Soft, Stretchable, and Pneumatically Triggered Thermochromic Optical Filters with Embedded Phosphorescence.

Phosphorescence is commonly used in nature to communicate using light. There are many ways to activate phosphorescence, including UV light, heat, and mechanical forces, but there are few methods to control phosphorescence once activated. Here, we present soft composite devices-which we call "optical filters"-for controlling the release of light by phosphorescence within a stretchable matrix. The filters consist of liquid metal wires, phosphorescent particles, and thermochromic pigments embedded in an elastomeric matrix. UV light initially activates the phosphorescence of rare-earth long-lasting luminescent particles. At room temperature, the thermochromic pigments block the phosphorescence from leaving the matrix. However, Joule heating of the liquid metal can change the opacity of the thermochromic pigments, which tunes the color, intensity, and wavelength of phosphorescence that exits the composite. In addition, the resistance of the liquid metal wires changes with physical deformation, thereby converting mechanical forces (strain, compression, and pneumatic inflation) into an optical response. Controlled phosphorescence, combined with the electrical conductivity of the liquid metal and the overall soft matrix, enables potential applications as an electronic skin for soft robotics, stretchable electronics, and prosthetics.