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Effects of diphenyliodonium salt addition on the adhesive and mechanical properties of an experimental adhesive
journal of dentistry 41 (2013) 653–658
Available online at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Effects of diphenyliodonium salt addition on the adhesive and mechanical properties of an experimental adhesive Alessandro D. Loguercio a,*, Rodrigo Stanislawczuk a, Friedrich G. Mittelstadt b, Marcia M. Meier c, Alessandra Reis a a
Department of Restorative Dentistry, State University of Ponta Grossa, Ponta Grossa, Parana´, Brazil Department of Research & Development, FGM Prod. Odont. Ltda, Joinville, Santa Catarina, Brazil c Department of Chemistry, State University of Santa Catarina, Joinville, Santa Catarina, Brazil b
article info
abstract
Article history:
Objectives: The aim of this study was to assess the microtensile bond strength (mTBS),
Received 12 January 2013
nanoleakage (NL), nano-hardness (NH) and Young’s modulus (YM) of resin–dentine bonding
Received in revised form
components formed by an experimental adhesive system with or without inclusion of
19 April 2013
diphenyliodonium salt (DPIH) in the camphorquinone–amine (CQ) system.
Accepted 20 April 2013
Methods: On 12 human molars, a flat superficial dentine surface was exposed by wet abrasion. A model simplified adhesive system was formulated (40 wt.% UDMA/MDP, 30 wt.% HEMA and 30 wt.% ethanol). Two initiator systems were investigated: 0.5 mol%
Keywords:
CQ + 1.0 mol% EDMAB and 0.5 mol% CQ + 1.0 mol% EDMAB + 0.2 mol% DPIH. Each adhesive
Adhesive
was applied and light-cured (10 s; 600 mW/cm2). Composite build-ups were constructed
Hybrid layer
incrementally and resin–dentine specimens (0.8 mm2) were prepared. For NL, 3 bonded
Diphenyliodonium salt
sticks from each tooth were coated with nail varnish, placed in the silver nitrate, polished
Microtensile bond strength
down with SiC papers and analysed by EDX-SEM. NH and YM were performed on the hybrid
Nanoleakage
layer in 2 bonded sticks from each teeth. The remaining bonded sticks were tested on mTBS
Adhesive properties
(0.5 mm/min). The data from each test were submitted to a Student t-test (a = 0.05). Results: No significant difference was found for mTBS between groups ( p > 0.05). Significant lower NL and higher NH and YM were found in the hybrid layer and adhesive layer produced with the iodinium salt-containing adhesive ( p < 0.05). Conclusions: The inclusion of the DPIH to the traditional CQ is a good strategy to improve the adhesive and mechanical properties of a simplified etch-and-rinse adhesive system. # 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Simplified etch-and-rinse adhesives are build-up by joining the components of the primer and the bonding resin into a single solution. The hydrophilic features of these simplified adhesives were increased as the primer/bond solution should be compatible to the intrinsically moist, acid-etched dentine.1
However, several questions have been raised regarding the wet bonding technique. Water and solvents need to be completely removed before adhesive light polymerisation and this is not achieved even when using solvent evaporation 10–12 times longer than those recommended by the manufacturers.2 Inhibition of HEMA polymerisation was detected with intrinsic water at concentrations > 5 vol.%, even with a 10-fold increase in the photo-initiators.3
* Corresponding author at: Universidade Estadual de Ponta Grossa – Mestrado em Odontologia, Rua Carlos Cavalcanti, 4748, Bloco M, Sala 64A – Uvaranas, Ponta Grossa 84030-900, Parana´, Brazil. Tel.: +55 42 3220 3741; fax: +55 42 3220 3741. E-mail address: [email protected] (A.D. Loguercio). 0300-5712/$ – see front matter # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jdent.2013.04.009
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journal of dentistry 41 (2013) 653–658
Entrapped water also precludes the formation of a high cross-linked polymer within the collagen fibre network4 probably due to phase separation of hydrophobic and hydrophilic moieties,5,6 rendering the polymer weaker than the one formed in a free-water environment. This subpolymerised polymer is more prone to the plasticising effects of water over time and makes the adhesive layer more permeable to water from the oral environment and the underlying bonded dentine.7 Incompatibility issues8 and faster degradation of resin–dentine bonds are the consequences, comparatively to their three-step version.9,10 In an attempt to optimise the bonding to dentine using simplified etch-and-rinse adhesives, various clinical procedures have been proposed11–13 and most of these approaches favour solvent evaporation and increase the degree of conversion in order to produce a strong polymer within the hybrid layer. Despite the immediate benefits of these several clinical approaches, reductions of bond strength values over time are still a problem.14–17 An important consideration about the formulation of the current adhesive systems is that they usually employ hydrophobic initiators (e.g., camphorquinone [CQ]), and co-initiators (e.g., ethyl-4-(dimethylamino)benzoate [EDMAB]).18 Therefore they are likely to be distributed preferentially to the hydrophobic domains, jeopardising the overall polymerisation of the adhesive.19 Diphenyliodonium salt is ionic in nature and as such, it may increase the compatibility between amphiphilic monomers (i.e., having both hydrophilic and hydrophobic characteristics) and initiators, especially in the presence of water.19 This salt is usually associated to CQ–amine system forming a three component photoinitiator system and it acts as an electron scavenger from the dye radical.20 Earlier studies showed that the addition of diphenyliodonium salt in the camphorquinone-amine system accelerate the polymerisation rate and improve the final degree of conversion, transition glass temperature, and crosslink density as well as some other mechanical properties of the adhesive, mainly when the polymerisation reaction takes place in a moist environment.19,21,22 Despite these positive findings, to extent to our knowledge, no study has so far evaluated the association of diphenyliodonium (DPIH) salt to CQ–amine system on the immediate resin–dentine bond strength, nanoleakage and mechanical properties of the hybrid and adhesive layer, which was the aim of the present investigation. The following null hypotheses were tested in this study: (1) there will be no differences in the nanohardness an Young’s modulus of the hybrid layer and adhesive layer between the DPIH-free and DPIH-containing adhesives and (2) there will be no differences in the resin– dentine bond strength and nanoleakage between the DPIHfree and DPIH-containing adhesive.
