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Surfactin application for a short period (10/20 s) increases the surface wettability of sound dentin

  • Larissa Marcelino
  • Julia Puppin-Rontani
  • François Coutte
  • M. Terêsa Machini
  • Augusto EtchegarayEmail author
  • Regina Maria Puppin-Rontani
Short Communication
  • 33 Downloads

Abstract

The aim of this study was to evaluate the effect of spreading the lipopeptide surfactin, for short time (10/20 s), on dentin wettability. Study groups were surfactin: 2.8; 1.4; 0.7; 0.35; and 0.175 mg/mL and a control group that received no treatment. Dentin discs (4 mm height) were prepared and polished with 600-grit SiC paper. Contact angle determinations were carried out after microbrush spreading of surfactin on dentin specimens for, respectively, 10 and 20 s. Excess liquid was removed, and after 60 s, the specimens were analyzed in a goniometer using the sessile drop method to measure the contact angle. Results were analyzed by two-way ANOVA (concentration × time) and t student, with α = 0.05. Lower contact angles were obtained for surfactin (0.7 mg/mL) spread for 10 s. However, no statistical difference was observed for surfactin (2.8 mg/mL) applied during 20 s. Higher contact angles were observed for surfactin (0.7 mg/mL) spread for 20 s. In conclusion, dentin wettability is dependent on spreading time and surfactin concentration.

Keywords

Dentin Contact angle Wettability Surfactin 

Introduction

Tooth is composed of mineral and organic materials, which are prone to demineralization and decay by the metabolism of pathogenic microorganisms, present in plaque biofilm. The process is intensified by fermentable carbohydrates and by lack of dental hygiene (Sbordone and Bortolaia 2003; Rangreez and Mobin 2019). In the case of cavities, there are different restorative protocols. For instance, enamel restoration tends to be more efficient than dentine restoration, due to the hydrophobic characteristics of the restorative material (Sofan et al. 2017). Dentin presents variable composition, from 65 to 70% minerals, and it has higher content of protein (collagen) and water when compared to enamel (Zhang et al. 2014). In addition, there are calcium/zinc-dependent collagenases that are activated in the presence of acid, a fact that also contribute to restorations’ failure, since these depend on the perfect establishment of a hybrid layer, formed between dentin and an adhesive (Mazzoni et al. 2015). Therefore, restorations on dentin are more prone to failure when compared to enamel restorations (Sofan et al. 2017).

In recent years, there is a tendency for minimal invasive methods, based on restorative materials obtained by in situ polymerization of an adhesive and resin (Frencken et al. 2012; Sofan et al. 2017). In dental restorations for optimum adhesion, the monomers of an adhesive system need to penetrate the collagen structure that has been exposed by acid treatment (Tjäderhane et al. 2013). It is important that all spaces originally filled by minerals be filled with the adhesive and not by water. If there is water (not removed) or yet infiltrated, the recently formed polymer will be degraded by hydrolysis (Carvalho et al. 1999; Matos et al. 2017). The hybrid layer is formed between collagen and the adhesive, before conjugation to a resin (Liu et al. 2011). Hybrid layer formation benefits from a surface that is not excessively wet or dry, allowing the penetration of the adhesive system through an exposed collagen, before the polymerization reaction (Tsujimoto et al. 2018). Dentin’s surface has a contact angle, which depends on surface energy. Acid conditioning contributes to wettability; however, it induces higher demineralization and reduces the chance for complete percolation of the adhesive (Namen et al. 2011). To increase surface’s energy and wettability, it is recommended to use surface active substances as the surfactants, which can modify the surface polarity (Ivanova and Starov 2011).

Surfactants are used in dental care. The most commonly used examples are the anionic sodium dodecyl sulphate (SDS), widely used in dentifrices and cetyltrimethylammonium chloride (CPC), used in mouthwash (Chen and Wang 2010). In clinical practice, there are protocols using cetrimide for root canal cleaning (María Ferrer-Luque et al. 2014) and demineralization combined with EDTA. The recommended concentration for cetrimide is below 0.25%, as the combination leads to increased demineralization (Akcay and Sen 2012).

