Ultra-sensitive, highly linear, and hysteresis-free strain sensors enabled by gradient stiffness sliding strategy

Flexible strain sensors are crucial for applications in artificial intelligence, electronic skins, and health monitoring due to their superior biocompatibility and wide sensing ranges compared to rigid counterparts. The ideal flexible strain sensor would possess high sensitivity and linearity across a broad range. Existing strain sensors fall into three categories based on their operating mechanism: piezoelectric, resistive, and capacitive. Piezoelectric sensors offer fast response but cannot detect static loads. Resistive sensors achieve high sensitivity but often suffer from nonlinearity and hysteresis, leading to measurement errors and complex calibration. Capacitive sensors inherently possess excellent linearity and low hysteresis but are limited by low sensitivity, with a theoretical gauge factor (GF) of 1. This research addresses the challenge of simultaneously achieving high sensitivity and linearity in capacitive strain sensors by introducing a novel design strategy.

Previous research on improving strain sensor sensitivity has focused on optimizing active materials. However, the mechanical structure of the substrate plays a critical role. Studies incorporating auxetic metamaterials or nacre-inspired architectures have shown some success in increasing sensitivity. However, most capacitive strain sensors utilize a parallel-plate structure, inherently limiting sensitivity improvements. While some efforts have integrated hierarchical structures or wrinkled electrodes to enhance GF, achieving both high sensitivity and linearity remains a challenge.

This study proposes a Gradient Stiffness Sliding (GSS) design strategy for capacitive strain sensors. The GSS method modulates the deformation distribution under uniaxial tension by creating a substrate with a gradient stiffness structure. A sensor is fabricated by combining a hard PDMS/PVC layer and a soft Ecoflex layer, allowing for sliding between them under strain. A flexible electrode and an ionic polyacrylamide (PAM)/LiCl layer are incorporated to form an electric double layer (EDL) at the electrode-ionic layer interface. Finite element analysis (FEA) is used to simulate the deformation and electric field distribution. The shape of the upper electrode is optimized to a trapezoid to address nonlinearity at small strains. The sensor's static and dynamic response, sensitivity, linearity, hysteresis, response time, and limit of detection are characterized using an LCR meter and mechanical testing equipment.

The GSS-designed sensor exhibits an ultra-high gauge factor (GF) of 9.1 × 10⁴ and exceptional linearity (R² = 0.9997) across the entire sensing range. The sensor demonstrates almost no hysteresis and a fast response time of 17 ms. FEA reveals that the enhanced sensitivity stems from the increased contact area between the electrode and the ionic layer under strain, leading to a significant increase in EDL capacitance. The nonlinearity observed at small strains is attributed to the fringe field effect at the electrode edges, mitigated by optimizing the electrode shape to a trapezoid. The sensor shows excellent stability over time and under repeated strain cycles. It exhibits a low limit of detection (LOD) of 0.003% and low sensitivity to strain rate. The large change in capacitance enables detection of subtle strains like those caused by gentle breathing. The sensor accurately monitors various human body signals, from facial expressions to large-scale movements.

The GSS design strategy fundamentally overcomes the limitations of low sensitivity in conventional capacitive strain sensors while preserving their inherent linearity. The utilization of the EDL significantly enhances sensitivity, achieving a GF several orders of magnitude greater than previously reported capacitive sensors. The near-perfect linearity (R² = 0.9997) eliminates measurement errors associated with pre-strain, simplifying calibration and enhancing the accuracy of strain measurements. The fast response time, low hysteresis, and high stability make this sensor highly suitable for various applications. The results highlight the effectiveness of structural design in enhancing sensor performance, offering a new avenue for developing high-performance flexible sensors.

This research presents a novel GSS design strategy for capacitive strain sensors that achieves ultra-high sensitivity and linearity simultaneously, overcoming the limitations of previous designs. The use of EDL and the optimized electrode shape significantly improves the sensor's performance. Future research could explore the application of this GSS strategy to other sensor types and investigate the use of different materials for further performance optimization and the development of new applications in flexible electronics.

While the GSS sensor demonstrates exceptional performance, some limitations exist. The current fabrication process might require further optimization for large-scale production. The accuracy of measurements at very high strains might be affected by the limitations of the LCR meter's measurement range. Further investigation into long-term stability and biocompatibility under various environmental conditions is also recommended.