2.
Methods and materials
2.1.
Tooth selection and preparation
The molars were disinfected in 0.5% chloramine, stored in distilled water and used within six months of extraction. A flat and superficial dentine surface was exposed on each tooth after wet grinding the occlusal enamel on #180-grit SiC paper. The enamel-free, exposed dentine surfaces were further polished on wet #600-grit silicon carbide paper for 60 s to standardise the smear layer.
2.2.
A model simplified adhesive system was formulated through intensive mixing of 40 wt.% UDMA (urethane dimethacrylate) and MDP (10-methacryloyloxydecyl dihydrogenphosphate), 30 wt.% HEMA (2-hydroxyethyl methacrylate) and 30 wt.% ethanol. Two initiator systems were investigated: 0.5 mol% of CQ (camphorquinone) + 1 mol% of EDMAB (ethyl-4-(dimethylamino) benzoate) and 0.5 mol% of CQ + 1.0 mol% EDMAB + 0.2 mol% DPIH (diphenyliodonium hexafluorphosphate). All initiators and co-initiators were supplied by Aldrich Chemical Co (Milwaukee, WI, USA) and used without further processing. Before applying the experimental adhesive system, the homogeneity of the solution was evaluated according to Spencer and Wang.5 After mixing all components, 16 ml of each adhesive was transferred to a small glass and mixed with 16 ml of water. The mixture was stirred for 10–15 s. Immediately after, each mixture was observed under 100 magnification (Eclipse E200, Nikon, USA). Loss in clarity was interpreted as evidence of phase separation. This experiment was done in quadruplicate. The acid etching (CondacAC 37%, FGM Prod. Odont., Ltda, Joinville, SC, Brazil) was applied for 15 s, following the surfaces were rinsed with distilled water for 15 s and excess was removed using oil-free compressed air, leaving the dentine slightly wet. Two coats of adhesives were applied onto the dentine with rubbing action for 20 s. After each coat, an air stream was applied for 10 s at a distance of 20 cm. The time lapse between the starting the adhesive application and light curing (VIP, Bisco, Schaumburg, USA at 600 mW/cm2) was approximately 40 s. The light curing was performed for 10 s. Resin composite build-ups (Opallis, FGM) were placed on the bonded surfaces (3 increments, 2 mm each), which were individually light activated for 40 s each. All bonding procedures were carried out by a single operator at 24 8C room temperature. The bonded teeth were stored in distilled water at 37 8C for 24 h. All teeth were sectioned in both ‘‘x’’ and ‘‘y’’ directions across the bonded interface with a diamond blade in an Isomet 1000 saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) at 400 rpm to obtain resin–dentine beams with a cross-sectional area of approximately 0.8 mm2. For each tooth, a total of approximately 25–30 resin–dentine bonded beams could be obtained. Five beams from each tooth were not tested in tensile force and left for nanoleakage evaluation (n = 3 beams) and nano-hardness test (n = 2 beams). The remaining beams were used for microtensile bond strength evaluation.
2.3. Twelve extracted, caries-free human third molars were used. The teeth were collected after obtaining the patient’s informed consent. The Institutional Review Board approved this study.
Restorative procedure
Microtensile bond strength test (mTBS)
Each resin–dentine beam was attached to the Geraldeli’s device (Odeme Biotechnology, Joac¸aba, SC, Brazil) with
journal of dentistry 41 (2013) 653–658
cyanoacrylate resin (Super Bonder Gel, Loctite, Sa˜o Paulo, SP, Brazil) and subjected to a tensile force in a universal testing machine (Kratos Dinamometros, Sa˜o Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min. The failure modes were evaluated at 40 (Eclipse E200, Nikon, USA) and classified as cohesive (failure exclusively within the dentine [CD] or resin [CR]), or adhesive/mixed (failure at resin/dentine interface and failure at resin/dentine interface that included cohesive failure of the neighbouring substrates [A/M]).
2.4.
Nanoleakage evaluation (NL)
All resin–dentine bonded sticks were immersed for 24 h in a 50 wt.% ammoniacal AgNO3 solution, rinsed and placed in photo-developed for 8 h under fluorescent light to reduce the diamine silver ions into metallic silver grains.23 The silverimpregnated sticks were mounted on aluminium stubs and polished with a 1000-, 1200-, 1500-, 2000- and 2500-grit SiC paper and 1 and 0.5 mm diamond paste (Buehler Ltd., Lake Bluff, IL, USA) using a polish cloth. They were ultrasonically cleaned with distilled water for 30 min (Dabi Atlante – 3L, Ribeira˜o Preto, SP, Brazil), desiccated in colloidal silica for 24 h and gold-sputtered (Sputter Coater IC 50; Shimadzu, Tokyo, Japan). Resin–dentine interfaces were analysed in a scanning electron microscope operated in the backscattered electron mode (SSX-500, Shimadzu, Tokyo, Japan) with an accelerating voltage of 12 kV. Three pictures were taken of each specimen. The first picture was taken in the centre of the stick. The other two pictures were taken 0.3 mm to the left and right of the first one. As three resin–dentine beams per tooth were evaluated and a total of 6 teeth were used for each experimental condition, a total of 54 images were evaluated per group. They were all taken by a technician who was blinded to the experimental conditions under evaluation. The relative percentage of nanoleakage within the adhesive and hybrid layer areas was measured in all pictures using the UTHSCSA ImageTool 3.0 software (Department of Dental Diagnostic Science at The University of Texas Health Science Centre, San Antonio, TX, USA).
