Untitled 1http://dx.doi.org/10.20396/bjos.v17i0.8652928 Volume 17 2018 e18370 Original Article 1 Universidade Federal do Rio Grande do Norte (UFRN), Departamento de Dentística, Av Sen Salgado Filho 1787 – Lagoa Nova, Natal, RN, 59056-000, Brazil. 2 FAIPE School, Rua dos Girassóis 86 – Jardim Cuiabá, Cuiabá, MT, 78043-132, Brazil. Corresponding author: Rodolfo Xavier de Sousa-Lima Av Sen Salgado Filho 1787, Natal, RN, 59056-000, Brazil. Tel./fax: +55 84 3215 4101. E-mail address: rodolfo_xsl@hotmail.com Received: March 06, 2018 Accepted: July 09, 2018 Polymerization capability of simplified dental adhesives with camphorquinone, phenyl- propanedione and bis-alkyl phosphine photoinitiators Boniek Castillo Dutra Borges1, Rodolfo Xavier de Sousa-Lima1, Géssica Dandara Medeiros de Souza1, Ana Carla Bezerra de Carvalho Justo-Fernandes1, Letícia Virgínia de Freitas Chaves1, Eduardo José Carvalho Souza-Junior2, Isauremi Vieira de Assunção1 Aim: this study aimed to evaluate the degree of conversion (DC) exhibited by novel formulations of dental adhesive systems including camphorquinone (CQ), phenyl-propanedione (PPD), and bis-alkyl phosphine oxide (BAPO) when cured by mono- or polywave light emitting diodes (LEDs). Methods: an adhesive model was formulated by mixing hydroxyethyl methacrylate (HEMA, 40 wt%) and bisphenol A glycidyl dimethacrylate (BisGMA, 60 wt%) in ethanol (30 wt%). Five materials were then formulated by adding the following photoinitiators: CQ (1 mol%), CQ/PPD (0.5/0.5 mol%), CQ/BAPO (0.5/0.5 mol%), PPD (1 mol%), and BAPO (1 mol%). The DC for each material was measured with Fourier transform infrared spectroscopy. Analysis of variance and Tukey’s post-hoc test were used to analyze the data (p < 0.05). Results: Except for CQ, the photoinitiators provided a significantly higher DC in the adhesive systems following photoactivation with a polywave LED. Conclusion: The use of alternative photoinitiators and a polywave LED improved the DC of the adhesive systems examined. Keywords: Spectroscopy, fourier transform infrared. Dental materials. Dental cements. 2 Borges et al. Introduction Dental adhesives are materials composed of monomers with both hydrophilic and hydrophobic groups, photoinitiators, inhibitors or stabilizers, solvent and, in some cases, inorganic fillers1. The classical model of dental adhesives is available in three application steps (acid etching, priming and bonding). Over time, the need to reduce the number of clinical steps during application of dental adhesives required the emer- gence of simplified materials to reduce the chair time. In this way, adhesives systems have gone through several changes in recent years, with the creation of new mono- mers and photoinitiating molecules, in an attempt to simplify bonding procedures without compromising adhesion to tooth substrates2. In general, the adhesive performance depends on the degree of conversion (DC), so that a high DC is fundamental to improving resistance of material degradation under in vivo clinical conditions. Low DC of dental adhesives is associated with high water sorption/solubility, as well as low bond strength values, low mechanical prop- erties, increased permeability, and even the occurrence of phase separation3. This conversion could be affected by many factors, including the photoinitiator systems and light wavelength of the curing unit used4. Thus, the development of simplified adhesive systems capable to show increased DC is detrimental. The most contemporary adhesive systems are activated by light within the blue band of the spectrum (400–500 nm) and they use camphorquinone (CQ) as a photoinitia- tor5. CQ is a solid yellow compound with an unbleachable chromophore group that can absorb light in the spectral range of approximately 400–500 nm, with a peak near 470 nm6,7. However, the yellow hue characteristic of CQ compromises its aesthetic performance and photoinitiators eventually degrade over time5,8. Therefore, alterna- tive photoinitiators such as phenyl propanodione (PPD), and bis-alkyl phosphine oxide (BAPO) have been investigated in an attempt to replace CQ or decrease the amount of CQ into dental materials without compromising the DC6,9,10. Most of the alternative photoinitiators that have been studied have an absorption peak in the ultraviolet region which extends slightly into the visible light spectrum (380–420 nm)5,11. Both the spectrum emitted by a light source and the absorption capacity of a photoinitiator have an effect on the polymerization process of com- posites, thereby influencing their properties5,11,12. As a result, cure efficiency can be compromised when narrowband light-emitting diodes (LEDs), such as conventional monowave LEDs are used, since these LEDs do not have light emission in the violet wavelength range5,11,13. Thus, the ability of conventional LEDs to activate photoiniti- ators that respond to ultraviolet light is limited. However, polywave LEDs emit dual peaks, with one additional peak being near 405 nm14-16, and this allows these LEDs to activate