A flexible quasi-solid-state thermoelectrochemical cell with high stretchability as an energy-autonomous strain sensor

Advancing flexible thermoelectrics for integrated electronics

With the increasing demand for energy and the climate challenges caused by the consumption of traditional fuels, there is an urgent need to accelerate the adoption of green and sustainable energy conversion and storage technologies. The integration of flexible thermoelectrics with other various energy conversion technologies plays a crucial role, enabling the conversion of multiple forms of energy such as temperature differentials, solar energy, mechanical force, and humidity into electricity. The development of these technologies lays the foundation for sustainable power solutions and promotes research progress in energy conversion. Given the complexity and rapid development of this field, this review provides a detailed overview of the progress of multifunctional integrated energy conversion and storage technologies based on thermoelectric conversion. The focus is on improving material performance, optimizing the design of integrated device structures, and achieving device flexibility to expand their application scenarios, particularly the integration and multi-functionalization of wearable energy conversion technologies. Additionally, we discuss the current development bottlenecks and future directions to facilitate the continuous advancement of this field.

Robust, Efficient, and Recoverable Thermocells with Zwitterion-Boosted Hydrogel Electrolytes for Energy-Autonomous and Wearable Sensing

The rapid growth of flexible quasi-solid-state thermocells (TECs) provides a fresh way forward for wearable electronics. However, their insufficient mechanical strength and power output still hinder their further applications. This work demonstrates a one-stone-two-birds strategy to synergistically enhance the mechanical and thermoelectrochemical properties of the [Fe(CN)6]3-/4--based TECs. By introducing Hofmeister effect and multiple non-covalent interactions via betaine zwitterions, the mechanical strength of the conventional brittle gelatin hydrogel electrolytes is substantially improved from 50 to 440 kPa, with a high stretchability approaching 250 %. Meanwhile, the betaine zwitterions strongly affect the solvation structure of [Fe(CN)6]3- ions, thus enlarging the entropy difference and raising the thermoelectrochemical Seebeck coefficient from 1.47 to 2.2 mV K-1. The resultant quasi-solid-state TECs exhibit a normalized output power density of 0.48 mW m-2 K-2, showing a notable improvement in overall performance compared to their counterparts without zwitterion regulation. The intrinsic thermo-reversible property also allows the TECs to repeatedly self-recover through sol-gel transformations, ensuring reliable energy output and even recycling of TECs in case of extreme mechanical damages. An energy-autonomous smart glove consisting of eighteen individual TECs is further designed, which can simultaneously monitor the temperature of different positions on any touched object, demonstrating high potential in wearable applications.

Secondary Chemical Cross-Linking to Improve Mechanical Properties in a Multifaceted Biocompatible Strain Sensor

A new conductive and transparent organohydrogel is developed with high stretchability, excellent mechanical, self-healing, antifreezing, and adhesive properties. A simple one-pot polymerization method is used to create polyacrylamide cross-linked through N,N'-methylenebis(acrylamide) (MBAA) and divinylbenzene (DVB). The dual chemical cross-linked gel network is complemented by several physical cross-links via hydrogen bonding and π-π interaction. Multiple chemical and physical cross-links are used to construct the gel network that allows toughness (171 kPa), low modulus (≈45 kPa), excellent stretchability (>1100%), and self-healing ability. The use of appropriate proportions of the water/glycerol binary solvent system ensures efficient environment tolerance (-20 to 40 °C). Phytic acid is used as a conductive filler that provides excellent conductivity and contributes to the physical cross-linking. Dopamine is incorporated in the gel matrix, which endows excellent adhesive property of the gel. The organohydrogel-based strain sensors are developed with state-independent properties, highly linear dependence, and excellent antifatigue performance (>100 cycles). Moreover, during the practical wearable sensing tests, human motions can be detected, including speaking, smiling, and joint movement. Additionally, the sensor is biocompatible, indicating the potential applications for the next generation of epidermal sensors.

