Real-Time Stress Visualization of Hydrogels Enabled by Supramolecularly Switched Stretch-Induced Phase Separation

The visualization of mechanical stress in soft materials is highly desirable; however, real-time optical readouts using conventional sensing approaches are problematic because mechanophore-based systems typically require strong threshold-type activation with slow recovery. Herein, we report supramolecular hydrogels that enable the continuous and reversible visualization of mechanical stress in real time via stretch-induced phase separation. Supramolecular switching mechanotransduction (SSM) has been proposed as the key mechanism. Mechanical stimuli are transduced into a distinct network state transition through host-guest complexes between β-cyclodextrin and adamantane as supramolecular switches. In this design, guest-functionalized polymers undergo on-off transition between the hydrated and dehydrated states via host-guest complexation and decomplexation. Responsive polymers, incorporated into the hydrogel network via supramolecular bonds, function as reversible cross-links and switches. Upon stretching, the hydrogels macroscopically transition from transparent to opaque owing to the dehydration-induced heterogeneity within the responsive domains. The linear and reversible changes in the opacity with applied stress-attributable to sacrificial and reversible supramolecular switching-enable the visualization of stress distributions. This design principle offers a platform for spatiotemporally resolved mapping of the mechanical states in hydrogels with an intuitive, instrument-free readout, and lays the foundation for monitoring, timely intervention, and safer operation of soft-material systems.