d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 702–707
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Contact angle and surface free energy of experimental resin-based dental restorative materials after chewing simulation Stefan Rüttermann, Thomas Beikler, Ralf Janda ∗ Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstraße 5, D-40225 Düsseldorf, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate contact angle and surface free energy of experimental dental resin
Received 2 September 2013
composites containing novel delivery systems of polymeric hollow beads and low-surface
Received in revised form
tension agents after chewing simulation test.
23 January 2014
Methods. A delivery system of novel polymeric hollow beads differently loaded with two
Accepted 25 March 2014
low-surface tension agents was used in different amounts to modify commonly formulated experimental dental resin composites. The non-modified resin was used as standard. Surface roughness Ra , contact angle , total surface free energy S , its apolar SLW , polar SAB ,
Keywords:
Lewis acid S+ and base S− terms were determined and the results prior to and after chewing
Dental resin composite
simulation test were compared. Significance was p < 0.05.
Chewing simulation
Results. After chewing simulation Ra increased, decreased, Ra increased for two test mate-
Contact angle
rials and S decreased or remained constant for the standard or the test materials after
Surface free energy
chewing simulation. Ra of one test material was higher than of the standard, and S of the test materials remained lower than of the standard and, indicating their highly hydrophobic character ( ≈ 60–75◦ , S ≈ 30 mJ m−2 ). SLW , and S− of the test materials were lower than of
the standard. Some of the test materials had lower SAB and S+ than of the standard.
Significance. Delivery systems based on novel polymeric hollow beads highly loaded with lowsurface tension agents were found to significantly increase contact angle and thus to reduce surface free energy of experimental dental resin composites prior to and after chewing simulation test. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Contact angle and surface free energy (SFE) are discussed to influence plaque formation on modern dental filling composites [1–5]. Since there is a positive correlation between SFE and
early plaque formation [5,6], dental resin composites differing in contact angle were broadly investigated and bacterial adherence of materials with high and low SFE was found to be significantly less [2,7–11]. But it was also reported that no correlation occurred between streptococcal adhesion and
∗ Corresponding author at: Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstraße 5, Geb. 18.13, D-40225 Düsseldorf, Germany. Tel.: +49 6723 6020 750; fax: +49 6128 48 04 35. E-mail addresses: [email protected], [email protected] (R. Janda).
http://dx.doi.org/10.1016/j.dental.2014.03.009 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 702–707
substratum surface roughness or SFE [12–14] and it was realized that the influence of SFE on bacterial adhesion decreased significantly after saliva coating [14,15]. The present work is related to earlier investigations reporting an entirely new approach to obtain resin materials with low SFE but acceptable physical properties [16] which also proved diminished bacterial adherence [10]. Both papers reported about a new delivery system of polymer hollow beads (Poly-Pore, AMCOL Health & Beauty Solutions, Arlington Heights, IL, USA) as carrier material, highly loaded with two different low-surface tension agents (Tego Protect 5000, Evonik Tego Chemie GmbH Essen, Germany and Dimethicone, Dow Corning Corp., Midland, MI, USA) which were added in small quantities of 5–6 wt.% to experimental dental resin composites. It was found that due to abrasion processes, simulated by polishing, the carrier hollow beads were destroyed and released the low-surface tension agents which flushed the surface and thus reduced the resin’s SFE [16]. Now the effect of the exposition of the low-surface tension agents on SFE was re-investigated under more clinically relevant conditions with a chewing simulation device. To simplify the comparison with the previous papers [10,16] the selected experimental resins were coded accordingly. The null hypothesis was that the SFE of the delivery system containing material did not differ from the standard after performing the chewing simulation test.
2.
Materials and methods
2.1.
Test material preparation (analog [16])
Four experimental light-curing resin-based restorative materials (A, B, C and E) were prepared (Tables 1 and 2) using a laboratory vacuum planet kneader (Herbst Maschinenfabrik GmbH, Buxtehude, Germany). Material ST, representing a common formulation for dental resin composites, was used as the standard. The preparation process of all test materials and ST was done under vacuum at 50 ◦ C. Preparation of the test materials: Poly-Pore was loaded with two different active agents (Tego Protect 5000, Evonik Tego Chemie GmbH or Dimethicone, Dow Corning Corp.) in varying quantities of the agents resulting in four different delivery systems (Table 2). The delivery systems of materials A and B contained Tego Protect 5000, and materials C and E contained Dimethicone as the active agent. ST was modified by replacing 5% or 6% of the glass filler with the respective delivery system. At first the monomers were mixed, next all other ingredients but the glass filler and the Poly-Pore delivery systems were added and dissolved in the monomer mix. Then the respective Poly-Pore delivery system and finally the glass filler were homogeneously dispersed in the mixture.
2.2. Preparation of the delivery systems (analog [10,16]) To obtain the four delivery systems (Tables 1 and 2) 80 g or 85.7 g, respectively, Dimethicone or Tego Protect 5000 were dissolved in 400 g butanone (Lot 244238, Brenntag GmbH, Mülheim, Germany), then 20 g or 14.3 g, respectively, PolyPore hollow beads were added. The mixture was stirred
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for approximately 10 minutes to optimally wet the sorption particles. The mixture was slightly warmed to evaporate the solvent, and stirring was stopped when it became too stiff. Then it was put in a drying closet at 50 ◦ C until a constant weight was obtained (generally 24 h). After this treatment, the loaded sorption materials, representing the delivery systems, appeared totally dry and powdery.
2.3.
