Nature offers a robust conceptual framework for designing next-generation adaptive, multifunctional sensing systems. Also, in sensing systems, Trojan materials add a functional dimension to the microstructure, enabling the development of high-performance humidity sensors without interfering with their macrostructure. Thus, based on a brief overview of how inspiration from plants, animals, and membranes can be used to engineer high-performance platforms for environmental humidity monitoring, combined with the functional dimension of Trojan materials, this review presents a critical framework detailing the key developments in the main categories of self-sensing materials within the scope of humidity sensors. The review addresses electronically and ionically conductive polymers, polymer composites with dispersed active fillers, and hydrogel-based or other water-compatible systems. Additionally, commercially available sensors are described, and the main challenges and future directions are identified. 1. Introduction Within this framework, Trojan materials act as hidden enhancers embedded in the active layer of the sensor. Their effects are largely invisible at the macroscopic level but can modulate electrical, ionic, or dielectric properties internally. In contrast to conventional modifications such as coatings or structural changes, Trojan materials work at the micro- or nanoscale, providing advances without changing the device’s external appearance [ 12]. Common mechanisms include: (i) Amplified water adsorption through hydrophilic nanoparticles or porous [ 13]; (ii) Enhanced ionic or protonic transport within polymeric matrices [ 14, 15]; and (iii) Dielectric modulation to improve capacitive, resistive or impedimetric signals [ 16, 17]. This approach could be inspired by biological systems such as camouflage or immune evasion, where critical functions are hidden yet remain highly effective [ 18]. In the literature, there are numerous review articles regarding humidity sensors [ 10, 13, 19]. However, there is a gap regarding a synthesis that highlights the convergence of three domains—biomimetics, Trojan materials, and sensors—that lead to the design and development of smart and sustainable humidity-sensing materials. Therefore, this review aims to focus on the integration of these three domains, providing a critical framework. Biomimetics provides innovative solutions for water management: leaf-inspired hierarchical architectures enhance water adsorption, transport, and sensor performance [ 18, 19, 20], while artificial spider silks enable controlled droplet transport and the development of new water-collection materials [ 9]. By replicating these structures, researchers have designed surfaces that capture and transport moisture with energy efficiency and response speed beyond those achievable by traditional synthetic methods. The integration of Trojan materials adds a functional dimension, enabling the development of humidity sensors in which water uptake alters the material’s internal architecture to signal changes without compromising the macroscopic structure of the sensors. This interfacial and confined charge mediated functionality can support the design of passive, low cost, and potentially self-powered devices, although these characteristics depend strongly on material selection and system integration. In practice, the self-powered operation of humidity sensors is typically realized through coupling with triboelectric, piezoelectric, or photovoltaic components that convert environmental stimuli into usable electrical signals [ 21, 22]. Similarly, polymer and biopolymer-based Trojan systems contribute to sustainability and cost reduction due to their abundance and low temperature processability. However, nanocomposite architectures involving compound carbon nanotube (CNTs), graphene, or metal–organic framework (MOFs) may still present scalability and fabrication complexity challenges [ 23, 24, 25]. This overview is important because the integration of biomimetics and Trojan materials into humidity sensors allows overcoming the limitations of conventional sensors (such as low sensitivity or mechanical fragility) through an intelligent design approach inspired by nature. Beyond the Introduction, this review includes seven additional sections: Section 2 catalogs bioinspired water transport strategies applicable to humidity sensors. Section 3 presents the critical framework and examples demonstrating how certain materials used in humidity sensors, by acting as Trojan materials, can revolutionize materials science. Section 4 discusses humidity sensing mechanisms applied to different classes of Trojan materials, describing the materials used, their primary mechanisms, and sensor performance. Section 5 covers and discusses classes of bioinspired self-sensing materials with Trojan materials. Section 6 reviews commercially available humidity sensors. Section 7 outlines the main challenges and future directions in humidity sensing, and Section 8 provides the main conclusions. 2. Nature-Inspired Materials: Concepts and Mechanisms Biomimetics bridges different disciplines by studying and applying nature’s structures, processes, and strategies to advance solutions in science and engineering domains [ 18]. The objective is to arise strategic principles from nature rather than imitate its forms: understanding how organisms overcome challenges and converting those mechanisms into efficient, nature-inspired applications and devices [ 26]. Lotus leaves stay spotless through extreme water repellence (super-hydrophobicity) [ 27], dragonfly wings annihilate bacteria with forests of nanoscale pillars [ 28] ( Figure 1). Cephalopods dynamically reconfigure skin color in milliseconds [ 29] while pangolins fuse hard armor with flexible joints to combine protection and agility [ 30]. Together, these systems show that biology typically achieves multifunctionality, adhesion, sensing, antifouling, and mechanical durability at extremely low energy costs. Comparative studies demonstrate recurring hierarchical architectures, graded complexity, and stimuli-responsive materials that can be directly applied to next-generation sensors [ 31]. Nature works on several levels: form, structure, processes, manufacturing, and ecosystem circularity [ 18]. Overall, structural principles shape functional design; process strategies enable production with less energy and pollution; and ecosystem thinking helps create sensors suitable for sustainable, circular systems. In contrast to conventional engineering, which typically assumes abundant energy and materials, biological strategies reliably achieve rich multifunctionality with minimal resources and a much smaller environmental footprint [ 32]. 