Surfactant Release from Hydrophilized Vinylpolysiloxanes

Abstract

Vinylpolysiloxane impression materials (VPS) exhibit an apolar (hydrophobic) backbone chemistry. Hence, surfactants are added to improve their hydrophilicity for impression-taking in moist environments. However, the mechanisms at the liquid-VPS-interface regarding the surfactant are unknown. We hypothesized that surfactant is leached from the VPS. Four experimental VPS formulations were fabricated containing 0 (control), 1.5, 3, and 5 wt% non-ionic surfactant. Samples were prepared (n = 6) and contact angles determined 30 min after mixing. After 60 sec, droplets were transferred onto the control. Mass spectrometry was used to analyze the droplets. Contact angles were inversely correlated with the surfactant concentration (p < 0.05). Droplets transferred from hydrophilized specimens onto the control showed similar contact angles. Surfactant could be clearly identified inside the droplets from the hydrophilized samples, however, not inside the control. Surfactants reduced the surface tension of the liquid in contact and did not change the surface properties of the VPS itself.

INTRODUCTION

A major challenge for a dental impression material is to wet the prepared tooth structure, especially in the area of the finishing line, to obtain a precise impression (Sumiya et al., 1997; Anusavice, 2003). Although there is no clear evidence as to which inherent properties of a material might specifically affect the wetting ability (Takahashi and Finger, 1991; Böning et al., 1998), hydrophilicity is regarded as a major influencing factor (Peutzfeldt and Asmussen, 1988; Oh et al., 2003; Kugel et al., 2007). In addition, the hydrophilicity of the set material is important to avoid entrapment of air during die casting (Norling and Reisbick, 1979; Oh et al., 2003; Kanehira et al., 2007). Hence, the hydrophilicity of an impression material may influence the precision of the impression (Petrie et al., 2003; Kugel et al., 2007) as well as the die (Chong et al., 1990; Vassilakos and Fernandes, 1993a; Oh et al., 2003), and thus affect the ultimate clinical success of fixed prosthetic restorations (Ragain et al., 2000; Mondon and Ziegler, 2003).

Vinylpolysiloxanes (VPS) are the most frequently used impression materials (Anusavice, 2003); however, in contrast to the clinical requirements, they are hydrophobic, which can be related to their apolar backbone chemistry (Anusavice, 2003; Rupp et al., 2005). To overcome this shortcoming, manufacturers add surfactants to these materials to improve their hydrophilicity (Mandikos, 1998).

The determination of contact angles by the sessile drop method is regarded as an appropriate means of measurement to assess the hydrophilicity of impression materials (Takahashi and Finger, 1991; Lee et al., 2004; Rupp et al., 2005; Kugel et al., 2007), whereas it is imperative to distinguish between contact angles determined on set vs. unset material (Takahashi and Finger, 1991; Mandikos, 1998; Lee et al., 2004). Studies published with experimental VPS formulations containing surfactants of different types and concentrations (Norling and Reisbick, 1979; Oh et al., 2003; Lee et al., 2004; Seo and Lee, 2006) concluded that the contact angles determined are dependent on the surfactant used and are inversely correlated with surfactant concentration.

It is assumed that surfactants diffuse through the material reaching the top surface, in turn increasing its hydrophilicity (Oh et al., 2003; Seo and Lee, 2006; Grundke et al., 2008). The subsequent processes and the final destination of the surfactant are very controversial (Vassilakos and Fernandes, 1993a): Some authors believe that the surfactant remains attached to the impression material’s surface (Norling and Reisbick, 1979; Oh et al., 2003; Lee et al., 2004), whereas others assume that there is a release into the liquid at the interface (Takahashi and Finger, 1991; Seo and Lee, 2006; Kanehira et al., 2007). Consequently, contact angles may be caused by (a) a reduction of the surface tension of the liquid, (b) an increased wettability of the solid’s surface, or (c) a combination of both (Takahashi and Finger, 1991), depending on the mechanisms at the interface.

Hence, the aim of this study was to investigate the mechanisms at the interface between the impression material’s surface and a water droplet with respect to a possible surfactant release. The null hypotheses to be tested were: (1) There is no surfactant diffusion into the water droplet, and (2) the contact angles obtained are independent of the VPS’s surfactant concentration.

