Synergistically Improving Flexibility and Thermoelectric Performance of Composite Yarn by Continuous Ultrathin PEDOT:PSS/DMSO/Ionic Liquid Coating

Filler-free cellulose nanofiber composite papers with excellent mechanical properties for efficient electromagnetic interference shielding

The vast majority of conductive polymer composites (CPCs) currently available for electromagnetic interference (EMI) shielding rely on inorganic conductive fillers to construct conductive networks. However, the strategy inevitably causes some compromises in the biocompatibility, biodegradability, and mechanical properties of CPCs. In this work, the filler-free and high conductive cellulose nanofiber (CNF) composite papers containing poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) doped by lithium bis(trifloromethanesulfonyl) imide (Li-TFSI) are reported. The resultant Li-TFSI@PEDOT:PSS/CNF (LPPC) composite papers exhibit an exceptional absolute EMI shielding effectiveness of 14,525.5 dB∙cm-1, surpassing the reported values of many CPCs-based EMI shielding materials containing inorganic fillers. Li-TFSI can induce the structural reorganization of PEDOT chains. The conductivity of Li-TFSI@PEDOT:PSS was boosted with the enhancement of the crystalline order and oxidation level of PEDOT chains. Furthermore, the obtained LPPC composite papers demonstrate outstanding mechanical properties with a tensile strength of 44.42 MPa and EMI shielding stability with a retention ratio of up to 97 %, which are desirable for EMI shielding in wearable devices. Therefore, this work provides a feasible strategy to construct filler-free CPCs-based EMI shielding materials, which are expected to provide electromagnetic protection for the next flexible devices.

Metal-Like Conductivity in Acid-Treated PEDOT:PSS Films: Surpassing 15,000 S/Cm

Although poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films with high conductivity have been obtained through conventional organic solvent and acid treatment, their conductivity has not yet exceeded 10000 S/cm. In this paper, by combining blade-coating and treating with high concentration and volatilizable trifluoromethanesulfonic acid (CF3SO3H), PEDOT:PSS films with ultrahigh conductivity of 15143 S/cm, comparable to some metals, were prepared. Characterizations of morphology and structure indicate the formation of a perfectly continuous fibrous network structure, highly oriented crystallization, and tightly packed π-π stacking of PEDOT chains after removing a vast amount of PSS, which contributes to boosting the electrical conductivity of the treated PEDOT:PSS film. The distinguished electrical properties and ultrahigh conductivity enable it to replace metal materials as electrodes for "all-polymer" capacitive piezoelectric sensors with outstanding pressure sensitivity. Moreover, by regulating the blade-coating condition, the CF3SO3H-treated PEDOT:PSS films exhibit excellent electrochemical performance, which is an ideal channel material in organic electrochemical transistors (OECTs). The CF3SO3H-treated PEDOT:PSS film-based OECT devices display a high transconductance of 50.6 ± 5.5 mS and carrier mobility of 9.3 ± 1.5 cm2V-1s-1. This study not only provides new insights into the development of a simple and efficient PEDOT:PSS film treatment method but also expands its application in flexible electronics. Especially, the present research offers a useful reference in preparing "all-polymer"-based flexible electronic devices.

Bioinspired Tunable Helical Fiber-Shaped Strain Sensor with Sensing Controllability for the Rehabilitation of Hemiplegic Patients

Fiber-based strain sensors, as wearable integrated devices, have shown substantial promise in health monitoring. However, current sensors suffer from limited tunability in sensing performance, constraining their adaptability to diverse human motions. Drawing inspiration from the structure of the spiranthes sinensis, this study introduces a unique textile wrapping technique to coil flexible silver (Ag) yarn around the surface of multifilament elastic polyurethane (PU), thereby constructing a helical structure fiber-based strain sensor. The synergistic interaction between the elastic PU core and the outer helical Ag yarn enhances the mechanical strength and stretchability of the sensor, while the external helical Ag yarn offers high conductivity. By adjusting the spacing of Ag yarn coils on the surface of the fiber-based sensor, we achieve precise control over both sensing sensitivity and strain range. Specifically, experimental results show that with a pitch of 1.25 mm, the strain range reaches up to 150%, and the gauge factor (GF) is 2.6; when the pitch is adjusted to 5 mm, within a 60% strain range, the GF value significantly increases to 9.3. Based on these excellent performance metrics, we further apply the sensor as a conductor in ECG monitoring garments, successfully verifying its practicality in cardiac monitoring. Additionally, we developed a smart glove for hand function rehabilitation training, utilizing wireless signal transmission to promote hand function recovery in hemiplegic patients. The sensor is also capable of effectively monitoring respiratory rate and pulse, showing broad prospects in the fields of rehabilitation medicine and smart healthcare.

