Photoluminescent Properties of Hydroxyapatite and Hydroxyapatite/Multi-Walled Carbon Nanotube Composites

Calcium Orthophosphate (CaPO4) Containing Composites for Biomedical Applications: Formulations, Properties, and Applications

The goal of this review is to present a wide range of hybrid formulations and composites containing calcium orthophosphates (abbreviated as CaPO4) that are suitable for use in biomedical applications and currently on the market. The bioactive, biocompatible, and osteoconductive properties of various CaPO4-based formulations make them valuable in the rapidly developing field of biomedical research, both in vitro and in vivo. Due to the brittleness of CaPO4, it is essential to combine the desired osteologic properties of ceramic CaPO4 with those of other compounds to create novel, multifunctional bone graft biomaterials. Consequently, this analysis offers a thorough overview of the hybrid formulations and CaPO4-based composites that are currently known. To do this, a comprehensive search of the literature on the subject was carried out in all significant databases to extract pertinent papers. There have been many formulations found with different material compositions, production methods, structural and bioactive features, and in vitro and in vivo properties. When these formulations contain additional biofunctional ingredients, such as drugs, proteins, enzymes, or antibacterial agents, they offer improved biomedical applications. Moreover, a lot of these formulations allow cell loading and promote the development of smart formulations based on CaPO4. This evaluation also discusses basic problems and scientific difficulties that call for more investigation and advancements. It also indicates perspectives for the future.

Preparation and Application of Apatite-TiO2 Composite Opacifier: Preventing Titanium Glaze Yellowing through Pre-Combination

In order to enhance the degree of binding reaction of TiO2 in titanium-containing ceramic glazes and prevent the reaction of its transformation into rutile to eliminate the yellowing phenomenon of the glaze surface, an apatite-TiO2 composite opacifier (ATO) was prepared through the mechanical grinding of hydroxyapatite and anatase TiO2. The properties, opacification mechanism, and yellowing inhibition of the prepared ceramic glazes were studied. The results show that the ATO is characterized by a uniform coating of TiO2 on the surface of the apatite and the formation of close chemical bonding between the apatite and TiO2. The ceramic glaze surface when using an ATO has a white appearance and excellent opacification performance. When an ATO was used, the L*, a*, and b* values of the glaze were 89.99, -0.85, and 3.37, respectively, which were comparable to those of a ZrSiO4 glaze (L*, a*, and b* were 88.24, -0.02, and 2.29, respectively). The opacification of the glaze was slightly lower than that of the TiO2 glaze (L* value was 92.13), but the appearance changed from yellow to the white of the TiO2 glaze (b* value was 9.18). The ceramic glaze layer when using an ATO mainly consists of titanite, glass phase, and a small amount of quartz, and the opacification mechanism is the crystallization of the generated titanite. ATOs can play an active role in solving the critical problem that arises when TiO2 replaces ZrSiO4 as an opacifier.

Effect of Magnesium Substitution on Structural Features and Properties of Hydroxyapatite

Hydroxyapatite (HAP) is the main mineral component of bones and teeth. It is widely used in medicine as a bone filler and coating for implants to promote new bone growth. Ion substitutions into the HAP structure highly affect its properties. One of the most important substituents is magnesium. This paper presents new results obtained using high-precision hybrid density functional theory calculations for Mg/Ca substitutions in HAP in a wide magnesium concentration range within a 2 × 2 × 2 supercell model. Experimental data on the mechanochemical synthesis of HAP-Mg samples with different Mg concentrations are also presented. A comparison between the experiment and the theory showed good agreement: the HAP-Mg unit cell parameters and volume decreased with increasing degree of Mg/Ca substitution. The changes in the distances between the Ca and O, Ca and H, and Mg and O ions upon Mg/Ca substitution in different calcium positions was analyzed. The resulting asymmetry and distortion of the cell parameters were evaluated. It was shown that bulk modulus, energy levels, and band gap depend on the degree of Mg substitutions in the Ca1 and Ca2 positions. The formation energies of Mg/Ca substitutions showed non-monotonic behavior that was different for Ca1 and Ca2 positions. The Ca2 position had a slightly higher probability (~5 meV/f.u.) of substitution than Ca1 position at a Mg concentration x = 0.5. At x = 1, substitution in both positions can coexist. The simulated IR spectra for different Mg/Ca substitutions showed that Mg in the Ca2 position changes the IR spectrum more significantly than Mg in the Ca1 position. Similar changes were recorded in the IR spectra of the synthesized samples. The electronic structure is shown to be sensitive to the number and position of substitutions, which may be used to tweak the optical properties of the HAP-Mg material.