Contact angle measurements for dentin that was treated with surfactants have been evaluated by Iglesias et al. (2019) using canine teeth. The authors used cetrimide (0.2 and 0.1%) and benzalkonium chloride (BZC) (0.008%) in sodium hypochlorite solution and obtained very small contact angles, using the water drop assay (Iglesias et al. 2019). However, the time of surfactant treatment was not informed. It is deduced that dentin specimens were imbibed in surfactant solutions. Cetrimide, CPC, and BZC are cationic surfactants, which have intrinsic antimicrobial activities given the amphipathic character and the presence of a positive charge at the polar moiety (Liu et al. 2019). The antimicrobial property of BZC leads to the development of an adhesive system that includes this surfactant within the monomer composition (Zhou et al. 2019). Surfactants have also been used in other adhesive systems, as in the case of cetrimide and the quaternary amine containing monomer 12-methacryloyloxydodecylpyridinium bromide (MDPB) (Rangreez and Mobin 2019). In addition, surfactants have been used to facilitate the diffusion of adhesives and thus contribute to greater stability of the dentin-adhesive hybrid layer (Tjäderhane et al. 2013). However, all these surfactants are toxic and may introduce the risk of allergic reactions. A not yet explored and candidate for dentin-surface conditioning is surfactin, a biodegradable surfactant that has low toxicity. In addition to having important biological properties, such as anti-inflammatory, antitumoral, and antiviral properties, the no-observed-adverse-effect level (NOAEL) of surfactin is 500 mg/kg in rats (Wu et al. 2017). In addition, it has been shown to present synergic effect against Streptococcus mutans and Candida albicans when used concomitantly with the antimicrobial natural product terpinen-4-ol (Bucci et al. 2018). Therefore, the aim of this work was to evaluate the effect of spreading surfactin micellar solutions on flat dentin surfaces, on its surface’s wettability. As far as we know, this is the first report on the use of surfactin in dental practice, aiming at the preparation of dentin for restoration.

Materials and methods

This study was conducted after Ethical Committee board of the Pontifical Catholic University of Campinas approved the protocol under the number 2.907.654.

Surfactin (70%) was purchased from Lipofabrik, Villeneuve d’Ascq, France. All other reagents were purchased from Sigma-Aldrich (Brazil). We used deionized water, pH 7.0 throughout the experiments.

Surfactin analysis

Surfactin (3.5 mg/mL) was treated with HCl 0.1 mol/L and centrifuged (14,000 rpm) on a Hettich model 320 Universal centrifuge (Germany). The precipitate was dissolved in 100 µL of 100% methanol and 5 µL were analysed by HPLC using a Thermo Separation Products (Thermo Scientific, USA) chromatography system composed by a Constametric 3500 pump, a Constametric 3200 pump and a Spectromonitor 3100 detector coupled to a Grace-Vydac C18 analytical column (5 μm, 300 Å, 0.46 cm × 25 cm). Acetonitrile 90% and TFA 0.01% were used as solvents. Peptide identities were confirmed by liquid chromatography coupled to electrospray ionization mass spectrometry (LC–MS). LC was performed on a Shimadzu system (Kyoto, Japan) composed by a TGU-20A3 degasser, two LC-20AD pumps, an 8125 Rheodyne and a CTO20A column oven connected to a C18 pre-column (4.6 mm, 12 nm, 5 × 2 mm) Shimpack GVP-ODS and a C18 column (4.6 mm, 12 nm, 150 × 2 mm) using the instrument described above. Gradient conditions were based on the use of solvent A [0.1% v/v, trifluoroacetic acid (TFA) in water] and solvent B [90% acetonitrile/0.09% TFA/water (v/v/v)]. Gradient was from 5 to 95% of solvent B in 30 min. Sample volume used was 5 µL. Mass spectrometry analysis conditions were 4500 V for capillary voltage and 220 °C for operation temperature.

