Humidity Sensing Using Polymers: A Critical Review of Current Technologies and Emerging Trends

Abstract

In the post-pandemic era, human demand for a healthy lifestyle and a smart society has surged, leading to vibrant growth in the field of flexible electronic sensor technology for health monitoring. Flexible polymer humidity sensors are not only capable of the real-time monitoring of human respiration and skin moisture information but also serve as a non-contact human–machine interaction method. In addition, the development of moist-electric generation technology is expected to break free from the traditional reliance of flexible electronic devices on power equipment, which is of significant importance for the miniaturization, reliability, and environmentally friendly development of flexible devices. Currently, flexible polymer humidity sensors are playing a significant role in the field of wearable electronic devices and thus have attracted considerable attention. This review begins by introducing the structural types and working principles of various humidity sensors, including the types of capacitive, impedance/resistive, frequency-based, fiber optic, and voltage-based sensors. It mainly focuses on the latest research advancements in flexible polymer humidity sensors, particularly in the modification of humidity-sensitive materials, sensor fabrication, and hygrosensitivity mechanisms. Studies on material composites including different types of polymers, polymers combined with porous nanostructured materials, polymers combined with metal oxides, and two-dimensional materials are reviewed, along with a comparative summary of the fabrication and performance mechanisms of related devices. This paper concludes with a discussion on the current challenges and opportunities faced by flexible polymer humidity sensors, providing new research perspectives for their future development.

1. Introduction

In the post-pandemic era, the demand for healthy lifestyles and a smart society has grown significantly, driving the rapid development of flexible, wearable electronic sensing devices [1,2,3]. Among these, flexible humidity sensors play a crucial role. They can not only be used to monitor human respiratory status and skin moisture but also help prevent contact with germs through non-contact human–computer interaction. Humidity refers to the amount of water vapor present in air or other gases. It can be expressed in several ways, including absolute humidity, relative humidity, and dew point temperature, with relative humidity being the most commonly used measure. Absolute humidity, or vapor density, is defined as the ratio of the mass of water vapor in the air to the volume of the air, and is typically measured in grams per cubic meter or grains per cubic foot. Relative humidity (RH) is the ratio of the actual moisture content of the air to the maximum moisture level it can hold at a given temperature and pressure. Humidity sensors measure an environment’s humidity by utilizing the physical or chemical interaction of hygroscopic materials with water molecules. Typically, humidity sensors need to meet several key criteria such as repeatability, high sensitivity, linearity, having low hysteresis, rapid response and recovery speeds, stability, being low cost, and ease of integration with control units. The development of devices that meet these criteria has long been a focal point in scientific research [4].

Flexible humidity sensors are generally composed of three main components: a flexible substrate, hygroscopic materials, and peripheral interface circuits. Of these, the development of and research on hygroscopic materials are undoubtedly the core of sensor technology. The early research on hygroscopic materials primarily focused on electrolytes, ceramics, porous metal oxides, and semiconductors. However, due to the existing processing techniques and inherent material limitations, these materials have not seen widespread practical application. In contrast, organic polymer materials, with their advantages of ease of processing and being low cost, have gained great popularity, particularly due to the rapid development of wearable flexible devices. The ability of polymers to integrate with various substrates has drawn increasing attention to their potential in humidity sensor development [5,6,7].

This article will focus on polymer-based sensitive material humidity sensors, covering their working principles, their types, and the research progress related to them in different fields, aiming to provide a reference and inspiration for researchers and engineering technicians in this field.

2. The Working Principle and Structure of Polymer Humidity Sensors

With the continuous advancement of science and technology, alongside the growing demand for applications in diverse environments, various polymer-based humidity sensors have been developed. These sensors can be classified based on their output signal types, which primarily include impedance-based, capacitive, optical, acoustic frequency, and voltage-based humidity sensors.

2.1. Capacitive Humidity Sensors

Capacitive humidity sensors consist of a substrate, a hygroscopic sensitive material layer, and electrode components. Based on their electrode configuration, these sensors can be classified into two structural types: parallel plate and interdigital electrodes, as illustrated in Figure 1a(i,ii) [8,9]. Each of these two structures offers distinct advantages. The sensitivity of parallel plate capacitive sensors is generally superior to that of sensors based on interdigital electrodes, while the interdigital electrode structure is easier to fabricate. It is important to note that structural parameters such as the width, length, and spacing of the interdigital electrodes, as well as the thickness of the sensitive film coating, directly impact the sensor′s performance.

The working principle of capacitive humidity sensors is as follows: the polymer layer, acting as the humidity-sensitive material, continuously adsorbs water molecules from the environment. As the number of adsorbed water molecules, which have a high dielectric constant, increases, the capacitance of the sensor rises exponentially [18].

2.2. Resistive/Impedence Humidity Sensors

Resistive/impedance humidity sensors have a similar structure to that of interdigital capacitive sensors. However, because resistive/impedance types of sensors do not need to account for parasitic capacitance or the electric field distribution of the device structure to the same extent, they offer greater flexibility and diversity compared to capacitive sensors with interdigital electrodes. This flexibility is particularly evident when they are used on flexible substrates, as shown in Figure 1b(i) [10], where the electrodes can simply consist of two conductive tapes symmetrically attached to the hygroscopic material. Resistive/impedance humidity sensors offer the advantages of a simple structure and an easy fabrication process. Figure 1b(ii) [11] illustrates a typical three-step method for the preparation of resistive/impedance humidity sensors: the first step involves preparing the sensitive material ink, the second step is the deposition of a stable interdigital electrode pattern on the flexible substrate, and the final step involves screen-printing the ink onto the electrode layer to complete the device.

