Properties Of Dental Resin Composites Research Articles

Preparation of Zinc Oxide Mesocrystal Filler and the Properties of Dental Composite Resins

Preparation of Zinc Oxide Mesocrystal Filler and the Properties of Dental Composite Resins

Characterization of the physical–mechanical properties of dental resin composites reinforced with novel micro-nano hybrid silica particles: An optimization study

In the study, dental composites of color A2 using Bis-GMA/UDMA/TEGDMA resins (ratios 70/10/20), and silica filler (70%wt, 75%wt, and 80%wt) which is a hybrid of two silica types in nano and micro dimensions were made using two different photoinitiators namely BAPO and camphorquinone. The optimum photoinitiator was selected based on the mechanical tests results after which the composites were subjected to the following tests: FTIR to evaluate polymerization degree, microhardness test, UTM, and SEM micrographs were taken to analyze the surface fracture of samples. The results of photoinitiator selection (flexural strength test) is 36.54 MPa, 37.62 MPa, and 75.08 MPa for BAPO + camphorquinone, camphorquinone, and BAPO respectively. The results show that the BAPO photoinitiator exhibits better results over camphorquinone and also BAPO/camphorquinone initiator systems. Then after choosing the photoinitiator system composites with different filler contents show higher mechanical strength than existing dental composites. The results of the mechanical tests for the composites with different filler contents synthesized after initiator system selection were significantly higher than the values specified in ISO 4049:2009 (102 MPa over 80 MPa). FTIR results indicate that the degree of conversion in these composite is 25.41%, 37.68, and 40.94% for composites with different filler amounts.

Effect of the Time of Salivary Contamination during Light Curing on Degree of Conversion and Microhardness of a Restorative Composite Resin

Saliva contamination is a major clinical problem in restorative procedures. The purpose of this study was to evaluate the effect of the time of salivary contamination during light curing on the degree of conversion and the microhardness of a restorative composite resin. Eight groups of 10 samples for measuring the microhardness and eight groups of 5 samples for evaluating the degree of conversion were prepared. The samples of each group were contaminated with human saliva at a certain time. The first group (T0) was contaminated before light curing. The specimens in groups T2–T30 were contaminated at 2, 5, 10, 15, 20 and 30 s after the start of light curing, respectively. The samples of group T40 were contaminated after light curing. The degree of conversion and the microhardness of the specimens were measured by Fourier transform infrared (FTIR) spectroscopy and Vickers hardness testing techniques, respectively. The results of this study revealed that there were no significant differences between the groups in terms of the degree of conversion of the composite resin. Consistent with the findings for the degree of conversion, significant differences in the microhardness between the groups were not found. In conclusion, from a clinical point of view, the results of our study showed that the time of salivary contamination (before, during or after light curing of composite resin) has no significant effect on the polymerization (degree of conversion) and one of the important mechanical properties of dental composite resins (microhardness).

Effects of deformation rate variation on biaxial flexural properties of dental resin composites.

ObjectivesThis study aimed to determine the biaxial flexural strength (BFS) of resin composite materials at distinct deformation rates.MethodsTwo micro-hybrid [Filtek Z100™ Restorative (Z100), Filtek™ Z250 (Z250)] and two nano-filled [Filtek™ Supreme XT Body (FSB), Filtek™ Supreme Translucent (FST)] composite resins were selected. Disc-shaped (12 × 1 mm) specimens were fabricated using nylon split moulds. Bar-shaped specimens (25 × 2 × 2 mm) were fabricated from each material to determine the flexural modulus. The specimens were tested for BFS and flexural modulus under dry and wet conditions after 1, 13, and 52 weeks.ResultsThe highest BFS was recorded for Z250 (162 ± 19 MPa), followed by FST (154 ± 16 MPa), Z100 (150 ± 18 MPa), and FSB (136 ± 18 MPa). The materials exhibited a clear trend of increase in BFS with deformation rate. Following immersion for 1 week, the BFS was 126 ± 18 MPa for Z100, and 124 ± 17 MPa for Z250, which were higher than those of FSB (99 ± 16 MPa) and FST (115 ± 19 MPa) under comparable conditions. There was a remarkable reduction in the flexural moduli of the specimens immersed for 1 week compared to those of the dry specimens: Z100 (from 18.3 ± 1.2 GPa for dry specimen to 15.7 ± 0.8 GPa after immersion for 1 week), Z250 (from 16.7 ± 0.8 GPa to 13.3 ± 1.4 GPa), FSB (from 13.7 ± 0.6 GPa to 11.0 ± 2.1 GPa) and FST (from 12.7 ± 2.3 GPa to 10.4 ± 1.0 GPa).ConclusionThis study concludes that the BFS and flexural moduli of resin-based dental restoratives decline when they are immersed in an aqueous medium until saturation with water. However, after equilibrium is established, the immersion medium does not affect the restorative materials further. Variations in deformation rate did not have a significant effect on the BFS of resin-based dental restoratives.