photoinitiators such as PPD and BAPO. In this way, the aim of this study was to evaluate the degree of conversion (DC) of novel formulations of dental adhesive systems including CQ, PPD, and BAPO when cured by mono- or polywave LEDs in order to test the hypothesis that the use of alternative photoinitiators and photoactivation with a polywave LED can lead to an increased DC. 3 Borges et al. Materials and methods Experimental design The response variable evaluated in this in vitro study was DC. Five photoinitiators (CQ, CQ/PPD, CQ/BAPO, PPD, and BAPO) (Table 1) and two types of LEDs (monowave and polywave) (Table 2) were tested. Formulation of the experimental adhesive systems Bisphenol A glycidyl dimethacrylate (BisGMA) and hydroxyethyl methacrylate (HEMA) (60:40 wt%) (Sigma-Aldrich, St. Louis, MO, USA) were mixed with ethanol (30 wt%)17. Then, five different materials were generated with the addition of these various pho- toinitiators: CQ (1 mol%), CQ/PPD (0.5/0.5 mol%), CQ/BAPO (0.5/0.5 mol%), PPD (1 mol%), and BAPO (1 mol%). Ethyl 4-(dimethylamino)benzoate (EDMAB) (Sigma-Al- drich) (1 mol%) was added to all of the prepared formulations to serve as a co-initiator. DC evaluation DC was evaluated with a Fourier transform infrared/attenuated total reflectance instrument (FTIR/ATR) (Spectrum 100, PerkinElmer, Shelton, CT, USA) at 24 ºC under 64% relative humidity. One drop of each adhesive system (n = 10 per photoinitiator and LED) was applied to the ATR surface and the solvent was evaporated for 10 s. Then, a thin glass plate (0.5 mm thick) was placed on the material and it was photo- activated for 10 s using a LED. The irradiance of the LEDs were measured by using a computer-controlled spectrometer (USB2000, Ocean Optics, Dunedin, USA) and was integrated using Origin 6.0 software (OriginLab, Northampton, USA). The absorption spectra of both the nonpolymerized and polymerized adhesive sys- tems prepared were obtained between 4000 and 650 cm-1 with 32 scans at 4 cm-1. Intensities of the aliphatic carbon-to-carbon double-bond absorbance peak (located Table 1. Characteristics of photoinitiators used in this study. Photoinitiator Absorption spectrum range (nm) Absorption intensity peak (nm) [7] Molar extinction coeficiente (L/mol cm) [7] CQ* 400 – 500 [7] 470 28±2 PPD** 350 – 480 [17] 398 150±10 BAPO*** 365 – 416 [7] 370 300±10 *Camphorquinone; **Phenyl-Propanedione; ***Bis-Alkyl Phosphine Oxide Table 2. Technical details of light emitting diodes used in this study according to the manufacturers. Comercial name/ Manufacturer Classification Spectrum range Intensity peaks Irradiance Bluephase G2, Ivoclar Vivadent, Schaan, Liechtenstein Polywave 385 - 515 nm (380 - 420; 420 - 490) 405 nm 460 nm 1200mW/cm2 Radii Cal, SDI, Victoria, Australia. Monowave 440 - 480 nm 460 nm 1313 mW/cm2 4 Borges et al. at 1638 cm-1) and the aromatic component (located at 1608 cm-1; reference peak) were recorded. DC (%) was calculated using the following equation18: R polymerized R nonpolymerized], DC (%) = 100 x [1 - ( )], where R represents the ratio between the absorbance peaks at 1638 cm-1 and 1608 cm-1. Statistical analysis Two-way analysis of variance (ANOVA) and Tukey’s post-hoc test were used (p < 0.05). Results and Discussion There were statistically significant differences in the interaction between photoinitia- tors x LEDs (p < 0.01). Table 3 shows the intergroup comparisons. Only the CQ adhe- sive system achieved a similar mean DC as the samples were photoactivated by Radii Cal and Bluephase G2, so that Bluephase G2 provided a higher mean DC than Radii Cal to the other adhesive systems. Bluephase G2 provided similar mean DC between adhesive systems. Radii Cal provided the highest DC to the CQ adhesive system, while the lowest DC was observed for the CQ/BAPO and BAPO adhesive systems. Thus, the hypothesis that the use of alternative photoinitiators that and photoactivation with a polywave LED can lead to an increased DC was accepted. The DC of an adhesive system is influenced by the activity of photoinitiators and the wavelength and intensity of the curing light that is applied19. In this study, only the adhe- sive systems that included CQ exhibited a similar DC between the samples that were photoactivated by Radii Cal and Bluephase G2. Conventional monowave LEDs, such as Radii Cal, have an emission band in the visible region which results in the emission of a single peak in a narrow spectral band20. In contrast, Bluephase G2 is a dual peak LED that provides additional light with a spectrum that nearly includes 405 nm14,15. CQ is activated within the visible light spectrum and has a peak absorbance near 470 nm6,7. Based on the data collected, it appears that both monowave and polywave LEDs are able to excite CQ. This corroborates with that found by Segreto et al.21 (2016) who tested different photoinitiator units and photoinitiator systems and concluded that both types of light (mono and polywave) are capable of activating CQ and PPD. Table 3. Degree of conversion (%) means (standard-deviation) of dental adhesive systems according to the photoinitiator system and the curing light. Photoinitiator system Light emitting diode Radii Cal Bluephase G2 CQ 77.8 (6.8) aA 77.3 (14.1) aA CQ/PPD 48.8 (7.4) bB 71.6 (7.7) aA CQ/BAPO 31.5 (11.5) cB 74.1 (6.9) aA PPD 47.2 (4.0) bB 74.2 (6.4) aA BAPO 27.6 (3.9) cB 81.6 (6.5) aA Means followed by different capital letters indicate statistically significant differences between curing lights for the same photoinitiator (p<0.05). Means followed by different lower case letters indicate statistically significant differences among photoinitiator systems for the same curing light (p<0.05). 