Resistive/impedance-based humidity-sensing mechanisms, which use polymers as the sensitive layer material, are generally divided into two categories. The first category involves the use of polyelectrolytes with ionic group structures as the sensor′s sensitive layer. In these cases, electrolytes in the polymer ionize in the presence of water molecules, leading to an increase in the electrical conductivity of the sensitive material. The second category uses hydrophilic polymers, the principle of which can be explained by the Grotthuss mechanism [19]. In this process, the migration of H₃O⁺ ions between the multiple layers of physically adsorbed water molecules on the surface of the polymer film significantly reduces the impedance, facilitating the adsorption of water molecules.

2.3. Frequency-Based Humidity Sensors

Frequency-based humidity sensors include the surface acoustic wave (SAW) and bulk acoustic wave (BAW) types. Their working principle is based on the absorption or desorption of water molecules by the sensitive film, which induces changes in the wave velocity and frequency of the device. The primary distinction is that BAW sensors transmit sound waves through the bulk of a quartz crystal or other piezoelectric material, and these propagate within the crystal itself. In contrast, SAW devices generate sound waves that propagate along the surface of the piezoelectric crystal, with the waves traveling parallel to the surface.

The quartz crystal microbalance (QCM) type is a representative of the BAW type. QCM sensors are mass-sensitive sensors that detects shifts in the resonant frequency of the piezoelectric material and oscillation circuit that are caused by mass changes as water molecules are absorbed by the sensitive material in response to humidity.

2.3.1. QCM Humidity Sensors

The mass sensitivity principle of QCM sensors is based on the well-known Sauerbrey equation [20]:

$$∆f=-{\displaystyle \frac{2{f_0}^2}{A\sqrt{{\rho }_q{\mu }_q}}}∆m$$

where Δf is the resonant frequency shift of the QCM, f₀ is the base resonant frequency, Δm is the mass change, A is the electrode surface area, and ${\rho }_q$ and ${\mu }_q$ are the density and shear modulus of the quartz crystal, respectively. This relation is only valid with the following conditions: (i) the Δf << f₀, (ii) the added mass is rigid, and (iii) the mass is evenly distributed over the sensing area.

Zhang et al. [12] reported a typical preparation process for a quartz crystal microbalance (QCM) humidity sensor. As illustrated in Figure 1c, the sensitive material solution is evenly sprayed onto the piezoelectric quartz crystal at the center of the device, followed by a simple thermal treatment to fabricate the QCM humidity-sensitive device.

2.3.2. SAW Humidity Sensors

The structure and fabrication process of a SAW-type humidity sensor are shown in Figure 1d(i,ii) [13,14]. The working principle is that, when the sensitive material adsorbs water molecules, the propagation speed or frequency of its surface acoustic wave will change, thereby reflecting the information on humidity changes. The mutual conversion between the ultrasonic waves and electrical signals is accomplished by preparing interdigital transducers (IDT) on the surface of the piezoelectric substrate.

2.4. Fiber Optic Humidity Sensors

Optical sensing technology, a relatively new and emerging field, has recently found applications in humidity sensing. Its working principle typically involves utilizing the optical properties—such as the reflection coefficient, frequency, or phase of light propagation—of sensitive materials that change in response to variations in environmental relative humidity [21,22].

Xia et al. [15] reported an excessively tilted fiber grating (ex-TFG) humidity sensor. Figure 1e(i) presents a three-dimensional schematic diagram of the proposed fiber optic humidity sensor, while (ii) shows a schematic diagram of the sensor testing device. This type of grating combines the advantages of fiber Bragg gratings and long-period fiber gratings, with a functional composite hygroscopic film coated on the ex-TFG that enhances the modulation effect of external humidity changes on the optical signal.

Compared with traditional capacitive and impedance humidity sensors, optical flexible humidity sensors offer obvious advantages such as resistance to electromagnetic interference, having a small size, and the ability to transmit signals over long distances and can be applied in some specific scenarios. However, due to the high production cost and limited accuracy of optical flexible humidity sensors, their widespread adoption across various environments remains challenging.

2.5. Voltage Humidity Sensors

The traditional humidity sensors discussed above are all passive sensors, requiring an external power supply to generate electrical signals in practical applications. Voltage-type humidity sensors, however, can convert environmental humidity directly into a voltage output, offering the potential for self-powered development. Currently, two primary mechanisms for humidity-driven power generation in voltage-output sensors have been explored: (1) the ionization of water molecules along a gradient, and (2) reliance on redox reactions.

2.5.1. Voltage Humidity Sensors Based on Ion Diffusion

The concept of power generation using moisture was first proposed by Qu et al. [23] in 2015. They discovered that graphene oxide films with a gradient variation in the concentration of oxygen-containing groups can generate an open-circuit voltage of 20–30 mV under specific humidity conditions. The structure of this type of moisture-powered humidity sensor is illustrated in Figure 1f(i) [16]. In this design, hydrophobic polystyrene sulfonate is prepared asymmetrically, and its polar functional groups react with water molecules, with OH⁻ binding to the functional groups via hydrogen bonds, thereby releasing protons. The proton concentration gradient leads to ion drift and current generation, creating a potential difference between the electrodes. The formation of this proton concentration gradient is crucial for the sensor′s performance.

Increasing the ion concentration gradient is key to enhancing the ion migration and producing a greater potential difference. With thoughtful material design by establishing an asymmetric distribution of functional groups or a water content gradient within the material, the ion concentration difference can be amplified, thus boosting the potential difference.

