehensive inventory of the progresses achieved so far is both opportune and of paramount importance, in order to not only gather and summarize information, but to also allow fellow researchers to compare with ease and filter the best solutions for the cation substitution of HA-based materials and enable the development of multi-functional biomedical designs. The review surveys preparation and synthesis methods, pinpoints all the explored cation dopants, and discloses the full application range of substituted HA. Special attention is dedicated to the antimicrobial efficiency spectrum and cytotoxic trade-off concentration values for various cell lines, highlighting new prophylactic routes for the prevention of implant failure. Importantly, the current in vitro biological tests (widely employed to unveil the biological performance of HA-based materials), and their ability to mimic the in vivo biological interactions, are also critically assessed. Future perspectives are discussed, and a series of recommendations are underlined.

Toxicity of lanthanum oxide nanoparticles to the fungus Moniliella wahieum Y12T isolated from biodiesel

Moniliella wahieum Y12^T, isolated from biodiesel was used as a model organism to assess the use of lanthanum oxide (La₂O₃) (60-80 nm) and silver oxide (AgO) (10-40 nm) nanoparticles as potential fungal inhibitors. This is the first study to investigate the use of nanoscale La₂O₃ as a eukaryotic bio-inhibitor. The AgO nanoparticles were relatively effective at inhibiting the growth of M. wahieum Y12^T. The half maximal effective concentration (EC₅₀) for AgO was 0.012 mg/mL as compared with 4.63 mg/mL of La₂O₃. Fluorescein diacetate analysis showed that AgO nanoparticles significantly reduced metabolic activity in M. wahieum Y12^T. The results of this study indicated that AgO nanoparticles can be a nonspecific inhibitor for the treatment of M. wahieum Y12^T, a eukaryotic biodiesel contaminant.

Functional hydroxyapatite bioceramics with excellent osteoconductivity and stern-interface induced antibacterial ability

Osteogenic Ag/HAp bioceramics possess significant bacteria-killing abilities under ultra-low Ag⁺concentrations and the stern-interface induced antibacterial mechanism was explicitly proposed.