The Physiochemical Properties of Dental Resin Composites Reinforced with Milled E-glass Fibers

The milled E-glass fibers (consist of 54.4 wt% SiO2, 14.5 wt% Al2O3, 17 wt% CaO, 4.5 wt% MgO, 8.5 wt% B2O3, 0.5 wt% Na2O) with 20 mesh and 50 mesh sizes were used to replace particle fillers in dental resin composites partially, with the aim to enhance fracture toughness of dental resin composites. The length distribution of milled E-glass fibers with different size was investigated with a polarizing microscope. The FT-IR analysis was used to determine the double bond conversion (DC) of dental resin composites. Water sorption (WS) and solubility (SL) were obtained until the mass variation of dental resin composites in distilled water kept stable. Flexural strength (FS) and modulus (FM) of dental resin composites were measured using a three-point bending set up. Fracture toughness (FT) of dental resin composites was measured according to the single-edge notched beam method. For both 20 mesh and 50 mesh milled fibers, the main aspect ratio was less than 5.2, which was considered to be the lowest aspect ratio for dental resin composites. The milled E-glass fibers showed none negative effect on DC, WS, and SL, and even some fibers containing dental resin composites had higher DC, lower WS and SL. The milled E-glass fibers could improve FM and FT of dental resin composites, but had no enhancement effect on FS. The milled E-glass fibers could be used to enhance FT of dental resin composites without influencing some other physiochemical properties negatively.

Aging effect of pH on the mechanical and tribological properties of dental composite resins

ABSTRACTThis study evaluated the microhardness and wear behavior alterations of dental resins exposed to strong acid and alkaline degradation regimens. Fifty specimens of AP-X, Z350, P60, VITA ZETA and VITA LC resins were distributed into 10 groups. The control group was not subjected to aging treatment, while the other nine groups were assigned to the following pH solutions: 1, 7 or 13 for 1, 13 or 20 d, respectively. Vickers microhardness and wear behavior of materials without aging served as the reference. Repeated measurements were conducted for other specimens. Scanning electron microscopy was used to observe the morphology. Data were analyzed by repeated-measures one-way ANOVA, followed by the least significant difference (LSD) t-test (P ≤ 0.05) for multiple comparisons. The aging process was aggravated over time. Alkaline medium significantly decreased the microhardness and wear resistance of resins, and led to serious surface damage. AP-X, P60 and Z350 immersed in different media showed a lower degradation than the nanoresin. The mechanical degradation of resin composites was aggravated with increased aging time. Strong alkaline media induced the largest reduction in mechanical and tribological properties. Hybrid resin composites with high filler loading exhibited better mechanical performance than nanoresins with low filler loading under different pH conditions.

Academy of Dental Materials guidance—Resin composites: Part I—Mechanical properties

ObjectiveThe objective of this project, which was initiated from the Academy of Dental Materials, was to review and critically appraise methods to determine fracture, deformation and wear resistance of dental resin composites, in an attempt to provide guidance for investigators endeavoring to study these properties for these materials. MethodsTest methods have been ranked in the priority of the specific property being tested, as well as of the specific test methods for evaluating that property. Focus was placed on the tests that are considered to be of the highest priority in terms of being the most useful, applicable, supported by the literature, and which show a correlation with clinical findings. Others are mentioned briefly for the purpose of being inclusive. When a standard test method exists, including those used in other fields, these have been identified in the beginning of each section. Also, some examples from the resin composite literature are included for each test method. ResultsThe properties for evaluating resin composites were ranked in the priority of measurement as following: (1) Strength, Elastic Modulus, Fracture toughness, Fatigue, Indentation Hardness, Wear—abrasion (third body) and Wear—attrition (contact/two body), (2) Toughness, Edge strength (chipping) and (3) Wear determined by toothbrush. SignificanceThe following guidance is meant to aid the researcher in choosing the proper method to assess key properties of dental resin composites with regard to their fracture, deformation and wear resistance.

