Recent progress on PEDOT‐based wearable bioelectronics

Abstract

Bioelectronics, which can perform monitoring and biological signal collection of strain, pressure, and temperature at the same time, have attracted more and more attentions. To meet the unique requirements of wearable characteristics, the bioelectronic device with excellent flexibility is desired. At present, the biggest challenge for the flexible device is the candidate of electrode materials. The classical conducting polymer poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) has showed a promising potential in the field of wearable bioelectronic devices due to its high conductivity, excellent flexibility, aqueous processability, and good biocompatibility. Here, we reviewed the recent progress of PEDOT‐based wearable bioelectronic sensors and focus on the role of different PEDOT‐based flexible materials in addressing the mechanical strain, pressure, and temperature requirements. The impact of contemporary bioelectronic research will be highlighted.

INTRODUCTION

With the mutual penetration of life science and electronic science, a new concept of “bioelectronics” is gradually derived.^1,2 Bioelectronics, including electronic biomedicine, artificial biological information model, and human–machine biological effects, contribute to solving a series of related problems in biology, medicine, and electronic engineering technology.^3,4 Electronic sensor devices are one of the most representative bioelectronics. And the electrode is a critical material for the electronic sensor. Traditional electronic devices usually employed metals, alloys, metal-oxide, and other materials as electrode. However, conventional electrode materials failed to meet the flexibility requirements of bioelectronic soft tissue systems. Therefore, the development of flexible bioelectronic electrode is important. Except for flexibility of electrode, the electrode is also expected to realize accurate responses to various stimuli and biological signals of biological tissues.⁵ The high conductivity of electrode is favorable for obtaining the accurate signals. Based on the above problems, the development of flexible bioelectronic electrode with high conductivity is extremely critical.

Conducting polymers have attracted great attention in the field of electronics due to their tunable electrical conductivity, good mechanical properties, good stability, and low cost.⁶ The main conductive polymers include polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT).^7,8 Among them, the conductivity of PPy (380 S cm⁻¹) and PANI (5 S cm⁻¹) are much lower than PEDOT:polystyrene sulfonate (PSS), which is up to 4000 S cm⁻¹. In addition, PPy and PANI are insoluble in water and even insoluble in some organic solvents, which limits the secondary doping for PPy and PANI.⁹ In contrast, PEDOT displays a tunable high electrical conductivity, excellent thermal stability, non-toxicity, low signal-to-noise ratio, and good biocompatibility, and it can be fabricated via a large-scale and low-cost technology. Moreover, the PEDOT-based flexible devices could be closely attached to the curved skin surface, and the flexible devices are stretchable and can monitor most of the mechanical deformations involving the body movements, greatly reducing the negative reactions of bioelectronic devices.¹⁰

PEDOT based on conducting polymer materials, whether gels, fibers, or films, have been widely used in strain, pressure, and temperature sensors related to bioelectronics.¹¹ Compared to other conductive polymer gels, PEDOT-based gels have superior absorbency and excellent water retention. Sensors based on PEDOT gels have certain characteristic advantages of stretchability, flexibility, viscosity, and tunable elastic modulus, which is easier to attach to biological surfaces than conventional sensors. Over the past few decades, the application progress of gels in flexible wearable sensors is reflected in flexible electrodes, strain/pressure sensors, and a range of substrates. For example, Xu and coworkers have achieved abundant results in the research of PEDOT:PSS gel, including the design and performance improvement of pure PEDOT:PSS gel and PEDOT:PSS gel mixed with other polymers after improvement in performance as a sensor.^12,13 PEDOT-based fibers have gained widespread attention due to their lightness, inherent flexibility, good skin compatibility, and simple fabrication. The traditional preparation methods of PEDOT:PSS fibers include wet spinning and electrospinning. The difference is that the PEDOT:PSS fibers obtained by electrospinning usually exist in the form of fiber membranes or fiber mats, which can be applied to make biosensors and stretchable sensors. However, the poor mechanical properties of PEDOT-based fibers seriously affect its application. In order to meet the most basic comfort performance and foremost electrical performance requirements of wearable products, compared with other conductive polymer films, PEDOT-based films have gained extensive attention as a representative member of the flexible family.¹⁴ In previous work, the mechanical properties of PEDOT films prepared from traditional technology were insensitive. Based on this, Sun and coworkers improved the electrical properties of the sensor well by embedding double cross-linked networks in the PEDOT films.¹⁵ Additionally, PEDOT-based film sensors were prone to crack under relatively large strain/pressure levels. To address the above issues, the selection of fabrication process, the chemical structure of the added elastomer, and the presence of some polymers are particularly important.

