Electrospun Conducting Polymers: Approaches and Applications

Inherently conductive polymers (CPs) can generally be switched between two or more stable oxidation states, giving rise to changes in properties including conductivity, color, and volume. The ability to prepare CP nanofibers could lead to applications including water purification, sensors, separations, nerve regeneration, wound healing, wearable electronic devices, and flexible energy storage. Electrospinning is a relatively inexpensive, simple process that is used to produce polymer nanofibers from solution. The nanofibers have many desirable qualities including high surface area per unit mass, high porosity, and low weight. Unfortunately, the low molecular weight and rigid rod nature of most CPs cannot yield enough chain entanglement for electrospinning, instead yielding polymer nanoparticles via an electrospraying process. Common workarounds include co-extruding with an insulating carrier polymer, coaxial electrospinning, and coating insulating electrospun polymer nanofibers with CPs. This review explores the benefits and drawbacks of these methods, as well as the use of these materials in sensing, biomedical, electronic, separation, purification, and energy conversion and storage applications.

While many researchers still prefer this route, for both economic feasibility and ease of customization, the demand for production-scale electrospinning equipment is also growing. Several companies worldwide now specialize in manufacturing, selling, and servicing electrospinning equipment [59,60]. These commercial electrospinning machines can produce roll to roll quantities of nanofibers of about 200 g/h, a 20-fold increase in rate over the 100 mg/h produced with lab-scale equipment [61,62].
A wide variety of inexpensive, commercially available ("commodity") polymers have been electrospun, but there are inherent difficulties associated with electrospinning CPs. This review focuses on the production of CP nanofibers via electrospinning, as well as the applications of CP nanofibers ( Figure 2). A brief background on CPs and the electrospinning process is provided, followed by a discussion of the approaches used to incorporate CPs into electrospun nanofibers. A summary of recent advances and suggestions for future work in electrospun CP applications conclude this review. Many thousands of publications on electrospun CP nanofibers are available; thus, this review focuses on some exemplary highlights of efforts in this area.

Inherently Conducting Polymers
CPs are highly conjugated polymers containing alternating single and double bonds, as shown in Figure 1; this sp 2 hybridized backbone is responsible for the unique electronic and optical properties of CPs. Resonance delocalization of electrons and cations (holes) formed during the oxidation (doping) process ( Figure 3) allows for the conductivity of CP systems to increase from the semiconducting regime to the metallic regime when oxidized [4]. Oxidation of the neutral polymer (Figure 3, top) initially forms the polymer radical cation, known as a polaron (Figure 3, middle). Further removal of electrons forms the polymer dication, known as a bipolaron (Figure 3, bottom). As the polymer oxidation state changes, CP properties including color, solubility, conductivity, volume, and reactivity, also change [45,97]. The process is typically reversible; CPs can typically be oxidized and reduced thousands to millions of times, enabling their use in a wide range of devices [98,99]. Several excellent reviews on the synthesis, characterization, properties, and applications of CPs are available [3,7,[99][100][101][102].  Redox processes, which are typically reversible, allow interconversion among neutral polymers (top), radical cations known as polarons (middle), and dications known as bipolarons (bottom). Reprinted from Ref. [45], available at https://www.mdpi.com/1996-1944/12/16/2629; accessed on 1 December 2022.
The rigid structure of most CPs makes them highly insoluble, and many are sensitive to moisture and air. They are also difficult to process and characterize; neat polyacetylene, albeit highly conductive, has been shown to be poorly suited for any technological applications [103][104][105].
Substituent groups can be incorporated into CPs to improve solubility and stability [28,29], control the band gap (Eg) [22], and decrease the oxidation potential [106][107][108] of the monomers and their resulting polymers. The stability, conductivity, and processability of these CP derivatives can also be improved through careful structural modification [109][110][111].
Polythiophenes have become one of the most widely used and researched CPs primarily because of their relative stability in oxygen and moisture [112]. While polythiophene (PT) is an insoluble material, polymerization of alkyl-substituted thiophenes with propyl or longer alkyl chains yields poly(3-alkylthiophene)s (P3ATs, Figure 4) that are soluble in organic solvents and mechanically processable. Long-chain alkoxy groups exhibit several advantages over alkyl substituents, such as increased solubility and a significant reduction in polymer Eg due to the electron-donating nature of the alkoxy substituent [109][110][111]. Poly(3-hexylthiophene) (P3HT) (Figure 4) is one of the most notable and widely researched CPs. Redox processes, which are typically reversible, allow interconversion among neutral polymers (top), radical cations known as polarons (middle), and dications known as bipolarons (bottom). Reprinted from Ref. [45], available at https: //www.mdpi.com/1996-1944/12/16/2629; accessed on 1 December 2022.
The rigid structure of most CPs makes them highly insoluble, and many are sensitive to moisture and air. They are also difficult to process and characterize; neat polyacetylene, albeit highly conductive, has been shown to be poorly suited for any technological applications [103][104][105].
Substituent groups can be incorporated into CPs to improve solubility and stability [28,29], control the band gap (E g ) [22], and decrease the oxidation potential [106][107][108] of the monomers and their resulting polymers. The stability, conductivity, and processability of these CP derivatives can also be improved through careful structural modification [109][110][111].
Polythiophenes have become one of the most widely used and researched CPs primarily because of their relative stability in oxygen and moisture [112]. While polythiophene (PT) is an insoluble material, polymerization of alkyl-substituted thiophenes with propyl or longer alkyl chains yields poly(3-alkylthiophene)s (P3ATs, Figure 4) that are soluble in organic solvents and mechanically processable. Long-chain alkoxy groups exhibit several advantages over alkyl substituents, such as increased solubility and a significant reduction in polymer E g due to the electron-donating nature of the alkoxy substituent [109][110][111]. Poly(3hexylthiophene) (P3HT) (Figure 4) is one of the most notable and widely researched CPs.  Incorporation of ether substituents to conjugated polyheterocycles further increases their electron density and reduces their oxidation potential [106]. To minimize steric interactions, researchers have focused primarily on poly(3,4-ethylenedioxythiophene) (PE-DOT) and its derivatives ( Figure 5). PEDOT is an insoluble conjugated polymer that can be functionalized via incorporation of alkyl and/or alkoxy substituents to impart solubility [113][114][115][116]. Doping PEDOT with poly(styrene sulfonate) yields stable, water-dispersible nanoparticles of poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Figure 5) that can be used as an electrostatic dissipation coating for photographic film [117]. PEDOT:PSS is now one of the most widely commercially used soluble conductive polymers in the world, with over 100 tons produced annually [7].

Electrospinning Fundamentals
The history of electrospinning and the development of electrospinning design assemblies have been thoroughly reviewed by Tucker et al. [118], Teo and Ramakrishna [53], and Ghosal et al. [119]. For a more complete foundation on electrospinning and the future of electrospinning, we recommend studies by Ramakrishna et al. [59,120], Baji et al. [121], Haider et al. [122], and Xue et al. [72]. A brief discussion of polymer and system parameters impacting electrospinning and how they pertain to CPs is covered in sections below.
Electrospinning setups are versatile and commercially scalable, with various manufacturers making and selling industrial-sized electrospinning equipment [123][124][125][126]. In its most basic form, an electrospinning setup simply requires a high voltage power supply, a syringe pump, a syringe, a needle, and a collection plate [127]. When an electrical charge is applied, Coulombic repulsion forces are generated in the polymer solution between Incorporation of ether substituents to conjugated polyheterocycles further increases their electron density and reduces their oxidation potential [106]. To minimize steric interactions, researchers have focused primarily on poly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives ( Figure 5). PEDOT is an insoluble conjugated polymer that can be functionalized via incorporation of alkyl and/or alkoxy substituents to impart solubility [113][114][115][116]. Doping PEDOT with poly(styrene sulfonate) yields stable, water-dispersible nanoparticles of poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Figure 5) that can be used as an electrostatic dissipation coating for photographic film [117]. PEDOT:PSS is now one of the most widely commercially used soluble conductive polymers in the world, with over 100 tons produced annually [7].  Incorporation of ether substituents to conjugated polyheterocycles further increases their electron density and reduces their oxidation potential [106]. To minimize steric interactions, researchers have focused primarily on poly(3,4-ethylenedioxythiophene) (PE-DOT) and its derivatives ( Figure 5). PEDOT is an insoluble conjugated polymer that can be functionalized via incorporation of alkyl and/or alkoxy substituents to impart solubility [113][114][115][116]. Doping PEDOT with poly(styrene sulfonate) yields stable, water-dispersible nanoparticles of poly (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Figure 5) that can be used as an electrostatic dissipation coating for photographic film [117]. PEDOT:PSS is now one of the most widely commercially used soluble conductive polymers in the world, with over 100 tons produced annually [7].

Electrospinning Fundamentals
The history of electrospinning and the development of electrospinning design assemblies have been thoroughly reviewed by Tucker et al. [118], Teo and Ramakrishna [53], and Ghosal et al. [119]. For a more complete foundation on electrospinning and the future of electrospinning, we recommend studies by Ramakrishna et al. [59,120], Baji et al. [121], Haider et al. [122], and Xue et al. [72]. A brief discussion of polymer and system parameters impacting electrospinning and how they pertain to CPs is covered in sections below.
Electrospinning setups are versatile and commercially scalable, with various manufacturers making and selling industrial-sized electrospinning equipment [123][124][125][126]. In its most basic form, an electrospinning setup simply requires a high voltage power supply, a syringe pump, a syringe, a needle, and a collection plate [127]. When an electrical charge is applied, Coulombic repulsion forces are generated in the polymer solution between

Electrospinning Fundamentals
The history of electrospinning and the development of electrospinning design assemblies have been thoroughly reviewed by Tucker et al. [118], Teo and Ramakrishna [53], and Ghosal et al. [119]. For a more complete foundation on electrospinning and the future of electrospinning, we recommend studies by Ramakrishna et al. [59,120], Baji et al. [121], Haider et al. [122], and Xue et al. [72]. A brief discussion of polymer and system parameters impacting electrospinning and how they pertain to CPs is covered in sections below.
Electrospinning setups are versatile and commercially scalable, with various manufacturers making and selling industrial-sized electrospinning equipment [123][124][125][126]. In its most basic form, an electrospinning setup simply requires a high voltage power supply, a syringe pump, a syringe, a needle, and a collection plate [127]. When an electrical charge is applied, Coulombic repulsion forces are generated in the polymer solution between charges of the same polarity; these forces destabilize the hemispherical droplet of the polymer solution at the tip of the nozzle to form a droplet of conical shape called the "Taylor cone" [128]. As the applied electrostatic field strength increases, the Coulombic repulsion force eventually exceeds that of the surface tension, resulting in the ejection of an electrically charged jet of polymer solution. This charged jet travels in a straight path for a small distance before bending instability forces it into a looping path. On the path to the collector plate, the charged jet elongates while drying out or solidifying, depositing ultrafine fibers on the collector [128]. Collection plates may be replaced with drums; at speeds of ≥1000 rpm, nanofiber alignment may occur ( Figure 6) [127,129,130].
Materials 2022, 15, x FOR PEER REVIEW 6 of 76 charges of the same polarity; these forces destabilize the hemispherical droplet of the polymer solution at the tip of the nozzle to form a droplet of conical shape called the "Taylor cone" [128]. As the applied electrostatic field strength increases, the Coulombic repulsion force eventually exceeds that of the surface tension, resulting in the ejection of an electrically charged jet of polymer solution. This charged jet travels in a straight path for a small distance before bending instability forces it into a looping path. On the path to the collector plate, the charged jet elongates while drying out or solidifying, depositing ultrafine fibers on the collector [128]. Collection plates may be replaced with drums; at speeds of ≥1000 rpm, nanofiber alignment may occur ( Figure 6) [127,129,130].

