Abstract

The deterioration of the mechanical properties of bone with age is related to several factors including the structure, organization and chemistry of the constituent phases; however, the relative contribution of each of these factors is not well understood. In this study, we have investigated the effect of chemistry (calcium deficiency) on the mechanical properties of single crystals of hydroxyapatite. Single crystals of stoichiometric crystals grown by the flux method and calcium-deficient platelet crystals grown using wet chemical methods were used as model systems. Using nanoindentation, we show that calcium deficiency leads to an 80% reduction in the hardness and elastic modulus and at least a 75% reduction in toughness in plate-shaped hydroxyapatite crystals. Measurement of local mechanical properties using nanoindentation and nanoscale chemistry through elemental mapping in a transmission electron microscope points to a direct correlation between the observed spatial variation in composition and the large scatter in the measured hardness and modulus values.

Abstract

Controlled nucleation and growth of hydroxyapatite (HA) crystals on electrospun fibers should play important roles in fabrication of composite scaffolds for bone tissue engineering, but no attempt has been made to clarify the effects of chemical group densities and the cooperation of two and more groups on the biomineralization process. The aim of the current study was to investigate into HA nucleation and growth on electrospun poly(dl-lactide) fibers functionalized with carboxyl, hydroxyl and amino groups and their combinations. Electrospun fibers with higher densities of carboxyl groups, combination of hydroxyl and carboxyl groups with the ratio of 3/7, and combination of amino, hydroxyl and carboxyl groups with the ratio of 2/3/5 were favorable for HA nucleation and growth, resulting in higher content and lower crystal size of formed HA. Carboxyl groups were initially combined with calcium ions through electrostatic attraction, and the introduction of hydroxyl groups could modulate the distance between carboxyl groups. The introduction of amino groups may lead to the inner ionic bonding with carboxyl groups, but can accelerate phosphate ions to form HA through a chelate ring with the calcium ion and carbonyl oxygen. The biological evaluation indicated that the mineralized scaffolds acted as an excellent cell support to maintain desirable cell–substrate interactions, to provide favorable conditions for cell proliferation and to stimulate the osteogenic differentiation.

Abstract

Bone tissue engineering (BTE) has been demonstrated an effective approach to generate bone tissue and repair bone defect in ectopic and orthotopic sites. The strategy of using a prevascularized tissue-engineered bone grafts (TEBG) fabricated ectopically to repair bone defects, which is called live bone graft surgery, has not been reported. And the quantitative advantages of vascularization and osteogenic environment in promoting engineered bone formation have not been defined yet. In the current study we generated a tissue engineered bone flap with a vascular pedicle of saphenous arteriovenous in which an organized vascular network was observed after 4 weeks implantation, and followed by a successful repaire of fibular defect in beagle dogs. Besides, after a 9 months long term observation of engineered bone formation in ectopic and orthotopic sites, four CHA (coral hydroxyapatite) scaffold groups were evaluated by CT (computed tomography) analysis. By the comparison of bone formation and scaffold degradation between different groups, the influences of vascularization and micro-environment on tissue engineered bone were quantitatively analyzed. The results showed that in the first 3 months vascularization improved engineered bone formation by 2 times of non-vascular group and bone defect micro-environment improved it by 3 times of ectopic group, and the CHA-scaffold degradation was accelerated as well.

Abstract

The use of bioceramics and cultured cells for tissue engineering is a novel approach, which is available in a wide variety of clinical situations. The approach requires apparent verification of the cellular functions occurring on the ceramic surface, and these functions could be monitored by microscopic observation of the cultured living cells on the ceramic material. However, such observation is difficult due to the opaque nature of ordinary ceramics. To overcome this drawback, we used transparent hydroxyapatite (tHA) ceramics as a culture substrate and a transgenic rat having an enhanced green fluorescent protein (EGFP)-expressing gene as the cell source. Marrow mesenchymal stem cells (MSC) were obtained from the rat and cultured on both tHA ceramics and a tissue culture polystyrene (TCPS) dish. One hour after the cell seeding, many MSC had attached and showed initial cell spreading. The attachment and spreading were more obvious 5 h after the seeding. Following the culture in the osteogenic condition, the cells differentiated into osteoblasts, which fabricated bone matrix on the culture substrate. The phenomena were similarly observed on both the tHA ceramics and TCPS substrata. These results confirm the excellent properties of tHA ceramics, which support cell attachment, proliferation, and differentiation. Transparent materials make us know the biological usefulness of ceramics in tissue-engineering field.

Abstract

Hydroxyapatite containing levels of titanium (TiHA) of up to 1.6 wt.% has been produced via a chemical co-precipitation route. The distribution of Ti was seen by transmission electron microscopy/energy-dispersive X-ray analysis to be uniform throughout as-prepared nanosized TiHA particles (20 nm × 100 nm). The incorporation of Ti into the HA structure was found to influence the ceramic microstructure on sintering and the grain size was found to decrease from 0.89 μm with HA to 0.63 μm with 0.8 wt.% TiHA (0.8 TiHA) and 0.45 μm with 1.6 wt.% TiHA (1.6 TiHA). Rietveld refinement analysis showed that there was a proportional increase in both the a and c axis with incorporation of Ti into the HA lattice structure, leading to an increase in the cell volume with the addition of Ti. Fourier transform-Raman analysis showed a slight increase in the ratio of O–H/P–O peaks on TiHA, in comparison with HA. A bone-like apatite layer was formed on the surface of TiHA after immersion in simulated body fluid for 3 days, which demonstrated the high in vitro bioactivity of TiHA. In vitro culture with primary human osteoblast (HOB) cells revealed that TiHA was able to support the growth and proliferation of HOB cells in vitro, with a significantly higher cell activity being observed on 0.8 TiHA after 7 days of culture in comparison with that on HA. Well-organized actin cytoskeletal protein was developed after 1 day of culture, and an increase in cell filopodia (attachment) was observed on TiHA sample surfaces. The results indicate that TiHA has great potential for biomedical applications.

Abstract

The synthesis of high-strength, completely dense nanograined hydroxyapatite (bioceramic) monoliths is a challenge as high temperatures or long sintering times are often required. In this study, nanorods of hydroxyapatite (HA) and calcium-deficient HA (made using a novel continuous hydrothermal flow synthesis method) were consolidated using spark plasma sintering (SPS) up to full theoretical density in ∼5 min at temperatures up to 1000 °C. After significant optimization of the SPS heating and loading cycles, fully dense HA discs were obtained which were translucent, suggesting very high densities. Significantly high three-point flexural strength values for such materials (up to 158 MPa) were measured. Freeze-fracturing of disks followed by scanning electron microscopy investigation revealed selected samples possessed sub-200 nm sized grains and no visible pores, suggesting they were fully dense.

A novel method for preparing highly transparent hydroxyapatite bioceramics at relatively low temperatures (325–450 °C), i.e. about 60% lower compared with other commonly used preparation methods, has been developed under ultrahigh pressure (UHP; 2.0–5.0 GPa). UHP can remarkably restrain grain growth, enhance density and microhardness and we have characterized the morphology, density and mechanical properties of the prepared ceramics. We reason that these bioceramics are highly transparent due to the negligible light scattering from the nanosized grains and pores in the samples.

Transparent nanocrystalline hydroxyapatite (HA) bodies with a relative density of 99.8% and a mean grain size of 81 nm exhibit an optical transmittance of 66% at a wavelength of 645 nm. Light scattering analysis reveals that grain boundary reflection is significant for nanograined crystals, and nanopores in HA substantially reduce the transmittance of visible light even though the wavelength of visible light is about 32–64 times the pore size.