Abstract:

Objective Ionic strain sensing devices are the most promising technology for human-computer interaction. However, current strain sensing devices still suffer from poor interface comfort and low durability of ionic hydrogel. Herein, this work proposes a novel strategy to break through the above bottleneck. A composite fabric was fabricated by encapsulating ionic hydrogel into a knitted fabric for wearable comfort and excellent strain sensing properties.
Method The ionic hydrogel composite fabric was a typical sandwich structure, where the ionic hydrogel was encapsulated by two layers of knitted fabric. The ionic hydrogel was polymerized by acrylamide in a thermal environment. Lithium chloride (LiCl) was used as conductive material. The ionic hydrogel composite fabric was found to have good strain sensing capability, stemming from its three-dimensional cross-linked mesh micro-structure. When the fabric was stretched, the mesh micro-structure of the ionic hydrogel was compressed. Consequently, the ions movement was obstructed, causing an increase in the resistance of the ionic hydrogel composite fabric.
Results The ionic hydrogel has a three-dimensional mesh-like porous structure, which can lock in a large amount of water and provide a medium of movement for ions. Meanwhile, its excellent elasticity and soft feel properties can be obtained by the unique loop structure of the knitted fabric substrate. The ionic hydrogel composite fabric had good strain-sensing properties (Fig. 2) indicated by the slope of the current change rate vs applied strain. The gauge factor gauge factor is a critical index to reflect sensitivity of the strain sensors. The gauge factor was 0.94 with strain ranging from 0% to 30%, then the gauge factor slowed down to 0.82 with strain ranging from 30% to 60%, and the gauge factor decreased to 0.37 with high strain ranging from 60% to 100%. Meanwhile, the response and recovery times for the composite fabric were 310 and 346 ms, respectively (Fig. 3). In order to evaluate the hysteresis performance of the composite fabric under high strain stimulus, the changes in relative current (ΔI/I₀, in which ΔI=I-I₀ and I are the currents before and after strain stimulus, respectively) of the composite fabric during loading-unloading cycles at a maximum strain of 100% was recorded (Fig. 4). The hysteresis was shown at low stretching ratio, which was due to the inability of the reticular microstructure of the ionic hydrogel to recover in time. The composite fabric exhibited wide sensing range (up to 100%)(Fig. 5). The ionic hydrogel composite fabric possesses stable electrical property, the current change ratio of the fabric maintained constant after 5 000 cyclic stretchings (Fig. 6). It was also found that the ionic hydrogel composite fabric was environmentally friendly, with the mass change ratio of the composite fabric being only 3.5%, while that for the pure ionic hydrogel being 76.5%.
Conclusion The design of ionic hydrogel composite fabric enables combined high strain-sensing and environment stability properties. Specifically, the composite fabric shows high sensitivity and wide working range, which is due to the three-dimensional mesh-like porous structure of the ionic hydrogel. In addition, the fabric substrates can be used as water-loss shield layer reducing the moisture loss ratio of the hydrogel. As the proof of concept, a wearable human-computer interaction device has been fabricated to monitor the human movement and recognize voice. Therefore, this work opens a new path for flexible strain sensing devices and has great potential in the field of wearable interaction.

Key words: ionic hydrogel, knitted fabric, thermal polymerization process, ionic hydrogel composite fabric, strain sensing