In vitro study of nonthermal atmospheric pressure plasma in improving the durability of the dentin-adhesive interface with an etch-and-rinse system

Danyang WANG (王丹杨), Na XIE (谢娜), Lin WANG (王琳),Peng WANG (汪鹏), Yanping ZUO (左艳萍), Chengfang TANG (唐成芳),Xinyang MA (马新扬), Wen XU (徐文), Fei LIU (刘飞),Qinhong WANG (王钦鸿) and Yang WANG (汪洋),6

1 Department of Prosthodontics,School of Stomatology,Xi’an Medical University,Xi’an 710021,People’s Republic of China

2 Department of Oral Medicine,School of Stomatology,Xi’an Medical University,Xi’an 710021,People’s Republic of China

3 Key Laboratory of Shaanxi Province for Craniofacial Precision Medicine Research, Department of Prosthodontics, College of Stomatology, Xi’an Jiaotong University, Xi’an 710004, People’s Republic of China

4 The Molecular Virology and Viral Immunology Laboratory, Xi’an Medical University, Xi’an 710021,People’s Republic of China

5 Key Laboratory of Shaanxi Province for Craniofacial Precision Medicine Research, Department of Pediatric Dentistry, College of Stomatology, Xi’an Jiaotong University, Xi’an 710004, People’s Republic of China

Abstract

Keywords: nonthermal atmospheric pressure plasma, dentin, bonding durability, collagen,hybrid layer

1.Introduction

Firm and long-term adhesion between enamel or dentin and dental prosthetic material is key to the success of dental adhesive restoration.Unlike enamel, dentin is a complex biological structure, which consists of 70% inorganic apatite crystallites, 20% organic matrix (comprising 90% type I collagen fibrils and 10% noncollagenous proteins), and 10%water(weight percentage,wt.%)[1].Dentin bonding relies on the formation of a compact and homogenous hybrid layer and resin tags [2].The treatment of dentin with acid conditioners can achieve a superficial demineralized dentin surface and exposed collagen fibrils.After that, the demineralized dentin collagen matrix should be entirely infiltrated with adhesive to form a composite structure after curing,which is the so-called‘hybrid layer’[3].The etch-and-rinse adhesive system and the self-etch adhesive system are two types of dentin-bonding strategies.In the etch-and-rinse system, the discrepancy between dentin demineralization and resin infiltration results in incompletely infiltrated zones along the bottom of the hybrid layer [3, 4].The denuded collagen fibrils from the incompletely infiltrated zones are not enveloped by resin [5].With time, these unprotected collagen fibrils are easily degraded by water or attacked by proteolytic enzymes,causing degeneration of the dentin-adhesive interface.Therefore, improving the crosslinking degree of dentin collagen fibrils and effectively reducing the exposure of unprotected collagen fibrils is a critical method to improve the bonding durability of the dentin-adhesive interface.

As an ‘effective’, ‘clean’ physical method, nonthermal atmospheric pressure plasma (NTAPP) is a recently developed surface treatment technology.Plasma contains highly reactive particles, including electronically excited atoms,molecules, ions, free-radical species, and active oxygen [6].Unlike conventional plasma, NTAPP does not extensively heat the surrounding environment during energy conversion,while the gas phase remains at room temperature.Therefore,NTAPP can be applied to biological tissues with high thermal sensitivity[7].Recent studies have demonstrated that NTAPP is an effective way of improving the surface characteristics of dentin as well as the immediate or aging dentin bond strength[8, 9].Previous studies revealed that plasma treatment can cause modification of the dentin collagen surface [10].However, the relationship between the surface modifications and the dentin-bonding stability after NTAPP treatment is still unknown and requires further study.

The present study aims to evaluate the effect of nonthermal helium plasma treatment on the dentin-bonding durability and collagen-degradation resistance.The null hypothesis we aimed to test was that nonthermal helium plasma treatment would have no effect on either the dentinbonding strength or the collagen-degradation resistance.

2.Materials and methods

Extracted, intact human third molars were collected with the patients’ informed consent under a protocol approved by the Xi’an Medical University Institutional Review Board.The teeth were stored in 0.9% saline containing 0.02% sodium azide at 4 °C and were used within one month of extraction.

