Flexible Capacitive Pressure Sensor with High Sensitivity and Wide Range Based on a Cheetah Leg Structure via 3D Printing

Biomimetic Flexible Capacitive Sensor with Loop Electrode and Snail Tentacle Structure for Enhanced Proximity and Pressure Sensing

Flexible electronics have garnered significant attention due to their promising applications in embodied intelligence, smart interaction, and human-machine interface (HMI). In particular, flexible sensors with a detectability of proximity and pressure hold great potential in areas such as touchless control, health monitoring, and robotic perception. Here, we present a biomimetic flexible capacitive sensor, called the loop electrode bioinspired snail tentacle sensor (LEBSTS). The loop-patterned electrode layer enhances proximity sensing via the fringing field effect, while the bioinspired snail tentacle-structured polydimethylsiloxane (PDMS) dielectric layer provides high sensitivity for pressure detection. The above design significantly improves sensing performance, achieving long-range proximity detection of 140 mm, a wide pressure-detection range from 0.9 Pa to 500 kPa, a high sensitivity of 2.844 kPa-1 (0-1 kPa), and a fast response time of 87.5 ms. Moreover, the sensor demonstrates good durability, maintaining stable performance over 4500 cycles. Furthermore, the potential application of the sensor was explored, including human motion monitoring, touchless HMI, Morse code transmission via LoRa wireless communication, and pressure visualization systems. This work provides insights into the development of a high-performance and multifunctional flexible sensor, offering a promising platform for advanced intelligent sensing applications.

Detection of Positive and Negative Pressure in a Double-Chamber Underwater Thruster

The aim of this paper is to develop a compact, rapid-response pressure sensor for underwater propulsion. Flexible pressure sensors are widely utilized in human-computer interactions and wearable electronic devices; however, manufacturing capacitive sensors that offer a broad pressure range and high sensitivity presents significant challenges. Inspired by the dermal papillary microstructure, a capacitive pressure sensor was prepared by infusing polydimethylsiloxane (PDMS) inside an anodic aluminum oxide (AAO) template and then demolding it. The resulting pressure sensor exhibits several key characteristics: high linearity in the range of -5.2 to 6.3 kPa, a comprehensive range for both positive and negative pressure sensing in air or water environments, a quick response time of 52 ms, a recovery time of 40 ms, and excellent stability. The sensor presented in this work is innovatively applied to detect underwater negative pressure, and it is employed for the swift detection of positive and negative pressure changes in underwater thrusters. This work highlights the promising potential of biomimetic flexible capacitive pressure sensors across various applications.

Flexible Pressure Sensors Enhanced by 3D-Printed Microstructures

3D printing has revolutionized the development of flexible pressure sensors by enabling the precise fabrication of diverse microstructures that significantly enhance sensor performance. These advancements have substantially improved key attributes such as sensitivity, response time, and durability, facilitating applications in wearable electronics, robotics, and human-machine interfaces. This review provides a comprehensive analysis of the sensing mechanisms of these sensors, emphasizing the role of microstructures, such as micro-patterned, microporous, and hierarchical designs, in optimizing performance. The advantages of 3D printing techniques, including direct and indirect fabrication methods, in the creation of complex microstructures with high precision and adaptability are highlighted. Specific applications, including human physiological signal monitoring, motion detection, soft robotics, and emerging applications, are explored to demonstrate the versatility of these sensors. Additionally, this review briefly discusses key challenges, such as material compatibility, optimization difficulties, and environmental stability, as well as emerging trends, such as the integration of advanced technologies, innovative designs, and multidimensional sensing as promising avenues for future advancements. By summarizing recent progress and identifying opportunities for innovation, this review provides critical insights into bridging the gap between research and real-world applications, helping to accelerate the evolution of flexible pressure sensors with sophisticated 3D-printed microstructures.

