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.