Ultrafast-Gelling Multifunctional Hydrogels for Integrated Self-Healing and Strain-Sensing Applications

Cosmin Gabriel Pauliuc1, Ana-Roxana Stefan1, Elena Iuliana Bîru1,2* , Horia Iovu1,2,3

1 Advanced Polymer Materials Group, National University of Science and Technology Politehnica Bucharest, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania, iuliana.biru@upb.ro

2 Research Institute of the University of Bucharest (ICUB), University of Bucharest, 91-95 SplaiulIndependentei, 050095, Bucharest, Romania

3 Academy of Romanian Scientists, 54 Splaiul Independentei, 050094, Bucharest, Romania

Introduction Wearable sensors and next-generation flexible bioelectronics have progressed rapidly thanks to advances in stimuli-responsive, or “smart,” materials. Among these, hydrogels stand out as ideal interfaces between electronic devices and the human body due to their intrinsic biocompatibility, tunable viscoelastic properties, and tissue-like softness. In particular, strain-sensitive hydrogels can reliably translate mechanical deformations into measurable signals, making them key components for continuous physiological monitoring and motion tracking—from subtle facial movements and pulse detection to large-scale joint motion.

Despite these advantages, it remains challenging to engineer hydrogels that simultaneously offer high stretchability, strong strain sensitivity, self-healing capability, and long-term electrical stability under repeated mechanical stress. To address this limitation, we developed a semi-interpenetrating polymer network (semi-IPN) hydrogel that leverages the structural synergy between a flexible synthetic polyacrylamide (PAM) network and a rigid biopolymer, carboxymethyl cellulose (CMC).

Experimental. Vanillin (VA) was first grafted onto CMC via a coupling approach while preserving free aldehyde groups to enable dynamic Schiff base interactions, as confirmed by FTIR and NMR. A semi-IPN hydrogel was then formed by acrylamide polymerization initiated with a KPS/FeSO₄ redox system in the presence of CMC-VA.

Gelation kinetics were evaluated by tube inversion and rheology, while frequency sweeps assessed viscoelastic behavior. Mechanical properties (tensile strength and elongation at break) were tested to determine structural integrity. Electro-mechanical sensing performance was analyzed by coupling an LCR meter with a universal testing machine, enabling real-time monitoring of resistance changes (ΔR/R₀) under dynamic strain.

Results and Discussion. Structural analyses confirmed successful vanillin grafting onto CMC and the formation of a stable semi-IPN network. The hydrogels exhibited tunableviscoelasticity with high strength and stretchability. Electromechanical tests showed sensitive, repeatable resistance changes (ΔR/R₀), enabling real-time monitoring of joint motions such as finger, elbow, and knee flexion.

Conclusions. A highly promising tissue-mimicking material for flexible bioelectronics is produced by the synergistic integration of the vanillin-modified CMC within the PAM network. This platform provides the mechanical resilience and electromechanical sensitivity needed for real-time human motion monitoring.