Study of the chemical interaction between a high-viscosity glass ionomer cement and dentin

Abstract Objective To investigate the chemical interactions between a high-viscosity glass ionomer cement (GIC) (KetacTM Molar Easymix, 3M ESPE, Seefeld, Bavaria, Germany) and human dentin. It was also analyzed the dynamics of GIC setting mechanism based on the time intervals required for the GIC and the GIC mixed with dentin to achieve stability. Material and Methods Each constituent of GIC – powder (P) and liquid (L) – and powdered dentin (D), as well as the associations P+L, D+L, and P+L+D in the concentrations of 29%, 50%, 65%, 78%, 82%, and 92% of GIC were analyzed with Fourier transform infrared (FTIR) and Raman spectroscopy. Results New optical absorption bands and/or Raman bands, which were not present in P, L, or D, were observed in the associations. The concentrations of 29% and 50% of GIC showed higher interaction, revealing that the amount of dentin influences the formation of new optical absorption or scattering bands. FTIR bands showed that the setting time to achieve bond stability was longer for the high-viscosity GIC (38±7 min) than for the sample with 29% of GIC (28±4 min). Conclusions The analysis revealed the formation of new compounds or molecular rearrangements resulting from the chemical interactions between GIC and dentin. Moreover, this study provides an effective method to evaluate the dynamics of the setting mechanism of GICs.


Introduction
The glass ionomer cement (GIC) can be defined as a water-based material resulting from an acidbase reaction between a fluoroaluminosilicate glass powder and an aqueous polyacid solution. 2,7,22 GICs have important properties such as adhesiveness to tooth structures, biocompatibility, thermal expansion coefficients close to teeth, and fluoride release. 18 Adhesiveness is one of the main characteristics by which this material is indicated. GIC adhesion to tooth Transform Infrared Spectroscopy (FTIR) 4 , and the intertubular diffusion of calcium ions of cement into dentinal structure was also observed with Raman spectroscopy. 1,22 Furthermore, X-ray studies showed ionic exchanges between carboxylate ions in the polyalkenoic acid and the calcium in the tooth. 17,28 However, the dynamics of the process, which could yield the time intervals required to achieve GIC and setting stability of GIC-dentin, as well as the characterization of the functional groups that may reveal the chemical interactions of the constituents of GIC with the dentin, were not investigated. Therefore, since FTIR and Raman spectroscopy are techniques that characterize the chemical structure of materials such as dental structures and cements, this research aimed to investigate the formation of chemical interactions between the dentin and a highviscosity GIC. This study also analyzed the dynamics of the GIC setting mechanism.

Material and methods
We performed in vitro tests using a high-viscosity GIC (Ketac TM Molar Easymix, 3M ESPE, Seefeld, Bavaria, Germany). Powdered dentin was obtained from human third molars extracted for orthodontic reasons, in accordance with the informed consent form signed by patients. This study was approved by the local Permanent Research Ethics Committee (resolution no. 681287 -CAAE no. 27637214.9.0000.0104).

Human dentin (D)
Teeth were sectioned in the mesial-distal direction with a water-cooled diamond disc (Diamond Wafering Blade, Diamond Series 15HC, Arbon Size ½ -12.7 cm, 10 cm x 0.3 mm, Buehler ® , Illinois, Chicago, USA), fixed to a cutting machine (IsoMet ® 1000 Precision Saw, Buehler ® , Illinois, Chicago, USA), and stored in a desiccator for 24 hours. We obtained powdered dentin using a spherical bur n o 4, driven at low speed, positioned in the central part of the tooth, 0.3 mm from the enamel and pulp chamber.

GIC powder + GIC liquid (P+L)
P+L samples were obtained according to the manufacturer's specifications, i.e., powder to liquid ratio of 2.4:1.
The samples were prepared in different concentrations of GIC and dentin to have a large variation between them. The amount of liquid was established to enable the mixing of the three components. samples.
Powdered portion was first mixed and then gradually incorporated into the liquid portion until a homogeneous mixture was achieved. 16 The mixture was stored at 37°C and 100% relative humidity for 1 hour to allow the setting. Subsequently, the samples were ground in a mortar with agate pestle to be prepared for FTIR and Raman spectroscopy.