Electrospinning Parameters
The electrospinning process is controlled by two sets of parameters: polymer parameters and process parameters [122]. These can be easily modified, enabling the electrospun nanofiber characteristics to be precisely tailored. Some mechanical properties of the nanofibers are affected by the polymer properties, while other mechanical properties are affected by the electrospinning parameters [131]. Nanofiber morphologies can be tailored to fit desired applications; this ease of tailoring is one aspect that makes electrospinning a versatile technique. By fine-tuning and precisely controlling the electrospinning parameters, electrospun nanofibers with desired properties and morphologies can be produced.

Polymer Parameters
For electrospinning purposes, relevant polymer characteristics include molecular weight, viscosity, surface tension, polydispersity, and conductivity. These properties play a role in fiber formation and fiber diameter, reduction of bead formation, and rate of nanofiber degradation [132,133]. Studies have shown that decreasing solution concentration reduces the diameter of the electrospun fibers [53]. Effects on rheological properties including electrical conductivity, dielectric strength, and surface tension in solutions have also been observed [134]. Numerous studies have been conducted, establishing the impact Figure 6. A typical electrospinning set up [130] using (a) a static grounded collector plate and (b) a grounded rotating drum collector. Reprinted from Ref. [130], available at https://doi.org/10.3390/ ma10111238; accessed on 1 December 2022.

Electrospinning Parameters
The electrospinning process is controlled by two sets of parameters: polymer parameters and process parameters [122]. These can be easily modified, enabling the electrospun nanofiber characteristics to be precisely tailored. Some mechanical properties of the nanofibers are affected by the polymer properties, while other mechanical properties are affected by the electrospinning parameters [131]. Nanofiber morphologies can be tailored to fit desired applications; this ease of tailoring is one aspect that makes electrospinning a versatile technique. By fine-tuning and precisely controlling the electrospinning parameters, electrospun nanofibers with desired properties and morphologies can be produced.

Polymer Parameters
For electrospinning purposes, relevant polymer characteristics include molecular weight, viscosity, surface tension, polydispersity, and conductivity. These properties play a role in fiber formation and fiber diameter, reduction of bead formation, and rate of nanofiber degradation [132,133]. Studies have shown that decreasing solution concentration reduces the diameter of the electrospun fibers [53]. Effects on rheological properties including electrical conductivity, dielectric strength, and surface tension in solutions have also been observed [134]. Numerous studies have been conducted, establishing the impact of molecular weights and solution concentrations on the variety of structures that are obtained via electrospinning [134].
Solution concentration/viscosity, charge density, and surface tension determine nanofiber diameter and surface morphology [135]. These parameters must be adjusted to prevent the formation of an electrospray and formation of beads during nanofiber formation [136]. Increasing solution viscosity reduces bead formation and increases fiber diameter, while increasing solution conductivity reduces bead formation and decreases fiber diameter ( Figure 7) [135]. For a homogeneous solution of a linear polymer, the Huggins equation [137] (1) describes the solution viscosity as where η sp is the specific viscosity, [η] is the intrinsic viscosity, c is the polymer concentration, and k H is the Huggins coefficient. Because [η] is related to the viscosity average molecular weight (M v ) of a linear polymer by the Mark-Houwink-Sakurada equation [138], [η] is dependent on the polymer structure, solvent, and temperature.
of molecular weights and solution concentrations on the variety of structures that are obtained via electrospinning [134]. Solution concentration/viscosity, charge density, and surface tension determine nanofiber diameter and surface morphology [135]. These parameters must be adjusted to prevent the formation of an electrospray and formation of beads during nanofiber formation [136]. Increasing solution viscosity reduces bead formation and increases fiber diameter, while increasing solution conductivity reduces bead formation and decreases fiber diameter ( Figure 7) [135]. For a homogeneous solution of a linear polymer, the Huggins equation [137] (1) describes the solution viscosity as where ηsp is the specific viscosity, [η] is the intrinsic viscosity, c is the polymer concentration, and kH is the Huggins coefficient. Because [η] is related to the viscosity average molecular weight (Mv) of a linear polymer by the Mark-Houwink-Sakurada equation [138], [η] is dependent on the polymer structure, solvent, and temperature.

Figure 7.
Increasing viscosity or conductivity (charge density) reduces bead formation [135]. Polymer solution viscosity is related to the extent of polymer chain molecular entanglement within that solution. Polymer chain entanglement and, therefore, solution viscosity can be increased by increasing either the concentration or the strength of intermolecular interactions (specifically, by incorporating functional groups that are capable of dipoledipole interactions). Viscous solutions are necessary to overcome the electrostatic and coulombic repulsion forces that stretch the electrospinning jet. At lower viscosities, the jet partially breaks up, and the free solvent molecules in the solution instead form spheres, resulting in the formation of beads [139]. Polymer solution viscosity is related to the extent of polymer chain molecular entanglement within that solution. Polymer chain entanglement and, therefore, solution viscosity can be increased by increasing either the concentration or the strength of intermolecular interactions (specifically, by incorporating functional groups that are capable of dipole-dipole interactions). Viscous solutions are necessary to overcome the electrostatic and coulombic repulsion forces that stretch the electrospinning jet. At lower viscosities, the jet partially breaks up, and the free solvent molecules in the solution instead form spheres, resulting in the formation of beads [139].

Process Parameters
Process parameters, such as needle tip diameter, shape, and location, flow rate of polymer solution, electric potential, solution temperature, and relative humidity also impact fiber formation. The distance between the needle tip and collector determines the extent of solvent evaporation from the nanofibers and deposition on the collector; collector rotation can result in nanofiber alignment during deposition [132]. Studies have shown that reducing the distance between the needle tip and collector increases the interconnectivity of the fibers [53]. In a study conducted by Beachley and Wen [156], a correlation was shown between increasing voltage and decreasing nanofiber diameters. They also showed Simple linear polymers like PE, PP, and PS are ruled by weak van der Waals forces [151,152] and require high polymer solution concentrations to be stable enough for electrospinning. Polymers with strong intermolecular forces such as polyamides (PA-6, for example) and polyesters (PCL, for example) [153,154] exhibit increased chain entanglement, allowing lower concentrations of these polymers to be used for electrospinning. Chain entanglement is also increased in branched polymers compared to their linear polymer counterparts [155].

Process Parameters
Process parameters, such as needle tip diameter, shape, and location, flow rate of polymer solution, electric potential, solution temperature, and relative humidity also impact fiber formation. The distance between the needle tip and collector determines the extent of solvent evaporation from the nanofibers and deposition on the collector; collector rotation can result in nanofiber alignment during deposition [132]. Studies have shown that reducing the distance between the needle tip and collector increases the interconnectivity of the fibers [53]. In a study conducted by Beachley and Wen [156], a correlation was shown between increasing voltage and decreasing nanofiber diameters. They also showed that an increase in voltage leads to increased uniformity of the nanofibers. Increasing rotational speed of the drum collector also increases fiber alignment; Matthews and coworkers showed [157] that, at speeds of less than 500 rpm, randomly oriented collagen fibers were obtained. When the rotational speed was increased to 4500 rpm, the collagen fibers showed a significant increase in alignment ( Figure 9). that an increase in voltage leads to increased uniformity of the nanofibers. Increasing rotational speed of the drum collector also increases fiber alignment; Matthews and coworkers showed [157] that, at speeds of less than 500 rpm, randomly oriented collagen fibers were obtained. When the rotational speed was increased to 4500 rpm, the collagen fibers showed a significant increase in alignment ( Figure 9).
Generally, nanofibers have high aspect ratios and large specific surface areas; depending on the chemical structure of the polymer, nanofibers may also have a high degree of flexibility and high mechanical strength [158]. Other mechanical properties such as Young's modulus, yield stress, and tensile stress have also been shown to increase with higher rotational speed and fiber alignment [53,159]. Alignment of electrospun nanofibers is critical for some bioengineering/biomedical applications. Directed neurite growth has been achieved on aligned electrospun polycaprolactone (PCL) nanofibers, facilitating nerve regeneration in a rat sciatic nerve injury [160]. Additionally, aligned nanofibers provide guided electron transport pathways, have particular strain-induced electronic properties, have high electron and thermal diffusivity rates, and can be easily tailored to various morphologies [158]. Organic solar cells that employ aligned electrospun nanofibers could improve charge transport and increase power conversion efficiencies without any thermal post-treatments [129,161].
It is possible to increase electrospinning temperature by heating the polymer solution, using a controlled temperature chamber [162,163] or a heated syringe [164,165]. Increasing temperature decreases surface tension and solution viscosity, resulting in decreased fiber diameters [164,165]. However, this effect suffers from a competing effect: increasing temperature also accelerates solvent evaporation, which can terminate fiber stretching prematurely, increasing fiber diameters [164,166]. Thus, careful control of temperature is needed to control fiber diameters.
Humidity can also have profound effects on nanofibers [167,168]. How a polymer solution interacts with moist air depends on the solvent and on the polymer hydrophilicity [168,169]. At high relative humidity (RH), hydrophilic and hygroscopic polymers in nonaqueous solutions undergo slow solidification because of the high water content in the surrounding air, resulting in smaller fiber diameters; at very high RH, the jet thins until capillary instability results in bead formation ( Figure 10A). Conversely, at high RH. water acts as a non-solvent for hydrophobic polymers, resulting in rapid solidification Figure 9. SEMs of electrospun collagen [157]. (A) Randomly oriented fibrils generated at less than 500 rpm. (B) Aligned fibrils generated at 4500 rpm. Reprinted and adapted with permission from [157]. Copyright 2002, American Chemical Society.
Generally, nanofibers have high aspect ratios and large specific surface areas; depending on the chemical structure of the polymer, nanofibers may also have a high degree of flexibility and high mechanical strength [158]. Other mechanical properties such as Young's modulus, yield stress, and tensile stress have also been shown to increase with higher rotational speed and fiber alignment [53,159]. Alignment of electrospun nanofibers is critical for some bioengineering/biomedical applications. Directed neurite growth has been achieved on aligned electrospun polycaprolactone (PCL) nanofibers, facilitating nerve regeneration in a rat sciatic nerve injury [160]. Additionally, aligned nanofibers provide guided electron transport pathways, have particular strain-induced electronic properties, have high electron and thermal diffusivity rates, and can be easily tailored to various morphologies [158]. Organic solar cells that employ aligned electrospun nanofibers could improve charge transport and increase power conversion efficiencies without any thermal post-treatments [129,161].
It is possible to increase electrospinning temperature by heating the polymer solution, using a controlled temperature chamber [162,163] or a heated syringe [164,165]. Increasing temperature decreases surface tension and solution viscosity, resulting in decreased fiber diameters [164,165]. However, this effect suffers from a competing effect: increasing temperature also accelerates solvent evaporation, which can terminate fiber stretching prematurely, increasing fiber diameters [164,166]. Thus, careful control of temperature is needed to control fiber diameters.
Humidity can also have profound effects on nanofibers [167,168]. How a polymer solution interacts with moist air depends on the solvent and on the polymer hydrophilicity [168,169]. At high relative humidity (RH), hydrophilic and hygroscopic polymers in nonaqueous solutions undergo slow solidification because of the high water content in the surrounding air, resulting in smaller fiber diameters; at very high RH, the jet thins until capillary instability results in bead formation ( Figure 10A). Conversely, at high RH. water acts as a non-solvent for hydrophobic polymers, resulting in rapid solidification and larger fiber diameters; at very high RH, phase separation occurs, which results in formation of porous fibers ( Figure 10B). and larger fiber diameters; at very high RH, phase separation occurs, which results in formation of porous fibers ( Figure 10B).