2.1. Plant-Inspired Materials Plant-inspired humidity sensors mimic natural water management mechanisms such as selective absorption and capillary-driven transport in leaf cuticles and root hairs. These biological systems allow effective humidity capture, directional transport, and regulation, inspiring advanced sensing platforms [ 33]. Pinecone-inspired hygroscopic systems use anisotropic swelling of cellulose-based materials for reversible actuation and sensing over 20–90% relative humidity (RH), with 10–60 s response times [ 34]. More recently, leaf-inspired hierarchical architectures have been shown to enhance water adsorption, transport, and overall sensor performance. Deng et al. developed a leaf-inspired self-powered humidity sensor that mimics vein networks and stomatal distribution for capillary-driven water transport. The device exhibits response and recovery times under 5 s and high sensitivity over 20–95% RH, outperforming conventional hygroscopic systems. Its self-powered design also makes it appropriate for wearable and remote sensing applications [ 20]. Likewise, Li et al. reported a bioinspired leaf vein-based humidity sensor using a hygroscopic polymer, in which the vein-like microstructure improves rapid humidity diffusion and signal amplification. This sensor is sensitive, particularly at low humidity levels (<30% RH), with response times of approximately 2–10 s and good repeatability over multiple cycles [ 35]. Cellulose-based sensors are inspired by plant systems, where cellulose is the main structural component for water interaction and transport. Its abundant hydroxyl groups strongly adsorb moisture through hydrogen bonding, and its porous, fibrous network promotes efficient water diffusion, similar to capillary flow in plants. Humidity-induced swelling and deswelling of cellulose produce measurable electrical changes, forming the basis of its sensing mechanism. These properties make cellulose a bio-derived material that translates plant water management strategies into sustainable, high-performance humidity sensors [ 36, 37]. Pure and chemically modified cellulose-based structures have also been extensively studied. A representative example is a flexible humidity sensor made from chemically functionalized cellulose paper, which shows stable sensing characteristics and high mechanical compliance, enabling potential use in environmental monitoring and wearable electronic platforms [ 38]. However, compared to composite architectures, such cellulose-based sensors generally operate over a more limited relative humidity range and exhibit reduced long-term stability. This limitation primarily results from the inherent tendency of cellulose to swell and undergo physicochemical degradation during prolonged exposure to high humidity conditions [ 38]. To address these limitations, cellulose has been combined with conductive polymers such as polyaniline. A resistive humidity sensor made from cellulose/polyaniline composite materials shows improved electrical responsiveness and increased sensitivity to environmental changes, especially regarding ambient humidity levels relevant to industrial and agricultural monitoring applications [ 39]. Similarly, inorganic–organic hybrids such as cellulose/TiO 2 sensors exploit the hydrophilicity and large surface area of cellulose, along with the chemical stability of metal oxides, resulting in reliable operation across a wide humidity range and improved durability in environmental applications [ 40]. Overall, these studies show that cellulose-based humidity sensors can cover broad operating ranges, typically from about 5% to 100% RH, depending on composition, while durability is significantly enhanced in hybrid systems that combine cellulose with conductive polymers, carbon nanomaterials, or metal oxides. Hydrogel-based sensors, inspired by plant tissues, offer high sensitivity across 10–100% RH due to their swelling behavior, though their response times (5–30 s) and long-term stability may be limited by dehydration [ 41]. Recent advances show that integrating plant-inspired features such as vein networks, cuticle-like selective absorption, and capillary-driven transport significantly enhances sensor performance. These strategies result in faster response times (<5 s), improved sensitivity at low relative humidity, and greater stability, making plant-inspired humidity sensors promising candidates for next-generation wearable, environmental, and agricultural monitoring systems. Humidity sensors inspired by leaf venation and stomatal architectures exhibit high sensitivity and rapid response dynamics, primarily due to their hierarchically organized microchannel networks, and can achieve improved environmental sustainability when fabricated from cellulose-based or hydrogel-based materials. Such bioinspired configurations enable spatially resolved, long-term humidity mapping across both natural ecosystems and the built environment. Nevertheless, several limitations persist, including the mechanical and chemical fragility of the constituent materials under prolonged moisture exposure, hysteresis caused by residual water trapped within microchannels, sluggish recovery at elevated relative humidity (RH), and the substantial fabrication complexity associated with faithfully reproducing microscopic biological structures. The same microchannels that provide efficient capillary transport also promote water retention, resulting in pronounced hysteresis and slow desorption, particularly near saturation, where sensor recovery can be markedly delayed. Furthermore, accurate emulation of leaf vein and stomatal geometries typically requires advanced microfabrication techniques or bio-replication processes, thereby increasing production costs and impeding large-scale, economically viable manufacturing. Cellulose-based hydrogels, with their porous, hydrophilic networks, enable spontaneous moisture uptake and release, allowing passive, energy-free humidity sensing through humidity-dependent ionic conductivity and dielectric permittivity. They provide biodegradability, mechanical flexibility, and high protonic conductivity via extended hydrogen-bond networks, but face challenges such as structural instability during cycling, moisture-induced softening and mechanical degradation, and hysteresis from slow water desorption. Reinforcing cellulose hydrogels with nanofillers (e.g., graphene oxide, MXenes, nanocellulose fibrils) and incorporating interpenetrating polymer networks improves mechanical robustness, environmental durability, and long-term sensing stability [ 42]. 