2.5.
Nano-hardness test
The resin–dentine beams were individually embedded in a self-cure polyester resin (Milflex, Milflex Indu´strias Quı´micas, Sa˜o Bernardo do Campo, SP, Brazil) and after 24 h the specimens were polished and cleaned as described earlier for the silver uptake test. For the nano-indentation measurements, the computer-controlled Nano Indenter XP (MTS, Oak Ridge, TN, USA) was employed mounted with a triangular pyramidal diamond indenter Berkovich. By means of the computer-controlled X–Y table, the dried specimen was transferred to the indenter. An accurate calibration of the distance between the microscope and the indenter was run before testing to ensure a precise transfer of the preprogrammed positions to the indenter. At the start of measurement two groups of nine indentation positions equally spaced each other were programmed by a remote video control (connected to the light microscope attached to the nano-indenter device) for each region. In order
655
to obtain precise measurements, the interval of each indentation was twice the size of the indentation in each region with the aim to avoid the corruption of the abutment.24 In the central area of the composite resin and adhesive layer, the surface approach rate of the nano-indenter was set to be 10 nm/s and the duration of the loading and unloading indentation was 5 s each. The pre-programmed distance among indentations for dentine, adhesive layer and composite resin was 20, 10 and 20 mm respectively with a load of 10 g. At the hybrid layer the surface approach rate was the same, however the load employed was 0.2 g and indentations were programmed to be performed at a distance of 3 mm. The reduction of the load was required to reduce the indentation size so that the indentations could be positioned entirely within the area of the hybrid layer. Epoxy resin replicas of all specimens were gold-sputtered and analysed under SEM according to previously described. It was necessary to verify the indentation geometry and the accurate positioning of the pre-programmed indentations. Those that were found out of the specified areas were excluded from the sample. The nano-hardness and Young’s modulus of each area were computed following the method of Oliver and Pharr.25
2.6.
Statistical analysis
The mean values of mTBS and nanoleakage from bonded beams originating from the same tooth were averaged for statistical purposes. The specimens with cohesive and premature failures were not included into the tooth mean. The same procedure was performed for the nano-hardness and Young’s modulus of the hybrid and adhesive layers. The data of each test were statistically analysed by a Student t-test (a = 0.05).
3.
Results
No signs of phase separation were observed in the DPIH-free or DPIH containing experimental adhesive (data not shown).
3.1. (%)
Microtensile bond strength (MPa) and nanoleakage
No significant difference was detected in the resin–dentine mTBS values for both adhesive formulations (Table 1; p = 0.62). The fracture pattern of the resin–dentine beams was very similar between the adhesive formulations. However, the use of DPIH-containing adhesive significantly decreased the amount of nanoleakage in the hybrid layer (Table 1; p = 0.001). Representative back-scattering scanning electron microscopy images captured at the resin–dentine interfaces are depicted in Fig. 1.
3.2.
Nano-hardness test
The means values and standard deviations for nano-hardness (GPa) in the composite resin and mineralised dentine were 1.14 0.1 and 0.82 0.2, respectively. For Young’s modulus, the mean values and the respective standard deviations (GPa)
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journal of dentistry 41 (2013) 653–658
Table 1 – Means (standard deviation) of the microtensile bond strength (n = 6 teeth), fracture pattern (number of beams) and nanoleakage (n = 6 teeth) for the experimental adhesives.
Microtensile bond strength (MPa)b Fracture pattern (number of beams)c Nanoleakage (%)
DPIH-freea
DPIH-containing
48.3 [4.2] A
53.2 [3.2] A
80/24/12/08
92/26/10/12
15.1 [2.7] a
11.2 [2.2] b
a
DPIH – diphenyliodonium hexafluorphosphate. Similar uppercase (microtensile bond strength) and lowercase (nanoleakage) letters indicate means statistically similar (Student-t test; p > 0.05). c Adhesive/mixed/cohesive in dentine and resin/premature failures. b
were 16.71 0.88 and 20.32 2.13 for the composite resin and mineralised dentine, respectively. The use of DPIH-containing adhesive significantly increased the nanohardness and Young’s modulus of the hybrid layer (Table 2; p < 0.001) and adhesive layer (Table 2; p < 0.002).
4.
