Working Hydration as a Design Variable in Hydrogel Fibers: Coupled Transport, Signal Fidelity, and Wearable Electronics

Hydrogel fibers combine a hydrated medium resembling tissue with axial continuity, anisotropy, and textile processability. Yet performance is often evaluated at a nominal water content that cannot represent water redistribution during operation. Unlike previous reviews centered on composition, fabrication, or application, this Review uses the working hydration state to link network evolution, transport, signal generation, integration, and reliability. It refers to water distribution, physicochemical state, and exchange dynamics under operating conditions. Four themes are examined: network evolution induced by hydration, axial and radial transport, signal generation and decoupling, and preservation of function from fibers to textiles and biointerfaces. Comparative analysis indicates that reliable operation requires preserved axial pathways, controlled radial exchange, spatial separation of coupled signals, reference channels, and calibration after hydration and deformation histories. Direct fiber evidence is distinguished from transferable evidence from soft devices. Major gaps include inconsistent equilibration criteria, nonstandard hydration cycling, insufficient synchronized operando measurements, limited interlaboratory reproducibility and human validation over extended periods, and scarce manufacturing statistics. Future research should prioritize models that account for hydration, standardized service testing, scalable textile processing, and validation across devices and users. This framework establishes working hydration as a basis for material design, signal interpretation, and reliability assessment.