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Review

Stimulus-Responsive Membranes: A Mini Review on Principles, Preparation Methods, and Emerging Applications

by
Yixin Wu
1,
Ziyu Wang
1,
Jian Zhou
2,3,
Qilin Gu
1,2,3,* and
Zhaoxiang Zhong
1
1
College of Chemical Engineering, Nanjing Tech University, Nanjing 211816, China
2
Nanjing Membrane Application Institute Co., Ltd., Nanjing 210009, China
3
NJTECH University Jiangsu Future Membrane Technology Innovation Center, Suzhou 215334, China
*
Author to whom correspondence should be addressed.
Separations 2025, 12(8), 219; https://doi.org/10.3390/separations12080219
Submission received: 9 June 2025 / Revised: 1 August 2025 / Accepted: 7 August 2025 / Published: 18 August 2025
(This article belongs to the Special Issue Design and Preparation of Sustainable Separation Membrane for Efficient Water Purification)
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Abstract

Membrane separation technology can be operated in moderate conditions with low energy consumption and has been widely explored and increasingly applied in the water treatment, food, chemical, and pharmaceutical industries. As an upgraded counterpart, stimulus-responsive membranes can respond to external stimuli (such as light, temperature, pH, electric field, magnetic field, etc.) and actively modulate their own physical and chemical properties, thus showing self-adaptive ability and improved performance. This review provides a comprehensive overview of the recent advancements in the design principles, fabrication methods, and applications of these stimulus-responsive membranes. The challenges and future directions in this field are also prospectively discussed, highlighting the potential for further innovation and industrial applications of stimulus-responsive membranes.

Graphical Abstract

1. Introduction

Separation is an indispensable part of everyday life and industry. Traditional separation technologies include distillation [1], adsorption [2], etc., but they all have the shortcomings of a large footprint, wide scale, and high energy consumption. Membrane separation technology, characterized by low energy consumption, low pollution, and a small footprint, is a fabulous alternative. In the past few decades, membrane separation technology has made great progress [3,4]. In addition to the conventional membrane process, increasing attention has been paid to stimulus-responsive membranes, inspired by the rapid response of chameleons to their natural environment. Through stimulation, such as temperature [5], pH [6], electric fields [7], magnetic fields [8], and light [9], the molecular-level properties of polymer reaction sites are altered. These advanced membranes can dynamically adjust their pore size, surface charge, or wettability in response to external stimuli (mentioned above), thereby providing the substantial advantage of self-adaptive ability over conventional static membranes.
To date, there have only been a few reviews of research on stimulus-responsive membranes. For example, Huang et al. [10] provided a detailed review of the stimulation mechanism of and methods for preparing stimulus-responsive membranes, as well as the membrane properties during the response process. They also listed the materials that responded to different stimuli. Pan [11] divided the stimulus-responsive membranes into two categories: physical stimulus response and chemical stimulus response, and provided a detailed review of them. Finally, the applications of stimulus-responsive membranes in the fields of environmental protection, medicine, and the food industry were also introduced. Due to their rapid development and the increasing attention they are receiving in various fields, a brief review of the latest progress is urgently required. A Mini Review of the basic principles and typical methods for stimuli-responsive membranes can provide quick access to the topic for colleagues from other disciplines.
In this review, we examine in detail the many recent contributions that demonstrate the important and rapidly developing field of stimulus-responsive membranes. Typical examples from the past few years are highlighted to keep readers updated on this field, mainly focusing on stimulus-response principles, preparation methods, and the potential applications of stimulus-responsive membranes. In addition, we project future development trends and outline potential issues that need to be solved.

2. Stimuli-Responsive Principles

Stimuli-responsive membranes are designed to exhibit reversible changes in their physical or chemical properties when exposed to specific triggers, such as temperature, chemical, light, electrical, and magnetic fields, as schematically illustrated in Figure 1.

2.1. Thermo-Responsive Stimulus

In thermo-responsive polymers, the change of temperature is a key factor in the occurrence of reversible phase separation. Such phase separation often occurs because the interaction between the polymer chain and the solvent varies with temperature. This ability to sense change has great practical significance. In general, the temperature at which phase separation occurs is defined as the critical solution temperature [17], which has been recognized as the most important characteristic for describing thermosensitive materials. Based on the critical solution temperature, thermo-responsive polymers are usually classified into two categories: lower critical solution temperature (LCST) [18] and upper critical solution temperature (UCST) [19]. From Figure 1a, we can see that the critical temperature of the LSCT polymer is at the lowest point of the miscibility gap, while that of the USCT polymer is at the highest point, indicating the difference between the two polymers.
LCST-type polymers, such as poly(N-isopropylacrylamide) [20], have a better interaction with polymer chains and solvents when the solution temperature is lower than the critical temperature. However, the situation changes when the temperature is higher. The polymer chain tends to collapse or become immiscible with the solvent because of the breaking of the hydrogen bond formed between the polymer and the solvent [18], which is unfavorable to the actual reaction. In contrast, for UCST polymers, such as poly(N-acryloylglycinamide) [21,22], hydrogen bonds within micelles are formed first, followed by the formation of inter-micellar hydrogen bonds during the cooling process. This leads to low miscibility between UCST polymers and solvents since the hydrogen bonds within micelles dissociate when heated above this critical solution temperature, resulting in high miscibility. These transformation processes can be observed by further measuring the enthalpy change during the phase transition through micro-differential scanning calorimetry (microDSC) [22].