Evaluation of the physical properties of dental resin composites using optical fiber sensing technology

ObjectivesThe characterization of the physical properties of dental resin composites is fraught with difficulties relating to significant intra and inter test parameter variabilities and is relatively time consuming and expensive. The main aim of this study was to evaluate whether optical fiber Bragg grating (FBG) sensing system may become a viable tool to study dental material characteristics. Of particular focus was the potential for the system to demonstrate a multi parameter all-in-one feature. MethodsA miniature FBG was embedded in six different dental resin composites and employed as a sensor to evaluate linear polymerization shrinkage, thermal expansion and water sorption. Six commercially available dental composites with different filler types and volume are evaluated. The tests are repeated with three sets of samples. The curing characteristics and residual strain gradient exhibited by the cured dental composites were also observed and commented. ResultsAmong the studied samples, SDR shows lowest polymerization shrinkage, while Beautifil FO3 shows the highest. The results also show clear distinction between particle filler type and fiber reinforcement based composites in their polymerization shrinkage properties. The agreement of the results with existing literatures show that FBG based system provides accurate results. Polymerization shrinkage rate of the samples are also obtained. Thermal expansion of the composites are measured using the FBG sensing method for the first time and is correlated with resin type, volume, filler type and glass transition temperature. The water sorption characteristics of the dental composite are also successfully measured using the FBG sensing method. The high level of repeatability and the low standard deviations shown in the results indicate good reliability with the use of FBG sensors. SignificanceThis study demonstrates how optical fiber technology can provide simple and reliable methods of measuring the critical physical properties of dental composites. In addition due to the embedding and preservation of the sensor within the samples multiple parameters can be tested for with the same sample. These features are expected to greatly assist material science researchers in dentistry as well as other biomedical fields. Of some interest the phenomenon of stress relaxation of dental composite at higher temperature was observed.

Antimicrobial and mechanical properties of dental resin composite containing bioactive glass.

The aim of this study was to evaluate the antimicrobial efficacy and mechanical properties of dental resin composites containing different amounts of microparticulate bioactive glass (BAG). Experimental resin composites were prepared by mixing resin matrix (70% BisGMA and 30% TEGDMA) and inorganic filler with various fractions of BAG to achieve final BAG concentrations of 5, 10 and 30 wt%. Antimicrobial efficacy was assessed in aqueous suspension against Escherichia coli, Staphylococcus aureus and Streptococcus mutans and in biofilm against S. mutans. The effect of incorporation of BAG on the mechanical properties of resin composite was evaluated by measuring the surface roughness, compressive strength and flexural strength. Under the dynamic contact condition, viable counts of E. coli, S. aureus and S. mutans in suspensions were reduced up to 78%, 57% and 50%, respectively, after 90 minutes of exposure to disc-shaped composite specimens, depending on the BAG contents. In 2-day-old S. mutans biofilm, incorporation of BAG into composite at ratios of 10% and 30% resulted in 0.8 and 1.4 log reductions in the viable cell counts compared with the BAG-free composite, respectively. The surface roughness values of composite specimens did not show any significant difference (p>0.05) at any concentration of BAG. However, compressive and flexural strengths of composite were decreased significantly with addition of 30% BAG (p<0.05). The results demonstrated the successful utilization of BAG as a promising biomaterial in resin composites to provide antimicrobial function.

Influences of iodonium salts on the properties of a hybrid composite resin containing BisS-GMA and expanding monomer modified epoxy

In order to reduce shrinkage and improve the mechanical properties of dental composite resins, we designed a hybrid resin formulation containing a novel matrix resin, BisS-GMA [bisphenol-Sbis( 3-methacrylato-2-hydroxypropyl)ether], and epoxy modified by a spiro-orthocarbonate (SOC) expanding monomer. Then, we tested the effects of an iodonium salt, diphenyliodonium hexafluorphosphate (DPIHFP), on the properties of the hybrid resin with seven different concentrations. The hybrid resin was polymerized by a ternary photo-initiator system. The volumetric shrinkage (VS), degree of conversion (DC) and compressive strength (CS) were assessed using AcuVol™, FTIR and a universal testing machine, respectively. The VS, DC and CS were improved with increasing DPIHFP concentration, but a high concentration of DPIHFP had a negative influence on the mechanical properties of the hybrid resin and offered no added improvement in the VS and DC. The best performance of a composite resin containing BisS-GMA and SOC-modified epoxy was achieved with 2wt% DPIHFP. The results also indicated that the resin containing BisS-GMA was superior to that containing Bis-GMA in terms of VS, DC and CS.

