A High-Performance Directional Strain Sensor Using Kirigami and Corrugated Structures for Real-Time Knee Joint Movement Detection

Abstract

This article presents the development and application of a flexible directional strain sensor designed using a combination of kirigami and corrugated structures to improve stretchability, sensitivity, and mechanical robustness. The sensor is embedded in a self-adhesive silicone elastomer, enabling reliable skin contact. It uses piezoelectric lead zirconate titanate (PZT) layers grown using a hydrothermal process, verified by X-ray diffraction (XRD) and polarization-electric field (P–E) curve analysis. We conducted extensive fundamental testing and assessed knee motion during squatting, standing, and knee bending cycles in lying-down status. The results showed that the sensor responded accurately to different stress levels, with accurate multidirectional and low cross-axis sensitivity. The reliability of the sensor was established in a 1000-cycle durability test. Notably, under the same bending angle of the leg, the larger signal was obtained for the motion from squatting to standing with increased knee joint loading compared to the leg outward folding motion in lying-down status. This innovative sensor has great potential for wearable health monitoring, motion tracking, and robotic systems, enabling reliable, real-time stress detection.