An autonomous biodegradable hygroscopic seed-inspired soft robot for visual humidity sensing

Visual sensors for relative humidity (RH) are of interest for distributed and autonomous environmental monitoring. Most of the visual humidity sensors are based on colorimetric sensing through the employment of hygroscopic inorganic pigments or photonic crystals (PCs). However, the toxicity of some inorganic pigments poses a risk to the environment especially if dispersed during in-situ measurements. On the other hand, the angle-dependent structural colours reading of the PCs, make these devices non suitable for autonomous and in-situ environmental monitoring.Here, we report the first visual humidity sensor using an artificial and hygroscopic seed-like robot (I-SeedPel) recently (2023) developed by our group for hygro-driven environmental exploration (https://doi.org/10.1002/advs.202205146).The I-SeedPel design is bioinspired to the hygroscopic and layered tissues of the Pelargonium appendiculatum seed and fabricated through additive manufacturing techniques using biodegradable polymers. The hygro-mechanical response of the I-SeedPel generates a reversible change of the geometrical features in the artificial seed structure (i.e., awn’s angular displacement and diameter variation) related to the RH. The variation of the geometric properties can be quantified and correlated to RH in a wide range (30–90 %), with an accuracy of 97–98 %, with a resolution of 0.17–0.52 % of RH and a good reproducibility (average RSD = 14.7 %).