Highly Sensitive Humidity-Responsive Actuator Comprising Aligned Electrospun Fibers Containing Metal-Organic Framework Nanoparticles

A humidity-responsive bilayer actuator was fabricated using the electrospinning method. We fabricated a polyvinylpyrrolidone (PVP)/poly(acrylic acid) (PAA) active layer with different alignments of fibers and thicknesses, and combined them with polyimide film. We observed the bending motion of the actuator while the relative humidity varied from 10% to 80%. While the PVP/PAA layer was electrospun with an increase in collecting speed and time, the degree of fiber alignment and the film thickness increased, respectively. We observed that a well-aligned or thick actuator exhibited a higher change in curvature throughout the experiment. However, a thick actuator had low sensitivity to relative humidity changes because of the low diffusion rate. To overcome this drawback, we added metal-organic framework (MOF) nanoparticles to the PVP/PAA solution before electrospinning to enhance the sensitivity of the actuator. As a result, the MOF-actuator was highly sensitive to humidity and showed larger curvature changes, generating a high actuating force of approximately 5.5 mN at the same thickness as the bare PVP/PAA actuator. We believe that the introduced actuator can be applied in robotics, energy harvesting, and sensors with controllable performance and high actuating force.