2.2. pH-Responsive Stimulus

pH-responsive materials can alter their surface charge and/or wettability in response to a change in pH levels, thus enhancing their ability to separate charged molecules or particles. These changes mainly arise from the ionized state of the chemical groups on the surface of the membrane at different pH conditions. When these changes occur, the present pH is defined as the transition or critical pH, which is determined by the pKa, becoming a vital criterion of the pH-responsive material. Furthermore, the pKa value refers to the pH value at which half of the ionizable groups are ionized. As illustrated in Figure 1b, when the pH exceeds the pKa, the polymer becomes negatively charged due to deprotonation. The repulsion between negative charges causes the polymer chains to expand, resulting in an extended conformation. The swollen conformation enhances the hydrophilicity and solubility of the surface polymer in water, leading to an increase in membrane flux. In contrast, at lower pH values (under acidic conditions), the polymer chains collapse due to protonation, causing the membrane pores to contract. This reduces hydrophilicity and solubility, resulting in a decrease in membrane flux.
Taking the poly(acrylic acid) (PAA) [23] as an example, it contains carboxylic acid groups (-COOH) on its backbone, which is the effective part. Under alkaline conditions (high pH), the carboxylic acid group ionizes into negatively charged carboxylic acid ions (-COO−). While under acidic conditions (low pH), the carboxylic acid group (-COOH) may not be charged and retain protonation, resulting in a significant reduction in negative charge. This change in charge affects the surface properties of the membrane and its ability to adsorb charged molecules.

2.3. Photo-Responsive Stimulus

Light is an efficient and controllable medium that can accurately transmit information. Owing to this, great interest in photo-responsive materials has been generated, leading to tremendous progress in the research on photo-responsive polymers [24,25]. Photo-responsive polymers are often composed of a combination of photo-responsive groups and bulk polymers [26,27]. Due to the characteristics of photosensitive groups, a stimulus-response mechanism is formed. The stimulation of different wavelengths of light alters the structure of the polymer, resulting in the transformation of its chain properties or catalytic reaction to produce active free radicals.
Polymers of this type include azobenzene [28], spiropyran [25], diarylethene [29], and so on. For example, as Figure 1c demonstrates, azobenzene and its derivatives can be transformed from a trans isomerization to a cis structure under ultraviolet light; the dipole moment increases, and the liquid crystal performance decreases. Under visible light, it restores to the trans isomer, which has a rod-like molecular structure and exhibits liquid crystal (LC) behavior and strong non-covalent aromatic–aromatic interactions. In addition, trans-azobenzene can form host–guest complexes with cyclodextrins (CD) [30], while cis-azobenzene leads to the dissociation of the inclusion compound. It follows in Figure 1c that reversible photoisomerism can lead to a switch between the two states of the photochromic portion, resulting in molecular changes in color, charge, group polarity, and size. These molecular changes can be scaled up into measurable macroscopic property changes. The differences in these optical properties endow the photo-stimulus-responsive membrane with different properties under different LC phases.

2.4. Electro-Responsive Stimulus

Electro-responsive membranes can change their physical or chemical properties when subjected to an applied electric field. One key advantage of electric fields is their ease of modulation in terms of amplitude, phase, and frequency, providing a versatile range of stimuli for controlling these membrane materials. The polar molecules or charged groups in the membrane are oriented under the action of the electric field, resulting in changes in the structure and properties of the membrane. This change in the membrane structure of molecular orientation is induced by the electric field. Figure 1d illustrates some functions and working principles of electro-responsive membranes. First, by adjusting the external voltage applied to the electro-responsive membrane, the electric field and charge density on the surface of the membrane can be regulated, thereby tuning mass transport, fouling mitigation, and fouling monitoring. Subsequently, certain solutes in aqueous solutions can undergo electrochemical reduction on the surface of the electro-responsive membrane. For instance, the generation of hydrogen and hydrogen peroxide can be achieved through such reduction processes. Additionally, under an applied external voltage, electrochemical oxidation of waterborne pollutants can occur on the membrane surface, thereby achieving pollution mitigation and self-cleaning functionality.
A prominent example of an electro-responsive polymer is poly(vinylidene fluoride) (PVDF) [31] along with its derivatives, such as copolymers of vinylidene fluoride with trifluoroethylene or tetrafluoroethylene. PVDF has piezoelectric and dielectric properties and thus is suitable for sensors and filter membranes. In addition to PVDF, other polymers and molecules like polyaniline (PANI), viologen, and polypyrrole (PPy) [32,33,34], as well as inorganic materials like BaTiO3 [35] and NaNbO3 [36] have also been identified as electro-responsive materials.

2.5. Magneto-Responsive Stimulus

Magnetic response materials have the advantages of remote control, low energy consumption, and fast response speed. In the current research, iron oxide is the most widely used magnetic response material, especially magnetite (Fe3O4) [16] and maghemite (γ-Fe2O3). Magnetic-responsive membranes are usually fabricated by grafting or coating these materials on the surface of the membrane. Magnetic particles on the surface of the membrane are loose and random (Figure 1(e1)). Thus, the membranes can be responsible for the external magnetic field variation. When the position of the magnetic pole of the applied magnetic field changes, the magnetic particles on the surface of the membrane move, and this movement is reversible. This change is shown in Figure 1(e2), where magnetic particles line up in a single direction according to the position of the magnetic field. In addition, the heating effect of magnetic nanoparticles will be induced under the high-frequency alternating magnetic field, which provides the possibility for the combination of magnetic nanoparticles and thermo-responsive materials.