Preparation of dentin specimens

For contact angle measurements, 55 human teeth (third molars), five for each experimental group, were randomly selected based on two criteria: tooth integrity, and absence of cracks and fractures. All teeth were donated by patients after signing the respective consent form. As received, teeth were stored in 0.1% (w/v) thymol for up to 30 days. Teeth were selected based on the above criteria and manually cleaned using periodontal curette to remove organic and inorganic residues. After, the specimens were washed with deionized water and treated on ultrasonic bath for 10 min to remove remaining contamination. These samples were stored in sterile deionized water at 4 °C for up to 30 days (maximum). To obtain flat dentin surfaces, teeth were fixed on acrylic slides with the help of a sticky wax (Asfer, Piracicaba, SP, Brazil) and low-melting material (Lysanda, Vila prudente, São Paulo, SP, Brazil). To produce dentin disks, the slides were assembled on an automatic cutting machine (Isomet 1000, Buehler, Illionois, USA, using a speed of 275 rpm and weight of 250 g. To produce 4 mm discs, teeth were precisely cut (under refrigeration), between their medium thirds and the enamel-cementum junction, thus removing the root and exposing flat dentin surfaces (Fig. 1).
Fig. 1

Details of dentin disc preparation using the Isomet 1000 machine. a Acrylic slide with a tooth specimen; b slide positioned before slicing; c flat dentin after precise assembly by the cutting machine

Dentin discs were further treated on an automatic water-cooled politriz (Aropol E, Arotec, Cotia, São Paulo, Brazil) for surface polishing using 600-grit SiC paper (Carborundum abrasives, Guarulhos, SP, Brazil). Polishing was carried out for 30 s to remove any remaining enamel fragments. After, the discs were washed with a stream of water to remove the smear layer, formed upon polishing.

Microbrush spreading of surfactin

Dentin surfaces were treated with 10 µL of the respective surfactin solution at pre-determined time and concentration. Experiments were designed using in 11 study groups (n = 5), as presented in Table 1. Spreading times were of 10 and 20 s as indicated. The control group did not receive any treatment. Active spreading was carried out manually using a microbrush (KG, Sorensen, Cotia, SP, Brazil) by the same operator throughout the experiments. After the pre-determined time (10 or 20 s), the remaining solution was removed by capillarity, using absorbent paper. Contact angle measurements were carried out by the sessile drop method. After 60 s, the specimen was positioned perpendicularly to a syringe coupled to the apparatus and was analyzed for contact angle determination (left and right angles) after the addition of a water drop (2.3 µL). The assay was performed on a Digidrop goniometer (Labometric, Leira, Portugal). Images were captured and evaluated using the software GBX DIGIDROP™ (GBX, Bourg de Péage, France). For statistical analysis, an average of both contact angles (left and right), obtained in each analysis, was treated as an experimental unit.
Table 1

Surfactin study groups (n = 5)

Study group

Characteristics

1

Control—no surfactin treatment

2

Surfactin 2.8 mg/mL 10 s

3

Surfactin 2.8 mg/mL 20 s

4

Surfactin 1.4 mg/mL 10 s

5

Surfactin 1.4 mg/mL 20 s

6

Surfactin 0.7 mg/mL 10 s

7

Surfactin 0.7 mg/mL 20 s

8

Surfactin 0.35 mg/mL 10 s

9

Surfactin 0.35 mg/mL 20 s

10

Surfactin 0.175 mg/mL 10 s

11

Surfactin 0.175 mg/mL 20 s

Statistical analysis

Data from contact angle measurements were analyzed using Biostat 4.0 software. The obtained values were submitted to variance analysis (two-way) ANOVA factorial a (time, 2 levels) × b (concentration, 6 levels). Tukey test and t student were used for independent samples. t student was used to evaluate experimental data. All tests were applied considering a significance level of 5%.

Results

In this work, the determination of contact angle on dentin surface was carried out in the presence and the absence of the lipopeptide surfactin, a natural product produced by Bacillus sp., which has strong surfactant activity. Figure 2 shows the HPLC characterization of surfactin used in these experiments.
Fig. 2

HPLC and mass spectrometry profiles of surfactin used in this work. In the analysis by mass-spectrometry, there is 1 min delay between the UV and mass-spectrometry outlines (a). Three isoforms of surfactin can be identified according to their molecular ions C13 (b), C14 (c) and C15 (d)

Both the UV and mass-spectrometry outlines show the elution of surfactin isoforms at the end of gradient. There are three isoforms that differ in the size of the beta-hydroxylated fatty acid, thus C13, C14, and C15, corresponding to molecular ions m/z [M + H+] 1008, 1022, and 1036, respectively, which are eluted in the same order (de Faria et al. 2011). According to the analysis, the isoform corresponding to C14 is present in higher amounts (Fig. 2a).