2.5.2. Voltage Humidity Sensors Based on Redox Reactions

Electrochemical humidity power generation sensors are inspired by the structure and working principle of metal–air batteries. The commonly used hygroscopic electrolyte consists of humidity-sensitive materials and electrolyte substances [24,25]. The humidity-sensitive material promotes the adsorption of water molecules, while the electrolyte facilitates ion conduction. When the material absorbs moisture, an anion diffusion channel is formed, triggering oxidation-reduction reactions at the electrodes, which in turn causes the metal electrode to lose electrons and generate a current.

Li et al. [17] employed an aSF/GO/LiBr composite film as the solid electrolyte, as shown in Figure 1f(ii). By connecting electrodes to both sides of the film, ion transfer channels are established within the material layer as water is absorbed, and oxidation-reduction reactions occur at the positive and negative electrodes. The reaction equations at the respective electrodes are as follows (Equations (1) and (2)):

$$O_2+4e^{-}+2H_{2}O\to 4O{H}^{-}$$

(1)

$$M\to M^{n+}+n{e}^{-}$$

(2)

where M refers to the active metal at the anode, and electrons are transferred to the cathode through the external circuit.

It is noteworthy that, although the two aforementioned types of humidity sensors have the potential for self-powered operation and do not require external power consumption, their active power output remains relatively low, and they are prone to interference. Additionally, challenges persist in extending their service life and maintaining their operational stability. Enhancing the output power while ensuring stable and sustained operation remains a persistent challenge.

3. Research Progress in Polymer Humidity Sensors

To date, a variety of materials such as ceramics [26], organic polymers, metal oxides [27,28], and two-dimensional materials [29,30,31] have been studied for use in humidity sensors. With the rapid development of flexible and wearable electronic technology, the application fields of humidity sensors have expanded, imposing more stringent requirements for their functionality and performance. Therefore, the development of humidity sensors that are fast-responding, are linear, exhibit low hysteresis, have high sensitivity, have low power consumption, and are economically and environmentally friendly has become a research focus for scholars in the field.

Due to their lightweight, simple preparation methods, low-cost large-scale production, and structural flexibility, polymers have gained significant attention in recent decades as humidity materials for flexible devices. There are many organic polymers used as hygroscopic materials, including conductive polymers such as polyaniline (PANI) and polypyrrole (PPy) [6,32,33], long-chain polymer electrolytes with ionizable groups, such as quaternary ammonium salts and sulfonate salts [34,35], and lipids like polyimide (PI) [36,37,38], polymethyl methacrylate (PMMA), and polyethylene terephthalate (PET) [39,40], as well as hydrophobic polymers like polydimethylsiloxane (PDMS) [41]. Biopolymers with a large number of hydrophilic groups, such as chitosan (CS) [42,43,44], cellulose [45], sodium alginate [46], and silk fibroin (SF) [17], are increasingly being used in humidity sensors due to their good biodegradability and non-toxicity to humans and nature.

Overall, polymer humidity sensors still have the following limitations.

(1)

Hydrophilic polymers experience significant chain/segment dissolution/migration/relaxation at high humidity levels, greatly restricting their application range.

(2)

When polymers are used as resistive/impedance humidity sensors, their conductivity is too low at low humidity levels, making detection difficult.

(3)

Individual polymer humidity sensors often suffer from low sensitivity or long response/recovery times, which is unfavorable for applications of flexible devices that require rapid and sensitive human health monitoring.

Single types of hygroscopic material are increasingly unable to address these challenges, making the combination of different materials a key direction for future development. The following is a summary of typical composite material approaches based on polymers: composites of polymers with differing properties, polymers with nanoporous structures, polymers combined with metal oxides, and polymers integrated with novel two-dimensional materials. These composite materials are effectively combined in various experimental designs, creative sensor structures, and configurations that harness these materials’ complementary properties.

3.1. Polymer-Polymer Humidity Sensors

Leveraging the advantages of different polymers, composite materials can achieve an enhanced overall performance. Moreover, optimizing the ratios and processing techniques of these polymers can further improve the performance of humidity sensors, enabling high sensitivity, rapid responses, and extended operational lifespans.

Hydrophilic polymers such as polyethyleneimine (PEI), sodium alginate, and chitosan exhibit excellent moisture-sensing responses due to their high affinity for water molecules. However, their long-term stability in high-humidity environments is limited, constraining their functional ranges [47,48,49]. In contrast, some polymers such as polyacrylonitrile (PAN), cellulose, PPy etc. [50,51,52], which demonstrate superior water resistance and mechanical stability, can effectively improve the long-term stability of humidity sensors. PAN, in particular, is an excellent supporting membrane with high mechanical stability. Aditya et al. [47] and Tang et al. [48] reported the fabrication of QCM humidity sensors by grafting hydrophilic polymers such as PEI and sodium alginate onto electrospun PAN nanofibers, which resulted in sensors with both high sensitivity and excellent mechanical stability. Cellulose, which is insoluble in water and possesses remarkable mechanical strength, can further enhance the structural stability of sensor films [49,50]. Liu et al. [52] fabricated CS/PPy QCM humidity sensors by combining chitosan, which has abundant amino and hydroxyl groups and excellent film-forming properties, with PPy, a polymer that is easy to synthesize and environmentally stable. These sensors exhibited a total frequency shift of 5132.12 Hz over a humidity range of 0–97% RH, along with good repeatability and stability, highlighting that combining polymers with superior flexibility and film-forming capabilities with highly conductive polymers can not only improve the response sensitivity of sensors but also enhance their durability and environmental stability.

