adhesion is defined as the process of cross - linking between an adhesive and a solid or liquid substrate .
close contact between the adhesive and the substrate is essential in achieving the strong adhesion .
in addition , the surface tension of adhesive should be less than the free energy of substrate surface .
adhesion in the etch - and - rinse systems is based on infiltration of the resin between the unsupported collagen fibers , by removing the smear layer and then production of a hybrid layer .
the spatial structure and arrangement of the collagen fibers influence the interfiber spaces and the efficiency of primer penetration into those spaces .
the self - etching systems are based on etching and contemporary penetration of acidic monomers .
acidic monomers do not dissolve the smear layer and the mineral components of dentin completely .
substrate characteristics and thickness , tensions in the resin - dentin interface , and hybrid layer characteristics affect bonding to dentin . higher bond strengths between resin and dentin can be achieved using different types of dentin bonding agents , using appropriate clinical methods such as multiple adhesive coatings , increasing the application time of bonding agents , delaying polymerization , and longer exposure times of the bonding system .
most of these methods result in a higher quality hybrid layer and a stronger polymer .
temperature has been reported as an effective factor on bond strength of composites to the dental tissue .
many studies have shown that different temperatures of the dentin bonding agent , drying air spray , restorative material and environment have on bond strength of resin composite to dentin .
they showed that the increase or decrease in temperature affects the bond strength of composite to dentin .
heat increases the kinetic energy of molecules which can affect characteristics of the bonding agent and the substrate .
it has been demonstrated that warm air drying evaporates the bonding agent solvent , resulting in stronger dentin - resin bonding .
in addition , warming the bonding agent has been reported to increase the bonding strength .
since the washing is an essential part of the adhesion process , it seems that the washing water temperature can be used to affect the temperature of the substrate ( i.e. , enamel or dentin ) .
the higher temperature may be able to decrease the bonding agent viscosity , which could lead to better penetration of bonding agent into both etched enamel and the exposed collagen in dentin and improve tooth - composite adhesion .
the purpose of this study was to evaluate the effect of different temperatures of irrigation water on micro - shear bond strength ( sbs ) of a two - step self - etching primer and a two - step etch - and - rinse adhesive system .
the null hypothesis was that sbs was not affected by the temperature of washing water in both of the bonding systems .
in this in vitro study , 24 intact human third molars were used which were extracted within a period of 3 months before the experiment .
the teeth were washed and cleaned under running water immediately and stored in a 0.2% thymol solution at room temperature ( 23c ) .
the teeth were individually embedded in self - cure orthodontic resin ( orthoresin , dentsply ltd . , weybridge , england ) with the buccal and lingual surfaces of crowns oriented toward the external block face to expose the buccal and lingual enamel .
flat dentin surfaces were created on mid - buccal and mid - lingual of each sample , using a low - speed diamond saw ( 180 grit dandiran , iran ) , under running water coolant .
the exposed dentin surfaces were polished with # 600-grit silicone - carbide paper for 60 s under running water to create a standard smear layer .
the samples were divided into three experimental groups ( 5  2c , 22  2c and 50  2c ) with eight teeth in each group .
then , they were stored in distilled water at room temperature . to set the washing water temperature to 5  2c ,
eight 50 ml syringes filled with distilled water were refrigerated at 5c for 24 h before the experiment .
to make hot water for the experiment , a calibrated compound heater ( hanau uh-1 115v vintage dental lab utility heater , usa ) was used .
the adhesive bonding agents , which were a two - step self - etching primer ( se , clearfil se bond kuraray medical inc . , okoyama , japan ) and a two - step etch - and - rinse adhesive ( single bond [ sb ] , 3 m espe , st .
paul , mn , usa ) [ table 1 ] , were stored in the temperature the manufactures recommended ( 5  2c ) until an hour before the experiments . in the se group ,
dentin surfaces were rinsed with distilled water at warm ( 50  2c ) , cold ( 5  2c ) or room temperature ( 22  2c ) for 20 s , and then blotted dry with a cotton pellet to reduce the temperature fluctuations . the surface temperature was measured with a digital thermometer with 94% accuracy ( scantemp 485 , dostmann electronic , china ) [ table 2 ] .
