A Review of Electroactive Polymers in Sensing and Actuator Applications

Abstract

Electroactive polymers (EAPs) represent a versatile class of smart materials capable of converting electrical stimuli into mechanical motion and vice versa, positioning them as key components in the next generation of actuators and sensors. This review summarizes recent developments in both electronic and ionic EAPs, highlighting their activation mechanisms, material architectures, and multifunctional capabilities. Representative systems include dielectric elastomers, ferroelectric and conducting polymers, liquid crystal elastomers, and ionic gels. Advances in fabrication methods, such as additive manufacturing, nanocomposite engineering, and patternable electrode deposition, are discussed with emphasis on miniaturization, stretchability, and integration into soft systems. Applications span biomedical devices, wearable electronics, soft robotics, and environmental monitoring, with growing interest in platforms that combine actuation and sensing within a single structure. Finally, the review addresses critical challenges such as long-term material stability and scalability, and outlines future directions toward self-powered, AI-integrated, and sustainable EAP technologies.

1. Introduction

1.1. Background and Importance of Electroactive Polymers (EAPs)

Electroactive polymers (EAPs) are smart materials capable of converting electrical energy into mechanical deformation and vice versa. Their high strain capability, flexibility, low density, and mechanical compliance make them ideal for applications in soft robotics, biomedical devices, aerospace structures, and energy harvesting systems [1,2,3,4].

Unlike conventional actuators made from metals, ceramics, or piezoelectric ceramics, EAPs offer superior compliance, biocompatibility, and adaptability to soft and dynamic environments. These characteristics have driven growing interest in their use for artificial muscles, haptic interfaces, flexible electronics, autonomous robots, prosthetics, and implantable medical systems [5,6,7,8].

1.2. Historical Development of EAPs

The origins of EAP research date back to 1880 when Wilhelm Roentgen first reported electrically-induced deformation in a rubber sheet [9]. Progress was incremental and sporadic until the mid-to-late 20th century when EAPS began to be used as actuators and sensors. Key historical milestones include:

• 1899: Sacerdote introduced the first engineered electric field-based strain response [9].
• 1925: Eguchi discovered the first piezoelectric polymer, Electret, composed of rosin and beeswax solidified under a DC field [9].
• 1950s–1960s: Discovery and exploration of piezoelectric and ferroelectric polymers such as polyvinylidene fluoride (PVDF) [10,11].
• 1970s–1980s: Introduction of Ionic Polymer-Metal Composites (IPMCs) and conductive polymers, enabling low-voltage actuation [12].
• 1990s: Emergence of Dielectric Elastomer Actuators (DEAs), which demonstrated high strain, energy density, and efficiency [4,13,14].
• 2000s–Present: Rapid advancements in 3D printing, nanotechnology, and bio-compatible polymers, expanding EAP use in biomedical implants, soft robotics, and multifunctional systems [3,6,7].

1.3. Classification of EAPs

EAPs are broadly categorized into Electronic EAPs and Ionic EAPs, based on their activation mechanism [13,15].

1.3.1. Electronic EAPs (Field Activated)

Electronic EAPs operate through Coulombic forces generated by external electric fields, leading to deformation without significant ionic movement [3,16]. They exhibit fast response times and operate effectively in dry environments, making them suitable for aerospace, haptic interfaces, and high-speed actuation systems [7,17].

• Dielectric Elastomers (DEs): Function as variable capacitors, demonstrating substantial strain responses (>100%), and are prominent in artificial muscles and soft robotics [6,10,14]. DEs typically require high voltages (1–10 kV) and low currents (<100 µA), necessitating specialized circuitry.
• Ferroelectric Polymers: PVDF and its copolymers exhibit piezoelectric properties valuable in energy harvesting, biomedical sensors, and MEMS [10,18].
• Liquid Crystal Elastomers (LCEs): Undergo molecular reorientation under electric fields, enabling programmable shape changes for adaptive optics, artificial muscles, and biomedical devices [8,19,20].

1.3.2. Ionic EAPs (Ion-Activated)

Ionic EAPs deform due to ion migration within the polymer structure under low-voltage stimuli (<5 V), excelling in biomedical, soft robotics, and underwater applications due to their ability to generate significant bending displacements [12,21].

• Ionic Polymer-Metal Composites (IPMCs): Hydrated membranes with conductive electrodes that bend under low-voltage stimulation, extensively applied in bio-inspired robotics and biomedical prosthetics [8,22,23].
• Conducting Polymers: Polypyrrole (PPy), polyaniline (PANI), and Poly (3,4-ethylenedioxythiophene) (PEDOT) undergo electrochemical redox reactions inducing volume changes suitable for bioelectronics and drug delivery [24,25].
• Ionic and Polyelectrolyte Gels: Responsive gels that swell or contract under electric stimulation, used in drug delivery, artificial skin, and soft actuator systems [15,26].

1.4. Importance of EAPs in Modern Technology

EAPs exhibit versatility across diverse technological fields, including:

• Biomedical Applications: artificial muscles, drug delivery systems, and soft robotic prosthetics [5,27,28].
• Aerospace and Robotics: morphing aircraft structures, bio-inspired actuators, and adaptive robotic systems [29,30].
• Flexible Electronics: wearable sensors, epidermal electronics, and haptic feedback interfaces [7,28].

