Bioceramics Development and Applications

Hydroxyapatite (HA) is a widely studied biomaterial for bone grafting and tissue engineering applications. The crystal structure of HA lends itself to a wide variety of substitutions, which allows for tailoring of material properties. Iron is of interest in ion substitution in HA due to its magnetic properties. The synthesis and characterization of iron-substituted hydroxyapatite (FeHA) have been widely studied, but there is a lack of studies on the sintering behaviors of FeHA materials compared to pure HA. Studying the sintering behavior of a substituted apatite provides information regarding how the substitution affects material characteristics such as stability and bulk mechanical properties, thereby providing insight into which applications are appropriate for the substituted material. In this study both pure HA and FeHA were synthesized, pressed into pellets, and then sintered at temperatures ranging from 900- 1300°C and 600-1100°C, respectively. The study thoroughly examined the comparative sintering behaviors of the two materials using density measurements, mechanical testing, X-ray diffraction, and electron microscopy. It was found that FeHA is considerably less thermally stable than pure HA, with decomposition beginning around 1200°C for pure HA samples, while at 700°C for the FeHA. The FeHA also had a much lower mechanical strength than that of the pure HA. An in vitro cell culture study was conducted by immersing FeHA powder in cell culture media, with HA powder at equivalent doses as a control, verified that FeHA is a biocompatible material. Although the FeHA would be unsuitable for bulk applications, it is a potential material for a variety of biomedical applications including drug delivery, cancer hyperthermia, and bone tissue engineering composites.

Premixed calcium phosphate cements (pCPC), where glycerol is used instead of water as mixing liquid, present better handling characteristics than water-based cements. However, the setting mechanisms of pCPC have not been described thoroughly. The aim of this paper is to increase the understanding of the setting mechanism of pCPC. The investigated cement starts to set when glycerol is exchanged with water via diffusion of glycerol out to the surrounding body fluid and water into the material. To better understand the water-glycerol exchange a method was developed where the setting depth of the cement was measured over time. Thermo gravimetric analysis (TGA) was used to determine the liquid exchange rate during setting. To study the influence of temperature on the crystalline end product, pCPC and water-mixed calcium phosphate cement (wCPC) were set at different temperatures and analyzed with X-ray diffraction (XRD). The setting depth measurements showed that the set layer of the pCPC grew with a speed proportional to t0.51 at 37°C. TGA results furthermore showed that less than 10% of the glycerol remained after 16 hours. Setting of pCPC at different temperatures showed that mainly brushite was formed at 5°C, a mixture of brushite and monetite at 21°C and mainly monetite at 37°C. It furthermore showed that brushite was the main phase after setting of wCPC, but some monetite was present in these cements. The study presents a new method for evaluation of pCPC that increases the understanding of their setting mechanism. Furthermore, the XRD results indicate that storage at 5°C could improve the shelf life of acidic pCPC.