- Email: [email protected]
Wettability modification of human tooth surface by water and UV and electron-beam radiation
Materials Science and Engineering C 57 (2015) 133–146
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Wettability modification of human tooth surface by water and UV and electron-beam radiation Gaby E. Tiznado-Orozco a,b, José Reyes-Gasga a,c,⁎, Florina Elefterie a, Christophe Beyens a, Ulrich Maschke a, Etienne F. Brès a a b c
UMET, Bâtiment C6, Université de Lille 1, Sciences et Technologies, 59650 Villeneuve d'Ascq, France Unidad Académica de Odontología, Universidad Autónoma de Nayarit, Edificio E7, Ciudad de la Cultura “Amado Nervo”, C.P. 63190 Tepic, Nayarit, Mexico Instituto de Física, UNAM, Circuito de la Investigación s/n, Ciudad Universitaria, 04510 Coyoacan, México, D.F., Mexico
a r t i c l e
i n f o
Article history: Received 5 July 2014 Received in revised form 20 February 2015 Accepted 10 June 2015 Available online 21 July 2015 Keywords: Wettability Enamel Dentin UV radiation EB radiation
a b s t r a c t The wettability of the human tooth enamel and dentin was analyzed by measuring the contact angles of a drop of distilled water deposited on the surface. The samples were cut along the transverse and longitudinal directions, and their surfaces were subjected to metallographic mirror-finish polishing. Some samples were also acid etched until their microstructure became exposed. Wettability measurements of the samples were done in dry and wet conditions and after ultraviolet (UV) and electron beam (EB) irradiations. The results indicate that water by itself was able to increase the hydrophobicity of these materials. The UV irradiation momentarily reduced the contact angle values, but they recovered after a short time. EB irradiation raised the contact angle and maintained it for a long time. Both enamel and dentin surfaces showed a wide range of contact angles, from approximately 10° (hydrophilic) to 90° (hydrophobic), although the contact angle showed more variability on enamel than on dentin surfaces. Whether the sample's surface had been polished or etched did not influence the contact angle value in wet conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Surface wettability indicates the ability of a liquid to wet the surface of a solid [1]. In biomaterials, surface wettability is an important property because it may indicate cells immobilization, drug delivery and gene transfer, among other applications [2,3]. For human tooth and skin, wettability is of interest in the study of pharmaceutical and cosmetic products [4]. In tooth, hydrophobicity affects initial water absorption and the adhesion of oral bacteria [5]. The success of any dental restorative treatment depends on the adhesion to the dental tissue; but the improvement of adhesion implies the need for acid etching of enamel surfaces [3]. On the other hand, the analysis of adhesive systems in dentistry could identify a way to eliminate, for example, the metal brackets [6]. The wettability of a material is usually determined by measuring the contact angle formed between the surface of the material and the line tangent to the curved surface of a liquid drop at the point of contact [7]. The contact angle depends on the surface energy of the material and the surface tension of the liquid. In addition, the surface energy is ⁎ Corresponding author at: Instituto de Física, UNAM, Circuito de la Investigación s/n, Ciudad Universitaria, 04510 Coyoacan, México, D.F., Mexico. E-mail addresses: [email protected] (G.E. Tiznado-Orozco), jreyes@fisica.unam.mx (J. Reyes-Gasga), elefterie_fl[email protected] (F. Elefterie), [email protected] (C. Beyens), [email protected] (U. Maschke), [email protected] (E.F. Brès).
http://dx.doi.org/10.1016/j.msec.2015.06.054 0928-4931/© 2015 Elsevier B.V. All rights reserved.
correlated with different physical parameters, such as elastic modulus, melting point, and heat and energy of vaporization [1,4,8]. The contact angle is also related to the surface roughness of the substrate, which can be modified during the sample preparation process [5]. A brief theoretical analysis of the relationship between the contact angle and the surface energy is given by Aronov et al. [9]. Contact angle measurement is probably the most popular method to determine the hydrophobicity and/or hydrophilicity of the surfaces of materials. A greater contact angle indicates hydrophobicity (poor wetting). Several authors [6,10] consider that a contact angle of less than 90° is an indication of a hydrophilic material, whereas a contact angle larger than 90° indicates a hydrophobic material. However, several others [11] consider that an angle of 65° is more appropriate for this differentiation. Wettability modification of hydroxyapatite (HAP, Ca10(PO4)6(OH)2) by electron irradiation has been reported by Aronov et al. [9,12]. These researchers showed that the use of low-energy electron irradiation modifies the wettability of the HAP surface in a wide range of contact angles, extending from 10° (hydrophilic) to 100° (hydrophobic). They indicated that the incident electrons generate electron/hole pairs, resulting in significant variation of the surface potential of the hydroxyapatite (the penetration depth is approximately 2 nm for 100 eV incident electrons) and giving rise to the wettability modification observed. Based on this information and our experience in HAP and human tooth material, we undertook the task of studying the contact angles
134
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
on the surface of the human tooth enamel and dentin before and after electron beam (EB) radiation. The samples were cut in such a way that their surfaces were transverse (T) and longitudinal (L) to the tooth surface. The surfaces of the samples were mirror-finish polished, and some of them were also etched with phosphoric acid to reveal their microstructure. The samples were placed under dry and wet conditions, and the EB radiation dose was set at three different values. Because ultraviolet (UV) radiation can modify the wettability of the surface of certain ceramics, and due to the importance of this radiation in dental work, we decided to include UV irradiation as a condition in this study. Human tooth enamel and dentin are composed mostly of natural HAP crystals [13]. Enamel is comprised of 96% HAP and 4% organic material, while dentin is 74% HAP and 26% organic material. Water content varies from 1 to 6% by weight in enamel and from 15 to 30% in dentin [14]. In enamel, HAP crystals have a tablet shape with dimensions of approximately 30 × 50 × 100 nm. These tablets are arranged in a textured manner and provide form to the enamel prisms. The organic material in enamel wraps the HAP crystals. In dentin, conversely, HAP crystals have a platelet-like shape, and they are smaller than those in enamel. These crystals are immersed in an organic matrix and their density is higher around the dentinal tubules than in the space between them. 2. Experimental procedure Human tooth enamel and dentin samples were taken from permanent human premolar teeth from persons 18 to 25 years old. All of the extractions were done for orthodontic or periodontal reasons. 2.1. Preparation of samples The health of the teeth was carefully and clinically examined before and after being split into halves using a low-speed diamond micrometer saw (IsoMet, Buehler). Enamel and dentin were mechanically separated from each other using a hand-guided dental drill and a light microscope (Carl Zeiss model Axiovert 25). Enamel and dentin were cut in such a way that the surfaces of the samples were flat and parallel (named “transverse”, or “T”, samples hereafter) or perpendicular (named “longitudinal”, or “L”, samples hereafter) to the tooth surface. Next, the surface of the samples was mirror finish-polished with silicon carbide paper and alumina slurries in a grinder–polisher (MiniMet, Buehler). Fig. 1 shows the light microscopy images of one of the L samples (Fig. 1A) and one of the T samples (Fig. 1B). Some of the samples were left as obtained after the mirror-finish polishing (named “polished”, or “P”, samples hereafter) and others were etched with phosphoric acid until their microstructure was
Fig. 2. SEM image of the surface of one of the samples just after polishing. The smear layer produced during polishing covers the microstructure of human tooth.
revealed (named “etched”, or “E”, samples hereafter). Fig. 2 shows the SEM image of one of the mirror finish-polished surfaces but, because of the smear layer produced during polishing, no contrast of the human tooth structure is observed. However, when these surfaces are etched with phosphoric acid, the enamel prisms and the dentinal tubules are observed (Fig. 3). All the samples were ultra-sonicated at 30 °C for 10 min in an ultrasonic bath (S30 H Elmasonic). The samples were kept in a desiccator following this procedure. Therefore, we worked with 4 enamel samples (2 in transversal direction: one polished and one etched; and 2 in longitudinal direction: one polished and one etched), and 4 dentin samples (2 in transversal direction: one polished and one etched; and 2 in longitudinal direction: one polished and one etched). The dentin samples, as well as the enamel ones, were labeled TE, TP, LE and LP according to the cutting direction and to the surface treatment. 2.2. The contact angle measurement For measuring the contact angle, a distilled water sessile drop measuring 7 mm in diameter was deposited on the surface of the sample by a syringe and the contact angle was measured using the Contact Angle Meter Digidrop (GBX Instrumentation Scientifique, France). For this equipment, the standard deviation for the contact angle measurements was ± 5°. Optical inspection of the drop was performed by a camera device and digital imaging techniques. The measurement of the contact angle was taken 60 s after the deposition of the drop, the estimated time for equilibrium to be reached. For data collection, measurements were made in two drops at the same time and the average was used for the graphs. After each measurement, the water drop was wiped with
Fig. 1. Light microscopy images of two of the human tooth samples. A) Enamel in the longitudinal direction. B) Dentin in the transverse direction. The area where the drops of water were placed is indicated by white rectangles.
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
135
Fig. 3. SEM images of the surface of samples etched with phosphoric acid. A) Enamel prisms in the transverse view. B) Enamel prisms in the longitudinal view. C) Dentinal tubules in the transverse view. D) Dentinal tubules in the longitudinal view.
Fig. 4. Flowchart indicating the experimental flux followed to measure the contact angle in the enamel and dentin samples (see text). Note the existence of the loop for n = 1, 2, and 3.
absorbent paper. Water was exchanged for each measurement session, and the instruments were cleaned twice with distilled water. The contact angle was measured first in the dried enamel and dentin samples (referred to as “dried samples” hereafter). These samples are named “without radiation”, or “WR”. Later on, the angle was measured again in the same samples, first after ultraviolet light radiation (UVradiation), and then after electron beam irradiation (EB-radiation). Afterward, this process was repeated in the same samples but under wet conditions (referred to as “watered samples” hereafter). Fig. 4 shows a schematic flow diagram to facilitate the understanding of the experimental procedure. Seventy-two hours passed between the UV and EB radiation processes, and the samples were kept in a desiccator during that time. For wet conditions, the samples were kept in water for 7 days after the last measurement of the contact angle in the dry condition. Again, 72 h elapsed between UV and EB radiations, but on this occasion the samples were kept in water. Between two consecutive EB radiation cycles, the samples were kept in water for 7 days. The contact angle was measured at 0, 2, 4, 8, 15, 30, 60, 90, 120, 150, 180, and 210 min. In the case of radiation, the contact angle was also measured at 15, 10, 5 and 3 min prior to irradiation.
Fig. 5. Light microscope images of a drop on the surface of the LE enamel sample. A) WR in dried conditions; B) in wet conditions after EB(x2) radiation. Note the hydrophilic state in (A) and the hydrophobic state in (B).
136
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
was used. The EB radiation conditions for the first treatment (named “x1” hereafter) were: voltage of 180 kV, speed of 9 m/min, air gap of 18 mm, and current of 1 mA for a surface dose of 16 kGy. The conditions for the second EB treatment (named “x2” hereafter) were: voltage of 180 kV, speed of 3 m/min, air gap of 10 mm, and current of 1 mA for a surface dose of 26 kGy. In the third EB treatment (named “x3” hereafter) the radiation conditions were: voltage of 180 kV, speed of 3 m/min, air gap of 10 mm, and current of 5 mA for a surface dose of 132 kGy. The EB irradiations were done in inert atmosphere (N2) and UV irradiations were done in ambient atmosphere. 3. Results
Fig. 6. Schematic definition of the parameters used in the interpretation of results. m is the slope of the line; ΔV is the variation of the contact angle before radiation; ΔH is the variation of contact angle after radiation; ΔG is the gap between the values before and after radiation (difference ΔH − ΔV); ΔG0 is the gap at t = 0.