Although these materials of PEDOT-based gels, fibers, and films have shown excellent properties and been widely studied, few reports have systematically summarized these results. Herein, we aim to summarize the recent progress of different PEDOT-based flexible materials in addressing the mechanical strain, pressure, and temperature requirements of wearable electronic sensors (Figure 1).

FIGURE 1. The types of poly(3,4-ethylenedioxythiophene) (PEDOT)-based materials used in the different sensors. Reproduced with permission from Refs.19,45,73–77 Copyright 2020, 2021, 2022, American Chemical Society

PEDOT-BASED BIOELECTRONICS

In this section, the PEDOT-based bioelectronics, including strain sensor, pressure sensor, and temperature sensor, were reviewed from the viewpoint of PEDOT gels, fibers, and films.

Strain sensor

Strain sensors can detect the deformation of objects through the response of electrical signals (resistance, current, capacitance, etc.). Traditional flexible wearable strain sensors mainly include capacitive sensors and resistive sensors. The difference is that capacitive is non-contact (induction) and resistive is contact.^16,17 Among them, capacitive sensors use medium sensing and are generally used for displacement monitoring. Tensile strain reduces the displacement of the two electrodes and leads to an increase in capacitance. Resistive sensors are usually limited by mechanical structure and temperature environment, and generally rely on resistance to conduct signals. When the sensing material was stretched, the resistance of the sensing material changed with the applied strain. After the strain is released, the sensing material restored to its original state with the resistance of the sensor.

PEDOT gel-based strain sensor

PEDOT-based conductive gels have shown great potential in bioelectronics, including artificial tissues, wearable/implantable sensing devices, and smart electronic skins due to their similar properties to biological tissues.¹⁸

The construction of integrated conductive network structures has proven to be a good approach to improve the tensile properties of PEDOT-based gels as strain sensors. For example, Dong et al. developed a double-network interpenetrating conductive hydrogel synthesized by multiple hydrogen bonding, composed of dual-cross-linked polyacrylamide and polyacrylic acid as well as PEDOT:PSS and graphene (GR). The hydrogel can be stretched to about 500% before fracture.¹⁹ The hydrogel was then sandwiched between two dielectric carbon nanotubes (CNTs) to make the sensor (Figure 2A). The sensor can be stretched to 100% of its original length, which can be used to monitor simple human motion. However, in most cases, larger stretch rates are required to meet the strain requirements as bioelectronic sensors. Zhang et al. developed a novel all-polymer conductive hydrogel with an interpenetrating polymer network structure (Figure 2B), which was prepared from conductive PEDOT:PSS modified with zwitterion (HEAA-co-SBAA).²⁰ This hydrogel can achieve high stretchability of 4000%–5000%. The mechanical recovery rate of 70%–80%, the tensile strength of 0.5 MPa, and the strong surface adhesion (1700 J m⁻²) are quickly achieved within 3 min. The conductive hydrogel-based strain sensor can be used for bioelectronic human–computer interaction and medical monitoring (Figure 2C). Based on a similar concept, Suneetha et al. uniformly dispersed capacitive PEDOT:PSS into microstructure gels, resulting in an increase of in the tensile stress of the hydrogel from 36.9 ± 2.3 kPa to 44.9 ± 2.6 kPa and stretchability increased from 2763% to 2873%.²¹ The hydrogels showed remarkable high stretchability by controlling the carboxymethylcellulose sodium salt-dopamine hydrochloride (CMC-DA). The gel adding PEDOT:PSS has the highest tensile strength and tensile stress when the CMC-DA content reaches 4 wt% (Figure 2D). It provides a broad development direction for the application of bioelectronics. Although the design of gels with integrated conductive network structures plays a guiding role in improving the strain performance of the gels in biological tissue systems, the network interactions and energy dissipation of the gels may affect the mechanical strength and fatigue resistance for repeatable stretches of the strain sensors. In addition, the strain sensors obtained by adding zwitterions dopants to the PEDOT gel precursor demonstrate better tensile properties than those doped with nanoparticles. It may be that the dispersion uniformity of the two in the gel precursor is different. Therefore, a great uniformity could improve tensile properties of PEDOT gel.