Effect of low shrinkage monomers on physicochemical properties of dental resin composites.

The aim of this study was to evaluate the effect of low shrinkage monomers on physicochemical properties of dental resin composites. Two low shrinkage resin composites: one with a crosslink branching monomer (Kalore, GC Corp) and a novel monomer (Venus Diamond, Heraeus Kulzer) were compared to a conventional resin composite formulation (Filtek Z250, 3M/ESPE). The volumetric shrinkage was evaluated by µCT analysis (n=5) and the physicochemical properties by degree of C=C conversion (DC), flexural strength (FS) and Young's modulus (YM) (n=10). All samples were light cured by a LED device (Radii, SDI) with 16 J/cm2. The results were analysed by one-way ANOVA and Tukey test for multiple comparisons (α=0.05). No statistical difference was found between µCT shrinkage values to Kalore (1.8%) and Venus Diamond (1.7%) (p≥0.05); Z250 presented statistical highest shrinkage value (2.0%). Kalore presented higher statistical DC (60.8%) than Venus Diamond (49.5%) and Z250 (49.6%). No statistical difference was found between FS or YM properties to Venus Diamond and Z250; Kalore presented statistical lowest FS and YM properties (p≥0.05). Using novel monomers seem to reduce polymerization shrinkage without affecting the physicochemical properties evaluated of resin composites rather than using crosslink branching monomers.

Formation of functionalized nanoclusters by solvent evaporation and their effect on the physicochemical properties of dental composite resins

ObjectiveThe aim of this work was to study the effect of silica nanoclusters (SiNC), obtained by a solvent evaporation method and functionalized by 3-methacryloxypropyltrimethoxysilane (MPS) and MPS+octyltrimethoxysilane (OTMS) (50/50wt/wt), on the rheological, mechanical and sorption properties of urethane dimethylacrylate (UDMA)/triethylenglycol dimethacrylate (TEGDMA) (80/20wt/wt) resins blend. MethodsSilica nanoparticles (SiNP) were silanized with MPS or MPS+OTMS (50/50wt/wt) and incorporated in an UDMA-isopropanol mix to produce functionalized silica nanoclusters after evaporating the isopropanol. The effect of functionalized SiNC on resins rheological properties was determined by large and small deformation tests. Mechanical, thermal, sorption and solubility properties were evaluated for composite materials. ResultsThe UDMA/TEGDMA (80/20wt/wt) resins blend with added SiNC (ca. 350nm) and functionalized with MPS showed a Newtonian flow behavior associated to their spheroidal shape, whereas the resins blend with nanoclusters silanized with MPS+OTMS (50/50wt/wt) (ca. 400nm) showed a shear-thinning behavior due to nanoclusters irregular shape. Composite materials prepared with bare silica nanoclusters showed lower compressive strength than functionalized silica nanoclusters. MPS functionalized nanoclusters showed better mechanical properties but higher water sorption than functionalized nanoclusters with both silane coupling agents, MPS and OTMS. SignificanceThe solvent evaporation method applied to functionalized nanoparticles showed to be an alternative way to the sinterization method for producing nanoclusters, which improved some dental composite mechanical properties and reduced water sorption. The shape of functionalized silica nanoclusters showed to have influence on the rheological properties of SiNC resin suspensions and the mechanical and sorption properties of light cured composites.

Investigation on the physical-mechanical properties of dental resin composites reinforced with novel bimodal silica nanostructures.

