Piezotronic Sensor for Bimodal Monitoring of Achilles Tendon Behavior

Abstract

Bimodal pressure sensors capable of simultaneously detecting static and dynamic forces are essential to medical detection and bio-robotics. However, conventional pressure sensors typically integrate multiple operating mechanisms to achieve bimodal detection, leading to complex device architectures and challenges in signal decoupling. In this work, we address these limitations by leveraging the unique piezotronic effect of Y-ion-doped ZnO to develop a bimodal piezotronic sensor (BPS) with a simplified structure and enhanced sensitivity. Through a combination of finite element simulations and experimental validation, we demonstrate that the BPS can effectively monitor both dynamic and static forces, achieving an on/off ratio of 1029, a gauge factor of 23,439 and a static force response duration of up to 600 s, significantly outperforming the performance of conventional piezoelectric sensors. As a proof-of-concept, the BPS demonstrates the continuous monitoring of Achilles tendon behavior under mixed dynamic and static loading conditions. Aided by deep learning algorithms, the system achieves 96% accuracy in identifying Achilles tendon movement patterns, thus enabling warnings for dangerous movements. This work provides a viable strategy for bimodal force monitoring, highlighting its potential in wearable electronics.

1. Introduction

Understanding complex external forces has promoted significant advancements in stress sensors [1–3], enabling their crucial applications in cutting-edge areas, such as human–computer interaction, electronic skin, bio-robotics, and health monitoring [4–7]. Notably, many scenarios require the simultaneous monitoring of dynamic and static stresses, placing stringent and urgent requirements on the performance of stress sensors [8–13]. In recent years, efforts to develop bimodal sensors capable of detecting both dynamic and static stresses have mainly focused on compounding multiple functional layers to capture signals separately [14–16]. According to different working principles, conventional bimodal sensors can be primarily categorized into piezoresistive–piezoelectric, piezoresistive–triboelectric, and piezoelectric–triboelectric configurations [17–20]. However, these devices often suffer from complex structures, severe signal crosstalk, and high manufacturing costs. Therefore, achieving bimodal sensing within a single material has emerged as a key focus of current research [21–23].

Piezotronic sensors, which leverage the unique piezotronic effect, have received widespread attention due to their brand-new regulation mechanism [24–26]. By coupling piezoelectricity with semiconductor properties, piezotronic sensors could exponentially modulate the carrier transport by strain-induced piezoelectric potential at the interface between the piezoelectric material and the metal electrode. This mechanism enables the direct correlation between mechanical stimuli and changes in electrical output, resulting in excellent sensitivity to mechanical inputs [27–29]. Various materials, including ZnO, MoS2, and GaN, have been explored for piezotronic sensors [30–32]. A landmark achievement was the development of the first piezotronic sensor based on a transverse single ZnO nanowire [33], which utilized piezoelectrically polarized charges at the interface of two back-to-back Schottky contacts to modulate electrical transport properties, achieving a gauge factor of up to 1250. More recently, a piezoelectric tunnel junction strain sensor based on HfO2 and n-ZnO demonstrates remarkable performance with a gauge factor as high as 4.8 × 10^5 and an on/off ratio of 478 at 0.10% strain [25]. Furthermore, Li ions-doped ZnO piezotronic sensor array enables large-scale integration and in-plane strain detection, demonstrating a broad application prospect [34]. As can be seen, ZnO has aroused intensive attention as a material of choice for piezotronic sensors, due to its rich properties, low cost, and ease of large-scale integration [35–37]. Despite these milestones, current research has predominantly focused on enhancing sensitivity and on/off ratios, while little attention has been paid to their possible bimodal response capabilities [38–40].

In this work, we present a bimodal piezotronic sensor (BPS) based on rare-earth Y-ion-doped ZnO, which achieves simultaneous detection of static and dynamic forces with a simplified structure and enhanced sensitivity. The sensor exhibits an ultrahigh gauge factor of 23,439, an on/off ratio of 1029, and a static force response duration of up to 600 s. As a proof-of-concept, the BPS is integrated into a wearable system for continuous monitoring of Achilles tendon behavior, achieving 96% accuracy in identifying movement patterns and enabling hazard warnings. This work provides a viable strategy for bimodal force monitoring, highlighting its potential in wearable electronics and medical diagnostics.