Recent developments, including flexible printed electronics and self-powered multifunctional EAP systems, promise expanded application scenarios, overcoming traditional limitations related to environmental stability and cyclic loading hysteresis [3,31,32,33].

Table 1. Comparison of Electronic and Ionic EAPs [6,13,15].

Property	Electronic EAPs	Ionic EAPs
Activation Mechanism	Electric Field	Ion Transport
Response Time	Fast (ms)	Slow (s)
Energy Efficiency	High	Moderate
Operating Voltage	High (>1 kV)	Low (<5 V)
Mechanical Strength	High	Moderate
Environmental Sensitivity	Low	High (requires hydration)

summarizes key properties of Electronic and Ionic EAPs, highlighting their distinct attributes [6,13,15].

2. Materials and Methods

Due to the variety of material types and activation mechanisms, the processes for creating and using EAPs vary. In the last decade, novel manufacturing methods and combinations of EAP materials have advanced the field and pushed the boundaries in terms of flexible electronics, control, and integration. The following section organizes the manufacturing methods into ionic and electric EAP materials and highlights current advances in the field.

2.1. Dielectric Elastomers

Dielectric elastomers (DEs) are a class of electroactive polymers that operate based on the principle of electrostatic compression. Structurally, they consist of a thin elastomeric dielectric film sandwiched between two compliant electrodes. When a high voltage is applied across the electrodes, electrostatic forces compress the film’s thickness and cause it to expand laterally, generating a mechanical strain. This actuation mechanism is governed by Maxwell stress, which is defined as [34,35,36]:

$$P={ϵ}_0{ϵ}_r{\left({\displaystyle \frac{V}{d}}\right)}^2$$

(1)

where P is the pressure, ${ϵ}_0$ the vacuum permittivity, ${ϵ}_r$ the relative permittivity of the elastomer, V the applied voltage, and d the film thickness. The resulting actuation strain can be approximated using:

$$\epsilon ={\displaystyle \frac{P}{Y}}$$

(2)

with Y being the Young’s modulus of the elastomer. Under optimized conditions, strains exceeding 300% have been reported [35].

In dielectric elastomers, voltage-induced charge accumulation on compliant electrodes results in compressive stress (Maxwell stress) across the polymer, leading to lateral expansion. As shown in Figure 1, this mechanism (b) enables significant deformation, particularly in circular geometries (a), where radial displacement is commonly exploited in artificial muscle applications.

The most commonly used dielectric materials for DEs include acrylic elastomers such as VHB 4905 and VHB 4910 (3M) [36,37], silicone-based elastomers like PDMS, and thermoplastic polyurethanes (TPU) [17]. Acrylics are known for their high dielectric constant but exhibit viscoelastic behavior, while silicones provide excellent elasticity but require additives to enhance their dielectric properties [8,37]. Recent innovations have introduced interpenetrating polymer networks (IPNs), which improve electromechanical performance and eliminate the need for mechanical pre-strain by introducing fixed internal stress during polymerization [14,16]. Nanocomposite elastomers, obtained by incorporating high-permittivity nanoparticles such as barium titanate (BaTiO₃), aluminum oxide (Al₂O₃), or graphene, have further enhanced dielectric properties and electromechanical coupling [18,22,38].

In terms of processing, traditional manufacturing methods for DE films include spin coating, roll-to-roll coating, and molding. More recently, additive manufacturing techniques, especially direct ink writing (DIW) and fused deposition modeling (FDM), have enabled the fabrication of elastomeric layers and integrated electrode structures with high spatial precision [17]. In TPU-based systems, FDM has shown promise using flexible filaments [16]. Mechanical pre-strain remains critical to achieve large actuation strains and is typically applied biaxially using stretcher frames. Alternative strategies explore patterned or directional pre-straining to introduce anisotropic deformation or out-of-plane motion [6,14,34].

Compliant electrodes used in DE actuators must maintain electrical conductivity under high strain. Materials commonly used include carbon grease, graphite, silver nanoparticle inks, conductive polymers such as PEDOT:PSS, and thin metallic films. These are typically deposited using spray coating, screen printing, inkjet printing, or transfer printing methods [39,40]. Recent approaches focus on stretchable multilayer electrodes and micro-patterned designs to enhance performance and reduce mechanical hysteresis. Self-healing and self-cleaning electrode materials are also being explored to increase durability in long-term applications [4,20,41].

Various actuator architectures have emerged to optimize electromechanical performance. These include stacked multilayer actuators for high force output, spring-roll actuators that offer compact form factors, and bistable structures such as bistable electroactive polymer actuators (BSEPs). BSEPs leverage a dual-mode energy landscape to enable fast, energy-efficient snap-through actuation [4,7,42].

Furthermore, advances in control strategies for DEAs have significantly improved their functional performance, especially in wearable and biomedical applications. Medina et al. [43] reviewed a wide range of modeling and control techniques, including linear controllers (e.g., PID, LQR), nonlinear adaptive schemes, and model predictive control. Particularly relevant are recent integrations of machine learning, such as convolutional neural networks (CNNs) for deformation estimation and deep reinforcement learning (DRL) to mitigate viscoelastic hysteresis and enhance trajectory tracking in untethered soft robotic systems. These developments highlight the growing synergy between materials engineering and AI-enabled control frameworks in real-time EAP actuator operation.