Ultra-Stretchable Polymer Fibers Anchored with a Triple-Level Self-Assembled Conductive Network for Wide-Range Strain Detection

Numerous strategies have been demonstrated to enhance the mechanical stretchability of electromechanical sensors for widespread applications in wearable electronics. However, ranging from composite to microstructural materials, their electromechanical sensing performances are usually vulnerable to large stretching deformations due to the low-ductility of the infilled conductive components and the modulus mismatch between the flexible polymer substrate and conductive fillers. Here, a novel design strategy is proposed to fabricate ultra-stretchable electromechanical composites constructed by a triple-level interaction conductive network (Tri-LICN) in buckled-TPU microfibers for strain sensors. The Tri-LICN is established by bridging one-dimensional cellulose nanocrystals (CNC) with zero-dimensional gold-nanoparticles (AuNPs) and two-dimensional MXene sheets using interface self-assembly and ultrasound-assisted anchoring to eliminate the modulus mismatching between the conductive material and polymer substrate. The buckled-TPU microfibers are introduced to improve the mechanical stretchability of composites by the external-stimuli-induced imbalance of the stretching conformation of TPU macromolecules. The Tri-LICN MXene/CNC/AuNPs@TPU composite sensor displays an enhanced strain sensitivity (GF~2514) with a fast response time (~150 ms) over a wide operational strain up to 200% and excellent durability over 1000 tensile cycles. Our finding offers a promising approach to enhancing the performance of stretchable sensors based on polymer materials, providing new opportunities for the development of next-generation electronics.

Fully 3D-Printed Soft Capacitive Sensor of High Toughness and Large Measurement Range

Soft capacitive sensors are widely utilized in wearable devices, flexible electronics, and soft robotics due to their high sensitivity. However, they may suffer delamination and/or debonding due to their low interfacial toughness. In addition, they usually exhibit a small measurement range resulting from their limited stiffness variation range. In this paper, soft silicone-based capacitive sensors are developed by using a customized multimaterial 3D printer. By curing silicone materials simultaneously, the continuous conductive and dielectric layers achieve a substantial interfacial toughness of 1036 J·m-2. The sensor with tilted thin-plate dielectrics exhibits interfacial toughness of 645 J·m-2 or 339 J·m-2 in the transverse or longitudinal direction, respectively. Additionally, the sensors demonstrate a broad measurement range from 0.85 Pa to 5000 kPa. This extended range is facilitated by the significant stiffness variation of the separated tilted thin-plate dielectrics, ranging from 0.56 kPa to 19.76 MPa. Two applications of these fully printed soft sensors, including an intelligent sensorized insole and a robotic hand combining both soft actuators and soft sensors are showcased. It is believed that the strategy, employing 3D printing for soft microstructured sensors, is a general approach not only applicable for improving the performance of soft sensors, but also conducive to designing powerful soft functional devices.

Traditional Chinese Medicine (TCM)-Inspired Fully Printed Soft Pressure Sensor Array with Self-Adaptive Pressurization for Highly Reliable Individualized Long-Term Pulse Diagnostics

Reliable, non-invasive, continuous monitoring of pulse and blood pressure is essential for the prevention and diagnosis of cardiovascular diseases. However, the pulse wave varies drastically among individuals or even over time in the same individual, presenting significant challenges for the existing pulse sensing systems. Inspired by pulse diagnosis methods in traditional Chinese medicine (TCM), this work reports a self-adaptive pressure sensing platform (PSP) that combines the fully printed flexible pressure sensor array with an adaptive wristband-style pressure system can identify the optimal pulse signal. Besides the detected pulse rate/width/length, "Cun, Guan, Chi" position, and "floating, moderate, sinking" pulse features, the PSP combined with a machine learning-based linear regression model can also accurately predict blood pressure such as systolic, diastolic, and mean arterial pressure values. The developed diagnostic platform is demonstrated for highly reliable long-term monitoring and analysis of pulse and blood pressure across multiple human subjects over time. The design concept and proof-of-the-concept demonstrations also pave the way for the future developments of flexible sensing devices/systems for adaptive individualized monitoring in the complex practical environments for personalized medicine, along with the support for the development of digital TCM.