In addition to combining mechanically stable polymers, other strategies are also commonly employed to address the poor water resistance of hydrophilic polymers. Researchers have chemically modified polymer-sensitive materials through grafting [53], copolymerization [54], crosslinking reactions [55,56,57,58], and the development of interpenetrating network structures to achieve an optimal balance between hydrophobicity and hydrophilicity [58,59,60]. He et al. [58] effectively addressed the issue of polyethyleneimine (PEI) becoming easily deliquescent under high humidity conditions by employing a Michael addition reaction between PEI and tannic acid (TA) in an aqueous solution (Figure 2a). As illustrated in Figure 2b, compared to PEI that is not crosslinked or inadequately crosslinked, the PEI−TA material retains structural stability even after 24 h of water immersion.

When hydrophilic polymers are employed in impedance-based humidity sensors, it is important to note that, in addition to the issue of polymer hydrolysis under high humidity conditions, there is often a challenge of excessively high sensor impedance at lower humidity levels due to insufficient water molecule adsorption, which impedes the formation of ionic channels. Incorporating conductive polymers can be an effective composite solution to address this challenge.

Yang et al. [61] successfully prepared quaternized and crosslinked poly (4-vinylpyridine) (QC-PVP) with an interpenetrating network structure using materials such as poly (4-vinylpyridine) and poly (glycerol methacrylate). The researchers then coated polypyrrole onto the QC-PVP using vapor-phase polymerization technology, forming a QC-PVP/PPy composite hygroscopic membrane. This innovative design reduced the sensor′s impedance in low humidity, improving its practicality in low-humidity environments. The researchers also investigated a double-layer composite structure combining QC-PVP with polyaniline (PANI) [62], similarly reducing the device′s resistance in response to humidity. Suresha K. et al. [63] developed a cellulose/PPy nanocomposite using a polymerization-induced adsorption method, opening new avenues for the application of cellulose in humidity sensing technologies. Additionally, this composite can enhance the electrical properties of sensors by chemical modification or through the use of incorporated conductive materials, such as electrolytes (KOH, CaCl₂, sodium carboxylate (COONa), etc. [64,65,66]) and highly conductively noble metal particles (Au, Ag, Pt, etc. [67,68,69]). It is worth noting that the effect of noble metal nanoparticles such as Au, promoting the adsorption of hydrogen atoms, can enhance the detection performance of sensors for water molecules [70].

A wide variety of polymers are utilized in humidity sensors, and

Table 1. Comparison of several typical polymer–polymer composite humidity sensors.

Sensing Materials	Sensor Type	Range (% RH)	Sensitivity /Response	Response/ Recovery Times (s)	Hysteresis (%)	Reference
PAN/PEI	QCM	30–75	164 Hz/% RH	13/7	-	[47]
SA-PAN	QCM	0–97	63.33 Hz/% RH	14.5/2.5	1.6	[48]
GG/EC-PVP	QCM	0–97	55.72 Hz/% RH	26/2	2.8	[50]
CA/KGM	QCM	0–97	-	12.3/0.1	0.64	[49]
PPy/CS	QCM	0–97	-	13/2	1.68
NaSPF/PDEB	SAW	20–85	~0.4 kHz/% RH	-	-	[51]
QC-PVP/PPy	Impedance	33–97	-	33/110	5	[61]
QC-PVP/PANI	Impedance	1–98	10,000	24/35	3	[62]
PEI/TA	Impedance	35–90	29,900	28/12	2	[58]
Cellulose/PPy	Capacitance	27.8–92.4	0.0852 pF/% RH	<416	-	[63]

summarizes several typical composite humidity sensors based on polymer combinations. Each polymer possesses distinct characteristics, along with specific limitations. In practical applications, it is essential to leverage the strengths of various polymer materials while mitigating their weaknesses. Achieving optimal sensor performance requires carefully selecting and combining materials, as well as employing appropriate fabrication techniques.

3.2. Nano- and Porous- Structured Polymer Humidity Sensors

Traditional organic polymer films, due to their semi-closed characteristics and relatively small specific surface area, have a limited adsorption capacity for water molecules. The incorporation of nano-porous structures can significantly increase the specific surface area available for humidity sensing in polymers, thereby enhancing their sensitivity to moisture. Nano-porous structures, such as mesoporous SiO₂, Cu-ZnS, and halloysite nanotubes (HNTs), not only exhibit high stability but also possess large specific surface areas, providing numerous active sites for sensitive groups. These features make them ideal frameworks for constructing polymer-based hygroscopic materials.

In recent years, Fei et al. [71] developed an MCM-41/PPy humidity sensor by employing self-polymerizing pyrrole vapor within MCM-41, a type of hexagonal mesoporous silica. Compared with sensors made solely from MCM-41 or PPy, this composite sensor demonstrated a superior capacitive humidity sensing performance. In 2022, the same team [72] further developed MCM-41/PEDOT nano-flower structures (Figure 3a), which reduced the response time of PEDOT-based humidity sensors by approximately 5 min and expanded their humidity sensing range.

Hemalatha et al. [73] synthesized PANI-coated Cu-ZnS using hydrothermal and in situ polymerization methods. As shown in Figure 3b, the zinc acetate, copper acetate, and thiourea formed seed nuclei, which subsequently produced ZnS solid porous microspheres under the appropriate hydrothermal conditions. The microspheres were then dispersed in an acidic aniline solution, where π–π* stacking interactions between aniline functional groups and aniline monomers facilitated the coating of PANI on the Cu-ZnS microspheres after polymerization. The resultant PANI/Cu-ZnS composite exhibited a significantly improved response and recovery performance, with low humidity hysteresis (~1.2% RH) and excellent stability.

