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Adhesion between flexible devices and skin surface facilitates portability of devices and reliable signal acquisition from human body, which is essential for medical therapy devices or monitoring systems. Here, we utilize a simple, cost-effective, and scalable layer-by-layer dip-coating method to fabricate a skin-adhesive multifunctional textile-based device, consisting of three parts: low-cost and easily available airlaid paper (AP) substrate, conductive MXene sensitive layer, and adhesive polydimethylsiloxane (PDMS). The adhesive layer of lightly cross-linked PDMS enables the device to form conformal contact with skin even during human joint bending. The smart textile device exhibits excellent electro-thermal and photo-thermal conversion performance with good cycling stability and tunability. Furthermore, the textile electronics show good electromagnetic interference (EMI) shielding properties due to the good electrical conductivity, as well as sensitive and stable pressure sensing properties for human motion detection. Consequently, this efficient strategy provides a possible way to design multifunctional and wearable electronic textiles for medical applications.

Cations and anions can accumulate at the two ends of an ionic conductor under temperature gradient, which is the so-called Soret effect. This can generate a voltage between the two electrodes, and the thermopower can be higher than that of the electronic conductors because of the Seebeck effect by 1-2 orders in magnitude. The thermoelectric properties of ionic conductors depend on the ionic thermopower, ionic conductivity, and thermal conductivity. Compared with other ionic conductors, like liquid electrolytes and hydrogels, ionogels made of an ionic liquid and a gelator can have the advantages of high thermopower and high stability. Great progress was recently made to improve the ionic conductivity and/or ionic thermopower of ionogels. They can be used in ionic thermoelectric capacitors (ITECs) to harvest heat. In addition, they can be integrated with electronic thermoelectric materials to harvest heat from both temperature gradient and temperature fluctuation, which can be caused by waste heat.

Dry epidermal electrodes that can always form conformal contact with skin can be used for continuous long-term biopotential monitoring, which can provide vital information for disease diagnosis and rehabilitation. But, this application has been limited by the poor contact of dry electrodes on wet skin. Herein, we report a biocompatible fully organic dry electrode that can form conformal contact with both dry and wet skin even during physical movement. The dry electrodes are prepared by drop casting an aqueous solution consisting of poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), poly(vinyl alcohol) (PVA), tannic acid (TA), and ethylene glycol (EG). The electrodes can exhibit a conductivity of 122 S cm

Tunable lens technology inspired by the human eye has opened a new paradigm of smart optical devices for a variety of applications due to unique characteristics such as lightweight, low cost, and facile fabrication over conventional lens assemblies. The fast-growing demands for tunable optical lenses in consumer electronics, medical diagnostics, and optical communications require the lens to have a large focal length modulation range and high compactness. Herein, for the first time, an all-solid tunable soft lens driven by highly transparent dielectric elastomer actuators (DEAs) based on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) and waterborne polyurethane (PEDOT:PSS/WPU) transparent electrodes is developed. The deformation of the tunable soft lens is achieved by the actuation of DEAs, mimicking the change of the surface profile of the human eye to achieve remarkable focal length variations. Upon electrical activation, this tunable soft lens can vary its original focal length by 209%, which is one of the highest among current tunable soft lenses and far beyond that of the human eye. This study demonstrates that transparent DEAs are capable of achieving focus-variation functions, and potentially useful in artificial robotic vision, visual prostheses, and adjustable glasses, which will induce significant effects on the future development of tunable optics.

Abundant heat is generated in household, industry and natural processes, and the majority of the heat is dissipated to environment as waste heat owing to the low heat utilization efficiency. Hence, to harvest waste heat is of great significance for the sustainable development. Among the heat-harvesting technologies including organic Rankine cycles, thermo-osmotic energy conversion and thermoelectric generators (TEGs), TEGs that can directly convert heat into electricity is the only feasible technology to harvest the low-grade heat that has a temperature below 200 oC and is about 2/3 of the total waste heat. The operational principle of TEGs is the Seebeck effect of electronic thermoelectric (TE) materials such as semiconductors or semimetals. Recently, ionic conductors, such as liquid ionic conductors and gel ionic conductors, emerged as the next-generation TE materials mainly due to their high thermopower. Their thermopower can be higher than that of the electronic TE materials by 2-3 orders in magnitude. In addition, they usually have a low thermal conductivity. Because the charge carriers in ionic conductors are cations and anions, the principle for the ionic thermopower and ionic TE conversion devices are notably different from the electronic counterparts. Very exciting progress has been made on the ionic TE materials and devices. This article provides a review on the ionic TE materials, including the thermopower mechanism, TE properties, and applications.

