A unidirectional moisture transport material is a specialized type of material designed to transport moisture from one side to the other while simultaneously preventing moisture from moving in the opposite direction. Among these innovative materials， pore-gradient unidirectional moisture transport materials stand out as particularly significant. These advanced materials achieve unidirectional water transport through a carefully engineered gradient of pore sizes within the material， a process driven by the Laplace pressure. Such materials are not only eco-friendly and stable but also operate without requiring any external energy input， making them highly applicable and valuable in fields such as directional water collection， liquid transport， and oil-water separation. This paper first introduces a detailed classification of the various unidirectional moisture transport mechanisms and explains the underlying theoretical mechanisms from an energy perspective. It then reviews and analyzes the different types of pore-gradient materials. Finally， the paper discusses both the current and future applications of unidirectional moisture transport materials， along with a comprehensive analysis of their limitations and potential development directions.
Contents
1 Introduction
2 One-way moisture conduction mechanism of porous gradient materials
2.1 Classification of unidirectional moisture conduction mechanism
2.2 The mechanism of unidirectional moisture conduction explained from the perspective of energy
3 Classification of aperture gradient materials
3.1 One-component substrate aperture gradient material
Water is the source of life, and obtaining the water necessary for survival from nature is a challenge that all living organisms must address. Over 3.5 billion years of evolution, many organisms in nature have developed systems capable of efficiently collecting and transporting water. For example, the vascular system in plants and the network of blood vessels in animals provide essential water and nutrients for the proper functioning of living organisms[1-2]. With advances in materials science, researchers have begun to mimic these natural phenomena, designing and developing a series of materials with directional water transport capabilities. Studies have shown that surfaces capable of directional water transport in nature typically exhibit physical pore size gradients or wettability gradients. These structures create surface energy gradients and Laplace pressure differences on droplets, driving the directional transport of liquids. For instance, the needle-like spines of cacti feature a conical multi-level structure that can direct droplets from the tip of the spine to its base[3-5]; spider silk undergoes "wet reconstruction" upon absorbing water, with the radius gradually increasing from the silk's attachment point to the fusiform node, and the Laplace pressure difference created by the curvature gradient from the attachment point to the spinneret can drive droplets toward the fusiform node[6-9]; the bumps and grooves on the back of desert beetles allow water droplets condensed on their backs to roll into hydrophobic grooves once they reach a certain size, enabling the collection of moisture from the air[10].
Currently, constructing materials with an internal pore size gradient is a major research focus in developing unidirectional moisture-wicking functional materials. Researchers have employed methods such as structural design and laser etching to create materials with internal pore size gradients. These gradient materials can be categorized into single-substrate pore size gradient materials and multi-substrate composite pore size gradient materials. Pore size gradient materials exhibit high stability and good scalability, showing promising applications in water collection[11-13],oil-water separation[14-18],and medical applications[19-21],among others. This article first elaborates on the unidirectional moisture-wicking mechanism of pore size gradient materials and the mainstream methods for preparing such materials, then introduces their applications in various fields and future development directions.
2 Unidirectional moisture-wicking mechanism of pore-gradient materials
2.1 Categories of unidirectional moisture-wicking mechanisms
Currently, the unidirectional moisture-wicking mechanism of materials is typically attributed to three types: differential capillary effect, wettability gradient effect, and biomimetic transpiration effect. Pore-gradient materials primarily rely on the differential capillary effect to achieve unidirectional liquid transport, using Laplace pressure to move liquids unidirectionally from regions with larger pores to regions with smaller pores[22].
2.1.1 Differential capillary effect
The differential capillary effect is caused by the pore size gradient structure present in unidirectional moisture-wicking materials. The smaller the pores, the stronger the differential capillary effect and the greater the Laplace pressure (Fc)[23]. When the structure of a unidirectional moisture-wicking material forms a pore size gradient in a specific direction, it guides the liquid to transport directionally from areas with larger pores to areas with smaller pores, which is the unidirectional moisture-wicking performance[24]. The Laplace pressure (Fc) is shown in Equation (1).
in the above equation, γwaterrepresents the surface tension of water, θ is the water contact angle, and ris the average pore size of the material's surface. As can be seen from this formula, reducing the material's pore size rincreases Fc, which facilitates the transport of water within the material. Constructing hierarchical micro- and nanostructures is an important method for achieving directional water transport in materials[25-26].
