All-Solution Processed Conformable Strain Sensor for Precise Muscle Movement Detection On Skin Surface

With the development of more precisely controlled artificial skin, soft robotics and human-machine interfaces, highly sensitive sensors to detect skin surface deformation has gained tremendous significance for their wide range of use in various fields such as device operation control, motion tracking and advanced interface. To keep pace with these trends, many studies have been introduced to suggest the next generation sensor for precise mapping of skin surface deformation. Over the past few years, major research has been conducted to measure and interpret bioelectric signals such as electrocardiography (ECG) and electromyography (EMG). These studies succeeded in obtaining accurate data by analyzing the signals generated by the human body, but when the size of the target muscles becomes smaller or the number and size of sensor electrodes become smaller, the accuracy of data decreases rapidly, making it difficult to obtain valid information. Also, for the EMG signal with a small signal-to-noise ratio (SNR), it was almost impossible to specify the exact point of signal occurrence, which is critical in real-time signal monitoring. Therefore, a reliable sensor capable of measuring the real-time movement of the skin without interfering with the fine movement of the muscles is highly desirable for next-generation electronics requiring immediate and close measurement of small and minor input signals.<br/>In this work, we propose an all-solution processed skin-conformable strain sensor that is highly reliable for low-strain skin deformation sensing. Our strain sensor is facile and large-area compatible based on the spray coating method, maintaining high cycle stability and moderate resistance by taking advantage of both silver nanowire (AgNW) and carbon black (CB) blended as a sensing material. A mixture of AgNW and CB dispersed in ethanol is spray-coated on the polyimide film. After vaporizing the solvents, polydimethylsiloxane (PDMS) is poured onto the sensor film, and the entire substrate is spin coated. After annealing, sensor film is embedded into PDMS and easily detached from the polyimide (PI) film. The fabricated sensor shows high cycle stability of over 6,000 cycles under repeated 5% strain with excellent linearity and gauge factor over 20. At the same time, the thickness of the sensor is under 300 μm, assuring high conformability and precise detection of skin deformation. To further demonstrate the feasibility of our sensor toward precise muscle movement deformation, data obtained from our sensor attached to the human muscle was compared to the actual skin deformation rate measured with a 3D digital image correlation (3D-DIC) device. Our sensor has succeeded in precisely tracking deformation at a given location without interfering with muscle movement. This study provides a new pathway to map skin deformation especially in the single muscle movement with a low deformation rate under 5%, which is clearly distinguished from other sensors applied to large strains such as joint flexure. Furthermore, our sensor can be applied to any type of deformable substrate with potential applications such as wearable devices, soft robotics and advanced user interface. The detailed methods and results will be discussed later.<br/>This work was supported by the Technology Innovation Program (No.20008801, Development of muscular function management solution based on electronic skin with EMG IMU and Strain sensor) funded By the Ministry of Trade, Industry & Energy(MOTIE, Korea).