Research infrastructure

We evaluate how effectively a composite material hardens under curing light by monitoring its chemical transformation in real time.

A small sample of the material is placed directly on a highly sensitive infrared sensor and exposed to a clinically relevant curing light. As the material transitions from a soft, moldable state into a solid polymer, the instrument continuously tracks the concentration of specific chemical bonds that are responsible for this transformation. This allows us to measure how fast and how completely the reaction occurs over time.

We can test materials at different layer thicknesses (for example, very thin films versus clinically relevant 2 mm layers, or 4 mm layers for some specific materials). This enables us to assess how effectively light penetrates through the material and how curing performance changes with layer thickness. In practical terms, this simulates real clinical situations where insufficient light penetration can lead to incomplete curing in deeper regions.

The method provides several key outputs:

• Extent of curing (degree of conversion): how much of the material has chemically reacted
• Curing speed: how quickly the material reaches its hardened state
• Reaction profile over time: whether curing is smooth, delayed, or inhibited
• Depth-dependent performance: how curing efficiency changes between surface and deeper layers

Because the measurement is continuous over a desired time period (several minutes to several hours), we obtain a full curing “profile” rather than a single endpoint value. This makes it possible to detect subtle differences between materials.

The setup replicates clinical conditions, including realistic light intensity and exposure times, ensuring that the results are directly relevant to real-world use.

From a business and product development perspective, this method allows us to:

• Rapidly compare new formulations of experimental materials against benchmarks
• Identify materials that cure faster or more completely
• Demonstrate performance advantages linked to composition or technology
• Verify product claims related to curing efficiency, depth of cure, and clinical reliability

In addition to basic curing performance, this approach can be used to derive indirect indicators of clinical behavior. By analyzing how quickly the material reaches its peak reaction rate and how the reaction progresses over time, we can identify curing profiles that may be associated with higher or lower internal stress development, which is relevant for marginal integrity and long-term stability. This enables early-stage screening of formulations for potential performance risks before more complex mechanical testing is required.

The method can also be adapted to simulate real-world clinical variability by adjusting parameters such as layer thickness, exposure time, and distance or positioning of the curing light. This allows us to evaluate how sensitive a material is to suboptimal curing conditions and to determine how reliably it performs in everyday clinical scenarios where ideal conditions are not always achieved.Beyond the initial light exposure, the system can be used to monitor post-cure behavior, capturing continued chemical reactions that occur after the light is switched off. This provides insight into how the material evolves over time, including whether additional conversion contributes to improved mechanical properties.

From a broader perspective, polymerization kinetics can be integrated with other measurements, such as light transmission or mechanical testing, to create a more comprehensive understanding of material performance. This enables a direct link between formulation, curing behavior, and functional outcomes, supporting both product optimization and differentiation.

Finally, the quantitative nature of this method makes it suitable for supporting product claims and regulatory documentation, such as demonstrating efficient curing at clinically relevant depths, high degree of conversion, or robust performance under varying conditions.

An infrared spectrometer equipped with a microscope allows us to map conversion across different regions of a specimen with high positional accuracy. For example, we can assess how curing efficiency changes with depth by measuring conversion at multiple points from the irradiated surface toward deeper layers, effectively visualizing curing gradients caused by light attenuation. In addition, the technique can be used to examine localized effects at critical interfaces, such as between adhesive layers and overlying composites, or in the vicinity of defects, voids, or inclusions, where curing may be compromised. This makes it possible to identify heterogeneities that are not detectable with bulk measurements. From a product development perspective, this provides a powerful tool for verifying uniformity of polymerization, optimizing layer thickness recommendations, and demonstrating reliable curing performance under simulated clinical conditions.

Mechanical performance of dental materials (or any other material of similar mechanical behavior) can be evaluated using a universal testing machine, which enables controlled loading of specimens under different types of mechanical stress while continuously recording force and deformation. Our primary application is the measurement of flexural strength and flexural modulus of resin composites using three-point bending tests. In this setup, standardized bar-shaped specimens are loaded until fracture, allowing us to determine both the maximum load-bearing capacity of the material and its stiffness during deformation. The method can also be combined with artificial aging procedures, such as long-term water storage, storage in aggressive chemical media, or thermocycling, to evaluate how mechanical properties change over time and under artificial aging conditions.