In all the samples, not only in those immersed in water or UV and EB irradiated, the contact angle was modified as a function of time. Fig. 5 presents two light microscope images of a drop on the surface of one of the human tooth enamel samples as an example of the hydrophilic (Fig. 5A) and of the hydrophobic (Fig. 5B) states.
2.3. UV and EB irradiation conditions
3.1. Definition of graph parameters
For UV irradiation, an AERI-Honle UV Technology equipment (Construction Mécanique, Power: 12 kW) was used. The UV radiation cycle speed was of 100 mm/s, and the UV irradiation dose was 1352 mJ/cm2 in the first treatment (named “x1” hereafter) and 2070 mJ/cm2 in the second treatment (named “x2” hereafter). For the electron beam (EB) irradiation, a COMET E-Beam Sources equipment (COMET Technology Model EBLab 180/210, Switzerland)
For a better presentation and reading of the contact angle versus time graphs, we find of great help to define some parameters (see Fig. 6). These parameters were taken only as qualitative indicators of the tendency of the graphs. First note that, within a given range of time t, the values of the contact angle can be represented, after the least-square method, by the line θ = mt + θ0, where θ0 is the intercept (i.e., there is a linear relationship
Table 1 Data for enamel samples in the dry condition (Fig. 7).a Sample enamel Dry
Etch
Polish
Type
WR
UV (x1)
EB (x1)
ΔHmax
L
ΔH = 38° (12–50)
ΔH = 8° (60–68) ΔV = 15° (45–60) ΔG0 = +15° ΔG = 0° [0, 210]: m = 0.0, θ0 = 65
38°
T
[0, 30]: m = 0.7, θ0 = 12 [30, 210]: m = 0.1, θ0 = 30 ΔH = 30° (15–45)
ΔH = 30° (55–85) ΔV = 15° (55–70) ΔG0 = +20° ΔG = −15° [0, 210]: m = 0.1, θ0 = 72
30°
L
[0, 30]: m = 0.7, θ0 = 12 [30, 210]: m = 0.1, θ0 = 30 ΔH = 20° (55–75)
T
ΔH = 10° (65–75) ΔV = 10° (65–75) ΔG0 = −5° ΔG = −10° [0, 210]: m = 0.0, θ0 = 68
ΔH = 17° (75–92) ΔV = 9° (76–85) ΔG0 = 0° ΔG = −10° [0, 8]: m = −1.3, θ0 = 86 [8, 210]: m = 0.1, θ0 = 78 ΔH = 12° (80–92) ΔV = 5° (75–80) ΔG0 = +10° ΔG = 0° [5, 210]: m = 0.0, θ0 = 90
20°
[0, 20]: m = 0.9, θ0 = 54 [20, 210]: m = 0.0, θ0 = 60 ΔH = 40° (30–70)
ΔH = 16° (40–56) ΔV = 15° (35–50) ΔG0 = +10° ΔG = −10° [0, 5]: m = 2.0, θ0 = 40 [5, 210]: m = 0.0, θ0 = 50 ΔH = 10° (50–60) ΔV = 4° (48–52) ΔG0 = +6° ΔG = −2° [0, 5]: m = −0.3, θ0 = 58 [5, 210]: m = 0.2, θ0 = 52 ΔH = 10° (70–80) ΔV = 4° (68–72) ΔG0 = 0° ΔG = −2° [0, 210]: m = 0.2, θ0 = 75
16° −10°
30° −15°
ΔHRmax ΔGmax
[0, 15]: m = 2.75, θ0 = 35 [15, 210]: m = 0.1, θ0 = 60 40°
40°
a Nomenclature used in Table 1: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; Δmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
within the contact angle and time; see also [15]). The slope m indicates the rate of increase or decrease of the contact angle value in (°)/min. Now let ΔV and ΔH be the range of variation for the contact angle values obtained before and after the radiation treatment, respectively (in the case of WR samples only ΔH is considered). Therefore, ΔHmax indicates the maximum variation of the contact angle values observed in each sample, and ΔHRmax indicates the maximum variation of the contact angle values after each type of radiation (or WR) in dry or wet conditions. The difference or gap between the values of the contact angle before and after irradiation can also be taken into account for each sample. Let ΔG represent this gap, which can be calculated as the difference between the minimum value of the contact angle in ΔH and the maximum value of the contact angle in ΔV. The value and sign of ΔG reflects the behavior of the contact angle after a period of time following irradiation: the sign “+” means an increase in the contact angle values relative to the values measured prior to irradiation (m N 0); the sign “−” means a decrease in the contact angle values (m b 0). The parameter ΔGmax indicates the maximum difference for a given type of radiation and sample conditions. The value of ΔG0 indicates the presence or absence of a “jump” in the contact angle value after irradiation. The value is calculated by the difference between the last value of the contact angle measured before irradiation and the first value measured after irradiation (i.e., at t = 0).
137
The values of m, θ0, ΔH, ΔV, ΔG0, ΔG, ΔHmax, ΔHRmax, and ΔGmax, have been included in Tables 1 to 4. Note that after a short time following the application of both UV and EB radiation, the values of m are close to zero. 3.2. The contact angle versus time graphs Figs. 7 and 8 show the graphs for enamel and dentin samples in dry conditions: WR (Figs. 7A and 8A) and with UV (x1) (Figs. 7B and 8B) and EB (x1) (Figs. 7C and 8C) radiation. In the graphs corresponding to the radiation treatments, the moment of irradiation is indicated by a vertical dark line set at t = 0. The values of the contact angle prior to radiation (to the left of the dark line and indicated by empty symbols) were also included in the graphs. Figs. 9 and 10 show the graphs for enamel and dentin samples in wet conditions: WR (Fig. 9A and 10A) and with UV (x1) (Figs. 9B and 10B) and EB (x1) (Figs. 9C and 10C) radiation. The graphs for enamel and dentin in wet conditions after the UV (x2) radiation and EB (x2) and EB (x3) radiation are shown in Figs. 11 and 12. The data obtained from Fig. 7 are shown in Table 1, those obtained from Figs. 9 and 11 are shown in Table 2, those obtained from Fig. 8 are shown in Table 3, and those obtained from Figs. 10 and 12 are shown in Table 4. As observed in the figures, most of the graphs are notably similar: the data show a more or less flat trend and they are contained between
Table 2 Data for enamel samples in the wet condition (Fig. 9). The (x2) and (x3) graphs are shown in Fig. 11.a Sample Enamel Wet
Etch
Type
WR
UV (x1)
UV (x2)
EB (x1)
EB (x2)
EB (x3)
ΔHmax
L
ΔH = 15° (45–60)
ΔH = 8° (65–73) ΔV = 20° (50–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 63
ΔH = 30° (62–92) ΔV = 6° (62–68) ΔG0 = 0° ΔG = −6° [0, 30]: m = 1.0, θ0 = 62 [30, 110]: m = 0.0, θ0 = 90 [110–210]: m = 0.0, θ0 = 82 ΔH = 10° (50–60) ΔV = 5° (55–60) ΔG0 = −5° ΔG = −10° [2,15]: m = 0.2, θ0 = 50 [15, 210]: m = 0.0, θ0 = 56
ΔH = 10° (65–75) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 70
ΔH = 15° (80–95) ΔV = 25° (55–80) ΔG0 = +15° ΔG = 0° [0, 8]: m = −0.6, θ0 = 94 [8, 210]: m = 0.0, θ0 = 87
ΔH = 7° (87–94) ΔV = 8° (72–80) ΔG0 = +10° ΔG = +7° [0, 210]: m = 0.0, θ0 = 90
30°
ΔH = 13° (50–63) ΔV = 5° (55–60) ΔG0 = 0° ΔG = −10° [0, 30]: m = −0.2, θ0 = 55 [30, 210]: m = 0.0, θ0 = 60
ΔH = 11° (80–91) ΔV = 10° (68–78) ΔG0 = +5° ΔG = +2° [10, 210]: m = 0.0, θ0 = 82
21°
ΔH = 17° (65–82) ΔV = 10° (60–70) ΔG0 = +5° ΔG = −5° [0, 60]: m = 0.0, θ0 = 70 [60, 180]: m = 0.1, θ0 = 63 ΔH = 10° (65–75) ΔV = 10° (60–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.1, θ0 = 65
ΔH = 25° (70–95) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −10° [0, 15]: m = 0.7, θ0 = 75 [15, 180]: m = 0.0, θ0 = 92 ΔH = 23° (65–88) ΔV = 15° (60–75) ΔG0 = 0° ΔG = −10° [0, 120]: m = 0.1, θ0 = 78 [120, 210]: m = −0.2, θ0 = 110
ΔH = 18° (62–80) ΔV = 2° (70–72) ΔG0 = 0° ΔG = −10° [0, 30]: m = −0.4, θ0 = 68 [30, 210]: m = 0.0, θ0 = 79 ΔH = 10° (80–90) ΔV = 0° (78) ΔG0 = +10° ΔG = +2° [4, 30]: m = 0.7, θ0 = 68 [30, 210]: m = 0.0, θ0 = 86
ΔH = 5° (75–80) ΔV = 5° (55–60) ΔG0 = +15° ΔG = +5° [0, 4]: m = 1.0, θ0 = 75 [4, 30]: m = −0.2, θ0 = 80 [30–180]: m = 0.0, θ0 = 75 ΔH = 10° (80–90) ΔV = 5° (75–80) ΔG0 = +10° ΔG = 0° [4, 30]: m = 0.2, θ0 = 76 [30, 210]: m = 0.0, θ0 = 87 ΔH = 17° (70–87) ΔV = 8° (72–80) ΔG0 = +8° ΔG = −10° [0, 15]: m = −0.7, θ0 = 87 [15, 210]: m = 0.0, θ0 = 76
ΔH = 13° (76–89) ΔV = 8° (70–78) ΔG0 = +10° ΔG = −2° [15, 210]: m = 0.0, θ0 = 80
25°
23°
21° −10°
30° −10°
18° −10°
17° −10°
ΔH = 15° (75–90) ΔV = 10° (60–70) ΔG0 = +10° ΔG = +5° [8, 60]: m = 0.0, θ0 = 85 [60, 120]: m = −0.2, θ0 = 95 [120, 210]: m = 0.0, θ0 = 60 15° +7°
[0, 20]: m = 0.9, θ0 = 42 [20, 210]: m = 0.0, θ0 = 60
T
ΔH = 15° (55–70)
[0, 210]: m =0.0, θ0 = 66
Polish
L
ΔH = 5° (73–78)
T
[0, 30]: m = −0.1, θ0 = 73 [30, 210]: m = 0.0, θ0 = 68 ΔH = 10° (80–90)
[0, 15]: m = 0.8, θ0 = 80 [15, 210]: m = 0.0, θ0 = 90
ΔHRmax Gmax
15°
ΔH = 21° (45–66) ΔV = 5° (50–55) ΔG0 = −5° ΔG = −10° [0, 30]: m = 0.6, θ0 = 43 [30, 210]: m = 0.0, θ0 = 62
a Nomenclature used in Table 2: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; ΔHmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
138
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Table 3 Data for dentin samples in the dry condition (Fig. 8).a Sample Dentin Dry
Etch
Type
WR
UV (x1)
EB (x1)
ΔHmax
L
ΔH = 35° (45–80)
ΔH = 10° (70–80) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 74
35°
T
[4, 60]: m = 0.3, θ0 = 56 [60, 210]: m = 0.1, θ0 = 78 ΔH = 10° (83–93)
ΔH = 5° (65–70) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −15° [0, 15]: m = −0.9, θ0 = 77 [15, 180]: m = 0.0, θ0 = 64 ΔH = 40° (60–100) ΔV = 0° (50) ΔG0 = +10° ΔG = +10° [0, 4]: m = −2.3, θ0 = 63 [4,15]: m = 3.4, θ0 = 40 [15, 150]: m = 0.1, θ0 = 92 ΔH = 10° (75–85) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 75
ΔH = 20° (65–85) ΔV = 15° (55–70) ΔG0 = 0° ΔG = −5° [0, 140]: m = −0.1, θ0 = 76 [140, 210]: m = 0.2, θ0 = 46
40°
ΔH = 10° (65–75) ΔV = 2° (68–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 70
35°
ΔH = 5° (95–100) ΔV = 10° (90–100) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 96
20°
[4,15]: m = 0.3, θ0 = 85 [15, 180]: m = −0.3, θ0 = 85
Polish
L
ΔH = 35° (45–80)
T
