Wearable electronics, particularly flexible strain sensors, are gaining significant traction for applications in human motion detection, soft robotics, electronic skin, and human-machine interfaces. Current strain sensors, however, often suffer from drawbacks such as high hysteresis, slow response times, poor long-term stability, and lack of transparency. These limitations restrict their practical applications. Resistive strain sensors, while common, exhibit non-linearity and significant hysteresis due to the dependence of resistance change on both geometric structure and material conductivity. They also suffer from pressure coupling effects. Capacitive sensors offer an advantage with better linearity and low hysteresis, as the capacitance change primarily depends on the geometry of the dielectric material and electrodes. However, many capacitive sensors are designed with a vertical sandwich structure, which limits transparency and electrode stability. This work aims to address these shortcomings by developing a novel, transparent, capacitive strain sensor with enhanced robustness and reliability.

The literature review discusses various materials used for stretchable substrates and electrodes in strain sensors. Common substrate materials include Polydimethylsiloxane (PDMS), Ecoflex, hydrogel, and various elastomers like epoxy aliphatic acrylate, aliphatic urethane diacrylate, polyvinylidene fluoride (PVDF), thermoplastic polyurethane (TPU), natural rubber latex, styrene-(ethylene-butylene)-styrene (SEBS), and polyimide. Conductive electrodes often utilize carbon materials (carbon black, graphene nanoplates, carbon nanotubes, graphene) or metallic materials (silver nanoparticles, liquid metal, Au, Ag nanowire/MXene composites). Different sensing mechanisms are also explored: optical, capacitive, piezoelectric, and resistive. The paper highlights the advantages and disadvantages of each type, emphasizing the limitations of resistive sensors and the potential of capacitive sensors, while noting the transparency limitations of existing capacitive designs.

The proposed sensor utilizes vertically aligned carbon nanotubes (VACNTs) to create 3D interdigital electrodes on a PDMS substrate. The fabrication process involves depositing a thin Fe seed layer on a silicon wafer, patterning the interdigital electrodes, synthesizing a VACNT forest using microwave plasma-enhanced chemical vapor deposition (PECVD), spin-coating PDMS to embed the VACNTs, curing the PDMS, and peeling the sensor from the wafer. The process includes steps to ensure the VACNTs are fully embedded and to protect the electrodes. The paper details the parameters of the fabrication process, including spin coating speeds, curing temperatures, and vacuum times. The resulting sensor's dimensions and transparency are characterized. The height of the 3D electrodes is measured using a Keyence 3D laser confocal microscope. Optical transparency is assessed from 380 nm to 900 nm. The 3D electrode resistance during stretching is also analyzed to assess stability. Theoretical modeling of the sensor's capacitance under strain is presented, using equations that account for the change in dimensions of the electrodes and substrate due to stretching and Poisson's effect. The model includes parameters such as permittivity, electrode dimensions (length, height, gap, width), and the number of electrodes. The model is compared with experimental results.

The fabricated strain sensor exhibits several key performance characteristics: (1) Ultralow hysteresis (0.35% maximum) across a wide range of strains; (2) Excellent pressure insensitivity (less than 0.8% capacitance variation under pressures up to 1 MPa), demonstrating reduced crosstalk; (3) Fast response time (approximately 60 ms); (4) Good long-term stability, maintaining performance over 1500 testing cycles; (5) Good transparency, with transparency values varying depending on the electrode gap size (61.6% for 80 µm gap, 57.6% for 50 µm gap, and 52.7% for 20 µm gap at 550 nm). The gauge factor (GF) also depends on the electrode gap, with smaller gaps leading to higher GF (0.637 for 20 µm gap, 0.505 for 50 µm gap, and 0.413 for 80 µm gap). The electrode resistance variation during stretching is less than 1.6%, indicating excellent electrode stability. The experimental results show good agreement with the proposed theoretical model. Human activity monitoring tests demonstrated successful detection of various motions, including finger, knee, elbow, wrist, and neck bending, as well as subtle mouth movements. The sensor shows consistent performance in detecting both large and small scale movements.

The findings demonstrate the successful fabrication and characterization of a high-performance capacitive strain sensor that overcomes several limitations of existing technologies. The low hysteresis, pressure insensitivity, and good transparency make this sensor particularly suitable for wearable applications in human activity monitoring. The 3D electrode structure significantly enhances robustness and stability. The good agreement between the theoretical model and experimental results validates the design and understanding of the sensor's behavior. The successful demonstration of human activity monitoring underscores the sensor's potential for practical applications in diverse areas.

This study successfully developed a novel transparent capacitive strain sensor with 3D interdigital electrodes, achieving superior performance in terms of low hysteresis, pressure insensitivity, and transparency. The sensor demonstrated excellent suitability for human activity monitoring, showing its potential for applications in wearable electronics, soft robotics, and healthcare. Future work could focus on exploring different electrode materials and geometries to further enhance sensitivity and transparency, as well as integrating the sensor into more sophisticated wearable systems for real-world applications.

While the sensor shows excellent performance, potential limitations include the fabrication complexity involved in creating the 3D electrode structure, which might limit large-scale production. Further investigation is needed to assess the long-term stability under more extreme environmental conditions and to explore the sensor's performance across a wider range of strain values and frequencies. The transparency is affected by electrode gap sizes, requiring careful optimization for different applications. The theoretical model assumes uniform electrode properties and perfect elasticity of the PDMS, which might slightly deviate from real-world conditions.