Comparison of the Fluoride Ion Release from Composite and Compomer Materials under Varying pH Conditions—Preliminary In Vitro Study

Abstract

Fluoride prevents the demineralization and supports remineralization of enamel. It is important to maintain a constant low level of fluoride in the oral cavity at all times. Dental restorative materials that are used for this purpose contain NaF in their composition, which is a source of fluoride ions that diffuse into body fluids and surrounding tissues. Two such materials, the flowable composite material Wave (W) (SDI, Hamilton Parkway Itasca, Australia) and compomer material Freedom (F) (SDI, Hamilton Parkway Itasca, Australia), were studied in regards to their release of fluoride ions into various solutions imitating the oral environment: artificial saliva solution with the addition of hydrated calcium chloride (CaCl₂) × 2H₂O in the pH range of 4.5 and 5.5; artificial saliva free of calcium chloride in the pH range of 4.5, 5.5, 6.0, 7.0, and 7.5; deionized water; and saline solution. The results were assessed over a period of 7 days, i.e., 168 h. The Freedom (F) compomer showed the highest cumulative release of fluoride ions into artificial saliva solution at pH 4.5 (31.195 ± 10.461 μg F/mm²) and the lowest into saline solution (3.694 ± 1.115 μg F/mm²). The Wave (W) composite material revealed the highest cumulative release of fluoride ions into deionized water (7.982 ± 2.011 μg F/mm²); its lowest cumulative emission was observed in artificial saliva solution at pH 7.0 (1.391 ± 0.489 μg F/mm²). The amounts of released fluoride from the Freedom (F) compomer were considerably higher compared to the Wave (W) composite material. The ability to release the largest amounts of fluoride ions in an acidic environment results from the erosion of the surface layer. Different calcium content in available experimental and commercial artificial saliva preparations may influence the obtained results. Both the flowable Wave composite and Freedom compomer released more fluoride in an acidic environment with a pH of 4.5–5.5, but with different dynamics—Wave material had its maximum on the third day while Freedom on the first day of the experiment.

1. Introduction

The importance of fluoride in the prevention of dental caries has been the subject of clinical trials for many years. The mechanism of the action of the fluoride in this context is based on inhibiting the demineralization of hard dental tissues and supporting their remineralization [1,2]. Fluoride exerts a significant influence on plaque bacteria; it inhibits the activity of the metabolic enzyme enolase and thereby decreases the amount of acid production. It also impairs glucose transport into their cells, by which it obstructs the formation of intracellular polysaccharide storage. All of these actions result in growth inhibition of the cariogenic micro-organisms [3]. Maintaining an appropriate level of fluoride in the oral cavity is therefore an important element in the prevention of dental caries. Dental restorative materials are available that contain fluoride ions [4] and are able to ensure their long-term release into the oral cavity [5]. In aqueous environment, fluoride ions diffuse from the resin into the inorganic part of the tooth. There, they combine with hydroxyapatite crystals and replace their hydroxyl groups, which leads to the formation of fluoroapatite [6], a structure more resistant to caries [5,7,8]. Compared to hydroxyapatite, the fluoroapatite crystals possess a more stable structure, lower solubility, better thermal stability, and a greater resistance to acid [3,9]. Their movement is a complex process based on the diffusion of fluids into and out of the material as well as the diffusion of fluoride ions into the dental tissues, acting as a carrier of fluoride.

The release of fluoride ions from dental materials used in restorative dentistry depends on several factors. In terms of the material itself, the number of fluoride-containing fillers, the type of resin matrix, as well as the curing mechanism of the material, including the polymerization process, all play an important role [10]. Significant in the release of fluoride from dental materials is the adsorption of fluids mainly in the matrix of the material, which allows the liquid particles to penetrate into the stable structure of the material and release fluoride ions through controlled diffusion. The properties of the resin matrix determine the diffusion rate and the volume of water absorbed [11,12,13]. Compomers, represented in our study by the Freedom (F) material, differ from resin composites to which the flowable Wave (W) material belongs by the type of organic monomers and inorganic fillers [14]. The composite material (W) mainly contains the monomer UDMA (urethane dimethacrylate), which is much less hydrophilic than the monomer TEGDMA (triethylene glycol dimethacrylate) [13] that is found in the compomer (F). The composite also contains HEMA (2-hydroxyethyl methacrylate), which has hydrophilic groups, absorbing water during polymerization. Previous studies have shown that the type of monomer significantly influences the process of fluoride release [15]. Furthermore, the compomer (F) includes a small number of functional monomers with carboxylic acid groups -COOH in their structure, which is why compomer materials are also known as composite resins modified with polyacids [16,17,18]. The material (F) does not contain any water [19], which is why the acid groups are subject to dehydration. After polymerization, they combine considerably well with the resin matrix [20], allowing an acid-base reaction with metal ions from the glass filler (strontium-fluoride glass).