2.2. Ferroelectric Polymers

Ferroelectric polymers, particularly poly(vinylidene fluoride) (PVDF) and its copolymers such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), are widely utilized in electromechanical systems due to their spontaneous polarization and pronounced piezoelectric response. This functionality arises from the alignment of molecular dipoles within the crystalline $\beta$-phase, characterized by an all-trans (TTT) conformation that facilitates strong dipole ordering [44,45]. In contrast, other polymorphs such as $\alpha$ and $\gamma$ exhibit disordered dipole orientations, yielding lower piezoelectric activity [6,46].

Strategies to enhance $\beta$-phase content have focused on both post-processing and material-level modifications. Traditional methods include mechanical stretching and high-voltage electrical poling (up to 100 MV/m), while more recent approaches such as thermal annealing, nanoconfinement, and template-guided crystallization offer finer control over phase morphology [3,47,48]. Electrospinning, in particular, has gained attention for simultaneously producing oriented nanofibers and inducing high $\beta$-phase content without additional treatments [44,49].

Nanocomposite engineering further enhances piezoelectric performance. Incorporating functional fillers such as ZnO, BaTiO₃, graphene oxide, or carbon nanotubes serves to nucleate β-phase formation and increase the dielectric constant. For instance, ZnO nanoparticles have been shown to raise $\beta$-phase fraction to over 80%, while BaTiO₃ nanorods contribute to interfacial polarization and improved electromechanical coupling [44,45].

As illustrated in Figure 2, the alignment of molecular dipoles under an electric field induces net polarization across the polymer film.

The piezoelectric behavior of PVDF-based materials is often described by the relation:

$$S=d_{31}·E$$

(3)

where S is the induced strain, $d_{31}$ the transverse piezoelectric coefficient (typically 20–30 pC/N for PVDF), and E the applied electric field. Although they exhibit lower strains than dielectric elastomers, ferroelectric polymers offer faster response times, excellent durability, and direct voltage output—desirable features for real-time sensing and biomedical integration [28].

Recent progress in additive manufacturing techniques such as direct ink writing (DIW) and fused deposition modeling (FDM) has enabled the fabrication of customized PVDF-based devices on flexible substrates [17,32]. Additionally, electrospun films with high porosity and mechanical compliance are particularly suited for wearable and implantable applications [50,51].

To ensure efficient transduction, compliant electrodes such as gold, silver, PEDOT:PSS, or carbon inks are applied via printing or evaporation [52,53]. Optimizing the electrode-polymer interface remains essential for energy transfer efficiency and long-term stability.

Overall, PVDF and its derivatives represent a robust platform for piezoelectric actuation and sensing. Their applicability continues to expand in areas such as soft robotics, energy harvesting, and bioelectronic systems. However, challenges remain in achieving scalable, environmentally stable devices with consistent electromechanical performance. Future directions include the development of green nanocomposites, improved phase-control strategies, and integration with AI-enabled platforms for multifunctional operation [39,54].

2.3. Liquid Crystal Elastomers (LCEs)

Liquid crystal elastomers (LCEs) are soft, stimuli-responsive materials that exhibit large, reversible deformations by coupling the anisotropic behavior of liquid crystals with the elasticity of crosslinked polymer networks. Their actuation arises from the reorientation of mesogenic domains, rod-like liquid crystal molecules, triggered by external stimuli such as heat, light, electric, or magnetic fields. Upon stimulation, mesogens undergo a nematic-to-isotropic phase transition, leading to contraction along the director axis and producing macroscopic shape changes [6,20].

The actuation strain, which can exceed 200% in optimized systems, strongly depends on the alignment of mesogens prior to crosslinking. Figure 3 illustrates the mechanism, showing the transition from a disordered state (off) to an aligned, contracted configuration (on), where the mesogens orient along a preferred direction.

LCE synthesis typically involves polymerizable mesogens that are commonly acrylates or siloxanes, combined with flexible crosslinkers to form anisotropic elastomer networks. The actuation performance is influenced by molecular architecture, crosslinking density, and the degree of pre-alignment. Side-chain LCEs typically offer greater actuation strain, while main-chain variants provide improved mechanical strength and thermal stability [20].

Mesogen alignment is critical and achieved through methods such as mechanical stretching, electric or magnetic field application during polymerization, shear flow during extrusion, or using alignment layers (e.g., rubbed polyimide) [20]. After alignment, the network is fixed via UV-curing or thermal crosslinking, locking in the anisotropic orientation.

Recent advances in additive manufacturing, particularly direct ink writing (DIW) and digital light processing (DLP, have enabled spatial control over mesogen alignment and composition. These techniques support the fabrication of 3D and 4D printed actuators capable of complex shape changes including bending, twisting, and folding [21,55]. In 4D printing, shape evolution occurs over time or under environmental stimuli, making it ideal for biomimetic and adaptive structures.