The Seebeck coefficient of the composites of PEDOT:PSS and two-dimensional poly-(heptazine imide) (KPHI) can be greatly enhanced after exposure to visible light. It is attributed to the light-induced electron transfer from KPHI to PEDOT:PSS.

Quasi solid-state gels with a deep eutectic solvent that has the advantages of non-volatility, low cost and easy preparation can exhibit high thermoelectric properties, comparable to that of ionogels consisting of ionic liquids.

Recently, organic materials have emerged as next-generation thermoelectric (TE) materials because of their unique advantages including low cost, high mechanical flexibility, low or no toxicity, and...

High mechanical ductility and high mechanical strength are important for materials including polymers. Current methods to increase the ductility of polymers such as plasticization always cause a remarkable drop in the ultimate tensile strength. There is no report on the ductilization of polymers that can notably increase the elongation at break while not lowering the ultimate tensile strength. Here, we report the salt-induced ductilization of an intrinsically conducting polymer, poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS). Treating highly conductive PEDOT:PSS with a salt such as sodium perchlorate can enhance its elongation at break from 8.5 to 53.2%, whereas it hardly affects the tensile strength. Moreover, the resistance of the ductilized PEDOT:PSS films is insensitive to the tensile strain before fracture and slightly increases by only ~6% during the cyclic tensile testing with the strain up to 30%. These effects are ascribed to the decrease in the Coulomb attraction between PEDOT + and PSS − by the salt ions.

Conductive stretchable hydrogels and ionogels consisting of ionic liquids can have interesting application as wearable strain and pressure sensors and bioelectrodes due to their soft nature and high conductivity. However, hydrogels have a severe stability problem because of water evaporation, whereas ionogels are not biocompatible or even toxic. Here, we demonstrate self-adhesive, stretchable, nonvolatile, and biocompatible eutectogels that can always form conformal contact to skin even during body movement along with their application as wearable strain and pressure sensors and biopotential electrodes for precise health monitoring. The eutectogels consist of a deep eutectic solvent that has high conductivity, waterborne polyurethane that is an elastomer, and tannic acid that is an adhesive. They can have an elongation at a break of 178%, ionic conductivity of 0.22 mS/cm, and adhesion force of 12.5 N/m to skin. They can be used as conformal strain sensors to accurately monitor joint movement and breath. They can be even used as pressure sensors with a piezoresistive sensitivity of 284.4 kPa-1 to precisely detect subtle physical movements like arterial pulses, which can provide vital cardiovascular information. Moreover, the eutectogels can be used as nonvolatile conformal electrodes to monitor epidermal physiological signals, such as electrocardiogram (ECG) and electromyogram (EMG).

It is of significance to develop stretchable conductors for flexible electronics. Although intrinsically conducting polymers like poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)...

Both a high Seebeck coefficient and high conductivity are important for thermoelectric materials that can be used to directly convert heat into electricity. Although light exposure can affect the Seebeck coefficient of some inorganic or organic materials with low conductivity, it cannot enhance the Seebeck coefficient of thermoelectric materials with high conductivity, and thus it has no practical significance. Here, we report the photo-enhanced Seebeck effect on flexible and highly conductive composites of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with TiO2 nanoparticles. When a composite of PEDOT:PSS with dispersed TiO2 nanoparticles of 66.9 wt% loading is exposed to UV light, its Seebeck coefficient can be enhanced from 23.5 to 94.3 μV K−1, and the UV exposure of a double-layer structure of PEDOT:PSS and TiO2 nanoparticles can enhance its power factor from 159 to 356 μW m−1 K−2. We propose that the enhancement in the Seebeck coefficient arises from the UV light-induced electron excitation from the valence band to the conduction band of TiO2 and the subsequent electron transfer from the conduction band of TiO2 into PEDOT:PSS. This can partially dedope PEDOT:PSS and thus increase the Seebeck coefficient, and the dedoping of PEDOT:PSS can be readily tuned by controlling the light intensity.