[0, 15]: m = −0.5, θ0 = 85 [15, 60]: m = 0.2, θ0 = 65 [60, 210]: m = 0.0, θ0 = 79 ΔH = 20° (50–70)
[4, 210]: m = 0.0, θ0 = 55
ΔHRmax ΔGmax
35°
ΔH = 7° (65–72) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −5° [0, 8]: m = −0.6, θ0 = 68 [8, 210]: m = 0.1, θ0 = 68 40° −15°
20° −15°
a Nomenclature used in Table 3: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; ΔHmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
60° and 90°. The most prominent changes in the contact angle values were observed at the beginning of each graphic and/or immediately after irradiation. Next, the contact angle reached a maximum value around which the data oscillated. Figs. 7, 9 and 11 show that the values of the contact angle in enamel increased remarkably in the dried samples (the values go from 10° to approximately 45° in samples TE and LE and from 50° to 70° in the TP and LP). However, the contact angle reached its maximum value (approximately 80°) after UV (x1) and EB (x1) radiations. In wet enamel samples, the maximum value was approximately 90°, and it was reached after UV (x3) and EB (x3) radiations. In the case of dentin, Figs. 8, 10 and 12 show that the variation of the contact angle values was not as high as it was in dried enamel samples. In fact, the values of the contact angle in the dried dentin samples stayed close to its maximum value (approximately 80°). Dentin samples were the only ones where values of over 100° were registered, and that happened after the EB radiation. It is important to note that it was only at the beginning of the experiment where there was a slight indication that the value of the contact angle had dependence on the surface preparation method. That is, for the dry WR samples it was observed that the value of the contact angle was higher in samples TP and LP, which were the mirror-finishpolished samples, than in samples TE and LE, which were the acid etched samples. However, in the wet samples, this tendency was not observed. In fact, wet dentin and enamel samples showed similar
behavior: the oscillation of the contact angle value was approximately 80° ± 10° and the maximum value was reached after EB (x3) radiation. Therefore, except for the dried and WR samples, the polishing and the etching of samples did not influence the contact angles. Another important fact to point out is that once the maximum contact angle value is reached in each treatment and after the “resting” time noted in the experimental section, the contact angle value did not decrease completely to the baseline observed in the dry sample: there was a “hysteresis” effect in the samples, which caused the contact angle value to remain close to the maximum value measured in the previous step. This effect is well observed in the data obtained before radiation. 3.2.1. Enamel graphs Let us comment on the data presented in Tables 1 and 2 regarding the parameters defined in Section 3.1 in order to obtain more information from the results presented for enamel in Figs. 7, 9 and 11. In enamel, the highest variation in ΔHmax (40°) was observed in the TP dried WR samples. In the case of ΔG, the dried samples after EB (x1) radiation showed a large gap (−15°). It is also worth noting that the value of ΔG in the dried enamel samples presented only negative signs, while ΔG0 were positive or zero. For wet enamel samples, a ΔHmax of 30° was observed in the LE sample but after UV (x2) radiation. The treatment with UV (×2) radiation showed the maximum angle variation ΔHRmax. In most of the treatments,
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
139
Table 4 Data for dentin samples in the wet condition (Fig. 10). The (x2) and (x3) graphs are shown in Fig. 12.a Sample Dentin Wet
Etch
Polish
Type
WR
UV (x1)
UV (x2)
EB (x1)
EB (x2)
EB (x3)
ΔHmax
L
ΔH = 20° (65–85) [0, 15]: m = 0.9, θ0 = 65 [15, 210]: m = 0.0, θ0 = 78
ΔH = 27° (65–92) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −10° [0, 210]: m = 0.1, θ0 = 72
ΔH = 20° (60–80) [4, 210]: m = 0.1, θ0 = 65
L
ΔH = 12° (70–82) [0, 5]: m = −1.3, θ0 = 77 [5, 30]: m = 0.2, θ0 = 70 [30, 210]: m = 0.0, θ0 = 80
ΔH = 15° (70–85) ΔV = 10° (60–70) ΔG0 = 20° ΔG = 0° [2,15]: m = 0.5, θ0 = 73 [15, 210]: m = 0.0, θ0 = 80 ΔH = 24° (70–94) ΔV = 16° (62–78) ΔG0 = +20° ΔG = −8° [0, 15]: m = −0.7, θ0 = 93 [15, 210]: m = 0.0, θ0 = 90 ΔH = 15° (65–80) ΔV = 13° (52–65) ΔG0 = +5° ΔG = 0° [0, 15]: m = 1.0, θ0 = 65 [15, 210]: m = 0.0, θ0 = 80
ΔH = 19° (71–90) ΔV = 20° (45–65) ΔG0 = +15° ΔG = +6° [0, 30]: m = −0.4, θ0 = 85 [30, 210]: m = 0.0, θ0 = 72 ΔH = 23° (77–100) ΔV = 20° (30–50) ΔG0 = +50° ΔG = +17° [4, 120]: m = 0.0, θ0 = 86 [150, 210]: m = 0.2, θ0 = 45 ΔH = 8° (76–84) ΔV = 7° (38–45) ΔG0 = +40° ΔG = +31° [4, 210]: m = 0.0, θ0 = 80
T
ΔH = 20° (50–70) [0, 15]: m = 0.3, θ0 = 48 [15, 210]: m = 0.0, θ0 = 65
ΔH = 27° (55–82) ΔV = 20° (40–60) ΔG0 = −5° ΔG = −5° [0, 8]: m = 1.8, θ0 = 68 [8, 210]: m = 0.1, θ0 = 65 ΔH = 35° (55–90) ΔV = 4° (68–72) ΔG0 = −8° ΔG = −17° [0, 5]: m = −2.3, θ0 = 63 [5, 30]: m = 1.2, θ0 = 50 [30, 210]: m = −0.1, θ0 = 86 ΔH = 7° (68–75) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −2° [0, 4]: m = −1.3, θ0 = 75 [4, 210]: m = 0.0, θ0 = 70
ΔH = 15° (75–90) ΔV = 5° (65–70) ΔG0 = +10° ΔG = +5° [0, 8]: m = −0.3, θ0 = 83 [8, 210]: m = 0.0, θ0 = 82
ΔH = 18° (70–88) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −5° [4, 210]: m = 0.1, θ0 = 75
25°
ΔHRmax ΔGmax
20°
ΔH = 10° (85–95) ΔV = 20° (60–80) ΔG0 = +8° ΔG = +5° [0, 30]: m = −0.2, θ0 = 92 [30, 210]: m = 0.0, θ0 = 92 ΔH = 13° (80–93) ΔV = 20° (30–50) ΔG0 = +40° ΔG = +30° [0, 15]: m = 0.5, θ0 = 75 [30, 210]: m = 0.0, θ0 = 92 ΔH = 15° (80–95) ΔV = 20° (55–75) ΔG0 = +10° ΔG = +5° [0, 5]: m = −2.6, θ0 = 82 [5, 30]: m = −0.5, θ0 = 95 [30, 210]: m = 0.0, θ0 = 82 ΔH = 25° (65–90) ΔV = 5° (30–35) ΔG0 = +40° ΔG = +30° [0, 8]: m = −2.0, θ0 = 83 [8, 30]: m = 1.1, θ0 = 56 [30, 210]: m = 0.0, θ0 = 88 25° +30°
27°
T
ΔH = 7° (68–75) ΔV = 10° (60–70) ΔG0 = −5° ΔG = −2° [0, 60]: m = 0.2, θ0 = 64 [60, 210]: m = −0.1, θ0 = 83 ΔH = 20° (80–100) ΔV = 8° (80–88) ΔG0 = −5 ΔG = −8° [0, 210]: m = 0.1, θ0 = 80
24° −8°
23° +31°
35° −17°
ΔH = 8° (74–82) ΔV = 7° (68–75) ΔG0 = 0° ΔG = −1° [0, 210]: m = 0.0, θ0 = 73
ΔH = 12° (70–82) ΔV = 4° (68–72) ΔG0 = 0° ΔG = −2° [0, 210]: m = 0.1, θ0 = 69
20° −8°
27°
35°
a Nomenclature used in Table 4: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔGmax: gap between the values before and after radiation at t = 0; Δmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
ΔGmax was of − 10° and ΔG0 was of + 10°. It is worth noting that the values of ΔG in wet enamel samples went from negative to positive, and those of ΔG0 were positive for EB(xn) radiations. In the case of UV radiation, the values of ΔG0 had negative signs.
3.2.2. Dentin graphs Now let us to comment on the data presented in Tables 3 and 4 obtained from Figs. 8, 10, and 12 for dentin. In dried dentin, the WR samples showed a high variation in the contact values. The sample with the highest ΔHmax (40°) was TE after UV(x1) radiation, which also corresponded to the sample with the highest ΔHRmax value (40°). In the case of ΔGmax, the dried samples after UV and EB radiation showed a gap of − 15°. It is worth noting that, as in the case of enamel, the values of ΔG in dried dentin samples had a negative sign, and ΔG0 was zero in most of the cases. In wet conditions, the highest value of ΔHmax(30°) was observed in the LE sample after UV (x1) radiation. In fact, UV (x1) was the treatment with the maximum value of ΔHRmax. After EB radiation, dentin presented ΔGmax values of +30°. It is important to note that the values of ΔG in wet dentin samples had a positive sign after EB (xn) radiation. The values of ΔG0 had positive sign, and they incremented with EB (xn) radiation (it was of +50° for sample TE and of +40° for the LP sample after the EB(x3) radiation). In the case of the UV radiation, the values of ΔG0 had negative sign.
4. Discussion The results indicated that enamel shows greater variation in the contact angle values than dentin, which correspond well with previously reported data [16–20]. The observed variation in enamel and not in dentin must be related both to the inorganic/organic material ratio in these tissues (enamel: approx. 24; dentin: approx. 2.8) and to the size and shape of the HAP grains (much smaller in dentin) [13,14]. To this end, with the work so far we have not been able to elucidate the relationship between these modified systems, the structure of the teeth and the organic and inorganic components of enamel and dentin. However, the variation in the contact angle decreased in wet enamel samples, whereas in dentin it remained notably similar in most of the samples, dry or wet. Therefore, in enamel, the pre-wetting of the sample has a significant role in increasing the contact angle, but not in dentin. Newman et al. [6] measured the contact angle on enamel surfaces and found values of 50° for water and 35° for epoxy resins on pumiced teeth. However, they also found that the contact angle became hydrophilic with a phosphoric acid treatment and that the tooth surfaces can be restored to their original hydrophobic condition by brushing with pumice. Evidently, surface wetting and surface treatment should be important factors in the contact angle experiments; however, in our case, except for the dried WR samples at the beginning of the experiment, we did not find at clear difference in the contact angle behavior between the polished and etched samples.