While LCEs are conventionally actuated thermally or optically, incorporation of conductive fillers such as carbon nanotubes or silver nanowires has enabled electrothermal actuation via Joule heating. This method provides localized, low-voltage actuation compatible with printed electronics [3].

Overall, the capacity of LCEs to deliver programmable, reversible, and directional deformations makes them ideal for applications in soft robotics, adaptive optics, wearable systems, and smart biomedical devices. Challenges remain in ensuring stable long-term performance and precise control of mesogen alignment at larger scales, but continued innovation in materials chemistry and fabrication is rapidly addressing these limitations.

2.4. Ionic Polymer-Metal Composites (IPMCs)

Ionic Polymer-Metal Composites (IPMCs) are soft electroactive materials capable of producing large bending deformations under low-voltage electrical stimulation, typically below 5 V. Their structure consists of a hydrated ionomer membrane—commonly Nafion or Flemion—coated on both sides with conductive metallic electrodes. When an electric field is applied, mobile cations and solvated water molecules migrate toward the cathode, generating internal pressure gradients that result in asymmetric swelling and macroscopic bending [3,8].

Figure 4 illustrates this actuation mechanism. In the absence of voltage, ions are evenly distributed. Upon stimulation, directional ion migration and localized swelling lead to bending toward the anode.

Nafion remains the benchmark ionomer due to its high ionic conductivity and chemical stability, but alternative membranes such as cellulose-based ionomers and hydrogel-polymer composites have been proposed to reduce cost and improve biocompatibility [56]. Fabrication generally involves surface roughening of the polymer, ion exchange to optimize mobility (e.g., replacing Na⁺ with Li⁺ or H⁺), and deposition of metallic electrodes via electroless plating using platinum or gold salts. Post-processing steps such as thermal annealing and hydration are critical to restoring optimal performance.

Recent innovations have focused on enhancing durability, flexibility, and responsiveness through alternative electrode materials. These include ultra-thin metallic films, metallic nanoparticles, and conductive polymers like polypyrrole or PEDOT:PSS, which improve compliance and reduce electrode delamination [57].

To expand design versatility, multilayer casting techniques and 3D printing of ionomer precursors have enabled more complex geometries and spatially resolved actuator-sensor integration [58]. However, operational challenges remain. Performance degradation over time, primarily due to water electrolysis, back-diffusion of ions, and mechanical fatigue, remains a significant limitation. Over extended cycles, these effects lead to reduced bending amplitude, electrode detachment, and hydration loss.

Mitigation strategies include the use of ionic liquids (instead of water), embedded hydration reservoirs, and polymer encapsulation to reduce solvent evaporation and maintain ionic conductivity over time [8,59]. Continued improvements in membrane formulation and electrode architecture are essential to enhance stability and enable long-term deployment in bioinspired robotics, artificial muscles, and implantable medical devices.

2.5. Conducting Polymers (CPs)

Conducting polymers (CPs) are electroactive materials that deform via reversible redox reactions. When electrically stimulated, they undergo oxidation or reduction, leading to ion exchange with the surrounding electrolyte and resulting in volumetric changes such as expansion or contraction. These materials operate under low voltages (typically <1.5 V), making them suitable for bioelectronic and wearable systems [25,60].

The most widely studied CPs include polypyrrole (PPy), polyaniline (PANI), and poly(3,4-ethylenedioxythiophene) (PEDOT). PEDOT:PSS, in particular, offers high electrical conductivity, good environmental stability, and processability in aqueous dispersions, making it ideal for flexible thin films. Blending CPs with elastomers or hydrogels improves their stretchability and mechanical robustness for use in soft actuators and sensors [60].

CP actuators can be fabricated via electropolymerization—where monomers are oxidized on a conductive substrate to form films with tunable thickness and morphology or via chemical synthesis followed by casting or printing. Additive manufacturing methods such as inkjet and screen printing have enabled scalable integration of CPs into flexible electronics [60].

Depending on the configuration, CP films may function as both actuator and electrode. Alternatively, they can be deposited onto flexible substrates such as indium tin oxide (ITO), gold-coated polymers, or carbon-based materials to enhance performance and durability [32].

To improve actuation output and cycling stability, recent efforts have focused on hierarchical microstructures. For example, acanthosphere-like PPy morphologies increase electroactive surface area, enhance ion transport, and maintain structural integrity under mechanical loading [61].

Miniaturized CP actuators have also been developed using lithography or soft molding, enabling applications in microrobotics, neural interfaces, and implantable drug delivery systems. These microscale actuators benefit from the intrinsic biocompatibility and electrochemical responsiveness of CPs, allowing precise, low-power control in compact biomedical devices [21].

While CPs offer high flexibility and tunable electrochemical properties, limitations include moderate actuation strain compared to dielectric elastomers and potential performance degradation over time due to overoxidation or dedoping. Research continues into stabilizing formulations, composite integration, and real-time control strategies to expand their long-term use in soft robotics and bioelectronic platforms.

2.6. Ionic and Polyelectrolyte Gels

Ionic and polyelectrolyte gels are soft, water-rich polymer networks capable of undergoing large, reversible deformations in response to electrical stimuli. Their actuation arises from ion migration, osmotic pressure gradients, and electrochemical effects within the gel matrix, which result in swelling, contraction, or bending [3]. These materials operate at low voltages and offer mechanical compliance compatible with biological tissues.