Solvation Engineering via Fluorosurfactant Additive Toward Boosted Lithium-Ion Thermoelectrochemical Cells

Lithium-ion thermoelectrochemical cell (LTEC), featuring simultaneous energy conversion and storage, has emerged as promising candidate for low-grade heat harvesting. However, relatively poor thermosensitivity and heat-to-current behavior limit the application of LTECs using LiPF6 electrolyte. Introducing additives into bulk electrolyte is a reasonable strategy to solve such problem by modifying the solvation structure of electrolyte ions. In this work, we develop a dual-salt electrolyte with fluorosurfactant (FS) additive to achieve high thermopower and durability of LTECs during the conversion of low-grade heat into electricity. The addition of FS induces a unique Li+ solvation with the aggregated double anions through a crowded electrolyte environment, resulting in an enhanced mobility kinetics of Li+ as well as boosted thermoelectrochemical performances. By coupling optimized electrolyte with graphite electrode, a high thermopower of 13.8 mV K-1 and a normalized output power density of 3.99 mW m-2 K-2 as well as an outstanding output energy density of 607.96 J m-2 can be obtained. These results demonstrate that the optimization of electrolyte by regulating solvation structure will inject new vitality into the construction of thermoelectrochemical devices with attractive properties.

Super-stretching and high-performance ionic thermoelectric hydrogels based on carboxylated bacterial cellulose coordination for self-powered sensors

Self-powered sensors that do not require external power sources are crucial for next-generation wearable electronics. As environment-friendly ionic thermoelectric hydrogels can continuously convert the low-grade heat of human skin into electricity, they can be used in intelligent human-computer interaction applications. However, their low thermoelectric output power, cycling stability, and sensitivity limit their practical applications. This paper reports a 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized carboxylated bacterial cellulose (TOBC) coordination double-network ionic thermoelectric hydrogel with lithium bis(trifluoromethane) sulfonimide (LiTFSI) as an ion provider for thermodiffusion, as LiTFSI exhibits excellent thermoelectric properties with a maximum power output of up to 538 nW at a temperature difference of 20 K. The interactions between the ions and the hydrogel matrix promote the selective transport of conducting ionic ions, producing a high Seebeck coefficient of 11.53 mV K-1. Hydrogen bonding within the polyacrylamide (PAAm) network and interactions within the borate ester bond within the TOBC confer excellent mechanical properties to the hydrogel such that the stress value at a tensile deformation of 3100 % is reaches 0.85 MPa. The combination of the high ionic thermovoltage and excellent mechanical properties ionic thermoelectric hydrogels provides an effective solution for the design and application of self-powered sensors based on hydrogels.

Design and Optimization Strategies for Flexible Quasi-Solid-State Thermo-Electrochemical Cells

Currently, efficient utilization of low-grade thermal energy is a great challenge. Thermoelectricity is an extremely promising method of generating electrical energy from temperature differences. As a green energy conversion technology, thermo-electrochemical cells (TECs) have attracted much attention in recent years for their ability to convert thermal energy directly into electricity with high thermal power. Within TECs, anions and cations gain and lose electrons, respectively, at the electrodes, using the potential difference between the hot and cold terminals of the electrodes by redox couples. Additionally, the anions and cations therein are constantly circulating and mobile via concentration diffusion and thermal diffusion, providing an uninterrupted supply of power to the exterior. This review article focuses mainly on the operation of TECs and recent advances in redox couples, electrolytes, and electrodes. The outlook for optimization strategies regarding TECs is also outlined in this paper.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

A Fully Self-Powered Wearable Leg Movement Sensing System for Human Health Monitoring

Energy-autonomous wearable human activity monitoring is imperative for daily healthcare, benefiting from long-term sustainable uses. Herein, a fully self-powered wearable system, enabling real-time monitoring and assessments of human multimodal health parameters including knee joint movement, metabolic energy, locomotion speed, and skin temperature, which are fully self-powered by highly-efficient flexible thermoelectric generators (f-TEGs) is proposed and developed. The wearable system is composed of f-TEGs, fabric strain sensors, ultra-low-power edge computing, and Bluetooth. The f-TEGs worn on the leg not only harvest energy from body heat and supply power sustainably for the whole monitoring system, but also serve as zero-power motion sensors to detect limb movement and skin temperature. The fabric strain sensor made by printing PEDOT: PSS on pre-stretched nylon fiber-wrapped rubber band enables high-fidelity and ultralow-power measurements on highly-dynamic knee movements. Edge computing is elaborately designed to estimate multimodal health parameters including time-varying metabolic energy in real-time, which are wirelessly transmitted via Bluetooth. The whole monitoring system is operated automatically and intelligently, works sustainably in both static and dynamic states, and is fully self-powered by the f-TEGs.