2.1.Generation of the nonthermal plasma jet

The plasma equipment was supplied by Suman Plasma Technology Co.,Ltd(Nanjing,China).Helium(He,99.99%)was used as the operating gas at a flow rate of 4 l min−1.The plasma jet was operated at an input power of 8 W.The peak voltage and frequency were about 7.0 kV and 54.9 kHz.A mobile base allowed the operator to keep a distance of 30 mm between the nozzle and the dentin surface.

2.2.Dentin specimen preparation and etch-and-rinse adhesion procedure

We removed the occlusal enamel of each tooth using a watercooled low-speed diamond saw(SYJ-150,Ke Jing,Shenyang,China) to expose the coronal dentin.The dentin surface was polished with 800-grit silicon carbide sandpaper to create a standard smear layer.After that,we etched the polished dentin surface with 37% phosphoric acid gel (Vericom Co., Ltd,Korea) for 15 s, rinsed it with water for 60 s, and kept it wet until the plasma treatment.The teeth were randomly divided into five groups(twelve teeth per group):I control(no NTAPP treatment), II NTAPP treatment for 5 s (T-5), III NTAPP treatment for 10 s(T-10),IV NTAPP treatment for 15 s(T-15),V NTAPP treatment for 20 s (T-20).The etched dentin surface was blotted dry with moistened Kimwipes tissues(Kimberly-Clark,Roswell,GA,USA),and then exposed to the plasma jet for the various times as described.After re-wetting with a moistened Kimwipes tissue for 15 s,the specimens were treated with the adhesive, strictly according to the manufacturers’ instructions (Adper Single Bond 2, 3M Oral Care;St Paul, MN, USA).Then, the adhesive was air-thinned and light-cured.The resin composite (Z250, 3M Oral Care, USA)was applied to the bonded interface in four or five layers.The composite was light-cured in 2 mm increments for 40 s each [11].

2.3.Micro-tensile bond strength testing

The bonded specimens were stored in distilled water at 37°C for 24 h.We immediately subjected the tooth-composite specimens of each group(six teeth per group)to micro-tensile bond strength (MTBS) tests.The other specimens of each group were tested after 10 000 thermocycles in sterile artificial saliva.Thermocycling was performed at 5°C and 55°C.The dwelling time and transfer time were 60 s and 10 s,respectively.It was estimated that approximately 10 000 thermal cycles correspond to one year of clinical function [12].

All the teeth were occlusogingivally sectioned into 0.8×0.8 mm2beams using a low-speed diamond saw.The beams of each group(16 beams per tooth)were subjected to a tensile force at a crosshead speed of 0.5 mm min−1as described previously [11].The bond strength was calculated and recorded in MPa.The average MTBS value of the beams from one tooth was calculated as a statistical unit.The mean±standard deviation (SD) for every group tested was determined from the six teeth within each group [13].

Table 1.Micro-tensile bond strength of each group (means±SD, MPa).N = 6 per group.For each horizontal row, values with the same lowercase letters indicate no significant difference (p ＞0.05).For each vertical column, values with same uppercase letters indicate nosignificant difference (p ＞0.05).

2.4.Failure mode analysis

The failure modes of the debonded specimens were assessed under a stereomicroscope (MLC-150, Motic, USA).We classified the failure modes as follows [14]: mixed failure,adhesive failure (interface dentin-adhesive), adhesive failure (interface resin-adhesive), cohesive failure.Cohesive failure represents failures in dentin or resin, and a mixed failure indicates a mixture of adhesive and cohesive failure on the same fractured surface.Then we observed four representative fractured beams from each group with MTBS close to the mean bond strength of that group under a field emission scanning electron microscope (FE-SEM, JSM-7500F, JEOL,Tokyo, Japan).