Structurally, these gels are composed of hydrophilic polymer chains crosslinked into three-dimensional networks. Typical materials include polyacrylamide, alginate, PAMPS (poly(2-acrylamido-2-methylpropane sulfonic acid)), and natural polymers such as gelatin and cellulose derivatives. Polyelectrolyte gels incorporate fixed ionic groups along their backbone, enhancing ionic conductivity and responsiveness. Integration of stimuli-responsive moieties, such as thermoresponsive (e.g., PNIPAM), pH-sensitive, or photoresponsive units, further broadens their actuation modalities [26].

A typical actuation mechanism is illustrated in Figure 5: when an electric field is applied, mobile ions within the gel migrate asymmetrically, generating local osmotic gradients that induce bulk deformation.

Fabrication techniques for ionic gels typically involve casting precursor solutions into molds followed by chemical or UV-initiated crosslinking. More advanced patterning methods, such as inkjet printing, soft lithography, and micromolding, enable integration with microfluidics or flexible substrates. These approaches allow for precise control over geometry and directionality of actuation, which is crucial for soft robotic and lab-on-chip devices [39,62].

Compliant electrode materials are critical for maintaining performance. Common options include PEDOT:PSS, silver nanowires, carbon cloth, or ITO films. In some designs, electrodes are embedded directly within the gel matrix to form monolithic actuator-electrode structures, which enhance durability and electrical efficiency [16].

These gels are especially promising in biomedical and microfluidic applications due to their softness, low driving voltage, and compatibility with aqueous environments. Applications include artificial skin, soft microvalves, biomimetic actuators, and responsive drug delivery systems [5].

Challenges remain in improving actuation speed, repeatability, and long-term mechanical stability under cyclic loading. Future work is focused on nanocomposite reinforcement, encapsulation strategies to reduce water loss, and hybrid systems combining gels with ionic liquids or conductive polymers for enhanced performance.

2.7. Current Landscape of EAP-Based Actuation

Across all classes of EAPs, significant strides have been made in translating material-level actuation mechanisms into functional systems. Dielectric elastomers dominate in high-strain soft robotics and wearable artificial muscles due to their rapid response and energy density. PVDF-based ferroelectric polymers enable compact, low-power actuation for tactile sensing and biomedical stimulation, while ionic polymer-metal composites (IPMCs) are particularly suited for underwater systems and prosthetics due to their low-voltage operation and biomimetic motion. Conducting polymers such as polypyrrole (PPy) and PEDOT support miniaturized actuation in drug delivery and neural interfaces, and ionic gels provide morphological actuation in lab-on-chip and microfluidic platforms. The convergence of additive manufacturing, responsive materials, and intelligent control strategies is accelerating the integration of EAPs into real-world biomedical, robotic, and wearable applications.

In particular, recent surveys in robotic prosthetics underscore the growing adoption of EAP-based soft actuators in assistive devices such as artificial limbs and exoskeletal components. These actuators offer low stiffness, high compliance, and intrinsic safety, properties critical for user comfort and interaction in daily use [63]. Compared to rigid, motor-driven systems, EAP actuators can produce more natural, lifelike motion while reducing fatigue during repetitive tasks.

Notably, dielectric elastomers and electrohydraulic systems have demonstrated excellent compatibility with wearable prosthetics, including trans-radial and trans-tibial limbs. Their mechanical compliance allows seamless integration into body-conforming structures, while digital manufacturing enables personalization via 3D scanning and printing [63]. These advances reduce the risk of mechanical injury from actuator failure or high inertia, positioning EAPs as a compelling alternative for clinical deployment [63].

Alongside hardware progress, there is a growing focus on embedding computational intelligence directly into EAP systems. AI-guided platforms are now enabling real-time deformation tracking, adaptive control, and sensor fusion in soft robotic actuators. For instance, convolutional neural networks (CNNs) have been trained on sensor feedback to estimate strain in DEAs with sub-millimeter accuracy, while reinforcement learning (RL) is used to compensate for viscoelastic hysteresis and improve trajectory tracking in untethered applications [43]. Similarly, machine learning models have been employed to predict electromechanical properties of conducting polymer actuators and to optimize process parameters from experimental data [64]. These approaches reduce dependence on complex analytical models, facilitating robust, autonomous performance in wearable and field-deployable systems.

Together, these multidisciplinary innovations, spanning materials, fabrication, actuation, and control, are rapidly transforming EAPs from laboratory prototypes into practical, intelligent technologies for human-centered and adaptive soft systems.

3. Sensing Applications of Electroactive Polymers (EAPs)

Electroactive polymers (EAPs) represent a transformative class of smart materials capable of transducing a wide range of mechanical, thermal, chemical, and biological inputs into electrical signals. Their intrinsic properties, including high flexibility, tunable conductivity, low weight, and compatibility with soft and biological interfaces, make them outstanding candidates for next-generation sensing technologies [52,65]. Unlike rigid conventional sensors, EAP-based platforms offer mechanical compliance, adaptability to non-planar surfaces, and multifunctionality, thereby enabling broad application across biomedical monitoring, environmental surveillance, and structural health diagnostics [15,33].

