Synergistic Microcrack Engineering and Auxetic Metamaterials for Enhanced Sensitivity in Stretchable Strain Sensors

Abstract

Piezoresistive strain sensors are promising candidates for stretchable wearable devices. Although elastomer-based composite piezoresistive strain sensors offer a broad sensing range, achieving high sensitivity remains a challenge. Herein, a method is proposed to enhance sensor sensitivity by forming microcracks compatible with the behavior of auxetic metamaterials through biaxial pre-stretching. By applying biaxial pre-stretching to simply fabricated multi-walled carbon nanotubes/Ecoflex composite strain sensors, large cracks are formed, resulting in a 56% increase in gauge factor, from 15.3 ± 2.1 to 23.8 ± 2.5, compared to uniaxially pre-stretched sensors. Circular auxetic metamaterials with an effective Poisson's ratio ranging from −0.485 to 0.385, depending on the radius of the inner circle, are designed and integrated into the biaxially pre-stretched strain sensor. The cracks formed through biaxial pre-stretching exhibit significant resistance changes even in response to movements perpendicular to the stretching direction induced by the auxetic metamaterial. This led to a 136% increase in gauge factor, from 23.8 ± 2.5 to 56.2 ± 8.9, with the sensor maintaining high stability over 20000 cycles. The crack formation strategy introduced through biaxial pre-stretching extends beyond the scope of this study, offering a guideline for designing cracks optimized for auxetic metamaterial integration in crack-based piezoresistive strain sensors.