Advances in Conductive Polymer-Based Flexible Electronics for Multifunctional Applications

Abstract

The rapid developments in conductive polymers with flexible electronics over the past years have generated noteworthy attention among researchers and entrepreneurs. Conductive polymers have the distinctive capacity to conduct electricity while still maintaining the lightweight, flexible, and versatile characteristics of polymers. They are crucial for the creation of flexible electronics or gadgets that can stretch, bend, and adapt to different surfaces have sparked momentous interest in electronics, energy storage, sensors, smart textiles, and biomedical applications. This review article offers a comprehensive overview of recent advancements in conductive polymers over the last 15 years, including a bibliometric analysis. The properties of conductive polymers are summarized. Additionally, the fabrication processes of conductive polymer-based materials are discussed, including vacuum filtering, hydrothermal synthesis, spray coating, electrospinning, in situ polymerization, and electrochemical polymerization. The techniques have been presented along with their advantages and limitations. The multifunctional applications of conductive polymers are also discussed, including their roles in energy storage and conversion (e.g., supercapacitors, lithium-ion batteries (LIBs), and sodium-ion batteries (SIBs)), as well as in organic light-emitting diodes (OLEDs), organic solar cells (OSCs), conductive textiles, healthcare monitoring, and sensors. Future scope and associated challenges have also been mentioned for further development in this field.

1. Introduction

Over the last decade, research in advanced polymers has gained great interest from researchers and industrialists [1,2]. Among them, conductive polymers are an intriguing blend of materials science and electronics, with distinctive features that might transform several sectors [3]. These polymers, usually organic molecules, display electrical conductivity while retaining the natural flexibility and processability of conventional polymers. Flexible electronics are electrical devices or circuits that are created to be capable of bending, stretching, or conforming to different surfaces [3,4]. Conductive polymers are essential for advancing flexible electronics because they provide a unique blend of electrical conductivity and mechanical flexibility. A conductive polymer used in flexible electronics is a substance that combines electrical conductivity with mechanical flexibility, making it ideal for flexible electronic devices [4]. This enables diverse applications satisfying the day-to-day life demands. These polymers consist of organic molecules organized in a polymer chain structure, enabling the flow of charge inside the material. These conductive polymers are used to produce flexible conductive polymer composites (CPCs), which are composed of conductive fillers, such as graphene, carbon nanotubes, metal oxides nanoparticles, other nanomaterials, etc. The CPCs showed the ability to be stretched or bent, and the manipulation of twisted materials has considerable promise for several applications in the fields of tactile sensing, healthcare, human–machine interaction, and soft robotics [1,5]. CPCs have attracted significant interest due to their enhanced electrochemical performance, ease of synthesis, excellent biocompatibility, large surface area, high electrical conductivity, improved carrier transport, and distinctive optical properties. As a result, they are increasingly used in the fabrication of various flexible electronics, as their superior physicochemical properties enable broad applications in energy storage and conversion systems such as supercapacitors, lithium-ion batteries (LIBs), and sodium-ion batteries (SIBs), as well as in organic light-emitting diodes (OLED), organic solar cells (OSCs), sensors, wearable textiles, and health monitoring systems.

In 1977, Hideki Shirakawa et al. announced the successful creation of polyacetylene (PA) with outstanding electrical characteristics, marking the beginning of research into conductive polymers [2]. Numerous intrinsically conductive polymers (ICPs), such as polythiophene (PTh), polypyrrole (PPy), and polyaniline (PANI), have been synthesized by researchers that are being utilized in developing flexible electronics [6]. For a flexible electronic device to function properly, it has to have mechanically elastic active materials and electrodes. Thin metals may exhibit remarkable mechanical flexibility, but their Young’s modulus is much greater than that of elastomers often employed as substrates for flexible electronic devices. The primary method of manufacturing flexible electronic devices involves the integration of inorganic or organic nanomaterials, such as metal oxides, metal nanoparticles/nanowires, carbon materials, and conductive polymers, into flexible substrates [7]. However, the use of these materials in complex stress scenarios is limited as a result of their insufficient tensile properties and durability [8,9,10]. This can be resolved by conductive hydrogels (CHs) that are now under extensive investigation as versatile conductors for electrical devices because to their inherent flexibility, exceptional tensile strength, and compatibility with biological systems. Conductive polymers also produces conductive hydrogels, called conductive polymeric hydrogels (CPHs) that conduct electricity, as they are made up of conducting polymers such as PANI, PPy, and poly (3, 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT:PSS). CPHs may be extensively adjusted in terms of electrical characteristics, mechanical properties, and functionality based on design and preparation procedures, making them crucial in flexible electronic systems.

This review begins by presenting the methodology of bibliometric analysis, followed by a detailed and rational discussion of the findings. It explores various intrinsic properties of CPs, including their electrical conductivity, magnetic characteristics, optical behavior, wettability, mechanical properties, microwave-absorbing capabilities, electron percolation networks, frequency domain conductivity spectra, and the effects of multiple (accelerated) aging conditions. Additionally, the review highlights key fabrication techniques for conductive polymer-based textiles, encompassing methods such as chemical vapor deposition, in situ polymerization, electrochemical polymerization, dip coating, spray coating, electrospinning, vacuum filtration, hydrothermal synthesis, and screen printing. Subsequently, the diverse applications of CPs in flexible electronics are examined, including their roles in energy storage and conversion systems such as supercapacitors, lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), organic-light emitting diodes (OLEDs) [11], organic solar cells (OSCs) [12], and strain sensors for healthcare and food processing monitoring. Furthermore, the utility of electrically conductive and smart textiles in health, sports, and fitness applications is discussed. Finally, the review concludes by outlining several promising future directions in these fields.

2. Methodology of Bibliometric Analysis

For the bibliometric analysis, a comprehensive search strategy included the use of key terms, such as several conductive polymers, conductive textile, and flexible electronics. The keywords were chosen to guarantee a careful examination of pertinent research on the topic. From May 2010 to May 2024, a search was carried out to find the most recent advancements and trends in conductive polymers and textiles along with flexible electronic devices throughout a 15-year period. The Scopus database, which is well known for its extensive coverage of academic literature, was the main tool used to find relevant publications. Articles with the designated keywords in the keywords section, title, or abstract were filtered and assigned a projected inclusion score for the research.

A comprehensive examination of relevant articles published from 2010 to 2024 involved incorporating a range of keywords. The search for “conductive polymer” yielded 179,109 papers, with the majority (19,852 publications) completed in 2023. The second-highest number of publications occurred in 2022, totaling 18,746. In contrast, 2010 had the fewest publications, with only 2183. This trend highlights the growing interest in this field over time. Figure 1a demonstrates a steady increase in publications over the past 15 years, particularly in research and review articles. A total of 126,881 research articles were published during this period. Figure 1b reveals that the most prominent fields of study include materials science, chemistry, and chemical engineering, with engineering also contributing significantly, accounting for 28,335 publications. The search for “conductive textile” yielded 23,043 papers, with 3236 publications predominantly completed in 2023. The second-highest number of publications, 3141, occurred in 2022, while 2010 had the fewest, with only 255 publications. This trend indicates a growing interest in this field over time. Over the past 15 years, the number of publications has steadily increased, particularly in research and review papers, as illustrated in Figure 1c. The most popular areas of research include materials science, chemistry, and engineering, as shown in Figure 1d. In contrast, decision sciences accounted for only 335 publications, indicating a relatively low level of activity in this domain.

A total of 163,427 papers were identified when searching for “flexible electronics.” The majority of the 15,080 publications were completed in 2023, followed by 14,136 publications in 2022. The year 2010 recorded the fewest publications, with only 2960. Figure 1e illustrates the number of publications over the past fifteen years, while Figure 1f highlights the most prominent fields, including engineering, materials science, and physics and astronomy. Additionally, chemistry and computer science have made significant contributions. In contrast, the fields of business, management, and accounting account for only 335 articles.

