The Influence of the Matrix on the Apatite-Forming Ability of Calcium Containing Polydimethylsiloxane-Based Cements for Endodontics

Abstract

This study aimed to characterize the chemical properties and bioactivity of an endodontic sealer (GuttaFlow Bioseal) based on polydimethylsiloxane (PDMS) and containing a calcium bioglass as a doping agent. Commercial PDMS-based cement free from calcium bioglass (GuttaFlow 2 and RoekoSeal) were characterized for comparison as well as GuttaFlow 2 doped with dicalcium phosphate dihydrate, hydroxyapatite, or a tricalcium silicate-based cement. IR and Raman analyses were performed on fresh materials as well as after aging tests in Hank’s Balanced Salt Solution (28 d, 37 °C). Under these conditions, the strengthening of the 970 cm−1 Raman band and the appearance of the IR components at 1455−1414, 1015, 868, and 600−559 cm−1 revealed the deposition of B-type carbonated apatite. The Raman I970/I638 and IR A1010/A1258 ratios (markers of apatite-forming ability) showed that bioactivity decreased along with the series: GuttaFlow Bioseal > GuttaFlow 2 > RoekoSeal. The PDMS matrix played a relevant role in bioactivity; in GuttaFlow 2, the crosslinking degree was favorable for Ca2+ adsorption/complexation and the formation of a thin calcium phosphate layer. In the less crosslinked RoekoSeal, such processes did not occur. The doped cements showed bioactivity higher than GuttaFlow 2, suggesting that the particles of the mineralizing agents are spontaneously exposed on the cement surface, although the hydrophobicity of the PDMS matrix slowed down apatite deposition. Relevant properties in the endodontic practice (i.e., setting time, radiopacity, apatite-forming ability) were related to material composition and the crosslinking degree.

Keywords: GuttaFlow 2; GuttaFlow Bioseal; RoekoSeal; apatite; bioactivity; bioglass; calcium phosphate dihydrate (DCPD); calcium silicates (CaSi); crosslinking; endodontic sealer; hydroxiapatite (HA); polydimethylsiloxane; root filling materials; vibrational IR and Raman spectroscopy.

Figure 1

The average IR spectra of orange (A) and white (B) pastes of RoekoSeal, GuttaFlow 2, and GuttaFlow Bioseal. The spectra are normalized to the absorbance of the 2963 cm⁻¹ band. The bands assignable to polydimethylsiloxane (pSi), monoclinic zirconia (Z), and bioactive glass-ceramic (BG) are indicated together with those specifically assigned to Si-H bonds (*). The insets show the spectral ranges where the modes ascribable to vinyl groups are reported to fall (i.e., C=C stretching at about 1600 cm⁻¹ and =CH stretching at about 3055 cm⁻¹). More detailed assignments are summarized in Table S1, Supplementary Material.

Figure 2

A₂₁₆₀/A₂₉₆₃ (A) and A₉₁₀/A₁₂₅₈ (B) absorbance ratios (average ± standard deviation) as calculated from the IR spectra of the orange paste and fresh (i.e., just mixed) samples of the commercial sealers under study. The different capital letters in each histogram represent statistically significant differences (p < 0.05) between orange pastes and small letters between the fresh samples.

Figure 3

The average IR spectrum of fresh (i.e., just mixed) GuttaFlow Bioseal; the spectra of its white and orange pastes are reported for comparison. The spectra are normalized to the absorbance of the 2963 cm⁻¹ band. The bands assignable to polydimethylsiloxane (pSi), monoclinic zirconia (Z), and bioactive glass-ceramic (BG) are indicated together with those specifically assigned to Si-H bonds (*). More detailed assignments are summarized in Table S1, Supplementary Material.

Figure 4

Average FT-Raman spectra of orange (A) and white (B) pastes of RoekoSeal, GuttaFlow 2, and GuttaFlow Bioseal. The spectra are normalized to the intensity of the 638 cm⁻¹ band. The bands assignable to monoclinic zirconia (Z), polydimethylsiloxane (pSi), bioactive glass-ceramic (BG), zinc oxide (ZnO), and gutta-percha (GP) are indicated. More detailed assignments are summarized in Table S2, Supplementary Material.

Figure 5

Average FT-Raman spectra of fresh RoekoSeal, GuttaFlow 2, and GuttaFlow Bioseal. The spectra are normalized to the intensity of the 638 cm⁻¹ band. The bands assignable to monoclinic zirconia (Z), polydimethylsiloxane (pSi), zinc oxide (ZnO), and gutta-percha (GP) are indicated. More detailed assignments are summarized in Table S2, Supplementary Material.

Figure 6

I₇₁₀/I₆₃₈ intensity ratio (average ± standard deviation) as calculated from the FT-Raman spectra of fresh commercial sealers under study. Different letters represent statistically significant differences (p < 0.05) between values.

Figure 7

Average IR spectra recorded on the surface of GuttaFlow Bioseal before (i.e., fresh) and after aging in HBSS for 28 days. The spectra are normalized to the absorbance of the 1258 cm⁻¹ band. The bands assignable to polydimethylsiloxane (pSi) and monoclinic zirconia (Z) are indicated together with those specifically assigned to unreacted Si-H bonds (*) and B-type carbonated apatite (■). The inset shows the fourth derivative IR spectra in the 810–770 cm⁻¹ range. More detailed assignments are reported in the text and summarized in Table S1, Supplementary Material.

Figure 8

Average micro-Raman spectra recorded on the surface of GuttaFlow Bioseal before (i.e., fresh) and after aging in HBSS for 28 days. The spectra were normalized to the intensity of the 638 cm⁻¹ band. The spectra of the aged sample were recorded using pinholes of 3000 μm (pin 3000) and 50 μm (pin 50). The bands assignable to monoclinic zirconia (Z) and polydimethylsiloxane (pSi) are indicated.

Figure 9

IR A₁₀₁₀/A₁₂₅₈ (A) absorbance ratio, and Raman I₉₇₀/I₆₃₈ (B) intensity ratio (average ± standard deviation) as calculated from the IR and micro-Raman spectra of the materials under study before (i.e., fresh samples) and after aging for 28 days in HBSS. Asterisks indicate significant differences (*: p < 0.01; **: p < 0.001; ***: p < 0.0001) between fresh and aged samples within the same material in a Tukey’s HSD test.

Figure 10

Mechanism of the hydrosilylation reaction.