2.2. Animal-Inspired Materials Recent advances in materials science and sensing increasingly use bioinspired strategies for fluid control, environmental monitoring and intelligent interfaces. Natural systems provide efficient solutions through micro- and nanoscale surface structures that regulate wettability and direct water transport. Biological surfaces such as insect wings and animal skins use hierarchical textures for water channeling, condensation, and self-cleaning, offering key design principles for engineered materials [ 43]. The Namib Desert beetle, with its patterned hydrophilic and hydrophobic regions that promote gravity-driven water condensation and transport, also inspires synthetic surfaces with controlled wettability for water harvesting [ 44, 45, 46]. Spider silk uniquely collects and transports water through spindle-knots and joints that generate surface energy and curvature gradients, driving directional microdroplet motion [ 47]. Artificial spider silks replicate this effect, enabling controlled droplet transport and new water collection materials [ 48, 49]. Another study reports a biocompatible humidity sensor that uses natural inner eggshell membrane (IESM) as both the substrate and active layer, leveraging its multilayer cross-linked fiber structure. Two sizes of inkjet-printed interdigital electrodes enable capacitive and impedance-based humidity detection from 0% to 90% RH. The sensor exhibits a fast response (≈2 s) and recovery (<10 s), depending on electrode size, and stable operation at 1 kHz and 10 kHz, demonstrating the effectiveness of the porous structure for moisture sensing. The biodegradable IESM provides a sustainable, practical approach to eco-friendly, high-performance humidity sensors for everyday use [ 50]. Inspired by the water collection and transport capabilities of spider silk structures, a sensitive fiber humidity sensor is created by surface modification of profiled fibers with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). The fiber humidity sensor exhibits a quick response (4 s), swift recovery (29 s), and a wide operating range (6–85% RH). This performance results from the moisture absorption benefits of the bionic groove structure, the weak hydrophilicity of the profiled fibers, and the rapid moisture adsorption and desorption capacity, as well as the structural stability under bending deformation provided by the continuous lamellar PEDOT:PSS [ 51]. Beyond structural bioinspiration, biological sensing has also driven the development of intelligent and adaptive systems. For example, bee-inspired multisensory integration has led to neuromorphic platforms that fuse humidity and visual inputs using spiking neuron models, enabling efficient environmental perception and tasks such as authenticity recognition [ 43]. Similarly, camels’ ability to detect moisture in arid environments has inspired durable neuromorphic humidity sensors that locate water sources by sensing humidity gradients, offering promise for environmental monitoring and autonomous navigation [ 52]. Other strategies focus on optical and wearable sensing: chameleon-inspired colorimetric sensors provide real-time humidity detection through structural color changes, enabling intuitive sensing without complex electronics [ 46], while skin-conformal, breathable humidity sensors enable continuous monitoring of respiration and perspiration for emotional state recognition and non-contact human–machine interfaces [ 53]. Insect wings and animal skins are promising models for humidity-adaptive surfaces because their hierarchical micro- and nano-textures naturally control water condensation, transport, and removal. Their capillary gradients guide droplets, while hydrophobic microstructures provide self-cleaning and antifouling functions. These designs can also enable condensation-driven cooling and moisture collection for low-power, sustainable sensing materials. However, transferring these complex structures to biomimetic materials is challenging. Replicating multiscale textures requires expensive nanofabrication, and artificial replicas often lack the elasticity and durability of natural tissues, degrading under repeated wetting or UV exposure. Integrating textured surfaces with flexible or electronic substrates is also technically difficult. Together, these studies reveal a convergence of bioinspiration, advanced materials, and intelligent systems, demonstrating that nature-derived structural design, functional materials, and sensory capabilities can produce devices with improved performance, adaptability, and multifunctionality across diverse applications. 