Crack‐Growing Interlayer Design for Deep Crack Propagation and Ultrahigh Sensitivity Strain Sensing

Abstract Strain signals contain rich physical and physiological information, requiring highly sensitive sensors to capture minute deformations. Low strain detection is especially important for applications involving small but informative deformations, such as biological tissues, micromechanical systems, or subtle human motions. Crack‐based sensors, inspired by the mechanoreceptors of spiders, offer exceptional sensitivity, flexibility, and scalability. However, achieving ultrahigh sensitivity in low strain regimes remains challenge. Here, a crack‐growing interlayer is presented that facilitates the formation of a deep cracks, a key parameter for enhancing the sensitivity of crack‐based sensors. By curing a polyimide (PI) precursor at low temperatures, a semi‐cured polyimide (SPI) interlayer is fabricated with low fracture energy due to reduced imidization. This enables the development of deep cracks penetrating 2.5 µm into the SPI, exhibiting a wide gap of 135 nm at 2% strain. The sensor achieves a gauge factor (GF) of 100 000 at 2% strain, demonstrating ultrahigh sensitivity in the low‐strain regime. Its application is further validated through attachment to the nose, where it successfully detects subtle tissue deformations during breathing. These results highlight the potential for widespread applications in fields requiring minimally obstructive, scalable, and conformal sensing systems, such as biomedical monitoring, micromechanical systems, and subtle human motion detection.