3. Properties of Conductive Polymers

Conductive polymers (CPs), such as Polyacetylene (PA), Polypyrrole (PPy), Polyaniline (PANI), Polythiophene (PTh), and Polyfluorenes (PFs), offer unique properties that make them ideal candidates for various electronic and electrochemical applications [13]. The application of various conductive polymers are summarized in Table 1. PA, the first discovered conductive polymer, is known for its high conductivity when doped, but its poor mechanical properties limit its use [14,15]. In contrast, PPy exhibits good conductivity, stability, and processability, making it widely utilized in sensors and capacitors. PANI is particularly notable for its tunable conductivity, environmental stability, and ease of synthesis, which has led to its use in energy storage devices and sensors. PTh and its derivatives are also valued for their high electrical conductivity, environmental stability, and versatility in organic electronics, including light-emitting diodes (OLEDs) and solar cells. Blends of polyaniline with other polymers, such as Poly(aniline-co-oxaniline), enhance the properties of the base material by improving mechanical strength, flexibility, and conductivity. Polyfluorenes (PFs) are characterized by their high luminescence and are often used in organic semiconductors and optoelectronic devices. These CPs can be engineered to improve conductivity, flexibility, and mechanical properties, positioning them for diverse applications in flexible electronics, sensors, energy storage, and conversion technologies. Various conductive polymers exist, each with distinct characteristics and uses. Common types include:

Polyacetylene (PA):

PA holds a significant place in the history of conductive polymers, as it was the first to be identified [16]. Known for its remarkable electrical conductivity, particularly when doped with various materials, PA has garnered considerable attention in the field of organic electronics. Its unique properties enable it to serve as an effective conductor, making it suitable for a variety of applications. Extensive research has focused on PA’s potential in organic solar cells, where it can facilitate efficient charge transport and enhance energy conversion rates. Additionally, its use in organic field-effect transistors (OFETs) is well-documented, where PA contributes to improved device performance. The ability to manipulate PA’s conductivity through doping processes allows researchers to tailor its electrical characteristics for specific applications, leading to innovations in flexible and lightweight electronic devices. Furthermore, PA’s ease of processing and integration into different substrates makes it an attractive choice for next-generation electronics. As research continues to explore its capabilities, PA remains a cornerstone in the development of organic electronic components, paving the way for more advanced technologies in the field. Its ongoing evolution reflects the broader potential of conductive polymers in shaping the future of electronics.

Polypyrrole (PPy):

PPy is a highly researched conductive polymer celebrated for its outstanding electrical conductivity and environmental resilience [17]. Its ability to maintain performance under various conditions makes it an ideal candidate for numerous applications in modern technology. One of the key advantages of PPy is its straightforward production process, which allows for cost-effective scaling and integration into various devices. PPy is widely employed in antistatic coatings, effectively preventing the buildup of static electricity in sensitive electronic components and materials. Its use extends to sensors, where its conductive properties enable precise detection of environmental changes, making it invaluable in applications ranging from chemical sensing to biosensing. Moreover, PPy plays a significant role in energy storage technologies, including batteries and supercapacitors, where it contributes to improved charge capacity and efficiency. The versatility of PPy, combined with its impressive electrical performance, positions it as a key material in the development of advanced electronic devices and systems. As research continues to advance, PPy’s potential applications are expected to expand, further solidifying its status as a leading conductive polymer in the field.

Polyaniline (PANI):

PANI is a well-researched conductive polymer renowned for its exceptional conductivity, ease of processing, and remarkable environmental durability [18]. Its unique properties make it a versatile material, suitable for a wide range of applications across various industries. One of the standout features of PANI is its ability to maintain conductivity in diverse environmental conditions, making it particularly valuable for applications requiring resilience against moisture, temperature fluctuations, and corrosive elements. PANI is extensively utilized in corrosion prevention, where its conductive nature forms a protective layer that inhibits rust and degradation of metals, prolonging their lifespan. Additionally, PANI is integral to the development of sensors and actuators, leveraging its sensitivity to changes in the environment for precise detection and response capabilities. In energy storage devices, PANI enhances charge capacity and efficiency, contributing to the advancement of batteries and supercapacitors. The combination of high conductivity, processability, and environmental stability positions PANI as a leading candidate in the field of conductive polymers. Ongoing research and innovation continue to uncover new applications for PANI, reinforcing its significance in modern technology and materials science.

Polythiophene (PTh):

PThs are a notable class of conductive polymers characterized by their high electrical conductivity, optical transparency, and impressive thermal stability [19]. These unique properties make PThs highly sought after in various applications, particularly in organic photovoltaics, light-emitting diodes (LEDs), and display technologies. They are perfect materials for cutting-edge electrical equipment where functionality and esthetics must coexist because of their ability to transmit energy effectively while maintaining transparency. Among the various derivatives of PTh, Poly(3,4-ethylenedioxythiophene) (PEDOT) stands out for its exceptional electrical conductivity, optical clarity, and flexibility. This makes PTh-PEDOT particularly valuable in the development of transparent conductive coatings, which are essential for applications in touchscreens and solar cells. Moreover, its compatibility with organic electrodes enhances the performance of various electrical devices. In the biomedical field, PTh-PEDOT is gaining attention for its potential use in biocompatible devices, where its conductive properties can be harnessed for applications such as biosensors and neural interfaces. The ongoing research and development of PThs underscore their significance in advancing next-generation electronics and energy solutions.

Polyaniline/polyaniline blends:

Polyaniline (PANI) blends with various polymers have emerged as versatile materials that enhance mechanical properties, processability, and stability without sacrificing electrical conductivity [20]. These blends effectively combine the advantageous features of PANI with those of other polymers, resulting in composites that are not only conductive but also exhibit improved flexibility and durability. This makes them particularly suitable for a wide range of applications, including flexible electronics, where adaptability and resilience are crucial. Additionally, PANI blends are increasingly utilized in the development of advanced sensors, offering enhanced sensitivity and performance in detecting various environmental stimuli. Their unique combination of properties also makes them effective for electromagnetic interference (EMI) shielding, protecting sensitive electronic devices from external electromagnetic radiation. The ability to tailor the mechanical and electrical characteristics of PANI blends through careful selection of polymer partners broadens their applicability, paving the way for innovative solutions in electronics, sensor technology, and EMI protection. Ongoing research into optimizing these blends continues to reveal new possibilities for their use in cutting-edge applications.

Polyfluorenes (PFs):

PFs are a class of conjugated polymers distinguished by their remarkable photoluminescence efficiency and relatively low electrical conductivity. This unique combination of properties makes them particularly valuable in optoelectronic devices, such as organic light-emitting diodes (OLEDs) and organic photovoltaics, where their ability to emit light efficiently is crucial for performance [21,22]. The photoluminescent properties of PFs allow for vibrant displays and energy-harvesting applications, contributing significantly to the development of next-generation electronic devices. Beyond their applications in OLEDs and photovoltaics, PFs are at the center of extensive research to enhance their conductivity and broaden their functionality. Scientists are exploring various modifications and derivatives of PFs to improve their electrical properties while maintaining their photonic characteristics. This ongoing investigation not only aims to expand their use in traditional electronic applications but also seeks to unlock new potentials in fields such as energy storage, sensing technologies, and biomedical engineering. As research progresses, PFs are set to play a crucial role in the evolution of advanced materials for diverse technological applications.

Table 1

Summarized the application of different conductive polymers.

Conducting Polymer	Electrical Conductivity (S/cm)	Applications	Ref.
Polyacetylene (PA)	4.4 × 10−5	Organic electronics: OLEDs, organic photovoltaics, organic field-effect transistors Sensors: Environment monitoring, healthcare diagnostics, industrial process control Smart textiles: Wearable electronics, military application	[23,24]
Polypyrrole (PPy)	1 × 10−8	Antistatic coatings Sensors Batteries Supercapacitors	[24,25]
Polyaniline (PANI)	1 × 10−7	Prevention of corrosion Actuators Energy storage devices	[20]
Polythiophene (PTh)	1 × 10−7	Transparent conductive coatings Organic electrodes Biomedical devices Electrical devices	[16]
PANI/PANI blends	1.9 × 10−4	Flexible electronics Electromagnetic interference shielding Mechanical characteristics	[20]
Polyfluorenes (PFs)	1 × 10−10	Biosensors Photonic device Conductive coating and film Organic photovoltaics OLEDs	[21,26]
Polyparaphenylene (PPV)	1 × 10−12	Photovoltaic cells Sensors OLEDs	[19,24]

Over the last decade, nanotechnology has gained prominence as a field of research owing to its considerable potential for many applications [27]. There are four primary types of nanostructures. Zero-dimensional (0D) structures differ from one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) structures, each of which has distinct spatial characteristics. The unique physical, chemical, and electrical characteristics of one-dimensional nanostructures have attracted significant interest, especially in the context of nanoscale systems. The device function of a 1D-nanostructure is a significant characteristic that enables its use as components in many kinds of nanodevices [28,29,30]. Considerable progress has been achieved in the field of one-dimensional nanostructures at the nanoscale and molecular-scale characteristics that might meet the requirements of modern society in the 21st century. The aforementioned examples include carbon nanotubes, inorganic semiconducting, and metallic nanowires, as well as conjugated polymer nanotubes [28,31]. Nanostructures have several potential applications in the fields of nanoelectronics, molecular electronics, nanodevices, nanocomposite materials, bio-nanotechnology, and medicine [28,31,32]. Conducting polymers such as PA, PANI, PPy, poly(p-phenylenevinylene), PEDOT, polyfuran, and other PTh derivatives are highly sought after in the fields of nanoscience and nanotechnology because of their distinctive characteristics, including conductivity, reversible doping–dedoping process, controllable chemical and electrochemical properties, and ease of processing [5,33,34,35]. Figure 2 demonstrates the chemical structure of the mentioned polymers.

Conductive polymers possess the ability to conduct electricity while also exhibiting remarkable electrical, magnetic, wetting, optical, mechanical, and microwave-absorbing properties. Figure 3 illustrates the characteristics of different conductive polymers.