2.3. Membrane-Inspired Systems Membrane-inspired humidity sensors are a versatile technology for environmental monitoring, employing biomimetic design and advanced materials to achieve high sensitivity, flexibility, and fast response. They emulate natural membranes that selectively interact with water molecules, swell, or change electrical properties with humidity, enabling precise detection of relative humidity in diverse settings. These biomimetic sensors are suitable for wearable electronics, environmental monitoring, and soft robotics. Wang et al. report a flexible humidity sensor based on an MXene/bamboo cellulose fiber (BCF) aerogel, fabricated by vacuum filtration and freeze-drying to create a porous, moisture-interactive structure. Its sensing mechanism relies on the hygroscopic expansion of BCFs, which changes the spacing between conductive MXene layers and thus the resistance. The sensor exhibits a sensitivity of 2.46% per %RH, response and recovery times of 260 s and 282 s, respectively, and stable performance over repeated cycles and prolonged ambient exposure [ 54]. Materials are crucial to the performance of these sensors. Polymeric membranes, such as polyvinyl alcohol (PVA) and polyethylene oxide (PEO), are widely used for their hygroscopic properties and mechanical flexibility. A low-cost, high-performance humidity sensor based on a PEO/PVA polymer composite has been specifically designed for temperature-independent operation. By fabricating sensors with different weight ratios, the optimized composition (40:60% PEO/PVA) demonstrates stable and reliable performance across a humidity range of 0–60% RH. The sensing mechanism relies on impedance variation, with fast response and recovery times of 9 s and 16 s, respectively. The sensor maintains consistent performance despite temperature fluctuations and demonstrates excellent long-term stability over 90 days [ 55]. A highly compliant porous ionic membrane (PIM) humidity sensor is fabricated using a PVA/KOH gel electrolyte which self-organizes into a well-defined three-dimensional porous structure that enhances moisture adsorption and transport. The humidity sensing mechanism is governed by changes in ionic conductance, which increases more than 70-fold as relative humidity rises from 10.89% to 81.75%, while maintaining a rapid and fully reversible response under ambient conditions. The device demonstrates excellent selectivity for humidity, with negligible sensitivity to temperature fluctuations from 0 to 95 °C and pressure variations between 0 and 6.8 kPa. The sensor is also successfully integrated into a non-contact switching system capable of detecting humidity gradients generated by an approaching fingertip, where its output shows a strong linear correlation with readings from a commercial reference instrument [ 56]. Bioinspired membrane-type humidity sensors face mechanical instability during cyclic swelling, hysteresis from trapped moisture, fabrication challenges in creating multiscale pores, environmental degradation, and lower electrical conductivity compared to inorganic sensors, which limits long-term performance. These biological inspirations reinforce the design logic of Trojan-type humidity sensors, in which confined interfacial phases and dynamic hydrogen-bond networks emulate the selective, efficient, and self-regulated ion and proton transport observed in natural systems such as membranes and ion channels [ 57]. Trojan-type humidity sensors are bioinspired because they replicate key functional attributes of biological moisture and ion transport systems. In cell membranes and ion channels, selective and efficient transport occurs within confined, hydrated nano-environments, where water acts as a critical mediator of rapid proton and ion movement through transient hydrogen-bond networks and proton relay processes. By emulating these structures and physicochemical processes, engineers can develop sensors with greater sensitivity, selectivity, and energy efficiency. These bioinspired approaches offer the potential for high performance using sustainable, abundant, and low-cost materials. Plant-, animal-, and membrane-inspired humidity sensors collectively demonstrate the significant role of bioinspiration in advancing sensing technologies, each offering distinct advantages based on their natural counterparts. Plant-inspired designs enable efficient water transport and responsiveness through hierarchical and hygroscopic structures, while animal-inspired systems provide high sensitivity and adaptability by mimicking skin, hair, or other moisture-responsive biological surfaces. Membrane-based sensors, often fabricated from natural or biocompatible materials, deliver excellent flexibility, breathability, and real-time interaction with moisture. Among these, plant-based hygroscopic mechanisms are particularly promising for continuous humidity sensing because they provide fast, reversible responses driven by naturally occurring gradients in structure and composition. These approaches support the development of highly sensitive, low-power, multifunctional devices, particularly well-suited for wearable and health-monitoring applications. However, challenges related to long-term stability, environmental robustness, and large-scale integration persist, underscoring the need for continued research to translate these bioinspired concepts into durable, versatile solutions for biomedical and environmental sensing. Concept and Examples of Trojan Materials in Humidity Sensors In the field of humidity sensors, several material systems reported in the literature can be interpreted as Trojan materials when analyzed through the lens of functional invisibility (e.g., [ 62, 63]). Here, the term “Trojan material” does not refer to a specific chemical composition, but rather to the functional role and mode of integration of a material within a sensing matrix. Specifically, a component in the sensor is considered a Trojan material when it is embedded within the host matrix without significantly altering the device’s macroscopic morphology or structure. It acts as a hidden functional phase that enhances humidity-sensing performance in several ways. The Trojan material can serve as an adsorption enhancer, an energy-barrier modulator within the sensor matrix, and an interfacial mediator for ionic transport, rather than merely contributing typical bulk properties such as increased porosity or conductivity. In humidity sensors based on MOFs, MOFs function as localized adsorption enhancers as their large specific surface area and high porosity provide more active sites [ 64, 65]. The pores act as the pathway for transporting water molecules [ 65]. Additionally, the adjustable structure of MOFs endows MOF-based humidity sensors with tunable hydrophilicity. While MOFs are often known for their low hydrophilicity, the introduction of