Flexible electronic devices have unique ultra-thin, bendable, and lightweight features, which pave the way for developing the next generation of human motion detection, health monitoring, and wearable devices. As an important medium for collecting external mechanical signals, flexible tension sensors correspond to an indispensable component of flexible sensing systems. Given the massive potential of the sensors for applications ranging from electronic skin to real-time medical health monitoring, it is urgently needed to develop flexible, and highly stretchable and sensitive tension sensors. In this regard, achieving a high sensitivity and a large tension range simultaneously is still a bottleneck to be broken. Metamaterial has caught much attention in recent years. As an artificial structure, its electromagnetic response can be manipulated by changing the structural parameters to lay the foundation for its applications in sensing scenarios. Compared with the conventional and natural materials that have difficulty in interacting with electromagnetic waves, it overcomes this limitation, and the enhanced light-matter interaction within metamaterial can significantly improve the sensing performance. The advantages including light weight, low cost, and portability of metamaterial sensors have attracted many researchers. However, a well-designed resonant structure with a high Q factor which is also highly sensitive to structural deformation is urgently needed to further increase the sensitivity of metamaterial sensors.

Firstly, a tension sensor based on electromagnetic metamaterial is designed. More specifically, the electromagnetic response of the proposed structure composed of a metallic resonator etched on the surface of polydimethylsiloxane (PDMS) can be dynamically tuned by stretching the flexible PDMS substrate. Then, deformation or tension sensing with high sensitivity can be achieved by the proposed metamaterial-based sensor. Meanwhile, we adopt a configuration containing multiple connected square patterns whose resonances are highly sensitive to the structural dimensions to improve the sensitivity. The finite element method is utilized to simulate the reflection spectrum of the model under the applications of different deformations with different tensions. Additionally, the electromagnetic response mechanism of the sensor is systematically studied by the surface current distributions. The sensor consisting of 4×10 cells is fabricated, and the extracted reflection spectra of the samples by employing different tensions are tested by applying the free space method. Furthermore, we conduct a comparison and analysis of the simulated results.

We propose a tension sensor based on electromagnetic metamaterial (Fig. 1). The surface current distribution on the metallic pattern of the sensor is simulated to investigate the resonance mechanism of the proposed metamaterial. By stretching the proposed flexible metamaterial, the structural dimensions along the tension direction will be enlarged proportionally, consequently resulting in the variation of peak resonance frequency (Fig. 2). Progressively, the influence of structural parameters on sensitivity of the proposed sensor is then analyzed (Fig. 3). The flexible metamaterial sensor with 4×10 cell arrays is fabricated by performing photolithography on a 0.3 mm thick PDMS substrate, and the free space method is leveraged to measure the reflection spectra of the sensor under different tension (Fig. 4). By increasing the tension on the sensor from 0 to 1.2 N, we experimentally observe that the resonance peak frequency experiences redshift from 109.23 GHz to 99.42 GHz, which agrees well with the simulated results which have a shift from 108.88 GHz to 99.08 GHz. Meanwhile, the fitted sensitivity from the measurement results is 8.43 GHz/N, which matches well with the sensitivity of 8.20 GHz/N in simulations (Fig. 5). Finally, the response spectra of the sample at different number of stretch-relaxation cycles are investigated, and the durability test shows that the sample repeatability can be maintained up to 100 repeated stretch-relaxation cycles (Fig. 6).

We put forward a flexible sensor based on metamaterial, and experimentally demonstrate its applications in tension sensing with high sensitivity. By stretching the flexible PDMS substrate of the proposed structure, the structural dimension of the metasurface fabricated on the substrate can be adjusted, which allows for resonance frequency tuning. Meanwhile, we experimentally demonstrate a tension sensitivity of 8.43 GHz/N of the proposed sensor by introducing multiple square patterns with resonances highly sensitive to their structural geometry. The proposed concept is certainly capable of identifying small tension variations, which means that by increasing the tension from 0 to 1.2 N, an obvious frequency redshift from 109.23 to 99.42 GHz is observed. Additionally, the investigation of sensing mechanisms reveals that the asymmetry in the resonant structure design leads to high-order dipole resonances with higher Q value and frequency selectivity. Moreover, the durability test indicates that the sample repeatability can be maintained for at least 100 stretch-relaxation cycles. Our proposed sensor, with advantages of high sensitivity, easy fabrication, low cost, and small size, is potentially useful in deformation or tension sensing for conformal applications.