Highly Stretchable and Compressible Cellulose Ionic Hydrogels for Flexible Strain Sensors

TLDR

Natural polymer hydrogels are attractive for electronics, yet their mechanical performance must be enhanced, and their properties can be tuned by adjusting cross‑link density. This study reports highly stretchable (∼126 % strain) and compressible (∼80 % strain) cellulose ionic hydrogels. The hydrogels were fabricated by free‑radical polymerization of allyl cellulose in a NaOH/urea aqueous solution, yielding chemically cross‑linked networks. The resulting hydrogels are ∼89 % transparent, conduct ∼0.16 mS cm⁻¹, operate at –20 °C without freezing, and function as reliable strain sensors for monitoring human activity, demonstrating suitability for flexible electronics.

Abstract

Stretchable and compressible hydrogels based on natural polymers have received immense considerations for electronics. The feasibility of using pure natural polymer-based hydrogels could be improved if their mechanical behaviors satisfy the requirements of practical applications. Herein, we report highly stretchable (tensile strain ∼126%) and compressible (compression strain ∼80%) cellulose ionic hydrogels (CIHs) among pure natural polymer-based hydrogels including cellulose, chitin, and chitosan via chemical cross-linking based on free radical polymerization of allyl cellulose in NaOH/urea aqueous solution. In addition, the hydrogels have good transparency (transmittance of ∼89% at 550 nm) and ionic conductivity (∼0.16 mS cm-1) and can be worked at -20 °C without freezing and visual loss of transparency. Moreover, the CIHs can serve as reliable and stable strain sensors and have been successfully used to monitor human activities. Significantly, the various properties of hydrogel can be controlled through rationally adjusting the chemically cross-linked density. Our methodology will prove useful in developing the satisfied mechanical and transparent CIHs for a myriad of applications in flexible electronics.