2.1.2 Moisture gradient effect
The wettability gradient refers to a multilayer structure composed of two or more parts, where each layer has distinctly different wettability, and the wettability of each layer gradually improves or deteriorates along the thickness direction. When a droplet first contacts the hydrophobic side of the material, due to the presence of a wettability gradient, the hydrophobic layer exerts resistance to droplet movement, while the hydrophilic layer exerts an attractive force on the droplet, enabling the liquid to be transported unidirectionally from the side with lower wettability to the side with higher wettability. Conversely, when the droplet first contacts the hydrophilic side of the material, it will wet the hydrophilic side, while the low wettability of the hydrophobic side hinders the transport of moisture toward that side, thus achieving unidirectional moisture conduction in the material.
2.1.3 Biomimetic Transpiration Effect
The biomimetic transpiration effect combines the humidity gradient effect and differential capillary effect, enabling rapid and efficient directional water transport within materials. It draws inspiration from the transpiration process in vascular plants found in nature. In vascular plants, transpiration facilitates gravity-defying water transport from soil to plant stems and leaves through passive capillary action. This process features multiple branching structures, with pores gradually narrowing from bottom to top, allowing water to be transported against gravity via capillary forces[27-29].
2.2 Explaining the unidirectional moisture-wicking mechanism from an energy perspective
Currently, the unidirectional moisture-wicking materials designed by various researchers all start with constructing an internal pore size gradient and a wettability gradient within the material. Liquid flows from the hydrophobic side to the hydrophilic side of the material, but not vice versa. This phenomenon can be explained from an energy perspective. When a droplet is placed statically on a surface, with one side exposed to air and the other in contact with the solid surface, the droplet converts surface energy into kinetic energy, providing the energy required for its movement[30].
To understand the mechanism of droplet motion, the energy conversion process in this motion can be represented as follows.
The surface energy of the material can be expressed as[31]:
among them, X, Y, and Z represent the contact areas between liquid/vapor, solid/vapor, and solid/liquid, respectively; γlv, γsv, and γsl denote the corresponding interfacial tensions between liquid/vapor, solid/vapor, and solid/liquid, respectively. When the water droplet is in its initial state, i.e., not in contact with the material surface, the surface energy of the water droplet is E0, Z0 = 0, and Y0 = 0. When the water droplet comes into contact with the material surface, Y1 = 0, and the surface energy of the water droplet in the above equation can be expressed as:
θ is the water contact angle on the material surface. Substituting equation (4) into equation (3) yields:
γlvcosθ+（X1-X0）γlv
simplifying, we get:
considering that the contact area between the water droplet and air has changed relative to the contact area with the material, the relationship between the contact area of the water droplet with air and the water contact angle can be derived using the concepts of differentiation and integration.
Let the radius of the water droplet be R, and the relationship between the contact area between the water droplet and air (X1) and the contact angle with water is:
$Z_{1}=\pi(R \sin \theta)^{2}$
substituting equations (8, 9, and 10) into equation (7) and simplifying yields:
as can be seen from the equation, E is related to the water contact angle θ. As the contact angle increases, E also gradually increases, and E is always less than 0, indicating that the surface energy decreases during the wetting process. The released energy disrupts the interface between water and the solid, providing kinetic energy for droplet movement and ultimately achieving wetting.
3 Classification of Pore-Gradient Materials
In recent years, with the advancement of science and technology, techniques such as laser etching and plasma treatment have become increasingly mature and are gradually being applied to material processing. As a result, pore-size gradient materials are no longer limited to composites of multiple materials; it has become possible to create pore-size gradients within a single substrate material. Therefore, pore-size gradient materials can be classified into those with a single substrate and those composed of multiple substrates. Table 1lists some of the technologies used in the preparation of directional liquid transport materials.
3.1 Single-base pore size gradient material
Single-base pore gradient materials refer to materials whose base consists solely of a single material with a pore gradient structure. To create such a pore gradient within the material, physical methods can be employed, such as laser etching techniques to fabricate directional water transport materials with graded micro- and nanostructures. Alternatively, one or more chemical substances can be introduced, with their concentrations gradually varying along the thickness direction, thereby forming a pore gradient within the material—for example, using chemical vapor deposition. Whether the approach involves physical processing or the introduction of new substances, the treatment process is always applied to a single material, without any composite of multiple materials. Typically, there is no significant difference in wettability between the two sides of the material.