140
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 7. Contact angle versus time graphs for enamel samples in the dried condition: A) without radiation; B) after the UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
The effect of UV radiation on enamel and dentin samples was observed immediately after irradiation. UV irradiation reduces the contact angle, although it recovers after 5 min in enamel and after 15 min in dentin. After recovery, the contact angle reached even higher values than those observed in WR samples. After EB radiation, the contact angle reaches the highest values in all samples. These values were 90° ± 10° in enamel and 80° ± 10° in dentin, regardless of whether the samples were dry or wet. Only the TP dentin dry sample reached a contact angle of 100°.
The results also indicated a cumulative trend in the contact angle values. This is a type of “contact angle hysteresis”. Although this effect was observed in all samples, it was more obvious in the dry samples. 4.1. The role of water The results clearly indicated that water by itself changed the wettability properties of the tooth surfaces. In dry samples, the low contact angle values observed can be attributed to an inherent hydrophilicity
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
141
Fig. 8. Contact angle versus time graphs for dentin samples in dried conditions: A) without radiation; B) after UV(x1) radiation; C) after EB(x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
of dry materials. The inorganic component of tooth structure (i.e., HAP) has a strong affinity to water [21], so during first wetting, the hydrophilicity might be due to hydrogen bonds between HAP and absorbed water [22]. Following Aronov [9,12], human tooth material surfaces and HAP surfaces should present a certain amount of electric charge; this electric charge is produced by polarization of the surface's atoms. Lee et al. [23], investigating the surface polarization of HAP, found that polarization
and relaxation of the surface extends to a distance of several unit cells into the bulk. The atomic forces are perfectly balanced in the bulk but not at the surface. According to their calculus, the polarization generated was − 34 mC/m2 and the surface energy was 1.296 J/m2 approximately. Vandiver et al. [24] found a negative charge for stoichiometric HAP through zeta potential measurements in powder suspensions, indicating that this negative charge increases the preferential concentration groups at the top of the surface. of PO3− 4
142
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 9. Contact angle versus time graphs for enamel samples in wet conditions: A) without radiation; B) after the UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
Therefore, when human tooth and HAP surfaces in dry conditions come into contact with water, the effect of charge is that the hydrogen groups. In a thin layer ions H+ bond to the oxygen atoms in the PO3− 4 of water, the generation of hydrogen bonds and an effective positive charge are produced on the HAP-related surfaces and the properties of wettability are modified, thereby generating hydrophilicity. After wetting and the passage of time, materials showed increased contact angles, indicating an increase in hydrophobicity. This could be the result of strong repulsive forces between the previously wet surface
and the new water. It is well known that after wetting, dried samples invariably show the traces of a layer of previous water on the tooth surface and this water layer modifies the surface energy [1]. 4.2. UV irradiation When UV waves interact with the water molecules in the thin film of water that covers the HAP surface, contact angle values decrease precipitously and the hydrophilic system is generated. However, once the
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
143
Fig. 10. Contact angle versus time graphs for dentin samples in wet conditions: A) without radiation; B) after UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
effect of the UV radiation disappears, the surface resets up the hydrophobic state, and the contact angle increases to its previous value. In dry samples, the disorder produced by the UV waves to the surface atoms of HAP is such that they cannot reach the wet conditions completely, but rather the radiation produces an almost permanent damage in the arrangement of the surface atoms of HAP, which is not observed when the surface is covered with a thin film of water.
4.3. EB radiation In the condition of EB radiation, a negative electrically charged beam is incident on an insulator surface. Therefore, the incident electrons are trapped on the surface, thus creating a negative charge accumulation. Aronov et al. [9,12] indicated that the HAP surface presents a P-type band gap semiconductor behavior, with an energy gap of approximately
144
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 11. Contact angle versus time graphs for enamel samples in wet conditions: A) after UV (x2) radiation; B) after EB (x2) radiation; C) after EB (x3) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
4 eV. Next, the strong influence of the injected electrons on the surface electric potential of the HAP substrate is due to charge trapping in the vicinity of the HAP surface. The charge trapped on localized centers leads to electric field generation near the surface according to the Laplace equation ∇ϕ = − E [25]. This induced electric field may reach the magnitude of E = 106 V/cm at the flat surface [9,12]. In this way, the molecule is then polarized by the electric field and the surface free energy is modified, producing the observed results.
4.4. “Contact angle hysteresis” The results indicated the presence of a “contact angle hysteresis”. This idea is not new as some researchers also observed it for quite a long time [26,27]. During the contact angle measurement, the surface tension acting on the solid, the liquid, and the solid–liquid interface is balanced. However, if the surface was previously wetted, the surface “roughness” is increased because of a trace of water or an oxidation
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
145
Fig. 12. Contact angle versus time graphs for dentin samples in wet conditions: A) after UV (x2) radiation; B) after EB (x2) radiation; C) after EB (x3) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
film on the surface left there from the previous measurement, giving rise to this “contact angle hysteresis”. This is in fact a proof that the measurement of the contact angle was always done in the same place of the sample. To fully understand the effects of tooth structures on the contact angle measurements, further studies are needed, including investigations on: (i) grain size, (ii) structure orientation, and (iii) the role of
the inorganic/organic ratio, and the measurements must always be conducted on fresh new samples. 5. Conclusions Water itself modified the wettability of the human tooth surfaces, reaching contact angles of up to 80°, but the contact angle showed
146
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
more variability on enamel than on dentin surfaces. UV and EB radiation also increased the hydrophobicity of human tooth materials, but EB radiation allowed the surfaces to reach the maximum contact angle values (90°, hydrophobic) and produced “jumps” of up to 30°. The microstructure of the human tooth surfaces, except in dried and WR samples, was not an influencing parameter. Acknowledgments JRG is grateful to DGAPA-UNAM (contract IN106713) for the financial support and to CONACYT (203257) and PASPA-DGAPA-UNAM (057/2013) for sabbatical support at the University of Lille 1. References [1] S. Hussain, Textbook of dental materials, Jaypee Brothers Publishers, 2008. 494. [2] H.A. Wege, J.A. Aguilar, M.Á. Rodriguez-Valverde, M. Toledano, R. Osorio, M.A. Cabrerizo-Vilchez, Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments, J. Colloid Interface Sci. 263 (2003) 162–169. [3] H.A. Wege, J.A. Holgado-Terriza, J.I. Rosales-Leal, R. Osorio, M. Toledano, M.A. Cabrerizo-Vilchez, Contact angle on dentin surfaces measured with ADSA on drops and bubbles, Colloids and Surfaces A: Physicochem. Eng. Aspects 206 (2002) 469–483. [4] A.W. Neumann, R. David, Y. Zuo, Applied surface thermodynamics, Surfactant Science, Second ed.CRC Press, 2012. [5] F. Namen, J. Galan Jr., J. Farias de Oliveira, R. Derossi Cabreira, F. Costa e Silva Filho, A. Balduino Souza, G. de Deus, Surface properties of dental polymers: measurements of contact angles, roughness and fluoride release, Mater. Res. 11 (2008) 239–243. [6] G.V. Newman, W.H. Snyder, C.E. Wilson, Acrylic adhesives for bonding attachments to tooth surfaces, Angle Orthod. 38 (1968) 12–18. [7] Y. Oshida, R. Sachdeva, S. Miyazaki, Changes in contact angles as a function of time on some pre-oxidized biomaterials, J. Mater. Sci. C Mater. Med. 3 (1992) 306–312. [8] A.V. Grosse, The relationship between surface tension and energy of liquid metals and their heat of vaporization at the melting point, J. Inorg. Nucl. Chem. 26 (1964) 1349–1361. [9] D. Aronov, R. Rosen, E.Z. Ron, G. Rosenman, Electron-induced surface modification of hydroxyapatite-coated implant, Surf. Coat. Technol. 202 (2008) 2093–2102.
[10] G.V. Reddy, N.S. Reddy, J. Itttigi, K.N. Jagadeesh, A comparative study to determine the wettability and castability of different elastomeric impressions materials, J. Contemp. Dent. Pract. 13 (2012) 356–363. [11] E.A. Vogler, Structure and reactivity of water at biomaterial surfaces, Adv. Colloid Interface Sci. 74 (1998) 69–117. [12] D. Aronov, R. Rosen, E.Z. Ron, G. Rosenman, Tunable hydroxyapatite wettability: effect on adhesion of biological molecules, Process Biochem. 41 (2006) 2367–2372. [13] R.Z. Le Geros, Calcium phosphates in oral biology and medicine, in: Howard M. Myers (Ed.), Monographs in Oral Sciences, vol. 15, Karger in Basel, New York, 1991. [14] D. Muster (Ed.), Calcium Phosphates in Oral Biology and Medicine, Hard Tissue Repair and Replacement, Elsevier, Amsterdam, 1992. [15] H.J. Busscher, A.W.J. Van Pelt, H.P. de Jong, J. Arends, Effect of spreading pressure on surface free energy determinations by means of contact angle measurements, J. Colloid Interf. Sci. 95 (1983) 23–27. [16] H.J. Busscher, H.M. Uyen, H.P. de Jong, J. Arends, G.A.M. Kip, Adsorption of aminefluorides on human enamel, J. Dent. 16 (1988) 166–171. [17] P.O. Glantz, The adhesiveness of teeth, J. Colloid Interface Sci. 37 (1971) 281–290. [18] A.W.J. Van Pelt, H.P. de Jong, H.J. Busscher, Dispersion and polar surface free energy of human enamel (an in vitro study), J. Biomed. Mater. Res. 17 (1983) 637–641. [19] H.P. de Jong, P. de Boer, H.J. Busscher, Surface free energy changes of human enamel during pellicle formation-an in vivo study, Caries Res. 18 (1984) 408–415. [20] H.P. de Jong, A.W.J. van Pelt, H.J. Busscher, The effect of topical fluoride application on the surface free energy of human enamel-an in vitro study, J. Dent. Res. 63 (1984) 635–641. [21] M. Mondon, C. Ziegler, Changes in water contact angles during the first phase of setting of dental impression materials, Int. J. Prosthodont. 16 (2003) 49–53. [22] S. Rüttermann, T. Trellenkamp, N. Bergmann, W.H.M. Raab, H. Ritter, R. Janda, A new approach to influence contact angle and surface free energy of resin-based dental restorative materials, Acta Biomater. 7 (2011) 1160–1165. [23] W.T. Lee, M.T. Dove, E.K.H. Salje, Surface relaxations in hydroxyapatite, J. Phys. Condens. Matter 12 (2000) 9829–9841. [24] J. Vandiver, D. Dean, N. Patel, W. Bonfield, C. Ortiz, Nanoscale variation in surface charge of synthetic hydroxyapatite detected by chemically and spatially specific high-resolution force spectroscopy, Biomaterials 26 (2005) 271–283. [25] J. Reyes-Gasga, R. García, L. Vargas-Ulloa, In-situ observation of fractal structures and electrical conductivity in human tooth enamel, Philos. Mag. A 75 (1997) 1023–1040. [26] F.M. Fowkes (Ed.), Contact Angle, Wettability and Adhesion, Advances in Chemistry Series, vol. 43, Am. Chem Soc., 1964 [27] D.F. Moore, Principles and Applications of Tribology, International Series on Materials Science and Technology, vol. 14, Pergamon Press, New York 1975, pp. 148–150.