FIGURE 2. (A) Sandwich-structured disk triboelectric nanogenerators (D-TENG).19 (B) Preparation process of poly(HEAA-co-SBAA)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) hydrogel sensors.20 (C) Schematic diagram of a real-time monitoring system.20 (D) Dynamical mechanical analysis (DMA) tensile stress–strain curves of hydrogels.21 (E) Structure diagram of PEDOT:PSS fibers.24 (F) The microscope images of electrodes using positron emission tomography (PET) or medical gauze as substrates.24 (G) Plot of measured capacitance versus time for samples with different R values.24 Reproduced with permission from Refs.19,20,21,24 Copyright 2020, 2021, 2022, American Chemical Society, Royal Society of Chemistry, Elsevier

PEDOT fiber-based strain sensor

PEDOT-based fibers are used as active electrode materials for flexible sensors due to their rather high flexibility and excellent environmental stability. Among them, PEDOT:PSS fiber has high agility but low conductivity, which limits its application in flexible sensor electrodes. Hence, developing highly conductive flexible fiber electrodes is necessary to prepare excellent strain sensors.

Recently, PEDOT:PSS fiber has been extensively studied. For example, Khademhosseini and coworkers reported a facile strategy to prepare PEDOT:PSS-injectable hydrogel fibers at room temperature for achieving the repairing process of organic bioelectronics.²² Furthermore, Zhao and coworkers fabricated a PEDOT:PSS fiber by a low-temperature freezing and dry annealing technology. The obtained fiber exhibited an electrical conductivity of up to 155 S cm⁻¹. To further improve the conductivity of PEDOT:PSS fiber, utilizing acid-induced PEDOT chain conformational transitions to enhance interchain interactions and physical cross-linking has been demonstrated as a feasible approach in previous reports.²³ Ren et al. injected PEDOT:PSS aqueous solution into concentrated sulfuric acid to form PEDOT:PSS hydrogel fibers, which were then annealed and dried to obtain PEDOT:PSS fibers.²⁴ The obtained PEDOT:PSS fibers have higher electrical conductivity of 380 S cm⁻¹ than the PEDOT:PSS hydrogel fibers, mainly due to the compact structure formed by the volume shrinkage caused by drying water loss. Figure (2E) shows its formation mechanism. The chain conformation of PEDOT:PSS fibers after concentrated sulfuric acid treatment was transformed, which may be due to the stronger cross-linking interaction between the chains induced by acid. Taking advantage of the high electrical conductivity of the fiber, which can be used as a flexible electrode for capacitor sensors for the assessment of thrombosis risk. Figure (2G) shows an image of the sensor's capacitance versus blood addition time. The capacitance of the sensor increases with the number of blood droplets and does not change much. Figure (2F) shows the microscope images of electrodes. Few cracks appeared on the PEDOT:PSS fiber surface. It is could be concluded that the capacitor sensor possesses good stability (up to a wide detection range of 500 μm) and high blood detection sensitivity (up to 230 aF μm⁻¹). However, it is worth noting that the electrical conductivity and mechanical properties of fibers and sensors cannot be achieved simultaneously.