hydrophilic ligands into the framework [ 70, 71, 72] or directly loading humidity-active materials into the MOFs [ 73, 74] allows for an increase in their overall hydrophilicity. In humidity sensors based on hybrid compound CNT yarns decorated with manganese oxide (MnO 2), the process of energy-barrier modulation can be observed [ 67]. The CNTs provide a highly conductive electrical pathway, enabling rapid electron migration. The MnO 2 offers active sites for the adsorption of H 2O molecules. At the junction between the CNT and the metal oxide (the semiconductor), a Schottky junction (a potential barrier) is formed. When water molecules are adsorbed at this interface, they alter the charge density and flatten the energy barrier. This allows electrons to flow much more easily, resulting in a sudden increase in conductivity. When semiconductor metal oxides are combined with graphene or reduced graphene oxide (rGO), the energy-barrier modulation mechanism resembles that of CNTs but gains an additional physical dimension due to graphene’s two-dimensional (2D) structure. While CNTs provide one-dimensional transport, graphene offers a conducting sheet with a vast theoretical surface area. When metal semiconductors are anchored onto this sheet, the dominant mechanism becomes the formation of a 2D–3D heterojunction with ultra-sensitive, dynamic tuning of the Fermi level. When a metal semiconductor such as TiO 2, Fe 2O 3, or ZnO [ 63, 67] is deposited onto the graphene sheet, electron transfer occurs from the semiconductor to the graphene to align their Fermi levels. As water molecules adsorb onto the metal oxide surface and inject electrons into the semiconductor, a Schottky barrier forms along the entire two-dimensional interface, causing graphene to experience an electronic doping effect. In dry conditions, these barriers act as highly resistive energetic bottlenecks, keeping the system in a contained equilibrium. However, when ambient humidity changes, water molecules interact with the metal oxide, triggering a significant injection of electrons from within the structure. This electronic infiltration abruptly narrows and flattens the trans-interfacial potential barrier. Due to graphene’s atomic thickness, the two-dimensional sheet undergoes an instantaneous “chemical gating” effect, dynamically shifting its Fermi level. This geometric and electronic modulation, distributed across the entire planar surface, allows charge carriers to bypass energy fluctuations via quantum tunneling, causing the material’s resistance to collapse and resulting in an ultra-fast, linear, and highly sensitive macroscopic conductivity transition triggered by an otherwise invisible stimulus. In both cases, the Trojan-like behavior appears at the Schottky junction. For the CNT, the yarn seamlessly hosts the metal oxide nanostructures as benign, passive surface decorations; however, this hybrid integration quietly introduces millions of hidden electronic gates—the Schottky potential barriers—deep into the conductive matrix of the sensor. Similarly, for the metal oxide and graphene/rGO sheets, the sensor matrix incorporates both materials at the macroscale as structural components and passive conductors. Nevertheless, this union conceals a latent energetic trap that is the formation of many nanojunctions and hidden Schottky barriers throughout the entire volume of the material. In humidity sensors where cellulose nanofibers (CNF) are incorporated into graphene oxide (GO)/cellulose nanofiber (GO/CNF) films to develop capacitive humidity sensors, CNFs serve as interfacial mediators for ionic transport and as localized adsorption enhancers [ 61]. GO and CNF form a heterostructure with channels and lamellar nanointerfaces. Introducing CNFs between GO sheets prevents the graphene from compacting or fully restacking. CNFs function as nanoscale spacers, creating an interconnected three-dimensional porous network of interfacial pathways. Water easily penetrates the interfaces formed between GO and CNF. The functional groups of both materials ionize the water, releasing protons (H +). This interfacial porous network serves as an ultra-fast mediator for the Grotthuss mechanism, allowing hydronium ions (H 3O +) to hop and migrate along the interfaces with minimal friction, which increases the capacitance and dielectric response of the sensor to high levels. Both materials are highly hydrophilic, enhancing localized adsorption. Integrating hydroxyl-rich CNF into the GO framework increases the available active sites for water interaction through hydrogen bond formation compared to pristine GO membranes. GO contains epoxide and hydroxyl groups on its basal surface and carboxyl groups at its edges. In summary, introducing CNF into the GO matrix creates a synergistic effect: the CNFs act as localized adsorption enhancers by chemically attracting more water molecules into the film’s interior due to the abundance of -OH groups, and simultaneously open physical channels that serve as interfacial mediators for ionic transport, dramatically accelerating proton hopping (Grotthuss mechanism) and the dielectric polarization required for capacitive transduction. A Trojan effect occurs here because, at the macroscale, the sensor membrane incorporates cellulose solely as a structural support additive—a natural, innocuous, and biocompatible polymer—to provide flexibility to the film and prevent the restacking of the GO sheets. The system regards CNFs strictly as passive mechanical spacers. However, they covertly introduce latent hydrophilic active sites and interfacial channels into the material. In the presence of humidity, these channels activate from the inside out, triggering a chain ionization and significant dielectric polarization that sharply increases the sensor’s capacitance. Chitosan, a natural polymer used to develop resistive humidity sensors, serves also as an interfacial mediator for ionic transport and a localized adsorption enhancer [ 62]. It acts as a polymeric framework that is biomimetically insulating, passive, and stable. However, chitosan contains several functional groups, such as amines and hydroxyls, that form hydrogen bonds with environmental water molecules. When