3.1. Electrical-Conducting Properties

Doping enables conducting polymers to attain metallic conductivity, significantly enhancing their electrical performance [32,36]. Notably, the electrical conductivity of a single nanofiber can exceed that of traditional materials, such as particle nanotubes or nanowires, by one or two orders of magnitude. This remarkable property makes nanofibers particularly valuable in various applications, including flexible electronics, sensors, and energy storage devices, where high conductivity is essential. The unique characteristics of doped conducting polymers allow for innovative designs and improved functionality in advanced technological applications. Chen and colleagues investigated the electrical conductivity of PANI nanotubes and observed a two-order-of-magnitude increase in the conductivity of a single nanotube. The addition of an insulating component to 1D-conducting polymer nanoparticles often leads to a decrease in electrical conductivity since the insulating component partially obstructs the conductive pathway [37]. The research findings indicated that the resistivity of composite nanowire pellets consisting of beta-napthalene sulfonic acid (NSA)-doped PANI/Fe₃O₄ had typical semiconducting characteristics, with an increase seen as the temperature fell. The decreased conductivity observed may be attributed to the increased dispersion of charge carriers inside the NSA-doped PANI/Fe₃O₄ nanoparticles. Several kinds of 1D-conducting polymer nanocomposite systems exhibited a comparable decrease in electrical conductivity [38,39,40]. One potential approach to enhance the conductivity of nanocomposites is the incorporation of a nanocomponent with elevated electrical conductivity into conducting polymers [37]. The electrical conductivities of CNT/PANI composite nanocables were investigated, revealing a positive correlation between the loading of CNTs and the conductivity of pure PANI. The addition of carbon nanotubes to conducting polymers has the potential to serve as a conducting link, hence enhancing electrical conductivity [41,42,43]. The conductivity of CNT/PANI nanocomposites exhibited a decrease with decreasing temperature, so exemplifying the characteristic behavior of semiconductors [44]. Composite nanocables composed of carbon nanotubes and PPy have shown similar results [37]. The potential enhancement of electrical conductivity in conducting polymers may be achieved by the incorporation of metal nanoparticles [45]. The electrical conductivity of single Au/PANI nanocables is much higher in comparison to that of a single CSA-doped PANI nanotube [37,46,47]. Functionalized PEDOT nanostructures were generated by the process of template-free electropolymerization on an indium-tin-oxide substrate. The results provided a clearer understanding of the relationship between the functional group, nanostructures, and electrical properties.

3.2. Magnetic Properties

Significant research has been devoted to understanding the magnetic properties of conducting polymers, as these properties are crucial for revealing insights into charge-carrying entities and unpaired spins [48,49,50]. The unique magnetic behavior of these polymers stems from their conjugated structures, which facilitate the delocalization of electrons. This delocalization can lead to the presence of unpaired spins, influencing the material’s conductivity and overall electronic properties. By studying these magnetic characteristics, researchers can better comprehend the mechanisms underlying charge transport in conducting polymers, paving the way for improved applications in fields such as spintronics, sensors, and advanced electronic devices. The magnetic properties of PANI/Fe₃O₄ composite nanotubes synthesized using ultrasonic irradiation were investigated [51]. The magnetic properties of PANI/Fe₃O₄ composite nanorods were investigated by the use of a self-assembly technique. The use of an ultrasonic irradiation method enhances the dispersion of Fe₃O₄ particles more efficiently compared to those formed by self-assembly. The superparamagnetic characteristics were observed in the nanotubes generated using ultrasonic irradiation. Similar investigations have been conducted on the magnetic properties of three specific conductive polymeric composites: PPy composites decorated with magnetic crystals, PANI nanotubes encapsulating cobalt nanowires, and PANI nanotubes enveloping nickel nanowires [52,53,54].

3.3. Optical Properties

The unique optical properties of conducting polymers have garnered significant attention due to their potential applications in nanophotonic devices. These polymers, particularly when formed into one-dimensional nanostructures, are highly suitable for fabricating advanced components such as photodetectors, photochemical sensors, and photonic wire lasers [55,56,57]. Their ability to efficiently absorb and emit light enhances the performance of these devices, making them critical for a wide range of applications in optoelectronics and sensing technologies. As research in this field advances, conducting polymers are poised to play a pivotal role in the development of innovative nanophotonic solutions. The optical properties of CdS/PANI composite nanocables were investigated, and the photoluminescence spectrum exhibited a strong resemblance to that of CdS nanowires, characterized by heightened signal intensities. The observed enhancement may be attributed to the migration of carriers generated by the light inside the PANI layer towards the CdS nanowires. Turac et al. demonstrated the synthesis of a novel polythiophene (PTh) derivative through electrochemical oxidative polymerization of 2,5-di(thiophen-2-yl)-1-(4-(thiophen-3-yl)phenyl)-1H-pyrrole. Here, researchers conducted an analysis on the optical contrast, switching time, k_max, and band gap of the chemical that was synthesized [58].

3.4. Wettability

Wettability is a crucial property that influences various applications, including self-cleaning surfaces, microfluidics, controlled drug delivery, and bio-separation [59]. In this context, conducting polymers often demonstrate hydrophilic characteristics, making them highly suitable for enhancing surface interactions. Their ability to attract and retain water can improve adhesion, facilitate fluid flow, and optimize drug release profiles in biomedical applications. By manipulating the wettability of conducting polymers, researchers can design advanced materials with tailored properties for specific functions, paving the way for innovative solutions in diverse fields such as healthcare, environmental remediation, and material science. Conducting polymers often exhibit hydrophilic properties [59]. Hydrophobic acids may be doped to make conducting polymers with superhydrophobic properties [60,61]. The creation of a surface capable of transitioning between superhydrophobic and superhydrophilic qualities may be achieved by the manipulation of the chemical composition of conducting polymers [62]. The wettability of films composed of PAN/PANI coaxial nanofibers has shown dual-responsive chemical characteristics [63]. Upon the application of a single-layered photopolymerized nanocomposite film comprising polystyrene and TiO₂ nanorods over surfaces exhibiting diminishing hydrophilicity, a notable shift occurs from a hydrophobic state to a hydrophilic one [64].

3.5. Mechanical Properties

Recent research has increasingly focused on the mechanical properties of individual nanotubes, revealing their remarkable strength and flexibility [64,65]. These unique characteristics make nanotubes highly desirable for various applications, including composite materials, nanotechnology, and electronics. Their exceptional tensile strength, often surpassing that of steel, combined with their lightweight and high flexibility properties, allows for innovative uses in enhancing the mechanical performance of materials. As scientists continue to explore and understand these properties, single nanotubes are poised to play a crucial role in advancing the development of stronger, lighter, and more durable materials across numerous fields, including aerospace, automotive, and biomedical engineering. In order to ascertain the elastic tensile modulus of PPy nanotubes, force–curve or resonance–frequency experiments were performed. A notable increase in the elastic modulus was seen when the thickness of the PPy nanotube wall or its outer diameter was reduced. Similar mechanical properties that vary with size have been reported in several individual nanofibers [66,67]. Inorganic nanowires, such as CuO, silicon, silver, and lead nanowires, have shown a comparable mechanical behavior that is dependent on their size [68,69,70].

3.6. Microwave-Absorbing Properties

The exploration of conducting polymers as novel microwave absorption materials is driven by their advantageous properties, such as low density and ease of fabrication [69]. These polymers can effectively absorb microwave radiation, making them suitable for applications in electromagnetic interference (EMI) shielding and stealth technology. Their lightweight nature enhances their usability in various sectors, including aerospace and automotive industries, where weight reduction is critical. Additionally, the simple production processes involved in creating conducting polymers facilitate their integration into existing manufacturing systems, paving the way for the development of efficient and effective microwave absorption solutions. This combination of properties positions conducting polymers as promising candidates for advanced materials in modern technology. The electromagnetic loss characteristics of PANI-NSA and PANI-NSA/glucose micro-nanotubes, when synthesized without a template, are exceptional. The research conducted revealed that PANI doped with a fiber-like morphology has an enhanced ability to absorb electromagnetic waves in comparison to PANI with a particle-like morphology. Based on the aforementioned findings, it can be inferred that conducting polymer nanotubes has the potential to function as effective microwave absorbers owing to their notable absorption capability, wide frequency range, and lightweight nature.

