Strength and Biocompatibility of Heparin-Based Calcium Phosphate Cement Grafted with Ferulic Acid

The biomimetic synthesis of carbonated apatites by biomolecule-based templates is a promising way for broadening apatite applications in bone tissue regeneration. In this work, heparin was used as an organic template to prepare uniform carbonate-based apatite nanorods (CHA) and graft ferulic acid (F-CHA) for enhanced bone mineralization. Next, by combining calcium phosphate cement (CPC) with different F-CHA/CPC ratios, a new type of injectable bone cement combined with F-CHA bioactive apatite was developed (CPC + F-CHA). The physicochemical properties, biocompatibility, and mineralization potential of the CPC + F-CHA composites were determined in vitro. The experimental results confirmed the preparation of highly biocompatible CHA and the compatibility of F-CHA with CPC. Although CPC + F-CHA composites with F-CHA (2.5 wt%, 5 wt%, and 10 wt%) showed a significant reduction in compressive strength (CS), compositing CPC with 10 wt% F-CHA yielded a CS suitable for orthopedic repair (CS still larger than 30 MPa). Spectroscopic and phase analyses revealed that the phase of the hydrothermally synthesized CHA product was not modified by the heparin template. Injection and disintegration tests indicated that the CPC + F-CHA composites have good biocompatibility even at 10 wt% F-CHA. D1 osteoprogenitor cells were cultured with the composites for 7 days in vitro, and the CPC + 10%F-CHA group demonstrated significantly promoted cell mineralization compared with other groups. Given these results, the use of over 10% F-CHA in CPC composites should be avoided if the latter is to be applied to load-bearing areas. A stress-shielding device may also be recommended to stabilize these areas. These newly developed biocompatible CPC + F-CHA have great potential as osteoconductive bone fillers for bone tissue engineering.

Characterization of Synthetic Hydroxyapatite Fibers Using High-Resolution, Polarized Raman Spectroscopy

In the Raman spectrum of B-type carbonated apatites, the ν1 CO32– mode (at ∼1070 cm–1) overlaps the ν3 PO43– band. The latter is readily observed where the CO32– content is low (up to ∼3 wt%). The CO32– content of bone is considerably higher (∼7–9 wt%). As a result, the ν3 PO43– band becomes completely obscured. The 1000–1100 cm–1 spectral range of carbonated apatite is frequently considered a combined ν3 PO43– and ν1 CO32– region. Here, high-resolution polarized Raman spectroscopy (step size of 0.74 ± 0.04 cm–1) provides new insights into synthetic hydroxyapatite (HAp) obtained as micrometer-sized fibers. Compared to bone mineral (deproteinized bovine bone), spectral features of HAp fibers are highly resolved. In particular, the ν3 PO43– band resolves into nine distinct sub-components: 1028, 1032, 1040, 1043, 1047, 1053, 1055, 1062, and 1076 cm–1. Parameters including full width half-maximum, intensity, area fraction, intensity ratio, and area fraction ratio vary between parallel and perpendicular polarized configurations. It is likely that the ν1 CO32– band of B-type carbonated apatites may contain a small but not insignificant contribution from the 1076 cm–1 sub-component of the ν3 PO43– band. Furthermore, the 1076 cm–1/1047 cm–1 ratio changes between parallel and perpendicular scattering configurations, suggesting that the contribution of the 1076 cm–1 sub-component may vary as a function of local orientation of bone mineral, thus skewing the ν1 CO32– band and compromising accurate estimation of carbonate-to-phosphate ratios in B-type CO32– substituted apatite.