A Battery-Free Stretchable Strain Sensor Using Plasmon-Based Structural Color

Soft and stretchable sensors offer superior skin conformability, enabling the capture of high-quality biometric signals. Among these, color-based sensors can be operated without batteries and read using devices like smartphone cameras, making them easy to use. While there have been advancements in strain sensors using structural colors, such as photonic crystals [1,2], their applications are limited due to challenges in tuning device softness.<br/><br/>Here, we introduce a battery-free, soft, and stretchable strain sensor using structural color that combines plasmon resonance and interference effects. The device structure comprises 4 stacked layers: a high-refractive-index (n=1.55) polymer, a low-refractive-index (n=1.33) polymer, gold nanoparticles, and a black silicone substrate. Structural coloration emerges from the interference of reflected light from gold nanoparticles, where plasmon resonance takes place, with the reflected light at the interface of the top high and low-refractive-index layers. Applied strain changes the thickness of a low-refractive-index polymer layer to induce different structural colors. This color can be adjusted by modifying the thickness of the low-refractive-index polymer layer and the gold nanoparticle size. Given our structural color layer is ultra-thin, under 1 µm, its flexibility is easily adjustable with additional PDMS layers. A particular device displayed a reflection peak of 730 nm (red) at 0% strain and 665 nm (green) at 60% strain. Moreover, our sensor adapted to finger joints and exhibited a distinct color shift during finger bending.<br/><br/>1. Yue, Y. et al. Mechano-actuated ultrafast full-colour switching in layered photonic hydrogels. Nat. Commun. 5, 4659 (2014).<br/>2. Park, T. H. et al. Block copolymer structural color strain sensor. NPG Asia Materials 10, 328–339 (2018).