Discussion
The most commonly employed photoinitiators are those in which radicals are formed in a binary-system containing an excited state of a dye, such as camphorquinone (CQ), and a coinitiator that acts as an electron donor.26,27 The photoinitiator can absorb light directly and the co-initiator interacts with the activated photoinitiator to generate a reactive free radical for polymerisation initiation.28 In this binary-system CQ requires a tertiary amine reducing agent, usually ethyl-4(dimethylamino) benzoate (EDMAB), for efficient polymerisation to occur.20 As described before,1 the simplified adhesive contains very hydrophilic molecules, like primers, and relatively hydrophobic molecules, forming the bond. When simplified etch-andrinse adhesives are used over moist dentine water solubilise,
in a certain limit, in the adhesive affecting the solubility of the different components. However, differences in the hydrophilicity of the organic matrix and the CQ–amine system in the adhesive formulation may lead to uneven distribution of the latter in hydrophobic domains, producing phase-separation and jeopardising the overall polymerisation of the simplified adhesive, due to the lower concentration of the CQ–amine system in the hydrophilic domain.5,19,22 When simplified etch-and-rinse adhesives are used, not only the residual water from the moist dentine impair the polymerisation reaction of the adhesive system,3,19 but also the amount of residual solvent2,29 presented in the adhesive formulation. The amount of retained solvent in experimental adhesives was shown to be three to four times higher when water is added to the adhesive formulation.2 In a way to minimise these problems, studies attempted to investigate a three-component photoinitiator system in which a diphenyliodonium salt is added to the traditional twocomponent CQ–amine system. The mechanism of action proposed for the addition of the diphenyliodonium salt reports that this co-initiator salt might act as an electron scavenger from the dye radical.20,30 In this three-component photoinitiator system, the resulting amine-based radical is active for polymerisation, but the dye-based radical is a terminating radical. The third component, the diphenyliodonium salt, which is an electron acceptor, serves both to regenerate the photosensitizer molecules (e.g., CQ) by replacing inactive terminating radicals with active phenyl initiating radicals, and also to generate additional active phenyl radicals.20,30 Other authors suggested that an exciplex between CQ* and diphenyl iodinium salt is initially formed, which reacts further with the amine, giving radical amine species that initiates the polymerisation and regenerates the diphenyl iodinium salt, forming a new exciplex with a new photo-activated CQ, being this an extra source of radicals for initiating polymerisation.31 The addition of diphenyliodonium salt was shown to increase the velocity of reaction and the percentage of monomer conversion significantly. While the maximum rate
Fig. 1 – Backscattering scanning electron micrographs of the adhesive interfaces of the experimental groups. More silver nitrate deposition (white hands) could be seen in the adhesive interface produced in the group DPIH-free (B). (Co = composite resin; AL = adhesive layer; HL = hybrid layer; De = dentine).
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journal of dentistry 41 (2013) 653–658
Table 2 – Means (standard deviation) of nanohardness (n = 6 teeth) and Young modulus (n = 6 teeth) evaluated in the hybrid and adhesive layer for the experimental adhesives. Hybrid layer a
Nanohardness (GPa)b Young modulus (GPa)
Adhesive layer
DPIH-free
DPIH-containing
DPIH-free
DPIH-containing
0.12 [0.01] A 2.72 [0.3] a
0.16 [0.03] B 3.21 [0.2] b
0.22 [0.02] A 4.72 [0.4] a
0.27 [0.05] B 5.38 [0.3] b
a
DPIH – diphenyliodonium hexafluorphosphate. Similar uppercase (nanohardness) and lowercase (young modulus) letters indicate means statistically similar for each substrate (hybrid layer or adhesive layer) (Student-t test, p > 0.05). b
of polymerisation occurred after 1.65%/s and final percentage of conversion was 44.3% for the binary CQ–amine system, the use of diphenyliodonium salt reached 2.73%/s and degree of conversion was 54.4%.31 When in contact with water or solvents, this difference was much more pronounced favouring the adhesive formulation with diphenyliodonium salt.19,21,22,29 It is likely that the ionic nature of the diphenyliodonium salt can increase the compatibility between hydrophilic and hydrophobic monomers and initiators, especially in the presence of water, leading to higher degree of conversion and high mechanical properties of the adhesive layer and helps in the interaction between amphiphilic monomers (hydrophobic and hydrophilic characteristics) improving the resin infiltration inside the collagen mesh.19,21,22 The results of the present investigation demonstrated that adhesive and hybrid layer produced with the DPIH-containing adhesive showed higher nano-hardness and Young’s modulus and the first null hypothesis of this study was rejected. As previously described by Spencer and Wang,5 when phase separation occurs the adhesive layer becomes opaque due to difference in the index of refraction of the different phases. This makes the light penetration difficult, which in turn affects the light curing procedure and the quality of the produced polymer. This is not observed when DPIH is added to the formulation, since it improves the miscibility of the components keeping the adhesive layer translucent (data not shown). Although the benefits of DPIH were not seen in the bond strength evaluation, reduced nanoleakage was observed in the interfaces produced with the iodonium-containing adhesive, leading us to partially reject the second null hypothesis. As nanoleakage evaluation provides good spatial resolution of submicron defects in resin infiltration or areas of inadequate polymerisation,7,32 one may speculate that this is an indirect finding of even polymerisation throughout the adhesive interface. There was found less nanoleakage in both the hybrid layer (classic nanoleakage33) and the adhesive layer (water trees7) in the DPIH-containing adhesive. According to Tay et al.,7 water trees in dentine adhesives, together with nanoleakage within the hybrid layers, represent water-rich interfacial regions from which the leaching of hydrophilic resin components may readily occur, indicating signs of potential degradation of the interfacial bond in the future. The regions of silver uptake probably represent areas of suboptimal conversion within the polymer matrix due to incomplete solvent removal,34 and may permit higher diffusional water fluxes within the hybrid layers that could accelerate water sorption and the extraction of residual monomers or loosely bounded oligomers.7,11 Based on this,
one might expect reduced degradation of the resin–dentine bonds for the diphenyliodonium salt-containing adhesive and future studies should be conducted to confirm this hypothesis. It is worth mentioning that not only diphenyliodonium salt has been evaluated as an alternative to the traditional hydrophobic CQ–amine system. Other photoinitiatiors as TPO and QTX has been added to adhesive systems.19,21,35,36 However, only mechanical properties and kinetics of polymerisation reaction was evaluated in these studies.19,21,35–37 Further studies should be conducted in order to evaluate the impact of such photoinitiators on the adhesive properties as well as mechanical properties of the adhesive and hybrid layers. In conclusion, the addition of the hydrophilic co-initiator diphenyliodonium hexafluorphosphate improved the mechanical properties of the hybrid layer and adhesive layer produced by the experimental etch-and-rinse adhesive and decreased the amount of nanoleakage in the adhesive interface.