Several transduction mechanisms at the core of EAP sensing lie that determine the type and sensitivity of signal output based on material class and stimulus type [15,52]. Four primary pathways have been widely exploited:

• Piezoelectric response: Ferroelectric polymers such as PVDF and P(VDF-TrFE) generate voltage upon mechanical deformation due to dipole realignment in the $\beta$-phase. Nanostructured fillers like ZnSnO₃ and BaTiO₃ further enhance piezoelectric output by increasing crystallinity and interfacial polarization [10,66].
• Chemiresistive and electrochemical sensing: Conducting polymers such as PEDOT:PSS, PPy, and PANI exhibit changes in conductivity via redox or doping/dedoping reactions when exposed to biochemical or gaseous analytes. This mechanism underlies biosensors for glucose, lactate, dopamine, and common environmental pollutants like NH₃ and NO₂ [53,60,65,67].
• Capacitive and impedance modulation: Dielectric elastomers and ion-conducting hydrogels exhibit variations in capacitance or impedance when stretched, compressed, or hydrated. These effects are key for real-time hydration, pressure, and tactile sensing applications [57,68,69,70].
• Hybrid multimodal systems: Recent nanocomposite platforms integrate PEDOT derivatives with graphene oxide, CNTs, or metal oxides such as ZnO. These systems enable simultaneous multi-parameter sensing in wearable or epidermal formats and often incorporate AI-enabled signal analysis pipelines [7,64,69,71].

These mechanisms can be tailored through compositional design, nanofiller integration, and data-driven optimization strategies, enabling high sensitivity, selectivity, and robustness across diverse sensing environments [22,72,73].

3.1. Biomedical and Wearable Sensing

EAPs have found a central role in wearable and biomedical sensors due to their biocompatibility, ease of miniaturization, and potential for real-time physiological monitoring. Conducting polymers such as polypyrrole (PPy), polyaniline (PANI), and PEDOT:PSS have been extensively employed in the development of electrochemical biosensors capable of detecting critical biomarkers such as glucose, lactate, dopamine, and pH levels [53,65]. These biosensors operate by transducing the binding or chemical interaction with target analytes into electrical signals, typically through redox reactions or changes in impedance or potential.

Recent studies have demonstrated that polymer material selection can significantly influence detection sensitivity and selectivity for specific biomarkers. PEDOT: PSS-based platforms, for instance, have shown high sensitivity towards neurotransmitters like dopamine and inflammatory cytokines such as interleukin-6 (IL-6), owing to their high conductivity, mechanical flexibility, and tunable redox properties [67,74]. PPy-based sensors have been successfully functionalized for lactate and sarcosine detection, critical for cancer diagnostics, due to their stable electrochemical window and ability to support enzymatic immobilization [60,75]. Additionally, molecularly imprinted polymers (MIPs) incorporating PEDOT derivatives have emerged as promising platforms for the selective recognition of biomarkers like spermine and prostate-specific antigens in non-invasive fluids such as saliva [74,76].

One of the most significant advances in this domain is the integration of EAPs into epidermal electronics, which are ultra-thin, skin-conformal sensors designed for long-term monitoring of vital signs [69]. For example, PEDOT:PSS coatings on flexible substrates have enabled high-fidelity detection of electrocardiograms (ECG), electromyograms (EMG), and hydration levels in real time. These sensors exhibit excellent signal-to-noise ratios and mechanical resilience, making them highly suitable for ambulatory health monitoring applications [24].

Another notable advancement is the development of ZnO nanoneedle/PVDF hybrid piezoelectric films, which demonstrate exceptional sensitivity to mechanical pressure as low as 4 Pa, enabling wireless heart rate monitoring via integrated Bluetooth transmission. The synergy between ZnO’s high permittivity and PVDF’s piezoelectricity enhances dielectric properties and accelerates polarization response time. These hybrid films employ reduced graphene oxide (rGO) electrodes to ensure long-term mechanical durability, making them highly promising for wearable wireless health systems [71].

EAPs have also been used in implantable sensors, such as those for neural recording and stimulation, where their soft mechanical properties reduce immune response and tissue damage. Devices based on ionic EAPs like IPMCs can detect muscle activation and pressure variations internally, thereby enabling smart prosthetics and rehabilitation monitoring [50]. In microfluidic systems, EAP sensors are embedded to monitor biochemical changes in lab-on-chip platforms, facilitating integrated diagnostics in compact, flexible formats [5].

3.2. Tactile and Haptic Interfaces

Mimicking the sensory capabilities of human skin, EAP-based tactile sensors have advanced significantly for use in robotics, prosthetics, and virtual reality. These sensors rely on changes in resistance, capacitance, or potential to detect pressure, shear, and strain. Dielectric elastomers (DEs), for example, function as variable capacitors whose dielectric layer deforms under pressure, thereby modulating capacitance [68]. Their high stretchability and sub-millisecond response times make them ideal for dynamic haptic feedback.

Tactile sensors using EAPs can be engineered in multilayer configurations to detect spatially resolved stimuli, effectively serving as artificial skins. Recent work in 3D printing and inkjet deposition of conductive polymers has allowed for scalable and customizable tactile sensing surfaces [50,68]. These sensors have been integrated into robotic grippers, prosthetic hands, and surgical tools, enabling fine manipulation and real-time feedback in unstructured environments [28,68].

