Printed electronic technologies draw tremendous attention worldwide due to their ability to surpass the limitations of traditional high-cost electronics, based on rigid silicon, and the manufacturing of various devices on flexible substrates. As a critical component of flexible electronics, electrodes fabricated on soft, bendable, and stretchable substrates are of great importance. Based on the fabrication process, this paper classifies the mainstream technologies into two categories: top-down and bottom-up. Top-down technologies include physical evaporation methods, printing technologies and soft lithography, while bottom-up technologies involve polymer-assisted-metal-deposition methods and ion-exchange methods, respectively. In contrast to top-down technologies that transfer functional ink onto substrates directly, the bottom-up method achieves a great improvement in the adhesion between substrates and metal electrodes. In this paper, the challenges of top-down technologies, including cost, synthesis, and choice of ink for printing technologies, the limited choice of metal for bottom-up technologies and the mass production of these methods, are also discussed.

Wearable electronic sensing devices are deemed to be a crucial technology of smart personal electronics. Strain and pressure sensors, one of the most popular research directions in recent years, are the key components of smart and flexible electronics. Graphene, as an advanced nanomaterial, exerts pre-eminent characteristics including high electrical conductivity, excellent mechanical properties, and flexibility. The above advantages of graphene provide great potential for applications in mechatronics, robotics, automation, human-machine interaction, etc.: graphene with diverse structures and leverages, strain and pressure sensors with new functionalities. Herein, the recent progress in graphene-based strain and pressure sensors is presented. The sensing materials are classified into four structures including 0D fullerene, 1D fiber, 2D film, and 3D porous structures. Different structures of graphene-based strain and pressure sensors provide various properties and multifunctions in crucial parameters such as sensitivity, linearity, and hysteresis. The recent and potential applications for graphene-based sensors are also discussed, especially in the field of human motion detection. Finally, the perspectives of graphene-based strain and pressure sensors used in human motion detection combined with artificial intelligence are surveyed. Challenges such as the biocompatibility, integration, and additivity of the sensors are discussed as well.

It is extremely challenging to directly observe how the relative position of the nanosheet changes the charge transport in the channel. Previous work on graphene stacking strain sensors relies on nanosheet slip to detect small mechanical signals. However, the direct experimental verification evidence is still inadequate. In this work, the sliding conductive transmission of graphene nanosheets slip is directly measured through an improved in situ transmission electron microscopy observation technique. By accurately manipulating the atomic scale of graphene nanosheets to achieve nanoscale sliding, the resistance change between nanosheets in the in situ observation system can be directly measured. Besides, a mechanical sensor based on graphene layer structure is designed, which demonstrates a high gauge factor (4303 at maximum strain of 93.3%), negligible hysteresis (5–10%), and excellent stability over 3000 stretch–release cycles. Besides, the precise control of characteristics offers a practical approach of retaining high stability from device to device. This strain sensor is applied in various cases, especially the health monitor of the human cervical vertebra.