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Available online at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/jden
Effects of diphenyliodonium salt addition on the adhesive and mechanical properties of an experimental adhesive Alessandro D. Loguercio a,*, Rodrigo Stanislawczuk a, Friedrich G. Mittelstadt b, Marcia M. Meier c, Alessandra Reis a a
Department of Restorative Dentistry, State University of Ponta Grossa, Ponta Grossa, Parana´, Brazil Department of Research & Development, FGM Prod. Odont. Ltda, Joinville, Santa Catarina, Brazil c Department of Chemistry, State University of Santa Catarina, Joinville, Santa Catarina, Brazil b
article info
abstract
Article history:
Objectives: The aim of this study was to assess the microtensile bond strength (mTBS),
Received 12 January 2013
nanoleakage (NL), nano-hardness (NH) and Young’s modulus (YM) of resin–dentine bonding
Received in revised form
components formed by an experimental adhesive system with or without inclusion of
19 April 2013
diphenyliodonium salt (DPIH) in the camphorquinone–amine (CQ) system.
Accepted 20 April 2013
Methods: On 12 human molars, a flat superficial dentine surface was exposed by wet abrasion. A model simplified adhesive system was formulated (40 wt.% UDMA/MDP, 30 wt.% HEMA and 30 wt.% ethanol). Two initiator systems were investigated: 0.5 mol%
Keywords:
CQ + 1.0 mol% EDMAB and 0.5 mol% CQ + 1.0 mol% EDMAB + 0.2 mol% DPIH. Each adhesive
Adhesive
was applied and light-cured (10 s; 600 mW/cm2). Composite build-ups were constructed
Hybrid layer
incrementally and resin–dentine specimens (0.8 mm2) were prepared. For NL, 3 bonded
Diphenyliodonium salt
sticks from each tooth were coated with nail varnish, placed in the silver nitrate, polished
Microtensile bond strength
down with SiC papers and analysed by EDX-SEM. NH and YM were performed on the hybrid
Nanoleakage
layer in 2 bonded sticks from each teeth. The remaining bonded sticks were tested on mTBS
Adhesive properties
(0.5 mm/min). The data from each test were submitted to a Student t-test (a = 0.05). Results: No significant difference was found for mTBS between groups ( p > 0.05). Significant lower NL and higher NH and YM were found in the hybrid layer and adhesive layer produced with the iodinium salt-containing adhesive ( p < 0.05). Conclusions: The inclusion of the DPIH to the traditional CQ is a good strategy to improve the adhesive and mechanical properties of a simplified etch-and-rinse adhesive system. # 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Simplified etch-and-rinse adhesives are build-up by joining the components of the primer and the bonding resin into a single solution. The hydrophilic features of these simplified adhesives were increased as the primer/bond solution should be compatible to the intrinsically moist, acid-etched dentine.1
However, several questions have been raised regarding the wet bonding technique. Water and solvents need to be completely removed before adhesive light polymerisation and this is not achieved even when using solvent evaporation 10–12 times longer than those recommended by the manufacturers.2 Inhibition of HEMA polymerisation was detected with intrinsic water at concentrations > 5 vol.%, even with a 10-fold increase in the photo-initiators.3
* Corresponding author at: Universidade Estadual de Ponta Grossa – Mestrado em Odontologia, Rua Carlos Cavalcanti, 4748, Bloco M, Sala 64A – Uvaranas, Ponta Grossa 84030-900, Parana´, Brazil. Tel.: +55 42 3220 3741; fax: +55 42 3220 3741. E-mail address: [email protected] (A.D. Loguercio). 0300-5712/$ – see front matter # 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jdent.2013.04.009
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journal of dentistry 41 (2013) 653–658
Entrapped water also precludes the formation of a high cross-linked polymer within the collagen fibre network4 probably due to phase separation of hydrophobic and hydrophilic moieties,5,6 rendering the polymer weaker than the one formed in a free-water environment. This subpolymerised polymer is more prone to the plasticising effects of water over time and makes the adhesive layer more permeable to water from the oral environment and the underlying bonded dentine.7 Incompatibility issues8 and faster degradation of resin–dentine bonds are the consequences, comparatively to their three-step version.9,10 In an attempt to optimise the bonding to dentine using simplified etch-and-rinse adhesives, various clinical procedures have been proposed11–13 and most of these approaches favour solvent evaporation and increase the degree of conversion in order to produce a strong polymer within the hybrid layer. Despite the immediate benefits of these several clinical approaches, reductions of bond strength values over time are still a problem.14–17 An important consideration about the formulation of the current adhesive systems is that they usually employ hydrophobic initiators (e.g., camphorquinone [CQ]), and co-initiators (e.g., ethyl-4-(dimethylamino)benzoate [EDMAB]).18 Therefore they are likely to be distributed preferentially to the hydrophobic domains, jeopardising the overall polymerisation of the adhesive.19 Diphenyliodonium salt is ionic in nature and as such, it may increase the compatibility between amphiphilic monomers (i.e., having both hydrophilic and hydrophobic characteristics) and initiators, especially in the presence of water.19 This salt is usually associated to CQ–amine system forming a three component photoinitiator system and it acts as an electron scavenger from the dye radical.20 Earlier studies showed that the addition of diphenyliodonium salt in the camphorquinone-amine system accelerate the polymerisation rate and improve the final degree of conversion, transition glass temperature, and crosslink density as well as some other mechanical properties of the adhesive, mainly when the polymerisation reaction takes place in a moist environment.19,21,22 Despite these positive findings, to extent to our knowledge, no study has so far evaluated the association of diphenyliodonium (DPIH) salt to CQ–amine system on the immediate resin–dentine bond strength, nanoleakage and mechanical properties of the hybrid and adhesive layer, which was the aim of the present investigation. The following null hypotheses were tested in this study: (1) there will be no differences in the nanohardness an Young’s modulus of the hybrid layer and adhesive layer between the DPIH-free and DPIH-containing adhesives and (2) there will be no differences in the resin– dentine bond strength and nanoleakage between the DPIHfree and DPIH-containing adhesive.
