Hofmeister Hydrogel Iontronic Sensor for Dual-Mode Motion Monitoring

Flexible pressure sensors hold great promise for applications in motion monitoring and human signal recognition. However, conventional devices with fixed structure-performance coupling often fail to meet the diverse demands of different body sites and multi-scenario monitoring. Herein, we propose a polyvinyl alcohol (PVA)-hydroxyethane diphosphonic acid (HEDP) hydrogel iontronic sensor (PHHIS) with modulus programmability regulated by the <i>Hofmeister effect</i>. By introducing weakly hydrated I^- and strongly hydrated SO₄^{2^-}, we constructed soft-mode and hard-mode PHHIS, enabling precise acquisition of both low- and high-pressure human signals. The soft mode achieves ultrahigh sensitivity (238.4 kPa^-1, 0-50 kPa), ideal for subtle signals such as pulse and vocalization, while the hard mode maintains stable linearity (1.5 kPa^-1, 0-800 kPa), suitable for limb movement monitoring. Eight classes of human signals were precisely recognized with 100% accuracy through feature extraction combined with linear discriminant analysis, hierarchical cluster analysis, and artificial neural network analysis. This work demonstrates the promise of <i>Hofmeister effect</i>-regulated hydrogel design for programmable modulus control, enabling high-performance signal recognition and advancing flexible iontronic sensors for intelligent health monitoring.