3.7. Electron Percolation Network

The electron percolation network in conductive polymers (CPs) refers to the interconnected pathway of conductive sites that allow electrons to travel through the polymer matrix [15,71,72]. In CPs, the percolation threshold is achieved when the material’s conductive domains form a continuous network, enabling efficient electron transport. This network is crucial for enhancing the electrical conductivity and electrochemical performance of CPs, especially in applications like sensors, energy storage, and flexible electronics. Various factors, such as the doping level, polymer morphology, and the incorporation of nanomaterials, can significantly influence the percolation behavior and, consequently, the material’s conductive properties. Combinations of electrically conducting and non-conducting materials can exhibit electrical percolation. Here, electrical conductivity is created in systems having randomly distributed elements [73]. Conventional rigid circuits cannot adapt to the dynamic forms and movements of their applications; this is essential in flexible electronics such as wearable devices, flexible displays, and smart textiles. Wu et al. worked on a polydimethylsiloxane-based nanocomposite for the use of flexible electronics. Here, a continuous conductive layers network was formed within the insulating matrix [74]. Biological semiconductors (BSCs) are multi-layered, three-dimensional networks of carbon nanotubes (SWNTs) functionalized with antibodies, as developed by Bruck et al. By monitoring variations in electrical resistance, these networks identify biological interactions like antigen–antibody binding [73]. Another study developed a flexible electrolyte fuel cell by using a polydimethylsiloxane-coated Ag nanowire network for the current-collecting layer [75].

3.8. Frequency Domain Conductivity Spectra

The frequency domain conductivity spectra assess any material’s electrical conductivity variation with frequency. These spectra are used to analyze the electrical properties of conductive polymers (CPs) by examining their response to alternating current (AC) at varying frequencies [76,77,78]. This technique provides valuable insights into the dynamic behavior of CPs, including charge transport mechanisms, dielectric properties, and conductive network interactions. In CPs, the frequency domain spectra reveal information about the relaxation time, impedance characteristics, and conductivity in different frequency ranges, helping to optimize the materials for applications in sensors, energy storage, and flexible electronics. The spectra also provide a means to study the effects of doping, morphology, and environmental conditions on the CPs’ conductivity behavior. Important characteristics including charge transport methods, relaxation phenomena, and the impact of bending on ohmic and Faradaic resistances are all shown by these spectra. Frequency domain conductivity spectra are necessary for flexible electronics because they exhibit how electrical characteristics, such charge transport and dielectric behavior, change with frequency, offering information about device efficiency and material performance. They help assess the impact on conductivity by bending or mechanical stress, making materials and systems to be more dependable and stable functioning. Furthermore, as shown by the improved performance of the flexible fuel cell in this study, these spectra are crucial for building flexible devices with enhanced functionality because they correlate structural influences, such as compressive forces, with electrical performance [79]. In order to model the frequency-dependent conductivity and permittivity of CNT-polymer-nanoparticle nanocomposites, Murugappan et al. created a multi-scale effective-medium theory. It demonstrated that conductivity rises and permittivity falls with frequency, with ideal nanoparticle loading improving electrical properties prior to negative effects [80].

3.9. Multiple (Accelerated) Aging Conditions/Effects

Testing materials or devices to a variety of harsher environmental conditions, such as high temperatures, high humidity, UV rays, or mechanical stress, is known as multiple (accelerated) aging conditions/effects. This is performed to simulate long-term wear and forecast the devices’ performance and durability over time. In order to optimize materials for prolonged reliability in real-world settings, these studies are essential for identifying degradation mechanisms including oxidation, polymer chain scission, or electrical property deterioration [81]. Multiple (accelerated) aging conditions are crucial for evaluating the long-term stability and performance of conductive polymers (CPs) in real-world applications [82,83,84]. These tests, which simulate environmental stressors such as high humidity, temperature fluctuations, UV exposure, and mechanical wear, help assess how CPs degrade over time. Under accelerated aging, the conductivity, mechanical flexibility, and chemical stability of CPs can significantly degrade, often due to oxidation, environmental contamination, or structural changes. These effects are studied to optimize material formulations for reliable performance in flexible electronics, energy storage, and sensors under harsh conditions. Flexible electronics must withstand a variety of (accelerated) aging conditions and effects in order to maintain their dependability, robustness, and functionality in real-world scenarios. In addition to environmental elements like heat, humidity, UV rays, and mechanical wear, flexible electronics are subject to dynamic stressors like bending, folding, and stretching [85]. Momber et al. conducted a series of tests to assess the performance of a dual-layer thin-film coating, consisting of a polyamide primer and a MoS₂/PTFE topcoat, under simulated Arctic offshore conditions. These tests included accelerated aging, adhesion, abrasion, and frost accretion evaluations. Additionally, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDX) were employed, alongside surface energy studies, to thoroughly examine the coating’s structural integrity and material behavior. In order to evaluate changes in surface morphology, chemistry, and frost accretion behavior, the accelerated aging settings act like harsh Arctic offshore environments, such as high cold, corrosion exposure, and mechanical wear [86].

4. Fabrication Techniques for Conductive Polymer-Based Textiles

Various techniques have been adopted to fabricate conductive polymer-based textiles, including chemical vapor deposition (CVD), in situ polymerization, electrochemical polymerization, dip coating, spray coating, electrospinning, vacuum filtration, hydrothermal synthesis, and screen printing. Here, CVD offers precise control over the polymer layer’s thickness and uniformity, making it ideal for high-performance textiles. In situ polymerization and electrochemical polymerization allow for the integration of conductive polymers directly onto the fabric, enhancing conductivity while maintaining flexibility. Other methods, such as dip coating and spray coating, enable the uniform application of conductive materials, facilitating the creation of patterned designs for specific functionalities. Electrospinning generates fine fibers that enhance surface area and conductivity, while vacuum filtration ensures strong adhesion of conductive layers to porous textiles. Hydrothermal synthesis allows for the growth of nanostructures on textiles, further enhancing their electrical properties. Additionally, screen printing provides a scalable approach for applying conductive inks in intricate designs. Descriptions of each of these fabrication methods have been provided in this review, and Figure 4 illustrates these diverse techniques, highlighting their significance in developing innovative conductive textiles.

4.1. Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) is a highly effective technique for depositing thin films of conductive polymers onto textile substrates, offering unique advantages for wearable and flexible electronic applications [87]. In this process, polymers such as PANI, PPy, and PEDOT are synthesized through chemical reactions in the vapor phase, resulting in a uniform polymer coating on the textile surface. This method provides excellent control over the thickness and composition of the deposited film, which is crucial when working with textiles that require flexible, lightweight, and conformal coatings. CVD on textile substrates ensures a uniform distribution of the conductive polymer, allowing for consistent electrical properties across the fabric. This is especially beneficial in applications like wearable sensors, smart textiles, and e-textiles, where uniform conductivity is necessary for reliable device performance. The ability to fine-tune the thickness and surface morphology through CVD also enhances the flexibility of the textile, preserving its mechanical properties while imparting conductivity. Furthermore, this technique can create durable polymer composites with high conductivity and stable surface properties, making it ideal for integrating conductive polymers into fabrics without compromising comfort, flexibility, or durability.

4.2. In Situ Polymerization

In situ polymerization is a widely employed technique for integrating conductive polymers directly onto textile fibers, offering strong adhesion and enhanced durability, making it ideal for applications in wearable electronics and medical textiles [88]. This method involves the polymerization of conductive monomers, such as PANI or PPy, directly onto the surface of textile fibers, often facilitated by the use of initiators or catalysts. The strong bond formed between the polymer and the textile substrate ensures that the conductive coating remains intact, even under mechanical stress, making the textiles highly durable and resistant to wear. This approach is particularly advantageous for fabrics that require high mechanical strength, such as those used in smart clothing, flexible sensors, and medical devices where reliability and longevity are critical. However, precise control of reaction conditions, including monomer concentration, temperature, and catalyst activity, is essential to avoid potential issues such as non-uniform polymer coatings. Achieving a uniform conductive layer is key to ensuring consistent electrical performance throughout the fabric. Despite these challenges, in situ polymerization is a highly effective method for producing conductive textiles with excellent durability, flexibility, and functionality.

4.3. Electrochemical Polymerization

Electrochemical polymerization is an advanced method for applying conductive polymer layers onto textile substrates by using an electrical current to initiate and control the polymerization of monomers [89]. This technique offers precise control over both the thickness and morphology of the conductive polymer layer, making it especially suitable for applications that require fine tuning of the conductive properties, such as in textile-based sensors, flexible circuits, and smart garments. The process involves immersing the textile, which must be conductive or treated with a conductive coating, in a monomer solution, and applying an electrical current to trigger the polymerization directly onto the fabric surface. This enables the formation of uniform and well-adhered conductive films, which are essential for ensuring reliable electrical performance in flexible and wearable devices. However, precise control over electrochemical conditions—such as current density, voltage, and reaction time—is crucial to achieving optimal results and avoiding issues like uneven coating or over-deposition. Despite its limitation to conductive or pre-treated substrates, electrochemical polymerization provides an efficient way to tailor the properties of conductive textiles, offering great potential for the development of high-performance, flexible electronic textiles used in cutting-edge wearable technologies.