As shown in Figure 3c, Niu et al. [74] developed nano-cone arrays of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) with arc-shaped hollow structures by using a combination of hot pressing and anodic aluminum oxide (AAO) template-transfer methods. These arrays were assembled into a high-performance, flexible capacitive humidity sensor. Unlike traditional capacitive structures where the electrode and dielectric layers are closely attached, the arc-shaped hollow structure enabled water molecules to be adsorbed over a large area of the sensitive material. The three-dimensional conical design significantly reduced the adsorption and desorption times to just a few seconds.

In a 2023 study, Mu et al. [38] modified halloysite nanotubes (HNTs) using silane coupling agents to facilitate the integration of HNTs with polyimide, forming an efficient humidity-sensing sensing layer. Combined with advanced MEMS technology, this sensor achieved an outstanding sensitivity of 0.87 pF/% RH in the 10–90% RH humidity range, which was 1.81 times greater than that of a pure PI humidity sensor. In addition to using nanoporous materials as templates to fabricate various nanostructured patterns, some specialized fabrication methods can also be employed to construct nanostructures. Chen et al. [76] developed a polyimide humidity sensor based on a nanoforest structure embedded on silicon substrate by using O₂ plasma reaction etching. The unique nanoforest morphology on the PI layer exhibited an especially large surface area and thus provided numerous active sites for adsorbing molecules. This innovative design resulted in a humidity sensor that was 15 times more sensitive than traditional polyimide sensors in the 10–90% RH range. Zhao et al. [75] used the same method to fabricate a nanoforest-structured PI on a flexible substrate (Figure 3d). Due to its ultra-high sensitivity, it can detect the humidity levels on fingertips.

It is worth noting that, in addition to forming nano-porous characteristics by compounding with these special structural materials, also it is also possible to utilize nano-film preparation techniques on polymers to achieve the same effect. The commonly used polymer nano-film techniques include electrospinning [77,78,79,80], electrostatic self-assembly [81,82,83,84], inkjet printing [85,86,87], etc. Nanotechnology has greatly enhanced the effectiveness of humidity sensors, due to allowing for a high specific surface area and porosity related to water molecule adsorption.

Table 2. Comparison of several typical nanostructured polymer humidity sensors.

Sensing Materials	Sensor Type	Range (% RH)	Sensitivity /Response	Response/ Recovery Times (s)	Hsteresis (%)	Reference
MCM-41/PPy	Capacitance	11–95	119 pF/% RH	915/100	-	[71]
MCM-41/PEDOT	Capacitance	11–95	10,000	165/115	6	[72]
Cu-ZnS /PANI	Capacitance	30–90	12 pF/% RH	42/24	1.2	[73]
HNTs/PI	Capacitance	10–90	0.87 pF/% RH	12/8	2.18	[38]
P(VDF-TrFE)	Capacitance	50–90	-	3.693/3.43	9.1	[74]
PI-Nanoforest	Capacitance	10–90	-	8/5	-	[76]
PANI/PVB-Nanofiber	SAW	20–90	75 kHz/% RH	1/2	-	[77]
PMMA-nanofiber	Optical fiber	35–85	0.28	16/126	-	[78]
PVA/PEDOT:PSS-nanofiber	Optical fiber	20–80	−0.990 nm/% RH	-	-	[79]

summarizes polymer-based humidity sensors fabricated using nano-porous structure templates or nanotechnology.

3.3. Polymer-Metal Oxide Humidity Sensors

Metal oxides possess active adsorption sites that readily interact with water molecules through the chemical adsorption of hydroxyl groups [88,89]. Therefore, incorporating metal oxides into organic polymer materials significantly enhances their physical adsorption capacity for water molecules. This leads to the formation of a water layer within the composite material, increasing the number of ion transport channels and stabilizing the impedance rate of change [90].

SnO₂ is a typical metal oxide semiconduction material and has a wide band gap, suitable electrochemical stability, and abundant oxygen vacancies, which is not only beneficial for absorbing water but also allows forming biocomposites more easily with polymer [91,92,93]. As shown in Figure 4a, Shukla et al. [94] synthesized a nano-sized SnO₂/PANI nanocomposite. Under the synergistic effects of SnO₂ and PANI [95,96], the nanocomposite exhibited a 10% improvement in humidity sensitivity compared to pure PANI. Feng et al. [92] wrapped SnO₂/PVA nanofibers onto excessively tilted fiber grating (Ex-TFG), which exhibited an ultrafast response of ∼67 ms.

Other metal oxides also possess similar properties, and there have been many reports on the characteristics of their composites with polymers. B. Chethana [70] investigated the humidity sensing properties of a core-shell structured polypyrrole/tantalum pentoxide (Ta₂O₅) composite (Figure 4b). They mechanically blended polypyrrole with Ta₂O₅, and the resultant PPy-Ta₂O₅ composite displayed a significant increase in porosity, water absorption, and material expansion when compared to pure PPy. The sensor achieved response and recovery times of 6 s and 7 s, respectively, and demonstrated stable performance over two months. Su et al. [98] utilized 1 μm polystyrene microspheres as a substrate to synthesize microstructured porous TiO₂ films. These were subsequently immersed in a sodium polystyrene sulfonate (NaPSS) solution, resulting in an inorganic/polymer composite film. The ordered arrangement of TiO₂ within the composite film yielded an exceptionally fast response time in the 33–90% RH range (Figure 4c). After 2 years, the team [101] further optimized the process, discovering that microspheres with a 100 nm diameter produced the best results. In 2022, Tong et al. [99] transformed spherical TiO₂ into a tubular structure and embedded it in hydrophilic cellulose with a porous structure. As depicted in Figure 4d, the tubular channels not only enhanced the water adsorption capacity of the material but also facilitated water molecule migration from regions of high to low relative humidity. This advancement, alongside the synergistic properties of cellulose and semiconductors, significantly improved the sensor′s humidity sensitivity.