2.
Methods and materials
2.1.
Tooth selection and preparation
The molars were disinfected in 0.5% chloramine, stored in distilled water and used within six months of extraction. A flat and superficial dentine surface was exposed on each tooth after wet grinding the occlusal enamel on #180-grit SiC paper. The enamel-free, exposed dentine surfaces were further polished on wet #600-grit silicon carbide paper for 60 s to standardise the smear layer.
2.2.
A model simplified adhesive system was formulated through intensive mixing of 40 wt.% UDMA (urethane dimethacrylate) and MDP (10-methacryloyloxydecyl dihydrogenphosphate), 30 wt.% HEMA (2-hydroxyethyl methacrylate) and 30 wt.% ethanol. Two initiator systems were investigated: 0.5 mol% of CQ (camphorquinone) + 1 mol% of EDMAB (ethyl-4-(dimethylamino) benzoate) and 0.5 mol% of CQ + 1.0 mol% EDMAB + 0.2 mol% DPIH (diphenyliodonium hexafluorphosphate). All initiators and co-initiators were supplied by Aldrich Chemical Co (Milwaukee, WI, USA) and used without further processing. Before applying the experimental adhesive system, the homogeneity of the solution was evaluated according to Spencer and Wang.5 After mixing all components, 16 ml of each adhesive was transferred to a small glass and mixed with 16 ml of water. The mixture was stirred for 10–15 s. Immediately after, each mixture was observed under 100 magnification (Eclipse E200, Nikon, USA). Loss in clarity was interpreted as evidence of phase separation. This experiment was done in quadruplicate. The acid etching (CondacAC 37%, FGM Prod. Odont., Ltda, Joinville, SC, Brazil) was applied for 15 s, following the surfaces were rinsed with distilled water for 15 s and excess was removed using oil-free compressed air, leaving the dentine slightly wet. Two coats of adhesives were applied onto the dentine with rubbing action for 20 s. After each coat, an air stream was applied for 10 s at a distance of 20 cm. The time lapse between the starting the adhesive application and light curing (VIP, Bisco, Schaumburg, USA at 600 mW/cm2) was approximately 40 s. The light curing was performed for 10 s. Resin composite build-ups (Opallis, FGM) were placed on the bonded surfaces (3 increments, 2 mm each), which were individually light activated for 40 s each. All bonding procedures were carried out by a single operator at 24 8C room temperature. The bonded teeth were stored in distilled water at 37 8C for 24 h. All teeth were sectioned in both ‘‘x’’ and ‘‘y’’ directions across the bonded interface with a diamond blade in an Isomet 1000 saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) at 400 rpm to obtain resin–dentine beams with a cross-sectional area of approximately 0.8 mm2. For each tooth, a total of approximately 25–30 resin–dentine bonded beams could be obtained. Five beams from each tooth were not tested in tensile force and left for nanoleakage evaluation (n = 3 beams) and nano-hardness test (n = 2 beams). The remaining beams were used for microtensile bond strength evaluation.
2.3. Twelve extracted, caries-free human third molars were used. The teeth were collected after obtaining the patient’s informed consent. The Institutional Review Board approved this study.
Restorative procedure
Microtensile bond strength test (mTBS)
Each resin–dentine beam was attached to the Geraldeli’s device (Odeme Biotechnology, Joac¸aba, SC, Brazil) with
journal of dentistry 41 (2013) 653–658
cyanoacrylate resin (Super Bonder Gel, Loctite, Sa˜o Paulo, SP, Brazil) and subjected to a tensile force in a universal testing machine (Kratos Dinamometros, Sa˜o Paulo, SP, Brazil) at a crosshead speed of 0.5 mm/min. The failure modes were evaluated at 40 (Eclipse E200, Nikon, USA) and classified as cohesive (failure exclusively within the dentine [CD] or resin [CR]), or adhesive/mixed (failure at resin/dentine interface and failure at resin/dentine interface that included cohesive failure of the neighbouring substrates [A/M]).
2.4.
Nanoleakage evaluation (NL)
All resin–dentine bonded sticks were immersed for 24 h in a 50 wt.% ammoniacal AgNO3 solution, rinsed and placed in photo-developed for 8 h under fluorescent light to reduce the diamine silver ions into metallic silver grains.23 The silverimpregnated sticks were mounted on aluminium stubs and polished with a 1000-, 1200-, 1500-, 2000- and 2500-grit SiC paper and 1 and 0.5 mm diamond paste (Buehler Ltd., Lake Bluff, IL, USA) using a polish cloth. They were ultrasonically cleaned with distilled water for 30 min (Dabi Atlante – 3L, Ribeira˜o Preto, SP, Brazil), desiccated in colloidal silica for 24 h and gold-sputtered (Sputter Coater IC 50; Shimadzu, Tokyo, Japan). Resin–dentine interfaces were analysed in a scanning electron microscope operated in the backscattered electron mode (SSX-500, Shimadzu, Tokyo, Japan) with an accelerating voltage of 12 kV. Three pictures were taken of each specimen. The first picture was taken in the centre of the stick. The other two pictures were taken 0.3 mm to the left and right of the first one. As three resin–dentine beams per tooth were evaluated and a total of 6 teeth were used for each experimental condition, a total of 54 images were evaluated per group. They were all taken by a technician who was blinded to the experimental conditions under evaluation. The relative percentage of nanoleakage within the adhesive and hybrid layer areas was measured in all pictures using the UTHSCSA ImageTool 3.0 software (Department of Dental Diagnostic Science at The University of Texas Health Science Centre, San Antonio, TX, USA).