2.5.Observation of the degradation resistance of the demineralized dentin collagen in sodium hypochlorite (NaClO)

One-third of the coronal part of twenty-seven teeth was removed.One-third of the remaining middle part of each crown was then sectioned into three dentin slabs(1.5×1.5×6 mm3).Two lines marked one surface of the dentin slab as the reverse side.Then 80 specimens were randomly divided into five groups(16 slabs per group), and the obverse side of each slab was prepared as described in section 2.2.After the preparation, the specimens of each group were re-wetted in sterilized distilled water for 15 s,and further immersed in 5%NaClO solution for different treatment times (0, 30, 60 and 120 s, four slabs per subgroup).After NaClO treatment, all specimens were fixed,dried, platinum-sputtered, and observed under FE-SEM.

2.6.Statistical analysis

Kolmogorov-Smirnov and Levene tests were first performed to confirm the normality and equality of the data variance.The results of MTBS were analyzed with a two-way analysis of variance (plasma treatment and aging treatment as tested variables).A one-way analysis of variance was performed for MTBS values within the same aging treatment.Post hoc correction for multiple comparisons was done using Tukey’s test.The independent-sample T-Test was separately performed for MTBS values within the same treatment groups.The results of the failure modes were evaluated using the Chi-Square test.Statistical significance for all tests was set at the 95% level, and all analyses were performed using the SPSS 18.0 software package (SPSS, Chicago, IL, USA).

3.Results

3.1.Micro-tensile bond strength

The MTBS values of different tested groups are summarized in table 1.Both the plasma and artificial aging treatment significantly influenced the MTBS(p＜0.001).Furthermore,the interaction between plasma treatment and aging treatment was also significant (p＜0.05).The MTBS values of all NTAPP treatment groups were significantly increased compared with the control group.The T-5 group presented statistically higher MTBS values than any other groups (p＜0.05).After thermocycling aging, the MTBS values of T-5 and T-10 groups were significantly higher than those of the other groups (p＜0.05), and a statistically significant difference was detected between these two groups (p＜0.05).The MTBS values of the T-15 and T-20 groups were significantly higher than that of the control group (p＜0.05),while no significant difference was detected between the two groups (p＞0.05).Using the same treatment manner, the MTBS values of the control, T-15, and T-20 groups were significantly decreased after aging treatment (p＜ 0.05).However, in the T-5 and T-10 groups, no significant difference was detected after aging (p＞0.05).

3.2.Failure mode analysis

The percentage of failure modes in each group is presented in table 2.We did not observe any cohesive failures in the dentin or resin.Significant differences were found between the groups regardless of artificial aging(p＜0.05).Mixed failure was the most common mode of failure in each group, and its percentage decreased after the aging treatment.Furthermore,there were more mixed failures in the plasma-treated groups than in the control group, regardless of whether artificial aging was performed.After aging, the percentage of resinadhesive failure mode in each group increased.Simultaneously, there was an increase in dentin-adhesive failure in the control,T-15,and T-20 groups.Specimens in the T-5 and T-10 groups showed no changes in the failure mode distribution after aging (p＞0.05).No dentin-adhesive failures were observed in the T-5 or T-10 groups.

The representative failure modes are presented in figure 1.Before the aging treatment, the control group showed a pattern of mixed fractures, which were mostlypresent on the top of the hybrid layer (figure 1(a)).However,in the control group, many dentin tubules were not wellinfiltrated with adhesive resin (finger, figure 1(a)).After the aging treatment, the bottom of the hybrid layer presented a characteristic pattern of mixed failure,with the dentin tubules opening widely and resin tags being entirely pulled out(hollow arrow, figure 1(b)).

The T-5 and T-10 groups displayed a similar pattern of mixed failure presented on the top of the hybrid layer, and well-infiltrated adhesive resin tags were observed in the dentin tubules, regardless of whether we performed artificial aging (figures 1(c) and (d)).The T-15 and T-20 groups also presented this mixed failure on the top of the hybrid layer before aging.Most of the resin tags formed adequately in the fractured interface, while a few dentin tubules were empty(figure 1(e)).After aging treatment, mixed fractures of the T-15 and T-20 groups appeared at the bottom of the hybrid layer,with many fractured resin tags(solid arrow,figure 1(f))and several empty dentin tubules (figure 1(f)).

3.3.Morphology of the collagen fibrils after NaClO treatment

As illustrated in figure 2, collagen fibrils presented with various levels of degradation in each group, with increasing NaClO treatment time.