Specimen preparation and chewing simulation
Ten disks (thickness: 1 ± 0.1 mm, diameter: 10 ± 0.1 mm) of each material were made and cured for 40 s on each side (Spectrum 800, Dentsply deTrey GmbH, Constance, Germany). The output of the curing device was routinely checked (bluephase meter, Ivoclar Vivadent AG, Schaan, Liechtenstein). Irradiances of 931 ± 90 mW cm−2 were measured, and no significant decrease in the output was observed. The cured specimens were stored for 14 days in water at 37 ◦ C and then one side was wet-polished (sterile water) with fine and superfine polishing discs (Super-Snap mini, Shofu Inc., Kyoto, Japan) for 1 min, with a grinding pressure of 40–50 g and 10,000 rpm (Endo-Mate TC, Nakanishi Inc., Tochigi, Japan). The degree of polymerization was not measured but the high irradiance of the light curing device ensures an optimal polymerization. Next the specimens were fixed with the polished sides up in the mounts of a chewing simulator (CS-4, SD Mechatronik GmbH, Feldkirchen-Westerham, Germany) with a MMA/PMMA resin (Technovit 4000, Heraeus Kulzer GmbH, Hanau, Germany). They were stressed for 650 cycles in water at 37 ◦ C to simulate a 24 h chewing period according to the literature, which correlates the number of cycles to a realistic period of time [17,18]. The 24 h were chosen to test the daily availability of the active agents. The test specimens were sliding under contact to a spherical antagonist (aluminum oxide, 5 mm diameter, Quick-Ohm Küpper & Co. GmbH, Wuppertal, Germany) for a linear distance of 8 mm (speed: 40 mm s−1 ) and a vertical load of 50 N resulting in a contact area of 0.6 mm × 8 mm and an average wear of 5.4 m (±2.5 m). This was repeated five times resulting in a worn area of 3 mm × 8 mm = 24 mm−2 to obtain sufficient surface to place a drop.
2.4. Contact angle and surface free energy (analog [10,16]) Contact angles were determined with the sessile drop method (Phoenix-Alpha contact angle goniometer, Surface Electro Optics – SEO – Corporation, Suwon-City, Korea). Immediately after placing the drop on the specimen’s surface ten pictures of each drop were photographed (magnification: 25-folds) in one second (CCD-camera: FireDragon, Toshiba-Teli Corporation, Tokyo, Japan) and analyzed (Image XP-software, Version 6.0 FW 012108, SEO Corporation). The drops were formed by a standardized dosing tube and, therefore, had always the same volume. Contact angle was measured one time on each of the ten disks. SFE was calculated from the measured after water storage. Apolar Lifshitz-van der Waals LLW , polar Lewis acid L+ and Lewis base L− terms of the test liquids were taken from the literature [19] and the specimens’ total SFE S , their apolar term SLW , polar term SAB , acid term S+ and base term S− were calculated according to the equation of van Oss et al.
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Table 1 – Raw materials. Code
Product/properties
Photoinitiator Stabilizer TTEGDMA
UV-stabilizer UDMA
Bis-GMA
CQ Amine Glass
Poly-
Tego
Dimeth Poly-Dimeth Poly-Tego
Batch
Company
␣,␣-Dimethoxy-␣-phenylacetophenone Pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] Tetraethyleneglycole dimethacrylate, standard monomer, functionality = 2, MW = 330 [g mol−1 ], good chemical and physical properties, very low viscosity (14 Pa s, 25 ◦ C), diluting 2-Hydroxy-4-methoxy-bezophenone 7,7,9-Trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecan1,16-diol-dimethacrylate, standard monomer, functionality = 2, MW = 471 [g mol−1 ], flexible, tough, very good chemical resistance, medium viscosity (10,000 mPa s, 25 ◦ C) Bis-GMA, standard monomer, functionality = 2, MW = 513 [g mol−1 ], rigid, very good chemical resistance, very high viscosity (4500 mPa s, 60 ◦ C) d,l-Camphorquinone Ethyl-4-(dimethylamino)-benzoate Strontiumborosilicate glass (Glass G0 18-093, d50 = 0.7 m), silaned (3-methacryloyloxypropyltrimethoxy silane), D = 2.6 [g cm−3 ] Poly-Pore, cross-linked polyallyl methacrylate, adsorber, hollow beads, diameter 20–40 m
0066162S 26099IC3
Ciba Specialities Chemical Inc., Basel, Switzerland
J1620
Cray Valley, Paris, France
411351/143302 330503057
Fluka, Buchs, Switzerland Rahn AG, Zürich, Switzerland
Tego Protect 5000, hydroxyfunctional polydimethylsiloxane, hydro- and oleophobic, D = 1.05 [g cm−3 ], surface tension L = 17.8 (0.7) mJ m−2 [16] Dimethicone 200/350 cst, polydimethylsiloxane, D = 0.965 [g cm−3 ], surface tension L = 16.6 (0.5) mJ m−2 [16] Poly-Pore loaded with 80% and 85.7% Dimethicone, D = 1.0 [g cm−3 ] Poly-Pore loaded with 80% and 85.7% Tego Protect 5000, D = 1.0 [g cm−3 ]
ES57608918
2008218303
0148990002 310170 Lab14701
L07070303AB
4962250 Experimental products
Schott Electronic Packaging GmbH, Landshut, Germany AMCOL Health & beauty Solutions, Arlington Heights, IL, USA Evonik Tego Chemie GmbH, Essen, Germany Dow Corning Corp., Midland, MI, USA University laboratory
Information is based on the manufacturers’ technical data sheets.
[20] by (cos + 1) × L = 2( SAB
(S+ S− )
(SLW LLW ) +
(S− L+ ) +
(S+ L− ))
and with =2 (Image XP-software). Prior to the chewing simulation the contact angles of specimens were measured with aqua dest. (laboratory product), glycerol (LOT II1097071, Thermo Scientific, Rockford, IL, USA), ethylene glycol (LOT 3289749, Carl Roth GmbH + Co. KG, Karlsruhe, Germany), and diiodomethane (LOT S82251, Sigma Aldrich GmbH, Taufkirchen, Germany). Chewing simulation with 650 cycles was done and was measured with aqua dest. After drying with oil-free air was measured again with glycerol, ethylene glycol and diiodomethane. After each organic liquid’s measurement it was wiped off with alcohol and another 650 wear cycles were performed for each liquid to assure a clean surface.
