All-Solution Processed Silver Nanowire/Carbon Black Strain Sensor Array for Human Motion Detection

Nowadays, motion detection has been more important in various domains such as soft robotics, e-skin, wearable electronics, and human-machine interfaces. In response to these demands, development of diverse types of sensors in an array configuration has been widely studied. However, the sensor array is often partially stretchable even if the sensors themselves are fully stretchable, owing to their rigid components such as contact pads, spacers, and interconnects. Additionally, most sensors are fabricated in thin film form which raises reliability issues and inhibits their practical usage in stretchable media. To address these challenges, we propose a stretchable strain sensor array that features micro-crack-based resistance strain sensors and precisely engineered 3D-printed flexible vertical contacts.
In this study, a silver nanowire (AgNW) /carbon black (CB) composite embedded in polydimethylsiloxane (PDMS) was used as a strain sensor for motion detection, biodegradable polyester-copper complex for vertical interconnect access (Via). AgNW/CB strain sensor is implemented as a sensitive sensor (Gauge Factor=7.26 @ 10% strain) with a cycle-stability (change of Gauge Factor=5.21% over 6,000 cycles), attributed to the high conductivity of AgNW and high surface adhesion of CB to both the substrate and itself. To provide contact between strain sensors and the circuit on the upper plane, we engineer conductive filament for 3D printing from Multi3D. The filament’s low resistance (0.006 ohm/cm) enables the fabrication of high-performance and high-resolution array configurations with flexibility. When attached to the skin, our fabricated strain sensor array has succeeded in precisely tracking three types of skin surface deformation without interfering with muscle movement owing to its flexibility and high stretchability. Furthermore, the size and shape of the strain sensor are easily tailorable according to a high degree of process freedom of solution process and 3D printing. We have developed reliable and conformable strain sensor arrays with flexible vertical contact that can be actually commercialized in the near future. This study provides a new pathway to fabricate a tailorable high-resolution strain sensor array with minimum interference to skin surface deformation, which is clearly distinguishable from previous studies whose interconnects are located in the same plane with the sensors.