Selenophene substitution of photovoltaics materials can improve their intermolecular interactions and thus offer a good opportunity to finely optimize their phase separation morphology to an ideal state. However, the performance of this substitution strategy is yet unclear in liquid crystalline donors, which have demonstrated high efficiency. The combination of those structural advantages may help to achieve state-of-the-art device performance. Herein, two new liquid crystalline small molecule donors are designed and synthesized with different substituents on benzo[1,2-b:4,5-b′]dithiophene (BDT) central units, including L1 with thiophene group and L2 with selenophene group, for investigating the effect of selenophene substitution on morphology and photovoltaic performance of liquid crystalline donors. Unlike the control donor L1, the selenide donor L2 shows a higher intramolecular interaction and presents a novel sematic liquid crystal phase, which results in a more favored morphology, better light harvest, and lower charge recombination. As a result, outstanding power conversion efficiency up to 15.8% is realized in the L2:Y6 based devices. Particularly promising is the fact that selenide-based devices with active layer thicknesses up to ∼300 nm can still output a high PCE of 14.3%, which is promising for roll-to-roll printing processes with large-scale production. To our best knowledge, the PCEs of 15.8% and 14.3% are the highest values reported to date in binary ASM-OSCs and thick-film ASM-OSCs, respectively. These excellent values demonstrated the superiority of selenophene in the construction of efficient small molecule liquid crystalline donors.

It is of significance to develop efficient heat-to-electricity conversion technology for sustainable development because of the abundant waste heat on Earth. Waste heat can induce a temperature gradient in the environment, and the temperature fluctuates with time. Thermoelectric (TE) generators based on the Seebeck effect of electronic conductors under a temperature gradient can directly convert heat into electricity, but their performance is still too low for large-scale practical application. Here, we report mixed ion-electron thermoelectric converters (MTECs) using flexible quasi-solid ionogels consisting of an ionic liquid (IL), gelatin and single-walled carbon nanotubes (SWNTs). The ionogels are ionically and electronically conductive due to the ionic liquid and SWNTs, respectively, and they have very low thermal conductivity. The MTECs can convert heat into electricity from both the temperature fluctuation causing the ion accumulation variation of the ionic liquids and the temperature gradient inducing the Seebeck effect of SWNTs. The ratio of the optimal output power density (P) of the MTECs to the thermal conductivity (λ) of the active material is used to evaluate the TE performance. The P/λ value of the MTECs with optimal SWNT loading in the ionogel is about 7.7 times of that of the control TE generators with a SWNT film.

Electronic textiles (E-textiles) have received extensive attention for use in human motion detection, wearable electronics, and artificial intelligence. However, fabrication costs and methodology are still key issues preventing the commercialization of highly sensitive and wearable sensors. Here, simple to prepare low-cost conductive MXene-coated airlaid paper (MXene AP) sensors were fabricated via a dip-coating technique for use as electronic skin. The two-layer MXene AP sensors with a three-dimensional structure display high sensitivity (7.65 kPa−1 at 0–3.3 kPa), a broad sensing range (up to 300 kPa), and a stable response for more than 1000 cycles, presenting good sensitivity, repeatability, and durability, and their mechanism was investigated. Furthermore, origami technology can be used to fold MXene AP into an arbitrary shape with good sensing performance, and multistate conformational changes and simulated human touch can also be detected using the resultant three-dimensional (3D) MXene AP sensors. The wearable MXene AP sensors can be used to monitor human body motion and to collect human physical signals sensitively and accurately in the form of electronic skin. These strategies of folding and layering textile-decorated conductive nanomaterials provide a reference for next-generation textile-based wearable electronics and electronic skin.

