Mechano‐Responsive Spiropyran Microbeads: A Facile Fabrication Strategy for Self‐Reporting Materials

Abstract Spiropyran (SP) mechanophores are attracting attention as next‐generation smart materials that can self‐diagnose stress or strain thanks to their capacity for stress visualization with superior sensitivity. However, at present, to achieve the self‐reporting functionality, it is considered essential that SP is chemically bonded to a host matrix, which has greatly limited its application. In this paper, mechano‐responsive SP beads that can render a material self‐reporting by means of simple physical mixing are presented. The synthesis of SP beads is achieved in a microemulsifying needle via dispersion polymerization, and their application to various polymers and aluminum through blending or surface coating methods is reported. The self‐reporting property of the specimen, evaluated by in situ measurements of color and full‐field fluorescence during deformation, allows both homogeneous and spatially heterogeneous stress distributions to be successfully visualized; the experimental measurements are in good agreement with the finite element simulations. It is also observed that the mechano‐response of SP beads is highly dependent on the stiffness of the matrix. The surface‐coating method is demonstrated to possess great advantages in terms of applicability, sensitivity, and scalability, facilitating accurate self‐diagnosis of the onset and propagation of damage in real time, even under complex stress conditions.