Latest Application Research of Hydroxyapatites

In nature, there are many biominerals with excellent performance. Most of these biominerals have simple composition, but have high structural organization, good strength and hardness. Among them, hydroxyapatite (HA), as the main inorganic component of human hard tissue, has attracted extensive attention in the biomedical field for its excellent biocompatibility, bioactivity, and bioconductivity.

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Structure and properties of hydroxyapatite

The crystal system of HA is hexagonal, belonging to the P63/m space group, with unit cell parameters a=b=0.943nm, c=0.688nm. The chemical composition is Ca₁₀(PO₄)₆(OH)₂, and there are 10 Ca²⁺ in a single crystal cell, which can be divided into Ca (I) and Ca (II) according to their position. Among them, 4 Ca (I) are located between two oxygen atoms of 6PO₄^3-, and the remaining 6 Ca (II) are connected to the remaining 6 oxygen atoms in PO₄^3-, and OH- is located around the plane formed by Ca²⁺ and PO₄^3-.

The theoretical Ca/P ratio of HA is 1.67, but due to the actual preparation process, the actual composition is very complex. HA is slightly soluble in water, weakly alkaline, highly soluble in acid, but insoluble in alkali. It has a strong ion exchange capacity, in which calcium ions are often exchanged by ions such as Cd²⁺, Hg²⁺, Sr²⁺, and Ba²⁺; OH- can be quickly exchanged by halogen ions such as F^- and Cl^-. At the same time, amino acids, organic acids, and proteins containing hydroxyl groups are also easy to react with HA.

Latest application research of hydroxyapatite

Hydroxyapatite has good biodegradability, biocompatibility, biological activity, safety and non-toxicity; at the same time, it has many excellent properties such as high strength, bone conduction, high temperature resistance, ferroelectricity and catalytic activity, and its development prospects are extremely bright. The performance of hydroxyapatite is still being explored and developed, which is conducive to the further development of hydroxyapatite materials with excellent structure and performance, and expanding its application range.

(1) Drug delivery carrier and tumor treatment

Hydroxyapatite has a high loading capacity for some drugs and has good biocompatibility and biodegradability, so it has the potential to be used as a drug carrier. In particular, modified hydroxyapatite can be used as a drug carrier for different diseases such as bone-related diseases, antibiotic carriers, anti-inflammatory, carcinogenic drugs, medical imaging, protein delivery agents, etc. The pioneering progress of drug delivery systems based on hydroxyapatite and hydroxyapatite composite nanostructures indicates its good future development.

(2) Hard tissue repair materials

Hydroxyapatite is highly similar to the inorganic components in bones, and has excellent biodegradability, osteoconductivity, biocompatibility and osteoinductivity, so it is one of the most popular bone replacement materials. Some researchers used ultra-long hydroxyapatite nanowires loaded with magnesium silicate as the raw material for preparing hydroxyapatite nanowires@magnesium silicate nanosheets (HApNW@MS) core-shell porous hierarchical nanocomposite materials (nanobrushes) for bone defect repair scaffolds. The scaffolds can not only promote the attachment and growth of rat bone marrow-derived mesenchymal stem cells, but also induce the expression of rBMSCs vascular endothelial growth factor genes and osteogenic differentiation-related genes. In addition, the scaffolds can significantly promote bone regeneration in vivo.

Ca²⁺, PO₄^3- and OH^- in hydroxyapatite can be replaced by other atoms without changing the crystal structure. The biocompatibility of modified nanostructured hydroxyapatite doped with various ions in orthopedic applications is improved. Pure hydroxyapatite scaffolds are brittle and have insufficient vascularization and bone induction effects. Studies have shown that modified hydroxyapatite has great application potential and development prospects in promoting bone regeneration and repair.

(3) Soft tissue repair

Hydroxyapatite has good biocompatibility and high solubility. After being implanted in the body, it can form chemical bonds with tissues at the interface. It is not only safe and non-toxic, but also can conduct bone growth. It is a typical bioactive ceramic material. When hydroxyapatite is used as a transdermal implant, it can closely adhere to epithelial tissue to prevent bacterial infection and inflammation. When hydroxyapatite is implanted into soft tissues such as muscles or ligaments, microcapillaries and non-inflammatory cells no longer exist.