Thermoelectric polymers have attracted great attention because of their unique merits including low thermal conductivity, low cost, non- or low toxicity and high mechanical flexibility. However, their thermoelectric properties particularly the Seebeck coefficient should be greatly increased for practical applications. Although dedoping and energy filtering can increase the Seebeck coefficient, they lower the conductivity because the Seebeck coefficient and conductivity are interdependent. Here, we report simultaneous enhancements in the Seebeck coefficient and conductivity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) that is the most popular thermoelectric polymer by adding ferroelectric barium titanate (BaTiO3) nanoparticles. The BaTiO3 nanoparticles can enhance the Seebeck coefficient from 23.8 to 40.7 μV K−1 and the power factor from 34.4 to 117.0 μW m−1 K−2, and the insulating BaTiO3 nanoparticles do not decrease but increase the conductivity of PEDOT:PSS. The conductivity of the continuous PEDOT:PSS phase in the composite films can be enhanced from 587 to 1552 S cm−1. The enhancement in the Seebeck coefficient is ascribed to the energy filtering of low-energy charge carriers in PEDOT:PSS due to the spontaneous electric polarization of BaTiO3, and the increase in the conductivity is attributed to the secondary doping of PEDOT:PSS related to the high dielectric constant of the BaTiO3 nanoparticles.

ConspectusIntrinsically conducting polymers can have important application in flexible electronic devices and systems owing to their high mechanical flexibility. Among them, poly(3,4-ethylenedioxyt...

Stretchable, skin-interfaced, and wearable strain sensors have risen in recent years due to their wide-ranging potential applications in health-monitoring devices, human motion detection, and soft robots. High aspect ratio (AR) silver nanowires (AgNWs) have shown great potential in the flexible and stretchable strain sensors due to the high conductivity and flexibility of AgNW conductive networks. Hence, this work aims to fabricate highly stretchable, sensitive, and linear kirigami strain sensors with high AR AgNWs. The AgNW synthesis parameters and process windows have been identified by Taguchi’s design of experiment and analysis. Long AgNWs with a high AR of 1556 have been grown at optimized synthesis parameters using the one-pot modified polyol method. Kirigami sensors were fabricated via full encapsulation of AgNWs with Ecoflex silicon rubber. Kirigami-patterned strain sensors with long AgNWs show high stretchability, moderate sensitivity, excellent linearity (R2 = 0.99) up to 70% strain and can promptly detect finger movement without obvious hysteresis.

Electronic textiles (e-textiles) have attracted great attention due to their promising application in areas like flexible electronics, wearable devices, healthcare and sports. They should be machine-washable and have high thermal conduction for heat management. But there are few reports on e-textiles with high thermal conductivity. Here, we report highly machine-washable e-textiles with high strain sensitivity and high thermal conduction. The e-textiles are prepared by anchoring carbon nanotubes (CNTs) into non-woven fabric (NWF) by nanosoldering and then depositing reduced graphene oxide (rGO) through chemical reduction of graphene oxide (GO). The rGO/CNTs/NWF textiles are highly sensitive to the strain and have a gauge factor of 32.65 at a tensile strain of 1.0%. They can detect a subtle strain as low as 0.01%. In addition, the rGO and CNTs attached by nanosoldering have good adhesion to NWFs. The e-textiles are highly machine washable and can be operated in water for a long time. Moreover, they can have a thermal conductivity of 2.90 W m−1 K−1, much higher than normal textiles. Their applications as wearable sensors for monitoring body motion and blood pulse are also demonstrated.