4.4. Dip Coating

Dip coating is a straightforward and cost-effective method used to apply conductive polymers onto textile substrates [90]. In this process, textiles are immersed into a conductive polymer solution, such as PPy or PEDOT, and then dried to form a conductive layer. This method is widely used for producing conductive textiles in applications where high precision and uniformity of the coating are less critical, such as in anti-static fabrics and basic electronic textiles. While dip coating is easy to implement and scalable for industrial use, one of its main challenges is achieving uniform coatings across the fabric surface. Variations in fabric porosity and uneven absorption of the polymer solution can result in non-uniformity, leading to inconsistent conductivity across the textile. Additionally, controlling the thickness of the polymer layer is difficult, particularly in large-scale applications, which may affect the overall performance in more sensitive or high-performance electronic textiles. Despite these challenges, dip coating remains a practical solution for many textile-based applications where simplicity and cost considerations outweigh the need for fine-tuned electrical properties. Its utility in producing functional fabrics for everyday electronics and anti-static applications highlights its relevance in the growing field of smart textiles.

4.5. Spray Coating

Spray coating is an advanced technique for applying conductive polymers onto textile substrates, offering greater control over the coating area and thickness compared to dip coating [91]. In this process, a solution of conductive polymer, such as PANI or PEDOT, is atomized and sprayed onto the fabric’s surface, allowing for targeted application and the creation of patterned conductive textiles. This method is particularly useful for large-scale production, as it enables efficient coverage of large fabric areas while maintaining the flexibility to coat specific sections for customizable designs in smart textiles. One of the key advantages of spray coating is its ability to fine-tune the thickness of the conductive layer, making it suitable for applications like wearable sensors, flexible circuits, and textile-based antennas. However, achieving a completely uniform coating across the fabric can still be challenging due to variations in the spray distribution and the porous nature of textiles. Overspray and material wastage are potential drawbacks of this method, which can lead to inefficient use of conductive polymer solutions, particularly in high-volume manufacturing processes. Despite these limitations, spray coating remains a valuable technique for producing functional, conductive textiles, especially when precise control over the coating pattern is required. Its application in wearable electronics and other smart textile innovations highlights its versatility and importance in the development of advanced textile technologies.

4.6. Electrospinning

Electrospinning is an innovative technique for producing fine fibers from a polymer solution under the influence of a high-voltage electric field [65,92]. When applied to textile substrates, electrospun fibers offer a unique combination of high surface area and flexibility. By incorporating conductive polymers such as PANI or PEDOT into the polymer solution, it is possible to create conductive nanofiber mats, making this method ideal for developing advanced textile applications like sensors, filtration systems, and energy storage devices. The electrospun fibers can be deposited onto textile substrates to enhance their conductivity, providing a lightweight and flexible structure without compromising the fabric’s original properties. The high surface area of these nanofibers makes them particularly useful in applications where efficient charge transfer, filtration efficiency, or sensitivity to external stimuli is required, such as in wearable electronics and environmental sensors. However, the method comes with certain challenges, including the difficulty in scaling up production due to the limited throughput of the process and the need for high-voltage equipment, which can raise safety concerns. Additionally, achieving precise control over the fiber diameter and uniformity across large areas can be difficult, impacting the overall performance of the electrospun fibers. Despite these limitations, electrospinning remains a promising technique for the creation of conductive nanofiber-based textiles, particularly in high-performance and niche applications.

4.7. Vacuum Filtration

Vacuum filtration is an effective method for depositing a uniform layer of conductive polymer onto porous textile substrates by filtering a polymer solution through the fabric under vacuum pressure [93]. This technique ensures strong adhesion between the conductive polymer and the textile fibers, making it ideal for applications that require reliable and durable conductive layers, such as flexible electronics and sensors. By allowing for precise control over the polymer deposition, vacuum filtration can create highly uniform coatings, enhancing the electrical properties of textiles used in wearable technology, medical devices, and environmental sensing applications. However, while vacuum filtration offers excellent adhesion and consistency in small-scale experiments, it faces limitations in scaling up for large-scale production. The process can be time-consuming, and achieving a perfectly uniform coating across larger textile areas may prove challenging. Despite these constraints, vacuum filtration remains a valuable technique for creating conductive textiles, particularly in applications where precision and reliability are prioritized over production speed.

4.8. Hydrothermal Synthesis

Hydrothermal synthesis is a specialized method for growing conductive polymer nanostructures on textile substrates under high-temperature and high-pressure water conditions [94,95]. This technique is capable of producing highly crystalline and uniform coatings, which are essential for advanced textile applications requiring superior conductivity and structural integrity. Due to its precision, hydrothermal synthesis is particularly suitable for textiles used in high-performance energy storage devices, wearable electronics, and advanced sensors, where both durability and electrical performance are critical. However, the process’s reliance on high temperatures and pressures, along with its time-consuming nature, presents challenges for scalability and large-scale production. These limitations make hydrothermal synthesis more suitable for research and niche applications, where the need for highly controlled, high-quality conductive coatings outweighs the drawbacks of limited throughput.

4.9. Screen Printing

Screen printing is an effective technique for applying conductive polymer pastes onto textiles, utilizing a stencil or screen to create specific patterns [91]. This method offers scalability and flexibility, allowing for the production of intricate designs and precise applications on various textile substrates. Commonly used conductive polymers, such as PP and PANI, can be incorporated, making this technique ideal for developing printed electronics on textiles, including antennas, sensors, and circuits for wearable technology. Despite its advantages, screen printing does have limitations, such as restricted resolution and potential ink wastage, which can affect the efficiency and sustainability of the process. Additionally, achieving the desired conductivity may necessitate the application of multiple layers, complicating production and increasing processing time. Nonetheless, screen printing remains a popular choice in the field of textile electronics due to its ability to create customized, high-performance conductive patterns.

5. Application of Conductive Polymers

Conductive polymers uniquely combine the advantageous qualities of traditional polymers—such as flexibility, processability, and low weight—with enhanced electrical conductivity. This synergy makes them versatile materials, enabling their application across numerous fields, including prominent areas such as energy storage and conversion, organic light-emitting diodes (OLEDs), organic solar cells (OSCs), conductive and smart textiles, food processing monitoring, and healthcare monitoring [96,97]. A summary of the conductive polymers, including types, characterizations, key finding, limitations, and applications is presented in Table 2. For instance, in the realm of organic electronics, conductive polymers are integral to the development of OLEDs and OSCs, where they facilitate efficient charge transport and energy conversion. In healthcare, these materials are being explored for innovative applications in biosensors and drug delivery systems, enabling responsive and controlled therapeutic solutions. Additionally, conductive polymers are increasingly utilized in the realm of smart textiles, allowing for the integration of electronic functionalities into fabrics, which can be used in wearable technology for health monitoring and interactive garments. Their diverse applications highlight the potential of conductive polymers to revolutionize various sectors, including electronics, energy, and healthcare. Table 3 provides an overview of the production components of flexible electronics. As research continues to evolve, the possibilities for innovative uses of conductive polymers expand, showcasing their importance in advancing technology and improving quality of life across multiple domains, as illustrated in Figure 5.

5.1. Energy Storage and Conversion

Energy storage and conversion systems such as supercapacitors [98,99,100], lithium-ion batteries (LIBs) [101,102,103], and sodium-ion batteries (SIBs) [104,105] are significantly enhanced by the integration of conductive polymers (CPs), such as polypyrrole (PPy) and polyaniline (PANI). CPs offer unique advantages due to their high electrical conductivity, tunable electrochemical properties, and mechanical flexibility. In supercapacitors, CPs serve as active electrode materials, enabling high charge storage through pseudocapacitive mechanisms and ensuring excellent cycle stability. In LIBs and SIBs, CPs function as conductive matrices that improve ionic transport, accommodate electrode volume changes during cycling, and enhance the overall capacity and rate performance. Their ability to synergize with nanomaterials, such as graphene or metal oxides, further elevates energy and power densities across these systems. The application of CPs, thus, represents a pivotal advancement in creating efficient, durable, and sustainable energy storage and conversion devices. The application of two distinct CPs, (1) PPy and (2) PANI, in energy storage devices such as supercapacitors, LIBs, and SIBs is illustrated schematically in Figure 6a–l.

5.1.1. Supercapacitors

CPs like PPy, PANI, and PEDOT are pivotal in advancing supercapacitor technologies due to their exceptional electrical conductivity, pseudocapacitive behavior, and mechanical adaptability [99]. These materials store energy through rapid and reversible redox reactions, making them ideal for high-performance energy storage systems. For instance, PPy combined with reduced graphene oxide (RGO) to form RGO/PPy nanocomposites on a graphite substrate has demonstrated excellent electrochemical stability, high capacitance, and mechanical flexibility, making it suitable for flexible supercapacitors [107]. Similarly, PANI’s tunable conductivity enhances charge storage when integrated with other materials. A notable example is PANI anchored with ligninsulfonate sodium (LS) within a chitosan–polyacrylamide (CS/PAAM) matrix, forming (LS-PANI/CS/PAAM), which achieved robust specific capacitance and significant energy density in flexible supercapacitors [106]. Therefore, CPs not only address challenges such as low energy density and cycling stability but also enable faster charge–discharge rates and improved durability, positioning them as cornerstone materials for next-generation supercapacitor technologies.

