"[A] film-type humidity sensor inspired by the photonic crystal structure of the cuticle of Hercules beetle was designed and fabricated. The reflective color of the fabricated sample changes from blue-green to red with the bandgap shift of 137 nm when increasing humidity, which could be discriminated by human eyes...This proposed bioinspired artificial sensor offers alternative sensing mechanism to quantify the environmental humidity in a simple and effective way." (Kim et al. 2010)At 25% humidity, the sensor appears blue-green, but at 98%, it changes to red.
"[C]onventional sensors need externally wired electronics for measurements, causing electrical noise and complicated fabrication process. However, the proposed humidity sensor measures directly the level of humidity using the spectrum of visible light without electrical transducers. Moreover, the simple fabrication process and visible measurement technique make the humidity sensor applicable to various areas." (Kim et al. 2010 [see associated strategy for citation])
In their paper, Kim et al. (2010) refer to this as the design principle: "The visible dry-state greenish coloration originates from a open porous layer located at 3 μm below the cuticle surface. This layer has three-dimensional photonic crystal structures, which are a network of filamentary strings, arranged in layers parallel to the cuticle surface [Fig. 1d]. In dry state, nanosized holes in the layer are occupied with air (refractive index 1) but the empty holes are filled with water (refractive index 1.33) under high humidity. The change in refractive index with respect to the humidity level induces the variation in the visible color."They then used a colloidal templating method to create a porous nanostructure similar to that found in the Hercules beetle, including mimicking the black melanin layer under the porous middle layer of the exocutile, and making the porous area hydrophilic.Watch Video of Beetle and Sensor
The humidity sensor would not need electricity to run. It would be used in small medical or agricultural devices, including in remote areas where there is no electricity.Edit Summary