Investigating Tensile Bonding and Other Properties of Yttrium Oxide Nanoparticles Impregnated Heat-Cured Soft-Denture Lining Composite In Vitro

Abstract

INTRODUCTION Resilient liners, which are layered between the denture base and oral mucosa, are composed of plasticized resins or silicone elastomers.[1] The resins are heat cured or auto-polymerized.[2] Long-term and short-term resilient liners have differences: long-term denture liners remain resilient for 30 days and up to 12 months, whereas short-term liners maintain their desirable properties for 7 days and up to 30 days.[3] Resilient denture liners are indicated for patients with sharp atrophied alveolar ridge, patients with thin oral mucosa who could not tolerate denture pressure and patients who have denture that causes pressing or indenture pressure mark.[4] The main drawback of denture liners is the loss of durable bond to denture base material, because the detachment of the liners from denture base is a common clinical problem; a reliable bond between the soft liner and the denture base is required for optimal denture function.[5] The common methods used to evaluate the bonding strength of denture soft liners to denture base materials include peel, shear, and tensile tests.[6] Tear resistance test evaluates the ability of material to resist tearing forces when subjected to a denture cleaning procedure.[7] The hardness test (shore A) is considered one of the nondestructive tests used to determine whether a resilient material can be used as a liner; this test evaluates the reaction force of a tested material to indentation.[8] Surface roughness can be an influence of the adhesion of microorganisms; a rougher surface indicates a higher biofilm accumulation.[9] Yttrium oxide (Y2O3) nanoparticles (NPs) are a highly insoluble, thermally stable, and air-stable substance with good chemical stability, resistivity, and breakdown strength.[10] This study aimed to evaluate the mechanical and surface properties of liner materials, namely, tensile bond strength, tear strength, shore A hardness, and surface roughness, after impregnation with Y2O3 NPs. The null hypothesis states that Y2O3 NPs do not affect the mechanical and surface properties of denture liners compared with the control group. MATERIALS AND METHODS A pilot study that involves tensile bond strength and surface roughness tests using four Y2O3 NPs concentrations (1, 1.5, 2, and 2.5 wt%) was conducted. The Y2O3 NPs concentrations of 1.5 and 2 wt.% were chosen because they revealed favorable improvements in tensile strength without affecting the surface properties of the soft-lining materials. Composites of the soft-lining materials (Vertex Soft, Vertex Dental, Netherlands) and Y2O3 NPs (US Research Nanomaterials, Houston, TX, USA) were prepared by adding the measured nano-Y2O3 to monomers. A probe sonication apparatus (Soniprep 150, Highland, UK; 120 W, 60 kHz) was used for 3 min to achieve a good distribution of NPs within the monomer. The monomer with Y2O3 NPs was immediately mixed with soft-lining powder to minimize the chance of particle aggregation. Fourier-transform infrared spectroscopy (FTIR) (Tensor 27, München, Germany) was performed to investigate the chemical interaction between Y2O3 NPs and poly(ethyl methacrylate) (PEMA). Scanning electron microscopy (SEM) (AIS2300, Anestron Advanced, Peabody, MA, USA) was performed to investigate the dispersion of Y2O3 NPs within the matrix of PEMA, and energy-dispersive spectrum (EDS) was performed to obtain the atomic and weight percentages of the composition of the nanocomposite. The tested specimens were fabricated to assess tensile bonding strength (10 specimens for the control group and 10 specimens each for the liners with 1.5 and 2.5 wt.% Y2O3 NPs). Bond strength was tested using a universal testing machine (WDW-20, Larger Technology Co., Ltd., Quanzhou, China) with a crosshead speed of 5 mm/min. The maximum load needed for failure was registered to determine the bond strength of the specimens. The following equation was applied: Bond strength (N/mm2) = F/A, where F is the maximum load at failure (N) and A is the cross-sectional area of the sample (mm2).[11] Acrylic blocks 10 mm × 10 mm × 83 mm width, depth, and length, respectively, were prepared according to the manufacturer’s instructions. Then, the acrylic blocks were positioned in the mold with 3 mm space in between blocks to be filled with the required amount of soft-lining materials, which were cured according to the manufacturer’s instructions. Thirty testing specimens were prepared according to the literature to test the tear strength.