Chen et al[33]found that the high-speed transport of water on the surface of Sarracenia trichomes is caused by their unique hierarchical microchannel structure (Figure 1). Inspired by the water transport process on the surface of Sarracenia trichomes, Chen et al. used mathematical modeling and material testing methods to perform numerical simulations and experimental tests on these hierarchical microchannels, revealing a method for achieving highly directional water transport by incorporating secondary microchannels into the main microchannels through laser-etched high and low ribs. The study found that the water transport speed on the surface of Sarracenia trichomes is approximately three orders of magnitude faster than that of cactus spines and spider silk. When the trichomes are placed in a humid environment, a thin film of water forms within the hierarchical microchannels, automatically initiating an ultra-fast water collection and transport process. After a stable film is formed, even without the appearance of large droplets, the subsequent transport of collected water becomes even faster. This work strongly demonstrates the potential of pore gradient structures in directional water transport.
Wang et al. used hot-filament chemical vapor deposition (HFCVD) technology to deposit diamond powder onto β-SiC films, synthesizing micro- and nanocrystalline diamond/β-SiC composite gradient films. Using this method, they prepared a 0.9 mm gradient-length nano-diamond/SiC film capable of directional transport of acidic or alkaline solutions[34]. Furthermore, they investigated the effect of surface roughness on water contact angles and found that the micro-convexities on the micro- and nanodiamond surfaces were small enough to keep droplets in the Cassie-Baxter state. However, the nanocrystalline diamond surface had a higher coverage compared to the microcrystalline diamond surface, resulting in a higher water contact angle and lower surface energy (Figure 2).
They continued their previous research approach, using a controlled chemical vapor deposition process to regulate the deposition temperature and gas-phase concentration, thereby designing and synthesizing multifunctional, robust diamond gradient films with gradually varying hierarchical structures and chemical compositions. The size and abundance of graded micro/nanoscale diamond crystals gradually increase along the length of the film, enabling the gradient surface to guide water droplets from the highly hydrophobic diamond-rich surface (contact angle 141°) to the hydrophilic SiC surface (contact angle 13°) (Figure 3).
3.2 Multi-base composite gradient material with aperture
Currently, a major approach to fabricating materials with directional liquid transport capabilities is to combine multiple materials into multilayered structures using techniques such as electrospinning. Compared to single-substrate materials, multi-substrate composites allow for adjustments in pore size, wettability, and thickness for each layer, all of which are critical factors influencing the material's ability to direct water transport. Additionally, increasing the number of layers in the composite can create a greater pore size gradient, further enhancing the material's directional transport capacity. In nature, vascular plants possess a hierarchical porous structure that achieves greater capillary force and lower flow resistance by gradually reducing the diameter of capillaries (Figure 4a), minimizing fluid flow resistance within the capillaries and maximizing the capillary force driving vertical water transport[39-41]. Inspired by the transpiration process of vascular plants, Miao et al. used polyurethane (PU)/boron nitride nanofiber (BNNS) with a hierarchical porous structure whose capillary pore diameter (Dx) gradually decreases to mimic vascular tissue and mesophyll tissue[42]. By electrospinning PU/BNNS fibers with diameters gradually decreasing from 2500 nm to 300 nm, they prepared a hierarchical biomimetic multilayer fiber membrane with high directional transport capability (Figure 4b). Furthermore, they combined the Lucas-Washburn equation with the Poiseuille laminar flow equation to describe the vertical wicking and horizontal diffusion behaviors in the fiber membrane[43-45]. Theoretically, this confirmed that the anisotropic driving force between inter-fiber channels leads to enhanced vertical wicking and reduced horizontal spreading.