Hydroxyapatite-based carriers for tumor targeting therapy

At present, targeted drug delivery is regarded as the most effective means of tumor treatment, overcoming the lack of conventional chemotherapeutics that are difficult to reach or enter into cancer cells. Hydroxyapatite (HAP) is the main component of biological hard tissue, which can be regarded as a suitable drug carrier due to its biocompatibility, nontoxicity, biodegradation, and absorbability. This review focuses on the cutting edge of HAP as a drug carrier in targeted drug delivery systems. HAP-based carriers can be obtained by doping, modification, and combination, which benefit to improve the loading efficiency of drugs and the response sensitivity of the microenvironment in the synthesis process. The drug adsorbed or in situ loaded on HAP-based carriers can achieve targeted drug delivery and precise treatment through the guidance of the in vivo microenvironment and the stimulation of the in vitro response. In addition, HAP-based drug carriers can improve the cellular uptake rate of drugs to achieve a higher treatment effect. These advantages revealed the promising potential of HAP-based carriers from the perspective of targeted drug delivery for tumor treatment.

Molecular Dynamics Simulation of the Thermal Behavior of Hydroxyapatite

Hydroxyapatite (HAP) is the main mineral component of bones and teeth. Due to its biocompatibility, HAP is widely used in medicine as a filler that replaces parts of lost bone and as an implant coating that promotes new bone growth. The modeling and calculations of the structure and properties of HAP showed that various structural defects have a significant effect on the properties of the material. By varying these structural heterogeneities, it is possible to increase the biocompatibility of HAP. An important role here is played by OH group vacancies, which are easily formed when these hydroxyl groups leave OH channels of HAP. In this case, the temperature dependence of the concentration of OH ions, which also determines the thermal behavior of HAP, is important. To study the evaporation of OH ions from HAP structures with increasing temperatures, molecular dynamics simulation (MDS) methods were used in this work. As a program for MDS modeling, we used the PUMA-CUDA software package. The initial structure of HAP, consisting of 4 × 4 × 2 = 32 unit cells of the hexagonal HAP phase, surrounded by a 15-Å layer of water was used in the modelling. Multiple and statistically processed MDS, running calculations in the range of 700-1400 K, showed that active evaporation of OH ions begins at the temperature of 1150 K. The analysis of the obtained results in comparison with those available in the literature data shows that these values are very close to the experiments. Thus, this MDS approach demonstrates its effective applicability and shows good results in the study of the thermal behavior of HAP.

Hydroxyapatite-based adsorbents: Applications in sequestering heavy metals and dyes

Hydroxyapatite (HAp) is a calcium phosphate material that was used primarily in bone regeneration and repair as a result of its chemical similarity with bone. However, HAp has emerged as a very promising adsorbent for sequestering contaminants like heavy metals, dyes, hydrocarbons as well as other emerging pollutants from wastewater as a result of its versatility and encouraging adsorptive properties. Contaminants like heavy metals and dyes have been a major source of environmental concern. Research studies involving the use of HAp as adsorbents for the adsorptive treatment of heavy metal- and dye-contaminated wastewater have become increasingly popular due to its eco-friendliness, easy synthesis, unique adsorption properties etc. Various methods are available for the synthesis of HAp and its composites with some of these methods used in combination with other methods to obtain more efficient HAp-based adsorbents. In this work, the adsorptive removal of heavy metals and dyes by HAp and its composites was extensively reviewed as well as the parametric effects of process factors like contact time, solution pH, temperature, solute concentration etc on the adsorption process. Kinetic, thermodynamic, and isotherm models for elucidating the adsorption process were also considered. Generally, from the works reviewed, HAp-based adsorbents were found to be very effective for sequestering heavy metals and dyes from solution and thus presents a low-cost option for adsorptive wastewater treatment.