In this study, we developed a composite elastomer exhibiting both mechanical toughness and a structural color change using submicron-sized spherical fine silica particles with a uniform size as fillers and arranging them in a periodic structure in the elastomer. In this composite elastomer, the fine silica particles formed colloidal crystals in a nonclosest-packed state, so that the composite elastomer was very flexible. We showed the possibility for use as strain and stress sensors that can measure the amount of strain applied to and the stress generated in the composite elastomer according to the position of the reflection peak produced by the composite elastomer. In this composite elastomer, both the fracture stress and the fracture strain were improved by increasing the amount of fine silica particles so that the fracture energy and toughness increased. As a result, the fracture energy of the composite elastomer containing 35 vol % fine silica particles was 13.5 times that of the system containing no fine silica particles. In the stress–strain curve observed by uniaxial stretching of this composite elastomer, a shoulderlike change was observed at different positions depending on the content of the fine silica particles, and further stretching resulted in a large energy dissipation. It was also found that after being strained beyond the shoulder position for the composite elastomer containing 44.9 vol % fine silica particles, the composite elastomer became tougher than before it was stretched. This composite elastomer may be toughened by a change in the interaction between the silica–polymer interface that resulted from uniaxial stretching and a dissipation of the energy caused by a change in the particle arrangement. The composite elastomer developed by the present novel method has the ability to be toughened by being subjected to a large strain once and can be a safe material that can avoid sudden fracture.

We propose a strategy to develop a colorless, transparent, and tough composite elastomer inspired by the cornea, which is the transparent front portion of the eyeball. The composite elastomer, in which 34 vol % hard silica particles with a uniform particle size are dispersed as a filler in a lowcrosslinking polymer network exhibits a fracture energy that is ∼13.5 times higher than that of a system without the silica particles. This strategy also makes the elastomer optically transparent, because the light scattered by each silica particle that forms an ordered structure in the polymer network is cancelled by interference. This research may pave the way for the development of optically transparent and durable materials for applications such as advanced medical devices and soft robots.

Organisms that alter body color undergo color change in response to environmental variations and stimuli by combining chromatophores that develop colors by various mechanisms. Inspired by their body color changes, we can develop sensors and optical materials that change colors in response to multiple stimuli, such as mechanical and light stimuli. In this study, we report on bioinspired composite elastomers that exhibit various color changes as the pigment color, structural color, and background color change. These composite elastomers exhibit structural colors due to their fine structures in which fine silica particles form colloidal crystals, and the structural colors reversibly change as the elastomers elongate. Furthermore, photochromic dyes can reversibly change color depending on the wavelength of irradiated light when they are introduced to the composite elastomers. Since the structural color is one of the three primary colors of light and the pigment color is the color that corresponds to the three primary colors of a pigment, each color becomes vivid when the background color is black or white. Thus, we clarify that the composite elastomers exhibit various color changes due to the combination of structural color change in response to the mechanical stimulus, pigment color change in response to light irradiation, and background color change.

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