Research & Development
Biocompatibility is the measure of the response of living tissue or cells to an interfacing substance or force. Measures of biocompatibility are cellular health and behavior. Material biocompatibility can be described by the material’s chemical, mechanical, and thermal properties, surface characteristics, density and size. The application will determine which of these characteristics come into play and how critical each is to a successful biological interface.
Color stability refers to the ability of a material to maintain its reflectance, reflected wavelength (color), and excitation purity over time. There are instruments that measure color and can detect changes over time. Materials which are used as dental restoratives are typically color matched to the existing dentition at the time of placement. Ideally, the match should be stabile over time and use. Factors that influence the change in color are chemical degradation and staining of the restorative material.
Compressive strength describes how a material responds to a load applied in compression. As the material is loaded, it deforms. Some materials deform differently than others. Brittle materials deform elastically until they break. Rubber-like materials deform elastically until the polymeric bonds start breaking at which point the material deforms both elastically and viscoelastically. Metals deform elastically and then plastically until they fail. The load divided by the area of the material under load determines the stress applied to the material. Stress is always, therefore, in units of force/area (e.g., Newtons/mm², and pounds/in²). The stress at rupture or failure is the compressive strength.
Diametral Tensile Strength
Diametral tensile strength describes how a material responds to a tensile load. Also referred to as the Diametral Compression Test for Tension, it is an alternative method of placing a brittle material in tension without gripping it on opposite sides and pulling (typical tensile test). When a disk of brittle material is compressively loaded through opposite sides of its edge, tensile stresses are generated inside the material and perpendicular to the direction of applied load, which will eventually cause a tensile failure.
Dimensional stability refers to a material’s ability to maintain its size and shape when subjected to environmental variables such as thermocycles of hot and cold or humidity changes and the result of polymerization or setting. It is an important characteristic of root canal fillers, impression materials and restorative materials. A root canal filler that expands too much in the presence of water could split a tooth root. Restorative materials that contract too much when they polymerize can be the cause of marginal leakage. Impression materials that change dimensions with time or exposure to humidity may not produce accurate impressions.
Elastic modulus is a material property which describes its spring-like characteristics. The modulus is a ratio of the stress and the strain or more generally, the load and the resultant deformation of the material caused by the load which could be tensile, compressive, torsional, or bending (flexure). It is measured by calculating the slope of the plot of an increasing load versus the resultant increasing deformation. Stiffness is a synonym for elastic modulus. The higher the elastic modulus of a material, the more stiff it is. Steel and rubber are examples of elastic materials. Steel has the higher elastic modulus.
Elastic recovery is a material property which describes how completely a material returns to its original, unloaded shape and size after a load (e.g. compressive, tensile or torsional) is removed. In dentistry, it is used to evaluate the ability of an impression material to be removed from the teeth in the mouth and tolerate being pulled past undercuts and away from inter-proximal spaces without being permanently distorted.
Fatigue is a condition that can cause a structure subjected to cyclic loading to fail at a stress below the materials single cycle ultimate strength. It is caused by additive failure generated by loads below the yield load (in the elastic range of the material). Corrosion, cracks, scratches in the surface, and inclusions in the material can all contribute to decreased fatigue strength. Fatigue testing is performed by subjecting the material to cyclic loading at various maximum load levels until failure occurs. A plot of the number of cycles to failure versus the maximum stress is a tool which can help predict failure in structures. The fatigue limit or endurance limit is the level of stress that theoretically yields infinite cyclic life.
Film thickness is the thickness of a material such as cement or root canal sealer when a specified quantity is compressed between two flat plates with a specified load. The more viscous the material, the thicker is the resultant film thickness. The variables that can affect film thickness are the particle size of the solid component, the mix ratio of powder to liquid, the viscosity of the liquid, the incorporation of air into the mix, and the setting time of the material. Film thickness affects the successful cementation of restorations onto prepared teeth. If too high, it may prevent the restoration from seating properly.
Flexural Strength and Flexural Modulus
Flexural strength and flexural modulus describe the capacity of a material to resist bending. There are several methods used to perform a flexural strength test. The three-point and four-point methods are similar in that they support a bar of the material to be tested at each end of a specified span. The four-point test generates a uniform stress field between the two load application points and is useful in materials that are flaw sensitive and the object is to determine the effect of the flaws on the ultimate strength. The three-point test is useful in determining the strength of a specimen at the point of load application. Another type of bending test is the cantilever bending test. The specimen is rigidly held at one end and the load is applied at the other end. The flexural modulus of an elastic material is the amount a material will deflect elastically per an amount of applied load.
Flow is a measure of the movement of a specific amount of material under a compressive load typically at a time before the material has set. It is useful for determining how well a material like impression material will spread out over a surface. Other materials that utilize this characteristic to describe their behavior are root canal sealers and fillers.
Friction is a resistance to relative movement between two bodies such as a tooth and a restorative material. Friction is proportional to the force pushing the two bodies together and is a result of mechanical impingement and molecular interaction between the materials of the two bodies. At a macroscopic level, it is affected by the interfacing material’s density, roughness, hardness, and molecular interaction. Two highly polished surfaces typically exhibit low friction unless there is a molecular interaction such as magnetism, coherence or adherence. Friction is important in dentistry because it is one of the factors that affects wear and durability of restorations.
Gloss is a surface property which is related to specular reflection from that surface. It is dependent on the material, the surface roughness, and the angle of illumination. Gloss measurements can be used to compare surface preparation techniques such as finishing and polishing.
Hardness is a measurement of a material’s resistance to indentation or penetration and is a function of the applied load, the strength of the material and the surface area being indented. There are several hardness techniques including Barcol, Brinell, Knoop, Rockwell, Shore and Vickers hardness.