In order to better improve the tensile mechanical properties of conductive fibers, some scholars proposed some schemes by combining with polymers such as polyurethan, polyacrylonitrile, and polydimethyl siloxane (PDMS).^25,26 Wang et al. successfully fabricated a conductive blended fiber composed of PEDOT:PSS and polyvinyl alcohol (PVA) by wet spinning process.²⁷ The tensile strength of the conductive fiber was increased from 115 to 145 MPa by increasing the doping concentration of dimethyl sulphoxide (DMSO). But the elongation decreased from 25% to 17%. In contrast, Gao et al. added 20 wt% PVA and 10 wt% ethylene glycol into the PEDOT:PSS solution, the obtained conductive fibers had the best tensile strength (210 MPa) and elongation at break (27.5%), which can be used for reference to prepare other novel PEDOT composites.²⁸ Moreover, the authors employed the fibers in creating an electric circuit in a fabric for detection of various physiological behaviors of the body.

As for the current research, the mechanical properties of PEDOT:PSS fibers formed by a single ratio are poor, and they are usually mixed with other polymers to improve fibers’ stretchability.

PEDOT film-based strain sensor

Stretchable PEDOT:PSS films are used as electrodes in bioelectronic devices for recording electrochemical physiological signals. As biosensors, PEDOT:PSS films have been studied for the analysis of various biomarkers, such as peptides, and various ions.²⁹

The PEDOT:PSS film formed by a single ratio is easy to show cracking behavior due to its brittleness, which is basically meaningless for the preparation of flexible wearable sensors. To solve this problem, conductive particles are usually embedded into an elastomer matrix to make composite materials to enhance stretchability.^6,30 It has been reported that the widely used conducting polymer PEDOT:PSS can be obtained by mixing poly(ethylene glycol), polyurethane (PU), or PVA for stretching. For example, Shaker et al. have used wet electrospinning to prepare stretchable conductive thermoplastic polyurethane (TPU)/PEDOT:PSS nanofiber membranes for the first time. The nanofibrous membrane was micropatterned and acted as a potential flexible strain sensor, maintaining 3 kΩ at room temperature and the strain could be stabilized from 20% to 40%.³¹ In order to increase the strain rate of the film, Taroni et al. also combined PEDOT:PSS and PU (Lycra), and developed a flexible and stretchable composite film by direct in situ polymerization. The sensor can detect a strain range of 10%–300% at a temperature of 30 K and it has a strain rate at break of 700%.³² Additionally, Hagler et al. produced conductive films using PEDOT:PSS screen-printing on TPU substrates by a printing method that did not require pre-stretching, substrate modification, or commercially formulated additives, and when stretched by 100%, the resistance of the film increased by about three times, and the authors used the properties of the system as a printed electrodermal activity sensor for monitoring physiological signals and tracking important health indicators.³³

Although great progress has been made in addressing the cracking behavior of PEDOT:PSS films, there are still some issues that need to be addressed for better application in bioelectronic sensors. The uniformity of the film is important for the stability of the performance of each area of the sensor. However, to date, no studies on uniformity control of PEDOT:PSS films applied to bioelectronic sensors.

Pressure sensor

In order to meet the development needs of wearable devices in the field of bioelectronics, the design and fabrication of flexible pressure sensors with high sensitivity and low cost have attracted extensive attention from the scientific and industrial circles.³⁴ A pressure sensor is a sensor that converts pressure into electrical signal output. Traditional pressure sensors are usually based on three mechanisms and the corresponding devices include piezoresistive, capacitive, and piezoelectric.³⁵ As the basis of bioelectronic skin, pressure sensor mainly depends on the conductivity of conductive material, and sensitivity is the most important feature of pressure sensor. Therefore, improving the sensitivity is particularly important for the research of bioelectronic sensor.³⁶

PEDOT gel-based pressure sensor

A flexible wearable pressure sensor has two important components: an elastic substrate that provides mechanical flexibility and a sensitive material that determines the sensing performance.³⁷ Its sensitivity can be further improved by structural tuning of sensitive materials. PEDOT-based conductive aerogels as a sensitive material for pressure sensors have been extensively studied in the field of flexible wearable pressure sensors due to their lightweight properties and tunable three-dimensional (3D) network structures.³⁸