The results of contact angle determinations are presented in Table 2 and Fig. 3. Anova test showed a significant interaction between time of surfactin application and concentration (P = 0.0006), indicating that the contact angle is dependent on surfactin concentration and spreading time. The results demonstrated that higher contact angles were obtained when higher spreading times and lower concentrations of surfactin were used. The smallest contact angles for dentin specimen were found when surfactin was applied for 10 s at 0.7 mg/mL or for 20 s at 2.8 mg/mL.
Table 2

Results of contact angle determinations

Surfactin concentration and the corresponding contact angles (°)

Time

2.8 mg/mL

1.4 mg/mL

0.7 mg/mL

0.35 mg/mL

0.175 mg/mL

10 s

66.2 ± 5.7 Bb

69.6 ± 2.8 Ea

56.8 ± 4.1 Aa

68.3 ± 7.7 Da

67.0 ± 4.3 Ca

20 s

58.3 ± 1.8 Aa

72.7 ± 6.3 Db

74.4 ± 4.3 Eb

67.4 ± 7.1 Bb

68.9 ± 4.8 Cb

Control

84.2 ± 3.1

    

Capital letters indicate significative difference among the contact angle values within the spreading time. Small letters indicate a significative difference for the same concentration of surfactin solution

Fig. 3

Evaluation of contact angle for the experimental groups using the t student`s test. Data represent mean values (n = 5) ± SD. Differences were considered significant when P < 0.001 (*)

Figure 4 shows representative images obtained during contact angle determinations for the control group and for samples that produced the lowest values, according to the time of application and surfactin concentration. The results demonstrate that surfactin-treated specimen presented better wettability.
Fig. 4

Contact angle analysis of selected specimens, a control and b and c surfactin-spread dentin discs, as indicated

Discussion

This work presents an alternative approach to restorative dentistry. It is based on the use of surfactin, a non-toxic biosurfactant that could be added to increase surface’s wettability, before application of the adhesive system. The aim of adhesive restorations is to promote an intimate contact between restorative materials and dental tissues. Considering that the final restoration will stand in the oral cavity, to attain a stable and robust dental filling, the adhesive system must be able to flow and wet the adherent surface to form a stable hybrid layer (Sofan et al. 2017).

In this work, specimen preparation did not differ substantially from restorative protocols, considering the potentially dry environment generated after exposing the dentin surface. Thus, sample preparation was based on removal of the crown, followed by a polishing step and removal of smear layer. Under these conditions, the dentin loses internal fluid, leading to surface dehydration and risk to lose wettability (Zhang et al. 2009). The addition of surfactant promotes hydration by modification of surface energy (Iglesias et al. 2019). We have intentionally carried out surface treatment for short periods (10 or 20 s) and solutions were purposely spread on flat and sound dentin surfaces, using a microbrush technique (Jordão-Basso et al. 2016). The experiment was designed considering a potential future application at clinical practice, which requires the use of adequate and timely planned protocols. The use of lipopeptides to modify different surfaces (for instance, Teflon and stainless-steel) has been studied before (Shakerifard et al. 2009). The authors used the three lipopeptides produced by Bacillus sp. (surfactin, fengycin, and iturin) and verified that the effects on surface characteristics vary from hydrophobic to hydrophilic and are not entirely dependent on surfactant concentration. Here, we verified that the treatment on flat dentin surface, by the spreading technique, is dependent on treatment time and surfactin concentration. However, all surface treatment resulted in lower contact angles (Table 2 and Fig. 3). The work demonstrated that, instead of using cetrimide, SDS, CPC or BCK that are synthetic surfactants, which may present allergic reactions (Wong and Watson 2001), it might be more appropriate to use surfactin, which is non-toxic and has other important biological properties (Wu et al. 2017).