Emerging applications include sensors that simultaneously detect pressure and temperature, using composite structures of EAPs with thermoresponsive hydrogels or carbon-based fillers. This multimodal sensing is essential in wearable electronics and human-machine interaction.

3.3. Soft Robotics and Self-Sensing Actuators

One of the most powerful aspects of EAPs is their ability to combine actuation and sensing in a monolithic structure. This property has been exploited to create self-sensing actuators in soft robotics, where feedback is crucial for closed-loop control. Dielectric elastomer actuators (DEAs) and liquid crystal elastomers (LCEs) have been structured to self-monitor displacement, force, and deformation [20,50,77].

Multifunctional designs using trilayer EAPs, where an internal ionic gel is sandwiched between conducting polymer electrodes, can produce a measurable voltage output upon mechanical deformation [25,52]. This signal can be used to estimate strain, bending angle, or contact force. Robotic systems utilizing such EAPs do not require external sensors, reducing complexity, wiring, and weight, which is crucial in autonomous systems and wearables.

Advanced systems now integrate EAP sensors into distributed arrays along robotic limbs, enabling proprioceptive sensing analogous to biological systems. These systems can detect contact location, applied force, and shape adaptation, which are essential for soft manipulation, exploration, and locomotion in constrained environments [6,50].

3.4. Environmental and Structural Health Monitoring

Electroactive polymers (EAPs) have demonstrated increasing potential in environmental sensing, especially for the detection of gaseous analytes, pollutants, and mechanical stress across large-scale structures. Piezoelectric polymers such as PVDF and its copolymers are extensively employed in structural health monitoring (SHM) systems to detect dynamic strain, fatigue cracks, and delamination in infrastructure elements like bridges, aircraft wings, and wind turbine blades [10,78]. Their ability to convert mechanical vibrations into voltage signals makes PVDF-based acoustic sensors ideal for real-time condition monitoring. Moreover, their low weight and flexibility allow seamless embedding into structural composites without compromising mechanical integrity [45].

In the chemical sensing domain, conducting polymers such as polypyrrole (PPy) and polyaniline (PANI) function as chemiresistive materials by modulating their electrical conductivity in response to the adsorption of volatile organic compounds (VOCs). While initial implementations focused on ammonia, nitrogen dioxide, and carbon dioxide detection [79], recent advances have significantly broadened the analyte spectrum.

For instance, PANI–WO₃ nanocomposites have been engineered for ultra-sensitive detection of ethanol and sarcosine in physiological environments, achieving low detection limits and excellent signal stability [75]. PPy nanostructures with acanthosphere-like morphologies have also demonstrated increased electroactive surface area and mechanical durability, supporting deployment in flexible or wearable VOC sensing platforms [61].

Hybrid sensor architectures that combine PPy or PANI with carbon nanotubes, graphene oxide, or metallic nanoparticles have shown enhanced selectivity, faster response times, and improved environmental stability. These systems are increasingly used in multi-sensor arrays for dynamic breath analysis, industrial hazard detection, and environmental diagnostics [59,80].

To interpret complex VOC mixtures and temporal response profiles from these sensor arrays, machine learning algorithms such as support vector machines (SVMs) and convolutional neural networks (CNNs) are being implemented. These AI-based classifiers facilitate real-time analyte identification with high accuracy and robustness in wearable or distributed sensing networks [64].

Additionally, EAP-based humidity and temperature sensors, particularly those using ionic gels and PEDOT composites, are being developed for smart agriculture, climate monitoring, and energy-autonomous systems [7,69]. Their compatibility with flexible substrates and ultra-low power operation makes them ideal for integration in wireless sensor networks or self-sustained platforms.

Taken together, these innovations reflect a shift from simple chemiresistive detection to intelligent, multifunctional sensing architectures. EAP-based systems now combine tailored morphologies, composite design, and AI-enhanced signal processing to address complex challenges in environmental monitoring with unprecedented adaptability and precision.

3.5. Advanced Platforms, Challenges and Future Directions

The frontier of EAP-based sensing lies in the integration of multifunctional design, energy autonomy, and artificial intelligence (AI). Recent advances have led to the development of self-powered sensing platforms that combine triboelectric and piezoelectric effects within EAP composites, enabling continuous operation without external batteries, particularly valuable for remote monitoring and wearables [68,81].

Material-level enhancements have also enabled greater chemical specificity and selectivity. Molecularly imprinted EAPs (e.g., MIP-PEDOT) incorporate recognition sites directly into the polymer matrix, allowing label-free, high-affinity detection of metabolites, proteins, and disease biomarkers [53,76]. These systems are compatible with real-time monitoring in non-invasive platforms such as saliva sensors, wearable patches, and lab-on-chip configurations.

At the system level, machine learning (ML) algorithms are increasingly used to analyze EAP signal data, classify stimuli, and optimize fabrication processes. For instance, random forest regressors and support vector machines have modeled the electrochemical behavior of PPy-based electrodes under different environmental and operational conditions [64]. Deep learning approaches, including convolutional neural networks, have been used to interpret multimodal input signals from hybrid EAP sensor arrays, enabling context-aware and adaptive sensing behavior [82].

