Surface roughness of pigmented and non-pigmented maxillofacial silicone elastomer before and after artificial aging

Maxillofacial prostheses are frequently made from silicone elastomer. The silicone material’s rough surface can lead to the accumulation of microorganisms, irritation of the surrounding soft tissue, and the progression of some microsites into mechanical failure and color deterioration. This in vitro study aimed to compare the surface roughness of M511 heat-vulcanized maxillofacial silicone before and after artificial aging. Eighty disk-shaped specimens were prepared and divided equally among five experimental groups based on pigmentation, with 16 samples in each group. The colorless specimens were prepared without adding any pigment. The four study groups were prepared using distinct pigments (red, blue, yellow, and their mixture). Half of the specimens in each group were tested before aging, while the other half was tested after 750 h of artificial aging. Surface roughness was measured using atomic force microscopy (AFM), and the silicone bonds and pigments were observed using Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Data were compared between groups using a two-way analysis of variance, independent sample t-test, and Tukey’s post hoc test, considering a p < 0.05 statistically significant. FTIR-ATR showed no differences between the pre-and post-aging groups, with similar peaks observed. The surface roughness significantly increased in the colorless, blue, and mixed pigment groups (p < 0.001), surface roughness increased by 155.66%, 14.83%, and 96.12% respectively for colorless, blue pigment, and the mixture of pigments after 750 h of artificial aging. In contrast, the surface roughness significantly decreased in the yellow pigment (90.98%) and red pigment (5.7%) (p ≤ 0.001) after aging. The yellow pigment showed the smoothest surface after aging, while colorless specimens showed the roughest surface. Two-way ANOVA showed that pigment type and aging time significantly affected surface roughness. Therefore, induced aging considerably impacted the surface topography of maxillofacial silicone. The pigment type and distribution in the silicone matrix affected its surface roughness. The AFM analysis is a simple and straightforward method for assessing surface roughness at the nanometer level.


Introduction
A maxillofacial prosthesis is considered an alternative treatment when surgical correction of maxillofacial abnormalities is not an option for a patient.Missing or damaged parts of the head and neck can result from surgery, injuries, or congenital malformations.Despite advances in reconstructive and plastic surgery, extra-oral prostheses made of synthetic materials are still required to replace the more complex facial components [1].The primary goal of facial prostheses is to provide patients with aesthetics and comfort while enhancing their sense of self-worth and quality of life [2,3].
Medical-grade maxillofacial prosthetic silicone elastomeric materials have been preferred for restoring the anatomy and esthetic functionality of craniofacial defects [4][5][6].However, various environmental conditions, such as ultraviolet (UV) light, temperature, and humidity, affect their physical and mechanical properties, limiting the prostheses' service life [7,8].The maxillofacial silicone elastomer degrades prematurely when exposed to UV light and temperature, damaging the margins or compromising surface texture.Therefore, the maxillofacial silicone elastomer's service life is reduced to an average of 6-18 months [9][10][11].
It is generally agreed that a maxillofacial prosthesis's color reproduction, form, texture, and translucency are the most important factors affecting its clinical success.To achieve these factors, it is necessary to add colors to the silicone using either intrinsic or extrinsic coloration techniques [12].Pigments are essential in transferring color to maxillofacial prostheses.Intrinsic pigmentation lasts longer and is preferred but is difficult to achieve.Several studies have assessed the effect of various intrinsic pigment types on the mechanical properties of various maxillofacial silicone elastomers to improve their properties and durability [13,14].
A material's roughness is a measure of the fine irregularities in its surface texture.Average surface roughness (R a ) is the deviation of the surface valleys and peaks in micrometers or nanometers; the surface is considered rough when the deviations are large and smooth when the deviations are small.A material's surface roughness can be measured using various techniques, including optical techniques, surface profilometers, scanning electron microscopy, and atomic force microscopy (AFM) [15][16][17].
Artificial accelerated aging is widely used to simulate the long-term effect of outdoor weathering using aggressive weathering components, such as UV radiation and high temperature [18,19].Understanding the impact of aging on the mechanical, physical, and optical properties of pigmented and non-pigmented maxillofacial silicones may help eliminate current uncertainty about the best follow-up recommendations for patients with silicone prostheses [12].
Babu et al [20] conducted a study revealing a significant reduction in surface roughness after 60 days of storage in a disinfection solution, contrasting with an increase observed after 1008 h of accelerated artificial aging.Similarly, Goiato et al [21] reported a decline in surface roughness following 60 days of storage in a chemical disinfection solution.Another study demonstrated a decrease in roughness for maxillofacial silicone elastomers processed against coated gypsum materials, with AFM analysis indicating noticeable variations in surface roughness [22].Alwan et al [23] observed a proportional increase in the surface smoothness of maxillofacial silicone with the increase in the duration of artificial aging.Additionally, a different study documented an increase in surface roughness of maxillofacial silicone after immersion in various storage conditions for six months at 37 °C [24].Two separate studies demonstrated an increase in surface roughness for M511 maxillofacial silicone after six months of exposure to natural outdoor weathering [16,25].
All previous in vitro and in vivo studies have evaluated maxillofacial silicone elastomer surface roughness at the micro-level of roughness (μm).In contrast, this study evaluated the impact of artificial aging on the surface roughness of pigmented and non-pigmented maxillofacial silicone elastomers at the nano-level (nm).Its null hypothesis was that artificial aging would not change the surface roughness of pigmented and non-pigmented M511 maxillofacial silicone.