Fourier transform infrared (FTIR) spectroscopy
The characterization of the molecular interactions between GIC and dentin was obtained with a VERTEX 70v FTIR research spectrometer (Buehler ® , Illinois, Chicago, USA). Each sample was diluted in potassium bromide (KBr) and compressed under 10 tons for 1 min, to produce pellets. We collected KBr-sample pellets spectra using an average of 128 scans, with a 4 cm -1 resolution in the spectral range between 400 cm -1 and 4000 cm -1 .

GIC setting dynamics
To investigate the setting dynamics, i.e., the time intervals required for the chemical bonds to stabilize, the P+L and the P+L+D mixture were used at 29% and 82% GIC concentrations, respectively.

Results
The results present the changes in the spectra that were more evident for concentrations of 29% and 50% of GIC, as shown by the orange and pink curves from data of the FTIR (Figure 1). The phosphate associated with the band around 1060 cm -1 almost disappeared in the samples with higher concentrations of dentin. The samples with 78%, 82%, and 92% of GIC showed a new band at 1060 cm -1 that increases proportionally to the GIC quantity until it resembles the GIC band displayed by the red curve.     We can note that changes in the spectra of the original materials were more evident in the specimens with higher amount of dentin because the characteristic band for it, i.e., the phosphate associated with the   setting. Therefore, such sites are required in the glass network so that the material can be vulnerable to acid attack and allow the formation of glass ionomer cement. 3,10,11,13,14 Hydrolysis of these bonds will release Ca 2+ and Al 3+ cations and form the orthosilicic acid that is being gelled until forming silica gel. The absorption bands at 3442 cm -1 (Si-OH) in the FTIR spectra confirm that silica gel was formed by acid degradation, namely, the Si-O-Al bonds of the glass network were broken by the polyacrylic acid with the consequent incorporation of a water molecule. These bonds suggest that the degree of gelation in the silicate network keeps growing during cement setting. 3,11,12,16 Note that the humidity of the environment was controlled to avoid the variation of this band, since there is free OH molecules bands in this same region.  bands at 1447 and 1409 cm -1 , respectively. The cement at the end of the reaction appears to be composed by several glass particles coated with a layer of silica gel that are present in a matrix composed by polyacrylate salts, responsible for setting of cement. 3,10,13,14,18 To strengthen the hypothesis abovementioned, the results of this study which demonstrate the occurrence of this type of interaction (P+L), are presented as follow: the band at 1409 cm -1 is a COO -Ca + bond (calcium polyacrylate), typically resulting from the GIC gelation reaction. As seen in Figures 4 and 5 Our results showed that the D+L interaction was more evident in the samples with 29%, 50%, and 65% of GIC, i.e., the greater the amount of dentin, the better the visualization of the bands. We can suggest that this interaction depends essentially on dentin; as GIC powder is added, bond interaction tends to decrease. Chemically, the GIC powder competes to interact with available dentin powder. Figure 5(B) illustrates this interaction, showing the decreasing character of the chemical bond dependent on the amount of dentin.
The shape of the band tends to resemble the D+L band that appears in this same location. The intensity of this bond is typical of dentin, i.e., of the organic compound. The area decreases with increased amounts of GIC powder, demonstrating that the lower the amount of dentin in the sample, the lower the intensity of this bond. The hypothesis of being a pure dentin bond was discarded because neither the powdered dentin nor the GIC presented any bands in this same region. Thus, one may suggest that this bond is only formed when the dentin and the GIC liquid are placed together.
Finally, the P+L+D interaction is a result of one of the main properties of GICs, that is, their adherence to tooth structures. Therefore, previous studies 20, 21 pointed out that the inclusion of GIC in the conditioned dentin cavity promotes an initial polar attraction with predominance of weak hydrogen bonds with the free carboxyl groups that appeared after GIC handling, which are responsible for the shiny appearance of the material. Consensus exists about the fact that bonding mechanism to enamel is practically a process resulting from ionic or polar forces. However, when the bonding mechanism to dentin is considered, these interactions Study of the chemical interaction between a high-viscosity glass ionomer cement and dentin 2018;26:e20170384 11/13 become more complex. 25 McLean and Wilson 8 (1977) suggested that bonding to the organic component would occur via hydrogen bonds or pendant metallic ions on the bridge between the carboxyl groups of the polyacid and the collagen molecules, which were observed in this study in the FTIR band at 1386 cm -1 and in the Raman band at 1319 cm -1 (CH 3 bond).