Marginal leakage is a measurement of how well a restorative system adapts to a prepared tooth. The typical methodology used is to thermocycle the prepared tooth specimen and then submerge it in a dye that will show where leakage has occurred when the tooth is sectioned.
Percent elongation is a measurement of the amount of stretch that a specific size specimen (e.g., dog bone shape) will tolerate before failure. It is useful for helping to understand the maximum load carrying capabilities of rubber-like and metallic materials.
Polymerization shrinkage is a measure of the change in dimension as a monomer is linked and or cross-linked during polymerization. Polymerization can be caused by chemical interaction, x-ray radiation, light, and thermal energy and is composition dependent. Polymerization shrinkage can be an issue with restorative composites and bonding agents and be the source of marginal leakage and low bond strengths.
Radiopacity is a measure of the attenuation of x-rays passing thru a subject material. It is often measured in millimeters of aluminum. A typical method of determining radiopacity is to position a step wedge of pure aluminum with steps increasing by 0.5 mm per step in the x-ray view alongside the subject material. The thickness of the material is measured and the photographic density of the x-ray of the specimen and of the step wedge is measured and compared. The comparison yields a radiopacity in terms of aluminum thickness. It is an important feature for dental materials that need to be identified during an x-ray, or if broken pieces are inhaled and swallowed (e.g. denture materials, restoratives and endodontic materials).
Setting time is a measurement of the elapsed time from the initiation of mixing to the setting of a material.
Shark Fin Flowability
The Shark Fin Flowability test measures the maximum height that an unset impression material will extrude into a wedge shaped space between two stainless steel plates while under a specific load. The test is a method of comparing the capability of impression materials to penetrate narrow cervices like interproximal spaces.
Shear Bond Strength
Shear bond strength is a measurement of how well one material bonds to another. It places the bond interface in shear. There are many versions of the test that utilize an anvil to load the side of a cylinder of material that is bonded at one of its ends to a substrate material. A commonly used version of this test is the Ultradent Shear Bond Test. Typical measurement units are in megaPascals (MPa).
Strain in Compression
Strain in compression is a measure of the flexibility of a material. A standardized load is applied to a cylindrical specimen. The pre-load height and loaded height of the specimen are compared and the percent change is recorded as the strain in compression. This test is often used to characterize the behavior of impression materials.
Surface roughness is a surface condition which is a measure of a surface’s deviation from purely smooth. The variables which are used to describe surface roughness are height, frequency and wavelength. The precision of the measurement is dependent on the resolution capabilities of the measuring instrument. Typical instruments used to measure surface roughness are surface-contacting profilometers that utilize a stylus that is dragged across the surface and non-contacting instruments such as interferometers, confocal microscopes, electron microscopes, and electrical capacitance systems. It is an important parameter as it provides information about how effectively materials can be finished and polished and is a variable that affects friction and wear.
Tear energy is a measure of the amount of energy required to tear a material. It is a calculation that is based on the force required to tear a uniform thickness of the material a specified distance.
Tear strength is a measure of material’s ability to resist tearing. Testing is performed on a material of uniform thickness that has been partially cut with a sharp blade that creates a tear initiation site. It provides information about the behavior of materials that may be required to resist tearing e.g., impression materials that must be pulled out of interproximal spaces intact. The units of tear strength are force divided by thickness.
Tensile Bond Strength
Tensile bond strength is a measure of the ability of a bond between two materials to withstand a tensile load. It is determined by applying a tensile load to a material molded into the shape of an inverted cone that has been bonded to a specific substrate. The inverted cone shape facilitates pulling the cone from the substrate. The fracture load is divided by the area of the base of the cone to determine the bond strength.
Tensile strength is a material property that describes its ability to resist deformation under tensile or pulling loads. It is calculated by using the maximum load along the load/deformation plot and dividing this by the nominal cross-sectional area of the specimen measured in a plane perpendicular to the load.
Thermal cycling is a technique used to produce thermal stress by causing the test material to expand and contract as it is cycled back and forth from a cold medium to a hot medium. In dentistry, this form of testing is used to evaluate how well bonded materials will maintain their bond strength as they are subjected to hot and cold cycling. The intent is to simulate the thermal variance the oral cavity might experience during the eating of hot food and the drinking of cold fluids.
Thermal degradation is a condition that results as materials and chemical components separate, cross-link, inter-react, or lose structural integrity as a result of temperature sensitive reactions. It is important that dental materials remain stabile as ambient environmental temperatures vary.
Toughness is a measure of the energy required to plastically deform a material until it fails (rupture or fracture). Brittle materials typically have low toughness whereas ductile materials have higher toughness.
Ultimate strength is a material property determined during load/deformation testing of a material or component. It is calculated by using the maximum load along the load/deformation plot and dividing this by the nominal cross-sectional area of the specimen measured in a plane perpendicular to the load. It can be determined for various types of loads (compressive, tensile, torsion, or shear). It is useful for determining the maximum load a product can sustain during a single load cycle.
Working time is the elapsed time from the initiation of mixing a settable material until it reaches a consistency where it is no longer appropriate for use. It is helpful in the successful use of impression materials and some restorative and endodontic materials.
Yield strength is a material property which describes the point during loading where a material starts to behave plastically (it deviates from the linear change in deformation with added load which is typical of elastic behavior). As with all strength properties, yield strength implies that the cross-sectional area has been taken into consideration by dividing the yield load by the initial area that lies in a plane perpendicular to the applied load. It is a parameter critical to the design of successful components and materials. If a material has too low a yield strength for a particular application, it will fail prematurely. A rough estimate for the fatigue strength of a material is the yield strength divided by two.