Materials
The materials used in this study were obtained from their respective manufacturers: parts A and B of the M511 heat-vulcanized maxillofacial silicone elastomer (Technovent Co. Ltd., Bridgend, UK) and dry pigments (red, blue, and yellow; Technovent Co. Ltd., Bridgend, UK).

Experimental design and sample preparation
Eighty disk-shaped specimens (2 mm thick, 20 mm diameter) [26,27] were prepared and divided equally among five experimental groups based on pigmentation, with 16 samples in each group.The colorless specimens were fabricated without adding pigment.The colored specimens were prepared by adding different pigments (red, blue, yellow, and their mixture).Figure 1 shows the distribution of these specimens.
Metal molds were prepared from 2 mm-thick cast iron sheets by laser-cutting.For each mold, two stainless steel plates with precise exterior dimensions were cut to sandwich the mold and withstand clamping force.
The M511 silicone elastomer was supplied by the manufacturer as a base (part A) and catalyst (part B), mixed in a 10:1 weight ratio (10 g part A to 1 g part B = 11 g total).Color accounted for 0.2% of the total weight of the silicone [27][28][29][30][31].
First, silicone parts A and B were measured with a digital electronic weight balance (Nimbus Analytical; Adam Equipment Inc., Oxford, CT, USA).The colorless specimens in the control group were prepared by mixing the weighed silicone parts A and B for 5 min at 360 rpm under a vacuum of −0.09 MPa in a vacuum mixer (AX-2000C; Aixin Medical Equipment Co. Ltd., Xiqing, Tianjin, China).The pigmented silicone specimens with each pigment were prepared by weighing the pigment, adding it to the weighted silicone part A, and mixing them without vacuum for 2 min to prevent the vacuum from sucking up any pigment before leaving them to rest under mixing and vacuum for 8 min.Next, the mixture was allowed to cool to room temperature because the mixer's rotation generated heat, decreasing its working time.Then, the weighted silicone part B was added to the mixture and mixed for an additional 5 min under vacuum.
After using a metal spatula to load the mixture into the molds, it was placed in a vacuum chamber for 2 min to remove any air bubbles that had formed during loading.Next, the molds were placed in a pressure pot (Pentola A Pressione Typodont; Leone S.p.A., Sesto Fiorentino, Florence, Italy) at 0.2 MPa for 2 min to smooth the surface of the mixture and pop superficial air bubbles.Then, the mold was sealed and subjected to 0.03 MPa hydraulic pressure for 5 min.Finally, the molds were sealed and clamped with G-clamps, and the material was polymerized in a hot air oven for 1 h (Memmert; Memmert GmbH+Co KG, Büchenbach, Germany) [32].
After being removed from the molds, the specimens were washed with water and cleaned with liquid detergent before being dried with tissue paper.They were then cut using scissors to remove any excess.Samples that had obvious defects were discarded before testing.Figure 2 shows all specimens, pigments, and M511 maxillofacial silicone elastomer.
Half of the specimens (eight per group) were tested for surface roughness before aging by AFM (JPK BioAFM; Bruker Optics, Berlin, Germany), which provides images with near-atomic resolution for measuring surface topography and can also quantify surface roughness down to the angstrom-scale [22].The other half of the samples were subjected to artificial aging in an aging chamber (QUV Accelerated Weathering Tester; Q-Lab, Cleveland, OH, USA) according to American Society for Testing and Materials (ASTM) G154 cycle 1  (approximate wavelength: 340 nm; energy irradiance: 0.89 W m −2 nm −1 ; the exposure cycle included 8 h of UV irradiation at 60 °C ± 3 °C black panel temperature and 4 h of condensation at 50 °C ± 3 °C black panel temperature) [33] for 750 h.Afterward, the aged samples were removed and examined for surface roughness using AFM.
The open-source Gwyddion software was used for AFM color rendering [34,35].AFM images were analyzed to determine 3D images, height profiles, height distributions, and statistical results.The height profiles were created by drawing a line across a specimen and measuring the height along it.The height distributions and their means were determined using Origin Lab software.