Beyond flexural testing, the same instrument can be adapted for a broad range of additional mechanical measurements by using appropriate fixtures, jigs, and specimen geometries. This includes:

• Compressive strength testing, relevant for materials that are more sensitive to compressive loading, e.g., glass ionomers
• Tensile testing, used to assess resistance to stretching and crack initiation
• Shear bond strength testing, widely used to evaluate adhesion between restorative materials and tooth tissues (dentin or enamel)
• Elastic modulus measurements, describing material rigidity and deformation behavior under load

Depending on the selected configuration, the system can evaluate restorative composites, adhesives, cements, ceramics, polymers, fiber-reinforced materials, and additively manufactured (“3D printed”) specimens. The instrument can further be used to investigate the effects of formulation changes, filler content, curing protocols, environmental aging, or storage conditions on mechanical durability.

Because the testing machine continuously records force-displacement behavior throughout the experiment, it provides mechanical profiles that can be used to investigate how materials deform before fracture, how brittle or ductile they behave, and how consistently they perform across repeated measurements.

The universal testing machine can also be adapted for fracture toughness testing, which evaluates a material’s resistance to crack propagation and catastrophic failure. Using specially designed specimens containing predefined notches or defects, the system measures how much force is required for an existing crack to grow through the material. This provides insight beyond conventional strength measurements, as two materials with similar flexural strength may behave very differently once microscopic damage or flaws are present. Fracture toughness testing is particularly relevant for brittle or highly filled dental materials, where clinical failure often originates from crack formation under repeated chewing forces.

Visible light transmittance through composite materials can be evaluated using a fiber-optic spectrometer system that measures how much curing light passes through a specimen in real time during light exposure. In this setup, the material is positioned between the curing light source and the detector, while the spectrometer continuously records the intensity and spectral distribution of transmitted light across the clinically relevant wavelength range used for light-curing. This enables precise assessment of how efficiently light penetrates through the material during polymerization.

The method is particularly useful for evaluating factors that influence curing efficiency, such as material composition, filler type, opacity, shade, and specimen thickness. By testing different layer thicknesses, we can determine how rapidly light intensity decreases with depth and estimate how effectively deeper regions of the material can be cured when increasing the layer thickness. This is especially important for bulk-fill materials and highly filled or more opaque composites, where insufficient light penetration may compromise polymerization in deeper layers. For experimental composite materials, sufficient light transmittance is essential for their potential clinical applicability because inadequate light penetration can result in incomplete curing and consequently compromise clinical usability. Therefore, this method can be used for confirming that newly developed materials allow reliable light-curing under clinically relevant conditions.

Because the system records the full light spectrum rather than only total intensity, it can also be used to investigate wavelength-dependent light transmission. For example, shorter wavelengths in the violet region typically scatter more strongly within composite materials and therefore penetrate less deeply than blue light. This allows us to compare how different materials interact with various components of the curing light spectrum and evaluate their compatibility with different curing lamps, which emit different wavelengths.

An additional advantage of the method is the ability to monitor how light transmission changes dynamically during curing. As the material polymerizes, its optical properties gradually change, often allowing greater light transmission over time due to refractive index changes within the resin matrix. However, the opposite behavior can also be intentionally designed into the material. Some modern composites are formulated to initially exhibit higher light transmittance, allowing deeper light penetration and more effective polymerization, followed by a gradual decrease in transmittance as the material sets. This reduction in translucency after curing can improve the final visual appearance by increasing opacity and better matching the optical properties of natural tooth structure. Because the present method monitors light transmission continuously throughout the curing process, it can detect and characterize such dynamic optical changes in real time. This enables targeted optimization of experimental materials in which optical behavior is deliberately adjusted depending on the intended clinical or aesthetic outcome.

From a product development perspective, light transmittance measurements enable optimization of translucency, curing efficiency, and depth-of-cure performance while balancing aesthetic requirements. It can also support comparative benchmarking and validation of curing recommendations.

A thermographic camera can be used for real-time monitoring of temperature changes during light-curing of composite materials. In this method, the camera continuously records the surface temperature of the specimen while it is exposed to the curing light, allowing detailed evaluation of both the maximum temperature reached and the dynamics of temperature development during illumination with the curing lamp. Because dental composites generate heat both from the curing lamp and from the exothermic polymerization reaction itself, the method provides indirect information on curing behavior and reaction kinetics.

The technique is particularly useful for comparing how different material compositions influence thermal behavior during curing. For example, changes in filler type, particle size, filler loading, optical properties, or photoinitiator systems may alter how rapidly heat develops and dissipates within the composite. The method can therefore help identify formulations that exhibit unusually rapid or excessive temperature rise, which may be relevant for evaluating potential thermal risks for pulp tissue.