3. Synthesis Strategies of Stimulus-Responsive Membranes

3.1. Blending

Blending is a common method for preparing stimuli-responsive membranes. It involves mixing the polymer or additive with the ceramic substrate. This method can adjust the pore structure and surface properties of the membrane to achieve a response to the stimulus, which is simple and direct.
Sun et al. [37] crafted a novel CO2-responsive copolymer polyvinylidene fluoride grafted 2-diethylaminoethyl methacrylate (PVDF-G-PDEAEMA) on the surface of a PVDF membrane, and a PVDF/PVDF-G-PDEAEMA composite membrane was fabricated. The water flux of the PVDF/PVDF-G-PDEAEMA blend membrane under CO2 pressure was significantly lower than that under N2 pressure, indicating that the blends were CO2-responsive, due to the presence of protonated tertiary amine groups in PDEAEMA chains. After the introduction of CO2, the chain segment of PDEAEMA in the membrane hole tended to be in the extended chain state owing to the protonation reaction, resulting in the shrinkage of the membrane hole, and the water flux was only about 100 L m−2 h−1. However, under the pressure of N2, the chain segment was in a state of chain collapse due to the deprotonation reaction, which led to the enlargement of the membrane pore, and the water flux significantly increased to about 170 L m−2 h−1. Through the protonation and deprotonation processes of the tertiary amine group, the cleaning process of the membrane could be implemented by the process of alternately passing N2 and CO2, and the flux recovery rate reached 89.9%. Although it was slightly lower than the recovery rate of acid cleaning (90.6%) and alkaline cleaning (95.2%), it was much higher than the water rinsing (60.8%), presenting its high practical significance. It was also pointed out that the cleaning efficiency of alternate ventilation depended on the solubility of carbon dioxide in water, which was worthy of further study.
Ji et al.’s [38] thesis primarily investigated the modification of polyvinylidene fluoride (PVDF) membranes using amphiphilic thermosensitive copolymers and inorganic nanoparticles. The SEM images (Figure 2) revealed that the addition of GO modified the membrane’s pore structure, shifting from predominantly large pores to a more finely distributed network of smaller pores. This indicates that the hydrophilic groups on the GO surface formed a relatively stable hydrogen bond network with PVDF-g-PNIPAAm, which improved the internal structure of the membrane, enhanced its antioxidant properties, and simultaneously increased the separation efficiency and lifespan of the membrane without compromising its temperature responsiveness.
Figure 3a shows the formation diagram of the PVDF/PVDF-g-PNIPAAm/GO membrane using the blending process. First, they synthesized an amphiphilic temperature-responsive copolymer, where hydrophilic poly(N-isopropylacrylamide) (PNIPAAm) side chains were grafted onto a hydrophobic polyvinylidene fluoride (PVDF) backbone. Subsequently, the PVDF-g-PNIPAAm copolymer and graphene oxide (GO) were blended with PVDF to fabricate temperature-responsive separation membranes. The temperature responsiveness of the membrane was further enhanced by adjusting the grafting ratio of PNIPAAm.

3.2. Self-Assembly

Membrane surface functional modification can be achieved by the self-assembly of stimulus-responsive materials into the sandwich through a simple vacuum-assisted filtration or layer-by-layer process. For example, Wang et al. [42] introduced a doped block copolymer (BCP) with definite molecular geometry into the bilayer membrane and obtained a variety of pore structures. A robust and simple strategy was demonstrated to design hybrid membranes for solute transport pathways. Their simulation results showed that under the action of external tension, the phase interface in the mixed membrane could be effectively activated to achieve the opening of the hole. Dopants self-assemble and are rearranged at the edge of the hole to obtain their favorable configuration. These findings provided the possibility to construct structurally complex membranes with multiple pore modifications through co-assembly of mixed BCP.
It is worth mentioning that research progress has been made in layer-by-layer assembly in recent years. The layer-by-layer (LBL) technique offers several key advantages, such as the ability to process membranes entirely in aqueous solutions, precise control over membrane thickness by simply adjusting the number of deposition cycles, and the potential to create stratified membranes with distinct layers. For example, Li et al. [39] fabricated a multi-stimuli-responsive multilayer antibacterial coating (MMT/PPPB-CHA)n through layer-by-layer self-assembly of inorganic nanolayered montmorillonite (MMT), cationic antibacterial agent chlorhexidine acetate (CHA), and poly(phenylene pyridinium bromide) (PPPB), as illustrated in Figure 3b. They deposited the coating layer by layer until they had multiple layers.

3.3. Polymerization

Through surface-initiated polymerization reactions, such as photoinduced electron transfer reversible addition broken chain transfer polymerization (SI-PET-RAFT), polymers with antifouling and vision-activated bactericidal properties can be formed on the membrane surface [43]. This method enables precise control of the growth direction and length of the polymer chain, thus giving the membrane a specific stimulus response.
Recently, Gilmer et al. [44] reported a one-step polymerization with a diepoxide and a mixture of two dianilines to fabricate epoxy nanofiltration membranes with cleavable disulfide bonds. They investigated the ability of various diamines and dioxides to prepare epoxy membranes that were robust but permeable to selected chemicals. They successfully synthesized Dithiodiamine A with a yield of 91% using 3,3′-dithiopropionate as the raw material. Fourier transform infrared spectroscopy (FT-IR) was used to monitor the polymerization, and a decrease in the transmittance of the epoxide peak was observed at 910 and 860 cm−1. When the spectra of peaks at 910 cm−1 and 860 cm−1 did not change further, the reaction was considered complete. The disulfide bond reacted with the chemical irritant cysteamine, altering the flux and selectivity of the chemical through the membrane. The splitting of disulfide bonds increased the pore size and flux, and decreased the selectivity of the membrane. In addition, a single epoxy membrane with two different separation properties could be obtained by adding disulfide bonds to the membrane, and it was successfully applied to the separation of three-component mixtures.
To simultaneously enhance the water permeability, selectivity, and permeation performance of polyamide nanofiltration membranes in treating textile wastewater, Mi et al. [40] introduced zwitterionic polymer nanoparticles (ZNPs) into the polyamide selective layer. They used the polymerization approach shown in Figure 3c. Groups with different electrical properties in molecules were connected through electrostatic interactions, forming a polymer network. Thus, a salt-responsive thin-film nanocomposite (TFN) membrane was fabricated by synthesizing ZNPs via inverse miniemulsion polymerization using zwitterionic monomers, amino monomers, and crosslinking agents. The microstructure and properties of the polyamide TFN membrane were effectively modulated by adjusting the ZNPs loading and NaCl concentration. The salt-responsive nature of ZNPs endowed the resulting TFN-ZNP (thin film nanocomposite-zinc nanoparticle) membrane with tunable microstructure and separation performance, which could be altered by changing the sodium chloride (NaCl) concentration in water. In a 10 g L−1 NaCl solution, the water permeability of TFN-ZNP increased to 1.7 times its pure water permeability, and the salt-responsive behavior was reversible. Furthermore, this salt-responsive characteristic imparted the membrane with excellent separation performance for salt/dye or dye/dye mixtures. The selectivity of TFN-ZNP for NaCl/Congo Red and Methyl Orange/Neutral Blue reached ~104 and ~127, respectively. The TFN-ZNP membrane with salt-responsive properties also exhibited improved salt resistance, as evidenced by a much higher flux recovery ratio (96.5%) when cleaned with NaCl solution compared to deionized water cleaning (85.6%). This study provided a novel paradigm for the engineering of advanced TFN membranes in applications such as dye desalination and wastewater treatment.