Compomer (F) differs from composite (W) yet another important detail, namely the inorganic component in the form of silanated reactive fillers FAS (fluoro-alumino-silicate fillers) [21], which, in addition to improving the mechanical properties, are also able to release fluoride ions.

In addition, since the compomer lacks water, it is just these silanated FAS fillers that facilitate water absorption in the oral cavity by increasing the material’s absorbability. Upon contact of the material with the oral environment, dehydrated acid monomers located on the periphery of the material release protons attacking the surfaces of the FAS fillers, which in turn leads to the release of calcium, aluminum, and fluoride ions, which do not participate in the polymerization process of the material [22]. It is the ion release mechanism through water absorption after the material’s polymerization—that is the main difference between composites and compomers [22]. The polymerization causes a binding process similar to the mechanism observed in composite resins; however, the subsequent water sorption leads to the ionization of acidic groups and an acid-base reaction, which releases fluoride in a similar way to glass ionomers [23]. The results of our study showed that the amounts of released fluoride from the compomer material (F) were much higher than from the composite material (W) in most of the environments in which the tested samples were placed. The matrix of the composite resin is much less hydrophilic and its incorporated fluoride ions are released only in small amounts [24], which can also be seen in the test results of this study. Vermeersch et al. observed that the fluoridated resin in the composite released the fluoride in small amounts, about 10 times less than the compomers during the first day of the study [25]. This was also confirmed by the results of other studies, in which the fluoridated resin released less fluoride ions into artificial saliva solution than a compomer [26,27].

The amount and rate of fluoride release from the material also depends on the pH of the solution in which the material is placed [28]. In the oral cavity, the pH of the plaque can reach the critical value of 4–4.5. A pH with a value of 4–5.5 is considered as cariogenic, 5.5–6 is potentially cariogenic, whereas a pH > 6 is safe [29].

The preliminary character of this study is manifested with the short-term evaluation of fluoride release and only in vitro character. The next step will be the study of long term release and preparing materials in a real tooth.

The main goal of this study was to determine the fluoride concentrations at the surfaces of selected dental materials—composite and compomer (Wave and Freedom, respectively)—and to determine the level and dynamic of release of the fluoride from these materials in different storage media—deionized water, 0.9% NaCl solution, and artificial saliva (AS). Variable pH spectrum of AS (from 4.5 to 7.5), prepared accordingly to prof. Kaczmarek recipe [30,31,32,33], is purposed to mimic different environments of human oral cavity. Changes in the pH of human saliva, a natural physiological process, have a significant impact on the level of release of fluoride ions from the surface of dental materials. Previous experiments showed that in artificial saliva with a pH > 7, the precipitation of calcium compounds leads to the formation of turbidities. After the calcium combines with fluoride, the availability of fluoride ions is reduced. It therefore seems reasonable to evaluate the release of fluoride ions into the calcium-free artificial saliva solution. Therefore, the tests were carried out with saliva resembling solution with different pH and addition of calcium salt as well as with deionized water and saline solution. Beside the fluoride release the aim of this study is also to assess the morphological changes at the material surfaces using scanning electron microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR).

It is expected that the highest fluoride concentrations will be for compomer material in acidic environment. Moreover, the structure and composition of materials should not be affected after short-term release while the surface should show some changes and degradation.

2. Materials and Methods

• Wave (W) [SDI, Hamilton Parkway Itasca, Australia] is a composite with a semi-fluid consistency. It contains 39.98% by weight of acrylic monomer, 60% of inorganic filler (silica), 0.01% of butylated hydroxytoluene and 0.01% of NaF.
• Freedom (F) [SDI, Hamilton Parkway Itasca, Australia] contains 22% by weight of acrylic monomer, 77% of inorganic filler (strontium-fluoride glass), 0.01% of butylated hydroxytoluene and 0.10% of NaF.