Zhang et al. [82] developed a composite humidity-sensitive film using Co₃O₄ and PSS through layer-by-layer self-assembly technology. Benefiting from the strong adsorption capacity of Co₃O₄ and the layered nanostructure of PSS, the film displayed a superior response performance across a broad relative humidity range of 11–97% RH (Figure 4f). The sensor does not require a heating device and operates effectively at room temperature, demonstrating excellent repeatability and long-term stability.

N. Irawati et al. [100] demonstrated that adding a nanostructured ZnO coating to the surface of a microfiber loop resonator fabricated from PMMA fibers markedly improved the sensitivity of sensors (Figure 4f). Sahoo et al. [76] also explored the role of ZnO in low-relative humidity environments, revealing that an electrostatic field generated by water adsorption at oxygen-deficient sites on ZnO surfaces promotes water molecule dissociation, enhancing their sensor’s performance.

Table 3. Comparison of several polymer/metal oxide humidity sensors.

Sensing Materials	Sensor Type	Range (% RH)	Sensitivity /Response	Response/ Recovery Times (s)	Hsteresis (%)	Reference
SnO2/PANI	Impedance	5–95	0.22 kΩ/% RH	26/30	-	[94]
SnO2/PVA	Optical fiber	35–75	0.43 dB/% RH	0.067/0.087	-	[92]
Ta2O5/PPy	Impedance	11–97	0.044 kΩ/% RH	6/7	2	[97]
TiO2/NaPSS	Impedance	11–95	100,000	2/20	-	[98]
Co3O4/PSS	Impedance	11–98	~1.5 MΩ/% RH	37/23	-	[82]
TiO2/CNC	Resistance	11–97	~190	34/18	-	[99]
PMMA/ZnO2	Optical fiber	20–89	0.1746 dBm/% RH	-	-	[100]

summarizes several classic polymer-metal oxide composite humidity sensors. While the introduction of metal oxides enhances the performance of humidity sensors, it is worth noting that certain mechanisms remain unclear. Metal oxides may exhibit sensitivity to other gas molecules besides water, which poses a challenge for impedance-based signal output humidity sensors. Improving the selectivity of metal oxide/polymer humidity sensors selectivity a significant challenge that must be addressed to enable their broader application.

3.4. Polymer-Two-Dimensional Material Humidity Sensor

Two-dimensional materials, including graphene oxide (GO), carbon nanotubes (CNTs), transition metal carbides or nitrides (MXenes), and metal-organic frameworks (MOFs), have demonstrated immense potential across various fields due to their exceptional properties. These materials feature high specific surface areas and hydrophilic groups, along with surface vacancies that facilitate the adsorption of water molecules through hydrogen bonding. Given these advantages, the development of composite structures that integrate polymers and two-dimensional materials for humidity sensors has garnered significant attention from researchers.

GO is particularly notable for its unique layered structure, high specific surface area, and rich oxygen-containing functional groups, which confer excellent hydrophilicity. In 2023, Han et al. [102] modified the PI aerogel with GO. The researchers used laser direct writing technology to reduce the GO on both the upper and lower surface layers of the aerogel into reduced graphene oxide (rGO), forming planar electrodes (Figure 5a). The resultant device was a self-supporting, flexible, parallel plate capacitive humidity sensor. In addition to its high sensitivity, this sensor has the potential to detect moisture in common solvents, presenting new possibilities for humidity detection applications. The chemical reduction of GO removes many oxygen-containing groups and restores aromatic carbon double bonds, facilitating electron mobility and improving conductivity in the resulting film [103,104]. The reduction process often leaves behind some oxygen-containing functional groups, making the rGO conductive and chemically active due to the resulting existence of defect sites [103]. Ravikiran et al. [104] showed that PPy/rGO composites prepared using a similar method exhibited superior sensitivity, a lower detection limit, and reduced hysteresis compared to both PPy and PPy/GO (Figure 5b).

Polymer-CNT composite materials have garnered significant attention in the field of flexible humidity sensing, largely due to their high sensitivity, rapid response, and excellent stability [108]. As shown in Figure 5c, Jin et al. [105] advanced the traditional “ice-casting” method, successfully fabricating CNFs/CNT (cellulose nanofibers/CNT) aerogel thin films with an oriented porous structure. The honeycomb structure not only enhances the structure’s air permeability but also accelerates moisture exchange between the gas and material, significantly improving the sensor′s response speed in practical applications. This innovative structural design represents a new direction in the development of high-performance flexible humidity sensors.

Yang et al. [11] introduced sodium ascorbate (SA) to modify multi-layer MXene, followed by a simple screen-printing of SA-MXene ink onto paper substrate to fabricate a highly sensitive humidity sensor. The enediol groups in SA form a charge transfer complex with surface and edge Ti atoms in the MXene, preventing MXene’s oxidation in humid environments and improving the sensor′s long-term stability. Zhao et al. [35] similarly demonstrated the outstanding performance of PDDA-modified MXene sensors, which are capable of detecting minute movements, such as finger motions as small as 1 mm (Figure 5d).

MOFs, with their high specific surface area, precisely controllable structure, persistent porosity, and tunable properties, are an ideal platform for constructing porous electrolytes and chemically processing sensitive films. Through an in situ thiolene click crosslinking reaction, Zhang et al. [106] developed a humidity sensor based on a polyelectrolyte film derived from UIO-66. The inherent pore size of UIO-66 and structural defects introduced by sulfonic acid enabled the rapid adsorption and desorption of water molecules, effectively allowing the sensor to recognize various states of human breathing (Figure 5e).