The aim of this study was to investigate the influence of bimodal silica nanostructures comprising of SiO2 nanoparticles (SiO2 NPs, ~70 nm) and SiO2 nanoclusters (SiO2 NCs, 0.07-2.70 μm) on physical-mechanical properties of resin-based composites (RBCs). SiO2 NPs and SiO2 NCs were prepared with the Stöber method and the coupling reaction, respectively, then silanized and employed as fillers to construct RBCs using a mixture of bisphenol A glycerolate dimethacrylate (Bis-GMA) and tri(ethylene glycol) dimethacrylate (TEGDMA) as the organic matrix. Results showed that the properties of RBCs were influenced by the filler ratios of bimodal silica nanostructures, and the appropriate amount of SiO2 NPs could effectively increase the activating light efficiency and filler packing density of RBCs. Among all experimental RBCs, RBC 50-20 (SiO2 NPs:SiO2 NCs=50:20, wt/wt) presented the highest degree of conversion (71.6±1.1%), the lowest polymerization shrinkage (2.6±0.1%), and the enhanced flexural strength (104.8±4.4 MPa), flexural modulus (6.2±0.3 GPa), and compressive strength (205.8±14.3 MPa), which were improved by 44%, 19%, 28%, 48%, and 42% in comparison with those of RBC 0-60 (SiO2 NPs:SiO2 NCs=0:60, wt/wt), respectively. Besides, in vitro cytotoxicity evaluation of RBC 50-20 indicated its acceptable cytotoxicity. Although the best performance was achieved by commercial Z350 XT, the introduction of bimodal silica nanostructures might provide the enhanced physical-mechanical properties of RBCs, compared with those of RBC 0-60 reinforced with unimodal SiO2 NCs.

English

The aim of this case report was to evaluate the depth of cure of composite resin cured within root canals by means of light-transmitting plastic posts. Light-curing composite resins must be adequately polymerized within the root canal to reinforce weakened roots. The physical properties of dental composite resins are of great concern because they affect their clinical performance. The light- transmitting plastic posts (Luminex, Weissman Technology) have enabled the transmission of light into the root canal to polymerize composite resin placed within it.

Novel Ag nanocrystals based dental resin composites with enhanced mechanical and antibacterial properties

The aim of this work was to investigate the effect of trace addition of oleic acid coated Ag nanocrystals (Ag NCs) on mechanical and antibacterial properties of dental resin composites. Composites (70wt% of silica loading) with 25ppm, 50ppm, 75ppm and 100ppm (wt) Ag NCs were prepared and the composite without Ag NCs served as a control. The experimental results showed that the addition of Ag NCs significantly improved the strength and modulus of the resin composite without compromising the shade. For the composite with 50ppm Ag NCs, flexural strength (140.3MPa), modulus (13.2GPa) and compressive strength (347.2MPa) were increased by 4.1%, 22.2%, 13.3%, respectively, compared with the control. The antibacterial test demonstrated that trace Ag NCs provided the resin composites with an antibacterial effect. Such strong and antibacterial dental resin composites might be advantageous to prevent secondary caries and be potential for future clinical applications.

Influence of Al<sub>2</sub>O<sub>3</sub> as Filler Loading on the Fracture Toughness of Light-Cured Dental Resin Composites

The purpose of the present study was to evaluate the effect of filler loading on the fracture toughness of light-cured dental resin composites (DRCs). The monomer Bis-GMA and TEGDMA (75/25 wt/wt) were mixed with treated micro-sized aluminum oxide (Al2O3) particles ranging from 40-60 wt% as filler. The composite samples were cured with LED for 80s. The bulk density (g/cm3) and apparent porosity (%) of the samples were determined according to the ASTM standard. The fracture toughness (KIC) values of the composites were determined using a single edge notched specimen in the three-point bending test. There was a significant difference in KIC of the composites with different loading of Al2O3 particles (p < 0.05). The highest amount of filler (60 wt%) reduced the KIC value to 1.5 MPa.m1/2. As a conclusion, the Al2O3 particles used as filler loading does have an effect on the mechanical properties of DRCs.