Direct deposition of conductive materials on substrates by dip coating or spray coating has proven to be an effective method to enhance the performance of pressure sensors.³⁹ For example, Ding et al. developed a conductive sponge PEDOT:PSS@molecular sieve (MS) by simple dip coating.⁴⁰ The main preparation process is to cut a piece of MS into different shapes. The samples are then immersed in the PEDOT:PSS aqueous dispersion, then the excess PEDOT:PSS dispersion in the sponges is squeezed out and finally the PEDOT:PSS-coated sponges are dried to obtain the conductive PEDOT:PSS@MS sponges (Figure 3A). Compared to pristine MS, the conductive PEDOT:PSS@MS exhibits a piezoresistive response up to 80% compressive strain and exhibits good reproducibility over 1000 cycles. And the conductive PEDOT:PSS@MS exhibits higher electrical conductivity, possibly due to the intertwined network structure of the conductive sponge. Using the properties of this system to monitor the bending motions of the body. When the body bends rapidly, the pressure sensor also responds quickly. In addition, the 3D porous structure has been shown to be an effective way to increase the gel pressure. For example, Zhang et al. assembled a 3D porous Ti₃C₂T_xMXene/PEDOT:PSS composite aerogel-type pressure sensor using a copper-assisted electrogelling method (Figure 3B).⁴¹ After adjusting the ratio of MXene/PEDOT:PSS, the sensitivity of the 3D sensor can reach 26.65 kPa⁻¹ and the response ability can reach 106 ms. Based on these advantages of pressure sensor, which can be attached to the fingertips of robots as artificial tactile interfaces, with potential applications in monitoring human physiological signals (Figure 3C).

FIGURE 3. (A) Schematic diagram of the preparation of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)@molecular sieve (MS).40 (B) Structural principles of MXene/PEDOT:PSS composite aerogel (MPCA).41 (C) Application of pressure sensor based on MPCA.41 Reproduced with permission from Refs.40,41 Copyright 2018, 2022, American Chemical Society

The above scheme shows advantages of simplicity and straightforwardness, while, the PEDOT coating could be peeled off from the substrate, resulting in poor stability. The new materials, such as new substrates and dopants, need to be explored to enhance interface bonding force.

PEDOT fiber-based pressure sensor

PEDOT fiber-based materials have received certain attention in the field of piezoresistive sensors due to their simplicity, low cost, and easy operation. At present, the method to increase the performance of PEDOT fiber-based pressure sensors requires the construction of various microstructures, porous structures, and nano-network structures in addition to selecting suitable elastic substrates.^42,43 Adding nanomaterials with certain microstructures to fabrics can improve the sensitivity of PEDOT fiber-based pressure sensors.

Chen et al. formed a bi-layered conductive fabric by stacking PEDOT nanowires (NWs) and cellulose nanofibers (CNF) on PEDOT:PSS fabric as a realization of pressure sensing for wearable devices for medical health monitoring systems (15.78 kPa⁻¹).⁴³ The preparation process is to first immerse the PEDOT fabric in FeCl₃ solution, and then modify the fabric surface with sodium dodecylbenzenesulfonate and deposit PEDOT NWs and CNF on the fabric surface, and finally use deionized water and anhydrous ethanol to clean the surface of the fabric and vacuum dry (Figure 4A). In contrast, Zhou et al. fabricated a flexible pressure sensor by a simple electrospinning technique. The sensor is composed of a composite nanofibrous membrane obtained by simultaneous spinning of PEDOT:PSS and polyamide 6 (PA6) (Figure 4B), which exhibits high sensitivity of up to 6554.6 kPa⁻¹, and a fast response time of 53 ms. The achievements are possibly due to the interweaving of the two fibers to form a multilayer network structure, abundant contact points were formed during the loading process. PEDOT:PSS/PA6 composite fiber films can be used as large-area sensor arrays for pressure space sensing (Figure 4C). By controlling the electrospinning time, flexible electrodes with different sensing properties can be realized, which can help treat diseases in patients with disabilities by tracking human movement (Figure 4D,E). It has great potential in health monitoring of weak physiological signals and body joint movements.⁴⁴

