A double-layer mechanochromic hydrogel with multidirectional force sensing and encryption capability

Hydrogel-based soft mechanochromic materials that display colorimetric changes upon mechanical stimuli have attracted wide interest in sensors and display device applications. A common strategy to produce mechanochromic hydrogels is through photonic structures, in which mechanochromism is obtained by strain-dependent diffraction of light. Here, a distinct concept and simple fabrication strategy is presented to produce luminescent mechanochromic hydrogels based on a double-layer design. The two layers contain different luminescent species—carbon dots and lanthanide ions—with overlapped excitation spectra and distinct emission spectra. The mechanochromism is rendered by strain-dependent transmittance of the top-layer, which regulates light emission from the bottom-layer to control the overall hydrogel luminescence. An analytical model is developed to predict the initial luminescence color and color changes as a function of uniaxial strain. Finally, this study demonstrates proof-of-concept applications of the mechanochromic hydrogel for pressure and contact force sensors as well as for encryption devices.Singapore-MIT Alliance for Research and Technology (SMART)This research was funded by a seed grant from the BioSystems and Micromechanics thrust of the Singapore-MIT Alliance for Research and Technology (BioSym-SMART). The authors thank Dr. Shahrouz Amini for his help in establishing mechanical testing and sensing devices and for insightful discussions. The authors also thank Dr. Lin Kan for help with MATLAB modeling. Experimental details as well as Figure S1 to S16 are included in the Supporting Information