Samples of the tested materials were made in the shape of cylinders with a diameter of 5 mm and a thickness of 2 mm using a polyethylene matrix. The samples were fabricated with the use of the LED Elipar II lamp (3M ESPE, St Paul, MN, USA) emitting light in the wavelength range of 400–515 nm with a maximum intensity of 800 mW cm⁻². Curing time was 40 s. After curing, the samples were polished and cleaned of any possible residues and their contact area was calculated in mm².

X-Ray Diffraction (XRD) measurements were made on the X’Pert PRO X-ray diffractometer (Cu Kα1, 1.54060 Å) by PANalytical.

The FT-IR spectra of the materials were recorded on a Thermo Scientific Nicolet iS50 FT-IR spectrometer (Waltham, MA, USA) method with DTGS/KBr detector and an Attenuated Total Reflectance (ATR) with a diamond crystal plate. The ATR-FT IR spectra were recorded between 4000 and 400 cm⁻¹ at room temperature with 4 cm⁻¹ spectral resolution. A total samples/background scans using OMNIC spectra software (Thermo Fisher Scientific Inc.) of 100 scans were accumulated for each spectrum.

The Scanning Electron Microscopy (SEM) investigations were performed using a Hitachi SU-70 system (Hitachi, Chiyoda, Tokyo, Japan). Images were taken at 15 kV of accelerating voltage using detector of secondary electrons.

To determine the exact amount of released fluoride ions, the ion-selective ORION (Thermo Fisher Scientific Co., Waltham, MA, USA) model 9609 electrode was used in combination with the microcomputer pH/ion meter CPI-551 Elmetron. The system was calibrated before each subsequent measurement, which was repeated three times to determine the mean value. Samples were subsequently immersed in the 5 mL of studied solutions and left without stirring in closed containers at 37 °C for an appropriate period of time until the determination of the released fluoride from the materials was carried out (measurements were taken after 3, 24, 48, 72, 69, and 168 h). Six samples of each material were prepared for each environment (total n = 108).

After the measuring period, 5 mL of the eluate was taken for evaluation, the sample was dried and transferred to a new solution for determination (5 mL). The concentrations of fluoride released from the materials were expressed in μmol/L and in relation to their surface area. The cumulative level of fluoride ion releases (i.e., total release over the whole given observation period) and the increments of releases were determined over the given measurement periods and with respect to the unit of time—1 h.

The evaluation of fluoride release from the Wave composite material (W) and the Freedom compomer (F) was performed in 9 environments differing in composition and pH, over a period of 7 days, i.e., 168 h.

The tested solutions consisted of artificial saliva without calcium salts at pH 4.5 (1), 5.5 (2), 6.0 (3), 7.0 (4), 7.5 (5), artificial saliva with the addition of calcium salts at pH 4.5 (6) and 5.5 (7) as well as deionized water (8) and saline solution (9).

The composition of the artificial saliva was as follows [33]: NaCl (0.4 g), KCl (4.0 g), urea (1 g), Na₂S × 9H₂O (0.005 g), NaH₂PO₄ × 2H₂O, and calcium ions in the form of CaCl₂ × 2H₂O (0.795 g). All solutions were prepared on the basis of deionized water. To establish the correct pH values, HCl and NaOH solutions were used.

All experiments were done six times and descriptive data was expressed as the mean and standard deviation (±SD). A one-way ANOVA for dependent samples (according time periods) was used for comparisons of continuous data between more than two groups. A Tukey post-hoc test was used for intergroup comparisons. Pearson’s correlation coefficients (r) were calculated to evaluate associations between the time of incubation and release of fluoride ions from study materials. Because Pearson’s correlation analysis assumes a linear correlation, whereas the ion re-lease vs. time dependence should resemble a logarithmic function, data for this analysis were logarithmically transformed. A 2-tailed p-value of p < 0.05 was considered statistically significant. Statistical analyses were conducted using Statistica v.13.3 (Tibco Software Inc., Palo Alto, CA, USA).

3. Results

Figure 1 shows the XRD pattern of the Wave and Freedom pellets light-cured for 40 s. All samples are amorphous, which is demonstrated by a broad diffraction peak centred at 2θ = 26°. Both materials do not contain any crystalline add-ons.

Figure 2 presents the FTIR spectra of Wave and Freedom materials before and after fluoride ions release. The characteristic bands for the dental resins are clearly visible. The bands with a wavenumber of 1000, 1159, and 1716 cm⁻¹ come from the vibrations of the C=O double bond, the band at 1296 cm⁻¹ is related to the vibration of the C-O-C molecule, the band located at 1653 cm⁻¹ corresponds to the C=C double bond characteristic for dental resins, and intense bands at 2856 and 2920 cm⁻¹ are related to the vibrations of the C-H bond. In addition, a weak band is visible around 3500 cm⁻¹ associated with the vibration of the O-H group. Spectra before and after release of fluoride ions are very similar, which shows that there are no structural changes after immersion in different solutions.