adhesive systems : compositions and batch number adhesive systems : application mode , water , and dentin surface temperature in se groups , the self - etching primer was applied to the surface by micro brush for 20 s. the primed surface was gently air - dried with 0.15 mpa oil - free compressed air and temperature of 21c .
the air syringe was kept at a distance of 10 cm from the sample for 5 s. then , the bonding agent was applied on the surface , following the manufacturer 's instructions , and then gently air - dried [ table 2 ] .
bonding procedure for sb groups was done on the other side of all teeth at the same temperature .
after acid etching with 35% phosphoric acid gel ( ultra - etch , ultradent products inc . , south jordan , ut , usa ) , the respective etchant for 20 s , the dentin surfaces were rinsed for 10 s , and then compressed air and water for another 10 s. in this stage , the samples were rinsed with distilled water with various temperatures for 20 s and dried with cotton pellets .
two coats of adhesive were used on the surfaces ; the first layer was agitated for 15 s and then air - dried at temperature of 21c for 5 s at a distance of 10 cm .
the second layer was used and dried gently . prior to light curing of bonding resin in each specimen ,
two hollow cylinders from micro - bore tygon tube ( norton performance plastic , oh , usa ) with 1 mm height and an internal diameter of 0.8 mm were placed on the mesial and distal ( 16 cylinders for each bonding agent at each temperature ) of treated dentin surfaces .
the adhesive resins were light - cured with a halogen light cure unit ( coltlux 75 , coltene , switzerland ) , with a power density of 1000 mw / cm for 10 s. resin composite ( filtek z250 , shadea2 , 3 m espe , st .
paul , mn , usa ) was carefully inserted into the cylinders and was subjected to curing light for 40 s , during which the light was moved to assure curing of the entire cylinder . after an hour of storing the specimens in distilled water at 37c , the tygon tubes were cut and removed gently .
the specimens were then incubated for 48 h in distilled water and thermo cycling ( deltatpo2 , nemo , mashhad ,
iran ) was done ( 1000 cycles between 5c and 55c ) with a dwell time of 30 s in each bath and a transfer time of 5 - 10 s between the two baths .
the specimens were fixed in a universal testing machine ( nonstop , bego , germany ) .
a thin steel wire ( 0.275 mm d ) was looped flush between the load cell projection and the composite cylinder to make contact with the lower half - circle of the cylinder and stay parallel to the loading cell movement direction .
the loop was in line with the force vector and perpendicular to the dentin - resin interface to apply shear force .
force was applied with wire loop at a crosshead speed of 1 mm / min until failure in the composite cylinder occurred .
the maximum load at the time of failure was recorded in kilograms ( kg ) , which was converted to mpa for each specimen .
the mode of failure was determined at 40 using a stereomicroscope ( leicams5 , wetzlar , germany ) and recorded as adhesive failure ( a ) , cohesive ( c ) , and mixed modes ( m ) on the same fractured surface .
the bond strength results were analyzed using a two - way analysis of variance ( anova ) to detect the overall effect of the test factors ( water temperature and bonding systems ) .
one - way anova and tukey 's post hoc test were used to determine the effect of temperature on bond strength .
paired sample t - test was used to compare the bond strength between two types of adhesive at the same temperature , and failure mode distributions were analyzed by fisher excat test .
two - way anova analysis showed that sbs was significantly influenced by temperature ( p = 0.022 , f = 3.981 ) and the interactions of temperature and type of adhesive system ( p = 0.013 , f = 4.602 ) .
sbs was not affected by type of adhesive system ( p = 0.261 , f = 1.283 ) .
micro - shear bond strength mean values standard deviation in mpa and fracture mode distribution one - way anova and post hoc tukey 's tests showed that in the sb groups , there were significant differences between sbs of the warm washing water group ( 50c ) and sbs of 5c and 22c washing water temperature groups ( p = 0.003 and p = 0.019 , respectively ) .