Indium selenide (In₂Se₃), a hydrophilic two-dimensional material with a large specific surface area, high dielectric constant, and excellent semiconductor characteristics, has emerged as a promising material for high-performance humidity sensors. Ali Khan et al. [107] reported a composite humidity sensor comprising In₂Se₃ and PEDOT:PSS. As illustrated in Figure 5f, water molecules interact with the hydrophilic sulfonic acid groups and selenium atoms, resulting in chemical adsorption in low humidity and physical adsorption at higher humidity levels. These interactions form multiple layers of water molecules, amplifying the capacitance changes due to hygroscopicity. The addition of In₂Se₃, with its high dielectric constant, significantly enhances the sensor′s sensitivity over a broader humidity range. Both In₂Se₃ and PEDOT:PSS exhibit inherent hygroscopic properties, and their combination in appropriate proportions results in an enhanced overall humidity sensitivity. Through a synergistic combination of chemical and physical adsorption processes, the sensor effectively covers a wide range of humidity levels with increased sensitivity.

Table 4. Summary of the hygroscopic performance of several typical polymer/two-dimensional material composites.

Sensing Materials	Sensor Type	Range (% RH)	Sensitivity /Response	Response/ Recovery Times (s)	Hsteresis (%)	Reference
PI/GO	Capacitance	11–98	1000	39/37	-	[102]
rGO/PPy	Resistance	11–97	0.042 Ω/% RH	2/4	4	[104]
CNFs/CNT	Resistance	0–75	71.5%	18/47	-	[105]
PVA/Mxene	Resistance	11–97	40	0.9/6.3	-	[2]
SA/Mxene	Capacitance	11–97	131.4%	9.38/12.94	-	[11]
UIO-66-S3-NH-AA	Impedance	33–95	1000	3.1/1.5	-	[106]
PEDOT:PSS/In2Se3	Capacitance	5–95	0.177 μF/ % RH	1.2/1.9	-	[107]

summarizes these typical two-dimensional material and polymer composite humidity sensors. The large specific surface area and abundant adsorption sites of two-dimensional materials significantly enhance the performance of humidity sensors. However, given the relatively short history of two-dimensional material research, the long-term stability of the polymer composites which incorporate these materials requires further exploration and verification.

4. Application of Polymer Humidity Sensors

With the rapid advancement of internet of things (IoT) technology and the growth of flexible wearable electronic devices, as well as the increasing demand for an enhanced quality of life, humidity sensors have gained significant attention. Beyond these sensors’ traditional roles in monitoring environmental humidity in fields such as industry and agriculture, these sensors are being developed for emerging applications, expanding their utility and value. This review focuses primarily on three key areas of application: human health, contactless switches, and moisture-based power generation.

4.1. Application in Human Healthcare Detection

The human respiratory system is regulated by the respiratory center in the brain, controlling the levels of oxygen and carbon dioxide in the arterial blood. Respiratory patterns can provide critical insights into a person’s health, as well as their physical and mental activity [1,109].The human body releases significant amounts of water vapor during respiration, making flexible humidity sensors an appealing non-contact method for monitoring breathing patterns. This innovative detection approach has garnered the attention of numerous researchers. Additionally, other aspects of human health, such as skin moisture detection and the monitoring of baby diapers [110,111] can be effectively addressed by integrating flexible humidity sensors into external devices.

Park et al. [112] developed a flexible substrate that allows a sensor to be attached under the nose, as shown in Figure 6a. The sensor is connected to a signal processing and communication module worn on the arm, enabling real-time data recording and analysis via a computer. The displayed capacitive response on the screen can effectively distinguish between two respiratory states: walking and running. As illustrated in Figure 6b(i), Zheng et al. [113] embedded a humidity sensor into a mask. A program was designed to automatically calculate the duration of data stability shown in the red box in Figure 6b(ii). Periods of no fluctuation are classified into six risk alert levels; upon reaching the sixth level, the sensor automatically sends an SOS alarm to a monitoring terminal, helping to prevent accidents.

Jeong et al. [111] developed a breathable nano-net humidity sensor designed for long-term skin moisture monitoring (Figure 6c(i)). Its ultrathin, flexible structure enables easy attachment to the skin, allowing for the continuous recording of moisture levels on the user’s skin surface. Figure 6c(ii) demonstrates how the sensor′s resistance changes in response to sweat production after the user finishes exercising.

4.2. Application of Humidity Sensors in Noncontact Human-Machine Interaction Methods

Human–machine interaction (HMI) has significantly transformed how humans interact with their environment. However, traditional contact-based interaction methods pose a risk of bacterial or viral infections, a concern that has become increasingly prominent, particularly following the outbreak of the novel coronavirus disease in 2019. To mitigate the risk of infectious disease transmission, the development of non-contact human–machine interaction methods has gained critical importance. Humidity sensors, which can sensitively detect humidity fields generated by breathing, sweat, and skin hydration, offer a flexible, low-cost, and adaptable solution for a variety of applications.