exposed to humidity, these functional groups open pathways in the matrix, promoting the autoionization of water and rapidly transforming the insulating polymer into a conductive electrolytic medium through the Grotthuss mechanism. The examples presented as Trojan materials fundamentally differ from standard functional fillers, dopants, or nanocomposite reinforcements. Standard functional fillers superficially alter bulk electrical or porous properties, whereas atomic dopants intrinsically modify the host crystal lattice and electronic band gaps. On the other hand, nanocomposite reinforcements are introduced strictly to improve mechanical and thermal stability while remaining entirely passive during chemical transduction. Table 2 provides a comprehensive and comparative overview of the adsorption mechanism, energy barrier modulation, ionic transport mechanism, governing physical laws, and performance impacts that distinguish Trojan materials from standard material modifications. 4. Humidity Sensing Mechanisms A comprehensive understanding of the physicochemical mechanisms underlying humidity sensing is fundamental to the rational design of high-performance sensing materials. Phenomena such as the Grotthuss proton-transfer mechanism in water, adsorption and desorption kinetics, proton transport, interfacial polarization, and humidity-dependent dielectric modulation act in concert to determine the macroscopic electrical response of polymeric and composite systems. At low relative humidity (RH), the sensing behavior is predominantly governed by the physisorption of water molecules mediated by van der Waals forces and hydrogen bonding to the active sites of the material. With increasing RH, chemisorption processes and the formation of multilayer water films become increasingly important, giving rise to confined and/or interfacial water layers that can undergo partial dissociation to produce H +/H 3O + ionic species. The migration of these ions occurs primarily through proton hopping via the Grotthuss mechanism, which markedly enhances ionic diffusion and leads to a substantial increase in electrical conductivity at elevated RH levels [ 75]. The magnitude and nature of this effect depend greatly on the material type. In proton-conducting or polyelectrolyte polymers, protonic transport primarily determines the humidity response, whereas in composite systems such as carbon-nano-onion polymer nanocomposites, additional phenomena, including carrier compensation and humidity-induced polymer swelling, become more significant [ 76]. However, excessive swelling of amorphous regions at high relative humidity can disrupt long-range electronic percolation, progressively shifting the conduction mechanism toward ionic dominance. This behavior is observed in sulfonate-functionalized polythiophenes such as in PEDOT:PSS, where increased humidity induces microphase separation between hydrophobic (primarily electronic) and hydrophilic (ionic, water-swellable) regions. As water content increases, ionic conductivity rises markedly, while electronic transport is maintained through percolating π-stacked networks [ 77]. A comprehensive understanding of these mechanisms is crucial for accurately interpreting the performance of humidity sensors based on conductive polymers and hydrogels, as the underlying molecular processes directly determine the resulting electrical response. In such hydrated materials, proton transport proceeds predominantly via H + migration along percolating networks of water molecules and/or polar functional groups embedded within the polymer matrix. Kreuer describes two main pathways: (i) vehicular migration, in which protons travel with hydrating water molecules, and (ii) the Grotthuss mechanism, where successive hydrogen-bond exchanges and rapid water reorientation enable proton hopping between adjacent hydrogen-bonded molecules without large-scale molecular diffusion [ 78]. This enables efficient conduction in hydrated systems such as chitosan or alginate. In hydrogels and ionically conductive polymers, dynamic hydrogen-bond networks form continuous channels for proton migration, with their density and mobility strongly dependent on moisture content and the balance of dissociated ions [ 79, 80]. This is characteristic of Trojan-type systems, where confined charges act as hidden agents controlling conductivity. Dielectric modulation arises from interfacial polarization (Maxwell–Wagner–Sillars effect) in composites: humidity-induced permittivity contrasts cause charge accumulation at interfaces, altering the sensor’s dielectric response [ 81]. In mixed ionic–electronic polymers, this polarization is amplified by reversible redox doping, which locally tunes energy barriers and enables self-regulated conductivity [ 82]. Water-assisted ionic conduction integrates these effects: water uptake increases the dielectric constant, lowers resistance, and creates additional ionic pathways. The coupling of water adsorption, polarization, and proton transport underlies the high sensitivity and stability of hydrogel- and hydrated polymer-based humidity sensors [ 83]. 5.1. Conductive and Ionic Polymers The moisture-sensing response of conductive and ionic polymers arises from the combined effects of water adsorption, proton migration, dielectric modulation, interfacial polarization, and the structural characteristics of the polymer matrix [ 89]. At low RH levels, water molecules are first adsorbed into polar functional groups, such as hydroxyl, sulfonic, and amine groups, through hydrogen bonds and weak intermolecular interactions, forming a tightly bound primary hydration layer [ 23, 75, 90]. Under these conditions, the adsorbed water remains localized and changes in conductivity are limited, as continuous charge transport pathways have not yet formed. As RH increases, greater water absorption occurs via physisorption, leading to the development of multilayered and interconnected water domains within the polymer structure [ 75]. The partial dissociation of these confined water regions generates mobile ionic species, including H + and H 3O +, which increase conductivity via proton hopping through the Grotthuss mechanism, along with the diffusion