Microstructured Self-Healing Flexible Tactile Sensors Inspired by Bamboo Leaves

Wearable electronic devices with multifunctions such as flexible, integrated, and self-powered play a crucial role in the fields of health monitoring, motion monitoring, and human-computer interaction. However, their core basic components, flexible pressure sensors, face challenges including poor long-term stability and insufficient real-time sensing accuracy. In order to solve the challenges of long-term, stable, and accurate sensing of the sensor, this paper prepares polydimethylsiloxane (SHPDMS) with intrinsic self-healing property and designs a high-sensitivity self-healing capacitive flexible pressure sensor with dual microstructures (grating microstructured electrodes and microporous dielectric layer) as the substrate based on SHPDMS. Specifically speaking, the self-healing of the sensor under mild conditions was realized by introducing reversible imine bonds with low bonding energy into the polydimethylsiloxane (PDMS) flexible substrate, which solved the problem of the material's long-term service durability. A grating-like microstructure was introduced into the flexible electrode by using a spotted bamboo taro leaf as a template, and a dual microstructure sensor was constructed by combining it with a microporous dielectric layer doped with single-walled carbon nanotubes. This way reduces the elastic modulus of the dielectric layer, improves the dielectric constant of the sensor under loading, and thus significantly improves the sensor's sensitivity and extends the measurement accuracy in a low-stress range. The prepared self-healing flexible sensor achieves a sensitivity of 3.6 kPa-1, a minimum detection limit of 5 Pa, a response recovery time of less than 80 ms, and stability over 5000 cycles, which exceeds most previously reported silicone rubber-based capacitive flexible sensors.

From materials to structures: a holistic examination of achieving linearity in flexible pressure sensors

As a pivotal category in the realm of electronics skins, flexible pressure sensors have become a focal point due to their diverse applications such as robotics, aerospace industries, and wearable devices. With the growing demands for measurement accuracy, data reliability, and electrical system compatibility, enhancing sensor's linearity has become increasingly critical. Analysis shows that the nonlinearity of flexible sensors primarily originates from mechanical nonlinearity due to the nolinear deformation of polymers and electrical nonlinearity caused by changes in parameters such as resistance. These nonlinearities can be mitigated through geometric design, material design or combination of both. This work reviews linear design strategies for sensors from the perspectives of structure and materials, covering the following main points: (a) an overview of the fundamental working mechanisms for various sensors; (b) a comprehensive explanation of different linear design strategies and the underlying reasons; (c) a detailed review of existing work employing these strategies and the achieved effects. Additionally, this work delves into diverse applications of linear flexible pressure sensors, spanning robotics, safety, electronic skin, and health monitoring. Finally, existing constraints and future research prospects are outlined to pave the way for the further development of high-performance flexible pressure sensors.

Advancements and Applications of Micro and Nanostructured Capacitive Sensors: A Review

Capacitors are essential components in modern electrical systems, functioning primarily to store electrical charges and regulate current flow. Capacitive sensors, developed in the 20th century, have become crucial in various applications, including touchscreens and smart devices, due to their ability to detect both metallic and non-metallic objects with high sensitivity and low energy consumption. The advancement of microelectromechanical systems (MEMS) and nanotechnology has significantly enhanced the capabilities of capacitive sensors, leading to unprecedented sensitivity, dynamic range, and cost-effectiveness. These sensors are integral to modern devices, enabling precise measurements of proximity, pressure, strain, and other parameters. This review provides a comprehensive overview of the development, fabrication, and integration of micro and nanostructured capacitive sensors. In terms of an electric field, the working and detection principles are discussed with analytical equations and our numerical results. The focus extends to novel fabrication methods using advanced materials to enhance sensitivities for various parameters, such as proximity, force, pressure, strain, temperature, humidity, and liquid sensing. Their applications are demonstrated in wearable devices, human-machine interfaces, biomedical sensing, health monitoring, robotics control, industrial monitoring, and molecular detection. By consolidating existing research, this review offers insights into the advancements and future directions of capacitive sensor technology.