3.4. Electrospinning

Electrospinning is a common method for preparing membranes, and has the advantages of simple operation, a wide application range, and high yield. It is a technique in which a polymer solution, subjected to a high-voltage electric field, undergoes stretching and deformation due to electrostatic forces. Nanofibers are subsequently formed as the solvent evaporates. Polymers are often used as raw materials, which can be classified into synthetic polymers, such as poly-vinyl alcohol (PVA) [45], and natural polymers, like collagen [46], while common solvents used in the process include formic acid, dichloromethane, hexafluoroisopropyl alcohol (HFIP), dimethylformamide (DMF), acetic acid, ethanol, trifluoroacetic acid, tetrahydrofuran, and distilled water [47].
The fabrication of stimuli-responsive membranes using electrospinning was first reported in 2015, involving the preparation of smart nanostructured electrospun polymer membranes from polymethylmethacrylate-co-poly(N, N-diethylaminoethyl methacrylate) (PMMA-co-PDEAEMA) [48]. Li et al. also demonstrated the successful fabrication of pH-responsive antibacterial P(AA-AM)/CA/GA nanofiber membranes using electrospinning technology [49]. The results indicated that the incorporation of P(AA-AM) improved the wetting properties of the membrane. The P(AA-AM)/CA/GA spun fiber membrane subjected to high-temperature treatment at 150 °C exhibited the highest fracture stress and tensile strength. This was attributed to the cross-linking reaction of the P(AA-AM)/CA/GA membrane induced by high-temperature treatment, thereby enhancing the mechanical properties of the membrane. Furthermore, the prepared P(AA-AM)/CA/GA nanofiber membrane was pH-responsive, with the release rate of GA varying in solutions of different pH values. The P(AA-AM)/CA/GA fiber membrane demonstrated the strongest GA release capability in a solution with pH = 5.8. Additionally, it was found that P(AA-AM)/CA/GA fiber membranes showed good antibacterial activity against E. coli and S. aureus. The higher the concentration of GA, the stronger the antibacterial activity was. This study provided insights into the design and preparation of pH-sensitive antibacterial materials using electrospinning technology.
Dou et al. [41] prepared a pH- and temperature-dual-responsive PAN/PNIPAm/PDMAEMA nanofiber membrane by blending poly(N-isopropylacrylamide) (PNIPAm) and poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) with polyacrylonitrile (PAN) through a one-step electrospinning process (shown in Figure 3d). The three primary raw materials were first mixed to form a precursor solution and were then fed into the collector under high voltage for electrospinning. The introduction of responsive polymers in appropriate proportions endowed the membrane with stimulus-responsive wettability to both pH and temperature. Specifically, due to the protonation of tertiary amine groups and changes in the membrane’s microstructure, the membrane exhibited hydrophilic behavior at pH = 2.0 or T = 50 °C, while it became hydrophobic at pH = 7.0 or T = 25 °C. The blended membrane demonstrated excellent separation performance for different types of oil–water mixtures, achieving separation efficiencies of over 99.5% for immiscible oil–water mixtures and 96.8% for surfactant-stabilized emulsions. Owing to its superior responsive properties, the resulting blended membrane exhibited outstanding antifouling performance and a high flux recovery rate of up to 98.9%. The dual-responsive blended membrane with tunable wettability combines high separation efficiency and high flux recovery, making it highly practical for the separation of emulsified oil–water mixtures.