2.5.
Surface roughness Ra (analog [16])
Surface roughness data of the materials prior to a chewing simulation test have already been published [16]. After chewing simulation and contact angle measurement the specimens’ surface roughness Ra was determined again (Surftest SJ-210 profilometer, diamond pick-up, tip radius 5 m, load 4 mN, Mitutoyo Corporation, Kawasaki, Japan). Each specimen was measured five times at different distances and in different directions (evaluation length 4 mm, stylus speed 0.5 mm s−1 ) in the wear area. For each measurement the stylus was automatically moved five times forward and backward along the same path. The data were filtered with a cut-off (c ) of 0.8 mm (Gauss profile filter) following ISO 4288.
Table 2 – Formulation in wt.% of the experimental resin-based restorative materials and the standard ST). ST
A
B
C
E
Raw material
73.00 – – – – 27.00
68.00 5.00 – – – 27.00
67.50 – 6.00 – – 26.50
68.20 – – 5.00 – 26.80
67.50 – – – 6.00 26.50
Glass Poly-Tego 80% Poly-Tego 85.7% Poly-Dimeth 80% Poly-Dimeth 85.7% Matrix
0
4.00
5.14
4.00
5.14
Active agent
705
After
45.9 (7.2) 32.2 (11.1)1,2 39.5 (12.0)1,3 19.5 (14.0)2 25.0 (12.5)2,3
Before
16.2 (2.8) 1.3 (0.6)1 1.6 (0.4)1 0.5 (0.4)1 0.5 (0.3)1
After
0.1 (0.1)1 0.6 (0.5)1,2 2.6 (2.7)2 1.6 (1.9)1,2 1.8 (1.4)1,2
S−
Table 4 – Means and (standard deviations) of surface roughness Ra . Underlined values within the rows indicate non significant differences (p > 0.05). The same subscript number within the columns indicates non significant differences between the materials. The before values were already published in Ruettermann et al. [16]. Material
Ra before [m]
ST A B C E
2.6.
0.15 (0.02)1 0.17(0.05)1 0.17 (0.04)1 0.23 (0.03)2 0.19 (0.04)1,2
Ra after [m] 0.16 (0.10)1 0.19 (0.08)1 0.21 (0.08)1 0.38 (0.15)1,2 0.60 (0.53)2
Statistical analysis
Before
0.4 (0.2)1 1.8 (0.7)2 1.6 (0.5)2 0.4 (0.4)1 0.3 (0.2)1
After
−0.4 (2.7)1 −0.8 (3.7)1 −12.6 (11.5)2 −7.6 (4.4)1,2 −11.1 (5.4)2
Before
3.
S , total surface free energy of the specimen. SAB , polar term of the specimens free surface energy. SLW , apolar term of the specimens free surface energy. S+ , acid term of the specimens free surface energy. S− , base term of the specimens free surface energy.
44.8 (3.6) 33.1 (3.2)1 19.5 (12.6)2 28.5 (6.0)1,2 22.6 (5.4)1,2
Before After Before
43.2 (2.3) 29.9 (2.7)1 29.7 (3.4)1 29.9 (2.2)1 29.6 (2.4)1 36.9 (5.4) 59.0 (8.4)1 61.6 (9.1)1,2 73.5 (15.0)1,3 76.3 (12.7)2,3 57.6 (4.3) 95.9 (4.0)1 97.4 (2.3)1 102.9 (2.7)1,2 105.6 (1.2)2 ST A B C E
S After Before
39.5 (2.4) 32.3 (2.4)1 32.1 (3.8)1 30.7 (1.8)1 30.1 (2.5)1
SLW
After
Means and standard deviations were calculated. Normal distribution was tested by Kolmogoroff–Smirnoff-test. Multivariate ANOVA and post hoc Scheffé’s test were calculated to find differences between the test materials’ SFEs. Univariate ANOVA and post hoc Scheffé’s test were calculated to find differences between the materials’ surface roughness and a t-test for unpaired samples was accomplished to find differences prior to and after chewing simulation. Statistics were calculated using IBM SPSS Statistics Version 21.0 (IBM Corporation, Somers, NY, USA). The statistical significance for all tests was set at p < 0.05.
45.2 (2.3) 34.1 (3.8)1 32.2 (3.2)1 36.1 (3.2)1 33.7 (1.5)1
3.7 (2.0) −2.4 (1.3)1 −2.3 (1.0) −0.8 (0.7)1 −0.6 (0.4)1
S+ SAB
Surface free energy [mJ m−2 ] Contact angle [◦ ] Material
Table 3 – Means and (standard deviations) of contact angles and surface free energy, values are rounded to valid digits. The same subscript number within the columns indicates non significant differences between the materials. Underlined numbers within the rows indicate non significant differences between the values before and after chewing simulation (p > 0.05). The before values were already published in Ruettermann et al. [16].
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Results
Surface roughness values prior to chewing simulation were already reported and discussed in Ruettermann et al. [16]. Ra of ST and the materials A and B did not differ prior to and after chewing simulation. Ra of materials C and E increased after chewing simulation but only material E had higher Ra than ST (Table 4). An average material loss of 2.4 m occurred after each chewing simulation process. The worn area was always larger (0.6 mm × 8 mm) than the wetting area of the drop (maximum diameter 0.4 mm) and, therefore, the contact area of the substrate and the drop has almost no curvature. Contact angle and SFE values prior to chewing simulation (Table 3) were already reported and discussed in Ruettermann et al. [16]. After chewing simulation decreased for all materials. Except for material H, all other test materials had higher but lower S and SLW than ST. The polar term SAB of material B and E and the base term S− of materials C and E were lower than of ST. Only the acid term S+ of material B was higher than of ST.
