The explosive development of flexible electronic products has brought significant changes to many technological and lifestyle fields, such as wearable electronic products, intelligent electronic skins for airplanes and robots, micro air vehicles, efficient energy harvesting and storage devices, human-computer interaction technology, and micro optoelectronic devices. Resistance strain sensors are widely used in wearable strain sensing devices due to their significant sensitivity, simple structure, and readout circuit. They are used for monitoring human motion signals and help to accurately detect various signals, including joint motion, sound, expression, breathing, and pulse. In order to obtain signals from weak pulse signals to high strain joint motion signals without affecting normal human activities, a flexible strain sensor with high sensitivity and wide range is required. However, increasing the sensitivity of the device may come at the cost of reducing its range, and vice versa. This is because increasing sensitivity accelerates the depletion of electroactive materials, while increasing range slows down their depletion.

In order to improve the sensitivity of strain sensors, a large number of previous studies have reported that crack based resistance strain sensors can achieve high sensitivity. Changing the electrode cracking rate and crack morphology during the stretching process is an effective means of adjusting the response of the resistance strain sensor, which can be achieved through the structural design of the sensor and the updating of the material system. In order to further improve sensitivity, various structural designs have been studied, including slit structures that mimic the sensory functions of scorpions and spiders, as well as metamaterials with negative Poisson's ratio. Despite these advances, the measurement range of these high-sensitivity sensors is still limited. In order to meet a wider range of needs, researchers have investigated the possibility of changing the behavior of electroactive materials in penetrating cracks by layering two-dimensional materials underneath the crack material. In addition, some have proposed introducing wrinkled or curved structures to alleviate the direct stress response of electroactive materials, thereby slowing down their dissipation rate. However, due to differences in mechanical properties, the introduction of various materials increases the risk of sensor failure, and the complexity of wrinkling and bending designs may complicate the manufacturing process, which may hinder commercial feasibility.

Liquid metals have excellent tensile and conductive properties and are widely used in the field of stretchable electrodes, including interconnect leads, self-healing conductors, and strain sensors. In order to pursue a wider measurement range, some studies have shifted their focus from crack based strain sensors to liquid metals. Characterized by excellent tensile performance, it can avoid stress mismatch within the material system, and liquid metal based strain sensors can achieve a range of over 500%. However, such a wide range is excessive for detecting human bacterial strain signals, typically below 100%, and is accompanied by decreased sensitivity, making it difficult to capture weak signals such as pulses. In addition, the excellent conductivity of liquid metals leads to a decrease in base resistance in sensitive cells, amplifying the strain response of leads and potentially introducing a large number of invalid signals during strain measurement. Given that high sensitivity often comes with a limited range, while a wider range often comes with lower sensitivity, developing strain sensors that are both highly sensitive and have a wide measurement range for human applications remains an important research area.

This study draws inspiration from historical bamboo slips, which are composed of rigid strips interwoven with flexible threads to form a cohesive and rolling entity. This transformation transforms the naturally hard bamboo into a softer shape, giving it the ability to bend and curl easily. The adaptability of bamboo slips has been enhanced, increasing their storage capacity and making them easy to archive and transport directly. Based on the ancient bamboo sliding construction technology, we propose a method of strategically placing soft wires along the relative edges of individual bamboo boards, which helps to assemble them into a unified structure. In the metaphorical application of this concept, liquid metal is used as a "rope" in our research to effectively "lock" the crack edges within the stretchable electrode metal layer. Edge locking liquid metal connects dispersed electrical fragments together to restore a smooth conductive path as a whole. This innovative strategy can reconfigure the electrical path from a direction parallel to the elongation axis to a direction perpendicular to the elongation axis, thereby dynamically modifying the conductive network to respond to mechanical deformation. Unlike the traditional strategy of changing the dispersion rate of electroactive materials by altering the material [or structure] of the sensor, this work proposes a new strategy of altering the established cleavage electrical pathway of electroactive materials, which can effectively regulate the response of flexible resistance strain sensors. This strategy is applicable to various elastic materials, and due to its excellent ductility (i.e. failure strain>0.5), thermoplastic polyurethane (TPU, Tecflex from Lubrizol, France) was used in this study ™SG-80A）。

Source: Sensor Expert Network