Characterization
The bonds and chemical groups in the silicone structure were determined using Fourier-transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR; Tensor 27; Bruker Optics, Ettlingen, Baden-Württemberg, Germany).Silicone surfaces were analyzed before and after artificial aging to detect any possible effects of aging on silicone structure.This method focuses electromagnetic radiation in the infrared spectrum on the specimen's surface.Once absorbed by a molecule, the energy associated with these wavelengths is converted into molecular vibration-rotation energy to identify the chemical groups in the material's structure [36,37].

Statistical analysis
Pre-and post-aging surface roughnesses were compared in each group using the independent sample t-test.The effects of color and time on surface roughness were assessed using a two-way analysis of variance (ANOVA).Post hoc Tukey's tests were performed for pairwise comparisons of means with p < 0.05.Statistical analyses were performed using IBM SPSS Statistics software (version 24; IBM Corp, Armonk, NY, USA).

Atomic force microscopy (AFM)
The 3D images, height profiles, and grain height distributions of all groups before and after 750 h of artificial aging are shown in figures 3, 4, and 5.

Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR)
FTIR-ATR analysis (figure 6) showed similar peaks for silicones in all groups before and after aging that were similar to the pure (colorless) silicone before aging, suggesting that they had an identical basic structure.The lack of new peaks suggests that aging and various pigmentations did not induce the formation of new products or byproducts in the silicone compositions, preventing product degradation.The FTIR-ATR showed Si-O-Si stretching at 1009 and 1080 cm −1 , CH 3 stretching at 2963 and 2907 cm −1 , a Si-CH 3 peak at 1258 cm −1 , and Si-(CH 3 ) 2 stretching at 787 cm −1 , which are all characteristic peaks of silicone rubber.

Surface roughness
The average surface roughness of all specimens is shown in table 1.The mean and standard deviation of surface roughness before and after artificial aging are shown for all groups in table 2, and the two-way ANOVA results are shown in table 3. The independent sample t-test showed significant increases (p < 0.001) in surface roughness for colorless (155.66%),blue (14.83%), and mixed pigment (96.12%) silicones after 750 h of artificial aging.The colorless silicone showed the roughest surface after artificial aging.In contrast, surface roughness decreased significantly for the yellow (p < 0.001) and red (p = 0.001) pigment silicones.The surface roughness decreased from 232.61 nm to 20.98 nm (90.98%) for the yellow pigment silicone and 63.85 nm to 60.21 nm (5.7%) for the red pigment silicone.The yellow pigment showed the smoothest surface after aging.Two-way ANOVA showed that pigment type and time significantly affected surface roughness.The power was 0.999 and 0.997 for pigment and time, respectively, meaning that 99.9% of the variance in surface roughness could be attributed to pigment type and 99.7% to aging time.The interaction effect of pigment and time on surface roughness was statistically significant, with a power of 1.000, meaning that 100% of the variance in surface roughness could be attributed to the joint effects of pigment type and aging time.