2.5.
Nano-hardness test
The resin–dentine beams were individually embedded in a self-cure polyester resin (Milflex, Milflex Indu´strias Quı´micas, Sa˜o Bernardo do Campo, SP, Brazil) and after 24 h the specimens were polished and cleaned as described earlier for the silver uptake test. For the nano-indentation measurements, the computer-controlled Nano Indenter XP (MTS, Oak Ridge, TN, USA) was employed mounted with a triangular pyramidal diamond indenter Berkovich. By means of the computer-controlled X–Y table, the dried specimen was transferred to the indenter. An accurate calibration of the distance between the microscope and the indenter was run before testing to ensure a precise transfer of the preprogrammed positions to the indenter. At the start of measurement two groups of nine indentation positions equally spaced each other were programmed by a remote video control (connected to the light microscope attached to the nano-indenter device) for each region. In order
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to obtain precise measurements, the interval of each indentation was twice the size of the indentation in each region with the aim to avoid the corruption of the abutment.24 In the central area of the composite resin and adhesive layer, the surface approach rate of the nano-indenter was set to be 10 nm/s and the duration of the loading and unloading indentation was 5 s each. The pre-programmed distance among indentations for dentine, adhesive layer and composite resin was 20, 10 and 20 mm respectively with a load of 10 g. At the hybrid layer the surface approach rate was the same, however the load employed was 0.2 g and indentations were programmed to be performed at a distance of 3 mm. The reduction of the load was required to reduce the indentation size so that the indentations could be positioned entirely within the area of the hybrid layer. Epoxy resin replicas of all specimens were gold-sputtered and analysed under SEM according to previously described. It was necessary to verify the indentation geometry and the accurate positioning of the pre-programmed indentations. Those that were found out of the specified areas were excluded from the sample. The nano-hardness and Young’s modulus of each area were computed following the method of Oliver and Pharr.25
2.6.
Statistical analysis
The mean values of mTBS and nanoleakage from bonded beams originating from the same tooth were averaged for statistical purposes. The specimens with cohesive and premature failures were not included into the tooth mean. The same procedure was performed for the nano-hardness and Young’s modulus of the hybrid and adhesive layers. The data of each test were statistically analysed by a Student t-test (a = 0.05).
3.
Results
No signs of phase separation were observed in the DPIH-free or DPIH containing experimental adhesive (data not shown).
3.1. (%)
Microtensile bond strength (MPa) and nanoleakage
No significant difference was detected in the resin–dentine mTBS values for both adhesive formulations (Table 1; p = 0.62). The fracture pattern of the resin–dentine beams was very similar between the adhesive formulations. However, the use of DPIH-containing adhesive significantly decreased the amount of nanoleakage in the hybrid layer (Table 1; p = 0.001). Representative back-scattering scanning electron microscopy images captured at the resin–dentine interfaces are depicted in Fig. 1.
3.2.
Nano-hardness test
The means values and standard deviations for nano-hardness (GPa) in the composite resin and mineralised dentine were 1.14 0.1 and 0.82 0.2, respectively. For Young’s modulus, the mean values and the respective standard deviations (GPa)
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journal of dentistry 41 (2013) 653–658
Table 1 – Means (standard deviation) of the microtensile bond strength (n = 6 teeth), fracture pattern (number of beams) and nanoleakage (n = 6 teeth) for the experimental adhesives.
Microtensile bond strength (MPa)b Fracture pattern (number of beams)c Nanoleakage (%)
DPIH-freea
DPIH-containing
48.3 [4.2] A
53.2 [3.2] A
80/24/12/08
92/26/10/12
15.1 [2.7] a
11.2 [2.2] b
a
DPIH – diphenyliodonium hexafluorphosphate. Similar uppercase (microtensile bond strength) and lowercase (nanoleakage) letters indicate means statistically similar (Student-t test; p > 0.05). c Adhesive/mixed/cohesive in dentine and resin/premature failures. b
were 16.71 0.88 and 20.32 2.13 for the composite resin and mineralised dentine, respectively. The use of DPIH-containing adhesive significantly increased the nanohardness and Young’s modulus of the hybrid layer (Table 2; p < 0.001) and adhesive layer (Table 2; p < 0.002).
4.
