irreversible catastrophic disasters. Thus, to ensure the long-term stability of structures, damage diagnosis technology that can accurately detect the occurrence of cracks in real time is urgently required. [1] Two types of diagnostic methods are often employed: contact diagnostics, in which an electrical signal is detected by a n attached strain gauge [2] or piezoelectric sensor, [3] and noncontact diagnostics, in which a camera [4] or LiDAR detector [4b,5] is employed. Recently, research on selfreporting materials, which can produce visible signs of deformation or damage via the incorporation of force-responsive molecules called mechanophores, began to be conducted. [6] These materials are now attracting increasing levels of attention because of their ability to monitor nonuniform stress/strain distributions in real time, without the need for expensive power sources and data-processing devices.Spiropyran (SP) is a type of mechanophore that exhibits a specific reaction in response to force. [6e,7] As shown in Scheme 1, when a force is applied, the SP molecule undergoes selective C-O bond cleavage and is converted into merocyanine (MC), which is accompanied by color and fluorescence changes. SP is pale yellow and does not fluoresce, but MC is blue (or purple) and exhibits strong fluorescence in the 550-700 nm wavelength region. The advantages of stress visualization, high sensitivity, and reversible response have resulted in SP being very actively studied recently. Since 2009, when Moore and co-workers [7b] reported the discovery of the self-reporting of external forces by chemically bonding SP to a polymethacrylate polymer structure, SP has been introduced into various polymers such as polyurethane (PU), [8] polydimethylsiloxane (PDMS), [9] polyamide (PA), [10] polyester (PET), [11] polyacrylates, [6a,12] and polystyrene. [13] For example, Braun and co-workers [8a] successfully used SP as a molecular probe of force and orientation by incorporating it into the hard and soft phase of a segmented PU, respectively. Ko and co-workers [14] dramatically improved the mechano-sensitivity of SP-linked PDMS by adopting a hierarchical nanoparticle-inmicropore architecture and applied it as a mechano-chromic electronic skin. The nonuniform stress states of a SP-linked polyethylacrylate were experimentally monitored by Creton and co-workers, [15] and their observations were in accord with finite element simulation results.
Spiropyran (SP) mechanophores are attracting attention as next-generationsmart 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 selfreporting by means of simple physical mixing are presented. The synthesis of SP beads is achieved in a microemulsifying needle via dispersio...