FIGURE 4. (A) Schematic for the preparation of the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/PEDOT nanowires (NWs)/cellulose nanofibers (CNF)–fabric (PPCF).43 (B) Morphological structure and sensing mechanism of the piezoresistive pressure sensor.44 (C) Schematic diagram of the flexible sensor array.44 (D) Applications of pressure sensors.44 (E) Practical application of the pressure sensors in health monitoring.44 (F) Molecular structure of PEDOT:PSS.47 (G) Variable range hopping (VRH) distance and carrier mobility of PEDOT:PSS films with nitrogen plasma treatment times.47 Reproduced with permission from Refs.43,44,47 Copyright 2016, 2020, 2022, Royal Society of Chemistry, American Chemical Society

The piezoresistive sensing mechanism of conductive nanofiber materials is based on the change in electrical resistance caused by the change in the contact area between conductors during compressive deformation. However, piezoresistive sensors fabricated from conventional electrospun fibers have low porosity and less resistance change with changes in ambient pressure, resulting in lower pressure sensor performance. Therefore, further exploration of practical applications based on electrospun fibers is required to improve their mechanical properties.

PEDOT film-based pressure sensor

Due to the biocompatibility of PEDOT-based films, it can be applied to the development of microelectrodes for biological interfaces,⁴⁶ and can also be used as scaffolds to control the proliferation of biological cells. At present, strategies such as vapor deposition, in situ polymerization, and impregnation are mainly used to form PEDOT:PSS films on fabrics to improve the electrical properties of pressure sensors.

The researchers reported that the piezoresistive sensing performance was enhanced by nitrogen plasma treatment of PEDOT:PSS micelles to modify PEDOT oligomers and PSS chains. For example, Wang et al. modified the surface of PEDOT:PSS film by nitrogen plasma for 3 min. Compared with the original film, the lowest resistance of the PEDOT:PSS pressure sensor was reduced, and the piezoresistive sensitivity and responsivity were significantly improved.⁴⁷ The nitrogen fixation plasma modification induces significant changes in the material properties of PEDOT:PSS micelles (Figure 4F). The reason is that, the surface modification by nitrogen plasma leads to the decrease of carrier mobility and the increase of 3D variable range hopping (VRH) distance (Figure 4G). Subsequently, in order to increase the sensitivity of the sensor again, Wang et al. added GR oxide into PEDOT:PSS to make composite films, determined the horizontal position of GR oxide in the PEDOT:PSS copolymer, reduced the VRH distance, enhanced the carrier mobility, and improved the cell size. For a 0.2 cm device, the sensitivity of the PEDOT:PSS/Graphene Oxide (GO) composite thin film sample increased from 0.016 to 0.428 kPa⁻¹. Using computer-aided design simulation technology, a miniature pressure sensor based on PEDOT:PSS/GO composite film is proposed. The sensor can monitor brain pressure during intracranial surgery in rats, and has potential applications in the biomedical field.⁴⁸

However, the limited mechanical flexibility of PEDOT:PSS films limit its research in the field of pressure sensors, and there are many other structures worth exploring.⁴⁹ In addition, these conducting polymer films are insensitive to the relative conduction mechanism and unstable under high- or low-pressure conditions, which limits their practical applications.

Temperature sensor

Temperature, as a key factor affecting the efficiency of biological electron transport, changes continuously in time and space.^50,51 As one of the basic means of monitoring biological temperature changes, temperature sensors use the principle of sensor resistance changes with temperature to detect the local temperature changes of biological tissues accurately and sensitively to the greatest extent within the minimum sensing range. According to the temperature changes of biological tissues, it can diagnose the potential disease hidden dangers of organisms, and be used for wound healing and health monitoring.^52,53 Therefore, flexible and wearable temperature sensors have become a research hotspot in the field of bioelectronics.

PEDOT gel-based temperature sensor

In recent years, thermosensitive conductive gels have synergized the dual properties of conductive materials and gels to become a unique material in fields such as bioelectrodes, supercapacitors, and fuel cells.^54,55 PEDOT:PSS is a well-known conductive polymer with excellent electrical properties, which can be used as a temperature sensing electrode.

