Elastoplastic Inverse Opals as Power-Free Mechanochromic Sensors for Force Recording

Light-weight, power-free mechanochromic sensors that can change and record the reflective color depending on the magnitude and rate of the applied force are fabricated from inverse opals by infiltrating the colloidal crystals of silica particles with uncrosslinked SU-8, followed by removal of the colloidal templates. The mechanical sensing range of the materials is high, 17.6-20.4 MPa. Due to elastoplastic deformation of the SU-8 films, the deformed structures and thus colors can be locked after the removal of the load, therefore establishing a quantitative relationship between the mechanical force and optical responses. In comparison, mechanochromic photonic gels reported in the literature typically detect force in the range of 10-100 kPa; once the load is removed, the structure and color return back to the original ones. The mechanochromic sensors are highly sensitive: the ratio of shift in the stopband wavelength to the change in applied strain is up to 5.7 nm per percent, the highest among literature. Comparison of finite element simulations with experiments confirms the elastoplastic deformation of the films and highlights that reconfiguration of pore shape under compression plays a key role in the mechanochromic response. Power-free and highly sensitive mechanochromic sensors that can quantitatively measure the magnitude of mechanical force are prepared from uncrosslinked SU-8 inverse opals. They can record impact forces by exhibiting different visible colors depending on the amount and rate of the applied forces. Experiments and finite element simulations attribute this to the elastoplastic deformation of the crystals.