Contents lists available at ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Wettability modification of human tooth surface by water and UV and electron-beam radiation Gaby E. Tiznado-Orozco a,b, José Reyes-Gasga a,c,⁎, Florina Elefterie a, Christophe Beyens a, Ulrich Maschke a, Etienne F. Brès a a b c
UMET, Bâtiment C6, Université de Lille 1, Sciences et Technologies, 59650 Villeneuve d'Ascq, France Unidad Académica de Odontología, Universidad Autónoma de Nayarit, Edificio E7, Ciudad de la Cultura “Amado Nervo”, C.P. 63190 Tepic, Nayarit, Mexico Instituto de Física, UNAM, Circuito de la Investigación s/n, Ciudad Universitaria, 04510 Coyoacan, México, D.F., Mexico
a r t i c l e
i n f o
Article history: Received 5 July 2014 Received in revised form 20 February 2015 Accepted 10 June 2015 Available online 21 July 2015 Keywords: Wettability Enamel Dentin UV radiation EB radiation
a b s t r a c t The wettability of the human tooth enamel and dentin was analyzed by measuring the contact angles of a drop of distilled water deposited on the surface. The samples were cut along the transverse and longitudinal directions, and their surfaces were subjected to metallographic mirror-finish polishing. Some samples were also acid etched until their microstructure became exposed. Wettability measurements of the samples were done in dry and wet conditions and after ultraviolet (UV) and electron beam (EB) irradiations. The results indicate that water by itself was able to increase the hydrophobicity of these materials. The UV irradiation momentarily reduced the contact angle values, but they recovered after a short time. EB irradiation raised the contact angle and maintained it for a long time. Both enamel and dentin surfaces showed a wide range of contact angles, from approximately 10° (hydrophilic) to 90° (hydrophobic), although the contact angle showed more variability on enamel than on dentin surfaces. Whether the sample's surface had been polished or etched did not influence the contact angle value in wet conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Surface wettability indicates the ability of a liquid to wet the surface of a solid [1]. In biomaterials, surface wettability is an important property because it may indicate cells immobilization, drug delivery and gene transfer, among other applications [2,3]. For human tooth and skin, wettability is of interest in the study of pharmaceutical and cosmetic products [4]. In tooth, hydrophobicity affects initial water absorption and the adhesion of oral bacteria [5]. The success of any dental restorative treatment depends on the adhesion to the dental tissue; but the improvement of adhesion implies the need for acid etching of enamel surfaces [3]. On the other hand, the analysis of adhesive systems in dentistry could identify a way to eliminate, for example, the metal brackets [6]. The wettability of a material is usually determined by measuring the contact angle formed between the surface of the material and the line tangent to the curved surface of a liquid drop at the point of contact [7]. The contact angle depends on the surface energy of the material and the surface tension of the liquid. In addition, the surface energy is ⁎ Corresponding author at: Instituto de Física, UNAM, Circuito de la Investigación s/n, Ciudad Universitaria, 04510 Coyoacan, México, D.F., Mexico. E-mail addresses: [email protected] (G.E. Tiznado-Orozco), jreyes@fisica.unam.mx (J. Reyes-Gasga), elefterie_fl[email protected] (F. Elefterie), [email protected] (C. Beyens), [email protected] (U. Maschke), [email protected] (E.F. Brès).
http://dx.doi.org/10.1016/j.msec.2015.06.054 0928-4931/© 2015 Elsevier B.V. All rights reserved.
correlated with different physical parameters, such as elastic modulus, melting point, and heat and energy of vaporization [1,4,8]. The contact angle is also related to the surface roughness of the substrate, which can be modified during the sample preparation process [5]. A brief theoretical analysis of the relationship between the contact angle and the surface energy is given by Aronov et al. [9]. Contact angle measurement is probably the most popular method to determine the hydrophobicity and/or hydrophilicity of the surfaces of materials. A greater contact angle indicates hydrophobicity (poor wetting). Several authors [6,10] consider that a contact angle of less than 90° is an indication of a hydrophilic material, whereas a contact angle larger than 90° indicates a hydrophobic material. However, several others [11] consider that an angle of 65° is more appropriate for this differentiation. Wettability modification of hydroxyapatite (HAP, Ca10(PO4)6(OH)2) by electron irradiation has been reported by Aronov et al. [9,12]. These researchers showed that the use of low-energy electron irradiation modifies the wettability of the HAP surface in a wide range of contact angles, extending from 10° (hydrophilic) to 100° (hydrophobic). They indicated that the incident electrons generate electron/hole pairs, resulting in significant variation of the surface potential of the hydroxyapatite (the penetration depth is approximately 2 nm for 100 eV incident electrons) and giving rise to the wettability modification observed. Based on this information and our experience in HAP and human tooth material, we undertook the task of studying the contact angles
134
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
on the surface of the human tooth enamel and dentin before and after electron beam (EB) radiation. The samples were cut in such a way that their surfaces were transverse (T) and longitudinal (L) to the tooth surface. The surfaces of the samples were mirror-finish polished, and some of them were also etched with phosphoric acid to reveal their microstructure. The samples were placed under dry and wet conditions, and the EB radiation dose was set at three different values. Because ultraviolet (UV) radiation can modify the wettability of the surface of certain ceramics, and due to the importance of this radiation in dental work, we decided to include UV irradiation as a condition in this study. Human tooth enamel and dentin are composed mostly of natural HAP crystals [13]. Enamel is comprised of 96% HAP and 4% organic material, while dentin is 74% HAP and 26% organic material. Water content varies from 1 to 6% by weight in enamel and from 15 to 30% in dentin [14]. In enamel, HAP crystals have a tablet shape with dimensions of approximately 30 × 50 × 100 nm. These tablets are arranged in a textured manner and provide form to the enamel prisms. The organic material in enamel wraps the HAP crystals. In dentin, conversely, HAP crystals have a platelet-like shape, and they are smaller than those in enamel. These crystals are immersed in an organic matrix and their density is higher around the dentinal tubules than in the space between them. 2. Experimental procedure Human tooth enamel and dentin samples were taken from permanent human premolar teeth from persons 18 to 25 years old. All of the extractions were done for orthodontic or periodontal reasons. 2.1. Preparation of samples The health of the teeth was carefully and clinically examined before and after being split into halves using a low-speed diamond micrometer saw (IsoMet, Buehler). Enamel and dentin were mechanically separated from each other using a hand-guided dental drill and a light microscope (Carl Zeiss model Axiovert 25). Enamel and dentin were cut in such a way that the surfaces of the samples were flat and parallel (named “transverse”, or “T”, samples hereafter) or perpendicular (named “longitudinal”, or “L”, samples hereafter) to the tooth surface. Next, the surface of the samples was mirror finish-polished with silicon carbide paper and alumina slurries in a grinder–polisher (MiniMet, Buehler). Fig. 1 shows the light microscopy images of one of the L samples (Fig. 1A) and one of the T samples (Fig. 1B). Some of the samples were left as obtained after the mirror-finish polishing (named “polished”, or “P”, samples hereafter) and others were etched with phosphoric acid until their microstructure was
Fig. 2. SEM image of the surface of one of the samples just after polishing. The smear layer produced during polishing covers the microstructure of human tooth.
revealed (named “etched”, or “E”, samples hereafter). Fig. 2 shows the SEM image of one of the mirror finish-polished surfaces but, because of the smear layer produced during polishing, no contrast of the human tooth structure is observed. However, when these surfaces are etched with phosphoric acid, the enamel prisms and the dentinal tubules are observed (Fig. 3). All the samples were ultra-sonicated at 30 °C for 10 min in an ultrasonic bath (S30 H Elmasonic). The samples were kept in a desiccator following this procedure. Therefore, we worked with 4 enamel samples (2 in transversal direction: one polished and one etched; and 2 in longitudinal direction: one polished and one etched), and 4 dentin samples (2 in transversal direction: one polished and one etched; and 2 in longitudinal direction: one polished and one etched). The dentin samples, as well as the enamel ones, were labeled TE, TP, LE and LP according to the cutting direction and to the surface treatment. 2.2. The contact angle measurement For measuring the contact angle, a distilled water sessile drop measuring 7 mm in diameter was deposited on the surface of the sample by a syringe and the contact angle was measured using the Contact Angle Meter Digidrop (GBX Instrumentation Scientifique, France). For this equipment, the standard deviation for the contact angle measurements was ± 5°. Optical inspection of the drop was performed by a camera device and digital imaging techniques. The measurement of the contact angle was taken 60 s after the deposition of the drop, the estimated time for equilibrium to be reached. For data collection, measurements were made in two drops at the same time and the average was used for the graphs. After each measurement, the water drop was wiped with
Fig. 1. Light microscopy images of two of the human tooth samples. A) Enamel in the longitudinal direction. B) Dentin in the transverse direction. The area where the drops of water were placed is indicated by white rectangles.
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
135
Fig. 3. SEM images of the surface of samples etched with phosphoric acid. A) Enamel prisms in the transverse view. B) Enamel prisms in the longitudinal view. C) Dentinal tubules in the transverse view. D) Dentinal tubules in the longitudinal view.
Fig. 4. Flowchart indicating the experimental flux followed to measure the contact angle in the enamel and dentin samples (see text). Note the existence of the loop for n = 1, 2, and 3.
absorbent paper. Water was exchanged for each measurement session, and the instruments were cleaned twice with distilled water. The contact angle was measured first in the dried enamel and dentin samples (referred to as “dried samples” hereafter). These samples are named “without radiation”, or “WR”. Later on, the angle was measured again in the same samples, first after ultraviolet light radiation (UVradiation), and then after electron beam irradiation (EB-radiation). Afterward, this process was repeated in the same samples but under wet conditions (referred to as “watered samples” hereafter). Fig. 4 shows a schematic flow diagram to facilitate the understanding of the experimental procedure. Seventy-two hours passed between the UV and EB radiation processes, and the samples were kept in a desiccator during that time. For wet conditions, the samples were kept in water for 7 days after the last measurement of the contact angle in the dry condition. Again, 72 h elapsed between UV and EB radiations, but on this occasion the samples were kept in water. Between two consecutive EB radiation cycles, the samples were kept in water for 7 days. The contact angle was measured at 0, 2, 4, 8, 15, 30, 60, 90, 120, 150, 180, and 210 min. In the case of radiation, the contact angle was also measured at 15, 10, 5 and 3 min prior to irradiation.
Fig. 5. Light microscope images of a drop on the surface of the LE enamel sample. A) WR in dried conditions; B) in wet conditions after EB(x2) radiation. Note the hydrophilic state in (A) and the hydrophobic state in (B).
136
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
was used. The EB radiation conditions for the first treatment (named “x1” hereafter) were: voltage of 180 kV, speed of 9 m/min, air gap of 18 mm, and current of 1 mA for a surface dose of 16 kGy. The conditions for the second EB treatment (named “x2” hereafter) were: voltage of 180 kV, speed of 3 m/min, air gap of 10 mm, and current of 1 mA for a surface dose of 26 kGy. In the third EB treatment (named “x3” hereafter) the radiation conditions were: voltage of 180 kV, speed of 3 m/min, air gap of 10 mm, and current of 5 mA for a surface dose of 132 kGy. The EB irradiations were done in inert atmosphere (N2) and UV irradiations were done in ambient atmosphere. 3. Results
Fig. 6. Schematic definition of the parameters used in the interpretation of results. m is the slope of the line; ΔV is the variation of the contact angle before radiation; ΔH is the variation of contact angle after radiation; ΔG is the gap between the values before and after radiation (difference ΔH − ΔV); ΔG0 is the gap at t = 0.
In all the samples, not only in those immersed in water or UV and EB irradiated, the contact angle was modified as a function of time. Fig. 5 presents two light microscope images of a drop on the surface of one of the human tooth enamel samples as an example of the hydrophilic (Fig. 5A) and of the hydrophobic (Fig. 5B) states.