Based on the theory of constructing longitudinal channel differences in materials by utilizing a gradient of pore sizes across multiple layers to achieve unidirectional moisture transport, Chen et al. introduced innovations in water transport channels. They performed hydrophilic finishing on PLA nonwoven fabric surface electrospun with PAN (OPAN) nanofibers, obtaining an ultra-hydrophilic HOPAN layer with radial moisture transport channels, enabling water molecules to spontaneously diffuse and rapidly transfer radially along the fiber membrane[46]. They also compared the average pore sizes of PLA nonwoven fibers and HOPAN fibers. It was observed that the average pore size of the HPLA layer was 14.9 μm, one order of magnitude larger than that of the HOPAN layer (1.72 μm). Consequently, capillary pores with varying pore sizes were created between the HPLA and HOPAN layers, further enhancing the material's unidirectional moisture-wicking effect[47](Figure 5Figure 5).
4 Applications of Pore-Gradient Materials
Pore-gradient materials, with their unique morphology, offer advantages such as stability and no need for energy input. As multifunctional materials for directional liquid transport, they have extensive applications in directional water collection[48],liquid transport, oil-water separation[49],and other fields.
4.1 Application of Pore-Gradient Materials in Directional Water Collection
Currently, the shortage of freshwater resources is becoming increasingly severe. Fog in nature accounts for nearly 10% of all freshwater on Earth[50], and recovering water from fog can effectively alleviate the water resource crisis. Therefore, it is necessary to develop high-performance water-collecting materials to address freshwater scarcity. Researchers have achieved directional droplet aggregation and enhanced droplet capture capabilities by optimizing material structures and modifying wettability gradients[51]. Among these, fog collectors fabricated using fibers as material substrates exhibit superior water-collecting performance[52-54]. In regions facing water scarcity, many plants and animals have evolved unique abilities to collect fog water[55]. Early studies have shown that natural organisms such as spider silk, cacti, Namib desert beetles, and lizards can collect fog water from the atmosphere, relying on their unique asymmetric hydrophilic/hydrophobic surface wettability and surface topological structures, which enable directional droplet movement on their surfaces. Inspired by these principles, researchers have developed materials for directional liquid transport by constructing hydrophobic/hydrophilic surface morphologies and designing internal pore size gradients within the materials.
Wu et al. prepared hydrophobic/superhydrophilic directional moisture-wicking fiber membranes by electrospinning a double-layer PAN nanofiber membrane and subjecting it to differential heat treatment[56]. By controlling the pore sizes of the two fiber layers, they compared two types of fiber membranes with different pore size gradients: one with large pores in the hydrophobic layer and small pores in the hydrophilic layer, and the other with small pores in the hydrophobic layer and large pores in the hydrophilic layer. They found that the water collection capacity of the directional wicking membrane with larger pores in the hydrophobic layer and smaller pores in the hydrophilic layer was nearly 1.7 times that of the directional wicking membrane with smaller pores in the hydrophobic layer and larger pores in the hydrophilic layer. This demonstrates that an appropriate pore size gradient can significantly enhance the material's unidirectional liquid transport capability (Figure 6).
For materials with different hydrophobic and hydrophilic properties on either side, the water collection process typically begins at the hydrophobic side, as liquid cannot be directionally transported from the hydrophilic side to the hydrophobic side. Therefore, Bao et al. developed a three-layer membrane wicking structure with bidirectional water transport characteristics for continuous and efficient fog water collection[48]. They used multilayer electrospinning technology to fabricate a sandwich-structured composite nanofiber membrane, consisting of two outer layers of hydrophobic PLLA membranes and an inner layer of ultra-hydrophilic PLLA thin film modified in situ by a hydrophilic finishing agent (TF) through electrospinning. Due to the synergistic effect of the fiber structure and the anisotropic wettability from the external hydrophobic water transport layers to the intermediate ultra-hydrophilic water collection layer, the resulting membrane exhibits bidirectional water transport performance and excellent water collection capability. Water can spontaneously permeate from the two external hydrophobic layers into the central ultra-hydrophilic layer while being prevented from traveling in the opposite direction. In this sandwich structure, the pore size of the outer transport layers is larger, whereas the pore size of the middle layer is smaller, which further enhances the material's directional water collection ability (Figure 7).
However, current directional water collection materials typically rely on electrospinning technology, and issues such as material durability and the stability of directional water collection performance remain unresolved. Additionally, aspects such as the desorption rate of water after directional collection and the extent to which adverse environmental factors affect water collection performance require further investigation[57]. Only after comprehensive experimental validation can these methods be applied to practical atmospheric water harvesting scenarios.
