Pressing and sintering of nanosized hydroxyapatite powders

Abstract

The paper examines the compactability and sintering of nanosized synthetic hydroxyapatite (HA) powders for increasing the durability and strength of HA ceramics. The granulation of HA powders and limitation of the applied pressure prevent delamination in pressing. Porous HA samples show homogeneous nanoporous structure, high compactability, and excellent sinterability, which is better than that of nanosized powders.Calcium phosphate materials are used in surgery and other modern engineering areas to repair bone defects and place coatings on metal implants, for prolonged drug delivery and nonviral gene transfer, and as chromatographic sorbents, catalyst carriers, gas sensors, and ion conductors. The development of materials for such various applications involves controlling their structure and porosity based on the study of sintering and pressing processes. For example, the biological applications of calcium phosphates are based on hydroxyapatite (HA) Ca 10 (PO 4 ) 6 (OH) 2 , which is the closest analog of the bone mineral in chemical composition and crystal structure, and many studies on calcium phosphate biomaterials are focused on reproducing the morphology and structure of particles and pores found in natural bone [1].The main bone phase is represented by HA in which the Ca : P ratio is close to 1.5 (as different from stoichiometric HA in which Ca : P = 1.67), and primary particles constitute lamellar crystals about 1.5-3.5 nm × × 5.0-10.0 nm × 40.0-50.0 nm with complex nanostructure [2]. The volume fraction of these crystals is different: from 97% in dental enamel or 40% in the densest cortical bone to 10-20% in cancellous (spongy) bone. Collagen and other organic substances constitute the rest.Hydroxyapatite (either synthetic or organic from bones) is the main component of most biomaterials used to restore bone tissue. In spite of the great variety of inorganic biomaterials proposed recently, none of them can equal HA in the affinity to the living body, minimization of the body's negative reaction to the implantation of even substantial amounts of this material, osteoconductivity, i.e., ability to bond with bone and support bone growth, and rate and strength of bonding with bone. For many years of application, no difference has been noted in the body's

Biodegradable Materials for Bone Repair and Tissue Engineering Applications

“…Hydroxyapatite (HA) is a bioactive and bioresorbable (variable rate and extent) calcium phosphate that forms the majority of the inorganic component of bone tissue [ 114 , 326 , 327 ] ( Figure 5 b). The atomic ratio for calcium to phosphate is 1.67 in HA [ 44 ].…”

Nanodimensional and Nanocrystalline Calcium Orthophosphates

“…Such nanostructured systems constitute a bridge between single molecules and bulk material systems [165]. For instance, powders of nanocrystalline apatites [166][167][168][169][170][171][172] and β-TCP [173] were found to exhibit an improved sinterability and enhanced densification due to a greater surface area. This is explained by the fact that the distances of material transport during the sintering becomes shorter for ultrafine powders with a high specific surface area, resulting in a densification at a low temperature.…”

Nanodimensional and Nanocrystalline Apatites and Other Calcium Orthophosphates in Biomedical Engineering, Biology and Medicine

“…Such nanostructured systems constitute a bridge between single molecules and bulk material systems [153]. For instance, powders of nanocrystalline apatites [154,155,156,157,158,159,160] and β-TCP [161] were found to exhibit an improved sinterability and enhanced densification due to a greater surface area. This is explained by the fact that the distances of material transport during the sintering becomes shorter for ultrafine powders with a high specific surface area, resulting in a densification at a low temperature.…”