Event-Driven Stretchable Strain Monitor Using Positive Piezoconductive Strain Switch

In this study, an event-driven strain monitor with low power consumption and independent operation is developed by integrating a positive piezoresistive composite switch, whose resistance decreases significantly with strain, and a stretchable contactless power supply system using a liquid metal coil. The device is capable of supplying energy to an internal lithium-ion battery under 30 % elongation via a contactless power transfer by magnetic field coupling. This device utilizes the characteristics of positive piezoresistive composite switches, whose resistance decreases up to 1/10000 with strain, to interrupt the current flowing through circuits and ICs until a strain above a threshold value is applied. This enables the device to operate with lower power consumption than existing devices using existing strain sensors in which resistance rises with strain.
In recent years, various stretchable devices have been developed and are expected to be implemented in society. However, stretchable devices have low durability in exchange for high stretchability, resulting in failure of proper functionality when subjected to strain exceeding the durability limit. Therefore, a system to detect excessive deformation is required for social implementation of stretchable devices. Most existing strain sensors have low resistance when relaxing, resulting in high current consumption in the unstrained state. In addition, it is necessary to constantly monitor changes in resistance using voltage divider circuits and measurement ICs to detect strain above a certain magnitude, which leads to increased complexity of detection circuits and higher power consumption. To solve these problems, there is a need for stretchable strain detection devices that can be implemented with a simple structure and wiring and operate with low power consumption.
In this study, positive piezoresistive composites were formed by mixing a large amount of nickel powder into a gel matrix formed by silicone rubber and ionic liquid. The amount of change in resistance with strain could be controlled by the mixing ratio of nickel powder to polymer and the aspect ratio of the processed composite. This facilitated the fabrication of devices that could detect a specific strain rate. In all mixing ratios, the resistance was exceeding 10 MΩ when relaxed, and changed to less than 100Ω when strained.
The liquid metal coils used in the contactless power transfer system were formed by a printing process using a stencil mask. To improve the printing characteristics, a paste of liquid metal formed by oxidation with stirring was used. The resistance of the printed 20-winding liquid metal coil was about 6 Ω, and a 30 % extension caused a 17 % increase in resistance. The self-inductance was about 6.5μH, which increased by about 8% due to the 30% elongation. The increase in self-inductance due to the elongation of the liquid metal coil is due to the fact that the coil was compressed in the direction perpendicular to the elongation axis and the distance between the wires decreased.
Finally, a low-power strain monitor that can be extended and retracted was developed by integrating a positive piezoresistive switch, a liquid metal coil, flexible power supply and control circuits, and a lithium-ion battery on a silicone rubber substrate. The non-contact power transfer performance of the device was demonstrated: when the device was extended by 30%, the charging rate was reduced to 66% of the relaxed rate, but the non-contact power transfer could be maintained. In addition, strain detection on a deformable plastic plate was performed as a demonstration of strain detection in a structure. By specifying a threshold value with a switch and voltage divider circuit, a control circuit was activated in response to strain above a certain level, and an LED was turned on. By incorporating infrared communication using IRLEDs in the device, it was also possible to switch external electronic devices by strain.