Bilayer Actuators Based on Interfacially Stable Poly(vinyl alcohol) Hydrogels: Concentration-Differentiated Bilayer Design and Sodium Citrate-Induced Salting Out Actuation for Soft Robotics

Abstract Hydrogel-based flexible actuators have attracted significant attention for their stimulus-responsive properties, yet the practical application of bilayer architectures remains hindered by interfacial instability caused by mismatched mechanical properties between heterogeneous layers. Inspired by the humidity-responsive asymmetric structure of pinecone scales, we fabricated a bilayer hydrogel actuator using PVA layers with different mass fractions. The actuator consists of discrete 5 and 15 wt % PVA layers, whose different polymer contents result in distinct network structures, contraction responses, and mechanical properties while retaining the same polymer chemistry. Through sequential freezing and thawing, the bilayer exhibited strong interfacial integrity, with an apparent interfacial shear strength exceeding 0.74 MPa. The actuation is driven by sodium citrate-induced salting out and enables continuous ion-responsive bending, reaching a stable bending angle of 115.1° with a bending rate of 3.8°/min. Demonstrations include biomimetic motions such as butterfly wing flapping and flower petal closure, highlighting potential applications in soft robotics, minimally invasive surgical tools, and adaptive wearable devices. This work provides a simple strategy for constructing interface-stable hydrogel actuators through concentration-differentiated bilayer design.