Introduction
CP electrospinning has not received as much attention as electrospinning of other polymers that are easier to work with. CPs tend to have rigid backbones, low molecular weights, and low degrees of chain entanglement, which make them inadequate for electrospinning purposes [4,72,[170][171][172]. There have been limited reports of successful electrospinning and nanofiber formation from pure CP solutions ("neat" electrospinning) [172][173][174][175][176]. Attempts to overcome these limitations by coaxial electrospinning, combining CPs with other, typically higher-molecular-weight polymers ("carrier polymers") to electrospin as blends, or coating nonconductive polymer nanofibers to form layered structures are discussed in detail below. A comparison of advantages and disadvantages of the different methods can be found in Table 1.

Introduction
CP electrospinning has not received as much attention as electrospinning of other polymers that are easier to work with. CPs tend to have rigid backbones, low molecular weights, and low degrees of chain entanglement, which make them inadequate for electrospinning purposes [4,72,[170][171][172]. There have been limited reports of successful electrospinning and nanofiber formation from pure CP solutions ("neat" electrospinning) [172][173][174][175][176]. Attempts to overcome these limitations by coaxial electrospinning, combining CPs with other, typically higher-molecular-weight polymers ("carrier polymers") to electrospin as blends, or coating nonconductive polymer nanofibers to form layered structures are discussed in detail below. A comparison of advantages and disadvantages of the different methods can be found in Table 1. Table 1. Advantages and disadvantages of approaches to preparation of CP nanofibers.

Neat Electrospinning
Neat electrospinning of CPs is challenging to achieve. In addition to their rigid backbones and low molecular weights, many CPs are insoluble in organic solvents [72,173]. This makes the process of preparing and attaining neat CP solutions for electrospinning difficult; attempts to prepare high concentrations of CPs typically lead to precipitation, while low concentrations of CPs typically either do not form fibers at all, or they form beaded nanofibers. Nonetheless, researchers have been able to generate neat electrospun nanofibers from a variety of CPs including PPy [177], PANI [172,173,175], and P3HT [176]. Spectroscopic analysis of electrospun PANI blends indicated that PANI is stable to the high voltages used in electrospinning; no overoxidation of the PANI was observed [189]. In fact, individual fibers of electrospun PANI exhibited conductivity two orders of magnitude higher than solution-cast PANI films prepared from the same solution [175], likely due to significant chain alignment occurring during fiber formation. Similar results were observed for a low-molecular-weight polypyrrole doped with dodecylbenzenesulfonate [190]. Addition of a large excess of dodecylbenzenesulfonate increased chloroform solubility to enable electrospinning, and, after removal of excess dodecylbenzenesulfonate, the resultant PPy nanofiber conductivity was approximately double that of PPy powder or solution-cast films.
González et al. [176] demonstrated early success electrospinning neat P3HT nanofibers. A 7 wt.% solution of regioregular P3HT was dissolved in chloroform and electrospun at 20 kV to yield nanofibers with average diameter of 670 nm ( Figure 11); larger beads are also evident in the fibers. The high surface area of the nanofibers makes them particularly susceptible to reaction with atmospheric oxygen and water vapors; electrospinning and device fabrication in an inert environment might be necessary for implementation. Liu et al. [191] were able to reduce P3HT fiber diameters to ca. 180 nm by electrospinning 1 wt.% solutions of P3HT at 5.8 kV; the team noted the same issues with beading and with oxygen and water exposure.
A novel method using binary solvents for electrospinning neat poly [2-methoxy-5-(2ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) was studied by Zhong et al. [192] They explored binary solvent mixtures of chloroform (a good solvent for MEH-PPV) and either methanol or isopropanol (poor solvents for MEH-PPV) and found that a lower surface tension and higher solution electrical conductivity favor fiber formation; thus, isopropanol was a better poor solvent than methanol to use in the binary mixtures. Using a 4:1 mixture of chloroform/isopropanol to dissolve MEH-PPV yielded bead-free nanofibers, while increasing chloroform content even slightly (to 5:1 or higher) yielded beaded nanofibers. Researchers hypothesized that addition of isopropanol caused polymer chain aggregation, increasing interchain interactions to the point that steric hindrance of the MEH-PPV backbone motions increased effective conjugation lengths [192][193][194], stabilizing physical (chain) entanglements and enabling electrospinning of the neat MEH-PPV solution without addition of a carrier polymer [192,195]. For aligned MEH-PPV nanofibers, the researchers were also able to use polarization anisotropy spectroscopy to determine that the MEH-PPV chains were preferentially aligned along the electrospun fiber axis. particularly susceptible to reaction with atmospheric oxygen and wa spinning and device fabrication in an inert environment might be n mentation. Liu et al. [191] were able to reduce P3HT fiber diameters to trospinning 1 wt.% solutions of P3HT at 5.8 kV; the team noted the sam ing and with oxygen and water exposure.  [176]. Reprinted with permission from Ref. [176]. Copyri sion from Elsevier.  [176]. Reprinted with permission from Ref. [176]. Copyright 2005 with permission from Elsevier.

Coaxial Electrospinning
Coaxial electrospinning has emerged as a successful technique to obtain CP-containing nanofibers. Coaxial electrospinning produces core-sheath-structured nanofibers by forming a fiber that consists of complementary materials in the core and sheath [196]. Coaxial electrospinning enables traditionally non-electrospinnable CPs to be able to be electrospun with an adjacent electrospinnable polymer. Two styles of coaxial electrospun nanofibers can be generated: (i) the core comprises a CP with the spinnable polymer as the postprocess removable shell, or (ii) the spinnable polymer is the core, while the CP is the shell [173]. Additional parameters must be considered for the coaxial electrospinning process, including compatibility of the solutions and needle diameter ratios [184]. Coaxial electrospinning has provided a successful alternative for researchers to obtain neat CP electrospun nanofibers, after simple post-treatment processes to remove the nonconductive polymer. An in-depth review of coaxial electrospinning and its applications was recently published by Steckl and Han [196] and is recommended reading for further insights. A summary of coaxial electrospinning references discussed below can be found in Table 2.
When blends of PANI and PMMA would not electrospin due to insufficient elasticity of the polymer solutions, Zhang and Rutledge [182] utilized coaxial electrospinning to generate PANI/PMMA nanofibers, using emeraldine base PANI (M w = 65,000 Da) dissolved in chloroform for the inner core and poly(methyl methacrylate) (PMMA, M w = 540,000 Da) dissolved in N,N-dimethylformamide (DMF) as the outer shell. Smooth and continuous PANI/PMMA nanofibers were obtained with diameters ca. 1440 nm. The PMMA outer shell could be removed via immersion in an isopropanol solution for 1 h to yield neat PANI nanofibers with decreased diameters of ca. 620 nm ( Figure 12) and conductivities of 50 S·cm −1 . Chen and coworkers used coaxial electrospinning to prepare nanofibers with P3HT cores and PMMA shells [197]. The PMMA shell could be removed after electrospinning by immersion in acetone. The researchers evaluated three different chlorinated solvents to prepare the P3HT solutions for electrospinning and found that chloroform and chlorobenzene, both good solvents for P3HT, allowed for chain stretching during electrospinning. This increased alignment of adjacent chains, producing highly oriented crystalline grains and smaller crystal sizes as compared to P3HT electrospun from 1,2,4-trichlorobenzene (TCB). This higher orientation resulted in a three-orders-of-magnitude increase in charge carrier mobility (from 10 −4 to 10 −1 cm 2 ·V −1 ·s −1 ) for the fibers prepared from chloroform and chlorobenzene relative to those prepared from TCB. P3HT nanofibers prepared from chloroform offered the additional benefit of enhanced ductility relative to the fibers prepared from chlorobenzene or TCB.
Coaxial needles can be used in the preparation of electrospun CP nanofibers in ways other than the typical ones described above. In a slightly different approach to electrospinning using coaxial needles, Lee and coworkers [198] conducted a study on electrospinning of P3HT (Mw = 87,000 Da) in CHCl3 at a concentration of 11-13%; at lower concentrations, beaded fibers formed. The researchers observed that, during electrospinning of neat P3HT, the needle tip became blocked after about 10 s, due to the P3HT crystallizing as the solvent evaporated. To combat this, they utilized a coaxial electrospinning setup in which additional CHCl3 was delivered via the outer, nonconductive nozzle, while P3HT solution in CHCl3 was delivered via the inner, conductive nozzle. Smooth, uniform P3HT nanofibers with an average thickness of ~500 nm were obtained. The coaxial setup allowed for neat P3HT nanofibers to be generated without the aid of a carrier polymer and with no adverse impacts to the electrical properties of the CP used. Chen and coworkers used coaxial electrospinning to prepare nanofibers with P3HT cores and PMMA shells [197]. The PMMA shell could be removed after electrospinning by immersion in acetone. The researchers evaluated three different chlorinated solvents to prepare the P3HT solutions for electrospinning and found that chloroform and chlorobenzene, both good solvents for P3HT, allowed for chain stretching during electrospinning. This increased alignment of adjacent chains, producing highly oriented crystalline grains and smaller crystal sizes as compared to P3HT electrospun from 1,2,4-trichlorobenzene (TCB). This higher orientation resulted in a three-orders-of-magnitude increase in charge carrier mobility (from 10 −4 to 10 −1 cm 2 ·V −1 ·s −1 ) for the fibers prepared from chloroform and chlorobenzene relative to those prepared from TCB. P3HT nanofibers prepared from chloroform offered the additional benefit of enhanced ductility relative to the fibers prepared from chlorobenzene or TCB.
Coaxial needles can be used in the preparation of electrospun CP nanofibers in ways other than the typical ones described above. In a slightly different approach to electrospinning using coaxial needles, Lee and coworkers [198] conducted a study on electrospinning of P3HT (M w = 87,000 Da) in CHCl 3 at a concentration of 11-13%; at lower concentrations, beaded fibers formed. The researchers observed that, during electrospinning of neat P3HT, the needle tip became blocked after about 10 s, due to the P3HT crystallizing as the solvent evaporated. To combat this, they utilized a coaxial electrospinning setup in which additional CHCl 3 was delivered via the outer, nonconductive nozzle, while P3HT solution in CHCl 3 was delivered via the inner, conductive nozzle. Smooth, uniform P3HT nanofibers with an average thickness of~500 nm were obtained. The coaxial setup allowed for neat P3HT nanofibers to be generated without the aid of a carrier polymer and with no adverse impacts to the electrical properties of the CP used.