4. Applications of Stimulus-Responsive Membranes

4.1. Water Treatment

With the increase in the world population and the continuous development of industry, the world’s water resources are becoming scarce, and people have gradually realized the importance of water treatment. Membrane technology plays an important role in the water treatment field. The research on stimulus-responsive membranes makes the contribution of membrane technology to water treatment more efficient. The integration of catalytic degradation and membrane separation technologies into a composite membrane presents a promising solution for the one-step treatment of complex wastewater.
Wang et al. [50] grafted a temperature-responsive polymer onto a nano-silver-modified ceramic substrate to enhance its dye removal efficiency and antifouling properties. Through continuous optimization of reaction conditions, the optimal grafting parameters were determined. The surface morphology of the membrane at each modification step was characterized using FESEM, revealing that the copolymer on the composite membrane surface transitions from a network-like structure at 20 °C to a brush-like structure at 85 °C. Furthermore, the fabricated composite membrane exhibited temperature-responsive behavior. When the temperature increased from 20 °C to 85 °C, the pure water flux of the membrane increased from 68.8 L m−2 h−1 to 434.4 L m−2 h−1 (at 0.1 MPa) (Figure 4a), representing a sixfold enhancement, and this temperature-responsive behavior was reversible. The composite membrane demonstrated a separation efficiency exceeding 98.0% for oil-in-water emulsions and a catalytic degradation efficiency exceeding 99.0% for dye solutions (Figure 4b). At 85 °C, the composite membrane achieved a separation efficiency of 95.4% for both oil and dyes, highlighting its significant potential for water treatment applications. This performance was attributed to the changes in the physical conformation and wettability of the grafted polymer at different temperatures, which promoted the release of foulants.
In addition, Li et al. [51] prepared a ceramic composite membrane (Ag@Fe-CM) with visible-light-responsive properties by using polydopamine (PDA) as a modifier to in situ generate iron oxyhydroxide (FeOOH) and silver nanoparticles (Ag NPs) on a ceramic substrate. The PDA coating facilitated the mineralization of Fe3+ into FeOOH nanorods and stabilized them on the membrane surface. When the molar ratio of Ag to Fe was 1:20 (5% Ag@Fe-CM), the composite membrane exhibited a loose surface structure, excellent hydrophilicity and permeability, with a pure water flux of 138.1 L m−2 h−1 bar−1 (Figure 4c). During cross-flow filtration, the composite membrane demonstrated effective removal of methylene blue (MB) through the synergistic effects of membrane adsorption, separation, and photo-Fenton degradation. At an H2O2 concentration of 10 mM, the 5% Ag@Fe-CM achieved a high MB removal rate of 93.2% under visible light, compared to Fe-CM (Figure 4d). This enhancement was attributed to the Ag nanoparticles boosting the photo-Fenton catalytic activity of FeOOH. Additionally, the composite membrane exhibited excellent self-cleaning capabilities. Dye molecules adsorbed or retained on the membrane surface could be degraded, allowing for easy flux recovery. After the photo-Fenton reaction, the flux recovery rate of the MB-fouled composite membrane reached 91.7%, and the MB removal rate remained above 88% after four cycles (Figure 4e), demonstrating superior self-cleaning performance and reusability. This study provided a meaningful approach for fabricating photo-Fenton ceramic composite membranes, offering broad application prospects for organic wastewater treatment.
It is worth mentioning that stimulus-responsive membranes have also achieved certain results in oil–water separation. Zhang et al. fabricated core-shell structured fibrous membranes composed of polyurethane (PU) and poly(methyl methacrylate)-block-poly(N-isopropylacrylamide) (PMMA-b-PNIPAM), referred to as PTFMs [52]. By integrating light-responsive silver nanoparticles (Ag NPs) and thermo-responsive copolymers, the resulting membranes demonstrated photothermally tunable water wettability while maintaining excellent recyclability. To evaluate the separation efficiency, dynamic light scattering (DLS) was employed to analyze the droplet size distribution of the original hexane-in-water nanoemulsion and the corresponding filtrate. As shown in Figure 4f, before separation, the droplet size ranged from 60 nm to 460 nm. When the laser was deactivated, the filtrate contained droplets smaller than 10 nm, confirming effective separation. Upon laser activation, the filtrate showed droplets between 10 and 44 nm, indicating adjustable separation performance. These findings demonstrated that the light-responsive behavior of the copolymer enables efficient and controllable oil-–water separation. The work highlighted the significant potential of photothermal-responsive composite membranes in advanced oil–water separation applications.

4.2. Gas Separation

Membrane separation is a simple and efficient technology for gas separation and has been widely studied [53,54,55]. Recent advancements have centered around the development of stimuli-responsive materials by embedding functional groups that respond to stimuli directly within the pores or on the surface of membranes. These functionalized membranes leverage the interactions between the pores to induce changes in their structural conformation or the reactivity of the stimuli-responsive groups. As a result, they can effectively control mass transfer and separation processes, offering enhanced performance and adaptability for various applications.
In a pioneering effort, Ying et al. [56] engineered an ultrathin, pressure-responsive composite membrane composed of two-dimensional metal–organic framework nanosheets (MONs) and graphene oxide nanosheets (GONs) for enhanced CO2 separation. This innovative membrane harnessed a CO2-induced “gate-opening” mechanism, enabling highly efficient gas separation. Figure 5a clearly illustrated the membrane’s response to pressure, where the carbon dioxide molecule was the key that opens the MON flexible channel. By strategically controlling the direction of gas permeation and capitalizing on the pressure-responsive phase transition of MONs, the researchers achieved a substantial boost in CO2 permeance and selectivity (Figure 5b). This study introduced a novel paradigm for smart membrane systems, grounded in the dynamic chemistry of structural transformation. Additionally, molecular dynamics simulations were utilized to further illuminate the flexible behavior and separation mechanisms of the membrane, shedding light on the diffusion pathways of gas molecules and their interactions with the membrane’s architecture.
Due to the superior gas adsorption capacity of MOFs and the controllable properties of photo-responsive materials, Xin et al. [57] fabricated mixed matrix membranes using Uio-66-NH2 and the photo-responsive material CE-Azo-Uio-66 as fillers, with Pebax as the base membrane. The introduction of crown ether groups provided additional transport sites for SO2 within the membrane, and the cavity structure of the crown ether exhibited strong adsorption for polar SO2 gas molecules while repelling non-polar N2 gas molecules, thereby enhancing the SO2/N2 selectivity of the mixed matrix membrane. When the CE-Azo-Uio-66 content was 20%, the mixed matrix membrane demonstrated optimal performance. Compared to the pure Pebax membrane, the SO2 permeability and selectivity of the Pebax/CE-Azo-Uio-66 membrane increased by 191% and 179%, respectively (Figure 5c). Furthermore, the Pebax/CE-Azo-Uio-66 membrane not only exhibited higher permeability and selectivity than the Pebax/Uio-66-NH2 membrane but also possessed photo-responsive characteristics. Under UV-Vis light switching conditions, the Pebax/CE-Azo-Uio-66-20% membrane showed regular and reversible changes in SO2 permeability and SO2/N2 selectivity, highlighting the photo-responsive behavior of the azo-containing mixed matrix membrane.