For more accurate sensing of biological signals, the sensor should be directly and stably attached to the skin surface.⁵⁶ This essentially requires flexible adhesion properties of biological temperature sensors. In order to solve this problem, some scholars have combined thermosensitive poly-N-isopropylacrylamide (PNIPAM) hydrogel, PEDOT:PSS and CNTs developed a novel adhesive that mimics the structure of octopus suckers as a flexible temperature sensor.⁵⁷ The preparation process includes the preparation of the mold for depositing the Ti film on the Si substrate and the preparation of temperature sensor with PNIPAM/PEDOT:PSS/CNT composite film as electrode (Figure 5A). Compared with the reported PEDOT:PSS/CNT-based temperature sensor (0.6%/°C), the addition of thermosensitive material (PNIPAM) enables the sensor to exhibit higher temperature sensitivity of 2.6%/°C in the range of 25°C–40°C. Figure (5B) shows the sensor resistance as a function of temperature. Without the help of other adhesives, the adhesion strength of the self-adhesive temperature sensor imitating the octopus suction cup structure can be as high as 13.4 kPa at 40°C. Based on this, the temperature sensor can be used continuously for at least 12 h. This research work greatly expands the application potential of temperature sensors in disease diagnosis, personalized medical diagnosis, and electronic skin.⁵⁸

FIGURE 5. (A) Fabrication process for temperature sensor.57 (B) Graph of relative resistance versus temperature.57 (C) Preparation process of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/waterborne polyurethane (WPU) composite fibers.65 (D) Fabrication processes of the PEDOT:PSS−polydimethyl siloxane (PDMS) temperature sensors.71 Reproduced with permission from Refs.57,65,71 Copyright 2018, 2020, 2021 Elsevier, American Chemical Society

In addition, in order to enhance the conductive function of the thermosensitive temperature sensor and make it suitable for bio-wearability, researchers tried to insert carbon-based materials into the microgels. However, due to the van der Waals attraction between the carbon materials, the polymer microgels is difficult to transfer electrons on carbon surfaces.⁵⁹ Later, inserting a conductive polymer (PEDOT:PSS) with better biocompatibility into microgels proved to be one of the effective ways to enhance the electrical sensing performance. The electron transfer properties of thermosensitive PNIPAM microgels were enhanced by conductive PEDOT modification by Mutharani et al. Due to the contracted and expanded structure of PNIPAM, the resulting conductive microgels exhibit thermally reversible transitions by controlling the internal temperature in the range of 20°C–40°C. In addition, by adjusting the solution temperature, the electrochemical process is effectively switched, enabling sensitive detection of the sensor.⁶⁰

Based on the above various studies on temperature sensors, it is found that the temperature change can be monitored by adding temperature-sensitive materials. However, the adhesion performance to the biological skin surface after the addition of temperature-sensitive materials is still worthy of improvement in order to be better applied to biological systems.

PEDOT fiber-based temperature sensor

PEDOT:PSS-based fibers are currently the most widely studied in the thermoelectric (TE) field due to their good environmental stability, processability, and high tunable electrical conductivity.^61,62

Müller and coworkers directly prepared PEDOT:PSS TE fibers by injecting PEDOT:PSS into aqueous sulfuric acid, which showed a power factor as high as 30 μW m⁻¹ K⁻². Sarabia-Riquelme et al. improved the power factor of the PEDOT:PSS fiber to 115 μW m⁻¹ K⁻² by introducing a drawing step.⁶³ However, Wen et al. first prepared PEDOT:PSS TE fibers by wet spinning followed by post-treatment with concentrated sulfuric acid increased the power factor to 147.8 μW m⁻¹ K⁻², which is 15 times that of its corresponding film.⁶⁴ Embedding PEDOT:PSS into an elastic polymer matrix for re-spinning has been shown to be a common approach to improve the tensile and TE properties of PEDOT:PSS-based fiber materials. Wen et al. proposed to use WPU to improve the TE properties of the resulting composite fibers.⁶⁵ Figure (5C) shows a schematic diagram of the wet spinning of PEDOT:PSS/WPU composite fibers. Under the optimal conditions, the power factor of PEDOT:PSS/WPU composite fiber was improved to 26.1 μW m⁻¹ K⁻², it shows some potential in the field of wearable bioelectronics.