5.1.2. Lithium-Ion Batteries

As possess high electrical conductivity, flexibility, and stability, they have emerged as promising materials for enhancing the performance of LIBs. By incorporating CPs, the conductivity of anode and cathode materials can be significantly enhanced, resulting in faster ion transport, improved charge–discharge rates, and enhanced cycling stability. For instance, a polypyrrole (PPy)-coated silicon nanoparticle composite used as an anode material mitigates volume expansion during lithium intercalation, thereby extending the battery’s lifespan while enhancing electron/ion transfer rates [102]. Similarly, polyaniline (PANI), with its tunable conductivity, forms effective composites with other materials, such as flower-like iron oxide. These composites demonstrate ultrahigh specific capacity and long-term durability due to the PANI coating, which reduces volume change, improves electrical conductivity, and facilitates rapid Li⁺ diffusion [103]. Therefore, the integration of CPs not only increases the specific capacity of LIBs but also enhances structural integrity, resulting in superior durability and efficiency.

5.1.3. Sodium-Ion Batteries

Similarly, CPs have shown immense potential in improving the performance of sodium-ion batteries (SIBs) due to their intrinsic electrical conductivity, mechanical flexibility, and ability to form stable composites with active materials. These properties are crucial for addressing the challenges associated with SIBs, such as lower energy density and slower ion diffusion compared to lithium-ion batteries. By incorporating CPs, the electrochemical performance of electrodes is significantly enhanced, facilitating improved ion transport, increased conductivity, and better charge–discharge rates, thereby extending cycling life. For instance, Ma et al. synthesized flower-like architectures comprising PPy-wrapped phosphorus-doped VS₂ composites (P-VS₂@PPy) as anode materials for high-performance SIBs [104]. The PPy coating enhanced electronic conductivity, mitigated volumetric fluctuations, and accelerated charge/ion transfer efficiency, resulting in exceptional sodium-storage capacity and long-term cycling stability. Similarly, PANI-mediated MnO₂ nanowires (PANI/MnO₂) coated on carbon cloth (CC) demonstrated remarkable SIB performance, offering both reasonable rate capacity and prolonged stability [105].

5.2. OLEDs

Organic light-emitting diodes (OLEDs) have gained significant traction in modern display technologies, particularly in televisions and mobile phones, due to their superior color reproduction, contrast, and energy efficiency [11]. One of the standout features of OLEDs is their flexibility, allowing them to be integrated into a variety of applications beyond traditional screens. This flexibility makes them especially suitable for wearable devices, where lightweight and adaptable displays enhance user experience and comfort. Moreover, OLEDs can be incorporated into intelligent systems, facilitating innovations in smart textiles and other emerging technologies. As research progresses, the potential for OLEDs to revolutionize not just display technologies but also wearable electronics and interactive interfaces continues to expand, paving the way for a new generation of user-friendly, visually engaging devices. Their versatility and performance highlight the important role of OLEDs in shaping the future of consumer electronics and smart applications. The flexible transparent electrode for OLEDs might potentially be composed of PEDOT: PSS, a material known for its excellent conductivity. By using a methanol solution containing benzoic acid, Kim et al.’s research successfully increased the conductivity of PEDOT: PSS thin films. Over 1500 S/cm of conductivity is shown in the treated PEDOT: PSS material [11]. A glass/PEDOT framework was utilized in the fabrication of 86 OLEDs. 4,4-bis(Ncarbazolyl) and PSS/1,1-bis[(di-4-tolylamino)phenycyclohexane] make up the hole-transport layer. The compound used is 1,1′-biphenyl:tris(2-phenylpyridine). The electron transport layer is made up of 2,2′,2”-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), followed by LiF and Al. The emission layer is made up of iridium (III) [CBP:Ir(pp y)₃]. The OLEDs show a brightness of 1000 cd/m² and a maximum current efficiency of 25.3 cd/A at a bias of 5.9 V. The flexible OLEDs can maintain 83% of their original brightness after 1400 bending cycles at a bending radius of 5 mm.

Table 2

Summary of the conductive polymers, types, characterizations, key finding, limitations, and applications.

Name of Polymer	Type	Characterizations	Key Findings	Limitations	Applications	Ref
PPy	Intrinsically	XRD, TGA, DSC	Easy synthesis, High conductivity	Brittle nature	Flexible sensors, actuators, wearable electronics,	[108]
PEDOT	Intrinsically	Raman, EIS, AFM	High stability, Good conductivity	Expensive monomers	Transparent electrodes, flexible OLEDs, stretchable electronic devices, bioelectronics	[109]
PA	Intrinsically	XPS, ESR, NMR	High conductivity	Instability in air	Molecular electronics, flexible transistors	[3]
PT	Intrinsically	FTIR, SEM, CV	Flexible, Moderate conductivity	Poor processability	Solar cells, flexible sensors	[108]
Poly(3-hexylthiophene)	Intrinsically	UV-Vis, GPC, AFM	High mobility, Good film-forming ability	Limited environmental stability	Organic transistors, photovoltaics, flexible electronic circuits	[110]
Poly(para-phenylene)	Intrinsically	UV-Vis, TGA, DSC	High thermal stability	Difficult synthesis	LEDs, field-effect transistors	[111]
Poly(para-phenylene vinylene)	Intrinsically	Raman, AFM, CV	High fluorescence	Low conductivity	OLEDs, solar cells	[112]
Polyfluorene	Intrinsically	UV-Vis, PL, GPC	High photoluminescence	Limited solubility	Blue LEDs, photodetectors, bioelectronics	[113]
Polycarbazole	Intrinsically	FTIR, CV, AFM	High thermal stability	Limited conductivity	Sensors, photovoltaics	[4,114]
PPy-DBSA	Conductive blend	XRD, TGA, DSC	Improved processability	Lower conductivity	Actuators, batteries	[108]
PEDOT-PSS	Conductive blend	Raman, EIS, AFM	High conductivity, Processable	High cost	Transparent electrodes, solar cells	[109]
Poly(thiophene-co-pyrrole) (PT-PPy)	Copolymer	UV-Vis, FTIR, SEM	Enhanced conductivity	Stability issues	Solar cells, actuators	[111]
PANI-PSSA	Conductive blend	UV-Vis, FTIR, SEM	Improved solubility	Reduced conductivity	Antistatic coatings, sensors	[115]
PPy-DBSA	Conductive blend	XRD, TGA, DSC	Improved process ability	Lower conductivity	Actuators, batteries	[116]
PEDOT-dodecylbenzene sulfonate	Conductive blend	UV-Vis, FTIR, SEM	Improved solubility	Expensive monomers	Antistatic coatings, sensors	[117]
Poly(aniline-co-pyrrole)	Copolymer	XRD, TGA, DSC	Combined properties	Complex synthesis	Flexible sensors, batteries	[118]
Poly(para-phenylene)-polystyrene	Copolymer	Raman, EIS, AFM	Improved film forming	Lower conductivity	LEDs, photovoltaics	[119]
Poly(3-hexylthiophene)-polyethylene glycol	Copolymer	FTIR, SEM, CV	Enhanced solubility	Lower thermal stability	Organic transistors, flexible sensors	[120]
Polyfluorene-polyvinyl alcohol	Copolymer	UV-Vis, TGA, DSC	Improved film forming	Limited conductivity	Blue LEDs, photodetector, flexible electronics	[121]

5.3. Organic Solar Cells (OSCs)

Organic Solar Cells (OSCs) have emerged as a promising alternative to traditional silicon-based solar technology, primarily due to their lightweight, flexible design, and cost-effectiveness. Utilizing organic materials as the active layer, OSCs offer the potential for efficient energy conversion while maintaining versatility in application [2,38,122]. Their flexibility allows for integration into a variety of surfaces, including building materials, textiles, and portable devices, making them suitable for innovative energy solutions in urban environments. Furthermore, OSCs can be produced using low-energy manufacturing processes, contributing to sustainable energy practices. Recent advancements in materials and fabrication techniques have led to significant improvements in their efficiency and stability, enhancing their commercial viability. As the demand for renewable energy sources continues to grow, OSCs present an exciting opportunity for harnessing solar power in diverse applications, ultimately contributing to a more sustainable future. Their adaptability and environmental benefits position OSCs as a key player in the evolution of solar technology. In a study conducted in 2012, the investigation focused on the use of sulfuric acid-treated PEDOT: PSS films as the transparent electrode and poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM) as the active layer in organic solar cells [38]. The photovoltaic efficiency of OSCs is similar to that of control devices that use ITO as the flexible electrode. The photovoltaic efficiency of non-fullerene organic solar cells is much higher in comparison to solar cells using fullerene or its derivatives. The efficiency of flexible non-fullerene organic solar cells was shown to be 91.92%. The efficiency of flexible OSCs treated with PEDOT: PSS and D- maltose was shown by Ge et al. to be 12.35%. The transparent electrode had a conductivity value of 2626 S/cm, whereas the substrate used was polyethylene terephthalate (PEDOT) [11].