of carrier ions [ 23, 75, 90]. The gradual transition from adsorption-dominated behavior under low-humidity conditions to efficient ionic transport under high-humidity conditions determines key detection characteristics, including sensitivity, hysteresis, response and recovery times, and long-term operational stability [ 90, 91, 92]. Beyond adsorption phenomena, the moisture response of conductive polymers depends strongly on morphology, crystallinity, doping level, phase segregation, and conductive percolation effects [ 84, 88]. Amorphous and porous domains generally favor water diffusion and adsorption due to their larger free volume and enhanced chain mobility [ 84], whereas highly crystalline regions restrict swelling and improve mechanical robustness [ 84]. Increased crystallinity therefore tends to enhance environmental stability and reduce signal drift, although often at the expense of lower sensitivity and slower adsorption kinetics [ 25, 84, 90]. Similarly, conductive polymers operating near the percolation threshold may exhibit very high humidity sensitivity because relatively small structural variations can induce large conductivity changes. However, these systems are also more susceptible to hysteresis and instability under cyclic hydration–dehydration conditions due to progressive disruption of conductive pathways [ 25]. CPs, particularly PANI, PEDOT:PSS, and polypyrrole (PPy), have attracted considerable attention due to their π-conjugated structures and humidity-dependent electronic transport properties [ 93]. In these materials, water adsorption modulates the protonation state, doping level, and interchain charge transport, resulting in measurable changes in resistance or impedance. Generally, conductive polymer systems offer relatively fast electronic responses, easy integration into resistive sensing platforms, and compatibility with low-temperature fabrication [ 84]. However, their performance is often limited by swelling-induced drift [ 94], hydrolysis [ 95], dedoping processes [ 96], and reduced long-term environmental stability under prolonged humid exposure [ 88, 97]. PANI-based humidity sensors are among the most widely studied conductive polymer systems because of their multiple oxidation states and reversible protonation-deprotonation behavior. These materials typically show a decrease in resistance with increasing relative humidity, as water adsorption enhances proton-assisted hopping between localized states and increases charge carrier density [ 39, 98]. Imali et al. demonstrated that PANI-based resistive humidity sensors operate over a wide humidity range of 0–97% RH, exhibiting a linear impedance response with high correlation (R 2 = 0.99) and a sensitivity of approximately 1.1701 Ω/%RH [ 98]. The devices also showed relatively low hysteresis (about 2.1%) and good repeatability over multiple cycles. For dynamic performance, response times ranged from approximately 150 to 220 s, while recovery times reached up to 150 s, depending on humidity conditions. Similarly, Ragazzini et al. developed cellulose/PANI composite sensors in which humidity detection is governed by water adsorption and proton-assisted electron hopping within the polymer network, resulting in increased conductivity as RH rises [ 39]. Their devices exhibited a linear response in the 30–50% RH range, with a sensitivity of approximately 13%, good reproducibility, and response/recovery times ranging from 90–370 s and 87–490 s, respectively. Although PANI-based sensors offer high sensitivity, simple fabrication, and wide operating ranges, prolonged exposure to humidity can induce desorption, hydrolysis, and microstructural rearrangements, which may progressively compromise electrical reproducibility and long-term stability [ 99]. PEDOT:PSS exhibits intrinsically high conductivity due to its phase-separated morphology, in which PEDOT-rich conductive domains are dispersed within a hydrophilic PSS matrix [ 100]. In these mixed ionic-electronic conductors, the main PEDOT structure, with conjugated π bonds, supports electronic transport, while the hydrated and structurally disordered PSS-rich regions facilitate ionic migration [ 101]. Water absorption causes swelling of the hydrophilic PSS phase and microstructural rearrangement, which can enhance ionic conductivity and dielectric response while preserving electronic transport through permeable PEDOT pathways [ 100]. Bornemann et al. reported that this behavior improves connectivity between PEDOT domains and increases charge carrier mobility, resulting in reproducible detection responses over a wide range of humidity, typically between 20% and 80% RH, demonstrating the suitability of PEDOT:PSS for outdoor humidity detection applications [ 100]. Compared to conductive polymer systems, ionic polymers generally exhibit higher sensitivity at high relative humidity. However, extensive water absorption can also lead to swelling, slow recovery dynamics, ion leaching, and long-term signal drift. Panwar et al. developed a PVA-based ionic sensing platform by incorporating sugarcane extract into the polymer matrix, then molding and thermally drying it to form an ionic membrane with silver electrodes [ 104]. The sugarcane extract introduced natural ionic species, sulfur- and iron-containing compounds, and fibrous structures from bagasse, which enhanced ionic conductivity, dielectric properties, and interaction with water within the hydrated polymer network [ 104]. Similarly, Yu et al. reported cross-linked polyionic liquid humidity sensors that demonstrated high sensitivity, low hysteresis (<5% RH), and stable cyclic operation due to controlled hydration dynamics and efficient ionic conduction pathways [ 91]. In a subsequent study, the incorporation of zwitterionic functionalities further improved adsorption–desorption reversibility and operational stability by stabilizing local hydration structures [ 103]. However, excessive water absorption can still cause hydrolytic degradation and mechanical fatigue during prolonged cyclic exposure. 