Facile preparation and high performance of wearable strain sensors based on ionically cross-linked composite hydrogels
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Published online 9 November 2020 | https://doi.org/10.1007/s40843-020-1507-0
Facile preparation and high performance of wearable strain sensors based on ionically cross-linked composite hydrogels 1,2
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Jiahui Bai , Ran Wang , Mingxi Ju , Jingxin Zhou , Lexin Zhang and Tifeng Jiao ABSTRACT Flexible sensors that can respond to multiple mechanical excitation modes and have high sensitivity are of great significance in the fields of electronic skin and health monitoring. Simulating multiple signal responses to skin such as strain and temperature remains an important challenge. Therefore, new multifunctional ion-crosslinked hydrogels with toughness and conductivity were designed and prepared in this work. A chemical gel with high mechanical strength was prepared by cross-linking acrylamide with N,N'-methylenebisacrylamide and ammonium persulfate. In addition, in or2+ der to enhance the conductive properties of the hydrogel, Ca , 2+ 3+ Mg and Al ions were added to the hydrogel during crosslinking. The double-layer network makes this ionic hydrogel show excellent mechanical properties. Moreover, the compo2+ site hydrogel containing Ca can reach a maximum stretch of −1 1100% and exhibits ultra-high sensitivity (Sp = 10.690 MPa ). The obtained hydrogels can successfully prepare wearable strain sensors, as well as track and monitor human motion. The present prepared multifunctional hydrogels are expected to be further expanded to intelligent health sensor materials. Keywords: hydrogel, ionic cross-linking, strain sensor, E-skin
INTRODUCTION In recent years, the demand for flexible sensors has been increasing, especially for electronic skin, human health monitoring, and wearable devices [1–7]. Most of these flexible sensors rely on conductive materials such as doped carbon-based materials [8–12], metal nanowires [13,14], and metal nanoparticles to convert mechanical signals to electrical signals [15]. A flexible sensor material needs to have biocompatibility, excellent mechanical
properties, high sensitivity and linear response to stress and strain, high conductivity under large strain, good stability, repeatability and other characteristics [16–18]. However, the current flexible conductive materials usually cannot integrate multiple performances, which is a huge challenge for most flexible stress-strain sensors. Conductive hydrogels are ideal materials for flexible sensors due to their outstanding biocompatibility, flexibility and mechanical properties, as well as excellent electron and ion transmission capabilities [19]. The sensing mechanism of flexible sensors is mainly made up of four parts: transistors [20,21], piezoresistance [22], piezoelectric sensor [23] and capacitor [24]. Among them, the piezoresistive sensor is the most widely used electronic strain sensor due to the advantages of simple structure, low price, and fast data output. However, traditional piezoresistive sensors also have many problems. For example, their mechanical properties and electrical conductivity cannot be taken into accou
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