Nanocellulose-assisted 3D-printable, transparent, bio-adhesive, conductive, and biocompatible hydrogels as sensors and moist electric generators

Transparent hydrogels have found increasing applications in wearable electronics, printable devices, and tissue engineering. Integrating desired properties, such as conductivity, mechanical strength, biocompatibility, and sensitivity, in one hydrogel remains challenging. To address these challenges, multifunctional hydrogels of methacrylate chitosan, spherical nanocellulose, and β-glucan with distinct physicochemical characteristics were combined to develop multifunctional composite hydrogels. The nanocellulose facilitated the self-assembly of the hydrogel. The hydrogels exhibited good printability and adhesiveness. Compared with the pure methacrylated chitosan hydrogel, the composite hydrogels exhibited improved viscoelasticity, shape memory, and conductivity. The biocompatibility of the composite hydrogels was monitored using human bone marrow-derived stem cells. Their motion-sensing potential was analyzed on different parts of the human body. The composite hydrogels also possessed temperature-responsiveness and moisture-sensing abilities. These results suggest that the developed composite hydrogels demonstrate excellent potential to fabricate 3D-printable devices for sensing and moist electric generator applications.

Swift Assembly of Adaptive Thermocell Arrays for Device-Level Healable and Energy-Autonomous Motion Sensors

The evolution of wearable technology has prompted the need for adaptive, self-healable, and energy-autonomous energy devices. This study innovatively addresses this challenge by introducing an MXene-boosted hydrogel electrolyte, which expedites the assembly process of flexible thermocell (TEC) arrays and thus circumvents the complicated fabrication of typical wearable electronics. Our findings underscore the hydrogel electrolyte's superior thermoelectrochemical performance under substantial deformations and repeated self-healing cycles. The resulting hydrogel-based TEC yields a maximum power output of 1032.1 nW under the ΔT of 20 K when being stretched to 500% for 1000 cycles, corresponding to 80% of its initial state; meanwhile, it sustains 1179.1 nW under the ΔT of 20 K even after 60 cut-healing cycles, approximately 92% of its initial state. The as-assembled TEC array exhibits device-level self-healing capability and high adaptability to human body. It is readily applied for touch-based encrypted communication where distinct voltage signals can be converted into alphabet letters; it is also employed as a self-powered sensor to in-situ monitor a variety of body motions for complex human actions. The swift assembly approach, combined with the versatile functionality of the TEC device, paves the way for future advancements in wearable electronics targeting at fitness monitoring and human-machine interfaces.

High-Power-Density Thermoelectrochemical Cell Based on Ni/NiO Nanostructured Microsphere Electrodes with Alkaline Electrolyte

Effective low-grade waste heat harvesting and its conversion into electric energy by the means of thermoelectrochemical cells (TECs) are a strong theme in the field of renewable energy investigation. Despite considerable scientific research, TECs have not yet been practically applied due to the high cost of electrode materials and low effectiveness levels. A large hypothetical Seebeck coefficient allow the harvest of the low-grade waste heat and, particularly, to use TECs for collecting human body heat. This paper demonstrates the investigation of estimated hypothetical Seebeck coefficient dependency on KOH electrolyte concentration for TECs with hollow nanostructured Ni/NiO microsphere electrodes. It proposes a thermoelectrochemical cell with power density of 1.72 W·m-2 and describes the chemistry of electrodes and near-electrode space. Also, the paper demonstrates a decrease in charge transfer resistance from 3.5 to 0.52 Ω and a decrease in capacitive behavior with increasing electrolyte concentration due to diffusion effects.

MXene and Carbon-Based Electrodes of Thermocells for Continuous Thermal Energy Harvest

Low-grade heat represents a significant form of energy loss; thermocells (TECs) utilizing the thermogalvanic effect can convert thermal energy into electricity without generating vibrations, noise, or waste emissions, making them a promising energy conversion technology for efficiently harvesting low-grade heat. Despite recent advancements, the reliance on high-cost platinum electrodes in TECs has considerably hindered their widespread adoption. Developing cost-effective electrodes that maintain the same thermoelectrochemical performance is crucial for the successful application of TECs. In this review article, the exploration of MXene materials as TEC electrodes is discussed first, emphasizing the immense potential of the MXene family for low-grade heat harvesting applications. Next, recent research on carbon-based electrodes is summarized, and morphological and structural optimizations are comprehensively discussed aiming at enhancing the thermoelectrochemical performance of TECs. In the concluding section, the challenges are outlined and future perspectives are offered, which provide valuable insights into the ongoing development of high-performance TEC electrodes using MXene and carbon-based materials.

