Design and functional testing of graphene/CNT TPU flexible piezoresistive sensor with nipple microstructure

Sedentary monitoring requires both tracking accumulated sitting time and confirming continuous seat occupancy. To overcome the limitation of software-based timing in directly sensing sitting pressure, this study presents a flexible piezoresistive sensor with a nipple microstructure, fabricated from graphene/CNT-TPU and embedded in a seat cushion to build a state-triggered sedentary reminder system. The device comprises a nipple-patterned TPU elastic substrate, a spray-coated graphene/CNT conductive layer, and a TPU encapsulation layer. Under pressure, recoverable deformation of the nipple structure and contact reconstruction of the carbon-based conductive network jointly modulate resistance. The pressure response shows segmented nonlinearity with slopes of 3.02, 1.35, and 0.58% kPa −1 . Stable response plateaus were obtained under stepwise loading at 5–100 kPa. Response and recovery times are about 180 and 260 ms, with stable periodic response after 7000 cycles. The sensor also distinguishes toe movements, mouse clicks, elbow and finger bending, and empty/500 mL bottle loads. Integrated into a seat cushion, it recorded continuous seat pressure from three subjects (45, 50, and 55 kg). The control system triggered an audible alert after 50 min of continuous pressure and ceased upon seat departure. These results demonstrate closed-loop integration of pressure sensing, occupancy detection, and sedentary alerting, providing a feasible flexible sensing solution for non-wearable seat occupancy monitoring.