The Wave material contains an acrylic derivative and also SiO₂. The Si-O-Si groups were observed at 1000–1250 cm⁻¹ (stretching vibration) and at 700–800 cm⁻¹ (deformation vibration). The vibration of the Si-O-Si group overlaps with C=O vibrations at about 1000 cm⁻¹ on the infrared spectrum [34]. Furthermore, the Freedom material also contains the acrylic derivatives as well as strontium-fluoride glass that is characterized with high transparency in the measured range up to 900 cm⁻¹ (the bands are not visible). Additionally, on the infrared spectra, the peaks were located at a range from 1025 to 1080 cm⁻¹ (stretching vibrations) and at 735–775 cm⁻¹ corresponding to PO₄³⁻ groups. It is related to the deposition of phosphate salts from an artificial saliva. The similar observations were reported regarding dental cements in the literature [35,36].

Figure 3 and Figure 4 present SEM micrographs of selected Wave and Freedom samples, respectively. The surface of the control sample is smooth for both materials (Figure 3a and Figure 4a). The most significant changes are visible for samples after fluoride ions release in artificial saliva in pH 4.5. The surface is strongly modified with some fractal-like shapes (Figure 3d and Figure 4d). Additionally deep craters are visible (Figure 3e and Figure 4e). Increase of pH causes lowering of fluoride ions release and therefore smaller changes in surface of samples.

The results of in-vitro release of an fluoride ions from composite Wave into nine environments differing in composition of the solution and pH in selected time periods were presented in

Table 1. Release of fluoride ions (µg/mm²/h) from composite Wave (W) into nine environments differing in composition of the solution and pH. Six samples were prepared for each solution. Descriptive data were presented as mean ± standard deviation (±SD).

Time (Hours)	AS pH 4.5 (1) (μg/mm2/h)	AS pH 5.5 (2) (μg/mm2/h)	AS pH 6.0 (3) (μg/mm2/h)	AS pH 7.0 (4) (μg/mm2/h)	AS pH 7.5 (5) (μg/mm2/h)	AS + Ca2+ pH 4.5 (6) (μg/mm2/h)	AS + Ca2+ pH 5.5 (7) (μg/mm2/h)	Deionized H2O (8) (μg/mm2/h)	0.9% NaCl (9) (μg/mm2/h)
1	0.940 ± 0.299	1.417 ± 0.371	1.433 ± 0.658	0.838 ± 0.288	0.677 ± 0.083	0.882 ± 0.174	0.353 ± 0.119	1.535 ± 0.113	1.500 ± 0.141
3	0.398 ± 0.095	0.686 ± 0.195	0.055 ± 0.004	0.149 ± 0.078	0.655 ± 0.265	0.216 ± 0.078	0.643 ± 0.006	0.673 ± 0.538	0.648 ± 0.128
24	0.036 ± 0.004	0.061 ± 0.013	0.007 ± 0.004	0.003 ± 0.001	0.012 ± 0.005	0.036 ± 0.013	0.044 ± 0.002	0.064 ± 0.014	0.049 ± 0.001
48	0.029 ± 0.006	0.035 ± 0.001	0.008 ± 0.001	0.007 ± 0.001	0.028 ± 0.002	0.039 ± 0.006	0.024 ± 0.008	0.055 ± 0.013	0.028 ± 0.002
72	0.054 ± 0.016	0.053 ± 0.003	0.005 ± 0.001	0.001 ± 0.000	0.024 ± 0.001	0.043 ± 0.011	0.039 ± 0.006	0.018 ± 0.001	0.017 ± 0.006
168	0.006 ± 0.000	0.007 ± 0.000	0.000 ± 0.000	0.000 ± 0.000	0.001 ± 0.000	0.010 ± 0.002	0.006 ± 0.001	0.021 ± 0.002	0.004 ± 0.001
Mean ± SD #	0.244 ± 0.364	0.376 ± 0.552	0.251 ± 0.591	0.166 ± 0.329	0.233 ± 0.327	0.205 ± 0.323	0.185 ± 0.244	0.394 ± 0.605	0.375 ± 0.565
p-value (ANOVA for dependent samples)	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001
post-hoc Tukey test	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 1, 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 1, 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 1, 24, 48, 72, 168 h

AS: artificial saliva; ^#: +SD are higher than means because all data for each group were analyzed.

and in Figure 5. ANOVA analysis for dependent samples showed statistically significant differences in F-ions release in certain time periods in tested incubation conditions (p < 0.0001 for all). The significantly highest levels of F-ions release were observed after 1 h and 3 h of incubation for all environments, especially into deionized water and 0.9% NaCl.