4.
Discussion
Chewing simulation test [21], contact angle [9,16,22–24] and surface roughness [5,14,16] measurements were done according to well-established methods. The contact angles of water and of each of the three liquids were measured prior to and after chewing simulation on the same specimen because the statistical distribution of the specimen’s imperfections remained constant. Measuring each liquid on a new specimen with a different statistical distribution of the imperfections
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and combining the results in one equation might reduce the reliability of the mathematical calculation and, therefore, of the result. Saliva coating of the specimens prior to determine the surface properties with the sessile drop method is principally not possible because this method requires the direct contact of the surface and the measuring liquids. Furthermore, as we reported already earlier [16], in vivo studies proved that supragingival plaque formation, and therefore, adhesion and biofilm formation was greatest for high SFE and lowest for low SFE substrata, even when in the presence of saliva [5,6]. The carrier material Poly-Pore and the low-surface tension agents (Tego Protect 5000, Dimethicone) which were used to manufacture the delivery systems have been thoroughly described in earlier publications [10,16]. From our previous study ST and the Poly-Pore delivery system containing experimental dental resins or specimens manufactured of these, respectively, were chosen because they performed best [16]. A gradient of hollow beads in the outermost specimens’ surfaces can be excluded since they were polished with a standardized and clinically relevant method. Although of ST and all test materials decreased after chewing simulation, the test materials’ remained significantly higher than ST’s once and stayed about or even above 60◦ . The decrease of after chewing simulation might be caused by altered surface properties due to the chewing process and, considering materials C and E, by increasing Ra . But the fact that of the test materials remained significantly higher and S significantly lower after chewing proved again our findings that due to the abrasion processes the carrier hollow beads were destroyed and released the lowsurface tension agents which flushed the surface and thus reduced the resin’s SFE [16]. The renewed “floating” surface created by the low-surface tension agents over-compensated the increase of surface roughness. According to the literature [24] materials with > 65◦ are considered to be very hydrophobic. The highly hydrophobic character of the test materials was also expressed by the S -values which were found to be about 30 mJ m−2 . This value is defined for totally hydrophobic material surfaces [24]. The S -values of the materials B and E even decreased significantly below 30 mJ m−2 after chewing simulating what was certainly a consequence of their higher content of low-surface tension agents. Hydrophobic resin composite surfaces are assumed to be more resistant against attack by water or water-soluble species [7,25,26] but the relation between and bacterial adhesion has also been challenged [2,7]. However, the test materials’ and S , as they were found in the present investigation, confirmed our earlier assumption, based on a polishing process, that the low-surface tension agents were set free during the abrasion process (chewing simulation) and flushed the material’s surface to form a thin and strongly hydrophobic layer [16]. As we also hypothesized earlier [16] this layer represents a new floating discrete surface that is expected to be destroyed but also to be renewed by ubiquitous abrasive forces during the service life of the filling and we also experienced that this new floating discrete surface provided strong repulsive forces and hindered bacteria to adhere [10]. As discussed earlier [16], in vivo studies reported that biofilm formation was greatest for high S and lowest for low
S surfaces even in the presence of saliva [5,6]. Therefore, based on the results of the present study it is very likely that biofilm formation will be hindered on the test materials’ surfaces because of their lower S . Although Streptococcus mutans or lactobacilli are sometimes reported to adhere directly on surfaces [27–29], the typical early colonizers of the oral biofilm [30] as the Actinomyces species naeslundii and viscosus and the Streptocci oralis, mitis and sanguinis [10] are reported to create adhesive forces to the tooth surface which are probably based on hydrophobic interactions [31–35]. Therefore, the reduced hydrophobic interactions of the test materials when compared with ST, indicated by lower SLW prior to and after chewing simulation, might make it more difficult for the aforesaid bacteria to adhere. Some authors reported high SAB to create strong bacterial adhesion [9,36]. In the present study lower SAB than ST were calculated for all materials prior to but only for materials B and E after chewing simulation. This can be explained by the higher concentration of the low-surface tension agents in materials B and E (B, E: 5.14 wt.%/A, C: 4.00 wt.%, Table 2) that over-compensated the hydrophilic interactions of the increased surface roughness. The acid component S+ of the liquid at the interface interacts with the base component S− of the solid and vice versa [37] and it was hypothesized that S− is important in the ability of bacteria to adhere to solid surfaces [9]. Since solids mostly have low S+ but high S− values it was assumed that low S− reduces bacterial adhesion [38]. Although the present study shows an increase after chewing simulation S− of all test materials remained significantly lower than of ST. Biofilm formation is very complex and does not only include the bacteria and material characteristics but also saliva and its protein components. Therefore, biofilm formation on the test materials versus ST needs to be studied in the oral cavity to prove the protective effect of the low-surface tension agents set free from the delivery systems.
5.
Conclusions
Under the conditions of this study the Poly-Pore-based delivery system and the low-surface tension agents Tego Protect 5000 and Dimethicone proved to influence contact angle and surface free energy of experimental dental resin composites after a chewing simulation test. Based on the results of this investigation in connection with earlier studies [10,16] and the discussed literature it can be concluded that the described delivery systems could be most appropriate to reduce SFE of dental resin composites even after chewing simulation and, therefore, might be helpful to reduce plaque accumulation. The null hypothesis was rejected.
Acknowledgement This study was supported by the DFG Deutsche Forschungsgemeinschaft, Bonn, Germany (Project No. RU 825/3-1).