Recent Development of Self-Powered Tactile Sensors Based on Ionic Hydrogels

Hydrogels are three-dimensional polymer networks with excellent flexibility. In recent years, ionic hydrogels have attracted extensive attention in the development of tactile sensors owing to their unique properties, such as ionic conductivity and mechanical properties. These features enable ionic hydrogel-based tactile sensors with exceptional performance in detecting human body movement and identifying external stimuli. Currently, there is a pressing demand for the development of self-powered tactile sensors that integrate ionic conductors and portable power sources into a single device for practical applications. In this paper, we introduce the basic properties of ionic hydrogels and highlight their application in self-powered sensors working in triboelectric, piezoionic, ionic diode, battery, and thermoelectric modes. We also summarize the current difficulty and prospect the future development of ionic hydrogel self-powered sensors.

A Novel Gel Thermoelectric Chemical Cell for Harvesting Low-Grade Heat Energy

The use of gel thermoelectric chemical cells to capture low-grade heat for conversion to electricity is an attractive approach. However, there are few studies on whether the distribution of redox species in the electrolyte has an effect on the performance of cells. Herein, this concern was discussed by constructing a novel gel thermoelectric chemical cell (Cu-C-cg). Using cellulose-like rice paper as a separator, a concentration gradient of electrolyte was carefully constructed, so that the concentration of potassium ferrocyanide gradually decreased from the hot electrode to the cold electrode while the concentration of potassium ferricyanide gradually increased. Through electrochemical measurement and analysis, it was found that the thermoelectric performance of this cell outperformed the cell without electrolyte concentration gradients. Meanwhile, this performance could be enhanced by the use of asymmetric electrodes composed of copper foil and carbon electrodes. After optimizing the conditions, the open-circuit voltage, output power, and Seebeck coefficient of the Cu-C-cg cell at 12 K temperature difference were 0.450 V, 183 μW, and 7.82 mV K^-1 , respectively. This work not only provides a novel idea in gel-based cell design but also an excellent thermoelectric chemical cell.

Thermal/Near-Infrared Light Dual-Responsive Reconfigurable and Recyclable Polythiourethane/CNT Composite with Simultaneously Enhanced Strength and Toughness

Thermoset polymers cross-linked by dynamic covalent bonds are recyclable and reconfigurable based on solid-state plasticity, resulting in less waste and environmental pollution. However, most thermoset polymers previously reported show thermal-responsive solid-state plasticity, depending much on external conditions and not allowing for local shape modulation. Here, the isocyanate modified carbon nanotubes (CNTs-NCO) are introduced into the polythiourethane (PCTU) network with multiple dynamic covalent bonds by in situ polymerization to prepare the composite with thermal/light dual-responsive solid-state plasticity, reconfigurability, and recyclability. The introduction of CNTs-NCO simultaneously strengthens and toughens the PCTU composite. Moreover, based on the photothermal properties and light-responsive solid-state plasticity, the PCTU/CNTs composite or bilayer sample could achieve complex permanent shape by locally precise shape regulation without affecting other parts. This work provides a simple and reliable method for preparing high-performance polymer composite with light-responsive solid-state plasticity, which may be applied in the fields of sensing and flexible electronics.

Stretchable and Self-Powered Temperature-Pressure Dual Sensing Ionic Skins Based on Thermogalvanic Hydrogels

Tactile sensors with both temperature- and pressure-responsive capabilities are critical to enabling future smart artificial intelligence. These sensors can mimic haptic functions of human skin and inevitably suffer from tensile deformation during operation. However, almost all actual multifunctional tactile sensors are either nonstretchable or the sensing signals interfere with each other when stretched. Herein, we propose a stretchable and self-powered temperature-pressure dual functional sensor based on thermogalvanic hydrogels. The sensor operates properly under stretching, which relies on the thermogalvanic effect and constant elastic modulus of hydrogels. The thermogalvanic hydrogel elastomer exhibits an equivalent Seebeck coefficient of -1.21 mV K^-1 and a pressure sensitivity of 0.056 kPa^-1. Combined with unit array integration, the multifunctional sensor can be used for accurately recording tactile information on human skin and spatial perception. This work provides a conceptual framework and systematic design for stretchable artificial skin, interactive wearables, and smart robots.

