Biological tissues can grow into and bond with the surface of biosurfactant ceramics firmly. The most common surface bioactive ceramics includes hydroxyapatite ceramics, bioactive 45S5 bioglass.
Bioglass is a material using silica (glass) as the host material, incorporated with calcium and phosphorous to fuse broken bones. The conventional 45S5 bioglass, signifies bioactive glass with 45%wt of SiO2 and 5:1 molar ratio of calcium to phosphorus. The surface of Bioglass or bioactive glass ceramics can form hydroxyapatite coating by chemical reaction in physiological environment.The bonding between bioglass and tissues occurs through the hydroxyapatite layer on the surface.
The characteristics of bioabsorbable bioactive ceramics are that they can be partially or completely absorbed in vivo and then replaced by new tissues, and can induce the growth of new bone in organisms. The most common bioabsorbable bioactive ceramics are beta tricalcium phosphate.
The structure of the synthetic hydroxyapatite is similar to that of the biological bone tissue. Therefore, synthetic hydroxyapatite has the same properties as the biological hard tissue. For example, CA: P ≈ 1.67, density ≈ 3.14, mechanical strength greater than 10MPa, non-toxic to organisms, no stimulation, good biocompatibility, not absorbed, can induce new growth of bones. Hydroxyapatite has been used in filling alveolar bone defect, repair and filling of brain surgery, etc. and used as ear bone chain and plastic surgery materials. In addition, it can be made into artificial bone nucleus to treat bone tuberculosis.
The dissolution of hydroxyapatite is harmless, and new bone can be formed by supplementing calcium and phosphate ions from body fluid, which can produce decomposition, absorption and precipitation reactions at the bone interface to achieve firm bonding.
As to the widely used biodegradable ceramic: β – tricalcium phosphate (β – TCP) is a high temperature phase of calcium phosphate, which belongs to cubic crystal system with Ca / P atomic ratio of 1.5. The biggest advantage of β – TCP is its good biodegradability, direct fusion with bone after implantation, without any local inflammatory reaction and systemic toxicity.
The ratio of calcium to phosphorus plays an important role in determining the solubility and absorption rate in vivo. Synthetic Beta TCP is easier to dissolve in vivo than synthetic hydroxyapatite and its solubility is about 10 ~ 20 times higher than hydroxyapatite. The degradation rate of β – TCP varies with its surface structure, crystal configuration, porosity and implanted animals, and its strength often weakens with degradation. It has been proved that changing the pore size and material purity can slow down the degradation rate and improve the biological strength of beta TCP.
The disadvantage of β – TCP bioceramics is that its mechanical strength is too low to withstand the impact of force. Mixing β – TCP with other materials such as HAP to make biphasic or multiphase bioceramics is one of the methods to improve its mechanical strength. It is generally believed that the osteoconduction effect of biphasic calcium phosphate (BCP) is better than that of HA or TCP. It can combine the advantages of both HA and beta TCP with similar chemical composition to natural bones.
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