A Photoswitchable Hydrogel Engineered by a UV‐Triggered Fe ³ ⁺ /Fe ² ⁺ Valence Cycle for Optically Modulatable Triboelectric Sensing

Stimuli‐responsive hydrogels with reversible property switching are highly desired for soft electronics, but achieving this with a single stimulus remains challenging. Herein, a novel PAM/CMC@Fe 3 + dual‐network hydrogel is engineered via a UV‐triggered Fe 3 + /Fe 2 + valence cycle. UV light reduces Fe 3 + to Fe 2 + , producing a soft adhesive state, while subsequent air oxidation re‑oxidizes Fe 2 + to Fe 3 + , restoring the tough solid state. By simply tuning the Fe 3 + ‑to‑citrate ratio, mechanical properties and self‑healing efficiency are effectively programmed, navigating the trade‑off between mechanical integrity and healability. Taking advantage of this photosensitivity, the gel serves as a high‑performance bio‑ink for photocuring 3D printing. More importantly, its photo‑tunable adhesion enables an optically modulatable triboelectric nanogenerator (TENG) as an adaptive sensor. The gel acts as an intelligent triboelectric layer whose output signals are dynamically tuned by light, achieving a peak power density of ∼2.6 nW/cm 2 . This work demonstrates the gel's potential in wearable e‑skin for motion monitoring and robotic tactile perception, establishing a versatile platform from fundamental dynamic chemistry to functional device integration for next‑generation adaptive soft electronics.