4.3. Biomedical Science

Stimuli-responsive nanofiber membranes prepared using the electrospinning method possess a large specific surface area and porosity, and can rapidly respond to environmental stimuli, making them ideal stimuli-responsive materials. They can efficiently load drugs and subsequently release them in a controlled manner under specific conditions [58], such as light, temperature, magnetic field, and so on [59,60]. This enables slow, on-demand, and targeted drug release, demonstrating significant potential in the biomedical field, for example, for drug delivery and cancer therapy.
Yi et al. [61] successfully constructed a smart nanofiltration membrane with pH-responsive and rechargeable antibacterial capabilities through the interfacial polymerization of 3-aminophenylboronic acid (APBA) and trimesoyl chloride (TMC) on a polysulfone substrate. The boronate ester complex served as the key component enabling pH responsiveness. Under neutral or alkaline conditions, diol-containing bactericides could be grafted onto the membrane surface, while under acidic conditions, the bactericides could be released from the membrane. The rapid and effective pH response of the boronate ester endowed the membrane with excellent rechargeable functionality, allowing this process to be cycled at least four times (Figure 6a). With a rejection of 0.93% to NaCl and 98.1% to CR, and with fast water permeation of 53.4 L m−2 h−1 bar−1 (Figure 6b), the APBA-TMC membrane demonstrated high separation performance for binary dye/salt systems, along with superior molecular sieving selectivity and rechargeable anti-biofouling properties, showcasing promise for sustainable water purification and biological systems.
In another work, Wang et al. [62] developed responsive hybrid poly(vinyl alcohol) hydrogel membranes incorporating poly(N-isopropylacrylamide-acrylic acid) microgels as functional valves through a straightforward mixing process followed by a freeze–thaw cycle. Within the membrane structure, the matrix poly(vinyl alcohol) chains penetrated and entangled with the microgels, which were firmly constrained within the hybrid hydrogel network. The rapid and sharp temperature responsiveness of the embedded microgels was largely preserved, endowing the hydrogel membranes with excellent temperature and pH responsiveness (Figure 6c,d). Furthermore, the hydrogel membranes exhibited outstanding anti-fatigue properties in both temperature- and pH-responsive flux tests, ensuring long-term performance. It showed almost constant flux transition between around 7.5 and 66 kg·m−2·h−1 under a specific temperature (37 °C) and constant flux transition between around 6 and 62 kg·m−2·h−1 under a pH of 3.5 (Figure 6e,f). This study demonstrated the significant potential of these hybrid hydrogel membranes in biomedical applications and provided a novel strategy for the future design and construction of responsive biomaterials.
Based on the above representative application of responsive membranes, a summary of the responsive substance, supports, preparation methods, and key benefits is listed in Table 1. The incorporation of responsive substances into the membranes and the trade-off between selectivity and permeability can be broken. In particular, it is possible to simultaneously improve gas permeability and selectivity [57] or rejection and water permeation [61]. The coupling of various stimuli has been primarily explored. A typical example is the combination of photo- and thermo-stimuli, which can be realized by adding a single photo-responsive substance through the photo-thermal effect [52]. The membranes that are dual-responsive to temperature and pH were realized by using P(NIPAM-AA) [62]. Therefore, the current multiple responsive membranes are mainly based on the addition of specific multifunctional materials, and a large space remains for the coupling of different stimuli.

5. Conclusions and Prospects

With the ability to respond quickly to environmental changes, the stimulus-responsive membrane represents a good application prospect. In this mini review, the stimulation mechanism of stimulation-responsive membranes is reviewed in detail, including thermo-, pH-, photo-, electrical-, and magnetic- responses. A series of methods for preparing stimulation-responsive membranes is introduced, including blending, solution casting, self-assembly, and electrospinning. Finally, we discuss the main applications of current stimulus-responsive membranes, which have great potential for high performance in water treatment, gas separation, and biomedical science applications, indicating that stimulus-responsive membranes will achieve faster development in the coming decades. The following aspects shall be considered in the future for accelerating the development of stimulus-responsive membranes and their applications.

5.1. Sensitivity and Selectivity

For gas-stimulated responsive membranes, when the gas concentration is low (such as at the ppm level), the membrane material often fails to respond quickly or significantly, resulting in weak or delayed signals. Furthermore, when confronted with complex gas mixtures (such as CO2/N2/O2) alone, the membrane may have cross-responses to multiple gases, making it difficult to accurately identify the target gas. The response speed and reversibility of gas membranes are limited because the gas diffusion rate and the structural changes of the membrane material require time, which affects real-time monitoring or rapid switching applications. Meanwhile, the performance of some materials (such as certain metal–organic frameworks [MOFs]) deteriorates after multiple adsorption–desorption cycles, resulting in a decrease in response stability. Highly selective ligands (such as biomimetic carriers) or composite structures (such as MOFs/polymer hybrid membranes) can be developed, and the gas–membrane interface interaction can be optimized to enhance the diffusion rate and response sensitivity.

5.2. Stability in Microstructure and Performance

Many stimulation-responsive membranes (such as hydrogel-based membranes) lack the mechanical strength, toughness, and durability to meet the needs of long-term or recycled use. Additionally, some membrane materials will degrade after multiple stimulus-response cycles, which will affect their long-term stability. In complex industrial or biological environments, membrane materials may be affected by high salt, high organic concentration, or extreme pH, resulting in reduced response performance. The possibility of incorporating nanoparticles (such as nanoclay, graphene, etc.) into the membrane matrix has been proposed to enhance its mechanical strength and toughness. Furthermore, scientists can delve into research on membrane materials with self-healing functionalities, such as acetophenone [63], to relieve this issue effectively.

5.3. Multiple Stimuli and Functional Membranes

At present, most stimulus-responsive membranes can only achieve a single or double stimulus response (such as a temperature or pH response). The development of multi-stimulus-responsive membranes is still at the preliminary stage, and more efforts should be dedicated to developing them. Stimulus-responsive membranes also face difficulties in expanding their application areas. Although stimulus-responsive membranes show great potential for water treatment, biomedicine, and other fields, their application in complex industrial processes (such as membrane distillation) is still limited by membrane contamination, wettability, and other issues. More efforts should be devoted to this field, including increased support from governments and enterprises, to overcome these limitations. Additionally, one can optimize complex industrial processes through the integrated application of process integration, modular design, and other innovative approaches.
To the above ends, it is also possible to consider combining different preparation methods to prepare multi-functional stimulus-responsive membranes. Also, sustainable materials can be green-prepared by using bio-based polymers (such as cellulose and chitosan) or low-energy consumption processes (such as photopolymerization), and large-scale production can be achieved by combining 3D printing and roll-to-roll processes to promote industrial applications.

