Liquid Metal Fibers with a Knitted Structure for Wearable Electronics

Abstract

Flexible conductive fibers have shown tremendous potential in diverse fields, including health monitoring, intelligent robotics, and human-machine interaction. Nevertheless, most conventional flexible conductive materials face challenges in meeting the high conductivity and stretchability requirements. In this study, we introduce a knitted structure of liquid metal conductive fibers. The knitted structure of liquid metal fiber significantly reduces the resistance variation under tension and exhibits favorable durability, as evidenced by the results of cyclic tensile testing, which indicate that their resistance only undergoes a slight increase (<3%) after 1300 cycles. Furthermore, we demonstrate the integration of these liquid metal fibers with various rigid electronic components, thereby facilitating the production of pliable LED arrays and intelligent garments for electrocardiogram (ECG) monitoring. The LED array underwent a 30 min machine wash, during which it consistently retained its normal functionality. These findings evince the devices' robust stable circuit functionality and water resistance that remain unaffected by daily human activities. The liquid metal knitted fibers offer great promise for advancing the field of flexible conductive fibers. Their exceptional electrical and mechanical properties, combined with compatibility with existing electronic components, open new possibilities for applications in the physiological signal detection of carriers, human-machine interaction, and large-area electronic skin.

Keywords: conductive fibers; knitted structure; liquid metal; microchannel injection; wearable electronics.

Figure 1

Preparation, structure, and excellent tensile stability of liquid metal fibers with a knitted structure. (A) The preparation and structure of a single liquid metal fiber. (B) The structural diagram and an image of liquid metal fiber (gray) stitched into cotton threads (red and green). (C) The application for wearable ECG monitoring. (D) The side and cross-section images of the liquid metal fiber. (E) Images of single and knitted liquid metal fibers at maximum stretching state, as well as their resistance change curves at different stretching states. (F) Images of knitted liquid metal fibers subjected to longitudinal stretching, folding, and twisting.

Figure 2

The electrical properties of single and knitted liquid metal fibers. (A) Resistance variation rate of single and knitted liquid metal fiber under various cyclic strains. (B) Resistance variation rate of knitted liquid metal fiber under various cyclic strains. (C) Resistance variation rate of knitted liquid metal fiber when being held at various strains. (D) Resistance variation rate of single and knitted liquid metal fibers under various pressures. (E) Resistance variation rate of knitted liquid metal fiber under cyclic tensile loading.

Figure 3

The knitted liquid metal fiber connected with LED devices. (A) The structural diagram and a photograph of the knitted liquid metal fiber connected with an LED device. (B) Photographs of the normal working LED array under various strains and twists. (C) The conformal LED array under different bending states of the elbow. (D) Photographs of the immersed in water and washed LED array.

Figure 4

Wearable ECG monitoring system. (A) The overall structure of the wearable ECG monitoring system. (B) The ECG acquisition circuit board and commercial Ag/AgCl electrodes. (C) The ECG monitoring system sewn onto tight-fitting clothes. (D) The ECG signals of the volunteer in different states (sitting, standing, lying and walking). (E) The heart rate of the volunteer during sleep.