Discussion
This study's results support rejecting the null hypothesis because artificial aging significantly changed the surface roughness of pigmented and non-pigmented maxillofacial silicone elastomers (table 2).Maxillofacial silicone elastomers, including both high-and room-temperature vulcanized, seem to degrade over time because of changes in their mechanical and physical properties, including surface topography [16,38].Our study chose the Cosmesil M511 maxillofacial silicone elastomer because of its texture, strength, durability, ease of handling, color, and patient comfort [39].
As a surface property, surface roughness is a good predictor of an elastomeric material's mechanical performance.A surface irregularity may be a nucleation site for cracks, corrosion, and bacterial contamination [24].
The ideal elastomer-colorant combination should provide acceptable esthetics and appropriate physical properties such as surface roughness.The colorant may improve the physical properties of the elastomer used to make a maxillofacial prosthesis, but the ideal colorant should not degrade its properties [40].
The FTIR-ATR test showed similar peaks before and after aging in all groups.The samples were compared to standard (colorless) silicone before aging, with several peaks showing similar characteristics (figure 6).This result indicates that artificial aging and different pigmentations produced no new components or by-products [14,36,37,41].
The AFM surface roughness analysis showed significant increases (p < 0.05) in mean roughness for colorless, blue, and mixed pigment specimens.In contrast, it showed significant decreases in mean roughness for yellow and red pigment specimens (table 2).Furthermore, it showed that aging and pigment type significantly affected the surface roughness of the silicone (table 3).
The yellow and red pigment specimens showed increased surface smoothness after artificial aging.Their surface roughness may have decreased over time due to continuous polymerization, which causes additional polymer chain arrangement and expansion, ultimately creating a finer and smoother silicone surface over time [16,17,20,21,23].A material with a rougher surface contains finer surface flaws, which can lead to cracks, corrosion, and bacterial contamination.A maxillofacial material with a smoother surface could maintain its mechanical qualities better, which would be beneficial.Plaque accumulation would be reduced when the material is used to replace facial defects because of its smooth surface [24].Surface roughness is a key parameter controlling bacterial adhesion to the silicone's surface, with rougher surfaces experiencing increased bacterial adhesion and colonization [42,43].
Our findings contrast with those of Babu et al, who found that the surface roughness of red-pigmented M511 maxillofacial silicone increased after 1008 h of artificial aging [20].Goiato et al reported a decrease in surface roughness after chemical disinfection, which is associated with the continuous polymerization process that promotes a more complete polymeric chain, making the silicone surface smoother over time [21].The colorless, blue, and mixed pigment silicone specimens showed significant increases in surface roughness (p < 0.05) after 750 h of artificial aging.This increase in surface roughness most likely reflects a structural change in the material due to prolonged exposure to the aging conditions within the aging chamber.These alterations may cause microcracks to form in the material's surface layer, decreasing its resistance to aging conditions, which exacerbates all other degradation effects [44].Our results are consistent with Al-Dharrab et al [24] and Fatalla et al [45].
Studies have shown that some bonds in silicone can be cleaved due to their lower bond energy than UV photon energy.Therefore, crosslinking reactions can occur at cleaved positions.Consequently, as the time a silicone material is exposed to UV light increases, its porous surface area and loose layer depth increase, and silicone oxide fillers disappear from its surface, increasing surface roughness [46,47].Pigments were incorporated into the polymer matrix through physical interactions rather than chemical bonding.Chemical bonding between polymers and pigments will result in reinforcement if a chemical reaction occurs between them.While pigment particles can be treated as inert media in physical interactions because their presence has been accounted for by simple geometric modification of the elastic chains in a polymer network, their distribution on the surface increases surface roughness [26,40].
This in vitro study had some limitations that should be considered.This study only examined the effect of a single factor: artificial aging.In addition, it only tested one maxillofacial silicone type and specific pigments.Future studies should examine the effects of additional factors, such as natural weathering and silicone cleaning agents.They should also assess the various maxillofacial silicone types and pigments.

Conclusions
This in vitro study found that artificial aging significantly affects the surface topography of maxillofacial silicone elastomer.The surface roughness for certain pigments (colorless specimens, blue pigment, and mixture of pigments) increased significantly by 155.66%, 14.83%, and 96.12% respectively for colorless, blue pigment, and the mixture of pigments, while for other pigments, yellow pigment (90.98%) and red pigment (5.7%) decreased significantly after 750 h of artificial aging.The increase in surface roughness depends on the type and distribution of pigments in the silicone matrix.The type of pigments and the aging time had a significant impact on the surface roughness.Following artificial aging, the surface of the silicone which contained yellow pigment showed the smoothest surface, while colorless silicone specimens showed the highest roughness value.The Atomic force microscopy (AFM) analysis is a simple and straightforward method for determining the surface roughness of maxillofacial silicone at the nanometer level.

Figure 1 .
Figure 1.The flow chart of specimen preparation and distribution.

Figure 2 .
Figure 2. The M511 maxillofacial silicone, pigments, and all specimens before and after artificial aging.

Figure 3 .
Figure 3. 3D images, height profiles, and grain height distributions obtained from atomic force microscopy (AFM) analysis.Colorless silicone (a) before and (b) after aging.Silicone with blue pigment (c) before and (d) after aging.

Figure 4 .
Figure 4. 3D images, height profiles, and grain height distributions obtained from atomic force microscopy (AFM) analysis.Silicone with red pigment (a) before and (b) after aging.Silicone with yellow pigment (c) before and (d) after aging.

Figure 5 .
Figure 5. 3D images, height profiles, and grain height distributions obtained from atomic force microscopy (AFM) analysis.Silicone with mixed (blue, red, and yellow) pigments (a) before and (b) after aging.

Figure 6 .
Figure 6.Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) before and after aging for all groups.

Table 1 .
Average surface roughness of all groups in nanometers (nm) before and after artificial aging.

Table 2 .
Mean values ± standard deviation of surface roughness before and after artificial aging for all groups.

Table 3 .
Two-way ANOVA of surface roughness. of freedom; F, F-statistic, the ratio of two mean squares that forms the basis of a hypothesis test; pvalue, probability significant at a 5% level of significance (p < 0.05)