Design and synthesis of vertical gradient of expansion structure based on polyimide humidity sensing layer with ultra-fast response, high sensitivity, and large deformation driven by humidity

The design of humidity actuators with fast reaction sensitivity (particularly actuation time) while retaining desirable mechanical qualities is critical for advanced smart manufacturing such as soft robots and smart devices, yet it remains a challenge. Here, we fabricated film-based humidity-responsive materials from hydrogenated pyromellitic dianhydrides (HPMDAs) via a simple two-step chemical imidization. The bending of the film is highly sensitive to humidity, and even a small amount of humidity based on a wet cotton swab can be observed from a distance of a few millimeters to bend the film. HPMDAs-based PI (PIs-HPMDAs) not only possess facile fabrication features and excellent properties, such as thermal stability, mechanical properties, and optical properties, but also deliver excellent deformation, including a fast response/recovery (0.02/0.94 s), large deformation (220 degrees), and sufficient bending stability (no detectable degradation after 1200 times). Taking advantage of these benefits, the relative humidity can be converted into a deformation signal of moving object, and it can be further turned into electric signals. Compared with conventional humidity sensors, such the polyimide-based humidity sensor is expected to be used in ambient electrical sensors and relays due to their simple fabrication, rapid response, and high stability. Our strategy of generating and exploiting internal stress in responsive polymers provides a promising platform for humidity actuators.