Co-electrospinning with Carrier Polymers
Typically, neat CP solutions for electrospinning must be at a low concentration to facilitate the dissolution of the CP chains. These low-concentration CP solutions are often not viscous enough for electrospinning due to lack of chain entanglement. A carrier or host polymer is a high-molecular-weight polymer that is used in electrospinning to help an electrospinning solution reach an adequate viscosity for electrospun nanofiber formation. High-molecular-weight carrier polymers reach chain entanglement at low concentrations; hence, the addition of small amounts of high-molecular-weight carrier polymer can enable electrospinning of CPs with minimal impact on CP properties [198]. Thus, CPs are often electrospun using carrier polymers, including polyacrylonitrile (PAN), poly(ethylene oxide) (PEO), poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), (PS), and poly(lactic-co-glycolic acid) caprolactone) (PLGA), as shown in Figure 8. A summary of co-electrospinning approaches discussed in this review can be found in Table 3.
One disadvantage of electrospinning using these carrier polymers is that, while the carrier polymers impart beneficial properties, at higher carrier polymer concentrations, the resultant nanofiber blends may not possess the desired CP properties, such as conductivity or electrochromism. In addition to increasing insulating content, electrospinning CPs with carrier polymers may also introduce surfactants or byproduct impurities [192].
The research group of Nobel Prize winner Alan MacDiarmid conducted some of the early investigations into electrospun CPs utilizing carrier polymers or coating nonconductive nanofibers. They electrospun blends of doped PANI with PEO (50-89%) [189] or PS (80%) [175] as a carrier polymer. Fiber diameters varied with the concentration and nature of the carrier polymer; while PANI/PEO blends yielded fiber diameters from 0.9 to 1.9 µm [189], PANI/PS blends yielded fiber diameters from 72 to 100 nm [175]. Conductivity was found to increase as concentration of PANI in the blends increased, and conductivities of fibers were slightly lower than conductivities of cast films of the same blends [189].

Coating with Conducting Polymers
Coating of electrospun nanofibers of commodity polymers (Figure 8) with CPs has become a highly active area of research due to potential applications in smart textiles, energy storage, and sensors. Various techniques for coatings are employed, such as dip coating, vapor phase deposition, and in situ polymerization [242,243]. By coating the electrospun nanofibers with CPs such as PPy, PEDOT, PANI, or PEDOT:PSS, a conductive textile is created that combines the mechanical properties of the electrospun polymer with the electronic properties of the CP. It is important to note that some researchers have found that, while the mechanical properties of the layered nanofibers are improved relative to pristine CPs, filling the fiber interstices with stiff CP may reduce breaking strain while enhancing ultimate stress and modulus [90,244]. A summary of reports of CP coatings on electrospun polymers can be found in Table 4.
Chemical compatibility must be considered when coating polymer nanofibers with CPs. Zha et al. [245] electrospun cellulose nanofibers and then used an in situ polymerization method to generate a coating of poly(N-vinylpyrrole) (PNVPy) for one set of fibers and a coating of P3HT on another. Both types of electrospun CP-coated nanofiber mats were found to exhibit favorable cell adhesion, proliferation and cytocompatibility with PC12 cells. However, PNVPy formed aggregated nanoparticles on the electrospun cellulose nanofibers while P3HT formed a continuous coating ( Figure 13). This difference was apparent in the performance of these two systems: cellulose/P3HT demonstrated increased hydrophilicity and more effective PC12 cell proliferation relative to cellulose/PNVPy. Regardless, both types of electrospun CP coated nanofiber mats showed positive outcome toward neural tissue engineering.
PEDOT was oxidatively polymerized onto electrospun nanofiber blend of nitrile butadiene rubber (NBR) and poly(ethylene glycol) dimethacrylate (PEGDMA) for bioelectronic sensor applications as demonstrated by Fallahi et al. [244] An increase in the overall elastic modulus of the PEDOT coated NBR/PEGDMA nanofiber mats was observed at ca. 3.8 MPa compared to uncoated NBR/PEGDMA nanofiber mats at just 0.7 MPa. Inversely, PEDOT-coated NBR/PEGDMA nanofibers mats exhibited a threefold decrease in elongation strain, 75% maximum strain compared to 122% maximum strain for uncoated NBR/PEGDMA nanofiber mats. For all electrospun nanofiber mats there was no significant reduction in hysteresis after 10 cycles at 20% elongation; more specifically, the PEDOT coated NBR/PEGDMA nanofibers mats showed stabilizing behavior even after 100 cycles. Mechanical and electrical stability measurements were conducted on aligned PEDOT-coated NBR/PEGDMA nanofibers mats; the change in resistance was measured over 100 extension and relaxation cycles at 20% strain rate. The conductivity was measured at ca. 6 S·cm −1 ; nanofiber alignment led to anisotropic electrical connectivity, creating stable and reversible changes in resistance. In addition to imparting electrical properties, the PEDOT coating also acts a reinforcing agent that leads to an increase in elastic modulus and improved shape recovery forces after stress removal. There was a decrease in flexibility due to the addition of PEDOT, but flexibility and stretching properties were still within favorable ranges for the purposes of this application.
In another example, electrospun polyurethane (e-PU) nanofibers were dip-coated with PEDOT:PSS as shown by Ding et al. [246] The effects of using one to three dip coatings were explored. The team investigated the conductivity effects when the conductively coated fabrics were stretched, twisted, and bended, and all demonstrated sufficiently stable conductivity to light up a small light-emitting diode (LED). The conductivity of the e-PU nonwovens increased relative to the number of dip coatings applied; one PEDOT:PSS dip coating yielded a conductivity of 0.3 S·cm −1 , while a second dip coating increased conductivity to 0.9 S·mm −1 , and a third dip coating increased conductivity to 2 S·cm −1 . Conversely, tensile strength saw a decrease in elongation to break as more dip coatings were applied, uncoated e-PU and one PEDOT:PSS dip coating exhibited ca. 400% elongation at break, while two dip coatings decreased elongation at break to ca. 230%, and three dip coatings reduced elongation at break to ca. 40%. This dip coating method produced PEDOT:PSS/PU fabrics exhibiting highly stable conductivities as stretchable conductors for flexible electronics comparable to those in recent literature [246,247].
In a slightly different approach, solutions of PMMA [248] or poly(acrylic acid) (PAA) [249] with the EDOT monomer were electrospun into aqueous oxidant solutions. This resulted in formation of EDOT-containing PMMA or PAA fibers that underwent subsequent EDOT polymerization upon heating to yield fiber mats with conductivities up to 1 S·cm −1 . It is also possible to co-electrospin polymers with oxidant and then expose the resultant fibers to monomer to produce CP coatings. For instance, poly(N-vinylpyrrolidone) (PVP) was electrospun with ferric tosylate to form fiber with average diameter of 600 nm [186]. PEDOT was then deposited on the resultant nanofibers via vapor-phase polymerization, increasing the average fiber diameter to 710 nm. Subsequent rinsing with methanol removed PVP and residual oxidant, collapsing the PEDOT fibers slightly and decreasing average fiber diameter to 350 nm ( Figure 14). The PEDOT fibers appeared to fuse together during the rinsing process to yield fiber mats with excellent conductivity, ca. 60 S·cm −1 .