The cumulated concentration of F-ions was significantly highest during incubation in deionized water and AS pH 5.5 (

Table 2. Cumulated release of fluoride ions (µg/mm²) from composite Wave (W) into nine environments differing in composition of the solution and pH. Six samples were prepared for each solution. Descriptive data were presented as mean ± standard deviation (±SD). Data for correlation analysis were logarithmically transformed.

Time (Hours)	AS pH 4.5 (1) (μg/mm2)	AS pH 5.5 (2) (μg/mm2)	AS pH 6.0 (3) (μg/mm2)	AS pH 7.0 (4) (μg/mm2)	AS pH 7.5 (5) (μg/mm2)	AS + Ca2+ pH 4.5 (6) (μg/mm2)	AS + Ca2+ pH 5.5 (7) (μg/mm2)	Deionized H2O (8) (μg/mm2)	0.9% NaCl (9) (μg/mm2)
1	0.9 ± 0.3	1.4 ± 0.4	1.4 ± 0.7	0.8 ± 0.3	0.7 ± 0.01	0.9 ± 0.2	0.4 ± 0.1	1.5 ± 0.1	1.5 ± 0.1
3	1.7 ± 0.5	2.8 ± 0.8	1.5 ± 0.7	1.1 ± 0.4	2.0 ± 0.6	1.3 ± 0.3	1.6 ± 0.1	2.9 ± 1.2	2.8 ± 0.4
24	2.5 ± 0.6	4.1 ± 1.0	1.7 ± 0.8	1.2 ± 0.5	2.3 ± 0.7	2.1 ± 0.6	2.6 ± 0.2	4.2 ± 1.5	3.8 ± 0.4
48	3.2 ± 0.7	4.9 ± 1.1	1.9 ± 0.8	1.4 ± 0.5	2.9 ± 0.8	3.0 ± 0.7	3.1 ± 0.4	5.6 ± 1.8	4.5 ± 0.5
72	4.5 ± 1.0	6.2 ± 1.1	2.0 ± 0.8	1.4 ± 0.5	3.5 ± 0.8	4.1 ± 1.0	4.1 ± 0.5	6.0 ± 1.8	4.9 ± 0.6
168	5.0 ± 1.0	6.8 ± 1.1	2.0 ± 0.8	1.4 ± 0.5	3.6 ± 0.8	5.0 ± 1.2	4.7 ± 0.6	8.0 ± 2.0	5.4 ± 0.7
Correlation (Pearson test)	r = 0.980; p < 0.001	r = 0.977; p < 0.001	r = 0.973; p = 0.001	r = 0.926; p = 0.007	r = 0.914; p = 0.010	r = 0.987; p < 0.001	r = 0.932; p = 0.006	r = 0.985; p < 0.001	r = 0.965; p = 0.002

and Figure 6 left). The lowest level of cumulative F-ions release was observed after incubation in AS pH 7.0 and AS pH 6.0 (Table 2). We observed statistically significant correlations between selected time points and fluoride ions release in all tested incubation environments (p < 0.001) (Table 2). The composite material Wave (W) revealed the highest cumulative release of fluoride ions to deionized water (8.00 ± 2.0 μg F/mm²), followed by an artificial saliva solution at pH 5.5 (6.8 ± 1.1 μg F/mm²) and saline (5.4 ± 0.7 μg F/mm²). The lowest emission was observed in artificial saliva solution with pH 7.0 (1.4 ± 0.5) (Table 2). After the first hour, the material released fluoride the fastest to deionized water and 0.9% NaCl (1.5 ± 0.1 μg F/mm²/h), and the slowest to artificial saliva with the addition of calcium ions at pH 5.5 (0.4 ± 0.1 μg F/mm²/h), (Table 1). On average, the increase in fluoride emission was the highest in the deionized water, 0.9% NaCl, and pH 5.5 environments (0.4 ± 0.6 μg F/mm²/h).

