Studies on the Interaction Between the Functional Monomer 4-Methacryloxyethyl Trimellitic Anhydride and Hydroxyapatite and Stability of the Obtained Hybrids

Abstract

Minimizing the risk of secondary caries in dentistry is achieved by using adhesive systems that provide a strong bond between the natural hard tissue and the restorative material. Evaluating the effectiveness of these systems requires studying both their interaction with dentin and enamel and their behavior in environments with varying acidity. In this work, the interaction of a reactive monomer, 4-methacryloxyethyl trimellitic anhydride (4-META), used in adhesive systems with both dentin-like hydroxyapatite (HA) and hydroxyapatite ceramics, was investigated. Kinetic studies showed that under experimental conditions, 4-META was hydrolyzed and amorphized. Dentin-like HA possessed greater adsorption capacity to 4-META than ceramic HA. Immersion of HA into a solution of 4-META led to formation of an acidic calcium phosphate phase over time in both systems. Studies on the solubility of the synthetic nanosized hydroxyapatite and its derivative with 4-META in 0.1 mol/L lactic acid, also containing CaCl₂, Na₂HPO₄, and NaF (pH 4.5), and in distilled water (pH 6.3) indicated the occurrence of dissolution, complexation, and crystallization processes, causing changes in the liquid and solid phases. The total Ca²⁺ concentration upon dissolution of hybrid HA-4-META in a lactic acid solution was three times lower than the total Ca²⁺ concentration upon dissolution of pure HA. This suggested that 4-META-treated dentin-like surfaces demonstrate greater resistance to dissolution in acidic environments compared to untreated surfaces, highlighting the potential for these hybrids in dental applications.

Keywords: 4-methacryloxyethyl trimellitic anhydride; adhesive; hydroxyapatite; kinetics; solid-state NMR.

Figure 1

X-ray powder diffraction pattern of HA65 and HA1000 (black lines). The red line corresponds to calcium hydroxyapatite (Ca₅(PO₄)₃OH) of ICSD database code 60521.

Figure 2

Low-temperature absorption characteristics of HA65 and HA1000: (a) adsorption–desorption isotherms and (b) pore size distribution.

Figure 3

Direct excitation ³¹P NMR spectra of HA65 (a) and HA1000 (b). The experimental spectra are given in black, while the simulated spectra are presented in red lines. The individual contributions of the different components obtained after the deconvolution of the spectra are given with colored lines. The insert shows an expanded region around the main resonance in the spectrum of HA1000, where the two overlapping signals, with individual contributions given in blue and gray in the deconvoluted spectrum, are also visible.

Figure 4

Direct excitation ³¹P NMR spectra: (a) HA65 and (HA65-4META)_24h; (b) HA1000 and (HA1000-4META)_24h.

Figure 5

¹H-¹³C CP MAS NMR spectra of the hybrid materials: (a) (HA65-4META) series; (b) (HA1000-4META) series compared to the spectrum of pure crystalline 4-META. The broad signal at 110 ppm in the spectra of HA1000-4META series is a background signal from the rotor KEL-F cap, which is visible due to the negligible intensity of the sample resonances.

Figure 6

Kinetic profiles of pH (a,b) and concentration of free Ca²⁺ ions (c,d) measured in situ.