5 Borges et al. Unlike CQ, the adhesive systems formulated with CQ/PPD, CQ/BAPO, PPD, and BAPO exhibited a higher DC when they were photoactivated by Bluephase G2 than with Radii Cal. Alternative photoinitiators such as PPD and BAPO have an absorption peak in the ultraviolet region (100–400 nm)22, specifically at 398 nm and 370 nm, respectively5. Thus, photoactivation with a polywave LED could promote an increased excitation of these photoinitiators, thereby increasing the generation of free radicals that initiate the polymerization reaction. However, the monowave LED, Radii Cal, provided a higher DC for the PPD adhesive system than the BAPO system. Thus, it is likely that PPD can also absorb light in the visible range of the light spectrum20, thereby accounting for the greater excitation of PPD by Radii Cal compared with BAPO. The results indicated that PPD was a viable alternative in the formulation of experimental adhesives, observing that it presents greater reactivity independent of the type of photoinitiator unit21. Despite the fact that BAPO is a Norirish Type I photoinitiator which generates free radi- cals via a photocleavage process that does not require a co-initiator23, a tertiary amine EDMAB was added to the BAPO-containing materials in the present study. EDMAB is capable of reacting with the oxygen that is dissolved in the monomer, thereby reducing an oxygen-mediated inhibition of polymerization23. Since CQ employs a mechanism that predominantly involves abstraction of a proton from the amine hydrogen, and PPD can undergo photocleavage and proton abstraction of the amine24, EDMAB was included with all of the photoinitiators tested so the same conditions would be compared. The findings obtained in this study are of great relevance, since DC is the main phys- ical property related to other biological, physical and mechanical properties such as sorption and solubility, long-term stability of the hybrid layer25, liberation of residual monomers and preservation of the complex dentin pulp4, bond strength to dentin9, elastic modulus and flexural strength of dental adhesives26. Confirming this state- ment, Schneider et al.27 (2009) evaluated the effect of the photoinitiator type on the maximum rate of polymerization (R(p)(max)), stress development (final stress and maximum rate, R(stress)(max)), DC and cross-link density (CLD) of materials con- taining CQ, PPD or CQ/PPD and conclude that CQ/PPD reduced the R(p)(max) and R(stress)(max) without a reduction in DC and CLD. In this way, the use of alterna- tive photoinitiator systems could be a promising way to reduce the stress developed during the composite’s polymerization without affecting the final properties. Thus, to be able to show that the insertion of alternative photoinitiators in conjunction with third generation LEDs are able to increase the degree of conversion is a posi- tive and relevant result for adhesive dentistry. The literature states, therefore, that the combination of alternative photoinitiators with the traditional camphorquinone/amine system improved the color stability of the model resin composites and maintaining their mechanical properties28,29. Despite the important finding obtained in this study regarding DC, further physical, mechan- ical and biological properties should be investigated to strength the effect of alternative photoinitiators on the performance of dental adhesives. Indeed, since acidic monomers such as methacryloyloxydecyl hydrogen phosphate (MDP), and glycerol dimethacrylate phosphate (GDMA-P) have been included in dental adhesive systems with Bis-GMA and/ or HEMA30,31 further studies should be conducted to evaluate the DC exhibited by other formulations including acidic monomers and alternative photoinitiators. 6 Borges et al. In conclusion, the use of alternative photoinitiators and polywave LED was found to improve the DC and decrease the yellowing effect of the experimental dental adhesive systems tested. Acknowledgements This study was not supported by any funding agency. All expenses were made through own initiative. References 1. Sofan E, Sofan A, Palaia G, Tenore G, Romeo U, Migliau G. Classification review of dental adhesive systems: from the IV generation to the universal type. Ann Stomatol (Roma). 2017 Jul 3;8(1):1-17. doi: 10.11138/ads/2017.8.1.001. 2. Pena CE, Rodrigues JA, Ely C, Giannini M, Reis AF. 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