Effect of Modification of Aluminum Borate Whiskers with Three Methods on Flexural Properties of Dental Resin Composites

This study aimed to compare three different methods for the modification of aluminium borate whiskers and investigate the effect of aluminium borate whiskers composites on flexural properties of dental resin composites. Method A was to mix aluminium borate whiskers and silicon dioxide (SiO2) nanoparticles directly. Method B was to modify the mixture of aluminium borate whiskers and SiO2nanoparticles with sol-gel process of tetraethoxysilane (TEOS). Method C was to modify the aluminium borate whiskers with SiO2nanoparticles which were repaired by sol-gel method of TEOS. The effects of the three methods were characterized by the TEM and SEM. There was a significant difference (p<0.05) in the flexural strength among three methods. Group C had a flexural strength of 149.59 ± 12.86 MPa (mean ± SD; n = 5), which is significantly higher than 95.28 ± 4.53 MPa for Group A and 123.14 ± 17.37 MPa for Group B. It is concluded that ceramic whisker reinforcement significantly improves the flexural properties of resin composites; different methods produce different effects.

Effect of alumina particles loading on the mechanical properties of light-cured dental resin composites

The purpose of this study is to evaluate the effect of alumina (Al2O3) loading on the mechanical properties of dental resin composites (DRCs). The DRCs were prepared based on Al2O3 particles and bisphenol A-glycidyl methacrylate (Bis-GMA) was used as the base monomer. The silane-treated Al2O3 particles were mixed with the resin matrix in proportions of 40, 50, and 60wt%, respectively. Resin matrix without filler was used as the control sample. The Vickers hardness (HV) and flexural modulus (FM) of the DRCs mixed with Al2O3 particles were found to be superior compared to the control sample; the values increased from 14.4 to 23.5kg/mm2 and 1.5 to 5.7GPa, respectively. However, the flexural strength (FS) values of DRCs were slightly decreased as the filler loading increased i.e. from 84.5 to 74.2MPa. The results also revealed statistically significant increases in the HV and FM. On the other hand, FS values showed significant decrease when filler loading was increased (P<0.05).

Effect of in-office bleaching agents on physical properties of dental composite resins.

The physical properties of dental restorative materials have a crucial effect on the longevity of restorations and moreover on the esthetic demands of patients, but they may be compromised by bleaching treatments. The purpose of this study was to evaluate the effects of in-office bleaching agents on the physical properties of three composite resin restorative materials. The bleaching agents used were hydrogen peroxide and carbamide peroxide at high concentrations. Specimens of each material were prepared, cured, and polished. Measurements of color difference, microhardness, and surface roughness were recorded before and after bleaching and data were examined statistically by analysis of variance (ANOVA) and Tukey HSD post-hoc test at P < .05. The measurements showed that hue and chroma of silorane-based composite resin altered after the bleaching procedure (P < .05). No statistically significant differences were found when testing the microhardness and surface roughness of composite resins tested (P > .05). The silorane-based composite resin tested showed some color alteration after bleaching procedures. The bleaching procedure did not alter the microhardness and the surface roughness of all composite resins tested.

Synthesis and study of properties of dental resin composites with different nanosilica particles size

Objectives The aim of this work was the synthesis of light-cured resin nanocomposites using nanosilica particles with different particle size and the study of some physical–mechanical properties of the composites. Methods Various types of silica nanoparticles (Aerosil) with average particle size of 40, 20, 16, 14, and 7 nm, used as filler were silanized with the silane 3-methacryloxypropyl-trimethoxysilane (MPS). The total amount of silane used was kept constant at 10 wt% relative to the filler weight to ensure the complete silanization of nanoparticles. The silanizated silica nanoparticles were identified by FT-IR spectroscopy and thermogravimetric analysis (TGA). Then the silanized nanoparticles (55 wt%) were mixed with a photoactivated Bis-GMA/TEGDMA (50/50 wt/wt) matrix. Degree of conversion of composites was determined by FT-IR analysis. The static flexural strength and flexural modulus were measured using a three-point bending set up. The dynamic thermomechanical properties were determined by dynamic mechanical analyzer (DMA). Sorption, solubility and volumetric change were determined after storage of composites in water or ethanol/water solution 75 vol% for 30 days. The TGA for composites was performed in nitrogen atmosphere from 30 to 700 °C. Results As the average silica particle size decreases, the percentage amount of MPS attached on the silica surface increases. However, the number of MPS molecules attached on the silica surface area of 1 nm 2 is independent of filler particle size. As the average filler particles size decreases a progressive increase in the degree of conversion of composites and an increase in the amount of sorbed water is observed. Significance The prepared composites containing different amount of silica filler, with different particle size, but with the same amount of silanized silica and organic matrix showed similar flexural strength and flexural modulus, except composite with the lowest filler particle size, which showed lower flexural modulus.