Recent advances in ionic thermoelectric systems and theoretical modelling

Converting waste heat from solar radiation and industrial processes into useable electricity remains a challenge due to limitations of traditional thermoelectrics. Ionic thermoelectric (i-TE) materials offer a compelling alternative to traditional thermoelectrics due to their excellent ionic thermopower, low thermal conductivity, and abundant material options. This review categorizes i-TE materials into thermally diffusive and thermogalvanic types, with an emphasis on the former due to its superior thermopower. This review also highlights the i-TE materials for creating ionic thermoelectric supercapacitors (ITESCs) that can generate significantly higher voltages from low-grade heat sources compared to conventional technologies. Additionally, it explores thermogalvanic cells and combined devices, discussing key optimization parameters and theoretical modeling approaches for maximizing material and device performance. Future directions aim to enhance i-TE material performance and address low energy density challenges for flexible and wearable applications. Herein, the cutting-edge of i-TE materials are comprehensively outlined, empowering researchers to develop next-generation waste heat harvesting technologies for a more sustainable future.

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.

High-Performance Polyaniline-Coated Carbon Nanotube Yarns for Wearable Thermoelectric Generators

Carbon nanotubes/polyaniline (CNTs/PANI) composites have attracted significant attention in thermoelectric (TE) conversion due to their excellent stability and easy synthesis. However, their TE performance is far from practical demands, and few flexible yarns/fibers have been developed for wearable electronics. Herein, we developed flexible CNTs/PANI yarns with outstanding TE properties via facile soaking of CNT yarns in a PANI solution, in which the PANI layer was coated on the CNT surface and served as a bridge to interconnect adjacent CNT filaments. With optimizing PANI concentration, immersing duration, and doping level of PANI, the power factor reached 1294 μW m-1 K-2 with a high electrical conductivity of 3651 S cm-1, which is superior to that of most of the reported CNTs/PANI composites and organic yarns. Combining outstanding TE performance with excellent bending stability, a highly integrated and flexible TE generator was assembled consisting of 40 pairs of interval p-n segments, which generate a high power of 377 nW at a temperature gradient of 10 K along the out-of-plane direction. These results indicate the promising application of CNTs/PANI yarns in wearable energy harvesting.

Photothermal, superhydrophobic, conductive, and anti-UV cotton fabric loaded with polydimethylsiloxane-encapsulated copper sulfide nanoflowers

Multifunctional textiles have attracted widespread attention with the improvement of awareness of health. Especially, the fluorine-free superhydrophobic and conductive cellulose fiber-based fabrics have received intensive interest due to their broad and high-value applications. Herein, the copper sulfide nanoflowers were in-situ deposited on cotton fabric followed by polydimethylsiloxane (PDMS) treatment for encapsulating CuS nanoflowers and obtaining superhydrophobicity, recorded as Cot@PTA@CuS@PDMS. Cot@PTA@CuS@PDMS possesses superhydrophobicity with contact angles of 153.0 ± 0.4°, photothermal effect, excellent UV resistance, good conductivity, and anti-fouling. Interestingly, the resistance of Cot@PTA@CuS@PDMS is significantly reduced from 856.4 to 393.1 Ω under simulated sunlight irradiation with 250 mW/cm2. Notably, the resistance can be slightly recovered after shutting off simulated sunlight. Besides, Cot@PTA@CuS@PDMS has efficient oil-water separation efficiency for corn germ oil and castor oil, respectively. Briefly, this work provides a novel, facile, and promising strategy to fabricate multifunctional fiber-based textiles with the reversible change of resistance under simulated sunlight irradiation, inspiring more scholars to control the resistance change of textiles by light irradiation.

Highly Conductive, Ultrastrong, and Flexible Wet-Spun PEDOT:PSS/Ionic Liquid Fibers for Wearable Electronics

Conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) fibers with high electrical conductivity, flexibility, and robustness are urgently needed for constructing wearable fiber-based electronics. In this study, the highly conductive (4288 S/cm), ultrastrong (a high tensile strength of 956 MPa), and flexible (a low Young's modulus of 3.8 GPa) PEDOT:PSS/1-ethyl-3-methylimidazolium dicyanamide (EMIM:DCA) (P/ED) fiber was prepared by wet-spinning and a subsequent H₂SO₄-immersion-drawing process. As far as we know, this is the best performance of the PEDOT:PSS fiber reported so far. The structure and conformation of the P/ED fiber were characterized by FESEM, XPS, Raman spectroscopy, UV-vis-NIR spectroscopy, and WAXS. The results show that the high performances of the P/ED fiber are mainly attributed to the massive removal of PSS and high degree of crystallinity (87.9%) and orientation (0.71) of PEDOT caused by the synergistic effect of the ionic liquid, concentrated sulfuric acid, and high stretching. Besides, the P/ED fiber shows a small bending radius of 0.1 mm, and the conductivity of the P/ED fiber is nearly unchanged after 1000 repeated cycles of bending and humidity changes within 50-90%. Based on this, various P/ED fiber-based devices including the circuit connection wire, thermoelectric power generator, and temperature sensor were constructed, demonstrating its wide applications for constructing flexible and wearable electronics.

