Sub-terahertz metamaterial stickers for non-invasive fruit ripeness sensing.

Fruits and vegetables account for around a third of all food loss and waste. Post-harvest, retail and consumer losses and waste could be reduced with better ripeness assessment methods. Here we develop a sub-terahertz metamaterial sticker (called Meta-Sticker) that can be attached to a fruit to provide insights into the edible mesocarp's ripeness without cutting into the produce. The fruit acts as a complex multilayer substrate to Meta-Sticker and, when excited by sub-terahertz signals, generates two distinct resonances: localized dipole resonance that correlates with the exocarp's refractive index; and propagating plasmon resonance that penetrates into the mesocarp and resembles the rare phenomenon of 'extraordinary transmission'. The Meta-Sticker accurately predicted the ripeness of different fruits with a cumulative normalized root mean square error of 0.54% of the produce tested. This study offers a non-invasive, low-cost and biodegradable solution for accurate ripeness assessment with applications in distribution optimization and food waste reduction.