When Sunlight Makes Materials Move: A Laboratory Experiment Demonstrating Solar-Driven Hydrogel Actuator

Hydrogels, owing to their softness, deformability, and responsiveness to external stimuli, are widely employed in soft robotics as materials for actuation, sensing, and self-healing, enabling biomimetic motion and intelligent control. This laboratory experiment introduces upper-division undergraduate students to the concept of a solar-driven hydrogel actuator by fabricating and demonstrating a temperature-responsive hydrogel. Students prepare a temperature-sensitive poly(N-isopropylacrylamide) (PNIPAm) hydrogel incorporated with polypyrrole, a black photothermal additive that enhances sunlight absorption and photothermal conversion ability. Under solar irradiation, the temperature gradient between the upper and lower hydrogel layers induces asymmetric volume changes, resulting in visible bending or deformation. Conducted over a 4 h session with 32 upper-division undergraduate students, this hands-on laboratory experiment allows students to explore the relationships among polymer chemistry, materials science, photothermal conversion, and soft actuator behaviors. This laboratory experiment not only cultivates students’ practical skills in hydrogel preparation and data analysis but also stimulates their interest in polymer chemistry and materials science through the visual observation of solar-driven hydrogel deformation. Furthermore, it encourages students to explore the potential applications of hydrogel actuators in future intelligent materials and soft robotics.