1. Introduction
Skin is a type of human tissue that comes into direct contact with the external environment, with functions such as protection, respiration, secretion and the perception of external stimuli
[1]. Skin allows moist vapor to effectively propagate from the inside to the outer atmosphere, while protecting the human body against the penetration of liquid water such as rainwater and snow. This special property is defined as “breathability”, which is of great significance in regulating the function of organisms. Skin has the ability to regulate body temperature to a certain extent
[2], but in extreme environments such as high temperature and extreme cold, the comfort and safety of the human body cannot be guaranteed. Therefore, waterproof breathable materials have been developed to ensure the normal operation of skin. It shows the relationship between skin and fabric in
[3]. Natural fibers were the earliest waterproof breathable materials, but barely meet people’s needs because of their small variety, strong hydrophilicity and poor adhesion to the skin. Compared with natural fibers, synthetic fibers occupy the main market of high-quality functional clothing due to their high strength and no deformation
[4]. With the development of science and technology, scientists developed the first waterproof breathable membranes (WBMs) in the 1970s and combined them with textile materials by laminating composite methods, while ensuring the waterproof breathable property and strength of the fabric.
At present, commercial WBMs are generally divided into two types, namely hydrophilic non-porous membranes and hydrophobic microporous membranes. There is no pore structure on the surface and inside of hydrophilic membranes, and liquid water cannot penetrate them. Hydrophobic microporous membranes, more widely used than hydrophilic non-porous membranes, have an appropriate pore size and high porosity. These membranes can distinguish between water droplets (diameter > 100 μm) and small gas molecules (diameter < 1 nm), so as to allow water vapor to transfer from the body to the outside and prevent water penetration
[5], thus forming a “microclimate” between the skin and fabric. WBMs provide protection and wearing comfort for the human body, and are widely applied in ski suits, nautical suits
[6], military clothing, police jackets
[7], etc.
By allowing water vapor to pass through and limiting the entry of liquid water, WBMs not only can be used as a waterproof breathable layer for clothing to ensure human comfort, but have also had significant development in fields such as construction materials
[8], batteries
[9], water treatment
[10] and electronic equipment
[11]. For example, in contrast to relatively expensive proton-exchange membranes, WBMs introduce oxygen and remove water vapor on the cathode side of membrane fuel cells (MFCs)
[12], which are a kind of gas diffusion layer material with easy preparation, low cost and stable cathode structure. In addition, with the development of science and technology, mechanical and electronic products have begun to use a large number of WBMs to protect internal electronic components, as they can block liquid, dust, etc., from entering the interior
[13]. More importantly, compared with completely sealing electronic devices to achieve waterproofing, the breathability of WBMs allows them to remove toxic gases generated by batteries, preventing internal leakage and affecting use.
Moreover, as the demand for fresh water increases, methods of water treatment are constantly developing. Among them, reverse osmosis (RO), forward osmosis (FO) and membrane distillation (MD) are common desalination methods. RO achieves seawater desalination by driving water molecules and ions through a hydrophilic membrane under high pressure
[14], and its water transport mechanism is explained by the solution–diffusion model. But recently, Elimelech’s team
[15] proposed that the migration of water and solvent in RO can be predicted by a solution–friction model, in which water molecules travel in clusters through tiny transient holes within the polymer that exert friction on them as they pass through. As a permeation-driven membrane process, FO has the characteristics of low energy consumption, good separation effect and simple operation. Al-Furaiji et al.
[16] prepared a thin-film composite (TFC) membrane with polyacrylonitrile (PAN) as the supporting layer. PAN-TFC membrane has excellent porosity, water flux and mechanical properties that are no less than those of the RO membrane. MD technology is a combination of membrane separation and thermal distillation
[17], which utilizes the temperature difference on both sides of the membrane to generate a vapor pressure difference
[18], enabling WBMs to separate liquid feed from a penetrant.
Common methods of preparation and processing of WBMs mainly include melt extrusion, biaxial stretching and electrospinning. The melt extrusion method has strong adaptability and uniform coating, but most processed membranes have poor water vapor permeability. Microporous membranes can be manufactured by biaxial stretching, but the preparation process is complicated and pore size adjustment is difficult. Currently, the electrospinning technique is considered to be the most effective method for fabricating WBMs. By adjusting the process parameters, electrospinning membranes with suitable fiber morphology and pore structure can be produced. In order to obtain WBMs with better comprehensive performance, two modification methods can be applied: doping modification and post-treatment. In the past few years, WBMs have developed rapidly in the textile industry, showing promising application prospects in fields such as outdoor clothing, protective clothing, wound dressing and smart clothing.