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[30]
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Contact angle and surface free energy of experimental resin-based dental restorative materials after chewing simulation Stefan Rüttermann, Thomas Beikler, Ralf Janda ∗ Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstraße 5, D-40225 Düsseldorf, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate contact angle and surface free energy of experimental dental resin
Received 2 September 2013
composites containing novel delivery systems of polymeric hollow beads and low-surface
Received in revised form
tension agents after chewing simulation test.
23 January 2014
Methods. A delivery system of novel polymeric hollow beads differently loaded with two
Accepted 25 March 2014
low-surface tension agents was used in different amounts to modify commonly formulated experimental dental resin composites. The non-modified resin was used as standard. Surface roughness Ra , contact angle , total surface free energy S , its apolar SLW , polar SAB ,
Keywords:
Lewis acid S+ and base S− terms were determined and the results prior to and after chewing
Dental resin composite
simulation test were compared. Significance was p < 0.05.
Chewing simulation
Results. After chewing simulation Ra increased, decreased, Ra increased for two test mate-
Contact angle
rials and S decreased or remained constant for the standard or the test materials after
Surface free energy
chewing simulation. Ra of one test material was higher than of the standard, and S of the test materials remained lower than of the standard and, indicating their highly hydrophobic character ( ≈ 60–75◦ , S ≈ 30 mJ m−2 ). SLW , and S− of the test materials were lower than of
the standard. Some of the test materials had lower SAB and S+ than of the standard.
Significance. Delivery systems based on novel polymeric hollow beads highly loaded with lowsurface tension agents were found to significantly increase contact angle and thus to reduce surface free energy of experimental dental resin composites prior to and after chewing simulation test. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Contact angle and surface free energy (SFE) are discussed to influence plaque formation on modern dental filling composites [1–5]. Since there is a positive correlation between SFE and
early plaque formation [5,6], dental resin composites differing in contact angle were broadly investigated and bacterial adherence of materials with high and low SFE was found to be significantly less [2,7–11]. But it was also reported that no correlation occurred between streptococcal adhesion and
∗ Corresponding author at: Heinrich-Heine-University, Medical Faculty, Centre of Dentistry, Department of Operative and Preventive Dentistry and Endodontics, Moorenstraße 5, Geb. 18.13, D-40225 Düsseldorf, Germany. Tel.: +49 6723 6020 750; fax: +49 6128 48 04 35. E-mail addresses: [email protected], [email protected] (R. Janda).
http://dx.doi.org/10.1016/j.dental.2014.03.009 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 702–707
substratum surface roughness or SFE [12–14] and it was realized that the influence of SFE on bacterial adhesion decreased significantly after saliva coating [14,15]. The present work is related to earlier investigations reporting an entirely new approach to obtain resin materials with low SFE but acceptable physical properties [16] which also proved diminished bacterial adherence [10]. Both papers reported about a new delivery system of polymer hollow beads (Poly-Pore, AMCOL Health & Beauty Solutions, Arlington Heights, IL, USA) as carrier material, highly loaded with two different low-surface tension agents (Tego Protect 5000, Evonik Tego Chemie GmbH Essen, Germany and Dimethicone, Dow Corning Corp., Midland, MI, USA) which were added in small quantities of 5–6 wt.% to experimental dental resin composites. It was found that due to abrasion processes, simulated by polishing, the carrier hollow beads were destroyed and released the low-surface tension agents which flushed the surface and thus reduced the resin’s SFE [16]. Now the effect of the exposition of the low-surface tension agents on SFE was re-investigated under more clinically relevant conditions with a chewing simulation device. To simplify the comparison with the previous papers [10,16] the selected experimental resins were coded accordingly. The null hypothesis was that the SFE of the delivery system containing material did not differ from the standard after performing the chewing simulation test.
2.
Materials and methods
2.1.
Test material preparation (analog [16])
Four experimental light-curing resin-based restorative materials (A, B, C and E) were prepared (Tables 1 and 2) using a laboratory vacuum planet kneader (Herbst Maschinenfabrik GmbH, Buxtehude, Germany). Material ST, representing a common formulation for dental resin composites, was used as the standard. The preparation process of all test materials and ST was done under vacuum at 50 ◦ C. Preparation of the test materials: Poly-Pore was loaded with two different active agents (Tego Protect 5000, Evonik Tego Chemie GmbH or Dimethicone, Dow Corning Corp.) in varying quantities of the agents resulting in four different delivery systems (Table 2). The delivery systems of materials A and B contained Tego Protect 5000, and materials C and E contained Dimethicone as the active agent. ST was modified by replacing 5% or 6% of the glass filler with the respective delivery system. At first the monomers were mixed, next all other ingredients but the glass filler and the Poly-Pore delivery systems were added and dissolved in the monomer mix. Then the respective Poly-Pore delivery system and finally the glass filler were homogeneously dispersed in the mixture.
2.2. Preparation of the delivery systems (analog [10,16]) To obtain the four delivery systems (Tables 1 and 2) 80 g or 85.7 g, respectively, Dimethicone or Tego Protect 5000 were dissolved in 400 g butanone (Lot 244238, Brenntag GmbH, Mülheim, Germany), then 20 g or 14.3 g, respectively, PolyPore hollow beads were added. The mixture was stirred
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for approximately 10 minutes to optimally wet the sorption particles. The mixture was slightly warmed to evaporate the solvent, and stirring was stopped when it became too stiff. Then it was put in a drying closet at 50 ◦ C until a constant weight was obtained (generally 24 h). After this treatment, the loaded sorption materials, representing the delivery systems, appeared totally dry and powdery.
2.3.