Thermoelectric (TE) polymers have unique advantages in converting heat into electricity. But their Seebeck coefficient is lower than that of inorganic TE materials by about one order of magnitude. Here, surface energy filtering is proposed to significantly enhance the Seebeck coefficient of TE polymers. The as-prepared poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, the most popular TE polymer) films from aqueous solution have a low Seebeck coefficient of 15–18 μV K−1 and a power factor of ∼0.008 μW m−1 K−2. Depositing Rhodamine 101, a zwitterion, on PEDOT:PSS films can enhance the Seebeck coefficient to 47.2 μV K−1 and the power factor to 401.2 μW m−1 K−2. The Rhodamine 101 effect on the Seebeck coefficient is consistent with its effect on the work function of PEDOT:PSS. The enhancement in the Seebeck coefficient is ascribed to the dipole moment of Rhodamine 101 and the interfacial dipole moment formed at the surface of the PEDOT:PSS films. This is confirmed by theoretical simulations. We call this mechanism surface energy filtering. Surface energy filtering can increase the mean energy of the transporting carriers and thus the Seebeck coefficient of the PEDOT:PSS films while only slightly affecting the electrical conductivity. It can thus significantly enhance the Seebeck coefficient and the power factor simultaneously.

To accurately evaluate the wellbeing condition of a person, sensing systems with versatile functions are needed. Apart from directly monitoring vital signs, an indirect monitoring approach by measuring human activities with wearable devices has emerged. Herein, a smart textile fabricated with a simple dip coating method is developed with multiple functionalities, such as energy harvesting, physical sensing, and even gas sensing. A maximum output power density of 2 W m−2 is achieved with a layer of PEDOT: PSS coated textile and PTFE under foot stepping at 2 Hz, and the matched impedance is as low as 14 MΩ. A height-varying multi-arch strain sensor with a large strain sensing range from 10% to 160% is developed. Furthermore, the arch-shaped strain sensors mounted on the human fingers are demonstrated to monitor hand gestures for American Sign Language interpretation and robotic hand control. Additionally, four textile-based sensors located at different body parts are employed to track human activities. Beyond that, the smart textile is also demonstrated to be a building block of a wearable CO2 sensor. Looking forward, this smart textile could be incorporated into real clothes as both energy harvesters and various functional self-powered sensors to enable smart clothes for healthcare monitoring applications.

Harvesting energy from the environment to power the self-sustained systems has long been desired1,2. Ionic thermoelectric (i-TE) material with mobile ions as charge carriers has the advantage to generate large thermal voltages at low operating temperatures3-5. Recent works improved the thermopower substantially by modifying the polymer matrix of the i-TE hydrogels6-9. But the mobile ions have not been systematically studied in the context of i-TE hydrogels. This study highlights the role of ions in i-TE hydrogels employing a polyvinyl alcohol (PVA) polymer matrix and a number of ion providers, e.g. KOH, KNO3, KCl, KBr, NaI, KI, and CsI. The relationship between the intrinsic physical parameters of the ion and the thermoelectric performance is established, indicating electronegativity of the cation and the ability to influence the hydrogen bond by the anion are two crucial factors. Among these i-TE hydrogels, PVA/CsI hydrogel exhibits the largest ionic Seebeck coefficient, reaching 52.9 mV K-1, which is the greatest of all i-TE materials reported till date. In addition, PVA/NaI hydrogel exhibits excellent TE properties, with a record ZT value of 5.09 at room temperature. This flexible, inexpensive hydrogel that compatible with large-scale manufacturing shows great promise for low-grade thermal energy harvesting.

Safer human-robot interactions mandate the adoption of proprioceptive actuation. Strain sensors can detect the deformation of tools and devices in unstructured and capricious environments. However, such sensor integration in surgical/clinical settings is challenging due to confined spaces, structural complexity, and performance losses of tools and devices. Herein, we report a highly stretchable skin-like strain sensor based on a silver nanowire (AgNW) layer and hydrogel substrate. Our facile fabrication method utilizes thermal annealing to modulate the gauge factor (GF) by forming multidimensional wrinkles and a layered conductive network. The developed AgNW-hydrogel (AGel) sensors sustain and exhibit a strain-sensitive profile (max. GF = ∼70) with high stretchability (200%). Due to its conformability, the sensor demonstrates efficacy in integration and motion monitoring with minimal mechanical constraints. We provide contextual cognizance of tooltip during a transoral procedure by incorporating AGel sensors and showing the fabrication methodology's versatility by developing a hybrid self-sensing actuator with real-time performance feedback.