Surfactin presents high superficial activity, low-toxicity, and affinity with protein surfaces (Zhang et al. 2017). The structure of these lipopeptides is characterized by a lactone ring composed of seven aminoacids, linked by an ester between the C-terminal carboxyl of the last amino acid and the hydroxyl side chain of a beta-hydroxylated fatty acid (Jacques 2011). Within the structure of surfactin, there are two negative charges, respectively, from the N-terminal glutamic acid and from the fifth residue (aspartic acid), which allow a perfect interaction with calcium ions (Bastrzyk et al. 2019). All these properties, including the reported antitumoral and anti-inflammatory properties of surfactins, inspired us to evaluate its potential applications for surface wettability in dentin restoration.

Surfactin has a critical micellar concentration (CMC) in water of 0.008 mg/mL (Heerklotz and Seelig 2001). Therefore, the solutions used in this work are micellar solutions, since the concentrations of surfactin used were from 2.8 to 0.175 mg/mL. In dental practice, micellar solutions of surfactants are normally used between 0.1 and 0.2%. The highest value of 2.8 mg/mL (0.28% w/v) was based on its low CMC and on the literature, considering the use of benzalkonium chloride (Tjäderhane et al. 2013). The authors used a high concentration (5 mg/mL) of BCK, although the CMC is variable (between 0.068 and 0.346 mmol/L). In this work, we have not used surfactin concentrations higher than 2.8 mg/mL since surfactin, although reported as non-toxic, especially when compared to other surfactants, shows hemolytic activity (Kracht et al. 1999). This biosurfactant finds various applications: from environmental remediation to chemical and pharmaceutical uses (Jacques 2011). In dental research, it has been demonstrated a synergic effect of surfactin on the antimicrobial properties of terpinen-4-ol (Bucci et al. 2018). However, surfactin has never been studied in dental restoration before, for instance, to improve dentin wettability. Wettability is a significant factor that influences the adhesion of universal adhesive systems, using the etch and rinse mode (Tsujimoto et al. 2018). This study presents an interesting alternative to optimize the wettability of dentin, which was verified by the measurement of contact angle of biosurfactant-modified dentin surfaces. There are no previous reports in the literature on the application of surfactin to improve adhesion in dental restoration. Therefore, the present work brings this innovative option. Moreover, surfactin found potential use in clinical applications. The experiments were designed for short spreading-time (10 and 20 s) and using microbrush spreading.

Conclusion

All surfactin solutions produced a more hydrophilic substrate when compared to the non-treated control. Therefore, the short spreading time of surfactin solutions has contributed for the wettability of dentin. This finding was confirmed by the small contact angles obtained for all specimens.

Notes

Acknowledgements

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.– (Master's scholarship to Marcelino, L) and by the Fundação de Amparo à Pesquisa de São Paulo (Fapesp 2015/14360-4, to Machini, MT). The authors would like to thank Dr. Cleber W. Liria for the help in mass spectrometry analysis and Dr. Sérgio L. Pinheiro for important suggestions to this study.

Compliance with ethical standards

Conflict of interest

The authors declare no conflict of interest.

Ethical approval

This work was approved by the Ethics Committee in Research with Humans of the Pontifical Catholic University of Campinas, under the number 2.907.654.

Informed consent

The free informed consent form for specimen donation was signed by all participants in this research.

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Copyright information

© Springer-Verlag GmbH Austria, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Center for Life Sciences, Postgraduate Program in Health SciencesPontifical Catholic University of Campinas (PUC-Campinas)CampinasBrazil
  2. 2.Department of Operative Dentistry, Piracicaba Dental SchoolUniversity of CampinasPiracicabaBrazil
  3. 3.Department of Pediatric Dentistry, Piracicaba Dental SchoolUniversity of CampinasPiracicabaBrazil
  4. 4.Univ. Lille, INRA, ISA, Univ. Artois, Univ. Littoral Côte d’Opale, EA 7394-ICV Institut Charles ViolletteLilleFrance
  5. 5.Department of Biochemistry, Institute of ChemistryUniversity of São PauloSão PauloBrazil

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