Mixed Ionic-Electronic Conducting Hydrogels with Carboxylated Carbon Nanotubes for High Performance Wearable Thermoelectric Harvesters

Recent Progress in Polymer Gel-Based Ionic Thermoelectric Devices: Materials, Methods, and Perspectives

Polymer gel-based ionic thermoelectric (i-TE) devices, including thermally chargeable capacitors and thermogalvanic cells, represent an innovative approach to sustainable energy harvesting by converting waste heat into electricity. This review provides a comprehensive overview of recent advancements in gel-based i-TE materials, focusing on their ionic Seebeck coefficients, the mechanisms underlying the thermodiffusion and thermogalvanic effects, and the various strategies employed to enhance their performance. Gel-based i-TE materials show great promise due to their flexibility, low cost, and suitability for flexible and wearable devices. However, challenges such as improving the ionic conductivity and stability of redox couples remain. Future directions include enhancing the efficiency of ionic-electronic coupling and developing more robust electrode materials to optimize the energy conversion efficiency in real-world applications.

Organic Porous Materials and Their Nanohybrids for Next-Generation Thermoelectric Application

Thermoelectricity offers a promising solution for reducing carbon emissions by efficiently converting waste heat into electrical energy. However, high-performance thermoelectric materials predominantly consist of rare, toxic, and costly inorganic compounds. Therefore, the development of alternating material systems for high-performance thermoelectric materials is crucial for broader applications. A significant challenge in this field is the strong interdependence of the various thermoelectric parameters, which complicates their simultaneous optimization. Consequently, the methods for decoupling these parameters are required. In this respect, composite technology has emerged as an effective strategy that leverages the advantages of diverse components to enhance the overall performance. After elaborating on the fundamental concepts of thermoelectricity and the challenges in enhancing the thermoelectric performance, the present review provides a comparative analysis of inorganic and organic materials and explores various methods for decoupling the thermoelectric parameters. In addition, the benefits of composite systems are emphasized and a range of low thermal conductivity materials with microporous to macroporous structures are introduced, highlighting their potential thermoelectric applications. Furthermore, the current development obstacles are discussed, and several cutting-edge studies are highlighted, with a focus on the role of high electrical conductivity fillers in enhancing the performance and mechanical properties. Finally, by combining low thermal conductivity materials with high electrical conductivity fillers can achieve superior thermoelectric performance. These insights are intended to guide future research and development in the field of organic porous materials and their nanohybrids in order to promote more sustainable and efficient energy solutions.

Dually-crosslinked ionic conductive hydrogels reinforced through biopolymer gellan gum for flexible sensors to monitor human activities

Human-machine interactions, monitoring of health equipment, and gentle robots all depend considerably on flexible strain sensors. However, making strain sensors have better mechanical behavior and an extensive sensing range remains an urgent difficulty. In this study, poly acrylamide-co-butyl acrylate with gellan gum (poly(AAm-co-BA)@GG) hydrophobic association networks and intermolecular hydrogen bonding interactions are used to fabricate dual cross-linked hydrogels for wearable resistive-type strain sensors. This could be an acceptable way to minimize the limitations in hydrogels previously identified. The robust fracture strength (870 kPa) and exceptional stretchability (1297 %) of the hydrogel arise from the collaborative action of intermolecular hydrogen bonding and hydrophobic associations. It also demonstrates exceptional resilience to repeated cycles of uninterrupted stretching and relaxation, retaining its structural integrity. The response and restoration times are 110 and 120 ms respectively. Furthermore, a wide sensing range (0-900 %), notable sensitivity across various strain levels, and an impressive gauge factor (GF) of 31.51 with high durability were observed by the dual cross-linked (DC) hydrogel-based strain sensors. The measured conductivity of the hydrogel was 0.32 S/m which is due to the incorporation of NaCl. Therefore, the hydrogels can be tailored to function as wearable strain sensors that can detect subtle human gestures like speech patterns, distinguish between distinct words, and recognize vibrations of the larynx during drinking, as well as large joint motions like wrist, finger, and elbow. Furthermore, these hydrogels are capable of reliably distinguishing and reproducing various printed text. These findings imply that any electronic device that demands strain-sensing functionality might make use of these developed materials.

Recent Advances of Stretchable Nanomaterial-Based Hydrogels for Wearable Sensors and Electrophysiological Signals Monitoring

Electrophysiological monitoring is a commonly used medical procedure designed to capture the electrical signals generated by the body and promptly identify any abnormal health conditions. Wearable sensors are of great significance in signal acquisition for electrophysiological monitoring. Traditional electrophysiological monitoring devices are often bulky and have many complex accessories and thus, are only suitable for limited application scenarios. Hydrogels optimized based on nanomaterials are lightweight with excellent stretchable and electrical properties, solving the problem of high-quality signal acquisition for wearable sensors. Therefore, the development of hydrogels based on nanomaterials brings tremendous potential for wearable physiological signal monitoring sensors. This review first introduces the latest advancement of hydrogels made from different nanomaterials, such as nanocarbon materials, nanometal materials, and two-dimensional transition metal compounds, in physiological signal monitoring sensors. Second, the versatile properties of these stretchable composite hydrogel sensors are reviewed. Then, their applications in various electrophysiological signal monitoring, such as electrocardiogram monitoring, electromyographic signal analysis, and electroencephalogram monitoring, are discussed. Finally, the current application status and future development prospects of nanomaterial-optimized hydrogels in wearable physiological signal monitoring sensors are summarized. We hope this review will inspire future development of wearable electrophysiological signal monitoring sensors using nanomaterial-based hydrogels.

Organic Thermoelectric Materials for Wearable Electronic Devices

Wearable electronic devices have emerged as a pivotal technology in healthcare and artificial intelligence robots. Among the materials that are employed in wearable electronic devices, organic thermoelectric materials possess great application potential due to their advantages such as flexibility, easy processing ability, no working noise, being self-powered, applicable in a wide range of scenarios, etc. However, compared with classic conductive materials and inorganic thermoelectric materials, the research on organic thermoelectric materials is still insufficient. In order to improve our understanding of the potential of organic thermoelectric materials in wearable electronic devices, this paper reviews the types of organic thermoelectric materials and composites, their assembly strategies, and their potential applications in wearable electronic devices. This review aims to guide new researchers and offer strategic insights into wearable electronic device development.

Versatile Mesoporous All-Wood Sponge Enabled by In Situ Fibrillation toward Indoor-Outdoor Energy Management and Conversion

The on-demand regulation of cell wall microstructures is crucial for developing wood as a functional building material for energy management and conversion. Here, a novel strategy based on reactive deep eutectic solvent is developed to one-step in situ fibrillate wood via disrupting the hydrogen bonding networks in cell walls and simultaneously carboxylating wood components, without significantly altering the native hierarchical structures of wood. Benefiting from its distinctive cell wall structure composed of individualized yet well-organized lignocellulose nanofibrils, in situ fibrillated wood exhibits a prominent mesoporous structure with a specific surface area of 81 m2/g. It represents a robust sponge material (5 MPa at 80% strain) with excellent durability. Due to the enhanced compressibility and charge polarization capacity, the in situ fibrillated wood (10 × 11 × 12 mm3) can generate a piezoelectric output voltage of up to 2 V under 221 kPa stress. The favorable microstructural characteristics render in situ fibrillated wood with highly thermal-insulating properties, high solar reflectivity, and mid-infrared emissivity, favoring outdoor passive cooling effects with a subambient temperature drop of 6 °C. Combining its controllable, durable, and eco-friendly attributes, our developed wood sponge represents a versatile structural material suitable for indoor/outdoor energy-saving applications.