[12] A universal testing machine with a crosshead speed of 500 mm/min was used to measure the force per unit thickness needed to tear initiation. Tear strength was determined as follows: Tear strength = F/D, where F is the maximum force required to the break sample (N) and D is the medium thickness of each sample (mm). Thirty specimens with 3 mm thickness and 30 mm diameter were prepared for shore A hardness testing. All specimens were immersed in distilled water and kept in an incubator for 48 h at 37°C before testing. A shore A durometer (Time Instruments TH200, Shandong, China) was used to evaluate the hardness of the lining materials. Five readings were performed for each sample. The contact time was 5 s following each penetration, and the average was considered the test value. Thirty specimens (65 mm × 10 mm × 2.5 mm) were prepared for roughness testing. The specimens were immersed in distilled water for 24 h at 37°C before testing. A profilometer device (TIME, TIME3200/3202 (TR200), Shandong, China) was used. Three measurements were performed for each sample, and the average value was calculated. The results were statistically analyzed by interpreting the mean, standard deviation, and bar chart plotting; in addition to one-way analysis of variance (ANOVA) table with multiple-comparison Bonferroni’s test applied. RESULTS The FTIR results revealed that Y2O3 NPs (1.5 and 2 wt.%) and the acrylic soft-denture lining materials had no chemical interaction or reaction [Figure 1].Figure 1: Fourier-transform infrared spectroscopy (FTIR) analysisThe SEM images of the control and experimental groups reveal a uniform distribution of Y2O3 NPs within the matrix [Figure 2].Figure 2: Scanning electron microscopy (SEM) analysisThe EDS diagram indicates the incorporation of Y2O3 NPs into the matrix of the soft-lining materials, as shown in Figure 3.Figure 3: Energy-dispersive spectrum (EDS) analysisTensile bonding strength increased with the addition of 1.5 and 2 wt.% Y2O3 NPs compared with the control group. One-way ANOVA table revealed a highly remarkable difference among control and experimental groups, with multiple-comparison analysis by Bonferroni test the experimental groups and the control revealed highly significant differences (P < 0.05), but no substantial difference was found between both experimental groups (P > 0.05) as shown in Figure 4.Figure 4: Bar chart representation for tensile bonding strength test for all groups (bars with similar letters represent a significant difference when P < 0.05)Figure 5 shows that the addition of Y2O3 NPs (1.5 and 2 wt.%) substantially increased the tear strength of the liner compared with the control group. ANOVA table revealed a significant difference (P = 0.002). Multiple-comparison test by Bonferroni shows significant differences between control group and 2wt.% experimental group (P < 0.05).Figure 5: Bar chart representation for tear strength test for all groups (bars with similar letters represent a significant difference when P < 0.05)The liner with 2 wt.% Y2O3 NPs showed the greatest mean surface hardness value among the tested liners, as shown in Figure 6. The hardness of the liners with 1.5 and 2 wt.% Y2O3 NPs had a considerably higher hardness value than the control group. ANOVA table shows a significant difference (P < 0.001). Bonferroni's multiple-comparison test revealed that similar results obtained with tensile bond strength applied for the surface hardness test.Figure 6: Bar chart representation for shore A hardness test for all groups (bars with similar letters represent a significant difference when P < 0.05)The liner with a higher Y2O3 NPs concentration (2 wt.%) had a higher roughness value than the other liners. Its surface roughness was remarkably different compared with that of the control but was not remarkably different compared with the liner with 1.5 wt.