2.3. UV and EB irradiation conditions
3.1. Definition of graph parameters
For UV irradiation, an AERI-Honle UV Technology equipment (Construction Mécanique, Power: 12 kW) was used. The UV radiation cycle speed was of 100 mm/s, and the UV irradiation dose was 1352 mJ/cm2 in the first treatment (named “x1” hereafter) and 2070 mJ/cm2 in the second treatment (named “x2” hereafter). For the electron beam (EB) irradiation, a COMET E-Beam Sources equipment (COMET Technology Model EBLab 180/210, Switzerland)
For a better presentation and reading of the contact angle versus time graphs, we find of great help to define some parameters (see Fig. 6). These parameters were taken only as qualitative indicators of the tendency of the graphs. First note that, within a given range of time t, the values of the contact angle can be represented, after the least-square method, by the line θ = mt + θ0, where θ0 is the intercept (i.e., there is a linear relationship
Table 1 Data for enamel samples in the dry condition (Fig. 7).a Sample enamel Dry
Etch
Polish
Type
WR
UV (x1)
EB (x1)
ΔHmax
L
ΔH = 38° (12–50)
ΔH = 8° (60–68) ΔV = 15° (45–60) ΔG0 = +15° ΔG = 0° [0, 210]: m = 0.0, θ0 = 65
38°
T
[0, 30]: m = 0.7, θ0 = 12 [30, 210]: m = 0.1, θ0 = 30 ΔH = 30° (15–45)
ΔH = 30° (55–85) ΔV = 15° (55–70) ΔG0 = +20° ΔG = −15° [0, 210]: m = 0.1, θ0 = 72
30°
L
[0, 30]: m = 0.7, θ0 = 12 [30, 210]: m = 0.1, θ0 = 30 ΔH = 20° (55–75)
T
ΔH = 10° (65–75) ΔV = 10° (65–75) ΔG0 = −5° ΔG = −10° [0, 210]: m = 0.0, θ0 = 68
ΔH = 17° (75–92) ΔV = 9° (76–85) ΔG0 = 0° ΔG = −10° [0, 8]: m = −1.3, θ0 = 86 [8, 210]: m = 0.1, θ0 = 78 ΔH = 12° (80–92) ΔV = 5° (75–80) ΔG0 = +10° ΔG = 0° [5, 210]: m = 0.0, θ0 = 90
20°
[0, 20]: m = 0.9, θ0 = 54 [20, 210]: m = 0.0, θ0 = 60 ΔH = 40° (30–70)
ΔH = 16° (40–56) ΔV = 15° (35–50) ΔG0 = +10° ΔG = −10° [0, 5]: m = 2.0, θ0 = 40 [5, 210]: m = 0.0, θ0 = 50 ΔH = 10° (50–60) ΔV = 4° (48–52) ΔG0 = +6° ΔG = −2° [0, 5]: m = −0.3, θ0 = 58 [5, 210]: m = 0.2, θ0 = 52 ΔH = 10° (70–80) ΔV = 4° (68–72) ΔG0 = 0° ΔG = −2° [0, 210]: m = 0.2, θ0 = 75
16° −10°
30° −15°
ΔHRmax ΔGmax
[0, 15]: m = 2.75, θ0 = 35 [15, 210]: m = 0.1, θ0 = 60 40°
40°
a Nomenclature used in Table 1: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; Δmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
within the contact angle and time; see also [15]). The slope m indicates the rate of increase or decrease of the contact angle value in (°)/min. Now let ΔV and ΔH be the range of variation for the contact angle values obtained before and after the radiation treatment, respectively (in the case of WR samples only ΔH is considered). Therefore, ΔHmax indicates the maximum variation of the contact angle values observed in each sample, and ΔHRmax indicates the maximum variation of the contact angle values after each type of radiation (or WR) in dry or wet conditions. The difference or gap between the values of the contact angle before and after irradiation can also be taken into account for each sample. Let ΔG represent this gap, which can be calculated as the difference between the minimum value of the contact angle in ΔH and the maximum value of the contact angle in ΔV. The value and sign of ΔG reflects the behavior of the contact angle after a period of time following irradiation: the sign “+” means an increase in the contact angle values relative to the values measured prior to irradiation (m N 0); the sign “−” means a decrease in the contact angle values (m b 0). The parameter ΔGmax indicates the maximum difference for a given type of radiation and sample conditions. The value of ΔG0 indicates the presence or absence of a “jump” in the contact angle value after irradiation. The value is calculated by the difference between the last value of the contact angle measured before irradiation and the first value measured after irradiation (i.e., at t = 0).
137
The values of m, θ0, ΔH, ΔV, ΔG0, ΔG, ΔHmax, ΔHRmax, and ΔGmax, have been included in Tables 1 to 4. Note that after a short time following the application of both UV and EB radiation, the values of m are close to zero. 3.2. The contact angle versus time graphs Figs. 7 and 8 show the graphs for enamel and dentin samples in dry conditions: WR (Figs. 7A and 8A) and with UV (x1) (Figs. 7B and 8B) and EB (x1) (Figs. 7C and 8C) radiation. In the graphs corresponding to the radiation treatments, the moment of irradiation is indicated by a vertical dark line set at t = 0. The values of the contact angle prior to radiation (to the left of the dark line and indicated by empty symbols) were also included in the graphs. Figs. 9 and 10 show the graphs for enamel and dentin samples in wet conditions: WR (Fig. 9A and 10A) and with UV (x1) (Figs. 9B and 10B) and EB (x1) (Figs. 9C and 10C) radiation. The graphs for enamel and dentin in wet conditions after the UV (x2) radiation and EB (x2) and EB (x3) radiation are shown in Figs. 11 and 12. The data obtained from Fig. 7 are shown in Table 1, those obtained from Figs. 9 and 11 are shown in Table 2, those obtained from Fig. 8 are shown in Table 3, and those obtained from Figs. 10 and 12 are shown in Table 4. As observed in the figures, most of the graphs are notably similar: the data show a more or less flat trend and they are contained between
Table 2 Data for enamel samples in the wet condition (Fig. 9). The (x2) and (x3) graphs are shown in Fig. 11.a Sample Enamel Wet
Etch
Type
WR
UV (x1)
UV (x2)
EB (x1)
EB (x2)
EB (x3)
ΔHmax
L
ΔH = 15° (45–60)
ΔH = 8° (65–73) ΔV = 20° (50–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 63
ΔH = 30° (62–92) ΔV = 6° (62–68) ΔG0 = 0° ΔG = −6° [0, 30]: m = 1.0, θ0 = 62 [30, 110]: m = 0.0, θ0 = 90 [110–210]: m = 0.0, θ0 = 82 ΔH = 10° (50–60) ΔV = 5° (55–60) ΔG0 = −5° ΔG = −10° [2,15]: m = 0.2, θ0 = 50 [15, 210]: m = 0.0, θ0 = 56
ΔH = 10° (65–75) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 70
ΔH = 15° (80–95) ΔV = 25° (55–80) ΔG0 = +15° ΔG = 0° [0, 8]: m = −0.6, θ0 = 94 [8, 210]: m = 0.0, θ0 = 87
ΔH = 7° (87–94) ΔV = 8° (72–80) ΔG0 = +10° ΔG = +7° [0, 210]: m = 0.0, θ0 = 90
30°
ΔH = 13° (50–63) ΔV = 5° (55–60) ΔG0 = 0° ΔG = −10° [0, 30]: m = −0.2, θ0 = 55 [30, 210]: m = 0.0, θ0 = 60
ΔH = 11° (80–91) ΔV = 10° (68–78) ΔG0 = +5° ΔG = +2° [10, 210]: m = 0.0, θ0 = 82
21°
ΔH = 17° (65–82) ΔV = 10° (60–70) ΔG0 = +5° ΔG = −5° [0, 60]: m = 0.0, θ0 = 70 [60, 180]: m = 0.1, θ0 = 63 ΔH = 10° (65–75) ΔV = 10° (60–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.1, θ0 = 65
ΔH = 25° (70–95) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −10° [0, 15]: m = 0.7, θ0 = 75 [15, 180]: m = 0.0, θ0 = 92 ΔH = 23° (65–88) ΔV = 15° (60–75) ΔG0 = 0° ΔG = −10° [0, 120]: m = 0.1, θ0 = 78 [120, 210]: m = −0.2, θ0 = 110
ΔH = 18° (62–80) ΔV = 2° (70–72) ΔG0 = 0° ΔG = −10° [0, 30]: m = −0.4, θ0 = 68 [30, 210]: m = 0.0, θ0 = 79 ΔH = 10° (80–90) ΔV = 0° (78) ΔG0 = +10° ΔG = +2° [4, 30]: m = 0.7, θ0 = 68 [30, 210]: m = 0.0, θ0 = 86
ΔH = 5° (75–80) ΔV = 5° (55–60) ΔG0 = +15° ΔG = +5° [0, 4]: m = 1.0, θ0 = 75 [4, 30]: m = −0.2, θ0 = 80 [30–180]: m = 0.0, θ0 = 75 ΔH = 10° (80–90) ΔV = 5° (75–80) ΔG0 = +10° ΔG = 0° [4, 30]: m = 0.2, θ0 = 76 [30, 210]: m = 0.0, θ0 = 87 ΔH = 17° (70–87) ΔV = 8° (72–80) ΔG0 = +8° ΔG = −10° [0, 15]: m = −0.7, θ0 = 87 [15, 210]: m = 0.0, θ0 = 76
ΔH = 13° (76–89) ΔV = 8° (70–78) ΔG0 = +10° ΔG = −2° [15, 210]: m = 0.0, θ0 = 80
25°
23°
21° −10°
30° −10°
18° −10°
17° −10°
ΔH = 15° (75–90) ΔV = 10° (60–70) ΔG0 = +10° ΔG = +5° [8, 60]: m = 0.0, θ0 = 85 [60, 120]: m = −0.2, θ0 = 95 [120, 210]: m = 0.0, θ0 = 60 15° +7°
[0, 20]: m = 0.9, θ0 = 42 [20, 210]: m = 0.0, θ0 = 60
T
ΔH = 15° (55–70)
[0, 210]: m =0.0, θ0 = 66
Polish
L
ΔH = 5° (73–78)
T
[0, 30]: m = −0.1, θ0 = 73 [30, 210]: m = 0.0, θ0 = 68 ΔH = 10° (80–90)
[0, 15]: m = 0.8, θ0 = 80 [15, 210]: m = 0.0, θ0 = 90
ΔHRmax Gmax
15°
ΔH = 21° (45–66) ΔV = 5° (50–55) ΔG0 = −5° ΔG = −10° [0, 30]: m = 0.6, θ0 = 43 [30, 210]: m = 0.0, θ0 = 62
a Nomenclature used in Table 2: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; ΔHmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
138
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Table 3 Data for dentin samples in the dry condition (Fig. 8).a Sample Dentin Dry
Etch
Type
WR
UV (x1)
EB (x1)
ΔHmax
L
ΔH = 35° (45–80)
ΔH = 10° (70–80) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 74
35°
T
[4, 60]: m = 0.3, θ0 = 56 [60, 210]: m = 0.1, θ0 = 78 ΔH = 10° (83–93)
ΔH = 5° (65–70) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −15° [0, 15]: m = −0.9, θ0 = 77 [15, 180]: m = 0.0, θ0 = 64 ΔH = 40° (60–100) ΔV = 0° (50) ΔG0 = +10° ΔG = +10° [0, 4]: m = −2.3, θ0 = 63 [4,15]: m = 3.4, θ0 = 40 [15, 150]: m = 0.1, θ0 = 92 ΔH = 10° (75–85) ΔV = 10° (70–80) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 75
ΔH = 20° (65–85) ΔV = 15° (55–70) ΔG0 = 0° ΔG = −5° [0, 140]: m = −0.1, θ0 = 76 [140, 210]: m = 0.2, θ0 = 46
40°
ΔH = 10° (65–75) ΔV = 2° (68–70) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 70
35°
ΔH = 5° (95–100) ΔV = 10° (90–100) ΔG0 = 0° ΔG = −5° [0, 210]: m = 0.0, θ0 = 96
20°
[4,15]: m = 0.3, θ0 = 85 [15, 180]: m = −0.3, θ0 = 85
Polish
L
ΔH = 35° (45–80)
T
[0, 15]: m = −0.5, θ0 = 85 [15, 60]: m = 0.2, θ0 = 65 [60, 210]: m = 0.0, θ0 = 79 ΔH = 20° (50–70)
[4, 210]: m = 0.0, θ0 = 55
ΔHRmax ΔGmax
35°
ΔH = 7° (65–72) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −5° [0, 8]: m = −0.6, θ0 = 68 [8, 210]: m = 0.1, θ0 = 68 40° −15°
20° −15°