no statistical difference was observed between the cold water group and the room temperature water groups .
there were no statistical differences between three different water temperatures of the se . according to the paired sample t - test ,
the sbs was higher in the sb at 50  2c compared with se ( p = 0.007 ) . also , the sbs in the se was significantly higher than the sb at 22  2c ( p = 0.031 ) . although higher bond strength in cold water was observed for the se , difference was not significant .
fisher exact test showed no significant difference between different groups which means water temperature and bonding agent had no effect on fracture modes .
it is shown in this study that temperature increase can impact the substrate , dentin , or smear layer , depending on the adhesive system .
in addition , temperature rise can change the physical properties and the chemical structure of the materials .
the etch - and - rinse systems , such as sb , bond to dentin by removing the smear layer and penetrating into the collagen mesh of the resin .
mild self - etching systems such as se , on the other hand , are only capable of dissolving the smear layer and simultaneously penetrating the resin by etching the surface .
the findings of this paper showed that the bond strength of se at both low temperature ( 5  2c ) and room temperature of washing water ( 22  2c ) is more than sb bond strength .
se contains 10-methacryloxydecyldihydrogen phosphate ( 10-mdp ) as a functional monomer that appears not only to interact more intensively with hydroxyapatite ( hap ) but also to have the most hydrolytically stable bond with calcium .
this interaction is more stable than the interaction caused by the long term connection of 10-mdp and hap .
since the calcium salt of the acidic molecule is less soluble , the molecular adhesion to hap of substrate is more stable and intense .
in addition , the hybrid layer has a substantially higher modulus for the self - etching systems .
this may be a possible explanation for the higher bond strength of these materials . also , use of phosphoric acid in high concentrations ( 35% and upper ) causes denaturation of the top layer of collagen , which causes the lower bond strength of sb . in the present study ,
the bond strength of sb was increased with increases in the washing water temperature and it was higher at 50  2c compared to that of se .
it should be noted that in etch - and - rinse systems , after acid etching and demineralization , the collagen fibers collapse and interpeptidal hydrogen bonds are generated that cause more collagen mesh shrinkage .
the hydrogen bond is broken with hydroxyethyl methacrilate ( hema ) and water molecules during resin adhesion . moreover ,
a demineralized dentin zone at the bottom of the hybrid layer created by insufficient infiltration of resin is thought to be the site where interfacial degradation is expected to begin .
heat can reduce the effect of these phenomena by both affecting the etched dentin surface and changing the physical properties of the bonding agent . based on previous studies , the hydrogen bond strength decreases and the hydrogen bond length increases as a result of an increase in the temperature .
these mean that the unfolding of the helix and breaking of the bond by hema and water occur more easily .
in addition , heat increases the free energy of surface that decreases the contact angle of surface - liquid , which in turn facilitates the liquid diffusion and infiltration .
the increase in the temperature might enable free radicals and propagate polymer chains to become more flexible as a result of decreased viscosity .
the higher temperature could also lessen the amount of bounded water retained in the resin .
reported that an increase of 8c in the temperature increased the spreading velocity of the sb and prime and bond nt by approximately 21% due to decreased viscosity at high temperature .
all of these impacts help a better penetration and diffusion of the resin that reduces the demineralized zone .
this consequently leads to better micromechanical lock and higher bond strength of resin - dentin .
showed the blow of the warm air causes the sb solvent evaporation , which increases the resin - dentin bond strength .
since sb is a water / ethanol - based system , the duration of heat transfer for effective evaporation of the solvent is not enough compared to direct air blowing . for se groups , at first , the ascending or descending alteration of the bond strength seemed to be associated with increase in temperature , but the higher bond strength at room temperature compared to low and high temperatures was observed .
it seems the bond strength variations induced by the various temperatures in the self - etching system such as se is a combination of the effects of alteration in substrate , acidic monomer and their interaction .
some experiments reported that the smear layer thickness is not effective on the function of self - etching systems , but other studies suggested that the smear layer thickness interferes with the self - etching bond strength .