Coating with Conducting Polymers
Coating of electrospun nanofibers of commodity polymers (Figure 8) with CPs has become a highly active area of research due to potential applications in smart textiles, energy storage, and sensors. Various techniques for coatings are employed, such as dip coating, vapor phase deposition, and in situ polymerization [242,243]. By coating the electrospun nanofibers with CPs such as PPy, PEDOT, PANI, or PEDOT:PSS, a conductive textile is created that combines the mechanical properties of the electrospun polymer with the electronic properties of the CP. It is important to note that some researchers have found that, while the mechanical properties of the layered nanofibers are improved relative to pristine CPs, filling the fiber interstices with stiff CP may reduce breaking strain while enhancing ultimate stress and modulus [90,244]. A summary of reports of CP coatings on electrospun polymers can be found in Table 4.
Chemical compatibility must be considered when coating polymer nanofibers with CPs. Zha et al. [245] electrospun cellulose nanofibers and then used an in situ polymerization method to generate a coating of poly(N-vinylpyrrole) (PNVPy) for one set of fibers and a coating of P3HT on another. Both types of electrospun CP-coated nanofiber mats were found to exhibit favorable cell adhesion, proliferation and cytocompatibility with PC12 cells. However, PNVPy formed aggregated nanoparticles on the electrospun cellulose nanofibers while P3HT formed a continuous coating ( Figure 13). This difference was apparent in the performance of these two systems: cellulose/P3HT demonstrated increased hydrophilicity and more effective PC12 cell proliferation relative to cellulose/PNVPy. Regardless, both types of electrospun CP coated nanofiber mats showed positive outcome toward neural tissue engineering. PEDOT was oxidatively polymerized onto electrospun nanofiber blend of nitrile butadiene rubber (NBR) and poly(ethylene glycol) dimethacrylate (PEGDMA) for bioelectronic sensor applications as demonstrated by Fallahi et al. [244] An increase in the overall elastic modulus of the PEDOT coated NBR/PEGDMA nanofiber mats was observed at ca. 3.8 MPa compared to uncoated NBR/PEGDMA nanofiber mats at just 0.7 MPa. Inversely, PEDOT-coated NBR/PEGDMA nanofibers mats exhibited a threefold decrease in elongation strain, 75% maximum strain compared to 122% maximum strain for uncoated NBR/PEGDMA nanofiber mats. For all electrospun nanofiber mats there was no significant reduction in hysteresis after 10 cycles at 20% elongation; more specifically, the PE-DOT coated NBR/PEGDMA nanofibers mats showed stabilizing behavior even after 100

Applications
Electrospinning allows a wide range of polymers to be used to form nanofibrous structures with high surface-to-volume ratios, porosity, and orientation-dependent properties. The methods discussed above have enabled the use of electrospun CPs in applications, from biological to wearable materials. The addition of CPs introduces tunable physical and electronic properties with the application of an electrochemical stimulus. One important consequence of this is that when CPs are electrospun by themselves or co-electrospun with another polymer, the higher conductivity of the polymer solution often re-

Applications
Electrospinning allows a wide range of polymers to be used to form nanofibrous structures with high surface-to-volume ratios, porosity, and orientation-dependent properties. The methods discussed above have enabled the use of electrospun CPs in applications, from biological to wearable materials. The addition of CPs introduces tunable physical and electronic properties with the application of an electrochemical stimulus. One important consequence of this is that when CPs are electrospun by themselves or co-electrospun with another polymer, the higher conductivity of the polymer solution often results in the formation of thinner fibers [251].

Bioomedical Applications
Electrospun nanofibers can be functionalized with bioactive molecules such as drugs and proteins. The addition of CPs is especially suitable for application to biological processes which are enhanced by the application of a current or heat.

Wound Healing and Therapy
Human dermal fibroblast (HDF) growth on electrospun nanofibers comprising PCL and a thiophene/pyrrole-based polymer (PDBTT) at a ratio of 15:1 were shown ( Figure 15) to be enhanced due to the ability of the nanofiber mat to conduct a photocurrent after absorption in the near-infrared region (NIR) [204]. This was due to the promotion of cellular processes and division as a result of the electric and magnetic fields induced by the photocurrent. Similar wound-healing properties were also observed by another group on rat wounds covered with membranes of electrospun PANI-co-poly(o-aminobenzenesulfonic acid)/poly(vinyl alcohol)/chitosan oligosaccharide, with almost complete healing after 15 days [278].  The ability of CPs to absorb in the NIR due to conjugation has also led to photothermal therapeutic applications. Hollow fibers of PPy were prepared via in situ polymerization of pyrrole on PCL electrospun mats followed by dissolution of the substrate PCL [252]. These fibers, along with polydopamine-coated mats of PCL [279], were shown to be effective in the photothermal killing of model cancer cells, with the fibrous structure assisting in the continuous heat flow all over the treated cells and the high surface area of the mat resulting in increased heating effects. In addition, these structures, as well as a PANI-coated electrospun PCL mat, were shown [253] to have a tunable binding interaction with the chemotherapeutic doxorubicin; the membranes exhibited high swelling ca- The ability of CPs to absorb in the NIR due to conjugation has also led to photothermal therapeutic applications. Hollow fibers of PPy were prepared via in situ polymerization of pyrrole on PCL electrospun mats followed by dissolution of the substrate PCL [252]. These fibers, along with polydopamine-coated mats of PCL [279], were shown to be effective in the photothermal killing of model cancer cells, with the fibrous structure assisting in the continuous heat flow all over the treated cells and the high surface area of the mat resulting in increased heating effects. In addition, these structures, as well as a PANIcoated electrospun PCL mat, were shown [253] to have a tunable binding interaction with the chemotherapeutic doxorubicin; the membranes exhibited high swelling capabilities and reduced hydrophilicity, indicating their suitability for drug delivery and localized treatment applications.

Tissue Scaffolds
The conducting polymers PPy, PTh, and PANI have shown good biocompatibility, as they have been found to be noncytotoxic [205,254,255]. Porous, aligned electrospun mats containing these polymers mimic the structure of the human extracellular matrix. This structure allows for the attachment of proteins and cells, leading to applications as scaffolds for tissue engineering. In addition, the enhanced conductivity of CP-containing oriented fibrous scaffolds allows for better communication between cells, enhancing tissue growth. This is particularly of interest in tissues in which the conduction of impulses or signals are important, such as in nerve, muscle, and bone tissue. The main issue faced in these types of applications is the weak mechanical properties of scaffolds made up entirely of CPs, which tend to have low molecular weights and poor mechanical properties, such as brittleness. These are remedied by the incorporation of biocompatible high-molecularweight insulating polymers such as PLGA, PEO, PCL, and poly(vinylidene difluoride (PVDF) [206].
Bone tissue scaffolds were synthesized from PANI deposited on electrospun PCL/ osteogenon/gelatin/calcium phosphate nanoparticles [255] and electrospun mats of PANI/ PVDF and PPy/PVDF [207]. Importantly, the porous structure of CP-containing scaffolds allows for the implantation of calcium phosphate nanoparticles and drugs in addition to promoting cell growth and enhancing intercellular communication through enhanced conductivity; growth of MC3T3 cells on the scaffold is readily apparent when comparing SEM images from before and after cell culture ( Figure 16) The CPs in these scaffolds also mimic the piezoelectric property of natural bone cells, which allows for better cell-scaffold communication [251]. PANI/PVDF and PPy/PVDF [207]. Importantly, the porous structure of CP-containing scaffolds allows for the implantation of calcium phosphate nanoparticles and drugs in addition to promoting cell growth and enhancing intercellular communication through enhanced conductivity; growth of MC3T3 cells on the scaffold is readily apparent when comparing SEM images from before and after cell culture ( Figure 16) The CPs in these scaffolds also mimic the piezoelectric property of natural bone cells, which allows for better cell-scaffold communication [251]. In the field of cardiac and muscle tissue engineering, the properties of CP-containing scaffolds that have emerged as important are the presence of charges on doped CPs, their fibrous structure, and their conductivity. Scaffolds of PANI/PLGA, PANI/PCL and PE-DOT:PSS/polydopamine/PCL blends [208] have been used in this regard. These scaffolds allow for the adhesion of growth factors, stimulated controlled cardiac heating, and not only the attachment and proliferation of cells but also successful elongation [209] and differentiation [254]. As can be seen in Figure 17A,B, a high degree of alignment was attained for PANI/PLGA nanofibers; increasing PLGA content increased alignment. Doped scaffolds, which were expected to have increased conductivity relative to neutral scaffolds, In the field of cardiac and muscle tissue engineering, the properties of CP-containing scaffolds that have emerged as important are the presence of charges on doped CPs, their fibrous structure, and their conductivity. Scaffolds of PANI/PLGA, PANI/PCL and PEDOT:PSS/polydopamine/PCL blends [208] have been used in this regard. These scaffolds allow for the adhesion of growth factors, stimulated controlled cardiac heating, and not only the attachment and proliferation of cells but also successful elongation [209] and differentiation [254]. As can be seen in Figure 17A,B, a high degree of alignment was attained for PANI/PLGA nanofibers; increasing PLGA content increased alignment. Doped scaffolds, which were expected to have increased conductivity relative to neutral scaffolds, promoted adhesion of cardiomyocytes, which associated with each other to form isolated cell clusters ( Figure 17C). The cardiomyocytes in each cluster beat synchronously, and application of an electric field caused the beating of all the clusters to synchronize when higher amounts of PANI were used ( Figure 17D). Reprinted with permission from Ref. [208]. Copyright 2013, with permission from Elsevier. Lastly, because of the central function of the nervous system in transmitting impulses from one part of the body to another, nerve growth applications have received considerable attention in the field of CP-based tissue scaffolds. PPy-coated electrospun PLGA and P3HT/poly(N-vinylpyrrole) [256] scaffolds ( Figure 18) have been used for this application. The alignment of the fibers in the scaffold enables the oriented growth of nerves ( Figure 18B), and electrical stimulation mimics the electrophysiological environment of native nerve cells and promotes their growth and proliferation; longer neurites are obtained from PPy-coated nanofibers ( Figure 18C) [256]. Figure 17. Cardiac tissue scaffold made of a PANI/PLGA blend [208]. (A) SEM of PANI/PLGA nanofibers before and after doping with HCl. (B) Histogram of fiber angle distribution of PANI/PLGA showing a high degree of alignment. (C) Beating frequencies of indicated red and blue cell clusters without electrical stimulation. (D) Same beating frequencies as (C) but with electrical stimulation applied (yellow) showing synchronization of beating between electrical stimulation and beating in the red and blue cell clusters. Reprinted with permission from Ref. [208]. Copyright 2013, with permission from Elsevier.

Electronic Devices
The enhanced charge transport properties as well as uniformity in morphology of electrospun nanofibrous CPs lead to their applications in the field of electronic materials, particularly in the fabrication of energy conversion, energy storage, and other electronic devices.

