Hydroxyapatite ceramics

Hydroxyaptite ceramics (Hydroxyaptite, referred to as HAP) is significantly different from other biological materials in that it is made of artificially synthesized powders with the same mineral composition as human bones and teeth. Inorganic materials.

Nano-hydroxyapatite artificial bone

 

1. The crystal structure of hydroxyapatite ceramics

Hydroxyapatite belongs to the hexagonal crystal system, and the theory becomes [Ca10(PO)6(OH)2, denoted as HAP], and the actual composition is very complicated. The density is 3.156g/cm3. Hydroxyapatite (HAP) has a more complex crystal structure, in which there are larger channels in the direction parallel to the c-axis. Due to the special crystal structure of hydroxyapatite, in addition to higher ion activity and stronger ion exchange capacity, it also has stronger surface adsorption properties.

2. Preparation process of hydroxyapatite ceramics

There are two methods for preparing hydroxyapatite ceramics: dry method and wet method, which are specifically divided into solid phase reaction method, hydrothermal reaction method and precipitation reaction method. The raw materials for the synthesis of hydroxyapatite are chemical reagents. As the P2O5 component, calcium hydrogen phosphate (CaHPO4•2H2O), diammonium hydrogen phosphate [(NH4)2HPO4], phosphoric acid (H3PO4), etc. can be introduced; the CaO component is in addition to calcium hydrogen phosphate, It can also be provided by calcium carbonate (CaCO3), calcium hydroxide [Ca(OH)2], and calcium nitrate [Ca(NO3)2]. According to the molecular formula of the theoretical chemical composition of hydroxyapatite, it can be calculated that its Ca/P=1.67. In order to make Ca/P=1.67, two or more raw materials must be used to synthesize hydroxyapatite.

(1) Hydrothermal reaction method

The CaHPO4 and CaCO3 were mixed at a ratio of 6:4 (mol) and subjected to 24h wet ball milling. Then pour the ball-milled slurry into the reaction kettle, add enough distilled water, and perform a stirring reaction at a constant temperature of 80-100°C, and after cooling, precipitate to obtain a white hydroxyapatite precipitate.

In addition, Hideki Aoki of Japan also directly used calcium hydrogen phosphate hydrolysis to prepare hydroxyapatite crystals with better crystallinity in an autoclave.

The reaction is at a neutral pH and a controlled temperature of about 200°C, and the corresponding pressure is about 15atm.

(2) Precipitation reaction method

This method uses Ca(NO3)2 and (NH4)2HPO4 as raw materials for the reaction to obtain a white precipitate.

It is also possible to use calcium hydroxide to react with phosphoric acid to synthesize hydroxyapatite.

(3) Solid phase reaction method

The solid-phase reaction method is basically the same as the preparation method of ordinary ceramics. According to the formula, the raw materials are finely ground and mixed, and the synthesis is carried out at high temperature. It should be pointed out that in the process of synthesizing hydroxyapatite, the temperature cannot exceed 1330°C, otherwise it will cause the decomposition of hydroxyapatite.

In addition to the above methods, other methods can also be used to prepare hydroxyapatite. Hydroxyapatite can also form composite hydroxyapatite ceramics with other materials to improve the performance of the material.

For example, although hydroxyapatite has superior biocompatibility, it has one of the biggest disadvantages of low mechanical strength, which limits its application. On the basis of satisfying biocompatibility, in order to improve the strength, it is mainly to change the microstructure of the material.

Consider the structure and development of composite materials. Adding other oxides to hydroxyapatite to form other crystalline phases inside the material to achieve the purpose of increasing strength is an effective method.

3. The properties of hydroxyapatite

The main mineral component of biological hard tissue is hydroxyapatite, and the structure of synthetic hydroxyapatite is similar to that of biological bone tissue. Therefore, they have the same performance.

(1) Density, refractive index

Ca/P=1.67, density is p=3.140~3.156g/cm3, mechanical strength is greater than 100MPa. Its refractive index is n=1.64~1.65.

(2) Chemical properties

Hydroxyapatite is slightly soluble in pure water and is weakly alkaline (pH=7~9). Easily soluble in acid but hardly soluble in alkali. With excellent ion exchange performance, Ca2+ is easily replaced by some heavy metal ions (that is, pollution ions) such as Cd2+, Hg2+, Sr2+, Ba2+, Pb2+, etc. In addition, the anion (OH-) is also easy to be replaced by F-, and its replacement speed is very fast, and it can also react with (COOH) groups of amino acids, proteins, organic acids, etc., so it can be used for the separation and purification of amino acids and proteins. Wait.

(3) Physical properties

The hardness of hydroxyapatite is 5, which is the same as that of ordinary glass.

Low-crystalline hydroxyapatite powder has a large specific surface area, for example, up to 100 square meters/g, and is used as a catalyst or phosphorus capture agent. When hydroxyapatite is heated in the air, low-crystalline fine powder crystallizes, and sintering occurs at about 1000°C. Melting and decomposition reactions occur at 1600~1700℃.

The modulus of elasticity of base apatite is about 2 times larger than that of human bone, and it can be called a hard material. However, the flexural strength is lower than that of human bone, and its impact resistance and toughness are also poor, which is a brittle material.

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