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Caries affects billions of individuals worldwide, thus pointing out the importance of advancements in restorative dentistry. Dental resin composites yield restorations with satisfying mechanical properties, therefore the focus of development has shifted to accelerated treatments and esthetic aspects. Challenges in matching tooth color arise due to limited options, application changes, and color variations over time. Single-shade composites with the 'chameleon effect' adapt their color to the surrounding enamel by closely matching the tooth's optical spectrum, enhancing color blending. Structural color, based on light interference, contributes to this effect. The study investigates the submicron filler particles' impact on optical properties and the chameleon effect. Four single-shade dental resin composite materials were investigated. Needle-like samples about 100 μm in diameter were prepared and imaged in a scanning electron microscope. Light transmission through the materials for wavelengths between 200 and 900 nm was measured using a spectrophotometer. Three-dimensional nanotomography data were obtained through transmission X-ray microscopy at the ANATOMIX beamline, Synchrotron SOLEIL, France in both absorption and Zernike phase contrast mode with 23 nm voxel size. The real space information was complemented with small-angle X-ray scattering. These experiments revealed substantial differences in the microscopic structure of the materials. In the case of Omnichroma, the filler consists of almost identical spheres with a diameter of 260 nm while Filtek Universal exhibits polydisperse, irregularly shaped fillers. Additionally, Venus Pearl One’s fillers have a polyhedral shape and a wide size distribution. Finally, the setups used did not reveal any clearly identified microstructure of the Chroma Fill composite. Although all investigated materials are known to exhibit the chameleon effect, their differences in micro- and nanostructure call into question previous hypotheses on the chameleon effect’s origin from structural color. While we have now a reasonable understanding of filler morphology, size distribution and spatial arrangement, more information is needed on the exact chemical composition of filler and matrix and their interaction with electromagnetic waves, including possible nonlinear effects.
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