We showed statistically significant differences in F-ion release in time periods of compomer Freedom into tested incubation environments (p < 0.0001 for all) (

Table 3. Release of fluoride ions (µg/mm²/h) from compomer Freedom (F) into nine environments differing in composition of the solution and pH. Six samples were prepared for each solution.

Time (Hours)	AS pH 4.5 (1) (μg/mm2/h)	AS pH 5.5 (2) (μg/mm2/h)	AS pH 6.0 (3) (μg/mm2/h)	AS pH 7.0 (4) (μg/mm2/h)	AS pH 7.5 (5) (μg/mm2/h)	AS + Ca2+ pH 4.5 (6) (μg/mm2/h)	AS + Ca2+ pH 5.5 (7) (μg/mm2/h)	Deionized H2O (8) (μg/mm2/h)	0.9% NaCl (9) (μg/mm2/h)
1	5.7 ± 2.5	4.9 ± 0.4	0.7 ± 0.1	1.1 ± 0.3	1.7 ± 0.2	1.6 ± 0.3	0.9 ± 0.0	0.4 ± 0.1	0.3 ± 0.0
3	4.0 ± 1.3	2.3 ± 0.1	0.5 ± 0.1	0.4 ± 0.0	1.1 ± 0.1	2.9 ± 0.3	0.8 ± 0.0	0.5 ± 0.0	0.2 ± 0.0
24	0.3 ± 0.1	0.2 ± 0.0	0.04 ± 0.0	0.03 ± 0.0	0.1 ± 0.0	0.1 ± 0.0	0.08 ± 0.0	0.1 ± 0.0	0.03 ± 0.0
48	0.2 ± 0.1	0.1 ± 0.0	0.03 ± 0.0	0.03 ± 0.0	0.04 ± 0.0	0.2 ± 0.0	0.1 ± 0.0	0.05 ± 0.0	0.03 ± 0.0
72	0.1 ± 0.0	0.1 ± 0.0	0.03 ± 0.0	0.01 ± 0.0	0.07 ± 0.0	0.1 ± 0.0	0.1 ± 0.0	0.03 ± 0.0	0.02 ± 0.0
168	0.0 ± 0.0	0.04 ± 0.0	0.01 ± 0.0	0.01 ± 0.0	0.02 ± 0.0	0.02 ± 0.0	0.01 ± 0.0	0.02 ± 0.0	0.01 ± 0.0
Mean ± SD #	1.7 ± 2.5	1.3 ± 1.8	0.2 ± 0.3	0.3 ± 0.4	0.5 ± 0.7	0.8 ± 1.1	0.3 ± 0.4	0.2 ± 0.2	0.1 ± 0.1
p-value (ANOVA for dependent samples)	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001	<0.0001
post-hoc Tukey test	p < 0.001 for 1 h vs. 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h p < 0.05 for 168 h vs. 24, 48, 72 h	p < 0.001 for 1 h vs. 24, 48, 72, 168 h p < 0.001 for 3 h vs. 24, 48, 72, 168 h	p < 0.001 for 1 h vs. 3, 24, 48, 72, 168 h p < 0.001 for 3 h vs. 1, 24, 48, 72, 168 h

AS: artificial saliva; ^#: +SD are higher than means because all data for each group were analyzed.

and Figure 5 right). The significantly highest levels of fluoride ions release was demonstrated after 1 h and 3 h of incubation for whole study groups, especially into AS pH 4.5 and AS pH 5.5 (Table 3). Also cumulated levels of fluoride ions release from compomer (F) were highest during incubation in AS pH 4.5 and AS pH 5.5, and significantly lower values of cumulated F-ions were observed in 0.9% NaCl and AS pH 7.0 solutions (

Table 4. Cumulated release of fluoride ions (µg/mm²) from compomer Freedom (F) into nine environments differing in composition of the solution and pH. Six samples were prepared for each solution. Descriptive data were presented as mean ± standard deviation (±SD). Data for correlation analysis were logarithmically transformed.

