Ultrasensitive Ionic Liquid Polymer Composites with a Convex and Wrinkled Microstructure and Their Application as Wearable Pressure Sensors

Flexible iontronic sensing

The emerging flexible iontronic sensing (FITS) technology has introduced a novel modality for tactile perception, mimicking the topological structure of human skin while providing a viable strategy for seamless integration with biological systems. With research progress, FITS has evolved from focusing on performance optimization and structural enhancement to a new phase of integration and intelligence, positioning it as a promising candidate for next-generation wearable devices. Therefore, a review from the perspective of technological development trends is essential to fully understand the current state and future potential of FITS devices. In this review, we examine the latest advancements in FITS. We begin by examining the sensing mechanisms of FITS, summarizing research progress in material selection, structural design, and the fabrication of active and electrode layers, while also analysing the challenges and bottlenecks faced by different segments in this field. Next, integrated systems based on FITS devices are reviewed, highlighting their applications in human-machine interaction, healthcare, and environmental monitoring. Additionally, the integration of artificial intelligence into FITS is explored, focusing on optimizing front-end device design and improving the processing and utilization of back-end data. Finally, building on existing research, future challenges for FITS devices are identified and potential solutions are proposed.

Progress of Ionogels in Flexible Pressure Sensors: A Mini-Review

This paper reviews the research progress on ionogels in flexible pressure sensors. Ionogels comprise solid carrier networks and ionic liquids (ILs) dispersed therein and have good non-volatility, high conductivity, thermal stability, a wide electrochemical window, and mechanical properties. These characteristics give ionogels broad application prospects in wearable electronic devices, intelligent robots, and healthcare. The article first introduces the classification of ionogels, including the classification based on ILs and solid carrier networks. Then, the preparation methods and processing technologies of ionogels, such as the direct mixing method, in situ polymerization/gel method, and solvent exchange method, are discussed. Subsequently, the article expounds in detail on the properties and modification methods of ionogels, including toughness, conductivity, hydrophobicity, self-healing, and adhesiveness. Finally, the article focuses on the application of ionogels in flexible pressure sensors and points out the challenges faced in future research. The language of this mini-review is academic but not overly technical, making it accessible to even researchers new to the field and establishing an overall impression of research. We believe this mini-review serves as a solid introductory resource for a niche topic, with large and clear references for further research.

High-Sensitivity and Wide-Range Flexible Pressure Sensor Based on Gradient-Wrinkle Structures and AgNW-Coated PDMS

Flexible pressure sensors have garnered significant attention due to their wide range of applications in human motion monitoring and smart wearable devices. However, the fabrication of pressure sensors that offer both high sensitivity and a wide detection range remains a challenging task. In this paper, we propose an AgNW-coated PDMS flexible piezoresistive sensor based on a gradient-wrinkle structure. By modifying the microstructure of PDMS, the sensor demonstrates varying sensitivities and pressure responses across different pressure ranges. The wrinkle microstructure contributes to high sensitivity (0.947 kPa-1) at low pressures, while the PDMS film with a gradient contact height ensures a continuous change in the contact area through the gradual activation of the contact wrinkles, resulting in a wide detection range (10-50 kPa). This paper also investigates the contact state of gradient-wrinkle films under different pressures to further elaborate on the sensor's sensing mechanism. The sensor's excellent performance in real-time response to touch behavior, joint motion, swallowing behavior recognition, and grasping behavior detection highlights its broad application prospects in human-computer interaction, human motion monitoring, and intelligent robotics.