Interestingly, the results of this study revealed the great influence of the amount of dentin in the formation of new bands. In other words, for these interactions occur, a minimum amount of dentin is required to react with the GIC. The study showed that most of the bands were more intense in the samples with concentrations of 29% and 50% of GIC. This means that when GIC powder is added to the sample, the intensity of the interactions decreased as function of the decreased dentin. Therefore, it is suggested that the formation of these bonds, or for them to be at least visualized, a greater amount of available dentin is necessary, as shown in Figure 7 proportions. That is, by increasing the amount of GIC, these bands decreased to some extent, usually in the proportions of 65%-78% of GIC, and as more GIC was added, an inversion in the intensity of this band took place. This suggests that the appearance of new bands reaches a limit, as shown in Figure 7(B).
In this figure, the L+P+D interaction is evident for concentrations of 29% and 50% of GIC, i.e., the greater the amount of dentin, the greater their interaction. This behavior is consistent, since this band is associated with the P-O bond present in dentin.
When the amount of dentin is reduced, this bond also tends to decrease until it reaches the minimum concentration of 65% to 78% of GIC, considered a limit concentration for this bond. If the amount of dentin is diminished even further, this bonding interaction tends to P+L, the reason why the more GIC is added, the greater the interaction. These findings suggest that the GIC tends to set more rapidly when in contact with the dentin. This seems to be related to the increased presence of minerals available for the reaction, which consumes the carboxyl groups of the cement faster. Thus, in clinical practice, it is important to guide the patient not to chew on the GIC restoration for at least 150 minutes (approximately five times the value of τ, which represents approximately 99% of the variation of the measure, according to the behavior of an exponential function). During this time, the material is still vulnerable and in the process of forming chemical bonds to the dentin. Thus, dentists must pay special attention when removing the matrix and not conduct finishing and polishing procedures immediately after the restoration to avoid breaking the newly formed GIG-tooth chemical bonds.
Within the limitations of this study, only one HVGIC was used, and with materials of different compositions the results could be different. On the other hand, these results demonstrated that the FTIR and Raman spectroscopy were able to identify the bonds in the physical mixture of the GIC with powdered dentin, significantly contributing to the elucidation of the chemical bonding mechanism. As there are few studies that characterize the bonds in this process, further research is still required to elucidate this important mechanism for the dental practice. Thus, the results presented here may serve as reference for future research, as an effective method to evaluate the dynamics of the setting mechanism of GICs is presented.

Conclusion
New bands were formed resulting from the glass ionomer cement and dentin interaction. It was also observed a close relationship between the emergence of new bands and the amount of dentin available, i.e., the higher the amount of dentin, the greater the likelihood of new bands being formed and, consequently, the greater the chemical interaction between the GIC and the dentin. Regarding the clinical practice, it was found that the setting time of high-viscosity GIC was longer than the GIC-dentin mixture. Furthermore, it was also observed that the setting time of high-viscosity GIC was longer than the GIC-dentin mixture, and the time for such bonds to achieve 99% stability was about 150 min. Based on the foregoing, this study suggests a methodology capable of interpreting data related to the study of the adhesion of various materials.