Ultrasensitive Piezoresistive and Piezocapacitive Cellulose-Based Ionic Hydrogels for Wearable Multifunctional Sensing

Abstract

Tactile sensors, namely, flexible devices that sense physical stimuli, have received much attention in the last few decades due to their applicability in a wide range of fields like the world of wearables, soft robotics, prosthetics, and e-skin. Nevertheless, achieving a trade-off among stretchability, good sensitivity, easy manufacturability, and multisensing ability is still a challenge. Herein, an extremely flexible strain sensor composed of a cellulose-based hydrogel is presented. A natural biocompatible carboxymethylcellulose (CMC) hydrogel endowed with ionic conductivity by sodium chloride (NaCl) was used as the sensitive part. Both the sensible layer and electrodes were investigated with an innovative approach for wearable sensor applications based on electrochemical impedance spectroscopy to find the best device configuration. The sensor, exploitable both as a piezoresistor and as a piezocapacitor, presents high sensitivity to external stimuli, together with an extreme stretchability of up to 600%, showing the best strain and temperature sensitivity among the ionic conductive hydrogel-based devices presented in the literature. The very high strain sensitivity enables the hydrogel to be implemented in wearable strain sensors to monitor different human motions and physiological signals, representing a valid solution for the realization of transparent, easily manufacturable, and low-environmental-impact devices.

Self‐Powered Integrated Tactile Sensing System Based on Ultrastretchable, Self‐Healing and 3D Printable Ionic Conductive Hydrogel

“…This approach is easy, low cost, and already explored in literature with other hydrogel structures. [21,[49][50][51] Nevertheless, the modification of photocurable formulations can have relevant effects, which may affect the properties and 3D printing. In this sense, the effect of the presence of salts was investigated by measuring the viscosity of the different formulations, since inks with low viscosity are preferable for the DLP technique.…”

“…These values were comparable to other ionically conductive sensors presented in the literature. [21,39,58,59] The Piezocapacitive behavior of PVA/AAc/NaCl sensors was mainly attributable to the electric double layers formed by ions at the interface between metallic electrodes and hydrogel. [9] The capacitance transfer curve (Figure S2b, Supporting Information) exhibited a rapid quasi-linear decrease among 0% and 50% strains, approximated through a linear relationship that highlighted a sensitivity of 1.29.…”

“…In literature, several examples of electronic skin exploiting the stretchability, transparency, and conductivity of hydrogels have been presented with the aim of reproducing the human tactile system and monitoring body movements. [19][20][21] Indeed, the deformation of the hydrogel polymeric network induced by external tensile or compressive stress modifies the ions' diffusivity and thus ionic conductivity. [21,22] Another important characteristic of hydrogels is the possibility to include self-healing properties, exploiting the high polymeric chain mobility.…”

“…[19][20][21] Indeed, the deformation of the hydrogel polymeric network induced by external tensile or compressive stress modifies the ions' diffusivity and thus ionic conductivity. [21,22] Another important characteristic of hydrogels is the possibility to include self-healing properties, exploiting the high polymeric chain mobility. [23,24] The possibility of recovering physical damages induced by stretching and deforming the sensor enhances its durability and lifetime application, further mimicking living matter.…”

DLP‐Printable Porous Cryogels for 3D Soft Tactile Sensing

“…[12,13] Hydrogels are 3D polymeric networks that contain water within their porous structure. [14,15] The chemical composition of the hydrogels can be tailored to meet different requirements, such as high transparency, [16] inherent softness, [17] biocompatibility, [18] and self-healing ability. [19] These features have been increasingly exploited in soft sensing, especially in the field of wearable electronics.…”