PEDOT film-based temperature sensor

To detect the temperature in E-skin devices in real time, many types of flexible temperature sensors have been developed. Detecting temperature through resistance changes of sensitive materials is the most common method of temperature measurement in skin-like electronics.^66,67 Various temperature sensors have been reported using carbon-based materials (CNTs and GR) as sensitive materials.^68,69 Kuzubasoglu et al. used inks made by CNT and PEDOT:PSS, which were directly deposited on the binder polyamide to make temperature sensors.⁷⁰ Compared with the CNT-printed and PEDOT:PSS-printed sensors, the CNT/PEDOT:PSS composite ink-printed sensor has a resistance change of up to 0.3%, up to 1000 cycles, and higher sensitivity than conventional PEDOT:PSS. The proposed thermal expansion strategy has been proved to be a feasible solution to improve the temperature sensitivity of PEDOT-based conductive thin film materials. That is, it is attached to a thermally expanding substrate with a high thermal expansion coefficient, to enhance thermally induced deformation.⁷¹ This approach has been applied to some sensors made of carbon-based materials, relying on temperature fluctuations to generate strain, enhancing the response of the sensors to external temperature changes. Here, Yu et al. fabricated a temperature sensor by drop-casting an aqueous PEDOT:PSS solution onto a PDMS substrate, which provided a high temperature sensitivity of 0.042°C⁻¹. The main preparation process is to deposit PEDOT:PSS droplets on PDMS substrates followed by immersion in sulfuric acid solution. Then, after adding copper wire, another layer of PDMS is cast on top, and finally pre-stretched to obtain (Figure 5D). The PDMS substrate generates more microcracks with increasing temperature, improving sensor temperature sensitivity significantly.⁷² These newly developed sensors have good high stretchability and good biocompatibility, which provide certain application value for wearable bioelectronics.

Although the research and development of PEDOT thin film-based temperature sensors has been extensive, the development of sensors under specific conditions (high temperature, high pressure, acid–base conditions) is still a problem that we need to solve.

CONCLUSIONS AND PROSPECTS

PEDOT-based conductive materials can effectively solve the inherent rigid and brittle failure problems of traditional conductive materials, and enable good surface contact with biological systems, thus offering great potential in wearable applications and electronic monitoring. Therefore, the exploration of PEDOT has attracted more and more attention in recent years. This paper reviews the recent progress in the construction strategies and applications of different types of PEDOT-based conductive materials in various sensors. By developing different coping strategies, flexible wearable bioelectronic devices with the most applicability can be prepared for different application scenarios.^73,74 Benefiting from the remarkable mechanical change and adaptability, PEDOT:PSS shows great development potential, especially in monitoring of human body as well as medical monitoring therapy. In addition, PEDOT:PSS-based flexible composites have successfully developed sensors that work from changes in resistance caused by changes in microstructural stress, pressure, and temperature, demonstrating the potential for practical applications.^75–77

Meanwhile, there are still a series of unavoidable challenges for practical applications. First, as a biocompatible material, PEDOT:PSS-based functional materials need to exhibit sufficient ductility and toughness to accommodate the shape changes of soft tissue targets. Second, large-area PEDOT:PSS-based bioelectronics devices are relatively less and the devices could not monitor soft tissue at any designated location. Finally, the accurate and strong signals of devices still need to be improved. Currently, it is necessary to innovate the unique design principle of the sensor itself, which can simultaneously make relevant signal feedback according to the biological strain, pressure, and temperature. And seeking for some new materials to composite with PEDOT to improve electrochemical and mechanical properties. All in all, PEDOT-based conductive materials have excellent chemical, physical, and other properties, as well as their unique conductive properties and biocompatibility. Though the bioelectronics now present some challenges, they also demonstrate a sufficient space for the extension of PEDOT:PSS bioelectronic sensors.

ACKNOWLEDGMENTS

This work is supported by National Natural Science Foundation of China (grant no. 51902134), Zhejiang Public Welfare Technology Application Research Program (LGJ22B040001), Fundamental Research Funds of Jiaxing University (no. CDN70518005), and the Innovation Jiaxing·Elite leading plan 2020.

CONFLICT OF INTEREST

The authors declare no conflict of interest.