MATERIALS & METHOdS

Materials

An experimental two-component VPS base formulation of type 3 viscosity (ISO4823, 2000) was fabricated (Heraeus Kulzer, Dormagen, Germany), consisting of VPS oligomers, SiH-polysiloxane oligomers (base paste only), filler particles, additives, and an organic Pt catalyst (catalyst paste only). This base formulation was divided into 4 parts, and surfactant was added to 3 of the 4 parts, leaving one part, without surfactant, as the control (Table 1).

The surfactant used was a non-ionic surfactant based on a block copolymer of siloxane and ether groups according to the manufacturer’s information [(CH₃)₃SiO-[(CH₃)₂SiO-]_m[CR₁R₂-CR₃R₄-O-]_n[Si(CH₃)₂-O-]_oSi(CH₃)₃]. The siloxane part caused the surfactant to be dispersible in the VPS. The formulations were filled into double-chamber automix cartridges.

The materials were used according to the manufacturer’s instructions under ambient laboratory conditions (23 ± 1°C, 50 ± 10% rel. humidity).

Determination of Contact Angles

The contact angles were determined by means of the sessile drop method with a DSA10 device (Krüss, Hamburg, Germany). For sample preparation, we dispensed 2 mL of freshly mixed impression material in a 1-mm-deep notch (15 mm × 60 mm) of a stainless steel block and leveled it with a Teflon-coated glass slide. The glass slide and the mould were thoroughly cleaned with ethanol prior to each sample preparation. The specimens (n = 6 per material) were allowed to set for 30 min at 23°C and were subsequently placed on the Peltier element inside the testing chamber of the DSA10 device at 23°C.

A droplet (4 μL) of de-ionized water (Fluka, Steinheim, Germany, Lot 1203160 42505188) was carefully placed onto the specimen surface from a needle tip. Video recording with a CCD-camera was begun immediately prior to droplet deposition and continued for 60 sec (12.5 fps). The video file obtained was subjected to Software Drop Shape Analysis (release 1.9, Krüss, Hamburg, Germany) to determine the contact angles continuously (n = 3 per specimen).

Subsequently, the specimens were removed from the chamber. The droplet was absorbed by means of a precision micro-pipette (type 4710, Eppendorf, Wesseling-Berzdorf, Germany) and immediately placed onto the surface of VPS-control located in the testing chamber. Contact angles were determined for another 30 sec as described previously. The tip of the pipette was discarded after use with each specimen.

Detection of Surfactant Release

The surfactant released into the droplet was determined by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

The specimens were fabricated as described previously. The VPS-control was immersed in 1 mL de-ionized water for 15 min. In cases of the formulations containing surfactant, a droplet (4 μL) of de-ionized water was placed onto the surface 30 min after specimen fabrication and left in contact for 60 sec. A 1-μL quantity of this droplet and additional de-ionized water (control) was placed on a steel target (MPT AnchorChip 800-384, Bruker Daltonics, Bremen, Germany) and mixed with 1 μL of matrix (2,5 dihydroxybenzoic acid + methylene diphosphonic acid, 5 mg/mL each). The samples were allowed to dry before the spectra were acquired in the positive ion reflectron mode inside the mass spectrometer (Ultraflex TOF/TOF, Bruker Daltonics). Approximately 300 single shots were added, and the spectra were analyzed (Flex-Control, Bruker Daltonics). External calibration was performed with a peptide calibration standard (Bruker Daltonics).

Statistical Analysis

Mean contact angle values (n = 6) were calculated at five-second intervals. We used the Kolmogorov-Smirnov test (p = 0.05) to check for normal distribution, and the Levene test (p = 0.05) to check the homogeneity of variances. A one-way ANOVA (p = 0.05) was used for statistical analysis of the data obtained from the contact angle measurements, with subsequent Tukey post hoc comparison (p = 0.05). All statistical analyses were carried out with SPSS for Windows (release 15.01, SPSS Inc., Chicago, IL, USA).