5.4. Stimuli-Responsive Inorganic Membranes

As we can see from the above discussion, most of the current research on stimulus-responsive membranes focuses on organic membranes, while there are few works on inorganic membranes that are known for their better stability. Therefore, the exploration of inorganic responsive membranes is of great significance for the expansion of application fields. Due to their superior chemical resistance, thermal stability, mechanical robustness, and high separation performance, ceramic membranes hold a pivotal position among inorganic membranes. At present, there are only a few research articles on the pH- [64] and photo- [51,65] response of ceramic membranes. It is crucial to emphasize the unique advantages of inorganic membranes in extreme environments, such as high temperatures, strong corrosion, and high mechanical loads, and to address the gap in existing stimuli-responsive inorganic materials. Novel functional inorganic materials can be developed by incorporating functional nanoparticles, engineering porous structure gradients, and applying surface grafting.

5.5. Large-Scale Fabrication and Application

The goal of stimulus-responsive membranes is to be used for practical applications in industry, and the cost of their materials and mass production should be considered, which is still currently higher than that of conventional materials. Small-scale laboratory experiments also struggle to simulate the actual production application environment. The types of stimulus-responsive membrane materials are still deficient. It is thus quite necessary to explore new responsive materials and build models that can simulate actual production to test the possibility of industrial applications.

Author Contributions

Conceptualization, Q.G,; investigation, Y.W., Z.W. and J.Z.; Data curation, Y.W.; writing—original draft preparation, Y.W.; writing—review and editing, Y.W., Z.W., J.Z., Q.G. and Z.Z.; supervision, Q.G.; project administration, Z.Z; funding acquisition, Q.G. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the financial support provided by the National Key Research and Development Project of China (2022YFB3808904), the National Natural Science Foundation of China (22308150), the Natural Science Foundation of Jiangsu Province (BK20220345), Key Research and Development Program of Jiangsu Province (No. BE2023360), and Jiangsu Future Membrane Technology Innovation Center (No.BM2021804).

