Do You Really Know Hydroxyapatite?

Posted by Johnson Brown on March 24th, 2022

As an important branch of biomedical materials, bioceramics are mainly used to manufacture hard tissue repair devices and artificial organs in vivo, and generally have good biocompatibility. According to the chemical activity in the physiological environment, bioceramics can be divided into three types:

Nearly inert bioceramics, only weak or no chemical reaction occurs when exposed to the physiological environment for a long time, and can maintain long-term stability;

Surface-active bioceramics, can form chemical bonds with tissues through biochemical reactions on their surfaces in a physiological environment;

Absorbable bioceramics, can be gradually degraded and absorbed in the physiological environment, and replaced by new tissues, so as to achieve the purpose of repairing and replacing damaged tissues.

At present, many bioceramics have been commercialized and put into use, and their main components are generally calcium phosphate ceramics. Although the morphology of bone is different in different parts

of the body, their physicochemical structures are basically similar. In terms of chemical composition, they are mainly composed of 35% organic components and 75% inorganic components, and these inorganic components mainly include calcium phosphate, carbonate, a small amount of fluoride and magnesium and sodium.

In calcium phosphate ceramics, hydroxyapatite (HAP) can form chemical bonding with cartilage tissue due to its molecular structure and calcium-phosphorus ratio, which are very similar to the inorganic components of normal bone and have excellent biological activity and osteoinductivity. In the implantation of bone defect, there is no fibrous tissue interface between bone tissue and HAP, and carbonate apatite is formed on the surface of implant body. This combination is a reaction transformation in the formation process of bonding interface. Therefore, HAP is internationally recognized as a bioactive ceramic material suitable for clinical application at present.

In the field of biomedicine, hydroxyapatite is used as biotechnology materials such as artificial bones, tooth roots, and biomaterial coatings due to its excellent bioactivity and biocompatibility. HAPs with different morphologies and pore structures are also widely used in ionic exchange, catalyst carrier, biomedicine and other fields.

Research Trends of Hydroxyapatite Bioceramics

Hydroxyapatite has a long history of research. As early as 1790, Wemer named the material apatite in Greek. The period from 1926 to 1972 was the stage of exploration and research by scholars from all over the world.

In 1972, Japanese scholar HidkeiAoki successfully synthesized hydroxyapatite and sintered it into porcelain. Soon, American scholar Jacrho also fired hydroxyapatite ceramics.

In 1974-1975, Akoi et al. found that fired hydroxyapatite ceramics had good biocompatibility. Since then, countries all over the world have carried out all-round basic research and clinical application research on hydroxyapatite materials. Countries such as Western Europe, the United States, Japan and Australia have established more than ten high-level, multidisciplinary national biomaterials and engineering centers, which have been included in the high-tech key new material development plan.

By the 21st century, the total output value of Japanese bioceramics may become one of the important pillars of the economy.

Classification of Hydroxyapatite Active Bioceramics

Dense HAP bioceramics (H type)

The preparation of this kind of ceramics is made by adding HAP substrate into additive and binder to make a certain particle cascade, and then pressurized forming in the metal mold. After drying, the green body is fired at 900 °C. Finishing, and then pressure sintering at about 1300 ?.

Dense HAP has certain processability and is extremely convenient in clinical use. However, because it can only form bone on the surface after being implanted in the human body and lacks the ability to induce bone formation, it can only be used as a scaffold for bone formation and is mainly used for artificial root implants.

Porous HAP bioceramics (DH type)

The ceramic has good biodegradability and large specific surface area, which is beneficial to the attachment of biological tissues, and appropriate pore size is more conducive to the growth of biological tissues and organs. The disadvantage is that the strength is low, and it can only be used for some parts with relatively low strength.

Composite HAP bioceramics (FH type)

It is similar to porous HAP ceramics, but the preparation method is different. The method is to select appropriate calcium-containing phosphate glass and calcium phosphate ceramics for compounding. It is mainly made by adding a certain proportion of CaO-P₂O₅-Al₂O₃ series glass body to high-purity HAP powder, and sintering at high temperature (the temperature is 200 lower than that of H type).

The porosity of this ceramic can reach 20-30%, the apparent pore diameter is 80-200 microns, and HAP crystals are enriched on its porous surface, so it has good biological activity and mechanical properties.

Hybrid HAP bioceramic (FHD type)

The ceramics are made by coating porous HAP fabric onto HAP core. It compensates for the disadvantages of porous HAP porous and dense HAP ceramics, takes into account the advantages of both, and obtains better results. Because porous HAP ceramics are conducive to rapid activity after being implanted into human tissues, but the mechanical strength of the material itself is lower than that of dense HAP ceramics, while the specific surface area of dense HAP ceramics is small and the biological activity is slow, based on the principle of combining the above conditions, artificial roots are made, and their mechanical strength is close to that of dense HAP ceramics, and the biological activity is equivalent to porous HAP and composite HAP ceramics.

Coated composite materials

To improve the mechanical properties and mechanics of HAP bioceramics, coated HAP and composite HAP materials have emerged. This material uses high-strength and high-toughness materials as the base material, and uses HAP as a coating. In another case, HAP is compounded with other materials with excellent toughness and similar structure to prepare more ideal HAP biomaterials.