Iron-Supported Asymmetric Iontronic Pressure Sensors with High Sensitivity and Extended Linearity

Iontronic pressure sensors (IPSs) are emerging as promising candidates for integration into wearable electronics and healthcare monitoring systems due to their high sensitivity and low power consumption. However, achieving high sensitivity across a broad linear pressure range remains a significant challenge. This study presents a novel IPS device with an asymmetric sandwich structure, which includes a three-dimensional electrode made of nickel manganese oxide/carbon nanotubes (NMO/CNT) and an embedded iron needles ionic dielectric layer. The proposed device demonstrates exceptional linearity over 1700 kPa, with a sensitivity exceeding 7700 kPa-1. It exhibits rapid response and recovery times in the millisecond range and maintains a consistent capacitive response over 15,000 loading-unloading cycles. Moreover, the device enables noncontact sensing in response to magnetic field variations, broadening its potential applications. The innovative IPS design effectively balances high sensitivity and a wide linear pressure range, rendering it suitable for various applications such as nonverbal communication aids and healthcare monitoring systems.

Microbubble-based fabrication of resilient porous ionogels for high-sensitivity pressure sensors

High-sensitivity flexible pressure sensors have obtained extensive attention because of their expanding applications in e-skins and wearable medical devices for various disease diagnoses. As the representative candidate for these sensors, the iontronic microstructure has been widely proven to enhance sensation behaviors such as the sensitivity and limits of detection. However, the fast and tunable fabrication of ionic-porous sensing elastomers remains challenging because of the current template-dissolved or 3D printing methods. Here, we report a microbubble-based fabrication process that enables microporous and resilient-compliance ionogels for high-sensitivity pressure sensors. Periodic motion sliding results in a relative velocity between the imported airflow and the fluid solution, converts the airflow to microbubbles in the high-viscosity ionic fluid and promptly solidifies the fluid into a porous ionogel under ultraviolet exposure. The ultrahigh porosity of up to 95% endows the porous ionogel with superelasticity and a Young's modulus near 7 kPa. Due to the superelastic compliance and iontronic electrical double-layer effect, the porous ionogel packaged into two electrodes endows the pressure sensor with high sensitivity (684.4 kPa-1) over an ultrabroad range (~1 MPa) and a high-pressure resolution of 0.46%. Furthermore, the pressure sensor successfully captures high-yield broad-range signals from the fingertip low-pressure pulses (<1 kPa) to foot high-pressure activities (>500 kPa), even the grasping force of soft machine hands via an array-scanning circuit during object recognition. This microbubble-based fabrication process for porous ionogels paves the way for designing wearable sensors or permeable electronics to monitor and diagnose various diseases.

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.

Shrinking Devices: Shape-Memory Polymer Fabrication of Micro-and Nanostructured Electrodes

Since their discovery in the 1940s, shape memory polymers (SMPs) have been used in a broad spectrum of applications for research and industry.[1] SMPs can adopt a temporary shape and promptly return to their original form when submitted to an external stimulus. They have proven useful in fields such as wearable and stretchable electronics,[2] biomedicine,[3] and aerospace..[4] These materials are attractive and unique due to their ability to "remember" a shape after being submitted to elastic deformation. By combining the properties of SMPs with the advantages of electrochemistry, opportunities have emerged to develop structured sensing devices through simple and inexpensive fabrication approaches. The use of electrochemistry for signal transduction provides several advantages, including the translation into inexpensive sensing devices that are relatively easy to miniaturize, extremely low concentration requirements for detection, rapid sensing, and multiplexed detection. Thus, electrochemistry has been used in biosensing,[5] pollutant detection,[6] and pharmacological[7] applications, among others. To date, there is no review that summarizes the literature addressing the use of SMPs in the fabrication of structured electrodes for electrochemical sensing. This review aims to fill this gap by compiling the research that has been done on this topic over the last decade.