Li et al. [59] developed a highly sensitive flexible humidity sensor. As shown in Figure 7a(i), the sensor demonstrates a significant change in its impedance response as a finger approaches or moves away from it. The sensor exhibits good repeatability, and can estimate the distance between the finger and the sensor based on the magnitude of the impedance response. Building on this capability, the team created a 3 × 3 sensor array that effectively tracks the position of an operator′s fingertip and simulates keyboard button functions (Figure 7a(ii)). Yang et al. [88] enhanced this sensor array by analyzing and processing the signals from the 3 × 3 humidity sensing array using a microcontroller, enabling the accurate display of numbers on a computer screen (with each sensor corresponds to a specific number). As illustrated in Figure 7b(i), the screen displays the three positions in the array where the user has pressed. The array can not only detect finger positions near the sensor but also recognize non-contact gestures. As shown in Figure 7b(ii), the array captures movements tracing the letters “J”, “L”, and “U”. The more sensors that are incorporated into the array, the more information that can be gathered based on humidity.

Li et al. [87] furthered this concept by fabricating a 15 × 10 humidity sensor array on a PET substrate using inkjet printing technology. An analyzer was connected to monitor the capacitive response of each sensor, and the team mounted the sensor matrix on a display screen. When a user traced the Chinese character “He” in the air above the screen, the sensor accurately recorded the finger′s trajectory and height changes. As shown in Figure 7c(i), after the finger movement is completed, a humidity reconstruction map is generated based on the capacitive signals from the array′s sensors, demonstrating that non-contact interfaces based on humidity sensors can accurately capture users′ writing. To further expand its application in 3D space, the sensor array can be fabricated into a spherical structure. As shown in Figure 7c(ii), the 3D sensor array can detect both the number and direction of fingers surrounding it.

4.3. Appliacation of Humidity Sensor in Technology of Moist-Electric Generation

As fossil fuel resources dwindle and environmental pollution worsens, the search for safe, reliable, and clean energy alternatives has become a central focus in global energy research [16,115,116]. Water, as an abundant and recyclable resource, is not only essential for life but also serves as Earth′s largest energy carrier [117,118]. Moist-electric generation (MEG) technology, which converts the chemical energy of ambient humidity into electrical energy, is gaining increasing attention [119]. By integrating MEG with humidity sensors, self-powered humidity sensors can be developed, eliminating the need for external power supplies. This makes the devices lighter, more energy-efficient, and well-suited for large-scale applications.

As depicted in Figure 8a, Li et al. [120] demonstrated that, in a cellulose/MXene/PSSA composite film, water molecules induce the release of free protons from sulfonic and hydroxyl groups. The protons diffuse along a concentration gradient, causing charge accumulation on the opposite side of the film, which generates an electric field. The strength of this electric field is approximately linearly correlated with the surrounding humidity; the maximum output voltage can reach 300 mV. However, for humidity-based power generation to be practically viable, it is essential not only to achieve output voltages that meet the device activation thresholds but also to provide stable and continuous power for self-powered devices. Zhong et al. [121] reported a quaternized cellulose aerogel self-powered humidity sensor that, thanks to the aerogel′s porous structure, enhances the moisture capture and facilitates rapid ion transfer. The device generates electrical signals over a broad humidity range (30–90% RH) and produces varying electrical potentials based on the humidity field around a nearby finger (Figure 8b(i)). The aerogel system has demonstrated highly stable power generation for over 4000 min (Figure 8b(ii)).

In addition to ion diffusion-based humidity power generation designs, electrochmecial humidity devices based on redox reaction have also attracted significant interest [123,124,125]. Electrochemical reactions rely on electrode potential and ion conduction, and ion conduction is influenced by the ion concentration and adsorbed water molecules [123]. Therefore, to achieve a stronger voltage output performance, alkali metal salts (such as LiBr [17], NaCl [126], and LiCl [124]) are used either individually or in combination with some hydrophilic materials to form the sensitive layer of electrochemical humidity sensors. Tai Huiling′s research group [123] reported a high-performance power generation humidity sensor based on a NaCl/sodium alginate (SA) humidity sensing electrolyte and Cu/Zn electrodes. As illustrated in Figure 8c(i), the device has a high response voltage (∼0.7 V), a large short circuit current (∼24 μA), and a large loading power (∼2.63 μW) at 91.5% RH. Zhang et al. [122] used NaCl and hydroxylated multi-walled carbon nanotubes (OH-MWCNTs) as a humidity-sensing electrolyte. The team developed a self-powered wireless humidity monitoring system (Figure 8c(ii)) for which the system power module is composed of an array formed by connecting several sensors in series and parallel with a supercapacitor. By generating voltage in the array under humidity, the capacitor is charged to power the peripheral low-power circuit modules. The system was capable of automatically detecting humidity changes and transmitting real-time data to a mobile phone app, offering users a convenient and continuous humidity-monitoring solution without an external power source.

5. Conclusions and Outlook

This article presents a comprehensive review of the latest research progress in polymer-based humidity sensors. It reviews the various principles and structures of humidity sensors and different types of composite constructions, such as compounding with other polymers, nano-porous structural materials, metal oxides, and two-dimensional materials. The review also highlights recent applications of high-sensitivity flexible humidity sensors for human health monitoring, non-contact human-machine interaction, and self-powered devices utilizing moisture-based power generation. It also outlines various fabrication methods and compares the performance of different humidity sensitive materials.

The application prospects for polymer humidity sensors in the future are exceedingly promising. However, researchers inevitably confront a range of challenges during their development.

(1)

Although composite materials can provide excellent humidity sensitivity performance, the research history of these materials is relatively short, and the stability of the complex structural characteristics during long-term operation needs further investigation;

(2)

Currently, humidity sensors face high demands for their sensitivity and response speed when dealing with new applications such as contactless switches. Furthermore, there is a lack of research on the relevant circuit design and signal processing in this area;

(3)

Electrochemical and ionic diffusion-based humidity sensors, which derive power from environmental moisture, currently encounter limitations in terms of power generation and operational duration, necessitating further advancements for practical deployment.