Specimen preparation and chewing simulation
Ten disks (thickness: 1 ± 0.1 mm, diameter: 10 ± 0.1 mm) of each material were made and cured for 40 s on each side (Spectrum 800, Dentsply deTrey GmbH, Constance, Germany). The output of the curing device was routinely checked (bluephase meter, Ivoclar Vivadent AG, Schaan, Liechtenstein). Irradiances of 931 ± 90 mW cm−2 were measured, and no significant decrease in the output was observed. The cured specimens were stored for 14 days in water at 37 ◦ C and then one side was wet-polished (sterile water) with fine and superfine polishing discs (Super-Snap mini, Shofu Inc., Kyoto, Japan) for 1 min, with a grinding pressure of 40–50 g and 10,000 rpm (Endo-Mate TC, Nakanishi Inc., Tochigi, Japan). The degree of polymerization was not measured but the high irradiance of the light curing device ensures an optimal polymerization. Next the specimens were fixed with the polished sides up in the mounts of a chewing simulator (CS-4, SD Mechatronik GmbH, Feldkirchen-Westerham, Germany) with a MMA/PMMA resin (Technovit 4000, Heraeus Kulzer GmbH, Hanau, Germany). They were stressed for 650 cycles in water at 37 ◦ C to simulate a 24 h chewing period according to the literature, which correlates the number of cycles to a realistic period of time [17,18]. The 24 h were chosen to test the daily availability of the active agents. The test specimens were sliding under contact to a spherical antagonist (aluminum oxide, 5 mm diameter, Quick-Ohm Küpper & Co. GmbH, Wuppertal, Germany) for a linear distance of 8 mm (speed: 40 mm s−1 ) and a vertical load of 50 N resulting in a contact area of 0.6 mm × 8 mm and an average wear of 5.4 m (±2.5 m). This was repeated five times resulting in a worn area of 3 mm × 8 mm = 24 mm−2 to obtain sufficient surface to place a drop.
2.4. Contact angle and surface free energy (analog [10,16]) Contact angles were determined with the sessile drop method (Phoenix-Alpha contact angle goniometer, Surface Electro Optics – SEO – Corporation, Suwon-City, Korea). Immediately after placing the drop on the specimen’s surface ten pictures of each drop were photographed (magnification: 25-folds) in one second (CCD-camera: FireDragon, Toshiba-Teli Corporation, Tokyo, Japan) and analyzed (Image XP-software, Version 6.0 FW 012108, SEO Corporation). The drops were formed by a standardized dosing tube and, therefore, had always the same volume. Contact angle was measured one time on each of the ten disks. SFE was calculated from the measured after water storage. Apolar Lifshitz-van der Waals LLW , polar Lewis acid L+ and Lewis base L− terms of the test liquids were taken from the literature [19] and the specimens’ total SFE S , their apolar term SLW , polar term SAB , acid term S+ and base term S− were calculated according to the equation of van Oss et al.
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Table 1 – Raw materials. Code
Product/properties
Photoinitiator Stabilizer TTEGDMA
UV-stabilizer UDMA
Bis-GMA
CQ Amine Glass
Poly-
Tego
Dimeth Poly-Dimeth Poly-Tego
Batch
Company
␣,␣-Dimethoxy-␣-phenylacetophenone Pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] Tetraethyleneglycole dimethacrylate, standard monomer, functionality = 2, MW = 330 [g mol−1 ], good chemical and physical properties, very low viscosity (14 Pa s, 25 ◦ C), diluting 2-Hydroxy-4-methoxy-bezophenone 7,7,9-Trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecan1,16-diol-dimethacrylate, standard monomer, functionality = 2, MW = 471 [g mol−1 ], flexible, tough, very good chemical resistance, medium viscosity (10,000 mPa s, 25 ◦ C) Bis-GMA, standard monomer, functionality = 2, MW = 513 [g mol−1 ], rigid, very good chemical resistance, very high viscosity (4500 mPa s, 60 ◦ C) d,l-Camphorquinone Ethyl-4-(dimethylamino)-benzoate Strontiumborosilicate glass (Glass G0 18-093, d50 = 0.7 m), silaned (3-methacryloyloxypropyltrimethoxy silane), D = 2.6 [g cm−3 ] Poly-Pore, cross-linked polyallyl methacrylate, adsorber, hollow beads, diameter 20–40 m
0066162S 26099IC3
Ciba Specialities Chemical Inc., Basel, Switzerland
J1620
Cray Valley, Paris, France
411351/143302 330503057
Fluka, Buchs, Switzerland Rahn AG, Zürich, Switzerland
Tego Protect 5000, hydroxyfunctional polydimethylsiloxane, hydro- and oleophobic, D = 1.05 [g cm−3 ], surface tension L = 17.8 (0.7) mJ m−2 [16] Dimethicone 200/350 cst, polydimethylsiloxane, D = 0.965 [g cm−3 ], surface tension L = 16.6 (0.5) mJ m−2 [16] Poly-Pore loaded with 80% and 85.7% Dimethicone, D = 1.0 [g cm−3 ] Poly-Pore loaded with 80% and 85.7% Tego Protect 5000, D = 1.0 [g cm−3 ]
ES57608918
2008218303
0148990002 310170 Lab14701
L07070303AB
4962250 Experimental products
Schott Electronic Packaging GmbH, Landshut, Germany AMCOL Health & beauty Solutions, Arlington Heights, IL, USA Evonik Tego Chemie GmbH, Essen, Germany Dow Corning Corp., Midland, MI, USA University laboratory
Information is based on the manufacturers’ technical data sheets.
[20] by (cos + 1) × L = 2( SAB
(S+ S− )
(SLW LLW ) +
(S− L+ ) +
(S+ L− ))
and with =2 (Image XP-software). Prior to the chewing simulation the contact angles of specimens were measured with aqua dest. (laboratory product), glycerol (LOT II1097071, Thermo Scientific, Rockford, IL, USA), ethylene glycol (LOT 3289749, Carl Roth GmbH + Co. KG, Karlsruhe, Germany), and diiodomethane (LOT S82251, Sigma Aldrich GmbH, Taufkirchen, Germany). Chewing simulation with 650 cycles was done and was measured with aqua dest. After drying with oil-free air was measured again with glycerol, ethylene glycol and diiodomethane. After each organic liquid’s measurement it was wiped off with alcohol and another 650 wear cycles were performed for each liquid to assure a clean surface.