Photovoltaics
The conjugation present in CPs enables them to absorb photons from the UV, visible, and infrared regions [206]. The uniform nanofibrous structure produced by electrospinning polymer solutions properly prevents the formation of agglomerates which serve to concentrate photon absorption. Moreover, the electrospinning process extends the chains of the polymers being electrospun, resulting in an increase in conjugation length in the polymer and an increase in the efficiency of optical absorption [199].
The increased surface area, porosity, and tunability of nanofibrous structures over non-electrospun systems also lends devices made from these structures many advantages in the field of energy conversion. The photons absorbed by CPs create pairs of electrons and positively charged "holes" in the system [280]. Increasing the surface area of materials with CPs has the effect of allowing enhanced photon absorption, complete charge separation, and easy doping of CPs. Incorporating electron acceptor materials such as TiO 2 into the device separates these electron-hole pairs, with the CPs serving the purpose of being the hole-transport layer. The increased porosity of nanofibrous structures allows for the incorporation of these nanoscale electron-acceptor materials, or vice versa [257].
A recent report [206] postulated the potential of CP nanofiber blends for use in photovoltaic devices: PANI, PPy, and PTh were each blended at 1% and 3% CP content with PAN and then electrospun into nanofibers. The incorporation of CPs into the fibers decreased average fiber diameters by as much as 43%, possibly due to the increased solution conductivity during electrospinning. Even at such low CP concentrations, the nanofibers exhibited reduced bandgaps and improved absorption and light refraction relative to bare PAN.
A hybrid solar cell made from a nanowire device of PEDOT:PSS-coated MEH-PPV layer on electrospun TiO 2 was found to have enhanced power conversion efficiency and charge conduction [257]. This structure also allows for the incorporation of many other nanoparticles for varied applications, such as in the case of a photovoltaic cell device based on co-electrospun silver nanoparticles in ethyl glycol and a PFO-based polymer, with the silver nanoparticles also acting as a solar concentrator [200]. Other examples include incorporation of a fullerene-containing polymer into nanofibers with P3HT and poly(vinylpyrrolidone) [210] and another application involving quantum dots [199]. The alignment of nanofibers in these kinds of devices also plays an important role in their function. Aligned structures enhance the collection and transport of charges, leading to an increase in the number of junctions between hole-transporting CPs and electron accepting nanoparticles, which further increases charge collection overall which can flow in ordered conductive pathways.

Capacitors
Nanofibrous, high-surface area CP-based materials can store charge through two mechanisms: through pseudo-capacitance, which is the capacity of the structure to undergo repeated reversible redox reactions, and electrical double-layer capacitance, which is the capacity of the structure to promote accumulation of charges, separate these charges, and allow for ion transport [258]. CP-based electrochemical capacitors take advantage of both these charge storage mechanisms to deliver higher power and higher energy storage per unit mass than batteries. PEDOT-coated electrospun poly(vinyl alcohol) (PVA)/graphene oxide nanofibers and PEDOT:PSS/PEO [211,258], PANI/carbon nanotubes/PEO and PANI/PE [212,213], and polyindole/carbon nanotube [214] composite materials have been used as collectors and electrodes in this regard. The high surface areas of the fibers lead to improved capacities relative to films of the same polymers, while the flexibility of the fibers may lead to conformal energy storage.
Fotia and coworkers attempted to electrospin neat camphorsulfonic acid-doped PANI but were only able to obtain electrosprayed PANI mats [281]. Instead, the electrospun nanofibers of blends of doped PANI with carrier polymers poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc) had between 50 and 75 wt.% PANI in the blends. Conductivities of these fibers were significantly lower than the electrosprayed pure PANI mats (10 −2 S·cm −1 ), which were much lower than doped PANI films (600 S·cm −1 ). The conductivity of the PANI/PMMA blended nanofibers (10 −7 S·cm −1 ) was two-orders-ofmagnitude lower than that of the PANI/PVAc blends (10 −5 S·cm −1 ), with only minimal differences found with changes in the weight percentage of PANI. The authors postulated that this was due to the much lower conductivity of pristine PMMA (10 −10 S·cm −1 ) relative to that of pristine PVAc (10 −6 S·cm −1 ) and to lower compatibility of PANI with PMMA. Specific capacitances of these nanofiber composites were relatively low, between 0.2 and 0.4 F·g −1 at 10 mV·s −1 . The team attempted to improve performance with addition of graphene oxide or iron oxide; however, oxide addition did not provide a significant improvement in specific capacitance and decreased conductivity, possibly due to aggregation of the oxides.
One approach to improving the capacitive performance of CP-based capacitors is by addition of conductive carbonaceous materials such as carbon nanotubes or graphene oxide. In this way, devices combine the pseudocapacitive behavior of the CP with electrical double-layer capacitive behavior of the carbonaceous materials [282,283]. Electrochemical capacitors were prepared by electrospinning poly(thioaniline) with PVA, with and without addition of graphene oxide [282]. Addition of graphene oxide increased capacitance from a respectable 116 F·g −1 at 0.6 A·g −1 to 166 F·g −1 . Addition of graphene oxide also increased cycle stability, from a 32% decrease after 5000 cycles without graphene oxide to only a 10% decrease when graphene oxide was added.

Field-Effect Transistors and Diodes
Near-one-dimensional field effect transistors (FETs) made from CPs could enable increased component density in circuits; fabrication of these devices from electrospun CPs is potentially simpler than using traditional lithographic techniques [191]. An FET was successfully constructed from co-electrospun P3HT and PMMA on SiO 2 ; the oriented, conductive nanofibrous structures of these P3HT-based nanofibers have been used as field-effect transistors [191].
Lee and coworkers prepared P3HT-based FETs via coaxial electrospinning of P3HT [198]. Neat P3HT nanofibers prepared via coaxial electrospinning of P3HT/CHCl 3 in the inner nozzle and chloroform in the outer nozzle demonstrated a field-effect mobility of 0.017 cm 2 ·V −1 ·s −1 and an on/off ratio of 100. The group also prepared P3HT/PCL blends with P3HT content ranging from 50% to 90% by weight [198]. Nanofibers electrospun from the P3HT/PCL blends were tested for use in FETs; as PCL content increased, the field effect mobility decreased from 0.017 cm 2 ·V −1 ·s −1 for 100% P3HT to 0.00047 cm 2 ·V −1 ·s −1 for 50% P3HT: a 97% reduction. The authors hypothesized that, as PCL content increases, phase separation occurs between the P3HT and PCL domains, allowing the PCL domains to act as defect sites and degrading hole mobility.
Chen and coworkers [200] found that adding a second electric field introduced additional extensional force that prolonged the liquid jet by 3-5 cm (Figure 19). This modified procedure produced thinner fibers (reduced from 2.11 to 1.39 µm) of highly extended and oriented P3HT/PMMA core/shell nanofibers. The PMMA shell could then be removed by solvent etching, producing 144 nm diameter P3HT nanofibers vs. 414 nm without the secondary electric field. The strong extensional forces provided by the secondary electric field apparently caused the P3HT chains to align along the jet, dramatically enhancing charge carrier mobility in FETs by more than three orders of magnitude to 1.62 × 10 −1 cm 2 ·V −1 ·s −1 .
Materials 2022, 15, x FOR PEER REVIEW 53 of 76 Figure 19. Use of a secondary electric field below the syringe (A) results in prolonged liquid jets (B,C) that enhance charge transport, resulting in dramatically enhanced mobility [200]. Reprinted with permission from Ref. [200]. Copyright 2015, John Wiley & Sons.
P3HT/poly(lactic acid) (PLA) has also been used on n-doped Si/SiO2 in order to form highly efficient diodes [215]. Interestingly, while neither P3HT or PLA electrospins well alone, blends of the two polymers produced long composite fibers. These fibers were deposited onto prepatterned gold electrodes on n-doped Si/SiO2 substrates to form versatile p-n diodes that could be tuned either optically or electrostatically.

Sensing
Another important application of electrospun CPs lies in the field of sensors for gases, ions, and even biological molecules. These rely on (a) secondary interactions between the analytes and the substrates, (b) the ability of the analytes to reduce/oxidize the substrates, or (c) the change in the electrochemical potential induced in the substrate by the analyte binding to a sensor immobilized on the substrate [284]. The sensing ability is measured as a function of changing conductivity/resistivity of the substrate or changing absorption or fluorescence of the substrate [45]. The high surface area of nanofibers improves sensing ability over thin films [131]. The nanofibrous structure of electrospun CPs has the added advantage of increasing the number of conduction pathways in the sensing material, lending it added sensitivity and faster response rate [285]. For a thorough discussion on the Figure 19. Use of a secondary electric field below the syringe (A) results in prolonged liquid jets (B,C) that enhance charge transport, resulting in dramatically enhanced mobility [200]. Reprinted with permission from Ref. [200]. Copyright 2015, John Wiley & Sons.
P3HT/poly(lactic acid) (PLA) has also been used on n-doped Si/SiO 2 in order to form highly efficient diodes [215]. Interestingly, while neither P3HT or PLA electrospins well alone, blends of the two polymers produced long composite fibers. These fibers were deposited onto prepatterned gold electrodes on n-doped Si/SiO 2 substrates to form versatile p-n diodes that could be tuned either optically or electrostatically.

Sensing
Another important application of electrospun CPs lies in the field of sensors for gases, ions, and even biological molecules. These rely on (a) secondary interactions between the analytes and the substrates, (b) the ability of the analytes to reduce/oxidize the substrates, or (c) the change in the electrochemical potential induced in the substrate by the analyte binding to a sensor immobilized on the substrate [284]. The sensing ability is measured as a function of changing conductivity/resistivity of the substrate or changing absorption or fluorescence of the substrate [45]. The high surface area of nanofibers improves sensing ability over thin films [131]. The nanofibrous structure of electrospun CPs has the added advantage of increasing the number of conduction pathways in the sensing material, lending it added sensitivity and faster response rate [285]. For a thorough discussion on the use of electrospun nanofibers as sensors, see the review article by Halicka and Cabaj [286].