Before NaClO treatment,the collagen fibrillar network of the control group remained relatively intact,both in the dentin tubules and on the surface of the peri-and intertubular dentin(figure 2(a)).After NaClO treatment for 30 s, the integrity of the open network structures was maintained in the dentin tubules and peritubular dentin.In contrast, the collagen fibrils of intertubular dentin were degraded (figure 2(b)).Almost all of the collagen fibrils were degraded after NaClO treatment for 60 s.We observed only a few single collagen fibrils in some dentin tubules (finger, figure 2(c)).After NaClO treatment for 120 s, the collagen fibrils were completely degraded.In those cases, we observed larger and funnel-shaped tubular orifices with many lateral branches on the dentin surface (figure 2(d)).

Before and after NaClO treatment for 30 s, the T-5 and T-10 groups had a similar collagen fibrillar network as the control group without NaClO treatment(figures 2(e)and(f)).Treatment with NaClO for 60 s maintained the integrity of the network structures in the dentin tubules and peritubular dentin(figure 2(g)).However, after NaClO treatment for 120 s, the collagen fibrils were further degraded.Furthermore, the undegraded collagen fibrils became fractured or curly (solid arrow, figure 2(h)).

Before NaClO treatment, there were some cap-shaped collagen clumps at the top of the tubule orifices in both the T-15 and T-20 groups (hollow arrow, figure 2(i)).After NaClO treatment for 30 s or 60 s, the collagen fibrils in the dentin tubules became fractured and decreased significantly.The cap-shaped collagen clumps remained in the dentin tubules (figures 2(j) and (k)).After NaClO treatment for 120 s, only a few cap-shaped collagen clumps could still be observed at the top of tubule orifices (figure 2(l)).

4.Discussion

The present study shows that nonthermal helium plasma treatment induces significant effects on the micro-tensile bond strength, failure modes, and collagen-degradation resistance.Thus, our null hypothesis should be rejected.

Figure 1.Representative FE-SEM micrographs of the debonded interfaces of specimens with different treatment: (a) specimen in control group without thermocycling,(b)specimen in control group with thermocycling, (c)specimen in T-5 group without thermocycling,(d) specimen in T-5 group with thermocycling, (e) specimen in T-20 group without thermocycling, (f) specimen in T-20 group with thermocycling.Ar: adhesive resin.Hy: hybrid layer.Solid arrow: fractured resin tags.Hollow arrow: empty dentin tubules.Finger: not well-infiltrated dentin tubules.Asterisk: intertubular dentin.

Gas-discharge plasma is an electrically neutral, highly ionized gas,which is produced by a high-frequency and highpressure electric field.It is composed of charged particles,ions, excited atoms and molecules, free radicals, active oxygen, ultraviolet (UV) and visible radiation, as well as chemically reactive neutral particles [15, 16].Unlike conventional plasma, NTAPP can discharge in atmospheric conditions and generate plasma jets at room temperature [7],which allows for the exposure of plasma treatment to living tissues [17].Current research into NTAPP treatment in dentistry includes sterilization, the surface modification of prosthetic materials, teeth whitening, periodontal treatment, and dentin bonding [7, 18-20].

Figure 2.Representative FE-SEM micrographs of the collagen fibrils treated with NaClO for various times: (a) the specimen in the control group without NaClO treatment, (b)-(d) specimens in the control group treated with NaClO for 30, 60 and 120 s respectively, (e) the specimen in the T-5 group without NaClO treatment, (f)-(h) specimens in the T-5 group treated with NaClO for 30, 60 and 120 s respectively,(i)the specimen in the T-20 group without NaClO treatment,(j)-(l)specimens in the T-20 group treated with NaClO for 30,60 and 120 s respectively.Solid arrow: fractured collagen fibrils.Hollow arrow: ‘cap-shaped’ collagen clumps.Finger: single collagen fibril.

