Highly Sensitive, Stretchable and Durable Sensors Based on Conductive Polymer Hydrogels

Abstract

In recent years, wearable sensor devices, which directly attach to human skin for precise and dynamic human motion and physiological signals monitoring, have experienced a rapid development and presented a great use in modern medical systems. Despite the great research progress, the wearable sensors often need synchronized deformation of conductive fillers and flexible substrates to enable the mechanical signals transformation. However, some of the matrices are not flexible and stretchable enough, thus constraining the sensitivity and high precision of devices. Therefore, a stretchable, durable, and highly sensitive material was urgently needed. In this light, conductive hydrogels, offering the advantages of good flexibility, stretchability and biocompatibility, have attracted great interest as body-worn sensors. Additionally, hydrogels enjoy the capacity of tuning their mechanical properties to perfectly match with human skin. Therefore, a large number of stretchable hydrogel-based sensors has been fabricated. However, only a few hydrogel sensors can widely realize commercial application, with insufficient mechanical strength and stretchability as one of the main reasons. In addition, the sensing performance is not satisfactory. Particularly, it is difficult to detect some subtle deformations due to easy interference by external environment, thus leading to poor long-term durability. In this thesis, a novel one-pot technique to synthesize ultrastretchable hydrogel-based strain sensors by integrating carbon nanofibers with a double-network hydrogel matrix was reported. Outstanding mechanical properties of Agar/polyacrylamide(PAAm) double-network (DN) hydrogel, combing with high strain sensitivity given by tunneling effect of carbon nanomaterials, enable it to be a durable human motion sensor. We also prepare a highly anisotropic nanofluidic ionic skin (ANIS) composing of polyvinyl alcohol (PVA) and cellulose nanofibril via thermal stretching method, displaying comparable modulus, higher fracture energy and anti-fatigue property with cartilage and skin. It shows good pressure-independent temperature sensing property. Additionally, anisotropic and ionic conductive PVA/poly(N-isopropylacrylamide) (PNIPAM) DN hydrogel films with both physically and chemically cross-linked networks are created for multifunctional devices via thermal stretching, immersing and etching method. Combining the strong mechanical property of PVA under prestretching and unique thermal sensitivity of PNIPAM, PVA/PNIPAM DN gel can be ideal candidate for multiple sensing upon strain, pressure and temperature.