The Impact of Hydroxyapatite Sintering Temperature on Its Microstructural, Mechanical, and Biological Properties

Abstract

Hydroxyapatite (HA), the principal mineral of bone tissue, can be fabricated as an artificial calcium phosphate (CaP) ceramic and potentially used as bioceramic material for bone defect treatment. Nevertheless, the production method (including the applied sintering temperature) of synthetic hydroxyapatite directly affects its basic properties, such as its microstructure, mechanical parameters, bioabsorbability, and osteoconductivity, and in turn influences its biomedical potential as an implantable biomaterial. The wide application of HA in regenerative medicine makes it necessary to explain the validity of the selection of the sintering temperature. The main emphasis of this article is on the description and summarization of the key features of HA depending on the applied sintering temperature during the synthesis process. The review is mainly focused on the dependence between the HA sintering temperature and its microstructural features, mechanical properties, biodegradability/bioabsorbability, bioactivity, and biocompatibility.

Keywords: bioabsorbability; bioactivity; bioceramics; biocompatibility; cytotoxicity; mechanical properties.

Figure 5

Confocal laser scanning microscope images presenting poor adhesion and abnormal morphology of: (a) mouse calvarial preosteoblasts (MC3T3-E1 cells) after cytoskeleton staining (red florescence—cytoskeleton, blue fluorescence—nuclei) and (b) human fetal osteoblasts (hFOB 1.19 cells) after live/dead staining (green florescence—live cells, red fluorescence—nuclei of dead cells) grown on the composite biomaterial made of hydroxyapatite (HA) sintered at 800 °C and β-1,3-glucan (unpublished microscope images related to our studies performed previously [34,117]).

Figure 6

Scanning electron microscope images showing preosteoblasts (red arrows) grown on the composite biomaterial made of hydroxyapatite (HA) sintered at 800 °C and β-1,3-glucan (7 days after cell seeding): (a) small number of MC3T3-E1 preosteoblasts on gypsum-free β-1,3-glucan/HA biomaterial, (b) excellent cell growth on gypsum-enriched composite that reduced calcium ion uptake by highly reactive HA from the culture medium (unpublished microscope images related to our studies performed previously [111]).

Figure 1

Schematic representation of the influence of the sintering process stages on changes in pore structure of hydroxyapatite (prepared based on the information found in [29,30,31]).

Figure 2

The impact of HA sintering temperature on its basic properties.

Figure 3

Scanning electron microscope images presenting microstructure of hydroxyapatite (HA) dependent on the sintering temperature: (a) rough and porous microstructure of HA sintered at 800 °C; (b) dense and non-porous microstructure of HA sintered at 1250 °C.

Figure 4

Confocal laser scanning microscope images of osteoblasts after live/dead staining (green florescence—live cells, red fluorescence—nuclei of dead cells) grown on hydroxyapatite (HA) granules sintered at: (a) 400 °C (poor adhesion, abnormal morphology, and survivability of the cells) and (b) 1000 °C (good adhesion and spreading of the cells, high cell viability).