a Nomenclature used in Table 3: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔG0: gap between the values before and after radiation at t = 0; ΔHmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
60° and 90°. The most prominent changes in the contact angle values were observed at the beginning of each graphic and/or immediately after irradiation. Next, the contact angle reached a maximum value around which the data oscillated. Figs. 7, 9 and 11 show that the values of the contact angle in enamel increased remarkably in the dried samples (the values go from 10° to approximately 45° in samples TE and LE and from 50° to 70° in the TP and LP). However, the contact angle reached its maximum value (approximately 80°) after UV (x1) and EB (x1) radiations. In wet enamel samples, the maximum value was approximately 90°, and it was reached after UV (x3) and EB (x3) radiations. In the case of dentin, Figs. 8, 10 and 12 show that the variation of the contact angle values was not as high as it was in dried enamel samples. In fact, the values of the contact angle in the dried dentin samples stayed close to its maximum value (approximately 80°). Dentin samples were the only ones where values of over 100° were registered, and that happened after the EB radiation. It is important to note that it was only at the beginning of the experiment where there was a slight indication that the value of the contact angle had dependence on the surface preparation method. That is, for the dry WR samples it was observed that the value of the contact angle was higher in samples TP and LP, which were the mirror-finishpolished samples, than in samples TE and LE, which were the acid etched samples. However, in the wet samples, this tendency was not observed. In fact, wet dentin and enamel samples showed similar
behavior: the oscillation of the contact angle value was approximately 80° ± 10° and the maximum value was reached after EB (x3) radiation. Therefore, except for the dried and WR samples, the polishing and the etching of samples did not influence the contact angles. Another important fact to point out is that once the maximum contact angle value is reached in each treatment and after the “resting” time noted in the experimental section, the contact angle value did not decrease completely to the baseline observed in the dry sample: there was a “hysteresis” effect in the samples, which caused the contact angle value to remain close to the maximum value measured in the previous step. This effect is well observed in the data obtained before radiation. 3.2.1. Enamel graphs Let us comment on the data presented in Tables 1 and 2 regarding the parameters defined in Section 3.1 in order to obtain more information from the results presented for enamel in Figs. 7, 9 and 11. In enamel, the highest variation in ΔHmax (40°) was observed in the TP dried WR samples. In the case of ΔG, the dried samples after EB (x1) radiation showed a large gap (−15°). It is also worth noting that the value of ΔG in the dried enamel samples presented only negative signs, while ΔG0 were positive or zero. For wet enamel samples, a ΔHmax of 30° was observed in the LE sample but after UV (x2) radiation. The treatment with UV (×2) radiation showed the maximum angle variation ΔHRmax. In most of the treatments,
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
139
Table 4 Data for dentin samples in the wet condition (Fig. 10). The (x2) and (x3) graphs are shown in Fig. 12.a Sample Dentin Wet
Etch
Polish
Type
WR
UV (x1)
UV (x2)
EB (x1)
EB (x2)
EB (x3)
ΔHmax
L
ΔH = 20° (65–85) [0, 15]: m = 0.9, θ0 = 65 [15, 210]: m = 0.0, θ0 = 78
ΔH = 27° (65–92) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −10° [0, 210]: m = 0.1, θ0 = 72
ΔH = 20° (60–80) [4, 210]: m = 0.1, θ0 = 65
L
ΔH = 12° (70–82) [0, 5]: m = −1.3, θ0 = 77 [5, 30]: m = 0.2, θ0 = 70 [30, 210]: m = 0.0, θ0 = 80
ΔH = 15° (70–85) ΔV = 10° (60–70) ΔG0 = 20° ΔG = 0° [2,15]: m = 0.5, θ0 = 73 [15, 210]: m = 0.0, θ0 = 80 ΔH = 24° (70–94) ΔV = 16° (62–78) ΔG0 = +20° ΔG = −8° [0, 15]: m = −0.7, θ0 = 93 [15, 210]: m = 0.0, θ0 = 90 ΔH = 15° (65–80) ΔV = 13° (52–65) ΔG0 = +5° ΔG = 0° [0, 15]: m = 1.0, θ0 = 65 [15, 210]: m = 0.0, θ0 = 80
ΔH = 19° (71–90) ΔV = 20° (45–65) ΔG0 = +15° ΔG = +6° [0, 30]: m = −0.4, θ0 = 85 [30, 210]: m = 0.0, θ0 = 72 ΔH = 23° (77–100) ΔV = 20° (30–50) ΔG0 = +50° ΔG = +17° [4, 120]: m = 0.0, θ0 = 86 [150, 210]: m = 0.2, θ0 = 45 ΔH = 8° (76–84) ΔV = 7° (38–45) ΔG0 = +40° ΔG = +31° [4, 210]: m = 0.0, θ0 = 80
T
ΔH = 20° (50–70) [0, 15]: m = 0.3, θ0 = 48 [15, 210]: m = 0.0, θ0 = 65
ΔH = 27° (55–82) ΔV = 20° (40–60) ΔG0 = −5° ΔG = −5° [0, 8]: m = 1.8, θ0 = 68 [8, 210]: m = 0.1, θ0 = 65 ΔH = 35° (55–90) ΔV = 4° (68–72) ΔG0 = −8° ΔG = −17° [0, 5]: m = −2.3, θ0 = 63 [5, 30]: m = 1.2, θ0 = 50 [30, 210]: m = −0.1, θ0 = 86 ΔH = 7° (68–75) ΔV = 5° (65–70) ΔG0 = 0° ΔG = −2° [0, 4]: m = −1.3, θ0 = 75 [4, 210]: m = 0.0, θ0 = 70
ΔH = 15° (75–90) ΔV = 5° (65–70) ΔG0 = +10° ΔG = +5° [0, 8]: m = −0.3, θ0 = 83 [8, 210]: m = 0.0, θ0 = 82
ΔH = 18° (70–88) ΔV = 5° (70–75) ΔG0 = 0° ΔG = −5° [4, 210]: m = 0.1, θ0 = 75
25°
ΔHRmax ΔGmax
20°
ΔH = 10° (85–95) ΔV = 20° (60–80) ΔG0 = +8° ΔG = +5° [0, 30]: m = −0.2, θ0 = 92 [30, 210]: m = 0.0, θ0 = 92 ΔH = 13° (80–93) ΔV = 20° (30–50) ΔG0 = +40° ΔG = +30° [0, 15]: m = 0.5, θ0 = 75 [30, 210]: m = 0.0, θ0 = 92 ΔH = 15° (80–95) ΔV = 20° (55–75) ΔG0 = +10° ΔG = +5° [0, 5]: m = −2.6, θ0 = 82 [5, 30]: m = −0.5, θ0 = 95 [30, 210]: m = 0.0, θ0 = 82 ΔH = 25° (65–90) ΔV = 5° (30–35) ΔG0 = +40° ΔG = +30° [0, 8]: m = −2.0, θ0 = 83 [8, 30]: m = 1.1, θ0 = 56 [30, 210]: m = 0.0, θ0 = 88 25° +30°
27°
T
ΔH = 7° (68–75) ΔV = 10° (60–70) ΔG0 = −5° ΔG = −2° [0, 60]: m = 0.2, θ0 = 64 [60, 210]: m = −0.1, θ0 = 83 ΔH = 20° (80–100) ΔV = 8° (80–88) ΔG0 = −5 ΔG = −8° [0, 210]: m = 0.1, θ0 = 80
24° −8°
23° +31°
35° −17°
ΔH = 8° (74–82) ΔV = 7° (68–75) ΔG0 = 0° ΔG = −1° [0, 210]: m = 0.0, θ0 = 73
ΔH = 12° (70–82) ΔV = 4° (68–72) ΔG0 = 0° ΔG = −2° [0, 210]: m = 0.1, θ0 = 69
20° −8°
27°
35°
a Nomenclature used in Table 4: WR: contact angle without radiation; UV: contact angle after UV radiation, EB: contact angle after EB radiation, ΔV: segment of variation of the contact angle before radiation; ΔH: segment of variation of contact angle values after radiation, ΔG: gap between the values before and after radiation; ΔGmax: gap between the values before and after radiation at t = 0; Δmax maximum variation segment; ΔGmax maximum gap after treatment; m slope of the line; θ0 contact angle at t = 0.
ΔGmax was of − 10° and ΔG0 was of + 10°. It is worth noting that the values of ΔG in wet enamel samples went from negative to positive, and those of ΔG0 were positive for EB(xn) radiations. In the case of UV radiation, the values of ΔG0 had negative signs.
3.2.2. Dentin graphs Now let us to comment on the data presented in Tables 3 and 4 obtained from Figs. 8, 10, and 12 for dentin. In dried dentin, the WR samples showed a high variation in the contact values. The sample with the highest ΔHmax (40°) was TE after UV(x1) radiation, which also corresponded to the sample with the highest ΔHRmax value (40°). In the case of ΔGmax, the dried samples after UV and EB radiation showed a gap of − 15°. It is worth noting that, as in the case of enamel, the values of ΔG in dried dentin samples had a negative sign, and ΔG0 was zero in most of the cases. In wet conditions, the highest value of ΔHmax(30°) was observed in the LE sample after UV (x1) radiation. In fact, UV (x1) was the treatment with the maximum value of ΔHRmax. After EB radiation, dentin presented ΔGmax values of +30°. It is important to note that the values of ΔG in wet dentin samples had a positive sign after EB (xn) radiation. The values of ΔG0 had positive sign, and they incremented with EB (xn) radiation (it was of +50° for sample TE and of +40° for the LP sample after the EB(x3) radiation). In the case of the UV radiation, the values of ΔG0 had negative sign.
4. Discussion The results indicated that enamel shows greater variation in the contact angle values than dentin, which correspond well with previously reported data [16–20]. The observed variation in enamel and not in dentin must be related both to the inorganic/organic material ratio in these tissues (enamel: approx. 24; dentin: approx. 2.8) and to the size and shape of the HAP grains (much smaller in dentin) [13,14]. To this end, with the work so far we have not been able to elucidate the relationship between these modified systems, the structure of the teeth and the organic and inorganic components of enamel and dentin. However, the variation in the contact angle decreased in wet enamel samples, whereas in dentin it remained notably similar in most of the samples, dry or wet. Therefore, in enamel, the pre-wetting of the sample has a significant role in increasing the contact angle, but not in dentin. Newman et al. [6] measured the contact angle on enamel surfaces and found values of 50° for water and 35° for epoxy resins on pumiced teeth. However, they also found that the contact angle became hydrophilic with a phosphoric acid treatment and that the tooth surfaces can be restored to their original hydrophobic condition by brushing with pumice. Evidently, surface wetting and surface treatment should be important factors in the contact angle experiments; however, in our case, except for the dried WR samples at the beginning of the experiment, we did not find at clear difference in the contact angle behavior between the polished and etched samples.