Time (Hours)	AS pH 4.5 (1) (μg/mm2)	AS pH 5.5 (2) (μg/mm2)	AS pH 6.0 (3) (μg/mm2)	AS pH 7.0 (4) (μg/mm2)	AS pH 7.5 (5) (μg/mm2)	AS + Ca2+ pH 4.5 (6) (μg/mm2)	AS + Ca2+ pH 5.5 (7) (μg/mm2)	Deionized H2O (8) (μg/mm2)	0.9% NaCl (9) (μg/mm2)
1	5.7 ± 2.5	4.9 ± 0.4	0.7 ± 0.1	1.1 ± 0.3	1.7 ± 0.2	1.6 ± 0.3	0.9 ± 0.02	0.4 ± 0.1	0.3 ± 0.01
3	13.8 ± 5.1	9.4 ± 0.6	1.7 ± 0.2	1.9 ± 0.3	3.8 ± 0.3	7.5 ± 1.0	2.6 ± 0.1	1.3 ± 0.1	0.6 ± 0.1
24	20.8 ± 8.0	13.8 ± 0.7	2.6 ± 0.4	2.7 ± 0.4	6.0 ± 0.5	10.2 ± 1.8	4.2 ± 0.1	2.7 ± 0.4	1.2 ± 0.2
48	24.9 ± 9.4	16.3 ± 0.8	3.5 ± 0.7	3.5 ± 0.4	7.0 ± 0.7	15.2 ± 2.6	5.8 ± 0.3	3.8 ± 0.6	1.9 ± 0.5
72	28.0 ± 9.9	19.0 ± 1.0	4.1 ± 1.1	3.8 ± 0.4	8.7 ± 1.0	18.8 ± 3.3	7.3 ± 0.3	4.6 ± 0.8	2.3 ± 0.5
168	31.2 ± 10.5	23.2 ± 1.1	4.6 ± 1.1	4.5 ± 0.6	10.2 ± 1.0	20.7 ± 3.4	8.2 ± 0.4	6.2 ± 1.4	3.7 ± 1.1
Correlation (Pearson test)	r = 0.957; p = 0.003	r = 0.979;	r = 0.957; p = 0.003	r = 0.979;	r = 0.957; p = 0.003	r = 0.979;	r = 0.957; p = 0.003	r = 0.979;	r = 0.957; p = 0.003

, Figure 6 right). Significant correlations between time of incubation and release of F-ions were presented in all tested environments (p < 0.001) (Table 4). The Freedom (F) compomer showed the highest cumulative release of fluoride ions into the artificial saliva solution at pH 4.5 (31.2 ± 10.5 μg F/mm²), followed by saliva at pH 5.5 (23.2 ± 1.1 μg F/mm²), and lowest into the solution saline (3.7 ± 1.1 μg F/mm²) (Table 4). In addition, the most differentiated increases in fluoride emission were observed both after 1 h and on average during the entire observation period in the environment of artificial saliva with pH 4.5 (5.7 ± 2.5 μg F/mm²/h and 1.7 ± 2.5 μg F/mm²/h, respectively) and in the solution saline (0.3 ± 0.01 μg F/mm²/h and 0.1 ± 0.1 μg F/mm²/h, respectively) (Table 3). The amounts of released fluoride from the Freedom (F) compomer were much higher compared to the Wave (W) composite material. The physiological saline solution and deionized water did not appear to favour the emission of fluoride ions, in their case the Freedom material (F) recorded low values of released fluoride.

Similarly in the case of the Wave material, artificial saliva solutions with a pH of 4.5 and 5.5 without the content of Ca²⁺ ions turned out to be most suitable for the intensification of the fluoride release from the compomer (F). The maximum concentration of fluoride ions was read at pH 4.5 and was 5.7 ± 2.5 μg F/mm². The alkaline pH of the Freedom and Wave materials did not stop fluoride release.

4. Discussion

The aim of the study was to understand the pattern and quantification of the release of fluoride ions from the Wave and Freedom materials placed in various environments as well as to assess the impact of pH on this process.

In the acidic environment of pH 4.5 and 5.5, the higher cumulative release of fluoride from the composite material (W) can be observed, respectively, in artificial saliva solutions without the addition of Ca^{2 +} ions at pH 4.5 (5.048 ± 1.010 μg F/mm²), pH 5.5 (6.835 ± 1.130 μg F/mm²) and compomer (F) at pH 4.5 (31.195 ± 10.461 μg F/mm²) and pH 5.5 (23.200 ± 1.107 μg F/mm²), respectively, in which the emission of fluoride ions from the compomer material (F) was significantly larger.

