A coordination-covalent dual-network enables weak gradient-strong response in PVA-based bilayer humidity actuator

Humidity-responsive actuators are crucial for next-generation wireless microdevices. However, conventional polyvinyl alcohol (PVA)-based bilayer actuators face challenges such as high actuation thresholds, poor interfacial stability, and a trade-off between robustness and sensitivity. To address these issues, we develop a novel bilayer actuator by integrating a dynamic boric acid (BA)-crosslinked network with an Fe 3+ -coordinated PVA framework. This design constructs a coordination-covalent dual network at the interface: Fe 3+ chelation enhances interlayer adhesion, while the dynamic borate ester bonds regulate PVA crystallization and swelling. The optimized actuator (PFB-5) exhibits exceptional performance, including rapid bending (≈90° within 30 s) under a subtle humidity gradient (contact angle difference ≈21°, RH ≈ 70%), and improved mechanical properties. This “weak gradient–strong response” characteristic enables the actuator to mimic biological motions, such as butterfly wing flapping and pinecone scale opening/closing, demonstrating its potential for bio-inspired soft robotics. Combined with a scalable fabrication strategy, this design offers a promising approach for advanced applications in soft robotics, wearable devices, and MEMS.