Because measurements are performed in real time, the system can also characterize the full thermal profile of the curing process rather than only a single endpoint temperature. This enables comparison of the time required to reach maximum temperature, the dynamics of heat development, and post-curing cooling behavior. In experimental materials research, thermographic analysis can be combined with polymerization kinetics and light transmittance measurements to better understand how optical, chemical, and thermal processes interact during curing.

Surface roughness measurements are used to evaluate how smooth or irregular the surface of a dental material is after different treatments, aging procedures, or environmental exposures. In this method, a surface profilometer moves a highly sensitive probe across the material surface and records microscopic height variations, allowing quantitative determination of roughness parameters such as Ra, which represents the average surface irregularity.

This technique is useful for investigating the surface degradation of dental restorative materials caused by water exposure, acidic beverages, toothbrushing, polishing procedures, chemical aging, or wear simulation. Because surface roughness strongly influences plaque accumulation, staining susceptibility, gloss retention, and long-term esthetics, the method provides clinically relevant information about material durability and surface stability.

The system can be used to compare how different resin matrices, filler types, filler particle sizes, filler loading, or bioactive additives influence surface degradation over time. It is also useful for evaluating the effectiveness of polishing protocols, surface coatings, or protective treatments, as well as for studying how experimental materials respond to acidic or abrasive conditions intended to simulate the oral environment.

Color measurements are used to quantitatively evaluate the optical appearance and color stability of dental restorative materials before and after aging, staining, or environmental exposure. In this method, a spectrophotometric device measures the reflected light from the material surface and determines its position within the CIE Lab* color space, which describes brightness (L*), red-green balance (a*), and yellow-blue balance (b*). By comparing measurements obtained at different time points, the overall color change (ΔE) can be calculated and related to clinically perceptible discoloration.

This technique is important for evaluating the esthetic stability of restorative materials exposed to beverages, food pigments, water aging, or other environmental factors that may affect long-term appearance. Because modern restorative dentistry places strong emphasis on esthetics, color stability is a critical parameter for assessing the long-term clinical acceptability of composite materials, especially in anterior restorations.

The method can be used to investigate how material composition influences staining susceptibility and optical stability. In addition to measuring overall color change, analysis of individual L*, a*, and b* components can provide more detailed insight into the direction and mechanism of optical changes occurring during aging or degradation.

Surface microhardness measurements are used to evaluate the resistance of dental materials to localized surface deformation and are commonly applied as an indirect indicator of surface mechanical integrity, degree of conversion, and degradation resistance. In this method, a microhardness tester presses a diamond indenter into the material surface under a controlled load for a defined period of time, after which the size of the resulting indentation is measured to calculate the hardness value.

The technique is useful for investigating how different environmental conditions, aging procedures, or material compositions influence surface properties over time. For example, exposure to water, acidic beverages, chemical agents, thermal cycling, or mechanical wear may soften the material surface and reduce its hardness. Conversely, some materials may continue undergoing post-cure reactions after initial setting, resulting in gradual hardening over time. Because of this sensitivity, microhardness measurements provide insight into the balance between material maturation and degradation processes.

From a clinical perspective, surface hardness is relevant because it is associated with wear resistance, scratch resistance, and long-term surface stability of restorations.

A dual asymmetric centrifugal mixer is used for rapid and highly homogeneous preparation of experimental dental materials, particularly resin composites. The system mixes materials through simultaneous rotation around two axes, generating strong internal shear forces without the use of mixing blades. This enables efficient incorporation of all composite components (fillers, resins, photoinitiators, and other additives) while minimizing air entrapment and improving material homogeneity.

An important advantage of the method is the ability to achieve simultaneous mixing and degassing, resulting in more uniform and bubble-free materials, which is particularly important for reliable mechanical, optical, and polymerization properties of dental composites. Because the process is highly reproducible and programmable, it is also useful for standardized preparation of experimental formulations during product development and comparative testing.

In our work, the dual asymmetric mixer is primarily used for the preparation of experimental resin composites with precisely controlled compositions. Starting from individual raw components, such as resin monomers, reinforcing fillers, photoinitiators, and various functional additives, we can systematically vary material composition depending on the investigated research question. For example, functional fillers such as bioactive glasses can be incorporated in different amounts to create composites capable of releasing therapeutic ions. This controlled preparation approach allows us to directly investigate how specific formulation changes influence properties such as polymerization behavior, mechanical performance, optical properties, or ion release, enabling structure-property relationship studies.

A UV-visible spectrophotometer can be used for quantitative colorimetric analysis of liquid samples by measuring how strongly a solution absorbs light at specific wavelengths. In a typical measurement, the sample is placed in a transparent cuvette and exposed to ultraviolet or visible light, while the instrument records the amount of light absorbed by the solution. Because many chemical reactions produce characteristic color changes, the measured absorbance can be directly related to the concentration of a target substance in the sample.

