Recent advances in flexible and soft gel-based pressure sensors

Abstract

Gels, as typical flexible and soft materials, possess the intrinsic merits of transparent bionic structures, superior mechanical properties and excellent elasticity and viscosity. Recently, gel-based materials have attracted significant attention as a result of their broad and promising applications in biomedical, energy storage, light emission, actuator, military and aerospace devices, especially the intelligent sensing for human-related applications. Among the various flexible and soft pressure sensors, gel-based ones have been gradually studied as an emerging hot research topic. This review focuses on the latest findings in the rapidly developing field of gel-based pressure sensors. Firstly, the classification and properties of the three types of gels and their corresponding fabrication methods are introduced. Secondly, the four basic working principles of pressure sensors are summarized with a comparison of their advantages and disadvantages, followed by an introduction to the construction of pressure sensors based on gel structures. Thirdly, the latest representative research on the three types of gel-based materials towards various wearable sensing applications, including electronic skin, human motion capture, healthcare and rehabilitation, physiological activity monitoring and human-machine interactions, is comprehensively reviewed. Finally, a summary of the remaining challenges and an outline of the development trend for this field are presented.

Functional Gel-Based Electrochemical Energy Storage

“…17−19 One of the main reasons is the versatility of the different types of gels that are classified by their properties based on composition, structure, and intermolecular interactions into hydrogels, organogels, ionogels, and aerogels. 20,21 Gels consist in general of a three-dimensional cross-linked polymer network, as illustrated in Figure 1A (inorganic, organic, or natural), coexisting with a fluid phase (liquid or gas) inhabiting its vacancies/pores to form a quasi-solid material (Figure 1B). 22−24 The frameworks' suitability for electrochemical applications must match a few physicalchemical criteria: high molecular weight, suitable mechanical properties, ionic conductivity, and thermal and electrochemical stability.…”

“…Among them, gels have drawn much attention due to properties such as high ionic conductivity, flexibility, toughness, tenacity, and self-healing . Such attractive properties are applied in various electrochemical devices such as batteries, supercapacitors, sensors, and others. − Recent years have witnessed a growing interest in developing flexible gel-based materials to enhance the performance and safety of electrochemical energy storage devices. , Several reports have presented the application of these materials in different parts of electrochemical setups such as electrodes, − electrolytes, − and as stand-alone systems. − One of the main reasons is the versatility of the different types of gels that are classified by their properties based on composition, structure, and intermolecular interactions into hydrogels, organogels, ionogels, and aerogels. , …”

Flexible, Breathable, and Hydrophobic Iontronic Tactile Sensors Based on a Nonwoven Fabric Platform for Permeable and Waterproof Wearable Sensing Applications

“…Tactile sensors that transduce external pressure or force stimuli into measurable electrical signals with the unique merit of flexibility are the most basic components and thus have drawn a great deal of research attention. However, most of the reported flexible tactile sensors are built on polymer thin films as substrate, which is airtight and impermeable and leads to discomfort wearing experience. − Notably, the fabrics have been manufactured and used as clothing for thousands of years and could potentially serve as an alternative substrate material to construct tactile sensors with the unique property of breathability due to its intrinsic internal open-ended porous structure. In addition, fabric tactile sensors would possess the advantages of lightweight, flexibility, elasticity, and deformability, which are also important considerations for practical wearable sensing applications …”

Flexible All‐in‐one Quasi‐Solid‐State Batteries Enabled by Low‐water‐content Hydrogel Films

“…[21] This feature gives rise to the potential of obtaining fascinating properties, such as hydrophilicity, superior water-retaining ability, and flexibility. [22,23] Thus, hydrogel electrolytes are ideal for flexible energy storage devices. However, shortcomings exist, more or less, for all the currently developed hydrogel electrolyte.…”