4.2 Applications of Pore-Gradient Materials in the Medical Field
Pore-gradient materials' ability to direct liquid transport can help remove wound exudate while preventing external contaminants from entering the wound[58], thereby promoting wound healing[59]. Moreover, in air filtration devices such as masks, the directional transport properties of these materials can effectively discharge water vapor from the internal environment, enhancing the user's physiological comfort and reducing internal air pressure, which facilitates gas exchange with the external environment[60-61]. Lan et al. prepared a graded-structure air filtration membrane using electrospinning[62]. The inner layer is a hydrophobic thin PCL fiber membrane, with a PCL/gelatin layer serving as the intermediate layer, and an outer layer composed of PCL/gelatin/ε-PL. The pore size of the inner fiber membrane is significantly larger than that of the intermediate and outer layers. This filtration membrane can rapidly inactivate bacteria trapped on the surface of the fiber membrane[63], and more importantly, it can also prevent moisture from the external environment from entering, thus reducing discomfort caused by dampness. Meanwhile, the smaller pore size of the outer layer achieves a PM2.5 filtration efficiency of 98.82% (Figure 8).
In recent years, sensors used for monitoring human health have become a major research focus; however, several issues still exist in practical applications. When wearing sensors closely to the body to monitor health conditions, sweat produced by the skin around the sensor can lead to signal degradation, sensor malfunction, and delamination between the device and the skin, severely affecting the accuracy of data collection[64]. To address these problems, Xu et al., inspired by liquid transport strategies found in living organisms, developed an electrode composed of a gold/thermoplastic polyurethane/cellulose membrane (Au/TPU/CM) with gradient pore size and surface energy. This electrode can unidirectionally remove sweat from the sensor/skin interface to the sensor surface, thereby preventing signal attenuation and discomfort caused by sweat accumulation[65]. By combining gradient porosity and surface energy gradients, this electronic skin can immediately "pump" sweat from the interface to the external environment. The resulting electrode exhibits excellent conductivity (2.68 Ω·sq-1), as well as outstanding water vapor permeability and water evaporation rates (2.2 times and 7.1 times that of cotton fabric, respectively). The ultra-fast sweat-wicking capability of this electronic skin not only enhances wearing comfort but also minimizes measurement errors caused by skin hydration and temperature changes due to sweating, eliminates the risk of short circuits in sensor arrays, reduces noise levels, and improves the accuracy and reliability of sensors (Figure 9).
The design of medical dressings needs to consider the integration of multiple functions, such as hemostasis[66],pain relief[67],and promoting wound healing[68],among others. Achieving synergistic effects of these multiple functions within limited dimensions and space presents another technical challenge. While enhancing the absorption of wound exudate, it is also crucial to improve the biocompatibility and safety of dressings, ensuring that they do not trigger immune responses after implantation in the body. However, due to the complexity and individual variability of living organisms, it is difficult to fully replicate real-world conditions in laboratory settings, necessitating large-scale clinical trials to evaluate the long-term effectiveness of medical dressings.
4.3 Applications of Pore-Gradient Materials in Oil-Water Separation
In recent years, the continuous increase in industrial oily wastewater discharge has caused widespread damage to ecological balance and public health[69-71]. Oily wastewater typically contains complex components, such as oil/water emulsions and oil/water multiphase mixtures. Membrane separation technology, with its advantages of low energy consumption, high separation efficiency, and convenient operation, has been widely applied in wastewater purification. In recent years, researchers have leveraged the pore size gradient effect to develop a new generation of specialized water-in-oil emulsion separation membranes that exhibit directional transport properties, excellent separation efficiency, and high permeation flux under lower driving pressures. These membranes can direct the transport of droplets upon contact with the surface, successfully enabling the directional collection of tiny droplets from oil-water mixtures. This holds significant importance for the remediation of oily wastewater.