(4) Biodiagnosis and biodetection

In biodiagnosis and biodetection, fluorescence imaging of cells and living organisms plays an important role. The composition of hydroxyapatite nanoparticles is relatively simple. In the specific synthesis, it can absorb organic fluorescent molecules and some luminescent genes. At the same time, hydroxyapatite is also one of the commonly used bioimaging agents and can be used in biodiagnosis and biodetection.

(5) Catalyst

Modified hydroxyapatite is a material that can have different properties according to different preparation methods to meet the design requirements of controlling porosity or hierarchical structure in different fields, especially in the field of heterogeneous catalysis. It is widely used and has aroused great interest. The special feature of hydroxyapatite structure is that it can accept a large number of anionic and cationic substituents and form solid solutions, making modified hydroxyapatite suitable for different applications.

(6) Thermal insulation and flame retardant materials

Hydroxyapatite has a melting point of up to 1650°C and low thermal conductivity, so its application potential in thermal insulation and flame retardant fields is very prominent. In recent years, researchers have developed a highly flexible, non-flammable hydroxyapatite paper made of ultra-long hydroxyapatite nanowires. This paper can be used for printing and writing, and is expected to permanently and safely store information such as archives and important documents.

(7) Bio-coating materials

Hydroxyapatite has excellent biocompatibility with natural bone tissue, so it is considered to be the most ideal coating material among polymer and metal biomaterials. Since the biocompatibility and bone bonding properties of hydroxyapatite coatings are enhanced, they are widely used in biomedical devices directly implanted on bones. Although hydroxyapatite coating process technology has been developed, its application development is still constrained by the time-consuming hydroxyapatite synthesis and the additional redundant coating process.

(8) Optical applications

The counterfeiting of certificates, documents, and banknotes is a worldwide concern. Researchers have developed a new type of ultra-long hydroxyapatite nanowire-based paper with excellent properties such as waterproofing, heat preservation, fire resistance, and luminescence, and have anti-counterfeiting applications. Products made from it often have unique properties, such as high flexibility, good processability, writing and printing capabilities, tunable emission colors, waterproofness, and fire resistance. In addition, some well-designed graphics that are invisible under normal ambient light but can glow under ultraviolet light can be embedded in the paper.

(9) Water treatment

Hydroxyapatite has adsorption and ion exchange capabilities, and it can be used to adsorb dye molecules and heavy metals/harmful ions in sewage. Researchers have developed a new type of environmentally friendly filter paper based on ultra-long hydroxyapatite nanowires and cellulose fibers with excellent filtration and adsorption properties for high-performance water purification.

(10) High-temperature proton conductor

Hexagonal hydroxyapatite has proton conductivity, and its hydroxyl protons can move along the c-axis at high temperatures. Some researchers have electrochemically deposited hydroxyapatite seeds, performed secondary hydrothermal crystallization on the seed layer to promote normal growth of the c-axis with the substrate, and performed tertiary hydrothermal crystallization on the seed layer to promote a-axis growth to fill the gaps between oriented crystals. Since protons are mainly transported along the c-axis direction of the hydroxyapatite crystal, the c-axis orientation of the crystal domain across the membrane thickness significantly enhances the conduction of protons.

(11) Other electrical properties related materials

Natural hydroxyapatite nanocrystals are the main component of bone - a known piezoelectric material. Synthetic hydroxyapatite is widely used in bone grafts and prosthetic pyroelectric coatings due to its strong bond with natural bone. Nanocrystalline synthetic hydroxyapatite films have recently been found to have strong piezoelectric and pyroelectric properties. The charge on hydroxyapatite affects cell activity, and this phenomenon can be translated into clinically relevant porous scaffold structures, demonstrating that improvements to clinically used biomaterials such as hydroxyapatite have potential benefits for patients.

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