5.4. Strain Sensors with Intrinsically Conducting Polymers (ICPs) for Healthcare Monitoring

Intrinsically conducting polymers (ICPs) have gained significant attention as a primary material for versatile strain sensors, particularly in healthcare monitoring applications. These conductive polymers, when integrated with soft or elastic substrates, exhibit a resistance that is highly sensitive to tensile strain [123]. This unique property enables the development of sensors that can accurately detect even minor deformations, making them ideal for real-time monitoring of physiological parameters. For instance, these strain sensors can be applied to wearable devices, providing valuable insights into a patient’s movement, muscle activity, and overall health. Their flexibility and biocompatibility further enhance their usability in medical applications, allowing for unobtrusive monitoring in various settings [97]. As research progresses, the potential for ICP-based strain sensors to revolutionize healthcare monitoring systems is becoming increasingly evident, paving the way for more personalized and responsive medical care that can adapt to the needs of individual patients. In general, ICP films deposited on a flexible substrate have a relatively low sensitivity to tensile forces. In their study, Fan et al. observed that PDMS including PEDOT: PSS, when subjected to a potent acid, had a sensitivity of 22 at a strain of 20%. The increased sensitivity seen may be attributed to the formation of PEDOT/PDMS composites at the interfaces [124]. The resistance exhibits a persistent rise after 400 cycles. In a study conducted in 2018, it was shown that the resistance cyclability could be greatly improved by the use of a triple-layer structure comprising blends of PVA, PEDOT, and PSS, as well as highly conductive PEDOT, PSS, and PDMS. At strain levels of 10%, 20%, and 30%, the strain sensors exhibit sensitivities of 26, 70, and 100, respectively [91]. The observed high sensitivity may be attributed to the formation of little fractures in the PEDOT:PSS films, which exhibit high conductivity, when subjected to strain. These fractures are then reconnected by the PVA-PEDOT: PSS material after the tension is alleviated. The researchers demonstrated the potential use of strain sensors in monitoring various physical movements, including joint motions such as those of the knee, finger, elbow, and palm [11]. In the study “electrical performance of PEDOT:PSS-based textile electrodes for wearable ECG monitoring: A comparative study by Reinel Castrillón et al. evaluated textile electrodes treated with PEDOT:PSS for ECG signal detection as shown in Figure 7a,b and Figure 8a,b [125]. Comparing these to commercial silver-plated nylon and Ag/AgCl electrodes, they assessed contact impedance, polarization, noise, and long-term performance. Results showed that PEDOT:PSS-treated electrodes effectively detected ECG signals with less than 2% error, maintaining functionality after 36 h despite higher contact impedance and polarization than commercial electrodes [91].

5.5. Strain Sensors with ICPs for Food Processing Monitoring

Extensive research has explored the application of flexible strain sensors, particularly those utilizing intrinsically conducting polymers (ICPs), in various monitoring contexts, including food processing [122,126,127]. These strain sensors are adept at detecting substantial geometric fluctuations, making them valuable tools for ensuring the integrity and safety of food products throughout the processing stages. By continuously monitoring structural changes, such as deformation or stress in food packaging or processing equipment, these sensors can provide real-time data that help identify potential issues before they escalate. This capability not only enhances food safety but also improves overall efficiency in food production. The adaptability of ICP-based strain sensors to different environments underscores their versatility and potential for widespread adoption in the food industry, where precision and reliability are paramount. As technology advances, these innovative sensors will likely play an increasingly vital role in maintaining quality and safety standards in food processing. The resistance of a strain sensor varies as a result of the separation of conductive fillers caused by strain. Starch-based food has the potential to experience substantial alterations in volume throughout the process of food preparation. By incorporating a conductive material into starch-based food, it is possible to track changes in volume throughout food preparation by monitoring variations in resistance. Zhang et al. recently confirmed this notion. They showcased the use of a versatile strain sensor for monitoring the production of starch-based meals [128]. The significance lies in the fact that starch-based food items, such as bread and steam buns, are extensively eaten globally, and the manner in which they are processed may have a substantial influence on the overall quality of the food. The development of stretchable strain sensors included the integration of PEDOT: PSS into starch. PEDOT: PSS exhibits biocompatibility and may be uniformly dispersed into starch owing to the presence of a hydrogen bond between them. PEDOT has a consistent chemical structure throughout several food processing techniques, including fermentation, steaming, storing, and refreshing. The PEDOT:PSS/starch blends demonstrate electrical conductivity, and their electrical resistance varies in accordance with variations in the quantity of food throughout the processing stage [128]. The inclusion of 0.3 wt.% PEDOT:PSS did not provide any significant effects on the fermentation process or the alterations in volume of the starch-based meals. The quantity of food may rise by 1.9 times, while the resistance of the PEDOT:PSS/starch mixes can increase by 2.8 times. The primary cause of the change in resistance is mostly attributed to the expansion of volume rather than fluctuations in pH. This is because the pH of the meal only decreases from 5.2 to 4.7 throughout the fermentation process. The volume of starch-based food rises when it is steamed. The electrical resistance of the PEDOT:PSS/starch blends exhibits variations in response to changes in volume. During the start of the steaming process, there is an increase in both temperature and food volume. There is a corresponding increase in the resistivity of the PEDOT:PSS/starch mixtures. The resistance of the mixtures achieves its highest value when the meal reaches its maximum volume during the steaming process. The temperature exhibits a decline subsequent to the completion of the steaming procedure. As the quantity of the meal reduces, there is a corresponding drop in the resistance of the steamed bread. The PEDOT:PSS/starch doughs’ resistance might be used for monitoring the storage and freshness of food [108]. Hence, these sensors have the potential to aid in the identification of optimal conditions for food preparation. This might lead to the attainment of optimum food quality while minimizing energy use.

5.6. Electrically Conductive Textiles and Smart Textiles Applications

Although a large number of electrically conductive textiles that are suitable for smart textiles have been documented in the literature, there are still few of these fabrics on the market. This article emphasizes commercially accessible smart textile products crafted from conductive polymers, particularly those designed for applications in healthcare, sports, fitness, and the automotive industry. These innovative materials exhibit a range of advanced properties, including superamphiphobic characteristics, electrical conductivity, electrochromic capabilities, and electromagnetic interference (EMI) shielding. Such attributes position them as promising solutions for specialized applications, enhancing functionality and performance in various fields. For instance, smart textiles can monitor health metrics in real-time, provide support during athletic performance, and they offer enhanced safety features in automotive settings. As technology continues to advance, the potential for these conductive textiles to integrate seamlessly into everyday products will likely increase, paving the way for a future where smart textiles become an integral part of our daily lives, improving both convenience and efficiency.

Ashleigh Naysmith et al. used green synthesis methods to produce silver nanoparticles (AgNPs) with lime peel extract and optimize their size and properties using a Plackett–Burman design shown in Figure 9a–c. The AgNPs were combined with PPy and applied to linen fabric to create a conductive textile. Results showed that the optimized AgNPs had consistent sizes, and the resulting e-textile exhibited low electrical resistance and potential for practical applications in electronic textiles. The study also highlighted the challenges in maintaining electrical properties under washing conditions [129].

5.7. Health, Sport, and Fitness Applications

Innovative textile wearable devices have been developed, drawing inspiration from the traditional Japanese kimono dyeing process. These advanced devices are engineered to collect high-quality electrocardiogram (ECG) readings in both clinical and ambulatory settings, effectively quantifying heart rates. By integrating conductive polymers into the fabric, these wearables ensure exceptional flexibility and comfort, allowing for continuous monitoring without hindering the user’s movement. This design not only enhances the user experience but also facilitates real-time health tracking, making it a valuable tool for both athletes and patients. The ability to obtain accurate and reliable ECG data on-the-go is particularly beneficial for personalized health monitoring, enabling timely interventions and better management of cardiovascular health. As the demand for wearable technology in health, sports, and fitness continues to rise, these innovative textile devices exemplify the potential of combining traditional craftsmanship with cutting-edge technology to create effective health-monitoring solutions.

In a study focused on the direct patterning of organic conductors on knitted textiles for long-term electrocardiography, a PDMS stencil was employed to restrict the dispersion of aqueous PEDOT on interlock knit polyester fabric, taking advantage of the stencil’s hydrophobic properties. The resulting electrodes made from PEDOT and coated with an ionic liquid gel exhibited minimal impedance when in contact with the skin. This innovative approach has led to the development of a new edema monitoring device by Edema ApS [130]. Furthermore, a device enables the measurement of volume changes in the lower extremities, providing valuable insights into fluid retention and the effectiveness of drainage therapy through precise scientific measurements [91]. Sensoria firm created smart socks embedded with textile sensors capable of detecting foot pressure. The sock has conductive fibers that transmit data to an anklet, which communicates with a smartphone app via Bluetooth. Creating durable and consistent textile sensors that can withstand machine wash cycles is crucial for advancing smart fabric technology [123]. Adidas used conductive threads in flexible clothing using knitting technology. Imperceptible and skin-friendly textile electrodes have the capability to detect impulses originating from the heart and other muscles. The collected data by the electrodes are wired through the garment and connected to a tiny device that is securely fastened to the sport’s top [123].