Highly Electrical Conductive PEDOT:PSS/SWCNT Flexible Thermoelectric Films Fabricated by a High-Velocity Non-solvent Turbulent Secondary Doping Approach

Materials based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) can be potentially employed as flexible thermoelectric generators (TEGs) to capture waste heat and generate electrical energy. Among various methods, secondary doping is an effective way to modulate its thermoelectric (TE) performance. Different from conventional measures such as dropping, soaking, and steam fumigation, strong shear is integrated with the doping process and termed high-velocity non-solvent turbulent secondary doping (HNTD). We systematically investigate the transformation of PEDOT:PSS during this procedure and the formation mechanism of its highly conductive pathway. It is illustrated that PEDOT:PSS experiences PSS swelling, the phase separation of PEDOT from PSS, the removal of isolated PSS, and the evolution of PEDOT to a linear conformation. These evolutions contribute to the substantial elevation of electrical conductivity (σ). Furthermore, by employing continuous single-walled carbon nanotube (SWCNT) networks as structural units, highly conductive flexible PEDOT:PSS/SWCNT TE thin films could be prepared without sacrificing the Seebeck coefficient (S). Additionally, the effect of HNTD and direct addition method on TE properties of composite films is also compared. Finally, the PEDOT:PSS composite film with 40 wt % SWCNTs by the HNTD method exhibits the maximized power factor (PF) of 501.31 ± 19.23 μW m^-1 K^-2 with σ of 4717.8 ± 41.51 S cm^-1 and S of 32.6 ± 0.13 μV K^-1 at room temperature. It is worth mentioning that the σ value 4717.8 ± 41.51 S cm^-1 is the highest among the composites based on commercial carbon fillers and organic semiconductors. Finally, a 6-leg TEGs prototype is assembled and illustrates an output power of 4.416 μW under a temperature difference (ΔT) of 58 K. It is thought that such a strategy may provide some guidelines for manufacturing PEDOT:PSS-based functional materials.

Washable PEDOT:PSS Coated Polyester with Submicron Sized Fibers for Wearable Technologies

The use of non-metallic conductive yarns in wearable technologies like smart textiles requires compliant washable fibers that can withstand domestic washing without losing their conductive properties. A one-pot coating with PEDOT:PSS conductive polymers was applied to polyester submicron fibers, increasing the water resistance and washability under various domestic washing conditions. Plasma treatment of the untreated samples improved the anchoring of the coating to the fibers, producing smooth and homogeneous coatings. The primary doping of PEDOT:PSS with ethylene glycol (EG), dimethyl sulfoxide (DMSO), and a non-ionic surfactant as well as the secondary doping of the composite fibers improved the sheet resistance at room temperature. The as-obtained composite materials showed similar mechanical properties as the parent fibers, indicating that the coating and post-treatment do not affect the overall mechanical property of the composite. The performance of the composites under different temperature and humidity conditions and washability using the standardized ISO 6330:2012 procedure for domestic washing and drying showed that the obtained composites are good candidates for reliable washable wearable technologies, such as all-organic washable Joule heaters in functional textiles.

Anion Size Effect of Ionic Liquids in Tuning the Thermoelectric and Mechanical Properties of PEDOT:PSS Films through a Counterion Exchange Strategy

Poly(3,4-ethylene dioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) thermoelectric thin films have attracted significant interest due to their solution-processable manufacturing. However, molecular-level tuning or doping is still a challenge to synergistically boost their thermoelectric performance and mechanically stretchable capabilities. In this work, we report a counterion exchange between ionic liquid bis(x-fluorosulfonyl) amide lithium (Li:nFSI, n = 1, 3, 5) with different sizes of anions and a PEDOT:PSS-induced bipolaron network, which significantly boosted the thermoelectric power factor from 0.8 to 157 μW m K^-2 at 235 °C and the maximum tensile strain from 3% to over 30%. The π-π* stacking of the PEDOT polymer chains was fine-tuned by the hydrophobic anions of nFSI^-, providing a technical route for constructing a bipolaron network and inducing the transition from hopping transport to band-like transport. Furthermore, we found that the stretchable capabilities, that is, ε_max, were connected to the gelation time of the PEDOT:PSS-Li:nFSI aqueous solution. Thus, more fluorine-containing groups resulted in longer gelation times and higher ε_max values, which significantly improved the processability of the solution-derived films.

Novel Wearable Pyrothermoelectric Hybrid Generator for Solar Energy Harvesting

Recently, wearable energy harvesting systems have been attracting great attention. As thermal energy is abundant in nature, developing wearable energy harvesters based on thermal energy conversion processes has been of particular interest. By integration of a high-efficient solar absorber, a pyroelectric film, and thermoelectric yarns, herein, we design a novel wearable solar-energy-driven pyrothermoelectric hybrid generator (PTEG). In contrast to those wearable pyroelectric generators and thermoelectric generators reported in previous works, our PTEG can enable effective energy harvesting from both dynamic temperature fluctuations and static temperature gradients. Under an illumination intensity of 1500 W/m² (1.5 sun), the PTEG successfully charges two commercial capacitors to a sum voltage of 3.7 V in only 800 s, and the total energy is able to light up 73 LED light bulbs. The volumetric energy density over the two capacitors is calculated to be 67.8 μJ/cm³. The practical energy harvesting performance of the PTEG is further evaluated in the outdoor environment. The PTEG reported in this work not only demonstrates a rational structural design of high-efficient wearable energy harvesters but also paves a new pathway to integrate multiple energy conversion technologies for solar energy collection.