According to our MTBS results, NTAPP treatment effectively increases the immediate dentin bonding strength and durability with an etch-and-rinse adhesive system(Single Bond 2), which is in accordance with the previous studies[6,21,22].The water contact angle values of dentin decrease dramatically after plasma treatment.This phenomenon may happen since the nonthermal plasma improves the hydrophilicity and wettability of the dentin surface.Therefore,NTAPP can be used to enhance the penetration of adhesive at the interface with dentin [23, 24].In this study, the fracture mode results of both the MTBS test and the FE-SEM examination confirm this mechanism.The FE-SEM images showed that the dentin tubules were filled completely with resin tags in the NTAPP treatment groups.However, we saw interspace between the resin tags and the dentin tubules in the control group.With optimal wettability, plasma treatment induced a more uniform hybrid layer and better interface integrity [24, 25].After thermocycling aging, the MTBS of the control group decreased significantly, but no significant decrease was observed in the T-5 and T-10 groups.The uniform and integral hybrid layer on the dentin-adhesive interface after NTAPP treatment was also observed in the morphology examination.In the T-5 and T-10 groups,we did not detect the dentin-adhesive failure mode.Furthermore,the hybrid layer was denser and more durable than that in the control group (figures 1(a)-(d)), irrespective of whether thermocycling aging was performed.

The intrinsic mechanism behind the finding that nonthermal plasma treatment increases the bonding strength and durability of the dentin-adhesive interface, may involve a chemical interaction between the adhesive monomers and the dentin surface [6, 10].Previous work suggested that nonthermal plasma treatment can introduce new functionalities,such as carboxyl and carbonyl groups, onto the collagen fibrils [6, 21, 24].It was hypothesized that the addition of carbonyl groups to the dentin surface could increase the hydrogen-bonding interactions of the collagen fibrils with the adhesive.Moreover, Ritts et al revealed another potential mechanism [6]: repulsive electrical forces from the increased number of carbonyl groups may cause the collagen fibrils to be partially separated into smaller fibril aggregates or individual fibrils.In addition, the antibacterial effect of NTAPP should be taken into account [20].Residual bacteria after tooth preparation might be able to survive for two years in the interface between tooth and restoration.This could reduce the microphyte-induced degeneration of the adhesive interface by NTAPP treatment.Therefore, NTAPP treatment could provide a highly efficient way to improve the penetration of adhesive into collagen fibrils and increase the interface interactions between the adhesive components and collagen.Consequently, it would dramatically enhance the bonding strength and durability of the dentin-adhesive interface.

To our knowledge, this is the first study to evaluate the NTAPP treatment effect on the degradation resistance of collagen under a NaClO challenge.NaClO has a non-specific effect on protein degradation [26].In the present study,the degradation resistance of NTAPP-treated collagen significantly increased in NaClO.As shown by FE-SEM(figure 2), the collagen of the control group was almost completely removed from the mineralized dentin after NaClO treatment for 60 s.In addition,after treatment for 120 s,larger‘funnel-shaped’tubular orifices with many lateral branches on the dentin surface were observed in the control group.This was due to the loss of demineralized peritubular dentin caused by NaClO.In contrast, collagen fibrils of the T-5 and T-10 groups presented well-formed open networks after NaClO treatment for 60 s.Moreover, many fractured collagen fibrils remained in the dentin tubules after treatment for 120 s.These results indicate that NTAPP treatment might improve the strength of dentin collagen.This is probably attributable to a plasma-induced crosslinking effect on the dentin collagen.NTAPP contains highly reactive components, such as electrons, ions, radicals, and UV radiation.Several of these components might play a role in plasma-induced crosslinking[27, 28].First, the UV radiation in the plasma could result in gelatin intermolecular crosslinking and the formatting of radicals on the aromatic residues of collagen and gelatin amino acids, such as tyrosine and phenylalanine [29, 30].Second, the radicals generated by the plasma might interact with the OH species, which could lead to the formation of hydroxyl compounds on the polymeric fibers.In turn, this may increase the number of crosslinking junctions through hydrogen bonding[31].Furthermore, the electrons of plasma also interacted with the substrate and formed free radicals in polymer chains,provoking the nearly instantaneous formation of radicals in the polymeric chains and initiating crosslinking [27].