Conflicts of Interest

Author Jian Zhou and Qilin Gu were employed by the company Nanjing Membrane Application Institute Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Stimuli-responsive principles. (a) Phase diagrams of thermo-responsive (➀) LCST and (➁) UCST polymers [12]. (b) Principle of pH-responsive membranes [13]. (c) Principle of photo-responsive membranes [14]. (d) Multiple functions and working principles of electro-responsive membranes [15]. (e) Alignment of magnetic nanoparticles on the membrane surface [16]. Reprinted with permission from Refs. [12,13,14,15,16].
Figure 1. Stimuli-responsive principles. (a) Phase diagrams of thermo-responsive (➀) LCST and (➁) UCST polymers [12]. (b) Principle of pH-responsive membranes [13]. (c) Principle of photo-responsive membranes [14]. (d) Multiple functions and working principles of electro-responsive membranes [15]. (e) Alignment of magnetic nanoparticles on the membrane surface [16]. Reprinted with permission from Refs. [12,13,14,15,16].
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Figure 2. SEM photographs of the membranes: (a1–f1) the upper surfaces of pure PVDF membranes, PVDF/PVDF-g-PNIPAAm membranes, 0.25 wt%, 0.50 wt%, 0.75 wt%, 1.00 wt% GO content PVDF/PVDF-g-PNIPAAm/GO membranes; (a2–f2) the cross-section of the pure PVDF membranes, PVDF/PVDF-g-PNIPAAm membranes, 0.25 wt%, 0.50 wt%, 0.75 wt%, 1.00 wt% GO content PVDF/PVDF-g-PNIPAAm/GO membranes. Reprinted with permission from Ref. [38].
Figure 2. SEM photographs of the membranes: (a1–f1) the upper surfaces of pure PVDF membranes, PVDF/PVDF-g-PNIPAAm membranes, 0.25 wt%, 0.50 wt%, 0.75 wt%, 1.00 wt% GO content PVDF/PVDF-g-PNIPAAm/GO membranes; (a2–f2) the cross-section of the pure PVDF membranes, PVDF/PVDF-g-PNIPAAm membranes, 0.25 wt%, 0.50 wt%, 0.75 wt%, 1.00 wt% GO content PVDF/PVDF-g-PNIPAAm/GO membranes. Reprinted with permission from Ref. [38].
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Figure 3. Main strategies for synthesizing stimulus-responsive membranes. (a) Schematic diagram of PVDF/PVDF-g-PNIPAAm/GO membrane formation [38]. (b) Schematic illustration of the multi-stimulus responsive multilayer antibacterial coating (MMT-PPPB-CHA)n [39]. (c) Schematic illustration of ZNPs fabrication [40]. (d) Fabrication of PAN/PNIPAm/PDMAEMA blend membrane [41]. Reprinted with permission from Refs. [38,39,40,41].
Figure 3. Main strategies for synthesizing stimulus-responsive membranes. (a) Schematic diagram of PVDF/PVDF-g-PNIPAAm/GO membrane formation [38]. (b) Schematic illustration of the multi-stimulus responsive multilayer antibacterial coating (MMT-PPPB-CHA)n [39]. (c) Schematic illustration of ZNPs fabrication [40]. (d) Fabrication of PAN/PNIPAm/PDMAEMA blend membrane [41]. Reprinted with permission from Refs. [38,39,40,41].
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Figure 4. (a) The pure water flux of the composite membrane cycled through different temperatures [50]. (b) Separation efficiency of composite membranes for the oil-in-water emulsions [50]. (c) The pure water flux of different membranes [51]. (d) The removal efficiency of MB by different membranes [51]. (e) The flux recovery ratio and membrane fouling resistance of different membranes [51]. (f) Size distribution of H/W nanoemulsion before and after separation under light off and on. Reprinted with permission from Refs. [50,51,52].
Figure 4. (a) The pure water flux of the composite membrane cycled through different temperatures [50]. (b) Separation efficiency of composite membranes for the oil-in-water emulsions [50]. (c) The pure water flux of different membranes [51]. (d) The removal efficiency of MB by different membranes [51]. (e) The flux recovery ratio and membrane fouling resistance of different membranes [51]. (f) Size distribution of H/W nanoemulsion before and after separation under light off and on. Reprinted with permission from Refs. [50,51,52].
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Figure 5. (a) Architecture of the CO2-gate-opening MOF-based membranes [56]. (b) Two-cycle CO2 permeance, N2 permeance, and CO2/N2 perm-selectivity of MON@GON-0.1-0.002 and GOM-0.02 membranes [56]. (c) TGA diagrams of Pebax/CE-azo-Uio-66 membranes [57]. Reprinted with permission from Refs. [56,57].
Figure 5. (a) Architecture of the CO2-gate-opening MOF-based membranes [56]. (b) Two-cycle CO2 permeance, N2 permeance, and CO2/N2 perm-selectivity of MON@GON-0.1-0.002 and GOM-0.02 membranes [56]. (c) TGA diagrams of Pebax/CE-azo-Uio-66 membranes [57]. Reprinted with permission from Refs. [56,57].
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Figure 6. (a) Stationary phase plating with E. coli [61]. (b) The permeance and the rejection performance of the APBA-TMC membrane [61]. Hydrodynamic diameter (Dh) and volume change ((V-V0)/V0) of the P(NIPAM-AA) microgels (c) as a function of pH value at 37 °C [62] and (d) as a function of temperature at a pH of 3.5 [62]. Flux examination of the membranes (e) in 10 switching cycles of experiments at 25 and 45 °C with pH 3.5 [62], and (f) in 10 switching cycles of experiments with pH 3.5 and 7.4 at 27 °C [62]. Reprinted with permission from Refs. [61,62].
Figure 6. (a) Stationary phase plating with E. coli [61]. (b) The permeance and the rejection performance of the APBA-TMC membrane [61]. Hydrodynamic diameter (Dh) and volume change ((V-V0)/V0) of the P(NIPAM-AA) microgels (c) as a function of pH value at 37 °C [62] and (d) as a function of temperature at a pH of 3.5 [62]. Flux examination of the membranes (e) in 10 switching cycles of experiments at 25 and 45 °C with pH 3.5 [62], and (f) in 10 switching cycles of experiments with pH 3.5 and 7.4 at 27 °C [62]. Reprinted with permission from Refs. [61,62].
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Table 1. A summary of the membrane mentioned in the cases.
Table 1. A summary of the membrane mentioned in the cases.
Response TypeResponsive SubstanceSupport LayerPreparation MethodKey BenefitsReference
Thermo-responsiveP[P(VMDMO)-co-P(DMAEMA)-co-P
(MATE)]		Nano-Ag decorated ceramic ultrafiltration substrateBlendingPure water flux increased from 68.8 L‧m−2‧h−1 (25 °C, 0.1 MPa) to 434.4 L‧m−2‧h−1 (85 °C, 0.1 MPa).[50]
Photo-responsivePDAAg NPs, FeOOHSelf-Assembly5% Ag@Fe-CM showed a higher removal efficiency (93.2%) of MB under visible light compared with pure Fe-CM.[51]
Photo, thermal- responsivePMMA-b-PNIPAMPTFMsPolymerizationUpon laser activation, the filtrate showed droplets between 10 nm and 44 nm.[52]
Photo-responsiveCE-Azo-Uio-66PebaxBlendingThe SO2 permeability and selectivity of the Pebax/CE-Azo-UiO-66 membrane increased by 191% and 179% compared to pure Pebax membrane.[57]
pH-responsiveboronate esterPolysulfonePolymerizationA rejection of 0.93% to NaCl and 98.1% to CR, and fast water permeation of 53.4 L‧m−2‧h−1‧bar−1.[61]
Thermo, pH-responsiveP(NIPAM-AA)PVABlendingExcellent temperature and pH responsiveness, and almost constant flux transition between around 7.5 and 66 kg·m−2·h−1 under certain temperature (37 °C) and constant flux transition between around 6 and 62 kg·m−2·h−1 under certain pH of 3.5.[62]
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Wu, Y.; Wang, Z.; Zhou, J.; Gu, Q.; Zhong, Z. Stimulus-Responsive Membranes: A Mini Review on Principles, Preparation Methods, and Emerging Applications. Separations 2025, 12, 219. https://doi.org/10.3390/separations12080219

AMA Style

Wu Y, Wang Z, Zhou J, Gu Q, Zhong Z. Stimulus-Responsive Membranes: A Mini Review on Principles, Preparation Methods, and Emerging Applications. Separations. 2025; 12(8):219. https://doi.org/10.3390/separations12080219

Chicago/Turabian Style

Wu, Yixin, Ziyu Wang, Jian Zhou, Qilin Gu, and Zhaoxiang Zhong. 2025. "Stimulus-Responsive Membranes: A Mini Review on Principles, Preparation Methods, and Emerging Applications" Separations 12, no. 8: 219. https://doi.org/10.3390/separations12080219

APA Style

Wu, Y., Wang, Z., Zhou, J., Gu, Q., & Zhong, Z. (2025). Stimulus-Responsive Membranes: A Mini Review on Principles, Preparation Methods, and Emerging Applications. Separations, 12(8), 219. https://doi.org/10.3390/separations12080219

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