5.2. Polymer Composites To better understand their sensing behavior, the dominant sensing mechanism in most composites involves water adsorption through chemisorption at low RH and physisorption at higher RH. At low RH, chemisorbed water forms tightly bound hydroxyl groups, enabling limited electronic or proton transport. As humidity increases, multilayer physisorption forms hydrogen-bonded water networks, facilitating proton hopping (Grotthuss mechanism) and significantly increasing ionic conductivity [ 108, 110, 125]. In polymer-carbon nanocomposites, additional effects such as humidity-induced swelling [ 117, 126] and modulation of conductive percolation pathways [ 117, 118] further contribute to the sensing response. Carbon-based materials, such as CNTs [ 118] graphene [ 117], and its derivatives-namely GO [ 106, 127] and rGO [ 128] have been widely used in humidity sensors as dopants for reinforcing polymer matrices. Their outstanding electrical conductivity, large specific surface area, tunable surface chemistry, and ability to form highly responsive percolation networks make them especially attractive for this application [ 129]. As a result, polymer-carbon nanocomposites often show enhanced humidity sensing performance, including higher sensitivity [ 117, 126, 128], faster response and recovery times [ 108, 130] and improved stability [ 106, 117]. Compared with many conventional polymer-only humidity sensors reported in the literature, which often exhibit response/recovery times ranging from tens of seconds to several minutes together with lower resistance variation at high RH levels, these nanocomposites can achieve response/recovery times below 20 s and sensing responses exceeding 90%, highlighting their competitive performance relative to current state-of-the-art flexible humidity sensors. For instance, Chethan et al. [ 106] developed an ultra-sensitive humidity sensor based on a PANI/GO composite, exhibiting a high sensing response (≈93%), fast response/recovery times (4 and 7 s, respectively), low detection limits, negligible hysteresis, high sensitivity, and excellent stability. These response and recovery times are considerably lower than those commonly reported for conventional polymer-based humidity sensors, demonstrating the effectiveness of GO incorporation in accelerating water adsorption/desorption kinetics and proton transport mechanisms. A CNT-based flexible humidity sensor with a core–shell CNT@CPM structure (using chitosan and PAMAM) was developed by Kim et al. [ 118] Figure 3A, exhibiting high sensitivity, response/recovery times below 20 s, low hysteresis (−0.29 to 0.30% RH), and outstanding mechanical durability (over 15,000 bending cycles), enabling reliable long-term performance and real-time respiratory monitoring in smart wearable applications. Such mechanical robustness exceeds that of many flexible humidity sensors reported in the literature, where performance degradation is frequently observed after only a few thousand bending cycles, reinforcing the suitability of this platform for wearable electronics and long-term monitoring applications. In another study, it was demonstrated that bacterial cellulose/GO composite films, prepared via a simple and low-cost dry film-forming method and subsequently reduced with L-ascorbic acid to enhance electrical conductivity, exhibit high humidity sensing performance, with a resistance change of up to ≈94%, fast response/recovery times (13 and 47 s, respectively), and effective noncontact detection of breathing patterns, highlighting their potential for health monitoring applications [ 116]. SMOs have also been extensively studied as fillers in polymer-based humidity sensors due to their chemical inertness, low cost, portability, ease of processing, and high humidity sensing performance across a wide temperature range [ 130]. Common examples include TiO 2 [ 40, 107, 131], ZnO [ 110, 111, 112], CuO [ 87, 112], tin oxide (SnO 2) [ 114, 132, 133], and nickel oxide (NiO) [ 115] which are typically incorporated as nanoparticles or nanorods. Their combination with polymers enhances humidity sensing by increasing water adsorption, modifying charge transport pathways, and strengthening polymer–water interactions. Compared with pristine polymer sensors, SMO-polymer composites generally exhibit broader operating humidity ranges, improved sensitivity, and enhanced linearity due to the higher density of active adsorption sites and improved interfacial charge transfer processes. A ZnO-coated PMMA microfiber humidity sensor fabricated using direct drawing and sol–gel methods has been reported, where ZnO coatings significantly improve performance. Sensitivity increases from 0.1191 dBm/% (uncoated) to 0.1791 dBm/% for ZnO nanostructures and 0.2159 dBm/% for ZnO nanorods. This nearly twofold increase in sensitivity compared with the pristine PMMA sensor highlights the strong contribution of ZnO nanostructures to water adsorption and signal transduction efficiency. The device also offers improved sensitivity, low-cost fabrication, and suitability for compact humidity sensing applications [ 111]. In another study, Manjunatha et al. [ 87] ( Figure 3B) reported a flexible resistive humidity sensor fabricated on a polyethylene terephthalate (PET) substrate using screen-printed PPy/CuO nanocomposite inks, where the incorporation of CuO enhanced hydrophilicity and sensing performance, with the optimal 1:1 composite exhibiting high sensitivity over a wide humidity range (22–97% RH), fast response and recovery times (≈50 and 60 s, respectively), excellent stability, and negligible hysteresis. The broad operating humidity range and low hysteresis indicate reliable sensor performance under fluctuating environmental conditions, which remains a significant challenge for many resistive humidity sensing platforms. Recently, Devesa et al. [ 40] developed a sustainable, cost-effective resistive humidity sensor using a casting method, by incorporating TiO 2 nanoparticles into cellulose extracted from potato peels, to improve water adsorption and sensing performance, resulting in enhanced sensitivity compared to pristine cellulose, as confirmed by humidity-dependent impedance measurements. Overall, combining polymers with functional nanomaterials enables the design of high-performance humidity sensors with tailored properties and enhanced performanc