Discussion
The most commonly employed photoinitiators are those in which radicals are formed in a binary-system containing an excited state of a dye, such as camphorquinone (CQ), and a coinitiator that acts as an electron donor.26,27 The photoinitiator can absorb light directly and the co-initiator interacts with the activated photoinitiator to generate a reactive free radical for polymerisation initiation.28 In this binary-system CQ requires a tertiary amine reducing agent, usually ethyl-4(dimethylamino) benzoate (EDMAB), for efficient polymerisation to occur.20 As described before,1 the simplified adhesive contains very hydrophilic molecules, like primers, and relatively hydrophobic molecules, forming the bond. When simplified etch-andrinse adhesives are used over moist dentine water solubilise,
in a certain limit, in the adhesive affecting the solubility of the different components. However, differences in the hydrophilicity of the organic matrix and the CQ–amine system in the adhesive formulation may lead to uneven distribution of the latter in hydrophobic domains, producing phase-separation and jeopardising the overall polymerisation of the simplified adhesive, due to the lower concentration of the CQ–amine system in the hydrophilic domain.5,19,22 When simplified etch-and-rinse adhesives are used, not only the residual water from the moist dentine impair the polymerisation reaction of the adhesive system,3,19 but also the amount of residual solvent2,29 presented in the adhesive formulation. The amount of retained solvent in experimental adhesives was shown to be three to four times higher when water is added to the adhesive formulation.2 In a way to minimise these problems, studies attempted to investigate a three-component photoinitiator system in which a diphenyliodonium salt is added to the traditional twocomponent CQ–amine system. The mechanism of action proposed for the addition of the diphenyliodonium salt reports that this co-initiator salt might act as an electron scavenger from the dye radical.20,30 In this three-component photoinitiator system, the resulting amine-based radical is active for polymerisation, but the dye-based radical is a terminating radical. The third component, the diphenyliodonium salt, which is an electron acceptor, serves both to regenerate the photosensitizer molecules (e.g., CQ) by replacing inactive terminating radicals with active phenyl initiating radicals, and also to generate additional active phenyl radicals.20,30 Other authors suggested that an exciplex between CQ* and diphenyl iodinium salt is initially formed, which reacts further with the amine, giving radical amine species that initiates the polymerisation and regenerates the diphenyl iodinium salt, forming a new exciplex with a new photo-activated CQ, being this an extra source of radicals for initiating polymerisation.31 The addition of diphenyliodonium salt was shown to increase the velocity of reaction and the percentage of monomer conversion significantly. While the maximum rate
Fig. 1 – Backscattering scanning electron micrographs of the adhesive interfaces of the experimental groups. More silver nitrate deposition (white hands) could be seen in the adhesive interface produced in the group DPIH-free (B). (Co = composite resin; AL = adhesive layer; HL = hybrid layer; De = dentine).
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journal of dentistry 41 (2013) 653–658
Table 2 – Means (standard deviation) of nanohardness (n = 6 teeth) and Young modulus (n = 6 teeth) evaluated in the hybrid and adhesive layer for the experimental adhesives. Hybrid layer a
Nanohardness (GPa)b Young modulus (GPa)
Adhesive layer
DPIH-free
DPIH-containing
DPIH-free
DPIH-containing
0.12 [0.01] A 2.72 [0.3] a
0.16 [0.03] B 3.21 [0.2] b
0.22 [0.02] A 4.72 [0.4] a
0.27 [0.05] B 5.38 [0.3] b
a
DPIH – diphenyliodonium hexafluorphosphate. Similar uppercase (nanohardness) and lowercase (young modulus) letters indicate means statistically similar for each substrate (hybrid layer or adhesive layer) (Student-t test, p > 0.05). b
of polymerisation occurred after 1.65%/s and final percentage of conversion was 44.3% for the binary CQ–amine system, the use of diphenyliodonium salt reached 2.73%/s and degree of conversion was 54.4%.31 When in contact with water or solvents, this difference was much more pronounced favouring the adhesive formulation with diphenyliodonium salt.19,21,22,29 It is likely that the ionic nature of the diphenyliodonium salt can increase the compatibility between hydrophilic and hydrophobic monomers and initiators, especially in the presence of water, leading to higher degree of conversion and high mechanical properties of the adhesive layer and helps in the interaction between amphiphilic monomers (hydrophobic and hydrophilic characteristics) improving the resin infiltration inside the collagen mesh.19,21,22 The results of the present investigation demonstrated that adhesive and hybrid layer produced with the DPIH-containing adhesive showed higher nano-hardness and Young’s modulus and the first null hypothesis of this study was rejected. As previously described by Spencer and Wang,5 when phase separation occurs the adhesive layer becomes opaque due to difference in the index of refraction of the different phases. This makes the light penetration difficult, which in turn affects the light curing procedure and the quality of the produced polymer. This is not observed when DPIH is added to the formulation, since it improves the miscibility of the components keeping the adhesive layer translucent (data not shown). Although the benefits of DPIH were not seen in the bond strength evaluation, reduced nanoleakage was observed in the interfaces produced with the iodonium-containing adhesive, leading us to partially reject the second null hypothesis. As nanoleakage evaluation provides good spatial resolution of submicron defects in resin infiltration or areas of inadequate polymerisation,7,32 one may speculate that this is an indirect finding of even polymerisation throughout the adhesive interface. There was found less nanoleakage in both the hybrid layer (classic nanoleakage33) and the adhesive layer (water trees7) in the DPIH-containing adhesive. According to Tay et al.,7 water trees in dentine adhesives, together with nanoleakage within the hybrid layers, represent water-rich interfacial regions from which the leaching of hydrophilic resin components may readily occur, indicating signs of potential degradation of the interfacial bond in the future. The regions of silver uptake probably represent areas of suboptimal conversion within the polymer matrix due to incomplete solvent removal,34 and may permit higher diffusional water fluxes within the hybrid layers that could accelerate water sorption and the extraction of residual monomers or loosely bounded oligomers.7,11 Based on this,
one might expect reduced degradation of the resin–dentine bonds for the diphenyliodonium salt-containing adhesive and future studies should be conducted to confirm this hypothesis. It is worth mentioning that not only diphenyliodonium salt has been evaluated as an alternative to the traditional hydrophobic CQ–amine system. Other photoinitiatiors as TPO and QTX has been added to adhesive systems.19,21,35,36 However, only mechanical properties and kinetics of polymerisation reaction was evaluated in these studies.19,21,35–37 Further studies should be conducted in order to evaluate the impact of such photoinitiators on the adhesive properties as well as mechanical properties of the adhesive and hybrid layers. In conclusion, the addition of the hydrophilic co-initiator diphenyliodonium hexafluorphosphate improved the mechanical properties of the hybrid layer and adhesive layer produced by the experimental etch-and-rinse adhesive and decreased the amount of nanoleakage in the adhesive interface.
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