Hysteresis‐Free and High‐Sensitivity Strain Sensing of Ionically Conductive Hydrogels

Abstract

Hydrogels are promising materials for soft and implantable strain sensors owing to their large compliance (E < 100 kPa) and significant extensibility (ε max > 500%) compared with other polymer networks. Further, hydrogels can be functionalized to seamlessly integrate with many types of tissues. However, most current methods attempt to imbue additional electronic functionality to structural hydrogel materials by incorporating fillers with orthogonal properties such as electronic or mixed ionic conduction. Although composite strategies may improve performance or facilitate heterogeneous integration with downstream hardware, composites complicate the path for regulatory approval and may compromise the otherwise compelling properties of the underlying structural material. Herein, hydrogel strain sensors composed of genipin‐crosslinked gelatin and dopamine‐functionalized poly(ethylene glycol) for in vivo monitoring of cardiac function are reported. By measuring their impedance only in their resistive regime (>10 kHz), hysteresis is reduced and the resulting gauge factor is increased by ≈50× to 1.02 ± 0.05 and 1.46 ± 0.05 from ≈0.03 to 0.05 for PEG‐Dopa and genipin‐crosslinked gelatin, respectively. Adhesion and in vivo biocompatibility are studied to support implementation of strain sensors for monitoring cardiac output in porcine models. Impedance‐based strain sensing in the kilohertz regime simplifies the piezoresistive behavior of these materials and expands the range of hydrogel‐based strain sensors.

Poisson–Nernst–Planck framework for modelling ionic strain and temperature sensors

“…Strain sensing has several applications in physiological monitoring including cardiac and respiratory output, 12,29,30 pressure and tactile sensing, 31–33 and muscular function. 34,35 Temperature can also be used as a physiological marker of health, 36 internal organ function, 37 and tracking wound healing.…”

“…[1][2][3][4] Their similarity in physiochemical properties to soft biological tissue has shown improved biocompatibility and integration with the human body in comparison to other materials. [5][6][7][8][9] As a result, hydrogel-based bioelectronics have been utilized in interfacing with the peripheral nervous system, 10,11 the cardiac system, 12 and the skin. 13,14 Hydrogels have been used as various components in sensors such as structural elements and substrates, 15,16 coatings and intermediate layers, 17,18 and sensing elements.…”

“…4,[21][22][23][24] As a result, conductive hydrogels are being explored to serve as the primary device components with common applications thus far including strain and temperature sensing. [25][26][27][28] Strain sensing has several applications in physiological monitoring including cardiac and respiratory output, 12,29,30 pressure and tactile sensing, [31][32][33] and muscular function. 34,35 Temperature can also be used as a physiological marker of health, 36 internal organ function, 37 and tracking wound healing.…”

“…[59][60][61][62][63] Ionic strain sensing has been explored for wearable devices, 33,64 soft robotics, 22,49 and cardiac output monitoring. 12 Recent work has demonstrated that ionically conductive gels can also be designed to accurately monitor body temperature through wearable devices. 32,65 Utilizing AC voltages and electrochemical impedance has been proposed as alternate measurement technique for hydrogel-based sensors (Fig.…”

Healable Ionic Conductors with Extremely Low‐Hysteresis and High Mechanical Strength Enabled by Hydrophobic Domain‐Locked Reversible Interactions

“…[6][7][8][9][10][11][12][13][14] Apart from high mechanical strength and toughness, low hysteresis is a unique characteristic of ionic conductor-based sensors, actuators, and other dynamic load-bearing materials. [15][16][17][18][19][20][21] Low hysteresis can guarantee a rapid and fully recovery of the deformed ionic conductors, which can decrease the fatigue caused by the accumulation of incomplete recovery and restore the changed ionic conductivity caused by deformation. [16,21] Therefore, low hysteresis can significantly improve the durability and reliability of stretchable ionic conductors.…”