2D stress-distribution imaging using 3D transparent stimulus-responsive color-changing rubber

Imaging of invisible mechanical stresses is a significant challenge in a variety of fields. 2D distribution imaging of compression stresses applied by irregularly shaped 3D objects is not achieved using conventional sensing materials and devices. In the present work, such a compression-stress distribution in the range of 0.1 kPa-5 MPa is imaged using 3D transparent silicone rubber containing stimuli-responsive color-changing conjugated polymer, layered polydiacetylene (PDA). Compression-responsive capsules, liquid droplets surrounded by solid particles, collapse with compression on the rubber. The outflowed interior liquid containing polyethyleneimine (PEI) oligomer is diffused into the bulk rubber through the free volume space of the rubber matrix. PEI serves as a guest for intercalation into the interlayer space of the layered PDA, i.e. chemical stress, directing the blue-to-red color change. As the red-color intensity increases with increasing applied compression stress, the strength is colorimetrically quantified. The transparent 3D device enables 2D distribution imaging of the compression stresses applied by irregularly shaped 3D objects in the millimeter to the centimeter scales. The device design can be applied to achieve 2D stress-distribution imaging in various length scales and strength ranges.