NTAPP treatment may be regarded as a double-edged sword.On one hand,with optimal operating conditions,it can effectively increase the dentin-bonding strength, bonding durability, and the mechanical strength of collagen.On the other hand, prolonged dentin surface exposure to plasma can decrease the bonding strength and change the surface morphology of demineralized dentin.In the present study,we demonstrate the adverse effect of NTAPP treatment in the T-15 and T-20 groups.First, the MTBS values of the T-15 and T-20 groups were significantly lower than those of the T-5 group and dramatically decreased after aging.Second,after the aging procedure, the characteristic pattern of adhesive failure presented at the bottom of the hybrid layer with many fractured resin tags and a few empty dentin tubules.Third, the dentin-adhesive failure mode was observed regardless of the aging procedure used and increased after aging.When the performance of adhesive interface decreased as dentin collagen degraded, the adhesive failure modes would significantly increase [32].These results indicate that NTAPP treatment for more than 15 s harms the demineralized dentin surface and leads to the performance degradation of the hybrid layer.Furthermore,in the T-15 and T-20 groups,some cap-shaped denatured collagens were observed at the top of the tubule orifices before NaClO treatment.The demineralized collagen fibrils are thermally sensitive biomaterial.The protein denaturation temperature is around 40 °C [33, 34].Our previous study demonstrated that energy could be deposited from the external circuit during discharges [35].Therefore, excessive thermal accumulation in the T-15 and T-20 groups might result in thermal melting and degeneration of the demineralized collagen fibrils.This is probably because plasma treatment can remove organic matter by breaking the C-C and C-H bonds [24].In addition, the high-energy particle beams of NTAPP and the high gas flow rate should also be taken into account in the destruction of collagen fibrils.

Although NTAPP treatment showed some adverse effects in the T-15 and T-20 groups, the MTBS values were still significantly higher than those of the control group,regardless of aging.These results indicate that the modification effect of NTAPP,rather than its melting effect,played the leading role in this study.This was confirmed in the FE-SEM images.When treated for more than 15 s, empty dentin tubules at the fractured interface might result from the cap-shaped aggregates, which can prevent the adhesive from infiltrating into the dentin tubules(figure 1(e)).However,the other resin tags bond firmly with the dentin tubules.After aging, there were many fractured resin tags in the T-15 or T-20 groups and empty dentin tubules in the control group.This phenomenon suggests that NTAPP treatment mainly promotes the penetration of the adhesive and stimulates the bonding between the adhesive and demineralized dentine.Even though collagens were melted to cap-shaped aggregates, they could still resist the dissolving effect of NaClO after treatment for 120 s.This demonstrates that NTAPP treatment mainly reinforces the mechanical strength of the dentin collagen.Prolonged exposure of the dentin surface to plasma can decrease the immediate dentin-bonding strength.However,the stability of the adhesive interface was still better than that of the control group.

In this study,the optimal time for treatment was 5 s.This is shorter than those in several previous studies(10-30 s)[6].The input power(8 W)may account for this result,as we used a higher power than those in previous studies(3-5 W)[6,36].Thus, the effective treatment time was shortened.To reduce the adverse effects of NTAPP, we increased the treatment distance (30 mm) from the nozzle, which was further away than those in previous studies (5-10 mm) [9, 21].Since the demineralized collagen fibrillar network would collapse if it became too dry, we used water to prevent this from happening.In this study, water was immediately applied to the dentin surface after NTAPP treatment.

5.Conclusion

Under optimal in vitro conditions, NTAPP treatment is effective in enhancing the dentin-bonding strength, improving the durability of the dentin-adhesive interface with the Single Bond 2 system, and modifying the degradation resistance of collagen under NaClO challenge.In this study,the optimal treatment time was 5 s.Further studies are required to evaluate the crosslinking mechanism of NTAPP on dentin collagen.

Acknowledgments

This work was supported by grants from National Natural Science Foundation of China (Nos.81701014, 81801310,31700076), the Basic Research of Natural Science Project funded by the Department of Science and Technology of Shaanxi Province (No.2017JM8038), and the Science and Technology Project funded by the Science and Technology Bureau of Weiyang District, Xi’an city (No.201846).We thank Xiaoyan WEI for operating the scanning electron microscope at the laboratory of Xi’an Center of China Geological Survey (Xi’an, China).