Zhang et al. developed a bilayer nanofiber membrane with directional moisture-wicking properties for oil-water separation by encapsulating TPU with CA, using TPU as the raw material[72]. The TPU serves as the hydrophobic layer, while the CA/TPU forms the hydrophilic layer. By adjusting the electrospinning parameters, a beaded structure was induced in the TPU fiber membrane, reducing the pore size of the TPU layer and creating a pore size gradient within the material. When an oil-water mixture comes into contact with the TPU layer, water molecules are transported directionally toward the CA/TPU layer due to differences in wettability and the pore size gradient. During this directional transport, the beaded structure of the TPU layer intercepts water droplets, exerting a repulsive effect on the liquid phase and preventing oil from entering the membrane, thereby imparting oil-repellent properties and achieving oil-water separation (Figure 10).
Not only fiber membranes are applied in the field of oil-water separation; three-dimensional gels with porous structures, high specific surface areas, and strong adsorption capacities are also widely used in the treatment of wastewater contaminated by petroleum pollutants. However, due to the limitations of traditional aerogels, the oil-water separation process is unsustainable, cumbersome, and costly, making it insufficient for practical applications. Inspired by the gradient porous structures observed in bamboo stems and glomerular filtration membranes, Zheng et al. prepared a 3D cellulose scaffold with a pore-size gradient structure (GPDS) from a homogeneous cellulose solution[73]. The pore-size gradient structure effectively reduced the density of GPDS (0.019 g/cm3), while significantly enhancing its specific surface area, porosity (98.82%), and compressive strength (up to 870 kPa at 70% strain). The oil/organic solvent absorption capacity of GPDS ranges from 13 to 25 g/g, achieving up to 99.8% separation efficiency for immiscible oil-water mixtures, with a flux exceeding 2000 L/m2·h, and stable surfactant-stabilized water-in-oil emulsions (with an efficiency of up to 97.7%). This 3D cellulose scaffold with a pore-size gradient, featuring excellent mechanical strength and separation performance, provides new insights for developing advanced oil-absorbing materials (Figure 11).
Currently, preparing fiber membranes with complex structures (hollow, porous, etc.) to achieve higher separation efficiency and longer service life is one of the hotspots in current research. There is still considerable room for improvement in the anti-fouling properties, mechanical strength, and thermal stability of oil-water separation materials. Further exploration of the structural design of oil-water separation materials and optimization of preparation processes are needed to produce materials with higher separation efficiency and longer service life at low cost and high production efficiency. Expanding their application scope in the field of oil-water separation, such as marine oil spill treatment[74],industrial wastewater treatment[75],and food processing wastewater treatment[76-78],can meet the demands of different industries for oil-water separation technology.
5 Conclusion and Outlook
The design of a pore size gradient structure causes the pore sizes within the material to vary gradually according to a specific gradient. This variation can generate a capillary pressure difference, thereby promoting the flow of liquids within the material along a particular direction. Compared to materials with uniform pore structures, pore size gradient structures can transport liquids more effectively and hold great application potential in scenarios where liquid flow direction needs to be controlled, such as drug delivery, water treatment, and air purification. By designing a pore size gradient structure within the material, it is possible to achieve directional liquid transport and enhance overall transport efficiency. This article summarizes the structural characteristics, functional principles, types, and potential application areas of pore size gradient materials capable of directional liquid transport. It also reviews the development and application of pore size gradient materials in related fields and proposes directions for future research.
(1) There is still room for improvement in the ability of pore gradient structures to regulate liquid flow. Currently, most pore gradient materials also feature a wettability gradient, leveraging differential capillary effects and variations in wetting properties within the material to achieve efficient directional transport of liquids. How to enhance the pore gradient's regulatory capacity for liquids is an issue that needs to be addressed. Biomimicry represents an important research direction. In nature, there are many sophisticated liquid transport systems (such as plant xylem vessels and animal blood vessels) capable of efficiently and stably directing liquid transport.
(2) Complexity of pore size gradient design. The pore structure of materials with a pore size gradient typically exhibits a gradual transition from large to small pores. This design requires precise control over the distribution of the gradient, and in different material systems, the design of the pore size gradient and its actual effect may vary, resulting in directional delivery performance that falls short of expectations.
(3) Difficulty in large-scale production. The process for manufacturing materials with gradient pore structures is relatively complex and costly. Achieving efficient, centralized production remains a challenge that needs to be addressed. Therefore, improving existing material preparation processes or adopting new technologies such as 3D printing is one approach to reducing production difficulties.
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