Gas and Humidity Sensors
Gases ranging from common potentially harmful or pollutant gases such as ammonia, volatile organic compounds, ethanol, liquified petroleum gas, organic solvents, and ethanol to warfare agent gases such as organophosphorus nerve agents, nitroaromatics, and hydrogen gas have been detected on sensors based on electrospun CPs. Ammonia [216,259], hydrogen [217], and liquified petroleum gas [209] have been found to exchange electrons with PANI and increase the electrical resistance of PANI/ZnO composites, while the same mechanism has been found operative in the detection of electronaccepting nitroaromatics [218] such as trinitrotoluene by electron-donating polyfluorene (PFO)-based materials in composites with PS and gelatin. Volatile organic compounds such as benzene and toluene have been demonstrated to swell nanofiber composites of PEDOT:PSS/carbon nanotubes/poly(vinylpyrrolidone) [219], PANI/isoindigo-based copolymer/PMMA [220], and PFO/PMMA [221], destroying conduction pathways in these materials and increasing electrical resistance. Ethanol has been detected with a sensor made from SnO 4 /PANI/poly(hydroxy-3-butyrate), with hydrogen bonding between ethanol and amine domains, as well as dopants, being the operating mechanism [222]. Lastly, the fluorescence of conjugated CPs like polyquinoxaline and poly(phenylene vinylene) (PPV) has been shown to be quenched by π-π interactions with organophosphorous nerve gas [260] and aromatic organic solvents [223].
Humidity sensors are widely implemented in applications including pharmaceuticals, paper manufacturing, electronics, textiles, packaging, and food [287]. Many types of organic materials have been used for resistive and capacitive humidity sensors [288], and CP nanofibers are only a small subset of the approaches used. Naval Research Laboratory scientists trying to develop CP-coated nanofiber-based chemical warfare agent sensors discovered that humidity significantly impacted the conductivity of the fibers [289]. Since then, many researchers have explored the use of CP nanofibers for humidity sensing. Aussawasathien and coworkers [224] noted that, when designing humidity sensors, it is important to ensure that the polymer nanofibers do not swell or dissolve in water. Thus, Li and coworkers prepared a thermally crosslinkable PANI-containing nanocomposite that was electrospun, crosslinked, and tested for humidity sensing [225]. While the sensors were effective at detecting high humidity levels in the absence of PANI, addition of PANI decreased baseline impedance, allowing lower detection limits. They found a two-ordersof-magnitude change in impedance when relative humidity changed from 22% to 97% and stated that the high surface area increased ability to rapidly absorb and desorb water, leading to more rapid detection.

Ion Sensors
Electrospun CP-based sensors have been used to detect a wide variety of ions, including phosphate, mercury (II), and superoxide. Electrochemical detection of phosphate was accomplished using PAN/PT composite nanofibers deposited onto indium tin oxide-coated glass [226]. As phosphate concentration increased, the oxidation potential of PT in the fibers decreased. Peak current response was found to increase as the PT content in the fibers increased.
High levels of the superoxide anion (O 2 − ) can result in inhibition of cell growth, mutagenesis, and cell death [290]. Santhosh and coworkers electropun PMMA nanofibers and oxidatively deposited a PANI coating on the surface (Figure 20). By utilizing hydrogen tetrachloroaurate as the oxidant, they were able to form gold nanoparticles in the PANI. The enzyme superoxide dismutase was then immobilized on the gold nanoparticles. The resultant composite could then be used as amperometric sensors for superoxide (O 2 .− ) [261].
The sensors had good detection limits (0.3 µM), rapid response times (4 s), and good stability and reproducibility.
PEER REVIEW 55 of 76 Figure 20. PMMA/PANI superoxide sensors were prepared by electrospinning PMMA and coating the resultant nanofibers with PANI via chemical oxidation of aniline with hydrogen tetrachloroaurate [261]. The enzyme superoxide dismutase was then bonded to the gold nanoparticles formed in the PANI during oxidation, and the enzyme was used to detect superoxide. Republished with permission of Ref. [261]; permission conveyed through Copyright Clearance Center, Inc.

Biosensors
The ability of CPs to be modified with a wide range of bioactive species has led to extensive development of CP biosensors [45], Sensors for the detection of additives, antigens, and biomolecules have also been developed based on electrospun CPs. These work by either quenching interactions with analytes or by changing electrochemical potential as a result of binding of analyte on another sensor immobilized on the surface of the material. Sudan dyes, which are illegal food additives, have been found to quench the fluorescence of a PPV/PVA composite [228]. Antibodies to immunoglobulin G, a measure of Figure 20. PMMA/PANI superoxide sensors were prepared by electrospinning PMMA and coating the resultant nanofibers with PANI via chemical oxidation of aniline with hydrogen tetrachloroaurate [261]. The enzyme superoxide dismutase was then bonded to the gold nanoparticles formed in the PANI during oxidation, and the enzyme was used to detect superoxide. Republished with permission of Ref. [261]; permission conveyed through Copyright Clearance Center, Inc.
A colorimetric sensor was developed using PANI/polyvinylbutyral/poly(amide-6) nanofibers for the detection of Hg(II). The interaction between mercury and PANI, a multistep process involving complexation of Hg 2+ to aniline nitrogens, translates to color changing for rapid visual evidence of mercury exposure ( Figure 20) [227]. The color change in the sensors could be observed with the naked eye, or colorimetric responses could be quantitatively converted to numerical values.

Biosensors
The ability of CPs to be modified with a wide range of bioactive species has led to extensive development of CP biosensors [45], Sensors for the detection of additives, antigens, and biomolecules have also been developed based on electrospun CPs. These work by either quenching interactions with analytes or by changing electrochemical potential as a result of binding of analyte on another sensor immobilized on the surface of the material. Sudan dyes, which are illegal food additives, have been found to quench the fluorescence of a PPV/PVA composite [228]. Antibodies to immunoglobulin G, a measure of immunity, and carcinoembryonic antigens, have been immobilized on PPy/PEO [229] and PEDOT:PSS/PVA nanofibers ( Figure 21) [230], respectively. Enzymes for the detection and possible degradation of glucose, such as glucose oxidase, have been immobilized on the surface of PEDOT/poly(L-lactide) [262], PAN/PPy [263], and poly-4-(4,7-di(thiophen-2-yl)-1H-benzo[d]imidazol-2-yl) benzaldehyde)/multiwalled carbon nanotubes/nylon 6,6 fibers [264]. A sensor for the detection of non-Hodgkin's lymphoma has even been developed, based on an oligonucleotide immobilized on nitrile butadiene rubber/poly[(ethylene glycol)dimethacrylate]/PEDOT [265].

Wearable Sensors
Sensors that can be worn on the human body have also been developed, for applications including the detection of harmful substances in an area [266], analysis of body fluids [201], monitoring of vital signs [268][269][270], and recording of motion [268]. Promising sensors are those that have rapid response times to external stimuli and are stable through continued use and cycling between loading and unloading of the stimulus [291]. As with other sensors, the wearable chemical sensors operate on the principle of the CP substrate undergoing electron transfer with the analyte in question [231,285]. Mechanical wearable sensors operate on the principle of piezoelectricity/piezoresistivity: the application of an external pressure to layers of CP brings various regions of the material in contact with each other [231,292,293]. An increase in the number of contacting regions as a result of an increase in pressure induces an increase in the conductivity of the material, which is then

Wearable Sensors
Sensors that can be worn on the human body have also been developed, for applications including the detection of harmful substances in an area [266], analysis of body fluids [201], monitoring of vital signs [268][269][270], and recording of motion [268]. Promising sensors are those that have rapid response times to external stimuli and are stable through continued use and cycling between loading and unloading of the stimulus [291]. As with other sensors, the wearable chemical sensors operate on the principle of the CP substrate undergoing electron transfer with the analyte in question [231,285]. Mechanical wearable sensors operate on the principle of piezoelectricity/piezoresistivity: the application of an external pressure to layers of CP brings various regions of the material in contact with each other [231,292,293]. An increase in the number of contacting regions as a result of an increase in pressure induces an increase in the conductivity of the material, which is then interpreted to extrapolate a pressure or frequency reading after calibration. The nanofibrous structure imparted by electrospinning, with large porosity and surface area, imparts greater sensitivity and faster reaction time of the sensor to the measurement of the analyte or behavior [292].
An ammonia detector was constructed out of in situ polymerized PPy on sintered electrospun PVP/vanadyl acetylacetonate/gold(III) chloride trihydrate [266]. Sweat pH was measured using coaxial electrospun blended PANI and polyurethane (PU) [201]. PEO/PEDOT:PSS [231] and PANI/PVDF composites [267] were used as strain sensors in order to detect the flexion of the wrist, and movement and tapping of the finger. Zhao and coworkers prepared a wearable piezoelectric sensor using three electrospun polymer layers ( Figure 22A) [268]. They electrospun nanofibers from a blend of PLA, silk fibroin, and collagen. They then used oxidative polymerization to coat some of the nanofibers with PPy. These were then used to prepare a composite device using the PLA/silk fibroin/collagen nanofiber mat as the outer layer and the PPy-coated mat as the inner layer. The resultant flexible, wearable piezoelectric sensor was able to detect minute variations of pressure in the body and environment such as pulse, respiration rate, and throat vibrations ( Figure 23B) [268]. Similar devices constructed by other groups have been used to detect blood pressure [269] and facial expressions [270].
5, x FOR PEER REVIEW 57 of 76 resultant flexible, wearable piezoelectric sensor was able to detect minute variations of pressure in the body and environment such as pulse, respiration rate, and throat vibrations ( Figure 23B) [268]. Similar devices constructed by other groups have been used to detect blood pressure [269] and facial expressions [270].

Separatory and Purifying Materials
The porous structure of electrospun fibers allows not only for the adsorption of analytes but also the incorporation of materials which can catalyze degradation reactions of substances [232,294]. Furthermore, the addition of CPs in these provides a medium for transport of charges from photocatalysts like TiO2 [295]. It is for this reason that electrospun CPs have found application in the field of separation and purification of materials.

Water Purification
CPs have been particularly used for water purification, especially as electrospun CPs such as PEDOT:PSS/PEO have been observed to be resistant to water degradation [232]. A nanofiber mat of PPy coated on electrospun PAN and impregnated with nonpurified multiwalled carbon nanotubes has been observed to synergistically decontaminate water. In this device, dissolved oxygen in water reacts with photoexcited electrons on PPy and other pollutants are degraded by radicals generated by charge-carrying "holes" on the

Separatory and Purifying Materials
The porous structure of electrospun fibers allows not only for the adsorption of analytes but also the incorporation of materials which can catalyze degradation reactions of substances [232,294]. Furthermore, the addition of CPs in these provides a medium for transport of charges from photocatalysts like TiO 2 [295]. It is for this reason that electrospun CPs have found application in the field of separation and purification of materials.

