Dispersion of hydroxyapatite nanoparticles in solution and in polycaprolactone composite scaffolds.

The dispersion behaviour of hydroxyapatite nanoparticles (HAP) and surface-modified HAP was studied in 1,4-dioxane (DO), water and poly(ε-caprolactone) (PCL) solutions and the relationship between these and the dispersion in composite PCL scaffolds prepared by thermally induced phase separation (TIPS) was examined. Investigation of the change in particle sizes by dynamic light scattering, showed that the modification of HAP by adsorption or covalent attachment of heparin via a 3-aminopropyltriethoxysilane (APTES) layer improved the dispersion stability of the particles in water/DO mixtures, while no improvement was observed in DO. The distribution of the particles within the composite scaffolds was determined using a combination of transmission electron microscopy and a calcium quantification method which was used to determine distribution of the particles in the vertical direction. While the scaffolds fabricated in DO had particles embedded within the walls of the scaffold, the scaffolds fabricated in a DO/water mixed solvent showed the particles partitioned to the surface of the scaffold walls, which is likely because the particles acted as interface stabilisers and were not miscible with the PCL rich phase. Therefore, it can be concluded that the polymer-solvent system used, as well as the phase separation mechanism that occurs, significantly influences the distribution of the particles in the scaffolds and thus the particle behaviour in solution is not necessarily a good predictor for the ability to fabricate scaffolds with a high degree of particle dispersion and hence for overall materials performance. Bulk crystallinity and compressive modulus were examined and it was determined that no significant changes occurred compared with the pristine PCL, while the surface bioactivity of the scaffolds had improved significantly, indicating that the particles were present at the polymer-solution interface.

Mode of heparin attachment to nanocrystalline hydroxyapatite affects its interaction with bone morphogenetic protein-2.

Heparin has a high affinity for bone morphogenetic protein-2 (BMP-2), which is a key growth factor in bone regeneration. The aim of this study was to investigate how the rate of release of BMP-2 was affected when adsorbed to nanosized hydroxyapatite (HAP) particles functionalized with heparin by different methods. Heparin was attached to the surface of HAP, either via adsorption or covalent coupling, via a 3-aminopropyltriethoxysilane (APTES) layer. The chemical composition of the particles was evaluated using X-ray photoelectron spectroscopy and elemental microanalysis, revealing that the heparin grafting densities achieved were dependent on the curing temperature used in the fabrication of APTES-modified HAP. Comparable amounts of heparin were attached via both covalent coupling and adsorption to the APTES-modified particles, but characterization of the particle surfaces by zeta potential and Brunauer-Emmett-Teller measurements indicated that the conformation of the heparin on the surface was dependent on the method of attachment, which in turn affected the stability of heparin on the surface. The release of BMP-2 from the particles after 7 days in phosphate-buffered saline found that 31% of the loaded BMP-2 was released from the APTES-modified particles with heparin covalently attached, compared to 16% from the APTES-modified particles with the heparin adsorbed. Moreover, when heparin was adsorbed onto pure HAP, it was found that the BMP-2 released after 7 days was 5% (similar to that from unmodified HAP). This illustrates that by altering the mode of attachment of heparin to HAP the release profile and total release of BMP-2 can be manipulated. Importantly, the BMP-2 released from all the heparin particle types was found by the SMAD 1/5/8 phosphorylation assay to be biologically active.

Attachment of poly(acrylic acid) to 3-aminopropyltriethoxysilane surface-modified hydroxyapatite.

Nano-sized hydroxyapatite (HAP) is of interest in biomaterials science due to its similarity to bone mineral. In this study, HAP modification using 3-aminopropyltriethoxysilane (APTES) was carried out in toluene and the effect of reaction time and curing temperature on the surface layers formed was investigated through X-ray photoelectron spectroscopy, Fourier transform infrared (FT-IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy. It is shown that the chemical composition is strongly influenced by the curing temperature; with low temperatures of 50 and 100 °C resulting in a fully condensed APTES layer, an intermediate temperature of 150 °C causing partial oxidation of the surface layer with the conversion of some amine functionality to amides while curing at a temperature of 200 °C additionally leads to thermal decomposition of the silane layer and a loss of the pendent amine groups. However, the stability of these particles in aqueous solution indicated a loss of the silane layer for samples cured at 150 °C or less and it is concluded that there is a trade-off between the availability of functionality for further chemical grafting and the stability for these APTES-modified HAP materials. Subsequent attachment of the polyelectrolyte poly(acrylic acid) (PAA) via both ionic interaction and covalent bonding using carbodiimide chemistry resulted in particles with more negative zeta potentials (-27 to -18 mV) compared to pure HAP, which were stable to dispersion in aqueous solution, both with respect to their chemical composition at the particle surface and to aggregation.