Strain sensors with good stability are vital to the development of wearable healthcare monitoring systems. However, the design of strain sensor with both duration stability and environmental stability is still a challenge. In this work, we propose an ultra-stable and washable strain sensor by embedding a coupled composite film of carbon nanotube (CNT) and Ti₃C₂T_x MXene into polydimethylsiloxane (PDMS) matrix. The composite strain sensor with embedded microstructure and uneven surface makes it conformal to skin, while the CNT/MXene sensing layer exhibits a resistance sensitive to strain. This sensor shows reliable responses at different frequencies and with long-term cycling durability (over 1,000 cycles). Meanwhile, the CNT/MXene/PDMS composite strain sensor provides the advantages of superior anti-interference to temperature change and water washing. The results demonstrate less than 10% resistance changes as the temperature rises from −20 to 80 °C or after sonication in water for 120 min, respectively. The composite sensor is applied to monitor human joint motions, such as bending of finger, wrist and elbow. Moreover, the simultaneous monitoring of the electrocardiogram (ECG) signal and joint movement while riding a sports bicycle is demonstrated, enabling the great potential of the as-fabricated sensor in real-time human healthcare monitoring.

The morphology of drops of graphene oxide (GO) inks produced by inkjet printing shows a distinctive coffee ring after drying when the mean diameter of the GO is below a critical size. Inks with larger diameter flakes do not show a coffee ring and the transition mean flake diameter decreases as the substrate temperature increases and when the printed drop size decreases. This behavior can be predicted with a model that compares the characteristic time for the agglomeration of high aspect ratio particles in suspension with the time scale for an evaporating liquid drop to begin receding during the drying process. The model is shown to accurately describe the transition from a coffee ring to a uniform dried deposit using a range of GO inks with mean flake size in the range 0.68–35.9 µm, drying temperatures of 20–60 °C, and drop sizes with contact diameter ranging from 30 to 800 µm.

Flexible strain sensors exhibit outstanding advantages in terms of sensitivity and stability by detecting changes in physical signals. It can be easily attached to human skin and clothed to achieve monitoring of human motion and health. However, general sensing materials shows low stretchability and cannot respond to signals under large deformation. In this work, a highly stretchable polymer composite was developed by adding small amount (0.17 wt.%) of silver nanowires (AgNWs) in stretchable conductive polymer materials. The conductivity of polymer/AgNWs composite is 1.3 S/m with the stretchability up to 500%. The stretchable strain sensor based on the polymer/AgNWs composite can respond to strain signals in real time, even for 1% strain response, and shows excellent stability over 1,000 loading/unloading cycles. Moreover, the strain sensor can be attached to human skin and clothed to monitor joints, throat and pulse of the human body. The human body electrocardiogram (ECG) signal was detected successfully with the polymer/AgNWs electrode, which is comparable to the signal obtained by the commercial electrode. Overall, the sensors enable monitoring of human movement and health. These advantages make it a potential application in wearable devices and electronic skin.

This review summarizes the development of copper inks in printing electronics, including classification of inks, printing and sintering methods, antioxidant strategies, and applications for flexible electronics.