RESULTS

Contact Angle Measurement

Mean contact angles varied depending on the experimental formulation tested (Table 2 ). VPS-control showed relatively constant contact angles above or close to 90°. All other experimental materials showed a decrease of contact angles down to 25° and less. The steepest decline of the contact angles was determined within the first 10–15 sec, with significant differences between the five-second intervals (p < 0.05). The lowest initial contact angles were observed for VPS-5.0 (p < 0.05).

After the droplet was transferred from the respective experimental formulation onto VPS-control, contact angles did not differ significantly between 60 sec and 65 sec for all materials apart from VPS-5.0 (p < 0.05).

One-way ANOVA showed a significant influence of the surfactant concentration on the contact angles (p < 0.05). The post hoc comparison revealed significant differences among all materials at all times (p < 0.05), except for VPS-3.0 vs. VPS-5.0 at the 65- to 90-second time intervals (p > 0.05).

Mass Spectrometry

A comparison of the spectra of de-ionized water and VPS-control revealed no indication of ingredients released from the experimental formulation lacking surfactant. Only matrix and background signals were detected in the mass range from 500 to 1000 Da (Fig. a ).

In contrast, spectra obtained from surfactant containing VPS samples showed 2 prominent signals from polyethylene glycol clusters (Figs. b–d ), between 511 and 952 Da as well as between 952 and 1391 Da. With increasing surfactant concentration, the clusters became more obvious.

DISCUSSION

The hydrophilicity of an impression material is regarded as a major factor influencing the wetting of a prepared tooth surface and, in turn, the precision of the impression obtained (Peutzfeldt and Asmussen, 1988; Mondon and Ziegler, 2003; Kugel et al., 2007). In addition, wetting of the impression’s surface by the aqueous gypsum slurry is important for accurate die casting (Norling and Reisbick, 1979; Pratten and Craig, 1989; Cullen et al., 1991; Oh et al., 2003). Although it is important to distinguish between these two aspects (Takahashi and Finger, 1991; Mandikos, 1998; Lee et al., 2004), both may affect the accuracy of the final restoration and thus the clinical success of the prosthodontic treatment (McCormick et al., 1989; Ragain et al., 2000).

Since VPS impression materials are known to be hydrophobic (Anusavice, 2003; Grundke et al., 2008), manufacturers add surfactants (as denoted by the wording “hydrophilized VPS”), insinuating that this is equivalent to good wetting of moist tooth surfaces, as well as superior wetting during die casting (Kanehira et al., 2007). While the latter property has been proven in various studies (Norling and Reisbick, 1979; Pratten and Craig, 1989; Cullen et al., 1991; Vassilakos and Fernandes, 1993a; Oh et al., 2003), the clinical relevance of adding surfactants remains controversial (Takahashi and Finger, 1991; Böning et al., 1998; Nichols, 1999; Mondon and Ziegler, 2003; Blatz et al., 2005; Beier et al., 2007; Kanehira et al., 2007).

It is believed that contact angles, determined by the sessile drop method, reflect hydrophilic properties of either set or unset impression material (Grundke et al., 2008), although the mechanisms are not yet fully understood (Kanehira et al., 2007). This is inextricably related to the question of what happens to the surfactant at the interface between the liquid and hydrophilized VPS formulations. Some authors believe that the surfactants are released from the impression material’s surface (Takahashi and Finger, 1991; Mondon and Ziegler, 2003; Seo and Lee, 2006; Kanehira et al., 2007), whereas others state that the surfactant remains attached to the impression material’s surface (Norling and Reisbick, 1979; Chai and Yeung, 1991; Oh et al., 2003; Lee et al., 2004; Grundke et al., 2008). However, there is no scientific evidence in the literature regarding what happens to the surfactant. To answer this important question, investigators have fabricated 4 different experimental VPS formulations of known surfactant concentrations in an amount reported to be efficient (Oh et al., 2003; Lee et al., 2004; Seo and Lee, 2006; Grundke et al., 2008).