The method is highly versatile and can be adapted to a wide range of analytical applications using appropriate reagents and reaction protocols. For example, assays such as Arsenazo III or malachite green can be used for determining calcium or phosphate ion concentrations, while numerous additional colorimetric methods are available for analyzing dissolved compounds, released ions, reaction products, or degradation processes. In essence, whenever a substance can be converted into a measurable color signal, it can potentially be quantified using this approach.

Water sorption and solubility measurements are used to evaluate how restorative materials interact with aqueous environments over time, which is relevant because these materials are continuously exposed to saliva in the oral cavity. According to the ISO 4049 methodology, specimens are immersed in water for a defined period, weighed, dried, and then weighed again. From the measured mass changes, it is possible to determine how much water the material absorbs (water sorption) and how much material is lost through dissolution or leaching processes (solubility).

These measurements are particularly important because excessive water sorption may degrade the material, while high solubility may indicate leaching of potentially toxic components. Both water sorption and solubility can affect long-term mechanical stability, dimensional stability, esthetics, and clinical durability of restorative materials. At the same time, controlled solubility may be desirable for certain functional materials, especially bioactive composites designed to release therapeutic ions.

In addition to standard ISO 4049 measurements of water sorption and solubility, the method can be expanded through long-term monitoring of specimen mass during prolonged water immersion. By repeatedly measuring mass changes over weeks or months, it becomes possible to characterize the kinetics of water uptake, dissolution, and long-term material stability. This approach is particularly useful for experimental bioactive materials containing reactive fillers, such as bioactive glasses, where ion release is expected to occur over extended periods. In such systems, long-term mass monitoring can provide indirect insight into degradation behavior, ion-release dynamics, and the balance between functional activity and structural stability.

Depth of cure measurements according to the ISO 4049 “scraping method” are used to evaluate how deeply a composite material can be effectively polymerized by a curing light under standardized conditions. In this method, a long cylindrical specimen is illuminated with a curing light from one side, after which the uncured material is mechanically removed and the length of the remaining hardened portion is measured. The reported depth of cure is calculated from the remaining cured length and serves as a practical indicator of the material’s curing capability.

This technique is important for assessing whether experimental or commercial materials can achieve sufficient polymerization in deeper regions of a restoration, where light intensity is reduced as light passes through the composite. Factors such as filler type, filler loading, particle size, opacity, photoinitiator system, and curing protocol may all influence the attainable depth of cure. The method is therefore widely used for evaluating bulk-fill capability, optimizing curing recommendations, and screening new formulations for adequate clinical performance. Because inadequate depth of cure may negatively affect mechanical properties, biocompatibility, and long-term durability, the method serves as an important screening tool for determining the clinical applicability of experimental restorative materials.

Although the ISO 4049 scraping test provides a relatively simple and standardized measurement, it can be combined with more advanced analyses, such as degree of conversion or microhardness measurements at different depths, to obtain a more detailed understanding of curing efficiency throughout the material.

Artificial aging procedures are widely used to evaluate the long-term stability and degradation behavior of experimental and commercial dental composites under accelerated laboratory conditions. These protocols are designed to simulate chemical, thermal, and hydrolytic challenges encountered in the oral environment and to assess how material properties change over time. Among the most commonly used methods, thermocycling exposes composite specimens to repeated temperature changes, typically between cold and hot water baths, thereby generating stresses due to differences in thermal expansion between the resin matrix and filler particles. This process can promote water ingress, filler/matrix interfacial degradation, and microcrack formation, making thermocycling a clinically relevant model for evaluating the durability of restorative materials. Thermocycling is commonly performed between 5 and 55 °C with repeated cycles intended to approximate intraoral thermal fluctuations.

In addition to thermocycling, chemical aging protocols can be used to accelerate specific degradation mechanisms. Immersion in aqueous media such as simulated body fluid or acidic solutions can model hydrolytic degradation and cariogenic conditions, while ethanol-water solutions are commonly used to induce rapid softening of the methacrylate network. Strongly alkaline solutions may further accelerate hydrolytic degradation of the resin matrix and filler/matrix interface.

Artificial aging can be combined with measurements of numerous material properties before and after aging to comprehensively evaluate material stability. Frequently investigated outcomes include macromechanical properties such as flexural strength and flexural modulus, micromechanical properties such as microhardness, and surface-related properties including roughness and color stability. Aging protocols are also commonly used in studies evaluating aesthetic stability, including color change, translucency, gloss retention, and staining susceptibility.