Model-based predictive frameworks such as NARMAX and finite element methods (FEM) are concurrently being applied to describe nonlinear EAP responses under dynamic loading, material fatigue, and environmental degradation [83]. These tools are essential for long-term reliability prediction, especially in applications with continuous mechanical cycling or hydration-sensitive operation (e.g., IPMCs) [8,59].

Sustainability considerations are also gaining prominence as EAP devices transition toward broader deployment. Biodegradable substrates, green solvents, and recyclable electrode materials are being explored to minimize ecological footprint [73,84]. Generative design algorithms now enable structural and material optimization with environmental constraints in mind, supporting low-impact and application-specific EAP architectures.

Despite these advances, challenges remain. Material degradation over time, fabrication reproducibility, and integration with wireless electronics are persistent limitations. Addressing them will require interdisciplinary collaboration between materials science, electronics, AI, and sustainable manufacturing.

Table 2. Comparison of representative electroactive polymer materials for sensing applications.

Material/System	Type	Stimulus	Transduction Mechanism	Typical Applications
P(VDF-TrFE)	Ferroelectric Polymer	Mechanical strain, pressure, vibration	Piezoelectric: dipole realignment under stress produces voltage	Structural health monitoring, wearable strain sensors, acoustic sensing [10,28]
PEDOT:PSS	Conducting Polymer	Biochemical, ionic, hydration	Redox reaction and impedance modulation (electrochemical)	Glucose and lactate biosensors, hydration sensors, ECG/EMG/EEG electrodes [53,65]
PPy, PANI	Conducting Polymers	Gas, vapor, humidity, mechanical	Chemiresistive: conductivity changes due to analyte adsorption or redox activity	VOC and toxic gas detection, humidity sensors, flexible breath analyzers [60]
IPMCs (Nafion-based)	Ionic Composite	Low-pressure stimuli, bending	Ionic migration induces potential differences and deformation	Soft pressure sensors, artificial skin, bio-inspired tactile systems [8,22]
DEs (e.g., VHB, PDMS)	Dielectric Elastomer	Strain, touch, pressure	Capacitive: dielectric deformation alters capacitance	Wearable tactile interfaces, robotic touch sensors, haptic feedback [6,68]
PEDOT/CNT or PEDOT/GO composites	Hybrid Nanocomposites	Multiparametric (strain, temp, chemical)	Synergistic electrical response (redox, percolation, electrochemical)	Multimodal wearable sensors, AI-integrated epidermal electronics [7,69]
Polyelectrolyte Gels/Ionic Gels	Soft Hydrogel Network	pH, ionic strength, biochemical gradients	Swelling/contraction due to osmotic and ionic imbalance	Lab-on-chip biosensors, soft fluidic actuators, artificial organ systems [5,26]
LCEs	Liquid Crystal Elastomers	Heat, light, strain, pressure	Mesogen realignment induces birefringence or mechanical strain	Optical sensors, shape-memory tactile interfaces, proprioceptive robotics [6,20]

summarizes representative EAP systems, highlighting their dominant transduction mechanisms, stimuli types, and application domains, reinforcing the versatility of these platforms across conventional and emerging sensing paradigms.

Each class of electroactive polymers offers distinct advantages and limitations that must be carefully weighed according to application requirements. Ferroelectric polymers such as PVDF provide excellent piezoelectric sensitivity and mechanical robustness, making them well-suited for dynamic mechanical sensing; however, they often require complex poling processes and are less effective in humid or aqueous environments. Conducting polymers like PEDOT:PSS and PPy exhibit strong electrochemical responsiveness and can be chemically functionalized for biosensing, though they may suffer from long-term signal drift and environmental instability. Ionic EAPs, including IPMCs and hydrogel-based systems, operate under low voltage and allow for large deformation sensing but are dependent on hydration levels and ion transport conditions. Hybrid nanocomposites, combining carbon-based materials, metal oxides, or responsive gels, can simultaneously address sensitivity, flexibility, and selectivity but often increase fabrication complexity and require careful interface control. As the field advances, the strategic selection and integration of EAP materials based on transduction mechanisms, operating conditions, and scalability will be key to optimizing next-generation sensing systems.

4. Conclusions

Electroactive polymers (EAPs) are transforming the field of smart materials by enabling seamless coupling between electrical inputs and mechanical responses. This review has outlined their fundamental mechanisms, material classifications, fabrication strategies, and application domains spanning soft robotics, biomedical systems, and integrated electronics.

Beyond showcasing exemplary implementations (Figure 6), the field’s rapid evolution reveals a shift from material innovation toward full system integration. EAPs are increasingly being embedded within intelligent platforms, capable of sensing, actuation, and adaptation in real time.

Key challenges, long-term stability, environmental robustness, and scalable processing remain open frontiers. Addressing them will demand synergies between materials science, sustainable engineering, and AI-enabled design.

As EAPs migrate from laboratory demonstrations to clinical tools, wearables, and autonomous machines, they stand poised to define the next generation of human-centered technologies: soft, responsive, and embedded with intelligence by design.