it seems the smear layer effect is associated with its density which affects the primer 's ability to etch through the smear layer .
although burs cutting forces generate friction and heat which make the gelatin status of modified collagen , the sole effect of temperature increase was not evaluated up to now .
thus , the gelatin status of the smear layer may increase by an increase in temperature and leads to difficult penetration of primer into this layer .
the resin infiltrates into the demineralized dentin and retains hap crystals around collagen fibers , and therefore prevents collagen fibers from being stained .
moreover , the less soluble calcium salt causes the acidic molecule adhesion to hap of substrate to be more stable and intense .
this reaction happens at room temperature , under high buffering ability of the smear layer neutralizes the acidic monomers partially and the mixture 's ph values increases .
this increase in ph may prevent more dissolution of hap . with an increased temperature ,
in this situation , acidity of environment is more than the acidity at room temperature .
likely , with an increase in the acidity of the environment , hap around collagen fibers dissolves and degradation of the bond may occur .
moreover , the interaction of acidic monomer and calcium for producing calcium salt is an exothermal reaction . at higher temperature ,
the participative molecules in exothermal reaction have less likelihood to combine and this leads to less production of calcium salt .
the reduced calcium salt level may result in difficulty in the stability of resin - dentin bond .
although some researches have attempted to explain the effect of increase rate and pressure of evaporation , on reduction of demineralization ability of adhesive , those results were not supported by garcia et al . experiment .
furthermore , the solvent in the se is water , which needs more time and warmer temperature for enough evaporation .
low temperature causes more buffering ability of the smear layer and reduction in the surface free energy .
low surface free energy reduces resin penetration into smear layer and hap 's insufficient wetting .
two important issues that should be considered are the effect of high temperature on pulp vitality , and dentinal fluid flow .
it seems the warmth of high temperature water , measured at pulp side of dentin , is not higher than the warmth caused by curing units in composite 's polymerization .
quartz tungsten halogen curing unit raises the temperature in resin - based composite to approximately 15.5 - 18.6c .
the higher intrapulpal temperature ( 8c ) for 10 s was reported during the application of a plasma arc lamp , with remaining dentin thickness of 1 mm . with considering to hydrodynamic theory during application of the warm water on the surface ,
the intertubular fluid flow reaches up to the surface and endangers the resin - dentin bond strength .
more scanning electron microscopy studies are required to investigate the effect of temperature changes on resin - dentin interface .
the sbs of studied etch- and -rinse system was increased by higher temperature of the washing water . however , the bond strength of clearfil se bond as a self - etching system did not change using different water temperatures .
the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article .
the authors of this manuscript declare that they have no conflicts of interest , real or perceived , financial or non - financial in this article .
 background : the purpose of this study was to evaluate the effect of washing water temperature on the micro - shear bond strength ( sbs ) of composite resin to dentin using a two - step etch - and - rinse system and a two - step self - etching system.materials and methods : in this in vitro study , the intact dentins of buccal and lingual surfaces of healthy third molars were exposed . 
 dentin surfaces were rinsed with different temperatures of distilled water ( 20 s ) before applying single bond ( sb ) or clearfil se bond(se ) . after applying the adhesive , composite cylinders ( 0.8 mm diameter and 1 mm length ) 
 were bonded to the teeth surfaces . after storing the specimens in 37c distilled water for 48 h and thermocycling , 
 sbs test was done . 
 data were analyzed using analysis of variance , post hoc tukey tests , paired samples t - test , and fisher exact test (  = 0.05).results : temperature and interaction of temperature and type of bonding agent affected the bond strength . 
 the bond strength of sb groups was significantly higher at 50c washing than 5c ( p = 0.003 ) and 22c ( p = 0.019 ) , but no significant difference was observed between se groups . 
 the bond strength of se was significantly higher at 22c than that of sb ( p = 0.031 ) , whereas the bond strength of sb was significantly higher at 50c than that of se ( p = 0.007).conclusion : the use of high - temperature washing water is an appropriate method to enhance bond strength in etch - and - rinse systems . 
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION
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