What Exactly Can Bionic Strategies Achieve for Flexible Sensors?

Flexible sensors have attracted great attention in the field of wearable electronic devices due to their deformability, lightness, and versatility. However, property improvement remains a key challenge. Fortunately, natural organisms exhibit many unique response mechanisms to various stimuli, and the corresponding structures and compositions provide advanced design ideas for the development of flexible sensors. Therefore, this Review highlights recent advances in sensing performance and functional characteristics of flexible sensors from the perspective of bionics for the first time. First, the "twins" of bionics and flexible sensors are introduced. Second, the enhancements in electrical and mechanical performance through bionic strategies are summarized according to the prototypes of humans, plants, and animals. Third, the functional characteristics of bionic strategies for flexible sensors are discussed in detail, including self-healing, color-changing, tangential force, strain redistribution, and interfacial resistance. Finally, we summarize the challenges and development trends of bioinspired flexible sensors. This Review aims to deepen the understanding of bionic strategies and provide innovative ideas and references for the design and manufacture of next-generation flexible sensors.

A Flexible Capacitive Pressure Sensor with Adjustable Detection Range Based on the Inflatable Dielectric Layer for Human-Computer Interaction

As an essential component in wearable electronic devices and intelligent robots, flexible pressure sensors have enormous application value in fields such as healthcare, human-computer interaction, and intelligent perception. However, due to the complex and ever-changing pressure loads borne by sensors in different application scenarios, this also puts great demands on the flexible response and adjustment ability of a sensor's detection range. Therefore, developing a flexible pressure sensor with a wide and adjustable detection range, which can be applied flexibly under different pressure loads, is also a major challenge in current research. In this paper, we propose a flexible pressure sensor with a wide and adjustable detection range based on an inflatable adjustable safety airbag as the dielectric layer. This sensor uses inflatable airbags prepared using 3D printing technology and silicone reverse molding technology as the dielectric layer and achieves high sensitivity (0.6 kPa-1 to 1.19 kPa-1), wide detection range (220-1500 kPa), and flexible performance applicability by adjusting the air pressure inside the dielectric layer. At the same time, its simple production process, convenient production, fast response time (100 ms), and good stability provide the possibility for the flexible application of sensors in different pressure detection. The experimental results indicate that the sensor has enormous potential for applications in wearable devices, healthcare, human-computer interaction, and intelligent perception recognition.

Improving the Resolution of Flexible Large-Area Tactile Sensors through Machine-Learning Perception

Industrial robots are the main piece of equipment of intelligent manufacturing, and array-type tactile sensors are considered to be the core devices for their active sensing and understanding of the production environment. A great challenge for existing array-type tactile sensors is the wiring of sensing units in a limited area, the contradiction between a small number of sensing units and high resolution, and the deviation of the overall output pattern due to the difference in the performance of each sensing unit itself. Inspired by the human somatosensory processing hierarchy, we combine tactile sensors with artificial intelligence algorithms to simplify the sensor architecture while achieving tactile resolution capabilities far greater than the number of signal channels. The prepared 8-electrode carbon-based conductive network achieves high-precision identification of 32 regions with 97% classification accuracy assisted by a quadratic discriminant analysis algorithm. Notably, the output of the sensor remains unchanged after 13,000 cycles at 60 kPa, indicating its excellent durability performance. Moreover, the large-area skin-like continuous conductive network is simple to fabricate, cost-effective, and can be easily scaled up/down depending on the application. This work may address the increasing need for simple fabrication, rapid integration, and adaptable geometry tactile sensors for use in industrial robots.