Thermoelectrochemical Seebeck coefficient and viscosity of Co-complex electrolytes rationalized by the Einstein relation, Jones-Dole B coefficient, and quantum-chemical calculations

The Seebeck coefficient (Se) and the viscosity of a redox electrolyte are the key characteristics of thermoelectrochemical cells that generate electric power from waste thermal energy. However, the recent upsurge of research in this field is seriously disconnected from the knowledge of solution chemistry explored in the previous century. Herein, we systematically investigate five redox couples of cobalt complexes containing different aromatic ligands and anions in γ-butyrolactone solvent to demonstrate how the Einstein relation of hydrodynamic theory and the Jones-Dole B coefficient obtained from viscosity measurements can be used to account for such electrolyte properties. In essence, we reveal that the outer-shell (solvent reorganization) and inner-shell (metal-ligand reorganization) contributions to the redox reaction entropy ΔS_rc (∝Se) can be quantified by the analyses using the B-coefficients and quantum-chemical simulations, respectively, while the distinct regimes found in the viscosity and conductivity are well accounted for by the Einstein relation, despite its classical hydrodynamic origin.

Environmentally Tolerant Ionic Hydrogel with High Power Density for Low-Grade Heat Harvesting

Harvesting low-grade heat by an ionic hydrogel thermoelectric generator (ITEG) into useful electricity is promising to power flexible electronics. However, the poor environmental tolerance of the ionic hydrogel limits its application. Herein, we demonstrate an ITEG with high thermoelectric properties, as well as excellent capabilities of water retention, freezing resistance, and self-regeneration. The obtained ITEG can maintain the original water content at ambient conditions (302 K, 65% relative humidity (RH)) for 7 days and keep unfreezing at a low temperature (253 K). It can even be self-regenerated and recovered to its original state after a water loss in high-temperature conditions. Furthermore, a high ionic Seebeck coefficient of 11.3 mV K^-1 and an impressive power density of 167.90 mW m^-2 are achieved under a temperature difference of 20 K. A high power density of 60.00 mW m^-2 can also be maintained even at 258 K. After drying and regeneration, ITEG-re could even exhibit a higher ionic Seebeck coefficient of 11.8 mV K^-1. Successful lighting of light-emitting diodes (LEDs) and charging of capacitors demonstrate the great potential of ITEG to provide continuous energy supply for powering flexible electronics.

Advances in Cellulose-Based Hydrogels for Biomedical Engineering: A Review Summary

In recent years, hydrogel-based research in biomedical engineering has attracted more attention. Cellulose-based hydrogels have become a research hotspot in the field of functional materials because of their outstanding characteristics such as excellent flexibility, stimulus-response, biocompatibility, and degradability. In addition, cellulose-based hydrogel materials exhibit excellent mechanical properties and designable functions through different preparation methods and structure designs, demonstrating huge development potential. In this review, we have systematically summarized sources and types of cellulose and the formation mechanism of the hydrogel. We have reviewed and discussed the recent progress in the development of cellulose-based hydrogels and introduced their applications such as ionic conduction, thermal insulation, and drug delivery. Also, we analyzed and highlighted the trends and opportunities for the further development of cellulose-based hydrogels as emerging materials in the future.

Ultraflexible PEDOT:PSS/Helical Carbon Nanotubes Film for All-in-One Photothermoelectric Conversion

Photothermoelectric (PTE) conversion can achieve the recovery of low-quality light or heat efficiently. Much effort has been devoted to the exploitation of the inorganic heterogeneous asynchronous (separate) PTE conversion system. Here, a full organic PTE film with a pseudobilayer architecture (PBA) according to the homogeneous synchronous (all-in-one) PTE conversion hypothesis was prepared via successive drop-casting a PEDOT:PSS/helical carbon nanotube (HCNT) mixture and PEDOT:PSS onto a vacuum ultraviolet treated substrate. Our results prove that the heptagon-pentagon pairs embedded in HCNTs promote a denser arrangement of the molecular chains of PEDOT, which enhances the crystallinity and affects the thermoelectric properties. The weak connection and hollow structure of HCNTs inhibit the dissipation of heat, and the zT value of the film reaches over 0.01. The PBA film shows better photothermal conversion performance than a neat PEDOT:PSS film and stably generates a temperature difference of over 25.68 °C without external cooling. A flexible PTE chip demo was manufactured, and the ideal open-circuit voltage (simulated via COMSOL) of that reaches over 1.5 mV under weak NIR stimulation (83.12 mW/cm2), which is the best value reported for an organic all-in-one PTE device, and the real maximum output power reaches 2.55 nW (166.01 mW/cm2). The chip has incredible ultraflexibility, and its inner resistance changes less than 1.42% after 10000 bending cycles and displays ultrahigh stability (similarity >90%) in a continuous periodic output. Our work fills the deficit of homogeneous synchronous PTE research for a PEDOT:PSS composite and is a preliminary attempt in an ultraflexible integrated all-in-one PTE chip design.