140
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 7. Contact angle versus time graphs for enamel samples in the dried condition: A) without radiation; B) after the UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
The effect of UV radiation on enamel and dentin samples was observed immediately after irradiation. UV irradiation reduces the contact angle, although it recovers after 5 min in enamel and after 15 min in dentin. After recovery, the contact angle reached even higher values than those observed in WR samples. After EB radiation, the contact angle reaches the highest values in all samples. These values were 90° ± 10° in enamel and 80° ± 10° in dentin, regardless of whether the samples were dry or wet. Only the TP dentin dry sample reached a contact angle of 100°.
The results also indicated a cumulative trend in the contact angle values. This is a type of “contact angle hysteresis”. Although this effect was observed in all samples, it was more obvious in the dry samples. 4.1. The role of water The results clearly indicated that water by itself changed the wettability properties of the tooth surfaces. In dry samples, the low contact angle values observed can be attributed to an inherent hydrophilicity
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
141
Fig. 8. Contact angle versus time graphs for dentin samples in dried conditions: A) without radiation; B) after UV(x1) radiation; C) after EB(x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
of dry materials. The inorganic component of tooth structure (i.e., HAP) has a strong affinity to water [21], so during first wetting, the hydrophilicity might be due to hydrogen bonds between HAP and absorbed water [22]. Following Aronov [9,12], human tooth material surfaces and HAP surfaces should present a certain amount of electric charge; this electric charge is produced by polarization of the surface's atoms. Lee et al. [23], investigating the surface polarization of HAP, found that polarization
and relaxation of the surface extends to a distance of several unit cells into the bulk. The atomic forces are perfectly balanced in the bulk but not at the surface. According to their calculus, the polarization generated was − 34 mC/m2 and the surface energy was 1.296 J/m2 approximately. Vandiver et al. [24] found a negative charge for stoichiometric HAP through zeta potential measurements in powder suspensions, indicating that this negative charge increases the preferential concentration groups at the top of the surface. of PO3− 4
142
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 9. Contact angle versus time graphs for enamel samples in wet conditions: A) without radiation; B) after the UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
Therefore, when human tooth and HAP surfaces in dry conditions come into contact with water, the effect of charge is that the hydrogen groups. In a thin layer ions H+ bond to the oxygen atoms in the PO3− 4 of water, the generation of hydrogen bonds and an effective positive charge are produced on the HAP-related surfaces and the properties of wettability are modified, thereby generating hydrophilicity. After wetting and the passage of time, materials showed increased contact angles, indicating an increase in hydrophobicity. This could be the result of strong repulsive forces between the previously wet surface
and the new water. It is well known that after wetting, dried samples invariably show the traces of a layer of previous water on the tooth surface and this water layer modifies the surface energy [1]. 4.2. UV irradiation When UV waves interact with the water molecules in the thin film of water that covers the HAP surface, contact angle values decrease precipitously and the hydrophilic system is generated. However, once the
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
143
Fig. 10. Contact angle versus time graphs for dentin samples in wet conditions: A) without radiation; B) after UV (x1) radiation; C) after EB (x1) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
effect of the UV radiation disappears, the surface resets up the hydrophobic state, and the contact angle increases to its previous value. In dry samples, the disorder produced by the UV waves to the surface atoms of HAP is such that they cannot reach the wet conditions completely, but rather the radiation produces an almost permanent damage in the arrangement of the surface atoms of HAP, which is not observed when the surface is covered with a thin film of water.
4.3. EB radiation In the condition of EB radiation, a negative electrically charged beam is incident on an insulator surface. Therefore, the incident electrons are trapped on the surface, thus creating a negative charge accumulation. Aronov et al. [9,12] indicated that the HAP surface presents a P-type band gap semiconductor behavior, with an energy gap of approximately
144
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
Fig. 11. Contact angle versus time graphs for enamel samples in wet conditions: A) after UV (x2) radiation; B) after EB (x2) radiation; C) after EB (x3) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
4 eV. Next, the strong influence of the injected electrons on the surface electric potential of the HAP substrate is due to charge trapping in the vicinity of the HAP surface. The charge trapped on localized centers leads to electric field generation near the surface according to the Laplace equation ∇ϕ = − E [25]. This induced electric field may reach the magnitude of E = 106 V/cm at the flat surface [9,12]. In this way, the molecule is then polarized by the electric field and the surface free energy is modified, producing the observed results.
4.4. “Contact angle hysteresis” The results indicated the presence of a “contact angle hysteresis”. This idea is not new as some researchers also observed it for quite a long time [26,27]. During the contact angle measurement, the surface tension acting on the solid, the liquid, and the solid–liquid interface is balanced. However, if the surface was previously wetted, the surface “roughness” is increased because of a trace of water or an oxidation
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
145
Fig. 12. Contact angle versus time graphs for dentin samples in wet conditions: A) after UV (x2) radiation; B) after EB (x2) radiation; C) after EB (x3) radiation. The dark line indicates the moment of radiation (set at t = 0). The solid symbols represent the values after radiation; the empty symbols represent the values before radiation.
film on the surface left there from the previous measurement, giving rise to this “contact angle hysteresis”. This is in fact a proof that the measurement of the contact angle was always done in the same place of the sample. To fully understand the effects of tooth structures on the contact angle measurements, further studies are needed, including investigations on: (i) grain size, (ii) structure orientation, and (iii) the role of
the inorganic/organic ratio, and the measurements must always be conducted on fresh new samples. 5. Conclusions Water itself modified the wettability of the human tooth surfaces, reaching contact angles of up to 80°, but the contact angle showed
146
G.E. Tiznado-Orozco et al. / Materials Science and Engineering C 57 (2015) 133–146
more variability on enamel than on dentin surfaces. UV and EB radiation also increased the hydrophobicity of human tooth materials, but EB radiation allowed the surfaces to reach the maximum contact angle values (90°, hydrophobic) and produced “jumps” of up to 30°. The microstructure of the human tooth surfaces, except in dried and WR samples, was not an influencing parameter. Acknowledgments JRG is grateful to DGAPA-UNAM (contract IN106713) for the financial support and to CONACYT (203257) and PASPA-DGAPA-UNAM (057/2013) for sabbatical support at the University of Lille 1. References [1] S. Hussain, Textbook of dental materials, Jaypee Brothers Publishers, 2008. 494. [2] H.A. Wege, J.A. Aguilar, M.Á. Rodriguez-Valverde, M. Toledano, R. Osorio, M.A. Cabrerizo-Vilchez, Dynamic contact angle and spreading rate measurements for the characterization of the effect of dentin surface treatments, J. Colloid Interface Sci. 263 (2003) 162–169. [3] H.A. Wege, J.A. Holgado-Terriza, J.I. Rosales-Leal, R. Osorio, M. Toledano, M.A. Cabrerizo-Vilchez, Contact angle on dentin surfaces measured with ADSA on drops and bubbles, Colloids and Surfaces A: Physicochem. Eng. Aspects 206 (2002) 469–483. [4] A.W. Neumann, R. David, Y. Zuo, Applied surface thermodynamics, Surfactant Science, Second ed.CRC Press, 2012. [5] F. Namen, J. Galan Jr., J. Farias de Oliveira, R. Derossi Cabreira, F. Costa e Silva Filho, A. Balduino Souza, G. de Deus, Surface properties of dental polymers: measurements of contact angles, roughness and fluoride release, Mater. Res. 11 (2008) 239–243. [6] G.V. Newman, W.H. Snyder, C.E. Wilson, Acrylic adhesives for bonding attachments to tooth surfaces, Angle Orthod. 38 (1968) 12–18. [7] Y. Oshida, R. Sachdeva, S. Miyazaki, Changes in contact angles as a function of time on some pre-oxidized biomaterials, J. Mater. Sci. C Mater. Med. 3 (1992) 306–312. [8] A.V. Grosse, The relationship between surface tension and energy of liquid metals and their heat of vaporization at the melting point, J. Inorg. Nucl. Chem. 26 (1964) 1349–1361. [9] D. Aronov, R. Rosen, E.Z. Ron, G. Rosenman, Electron-induced surface modification of hydroxyapatite-coated implant, Surf. Coat. Technol. 202 (2008) 2093–2102.
[10] G.V. Reddy, N.S. Reddy, J. Itttigi, K.N. Jagadeesh, A comparative study to determine the wettability and castability of different elastomeric impressions materials, J. Contemp. Dent. Pract. 13 (2012) 356–363. [11] E.A. Vogler, Structure and reactivity of water at biomaterial surfaces, Adv. Colloid Interface Sci. 74 (1998) 69–117. [12] D. Aronov, R. Rosen, E.Z. Ron, G. Rosenman, Tunable hydroxyapatite wettability: effect on adhesion of biological molecules, Process Biochem. 41 (2006) 2367–2372. [13] R.Z. Le Geros, Calcium phosphates in oral biology and medicine, in: Howard M. Myers (Ed.), Monographs in Oral Sciences, vol. 15, Karger in Basel, New York, 1991. [14] D. Muster (Ed.), Calcium Phosphates in Oral Biology and Medicine, Hard Tissue Repair and Replacement, Elsevier, Amsterdam, 1992. [15] H.J. Busscher, A.W.J. Van Pelt, H.P. de Jong, J. Arends, Effect of spreading pressure on surface free energy determinations by means of contact angle measurements, J. Colloid Interf. Sci. 95 (1983) 23–27. [16] H.J. Busscher, H.M. Uyen, H.P. de Jong, J. Arends, G.A.M. Kip, Adsorption of aminefluorides on human enamel, J. Dent. 16 (1988) 166–171. [17] P.O. Glantz, The adhesiveness of teeth, J. Colloid Interface Sci. 37 (1971) 281–290. [18] A.W.J. Van Pelt, H.P. de Jong, H.J. Busscher, Dispersion and polar surface free energy of human enamel (an in vitro study), J. Biomed. Mater. Res. 17 (1983) 637–641. [19] H.P. de Jong, P. de Boer, H.J. Busscher, Surface free energy changes of human enamel during pellicle formation-an in vivo study, Caries Res. 18 (1984) 408–415. [20] H.P. de Jong, A.W.J. van Pelt, H.J. Busscher, The effect of topical fluoride application on the surface free energy of human enamel-an in vitro study, J. Dent. Res. 63 (1984) 635–641. [21] M. Mondon, C. Ziegler, Changes in water contact angles during the first phase of setting of dental impression materials, Int. J. Prosthodont. 16 (2003) 49–53. [22] S. Rüttermann, T. Trellenkamp, N. Bergmann, W.H.M. Raab, H. Ritter, R. Janda, A new approach to influence contact angle and surface free energy of resin-based dental restorative materials, Acta Biomater. 7 (2011) 1160–1165. [23] W.T. Lee, M.T. Dove, E.K.H. Salje, Surface relaxations in hydroxyapatite, J. Phys. Condens. Matter 12 (2000) 9829–9841. [24] J. Vandiver, D. Dean, N. Patel, W. Bonfield, C. Ortiz, Nanoscale variation in surface charge of synthetic hydroxyapatite detected by chemically and spatially specific high-resolution force spectroscopy, Biomaterials 26 (2005) 271–283. [25] J. Reyes-Gasga, R. García, L. Vargas-Ulloa, In-situ observation of fractal structures and electrical conductivity in human tooth enamel, Philos. Mag. A 75 (1997) 1023–1040. [26] F.M. Fowkes (Ed.), Contact Angle, Wettability and Adhesion, Advances in Chemistry Series, vol. 43, Am. Chem Soc., 1964 [27] D.F. Moore, Principles and Applications of Tribology, International Series on Materials Science and Technology, vol. 14, Pergamon Press, New York 1975, pp. 148–150.