Highly Stretchable and Compressible Cellulose Ionic Hydrogels for Flexible Strain Sensors

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.

Cyber–Physiochemical Interfaces

“…Compared with electronic conduction widely used in current electronics, ionic conduction is a prevailing mechanism in nature. It is advantageous in terms of its inherent stretchability, biocompatibility, and conformability at macro‐ (tissues), micro‐ (cells), and molecular scale . By combining ionically and covalently cross‐linked networks, supramolecular polymeric hydrogel exhibits stretchability up to tensile strain ≈2000% and fracture energy of ≈9000 J m −2 .…”

Cellulose Hydrogels by Reversible Ion‐Exchange as Flexible Pressure Sensors

“…[14b,16] Such properties make cellulose hydrogel a promising candidate to fabricate smart materials. [ 17 ]…”

Highly Stretchable, Strain-Sensitive, and Ionic-Conductive Cellulose-Based Hydrogels for Wearable Sensors

“…Smart and sensitive wearable devices that can transduce mechanical deformation into electrical signals [1,2] have attracted great interest in the fields of personalized health monitoring [3,4], electronic skins [5], and human motion detection [6], among others. The development of sensitive wearable devices has created high demand for flexible soft materials with high stretchability, strain sensitivity, and ion transport [2,7]. Notably, hydrogels, a novel kind of soft material consisting of cross-linked networks of hydrophilic polymers in water, have attracted worldwide attention for their liquid-like transport and solid-like mechanical properties [8].…”

“…Notably, hydrogels, a novel kind of soft material consisting of cross-linked networks of hydrophilic polymers in water, have attracted worldwide attention for their liquid-like transport and solid-like mechanical properties [8]. Especially, ionic conductive hydrogels that are swollen with electrolyte solutions [7,9,10] have high transparency and have drawn particular interest [11,12]. There are two categories of hydrogels including synthetic polymer-and biopolymer-based hydrogels [13].…”

“…However, cellulose-based hydrogels, especially pure-cellulose-based hydrogels, are usually lacking in mechanical properties (e.g., mechanical strength and stretchability) [15,17,18], which would result in unstable electrical behavior [7,15,[19][20][21]. To improve the mechanical performance of cellulose-based hydrogels, many studies have been done.…”