% Y2O3 NPs. ANOVA table revealed that highly significant differences among all groups further multiple comparisons by Bonferroni’s test show that similar results of tensile bond strength and surface hardness test [Figure 7].Figure 7: Bar chart representation for surface roughness test for all groups (bars with similar letters represent a significant difference when P < 0.05)DISCUSSION The effect of Y2O3 modification on the mechanical and surface properties of soft, acrylic-based denture lining materials was investigated. Denture liners are supposed to increase patients’ denture acceptance throughout functional stress distribution. Many problems are correlated to soft liners, such as poor bond strength to denture base and poor tearing strength.[13] Soft-denture lining materials should have excellent adhesion to the denture base, because failure of adhesion could lead to functional and hygienic problems.[14] Bond adhesion to denture base was remarkably improved by the incorporation of 1.5 and 2 wt.% Y2O3 NPs, which could be attributed to the van der Waals force between nanofillers and the polymer.[15] According to Jacobsen et al.,[16] the penetration of soft-lining materials is inversely proportional to the viscosity of liners. This relationship could explain the influence of Y2O3 NPs on the flowability of lining materials. The increase in stiffness will permit the easy adaptation of the lining materials to the denture base and provide good contact for the molecules of chemically similar polymers across the interface with acrylic resin.[1718] The highly remarkable increase in the durability and marginal integrity of lining materials with the addition of 1.5 and 2 wt.% Y2O3 NPs may be attributed to the interfacial bonding by the formation of van der Waals forces between nanofillers and the polymer, which increase the resistance of polymer chains to rupture under tearing forces.[15] When tearing is propagated, nanofillers inside the polymer matrix scatter the strain energy near the tips of the growing cracks; therefore, a large force will be required to break the polymer matrix.[19] The remarkable increase in tear strength was directly proportional to the nanofiller concentration because the fine size of the fillers produced a greater interfacial bonding along with good dispersion and homogeneity in the bonds.[2021] One of the most important properties of soft-denture lining materials is hardness because it lessens the impact of absorption.[2223] In this study, Y2O3 NP concentration was directly proportional to the hardness of the liner. The increase in hardness may be attributed to the NP distribution within the matrix. A decrease in interparticle distance with the increase in the bonding strength between particles makes the particles cluster together within the spaces of the soft-lining matrix and subsequently leads to an increase in hardness.[8242526] Surface roughness is an important characteristic of denture lining materials because it affects the adhesion of microorganisms and the development of pathogenic diseases, such as denture stomatitis, as a result of interference with proper oral hygiene maintenance.[8252627] In this study, the addition of Y2O3 NP resulted in an unremarkable increase in surface roughness. The small concentration of Y2O3 NPs impregnated in the soft liners resulted in the good dispersion of NPs within matrix and caused a nonsubstantial increase in surface roughness.[2128,29] CONCLUSION Within the limitations of this study, it is concluded that the addition of Y2O3 NPs into the soft-lining materials increased the tensile bond strength, tear strength, and hardness but not the surface roughness of soft, acrylic-based denture lining materials. RESEARCH APPLICABILITY According to the obtained results, the characterizations and properties of prepared heat-cured soft-denture lining composite make it suitable and applicable to clinical practice as modified soft-denture lining material. RESEARCH LIMITATION This study did not investigate the biological test. Any researcher cannot gamble and introduce modified material depending on testing mechanical or physical properties. A plan should be applied for testing the biological properties to support the results of our study. ACKNOWLEDGMENT Nil. FINANCIAL SUPPORT AND SPONSORSHIP Nil. CONFLICTS OF INTEREST There are no conflicts of interest. AUTHORS CONTRIBUTIONS Authors equally contributed to the paper. ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT Not applicable. PATIENT DECLARATION OF CONSENT Not applicable. DATA AVAILABILITY STATEMENT The data that support the study results are available from the corresponding author (Prof. Dr. Abdalbseet A Fatalla, e-mail: [email protected]) on request.

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