Water Purification
CPs have been particularly used for water purification, especially as electrospun CPs such as PEDOT:PSS/PEO have been observed to be resistant to water degradation [232]. A nanofiber mat of PPy coated on electrospun PAN and impregnated with nonpurified multiwalled carbon nanotubes has been observed to synergistically decontaminate water.
In this device, dissolved oxygen in water reacts with photoexcited electrons on PPy and other pollutants are degraded by radicals generated by charge-carrying "holes" on the carbon nanotubes [271]. In situ polymerized PEDOT on TiO 2 nanofibers have been found to improve the photocatalytic performance of bare TiO 2 and induce the degradation of the model pharmaceutical contaminant phenazopyridine in water [272]. In situ polymerized PANI on electrospun PS was found to be able to be de-doped and complexed by heavymetal ions such as Hg(II), Cd(II), Pb(II), Cr(VI), and Cu(II), resulting in their remediation from water [273]. PANI/PAN membranes, manufactured through coating of in situ polymerization of PANI on electrospun PAN and electrospinning of a blend of PANI and PAN, were observed to effectively separate industrial oils from water due to the presence of an electric double layer and hydrophilic groups on PANI [233].

Biopurification
The separatory properties of electrospun CPs can be applied to the isolation and potential degradation of biological contaminants. PANI-coated PU electrospun fibers were used in a microfluidic device to extract and spectrophotometrically detect morphine, codeine, and papaverine in urine [274]. PANI-coated polycaprolactam nanofibers were used to adsorb angiotensin II receptor antagonists like valsartan, losartan, and irbesartan from blood plasma via ion exchange [275].
During kidney failure, protein-bound uremic toxins (PBUTs) accumulate in the blood and are difficult to remove with traditional dialysis techniques [234]. Yen and coworkers used PEDOT:PSS-containing nanofibers to create a bioelectronic interface, facilitating electrical stimulation to remove PBUTs via an improved hemodialysis approach [234]. The team electrospun a mixture of multiwalled carbon nanotubes (MWCNTs), PEDOT:PSS, PEO (carrier polymer), and (3-glycidyloxypropyl)trimethoxysilane (to promote adhesion of the fibers to the substrate) directly onto conventional poly(ether sulfone) dialysis membranes. The resultant nanofiber mats exhibited excellent biocompatibility and proved very effective at removing uremic toxins.

Smart Devices
Electrospun CP-based nanofibers have been used in the development of "smart" devices which have tunable optical properties based on the application of a changing electric field or temperature. These devices not only have quick response times to changes in stimuli but also stability in the response after multiple cycles of loading/unloading of the stimulus.

Pressure-and Temperature-Dependent Applications
Electrospun CP-based nanofibers have been used in the field of clothing, and, interestingly, in the harvesting of energy from human motion or the environment. Devices based on PEDOT and polycarbazole (PCZ) have good strain capabilities for stretchability and robustness, structural integrity, and washability with retention of properties. A PEDOT:PSS-coated composite of multiwalled carbon nanotubes/lauric acid/thermoplastic urethane has been shown to store energy and convert an applied voltage to thermal energy, and release the energy when the electric stimulus is removed [276]. The addition of PCZ to electrospun PVDF was shown to stabilize the beta phase of PVDF with the highest piezoelectric coefficient and thus enable the material to harvest energy from periodic vertical compressive motions like toe and heel motions, wrist bending, and finger movements, at a level where it is able to light up multiple light-emitting diodes ( Figure 24) [235]. A composite of PEDOT:PSS/PVA coated with silver nanoparticles on a poly(ethylene terephthalate) film was demonstrated to convert heat to electricity via phonon scattering at the silver-CP interface [236]. Devices made from electrospun CPs have been observed to quickly and uniformly heat up as a consequence of the application of voltage. The application of a voltage across a device made from PPy coated on electrospun PLA has been reported to induce quick uniform heating and resultant deformation, with the deformation retained until the applied voltage is removed and the material returns to room temperature ( Figure 25) [277]. Conversely, a device has been made which changes its electrical properties with changes in temperature. A mixture of PEDOT:PSS and poly(N-isopropylacrylamide-co-Nmethylol acrylamide) was electrospun into mats and was observed to change in surface resistance in response to temperature changes with a high sensitivity in the range of 20-50 °C [237]. Devices made from electrospun CPs have been observed to quickly and uniformly heat up as a consequence of the application of voltage. The application of a voltage across a device made from PPy coated on electrospun PLA has been reported to induce quick uniform heating and resultant deformation, with the deformation retained until the applied voltage is removed and the material returns to room temperature ( Figure 25) [277]. Conversely, a device has been made which changes its electrical properties with changes in temperature. A mixture of PEDOT:PSS and poly(N-isopropylacrylamide-co-N-methylol acrylamide) was electrospun into mats and was observed to change in surface resistance in response to temperature changes with a high sensitivity in the range of 20-50 • C [237].

Optical Applications
Electrospun CP-based nanofibers have been observed to undergo color changes on the basis of their oxidation state with the uniformity, alignment, and compactness of the device. These factors have been observed to affect not only the distribution of color but also the mechanical properties of the resultant material [238]. These have potential applications in the fields of smart textiles. PANI/PVA electrospun composite nanofibers were observed to change in color from green to blue to yellow [239]. The structure of the electrochromic device constructed from these composite nanofibers is composed of a PMMA gel electrolyte, composed of LiClO 4 and PMMA, coated onto the PANI/PVA nanofibers. The same behavior was observed in a device constructed from electrospun PANI/silk fibroin nanofibers, with the addition of good processability and conductivity sufficient to light up LEDs ( Figure 26) [239]. A color change from light brown to transparent was observed as an applied voltage was increased across a device made from electrospun mixtures of PPy/WO 3 synthesized in ionic liquids [240]. Fluorescence was also observed in devices with electrospun CPs. Blue emission was observed in an electrospun blend of a PFO-based polymer and acrylonitrile butadiene rubber, with retention of fluorescence after 200 cycles of stretching and releasing. The addition of acrylonitrile butadiene rubber served to increase flexibility and reduced aggregation [241]. Reprinted with permission from [277]. Copyright 2018, American Chemical Society.

Optical Applications
Electrospun CP-based nanofibers have been observed to undergo color changes on the basis of their oxidation state with the uniformity, alignment, and compactness of the device. These factors have been observed to affect not only the distribution of color but also the mechanical properties of the resultant material [238]. These have potential applications in the fields of smart textiles. PANI/PVA electrospun composite nanofibers were observed to change in color from green to blue to yellow [239]. The structure of the electrochromic device constructed from these composite nanofibers is composed of a PMMA gel electrolyte, composed of LiClO4 and PMMA, coated onto the PANI/PVA nanofibers. The same behavior was observed in a device constructed from electrospun PANI/silk fibroin nanofibers, with the addition of good processability and conductivity sufficient to

Conclusions and Future Outlook
Electrospinning polymers leads to high surface area and potential for alignment, giving rise to applications in textiles and filtration. The additional benefits of tunable optoelectronic properties of CPs lead to even more nanofiber applications, including sensors, tissue engineering, drug delivery, electrochromics, energy storage, and photovoltaics. Thus, even though electrospinning CPs is very challenging, researchers continue to develop approaches to overcome the barriers.
Rigidity and low molecular weight are the two main challenges for neat electrospinning of CPs; instead of regular nanofiber formation, bead-on-a-string geometries are formed [176,191], or worse, no fiber formation occurs at all. While there are very few instances of neat CP electrospinning, several workarounds are commonly employed. Blending a CP with a carrier polymer, even at very low concentrations, can form good nanofibers with surprisingly high conductivities [198]. At higher carrier polymer concentrations, many of the desirable properties of the CP (conductivity, electroactivity) are lost. Instead, coaxial electrospinning can be used to produce core/shell nanofibers, and removal of the carrier polymer can produce neat CP nanofibers [173,182]. Alternatively, CP coatings can be deposited on electrospun commodity polymers, yielding an electroactive surface that can provide a combination of the mechanical properties of the commodity polymer with the optoelectronic properties of the CP. Coating deposition methods include solution-based chemical oxidative polymerization [242][243][244][245], vapor-phase deposition [186], and dip coating [246,247].

Conclusions and Future Outlook
Electrospinning polymers leads to high surface area and potential for alignment, giving rise to applications in textiles and filtration. The additional benefits of tunable optoelectronic properties of CPs lead to even more nanofiber applications, including sensors, tissue engineering, drug delivery, electrochromics, energy storage, and photovoltaics. Thus, even though electrospinning CPs is very challenging, researchers continue to develop approaches to overcome the barriers.
Rigidity and low molecular weight are the two main challenges for neat electrospinning of CPs; instead of regular nanofiber formation, bead-on-a-string geometries are formed [176,191], or worse, no fiber formation occurs at all. While there are very few instances of neat CP electrospinning, several workarounds are commonly employed. Blending a CP with a carrier polymer, even at very low concentrations, can form good nanofibers with surprisingly high conductivities [198]. At higher carrier polymer concentrations, many of the desirable properties of the CP (conductivity, electroactivity) are lost. Instead, coaxial electrospinning can be used to produce core/shell nanofibers, and removal of the carrier polymer can produce neat CP nanofibers [173,182]. Alternatively, CP coatings can be deposited on electrospun commodity polymers, yielding an electroactive surface that can provide a combination of the mechanical properties of the commodity polymer with the optoelectronic properties of the CP. Coating deposition methods include solutionbased chemical oxidative polymerization [242][243][244][245], vapor-phase deposition [186], and dip coating [246,247].
CPs, commodity polymers, and, in some cases, nanoparticles, can be combined in seemingly innumerable ways to form electrospun nanofibers with a wide range of properties. The ability to control the properties by changing the formulation is one of the reasons these materials are of such interest. Efforts will continue to directly electrospin neat CPs by increasing molecular weight and improved solubility, with the possible end goal of enhanced conductivity in aligned CP nanofibers. As the utility of these materials continues to become more widely known, additional unexpected applications will arise. When applications of CP nanofibers are further developed, an increased focus on production-scale electrospinning equipment is sure to follow.  Acknowledgments: The authors would like to thank Karen Lewis and David Irvin for helpful feedback regarding this work. Additional thanks go to the reviewers for their careful analysis of this manuscript and their invaluable comments and suggestions.

Conflicts of Interest:
The authors declare no conflict of interest.