During the release of fluoride from composite materials, an initial strong increase in fluoride ions is noticeable in the release medium, i.e., “burst effect” [37]. For the Wave material, the highest release kinetics occurred on the third day of the experiment in solutions with a pH of 4.5 and 5.5. On the other hand, in the case of Freedom, the release of fluorides occurred at pH 4.5 and 5.5 on the first day of the experiment. The research shows that there is a dependence of the pH of artificial saliva on the amount of released fluoride ions. Low pH values of 4.5–5.5 favor the increase of fluorine ions in the tested medium, which corresponds to cariogenic states. These results are consistent with the research conducted by J.L. Moreau and H.H.K. Xu published in [38] on resin-modified glass ionomers (Viremer, Fuji II LC, Ketac Nano), compomer (Dyract Flow) and composite (Heliomolar) and in [39]. At higher pH values of the solutions (6.0–7.5), the release of fluoride ions slower, which makes the Wave and Freedom materials a reservoir of these ions, “sparing” them when the saliva pH is physiological or elevated and cariogenic processes are not intensified.

Earlier studies by Silva et al. also showed that compomer material presents a higher fluoride release in acidic environment with pH 4.6 (pH-cycling) than in distilled/deionized water [40]. Similarly, other studies discovered that compomer material releases a significantly higher level of fluoride in acid demineralising solution than in neutral remineralising solution [41].

The results of our study are consistent with the observations of other researchers [42], indicating that the rate of fluoride release from compomer materials is dependent on the pH of the environment surrounding the material. This is undoubtedly related to the fact that the decrease in the pH of the solution increases the solubility of the material through hydrolytic degradation taking place in the matrix of the material, and more precisely at the interface between the matrix and the filler [43]. The amount of released fluoride from the material depends not only on the source and concentration of fluoride, but also on whether the fluoride ions can diffuse from the internal structure of the material [42]. The amount of released fluoride from the composite material (W) is significantly lower in the acidic environment of artificial saliva than from the compomer (F), which may be due the lower hydrophilicity of the organic monomers and the higher content of inorganic filler. Previous studies have shown that the amount of filler is negatively correlated with the process of water sorption [44]. A more extensive cross-linking of the polymer chains of the filler molecules in the composite works as a barrier through which less water diffusion takes place, thereby releasing less fluoride, especially in the environment of artificial saliva. A solution of deionized water, i.e., a solution devoid of ionic strength and other elements, proved to be the best medium for the composite material (W), where the cumulative amount of released fluoride was the highest (8.0 ± 2.0 μg F/mm²). The release of fluoride ions from materials used in restorative dentistry also depends on the roughness of the material, the surface preparation, and the finishing procedure [45]. The roughness of the materials increases during the release of fluoride ions to the surrounding solutions, where the smallest change is observed in the case of composites containing inorganic silica, i.e., in the case of composite material (W). Compomer materials that combine the features of composites and glass ionomer materials are more coarse [46]. Therefore, the release of fluoride ions and hydrolytic degradation will take place much easier in these materials and the emission of fluoride ions will be higher [47].

The selection of the appropriate medium for in vitro fluoride release testing from materials is critical. Different calcium content in available experimental and commercial artificial saliva preparations may influence the obtained results, therefore it is important to be aware of this. The shape of the samples is also important—in this experiment, samples were cylindrical and had released fluoride ions from three surfaces. In real conditions in the oral cavity, these materials can release only from one surface and it is possible that amount of fluoride ions will be much lower. Moreover, all experiments were performed at 37 °C, while in the oral cavity, there can be significant changes in temperature caused, for example, by drinking cold or hot drinks, which could affect release of fluoride ions.

5. Conclusions

The amounts of released fluoride from the Freedom (F) compomer were considerably higher compared to the Wave (W) composite material. The environment and changes in pH had a great impact on the dental materials and their release of fluoride in our study. Both the flowable Wave composite and Freedom compomer released more fluoride in acidic environment with a pH of 4.5–5.5 but with different kinetics—the Wave material had its maximum on the third day while Freedom on the first day of the experiment. It is in that cariogenic environment when the presence of fluoride is the most desirable and necessary to inhibit the demineralisation process and limit the development of caries. It is safe to say that the acidic environment significantly increases the rate of fluoride ion emission. In the case of increased pH to a value > 6, which is considered a safe environment in the oral cavity in the prevention of caries, the process of fluoride release from the tested materials is not ceased, yet decelerated, preventing the stored amount of fluoride from depleting prematurely.

The ability to release the largest amounts of fluoride ions in an acidic environment results from the erosion of the surface layer, which was confirmed by the SEM image. On the other hand, maintaining a smooth surface of the samples of both tested materials—incubated in a physiological saline solution—was accompanied by the lowest release of fluoride.