2.5.
Surface roughness Ra (analog [16])
Surface roughness data of the materials prior to a chewing simulation test have already been published [16]. After chewing simulation and contact angle measurement the specimens’ surface roughness Ra was determined again (Surftest SJ-210 profilometer, diamond pick-up, tip radius 5 m, load 4 mN, Mitutoyo Corporation, Kawasaki, Japan). Each specimen was measured five times at different distances and in different directions (evaluation length 4 mm, stylus speed 0.5 mm s−1 ) in the wear area. For each measurement the stylus was automatically moved five times forward and backward along the same path. The data were filtered with a cut-off (c ) of 0.8 mm (Gauss profile filter) following ISO 4288.
Table 2 – Formulation in wt.% of the experimental resin-based restorative materials and the standard ST). ST
A
B
C
E
Raw material
73.00 – – – – 27.00
68.00 5.00 – – – 27.00
67.50 – 6.00 – – 26.50
68.20 – – 5.00 – 26.80
67.50 – – – 6.00 26.50
Glass Poly-Tego 80% Poly-Tego 85.7% Poly-Dimeth 80% Poly-Dimeth 85.7% Matrix
0
4.00
5.14
4.00
5.14
Active agent
705
After
45.9 (7.2) 32.2 (11.1)1,2 39.5 (12.0)1,3 19.5 (14.0)2 25.0 (12.5)2,3
Before
16.2 (2.8) 1.3 (0.6)1 1.6 (0.4)1 0.5 (0.4)1 0.5 (0.3)1
After
0.1 (0.1)1 0.6 (0.5)1,2 2.6 (2.7)2 1.6 (1.9)1,2 1.8 (1.4)1,2
S−
Table 4 – Means and (standard deviations) of surface roughness Ra . Underlined values within the rows indicate non significant differences (p > 0.05). The same subscript number within the columns indicates non significant differences between the materials. The before values were already published in Ruettermann et al. [16]. Material
Ra before [m]
ST A B C E
2.6.
0.15 (0.02)1 0.17(0.05)1 0.17 (0.04)1 0.23 (0.03)2 0.19 (0.04)1,2
Ra after [m] 0.16 (0.10)1 0.19 (0.08)1 0.21 (0.08)1 0.38 (0.15)1,2 0.60 (0.53)2
Statistical analysis
Before
0.4 (0.2)1 1.8 (0.7)2 1.6 (0.5)2 0.4 (0.4)1 0.3 (0.2)1
After
−0.4 (2.7)1 −0.8 (3.7)1 −12.6 (11.5)2 −7.6 (4.4)1,2 −11.1 (5.4)2
Before
3.
S , total surface free energy of the specimen. SAB , polar term of the specimens free surface energy. SLW , apolar term of the specimens free surface energy. S+ , acid term of the specimens free surface energy. S− , base term of the specimens free surface energy.
44.8 (3.6) 33.1 (3.2)1 19.5 (12.6)2 28.5 (6.0)1,2 22.6 (5.4)1,2
Before After Before
43.2 (2.3) 29.9 (2.7)1 29.7 (3.4)1 29.9 (2.2)1 29.6 (2.4)1 36.9 (5.4) 59.0 (8.4)1 61.6 (9.1)1,2 73.5 (15.0)1,3 76.3 (12.7)2,3 57.6 (4.3) 95.9 (4.0)1 97.4 (2.3)1 102.9 (2.7)1,2 105.6 (1.2)2 ST A B C E
S After Before
39.5 (2.4) 32.3 (2.4)1 32.1 (3.8)1 30.7 (1.8)1 30.1 (2.5)1
SLW
After
Means and standard deviations were calculated. Normal distribution was tested by Kolmogoroff–Smirnoff-test. Multivariate ANOVA and post hoc Scheffé’s test were calculated to find differences between the test materials’ SFEs. Univariate ANOVA and post hoc Scheffé’s test were calculated to find differences between the materials’ surface roughness and a t-test for unpaired samples was accomplished to find differences prior to and after chewing simulation. Statistics were calculated using IBM SPSS Statistics Version 21.0 (IBM Corporation, Somers, NY, USA). The statistical significance for all tests was set at p < 0.05.
45.2 (2.3) 34.1 (3.8)1 32.2 (3.2)1 36.1 (3.2)1 33.7 (1.5)1
3.7 (2.0) −2.4 (1.3)1 −2.3 (1.0) −0.8 (0.7)1 −0.6 (0.4)1
S+ SAB
Surface free energy [mJ m−2 ] Contact angle [◦ ] Material
Table 3 – Means and (standard deviations) of contact angles and surface free energy, values are rounded to valid digits. The same subscript number within the columns indicates non significant differences between the materials. Underlined numbers within the rows indicate non significant differences between the values before and after chewing simulation (p > 0.05). The before values were already published in Ruettermann et al. [16].
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 702–707
Results
Surface roughness values prior to chewing simulation were already reported and discussed in Ruettermann et al. [16]. Ra of ST and the materials A and B did not differ prior to and after chewing simulation. Ra of materials C and E increased after chewing simulation but only material E had higher Ra than ST (Table 4). An average material loss of 2.4 m occurred after each chewing simulation process. The worn area was always larger (0.6 mm × 8 mm) than the wetting area of the drop (maximum diameter 0.4 mm) and, therefore, the contact area of the substrate and the drop has almost no curvature. Contact angle and SFE values prior to chewing simulation (Table 3) were already reported and discussed in Ruettermann et al. [16]. After chewing simulation decreased for all materials. Except for material H, all other test materials had higher but lower S and SLW than ST. The polar term SAB of material B and E and the base term S− of materials C and E were lower than of ST. Only the acid term S+ of material B was higher than of ST.