Figure 10 displays health, sport, and fitness applications [123,130]. The Life Tech jacket, a novel and advanced three-in-one garment, was created by Seymourpowell specifically for Kolon Sport, a Korean outdoor sportswear company. This jacket provides protection for those in challenging circumstances. Textile electrodes were developed with the purpose of quantifying human bio-potentials [131]. Different types of pastes were used for the electrode networks, including a polyurethane paste that could be printed on a screen, a silver conductor paste that could be printed on a screen, and a conductive rubber paste that could be printed using a stencil. The woven cloth used as the printed substrate was “Escalade,” which had a density of 295 g/m², was woven in a 1 twill pattern, and had a thickness of 410 µm. In order to maintain an electrical connection with the skin surface, electrode sites were subjected to stencil printing for the purpose of conductive encapsulation. Carvalho et al. outlined a garment specifically engineered for utilization in health-related, high-hazard zones, and surveillance during athletic endeavors. The development of fundamental weft structures, including jersey, basic pique, and locknit was undertaken [132]

Table 3

Overview of the production components of flexible electronics.

Flexible Electronics	Manufacturing Elements	Ref.
OLEDs	PEDOT Methanol solution of benzoic acid PSS/1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC) is used as the hole-transport layer with 4,4-bis (Ncarbazolyl). 1,1′-biphenyl:tris(2-phenylpyridine) iridium(III) [CBP:Ir(pp y)3] TPBi	[66]
OSCs	Sulfuric acid PEDOT:PSS films Poly(3-hexylthiophene):phenyl-C61-butyric acid methyl ester (P3HT:PCBM)	[66]
Healthcare monitoring	ICPs Soft or elastic polymer PDMS containing PEDOT:PSS treated with a strong acid had a sensitivity of 22 at 20% strain (PVA–PEDOT:PSS blends)/(highly conductive PEDOT:PSS)/ PDMS	[133]
Food processing monitoring	PEDOT:PSS Hydrogen link	[67,68,69,70]
Flexible super capacitors	PANI PTh PPy	[11,122,126,127,128,134,135]
Textiles and smart textiles applications	PDMS PEDOT:PSS DuPont microcircuit materials Polyamide	[91,123,130,136,137,138,139,140]

6. Challenges and Limitations

Mechanical, electrical, fabrication, material, economic, and environmental factors are only a few of the difficulties and constraints that conductive polymers and flexible electronics face. Ensuring mechanical robustness, adherence to different substrates, and longevity can be challenging, particularly under complicated stress settings [141]. Stability and homogeneity over time are difficult to achieve in electrical systems. Hydrothermal synthesis and electrospinning are two fabrication processes that necessitate exact control over conditions and frequently confront scaling challenges [142]. CPs provide issues in additive manufacturing due to their low processability, which means that, in order to preserve conductivity, CPs must be combined with other polymers and nanoadditives. It is challenging to control the electrical conductivity and structural order of pure CPs. Therefore, large-scale production and innovative synthesis techniques are needed. Systematic research and scalable, economical manufacturing techniques are required when combining CPs with other nanomaterials to create multifunctional nanocomposites. The problems of selectivity, irreversibility, and stability that gas and biosensors encounter call for enhanced sensor functionality and a deeper comprehension of material interactions. Providing long-term protection in severe conditions with individual CP coatings is challenging, which emphasizes the necessity for better protection of CP nanocomposites. Creating dependable large-scale manufacturing systems and comprehending CP interfaces are essential for proceeding with CP-based materials and their applications [143].

7. Future Directions

Several approaches address the problems that conductive polymers (CPs) and flexible electronics encounter. Advanced material compositions and coating processes can produce improved mechanical robustness and adhesion. Better doping strategies and nanoengineering are crucial to ensuring electrical stability and homogeneity. Scaling issues can be solved by automating and precisely controlling the fabrication process. Processability is improved in additive manufacturing by combining CPs with other polymers and nanoadditives and by investigating novel printing methods. Innovative synthesis techniques and dependable large-scale manufacturing technologies are essential for controlled, large-scale production. The enhancement of multifunctionality in CPs is achieved through the synthesis of hybrid nanocomposites and systematic research on CP–nanomaterial interactions. By focusing on material engineering and understanding the dynamics of these interactions, researchers can improve sensor performance by increasing selectivity, stability, and reversibility. This multifaceted approach not only elevates the capabilities of CP-based sensors but also addresses the growing demand for sustainable practices. The incorporation of economical production techniques and environmentally friendly materials reflects a commitment to balancing performance with cost-effectiveness and ecological responsibility. This integrated strategy paves the way for the development of advanced sensing technologies that are not only effective but also sustainable, catering to various applications across industries.

8. Conclusions

Conductive polymer-based flexible electronics are very promising and have a large market. The collaboration of conductive polymers with flexible electronics represents a noteworthy achievement in electronic materials research. The remarkable characteristics of conductive polymers, such as elasticity and adjustable conductivity, have created very flexible and versatile electronic systems. Combining wearable sensors with flexible screens creatively solves difficulties across many sectors. Incorporating various nanomaterials with their different nanostructures into conductive polymers profoundly impacts their application in flexible electronics. Different nanostructured metal oxides and other carbon nanomaterials (carbon nanotubes, graphene, etc.) could offer unique benefits that can enhance the electrical, mechanical, and optical properties of conductive polymers [144]. By selecting the appropriate nanostructure based on the application’s specific requirements, it is possible to develop advanced flexible electronic devices with improved performance and functionality. Continued research and development in nanostructured conductive polymers will likely lead to further innovations and applications in the field of flexible electronics. The future looks promising for the widespread use and development of conductive polymers in flexible electronics, thanks to ongoing research and technical developments. This article could be a trustworthy resource for future researchers conducting research on conductive electronics-based flexible electronics-based multifunctional applications.

Author Contributions

M.A.S., M.M.R. and M.I.U.H.: conceptualization, supervision, writing—original draft preparation, writing—review and editing, visualization, and figure preparation. M.T.H., I.H., M.S.S., M.S.R. and N.U.: writing—original draft preparation and figure preparation. S.W.D.: language refinement, proofreading, and manuscript review and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare that there are no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Bibliometric analysis by keyword: (a,b) Conductive polymer; (c,d) Conductive textiles; (e,f) Flexible electronics.

Figure 2. Structures of the prevalent conductive polymers.

Figure 3. Graphical illustration of the characteristics of conductive polymers.

Figure 4. Schematic fabrication techniques for conductive polymer composites for multifunctional applications. (a) Chemical Vapor Deposition (CVD), (b) Electrospinning, (c) Vacuum filtration, (d) Electrodeposition, (e) Hydrothermal synthesis, (f) Dip coating, (g) Screen printing, (h) Spraying, and (i) In-situ polymerization.

Figure 5. Applications of conducting polymers.

Figure 6. Application of two distinct conductive polymers in three different energy storage and conversion systems. (a) Polyaniline (PANI) anchored with ligninsulfonate sodium (LS) within a chitosan-polyacrylamide (CS/PAAM) matrix (LS-PANI/CS/PAAM) for flexible supercapacitors, and (b) corresponding electrochemical characterization through cyclic voltammetry (CV) at various scan rates [106]. (c) Reduced graphene oxide (RGO) combined with polypyrrole (PPy) to form RGO/PPy nanocomposites for flexible supercapacitors, and (d) corresponding CV for electrochemical characterization [107]. (e) PANI composites with flower-like iron oxide (PANI/Fe2O3) as an anode materials in LIBs, and (f) corresponding charge–discharge profile for showing the electrochemical performance [103]. (g) PPy-coated silicon nanoparticle composite as an anode material of LIBs, and (h) corresponding charge–discharge profile for showing the electrochemical performance [102]. (i) PANI-mediated MnO2 nanowires (PANI/MnO2) as an anode material for SIBs, and (j) corresponding glavanostatic charge–discharge (GCD) profile at various current densities exhibiting rate performance [105]. (k) Flower-like architectures of PPy-wrapped phosphorus-doped VS2 composites (P-VS2@PPy) serve as anode materials for SIBs, and (l) corresponding rate characterization performance via GCD profile at different current densities [104].

Figure 7. Textile electrodes treated with PEDOT:PPS for ECG signal detection. (a) Digital image of developed textile electrode, and (b) circuit design [125].

Figure 8. ECG system acquisition and control using CP-based conductive textile electrode for the signal detection in the human body. (a) Contact impedance measurement scheme, and (b) ECG signal measurement scheme [125].

Figure 9. (a) Experimental flow diagram, (b) SEM image of PPy-Silver nanoparticles, and (c) electrical resistance change under bending of the developed conductive textile fabric [129].

Figure 10. (a) Textile wristband with a PEDOT: PSS electrode, (b) Adidas textile electrode for sport top, and (c) fitness shirt with biometric function [91].

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