High-Sensitivity and Wide-Range Flexible Ionic Piezocapacitive Pressure Sensors with Porous Hemisphere Array Electrodes

The development of high-performance flexible pressure sensors with porous hierarchical microstructures is limited by the complex and time-consuming preparation processes of porous hierarchical microstructures. In this study, a simple modified heat curing process was first proposed to achieve one-step preparation of porous hemispherical microstructures on a polydimethylsiloxane (PDMS) substrate. In this process, a laser-prepared template was used to form surface microstructures on PDMS film. Meanwhile, the thermal decomposition of glucose monohydrate additive during heat curing of PDMS led to the formation of porous structures within PDMS film. Further, based on the obtained PDMS/CNTs electrodes with porous hemisphere array and ionic polymer dielectric layers, high-performance ionic piezocapacitive sensors were realized. Under the synergistic effect of the low-stiffness porous hemisphere microstructure and the electric double layer of the ionic polymer film, the sensor based on an ionic polymer film with a 1:0.75 ratio of P(VDF-HFP):[EMIM][TFSI] not only achieves a sensitivity of up to 106.27 kPa-1 below 3 kPa, but also has a wide measurement range of over 400 kPa, which has obvious advantages in existing flexible piezocapacitive sensors. The rapid response time of 110 s and the good stability of 2300 cycles of the sensor further elucidate its practicality. The application of the sensor in pulse monitoring, speech recognition, and detection of multiple dynamic loads verifies its excellent sensing performance. In short, the proposed heat curing process can simultaneously form porous structures and surface microstructures on PDMS films, greatly simplifying the preparation process of porous hierarchical microstructures and providing a simple and feasible way to obtain high-performance flexible pressure sensors.

Normal-Direction Graded Hemispheres for Ionic Flexible Sensors with a Record-High Linearity in a Wide Working Range

Flexible pressure sensors developed rapidly with increased sensitivity, a fast response time, high stability, and excellent deformability. These progresses have expanded the application of wearable electronics under high-pressure backgrounds while also bringing new challenges. In particular, the nonlinearity and narrow working range lead to a gradually insensitive response, principally because the microstructure deforms inconsistently on the device interfaces in the whole working range. Herein, we report an ionic flexible sensor with a record-high linearity (R2 = 0.99994) in a wide working range (up to 600 kPa). The linearity response comes from the normal-direction graded hemisphere (GH) microstructure. It is prepared from poly(dimethylsiloxane) (PDMS)/carbon nanotubes (CNTs)/Au into flexible and deformable electrodes, and its geometry is precisely designed from the linear elastic theory and optimized through finite element simulation. The sensor can achieve a high sensitivity of S = 165.5 kPa-1, a response-relaxation time of <30 ms, and superb consistency, allowing the device to detect vibration signals. Our sensor has been assembled with circuits and capsulation in order to monitor the function state of players in underwater sports in the frequency domain. This work deepens the theory of linearized design of microstructures and provides a strategy to make flexible pressure sensors that have combined the performances of ultrahigh linearity, high sensitivity, and a wide working range.

One-Step Laser Direct-Printing Process of a Hybrid Microstructure for Highly Sensitive Flexible Piezocapacitive Sensors

Microstructures can effectively improve the sensing performance of flexible piezocapacitive sensors. Simple, low-cost fabrication methods for microstructures are key to facilitating the practical application of piezocapacitive sensors. Herein, based on the laser thermal effect and the thermal decomposition of glucose, a rapid, simple, and low-cost laser direct-printing process is proposed for the preparation of a polydimethylsiloxane (PDMS)-based electrode with a hybrid microstructure. Combining the PDMS-based electrode with an ionic gel film, highly sensitive piezocapacitive sensors with different hybrid microstructures are realized. Due to the good mechanical properties brought about by the hybrid microstructure and the double electric layer induced by the ionic gel film, the sensor with a porous X-type microstructure exhibits an ultrahigh sensitivity of 92.87 kPa^-1 in the pressure range of 0-1000 Pa, a wide measurement range of 100 kPa, excellent stability (>3000 cycles), fast response time (100 ms) and recovery time (101 ms), and good reversibility. Furthermore, the sensor is used to monitor human physiological signals such as throat vibration, pulse, and facial muscle movement, demonstrating the application potential of the sensor in human health monitoring. Most importantly, the laser direct-printing process provides a new strategy for the one-step preparation of hybrid microstructures on thermal curing polymers.