MALDI-TOF-MS (Karas and Bahr, 1990; Cohen and Gusev, 2002) was applied to determine potential ingredient release from the experimental VPS formulations. Surfactant was clearly identified inside the water droplet which had been in contact with the hydrophilized VPS formulations, as indicated by the 2 polyethylene clusters in the spectra (probably representing 2 fractions of the surfactant added), but not in VPS-control. This is the final proof that surfactant is leached by diffusion from the set impression material’s surface at the interface with the water droplet. In addition, the contact angles of the droplets, transferred from the corresponding VPS specimens at 60 sec, were very similar to the values obtained subsequently on the VPS-control specimens at 65 sec. It can be concluded that the extrinsic surfactant added caused primarily a reduction in the surface tension of the water droplet (Chai and Yeung, 1991; Seo and Lee, 2006), but not an increase of the impression material’s hydrophilicity. Analysis of recent scientific data showing an increase of contact angles after the rinsing of specimens stored in water or disinfectant (Seo and Lee, 2006; Kanehira et al., 2007) strongly supports this hypothesis.

The high contact angles of VPS-control reflect the apolar backbone chemistry of the siloxanes (Norling and Reisbick, 1979; Oh et al., 2003; Lee et al., 2004). In contrast, during the observation period, all formulations containing surfactant showed a significant decrease of contact angles, up to 70°. A higher surfactant concentration was inversely related to lower contact angles, which is in accordance with reports in the literature (Norling and Reisbick, 1979; Lee et al., 2004; Seo and Lee, 2006). However, there seems to be a threshold for the surfactant concentration above which no further decrease of contact angles can be expected.

This observation additionally questions contact angle measurement as an appropriate method to describe the hydrophilicity and wetability, respectively, of hydrophilized VPS. Hence, contact angle measurement probably reflects the ability of the VPS to release surfactant, reducing the surface tension of, e.g., water or gypsum slurry (Vassilakos and Fernandes, 1993a). Hypothetically, it may be speculated that surfactant molecules also remain inside the material’s surface, at least partially. This, however, cannot be derived from the current experiment. In sum, the results indicate that both null hypotheses must be rejected.

The present study was conducted with set material. Hence, the results obtained limit the interpretation primarily to wetting of the set impression’s surface by the aqueous gypsum slurry and did not allow for direct conclusions to be drawn about the interaction between saliva and impression material during clinical application. Never theless, it is postulated that the results can be extrapolated, at least in part, to what happens when an impression is taken, since the contact angles determined on freshly mixed materials are reportedly lower compared with those of set materials (Rupp et al., 2005; Grundke et al., 2008). Taking into consideration the mechanisms revealed in the current experiment, this indicates surfactant release without changing the hydrophilicity of the impression material’s surface. In addition, the absence of a difference between unhydrophilized and hydrophilized VPS formulations with respect to the detailed reproducibility of wet surfaces and clinical results might be taken to corroborate this hypothesis (Takahashi and Finger, 1991; Böning et al., 1998; Stewardson, 2005).

Since a specific surfactant was used in the present study, the results regarding decreased contact angles cannot necessarily be extrapolated to different surfactant types (Norling and Reisbick, 1979). Finally, it must be borne in mind that saliva contamination might change the surface properties of a set impression material (Vassilakos and Fernandes, 1993b).

To the authors’ knowledge, this is the first study that has unequivocally demonstrated surfactant release from impression material, thus providing important fundamental data. Further studies are urgently needed to identify the key properties of VPS impression materials, responsible for a reliable wetting of moist tooth structure.

Under the above-mentioned limitations, the following conclusions can be drawn:

Surfactant incorporated into a hydrophilized VPS formulation is released from its surface and diffuses into the liquid in contact.

Extrinsic surfactants added to VPS formulations do first and last not increase the wettability of the impression material’s surface, but reduce the surface tension of the liquid in contact.

Table 1.

Surfactant Concentrations of the Experimental Batches* Tested

Abbreviation	Lot (base/catalyst)	Surfactant Content [%]
*All experimental batches were fabricated by Heraeus Kulzer (Dormagen, Germany) and supplied in automixing cartridges. VPS = Vinylpolysiloxane.
VPS-control	MHE1032 / MHE1037	no surfactant
VPS-1.5	MHE1114 / MHE1116	1.5% surfactant
VPS-3.0	MHE1115 / MHE1116	3.0% surfactant
VPS-5.0	MHE1115-1 / MHE1116	5.0% surfactant

Table 2.

