Stiffness-gradient adhesive structure with mushroom-shaped morphology via electrically activated one-step growth

Reptiles in nature have evolved excellent adhesion systems to adapt to complex natural environments, inspired by which high-performance bioinspired dry adhesives have been consistently created by precisely replicating the natural structures. Stiffness gradient, as a special feature evolved in reptilian adhesion systems, offers significant advantages in enhancing adhesion adaptation and stability. However, it remains a challenge to accurately replicate the geometrical morphology and soft-rigid composite properties of stiffness gradient structures, which limits the engineering applications of bioinspired adhesives. Here, a stiffness gradient adhesive structure with mushroom-shaped morphology via electrically activated one-step growth is proposed. Under the action of electric field, the liquid-phase polymer grows rheologically to realize the mushroom-shaped structural morphology, and the nanoparticles inside the polymer are aggregated toward the top by dielectrophoresis to realize the stiffness gradient distribution of rigid top and soft bottom. Due to the adaptation of the soft part to the interfacial contact and the effective inhibition of peeling by the rigid part, the proposed stiffness gradient structure improves the adhesion strength by 3 times in the parallel state and by 5 times in the nonparallel state compared to the conventional homogeneous structure. In addition, the application of adhesive structures in wall-climbing robots was demonstrated, opening an avenue for the development of dry adhesive-based devices and systems.