4.
Discussion
Chewing simulation test [21], contact angle [9,16,22–24] and surface roughness [5,14,16] measurements were done according to well-established methods. The contact angles of water and of each of the three liquids were measured prior to and after chewing simulation on the same specimen because the statistical distribution of the specimen’s imperfections remained constant. Measuring each liquid on a new specimen with a different statistical distribution of the imperfections
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and combining the results in one equation might reduce the reliability of the mathematical calculation and, therefore, of the result. Saliva coating of the specimens prior to determine the surface properties with the sessile drop method is principally not possible because this method requires the direct contact of the surface and the measuring liquids. Furthermore, as we reported already earlier [16], in vivo studies proved that supragingival plaque formation, and therefore, adhesion and biofilm formation was greatest for high SFE and lowest for low SFE substrata, even when in the presence of saliva [5,6]. The carrier material Poly-Pore and the low-surface tension agents (Tego Protect 5000, Dimethicone) which were used to manufacture the delivery systems have been thoroughly described in earlier publications [10,16]. From our previous study ST and the Poly-Pore delivery system containing experimental dental resins or specimens manufactured of these, respectively, were chosen because they performed best [16]. A gradient of hollow beads in the outermost specimens’ surfaces can be excluded since they were polished with a standardized and clinically relevant method. Although of ST and all test materials decreased after chewing simulation, the test materials’ remained significantly higher than ST’s once and stayed about or even above 60◦ . The decrease of after chewing simulation might be caused by altered surface properties due to the chewing process and, considering materials C and E, by increasing Ra . But the fact that of the test materials remained significantly higher and S significantly lower after chewing proved again our findings that due to the abrasion processes the carrier hollow beads were destroyed and released the lowsurface tension agents which flushed the surface and thus reduced the resin’s SFE [16]. The renewed “floating” surface created by the low-surface tension agents over-compensated the increase of surface roughness. According to the literature [24] materials with > 65◦ are considered to be very hydrophobic. The highly hydrophobic character of the test materials was also expressed by the S -values which were found to be about 30 mJ m−2 . This value is defined for totally hydrophobic material surfaces [24]. The S -values of the materials B and E even decreased significantly below 30 mJ m−2 after chewing simulating what was certainly a consequence of their higher content of low-surface tension agents. Hydrophobic resin composite surfaces are assumed to be more resistant against attack by water or water-soluble species [7,25,26] but the relation between and bacterial adhesion has also been challenged [2,7]. However, the test materials’ and S , as they were found in the present investigation, confirmed our earlier assumption, based on a polishing process, that the low-surface tension agents were set free during the abrasion process (chewing simulation) and flushed the material’s surface to form a thin and strongly hydrophobic layer [16]. As we also hypothesized earlier [16] this layer represents a new floating discrete surface that is expected to be destroyed but also to be renewed by ubiquitous abrasive forces during the service life of the filling and we also experienced that this new floating discrete surface provided strong repulsive forces and hindered bacteria to adhere [10]. As discussed earlier [16], in vivo studies reported that biofilm formation was greatest for high S and lowest for low
S surfaces even in the presence of saliva [5,6]. Therefore, based on the results of the present study it is very likely that biofilm formation will be hindered on the test materials’ surfaces because of their lower S . Although Streptococcus mutans or lactobacilli are sometimes reported to adhere directly on surfaces [27–29], the typical early colonizers of the oral biofilm [30] as the Actinomyces species naeslundii and viscosus and the Streptocci oralis, mitis and sanguinis [10] are reported to create adhesive forces to the tooth surface which are probably based on hydrophobic interactions [31–35]. Therefore, the reduced hydrophobic interactions of the test materials when compared with ST, indicated by lower SLW prior to and after chewing simulation, might make it more difficult for the aforesaid bacteria to adhere. Some authors reported high SAB to create strong bacterial adhesion [9,36]. In the present study lower SAB than ST were calculated for all materials prior to but only for materials B and E after chewing simulation. This can be explained by the higher concentration of the low-surface tension agents in materials B and E (B, E: 5.14 wt.%/A, C: 4.00 wt.%, Table 2) that over-compensated the hydrophilic interactions of the increased surface roughness. The acid component S+ of the liquid at the interface interacts with the base component S− of the solid and vice versa [37] and it was hypothesized that S− is important in the ability of bacteria to adhere to solid surfaces [9]. Since solids mostly have low S+ but high S− values it was assumed that low S− reduces bacterial adhesion [38]. Although the present study shows an increase after chewing simulation S− of all test materials remained significantly lower than of ST. Biofilm formation is very complex and does not only include the bacteria and material characteristics but also saliva and its protein components. Therefore, biofilm formation on the test materials versus ST needs to be studied in the oral cavity to prove the protective effect of the low-surface tension agents set free from the delivery systems.
5.
Conclusions
Under the conditions of this study the Poly-Pore-based delivery system and the low-surface tension agents Tego Protect 5000 and Dimethicone proved to influence contact angle and surface free energy of experimental dental resin composites after a chewing simulation test. Based on the results of this investigation in connection with earlier studies [10,16] and the discussed literature it can be concluded that the described delivery systems could be most appropriate to reduce SFE of dental resin composites even after chewing simulation and, therefore, might be helpful to reduce plaque accumulation. The null hypothesis was rejected.
Acknowledgement This study was supported by the DFG Deutsche Forschungsgemeinschaft, Bonn, Germany (Project No. RU 825/3-1).
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