Mean Contact Angles (n = 6) of the Experimental VPS Formulations, Including Standard Deviations in Parentheses

Application*/Time [sec]^#	VPS-control	VPS-1.5	VPS-3.0	VPS-5.0
* Direct application of de-ionized water onto the specimen from the needle tip vs. indirect application, i.e., transferring the droplet from the corresponding experimental vinylpolysiloxane (VPS) formulation after 60 sec onto the VPS-control specimen.
^# Time elapsed after the first contact with the corresponding VPS formulation. For the indirect application, 65 sec represents 5 sec after the droplet was transferred onto the VPS-control specimen. Identical lower-case superscript letters denote contact angles at different drop ages, which do not differ significantly within one experimental formulation (i.e., columns) at different times after droplet deposition (Tukey’s test, p > 0.05).
Direct
0	95.7 (1.1)^a	93.0 (1.0)^a	89.4 (1.5)^a	54.7 (2.1)^a
5	92.7 (1.0)^b	55.2 (1.2)^b	25.4 (2.3)^b	14.6 (1.1)^b
10	92.7 (0.8)^b	46.9 (1.1)^c	15.9 (1.2)^c	11.8 (0.8)^c
15	92.7 (0.8)^b	42.4 (1.1)^d	13.8 (1.0)^cd	10.6 (0.7)^cd
20	92.6 (0.7)^bc	39.3 (1.1)^e	13.1 (0.7)^de	10.1 (0.8)^cdefg
25	91.7 (0.6)^bcd	36.5 (1.1) ^f	12.5 (0.6)^de	9.6 (0.8)^defg
30	91.4 (0.6)^bcd	34.4 (1.3)^f	12.1 (0.6)^de	9.2 (0.8)^defg
35	91.1 (0.7)^cd	31.8 (0.8)^g	11.8 (0.6)^de	9.0 (0.8)^defg
40	90.9 (0.7)^def	30.2 (1.4)^gh	11.6 (0.6)^e	8.8 (0.8)^defg
45	90.9 (0.7)^defg	28.2 (1.3)^hi	11.4 (0.6)^e	8.4 (0.9)^efg
50	90.7 (0.7)^defgh	26.4 (1.3)^ik	11.2 (0.6)^e	8.4 (0.8)^efg
55	90.7 (0.7)^defgh	24.8 (1.0)^kl	11.1 (0.6)^e	8.3 (0.9)^fg
60	90.7 (0.7)^defgh	23.4 (0.7)^lm	10.9 (0.6)^e	7.9 (0.7)^g
Indirect
65	89.6 (0.8)^efgh	21.6 (1.4)^mn	12.2 (1.2)^de	10.4 (1.4)^cdef
70	89.5 (0.7)^fgh	21.4 (1.3)^mn	12.0 (1.1)^de	10.7 (0.9)^cd
75	89.4 (0.7)^fgh	21.3 (1.2)^mn	11.9 (1.1)^de	10.6 (1.0)^cd
80	89.3 (0.7)^gh	21.1 (1.2)^mn	11.8 (1.1)^de	10.5 (1.1)^cde
85	89.4 (0.6)^h	20.9 (1.2)ⁿ	11.9 (1.2)^de	10.4 (1.2)^cdef
90	89.2 (0.6)^h	20.8 (1.2)ⁿ	11.6 (1.1)^e	10.4 (1.1)^cdef

Figure.

Spectra obtained from MALDI-TOF mass spectrometry. Spectra from VPS-control (a) revealed background and matrix signals only, whereas strong polyethylene glycol clusters were found in the experimental VPS formulations that contained surfactant (b–d).

Footnotes

Notes

Acknowledgements

The authors are grateful for the support of Dr. Martin Grunwald and Dr. Andreas Grundler (Heraeus Kulzer, Dormagen, Germany), who fabricated the experimental formulations according to the authors’ requirements. The study was undertaken with institutional support of the Department of Prosthetic Dentistry (Justus-Liebig-University, Giessen, Germany).

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