Self-Reporting Multiple Microscopic Stresses Through Tunable Microcapsule Arrays.

Self-reporting materials have emerged as a promising tool for real-time monitoring of stress and damage in structural materials. When critical stress is applied to these materials, an optical response is triggered - for example by dye release, or molecular cleavage. A key challenge is to extend these systems to respond to multiple different stress levels. To achieve this, a novel microcapsule-based assembly strategy is presented. Microfluidic synthesis is used to create microcapsules that release dye at a precise level of applied force. Subsequently, capillary assembly is used to combine microcapsules with different stress-responsiveness and different fluorescent dyes into chains, which are uniformly patterned into regular arrays, and embed these into the self-reporting materials. Through indentation experiments, it is shown that these materials can distinguish and record spatially resolved local stresses based on the fluorescence emitted upon microcapsule rupture. Crucially, the technique's accuracy is significantly improved when microcapsules are spatially organized within the material. This versatile technique can be applied to a range of different materials, via the use of thin coatings containing the regularly patterned microcapsule chains.