Regulating Thermogalvanic Effect and Mechanical Robustness via Redox Ions for Flexible Quasi-Solid-State Thermocells

The design of power supply systems for wearable applications requires both flexibility and durability. Thermoelectrochemical cells (TECs) with large Seebeck coefficient can efficiently convert low-grade heat into electricity, thus having attracted considerable attention in recent years. Utilizing hydrogel electrolyte essentially addresses the electrolyte leakage and complicated packaging issues existing in conventional liquid-based TECs, which well satisfies the need for flexibility. Whereas, the concern of mechanical robustness to ensure stable energy output remains yet to be addressed. Herein, a flexible quasi-solid-state TEC is proposed based on the rational design of a hydrogel electrolyte, of which the thermogalvanic effect and mechanical robustness are simultaneously regulated via the multivalent ions of a redox couple. The introduced redox ions not only endow the hydrogel with excellent heat-to-electricity conversion capability, but also act as ionic crosslinks to afford a dual-crosslinked structure, resulting in reversible bonds for effective energy dissipation. The optimized TEC exhibits a high Seebeck coefficient of 1.43 mV K^-1 and a significantly improved fracture toughness of 3555 J m^-2, thereby can maintain a stable thermoelectrochemical performance against various harsh mechanical stimuli. This study reveals the high potential of the quasi-solid-state TEC as a flexible and durable energy supply system for wearable applications.

Paper-Based Ionic Thermocouples for Inexpensive and High-Precision Measurement of Temperature

Accurate and yet cost-effective temperature measurements are required in various sectors of academia and industry. Thermocouples (TCs) are most widely used for temperature measurements; however, their low temperature sensitivity and high thermal conductivity should be improved to ensure the reliable measurement of output voltage for small temperature differences. To address this, a paper-based ionic thermocouple (P-iTC) presented here utilizes a pair of paper strips soaked with the electrolytes of potassium ferri-/ferrocyanide and iron (II/III) chloride redox couples, which are used as p- and n-type elements, respectively. The fabricated P-iTC provides 70× higher temperature sensitivity (α, 2.8 mV/K) and 30× lower thermal conductivity (k, 0.8 W/m K) than those of commercial K-type TCs, thereby yielding a remarkably high α/k ratio of 3.5 mV m/W. Reliable sensing performance is measured during three weeks of operation, which indicates that the P-iTC should be stable in long-term operation. To demonstrate the practicality of the P-iTC, a 3 × 3 planar array of P-iTCs is fabricated to monitor the temperature profile of a surface in contact with heat sources. Using pencil-drawn graphite electrodes on paper, a highly cost-effective P-iTC with the material cost of ∼0.5 cents per device is also fabricated, which is successfully used to monitor cold chain temperatures while retaining its excellent temperature-sensing performance.

Highly Stretchable, Fast Self-Healing, and Waterproof Fluorinated Copolymer Ionogels with Selectively Enriched Ionic Liquids for Human-Motion Detection

The development of waterproof ionogels with high stretchability and fast self-healing performance is essential for stretchable ionic conductors in sophisticated skin-inspired wearable sensors but can be rarely met in one material. Herein, a semicrystalline fluorinated copolymer ionogel (SFCI) with extremely high stretchability, underwater stability, and fast self-healability was fabricated, among which hydrophobic ionic liquids ([BMIM][TFSI]) were selectively enriched in fluoroacrylate segment domains of the fluorinated copolymer matrix through unique ion-dipole interactions. Benefiting from the reversible ion-dipole interactions between the [BMIM][TFSI] and fluoroacrylate segment domains as well as the physical cross-linking effects of semicrystalline oligoethylene glycol domains, the SFCI exhibited ultrastretchability (>6000%), fast room-temperature self-healability (>96% healing efficiency after cutting and self-healing for 30 min), and outstanding elasticity. In addition, the representative SFCI also exhibited high-temperature tolerance up to 300 °C, antifreezing performance as low as -35 °C, and high transparency (>93% visible-light transmittance). As a result, the as-obtained SFCI can readily be used as a highly stretchable ionic conductor in skin-inspired wearable sensors with waterproof performance for real-time detecting physiological human activities. These attractive features illustrate that the developed ultrastretchable and rapidly self-healable ionogels with unique waterproofness are promising candidates especially for sophisticated wearable strain sensing applications in complex and extreme environments.