Application of β-Tricalcium Phosphate in Adhesive Dentin Bonding

Abstract

The study aimed at synthesizing β-tricalcium phosphate (β-TCP) nanoparticles and comparing the mechanical properties and dentin interaction of two adhesives: experimental adhesive (EA) and EA with 5 wt.% β-TCP nanoparticles (β-TCP-5%). These filler nanoparticles were synthesized and then characterized with scanning electron microscopy (SEM) and micro-Raman spectroscopy. The β-TCP nanoparticles were incorporated in the adhesives to form two groups: gp-1: EA (control) and gp-2: β-TCP-5%. These adhesives were characterized by SEM, energy-dispersive X-ray (EDX) spectroscopy and were also assessed for their micro-tensile bond strength (μTBS) with (TC) and without thermocycling (NTC). Fourier Transform Infrared (FTIR) spectroscopy was performed to evaluate the degree of conversion (DC) of two adhesives. The β-TCP filler was seen as irregularly shaped agglomerates on SEM. The micro-Raman spectra revealed characteristic peaks associated with β-TCP nanoparticles. Both adhesives presented suitable dentin interaction, which was demonstrated by the formation of resin tags of variable depths. The EDX analysis verified the existence of calcium (Ca) and phosphate (P) for the β-TCP-5% group. The greatest μTBS values were shown by β-TCP-5% group samples when they were non-thermocycled (NTC) (β-TCP-5%-NTC: 34.11 ± 3.46) followed by the thermocycled (TC) samples of the same group (β-TCP-5%-TC: 30.38 ± 3.66), compared with the EA group. Although the DC presented by β-TCP-5% group was comparable to the EA group, it was still lower. The addition of β-TCP nanoparticles in the adhesive improved its μTBS and resulted in a suitable dentin interaction, seen in the form of hybrid layer and resin tag formation. Nonetheless, a decreased DC was observed for the β-TCP-5% adhesive. Future studies probing the effect of different filler concentrations on various properties of the adhesive are warranted.

Keywords: adhesive; calcium; dentin; phosphate; tricalcium phosphate.

Figure 1

(A) Low (3505x) and (B) high magnification (6531x) SEM view of the synthesized nano β-tricalcium phosphate particles. The powder of β-tricalcium phosphate showed agglomerated morphology of various irregular sized polygonal crystals ranging from 500 nm to 1500 nm formed by sol-gel synthesis.

Figure 2

Raman spectrum of the nano-β-tricalcium phosphate indicates the labeled peaks as characteristic of the internal vibration of the PO₄³⁻ tetrahedric groups of the β-TCP molecule. The symmetric stretching (ν₁) of P-O bonds of the tetrahedron corresponds to the peaks with the highest intensity at around 950 cm⁻¹ and 970 cm⁻¹. The asymmetric stretching (ν₃) has a lower intensity and is in the 1015–1090 cm⁻¹ range.

Figure 3

Representative SEM images of the bonded resin dentin interface using (A) unmodified dentin adhesive (EA) and (B) 5 wt.% nano-β-tricalcium phosphate modified dentin adhesive. Note the addition of nanocrystals (agglomerations in the image as indicated by pointers) did not significantly affect dentin bonding at the hybrid layer (HL) with well-formed resin tags (RT).

Figure 4

Representative line EDX along the resin dentin interface/hybrid layer for (A) unmodified dentin bonding agent (EA) and (B) 5 wt% nano-β-tricalcium phosphate modified dentin bonding agent. The modified dentin bonding agent indicates the presence of calcium (Ca), phosphorus (P), and other tooth related elements like silica filler (Si) and carbon (C).

Figure 5

FTIR spectrum of polymerized and unpolymerized unmodified (EA) and βTCP nanocrystal modified adhesive. The degree of conversion was calculated by estimating the changes in peak height ratio of the absorbance intensities of aliphatic C=C peak at 1638 cm⁻¹ and that of an internal standard peak of aromatic C=C at 1608 cm⁻¹ during polymerization, in relation to the uncured adhesive as indicated by dotted lines.