2. Applications of WBMs in the Textile Field
The application of WBMs in the textile field is generally realized by compounding them with fabrics and forming waterproof breathable fabrics through lamination technology
[19]. The market value of waterproof breathable fabrics was USD 1.43 billion in 2015 and is expected to grow at a rate of approximately 6% per year to USD 2.3 billion by 2024
[3], making them a promising textile material. There are three types of waterproof breathable fabrics
[20]: high-density waterproof fabrics, waterproof-coating finished fabrics and WBM composite fabrics. High-density waterproof fabric is made of absorbent and hydrophilic yarn or microfiber synthetic yarn, resulting in a small aperture to maximize the resistance to wind and rain but poor water penetration; waterproof-coated fabrics have excellent water resistance but poor water vapor permeability
[21]. Due to excellent water resistance and water vapor permeability, WBMs are laminated onto the fabric support layer and fixed under high temperature and pressure, displaying superior waterproof and breathable properties. Therefore, WBM composite fabrics have become the mainstream product for waterproof breathable textiles in the current market. Recent research progress on WBMs is summarized in
Table 1. Waterproof breathable textiles make functional clothing with both water resistance and water vapor permeability, playing an indispensable role in outdoor sports
[22], special protection
[23], medical conservation
[24] and self-cleaning
[25] fields.
Table 1. Summary of WCA, hydrostatic pressure and WVRT of WBMs.
2.1. Daily Outdoor Sportswear
To protect waterproof breathable membranes from physical damage and mechanical wear, they are usually sandwiched between layers of fabric
[35]. However, if punctured, their performance and water resistance will be greatly reduced. Outdoor sportswear and casual wear are relatively common waterproof and breathable materials in daily life, which need to provide a higher level of comfort as well as weather protection, because consumers pay more attention to comfort when wearing such high-performance clothing
[22]. It is worth mentioning that a hydrostatic pressure of 9.8 kPa and WVTR of 5 kg m
−2 d
−1 are the minimum acceptable levels for sportswear and raincoats; otherwise, wearers may experience heat stress.
The waterproof jacket is one of the most widely used and well-known waterproof breathable fabrics in daily life. The main purpose of these jackets is not only to protect people from water and wind, but also to maintain comfort while wearing, which largely depends on the “breathability” of the outer shell. At present, both hydrophilic non-porous and hydrophobic microporous membranes can be used to fabricate waterproof jackets
[36]. Elise et al.
[37] designed a climbing waterproof jacket specifically developed for women, which combined hydrophilic PU membranes with a lining through lamination technology and waterproofing the edges. The jacket has waterproof protection while still maximizing airflow through the clothing, ensuring the comfort of wearing. Then, hydrophobic microporous membranes, represented by PTFE membranes (such as Gore-Tex)
[38], are constructed to act as a filter to block pollutants. A major breakthrough in WBM fabrics is the development of self-sealing fabrics
[39], which can solve the problem of reduced waterproofness and breathable performance caused by internal WBM damage after sewing on coats, tents, etc.
In order to ensure that the cloth has high permeability, waterproofing, anti-pollution and antibacterial properties, Liu et al.
[40] proposed a multifunctional fabric with cotton fabric as the raw material, one side with a super-hydrophobic treatment and the other side with a porous membrane coating. The super-hydrophobic layer is coated with SiO
2/polydimethylsiloxane (PDMS), resulting in a water contact angle of 161° and a sliding angle of less than 5°, effectively preventing the entry of external water. The porous layer is cellulose acetate film with micropores, and the high porosity ensures the breathability of the fabric. Then, a metallic silver network deposited on the porous layer achieves the triple effect of infrared reflection, flexible heating and antibacterial properties. The fabric is likely to be widely used in fields such as clothing and outdoor sports.
2.2. Special Protective Clothing
As a waterproof and breathable layer of protective clothing, WBMs also play a great role in the field of protection
[41], helping staff who need to work at high temperatures to effectively discharge surface sweat and reduce the occurrence of heat stress
[42]. Heart failure caused by heat stress is one of the most common causes of death of fire personnel, so the WVTR of firefighting clothing should be around 10 kg m
−2 d
−1 to ensure the safety of high-temperature operation. In military operations and terrorist attacks, soldiers are at high risk of being harmed by toxic substances
[23]. To ensure the life safety of workers in dangerous environments, there are high requirements for the anti-liquid permeability of fabrics, which can be achieved by improving the waterproof and corrosion resistance of WBMs. As protective clothing, it is necessary to have good elasticity and wear resistance to prevent puncturing with sharp objects from the outside. Membranes based on inorganic chalcogenide nanotubes and metal oxide nanoparticles
[3] are excellent candidates for bulletproof vests.
Li et al.
[31] proposed an easy method to produce a novel electrospinning composite fiber membrane with high waterproof and breathable properties, which is composed of polyurethane (PU), end-fluorinated polyurethane (FPU) and carbon nanotubes (CNT). Based on the use of FPU and CNT, more rough structures were formed on the fiber surface, resulting in a hydrostatic pressure of up to 108 kPa and a WVRT exceeding 9.2 kg m
−2 d
−1, with good bursting strength (47.6 kPa) and tensile strength (12.5 MPa). The results showed that the FPU/PU/CNT fiber membrane was a promising candidate material for protective clothing.
Furthermore, using PU and FPU as raw materials, Li et al.
[33] obtained a hydrophobic fiber membrane with breathable and waterproof properties through electrospinning. The porous structure of the membrane was adjusted by modulating relative humidity (RH) and electrospinning time. When the RH reached 60%, the membrane showed 60% porosity and 1.2 μm pore size. The composite fiber membrane was waterproof and breathable, and had good mechanical properties (tensile strength: 10 MPa; elongation: 353%), so it could be used as a new material for the manufacture of protective clothing.
2.3. Wound Dressing
For occupations that must resist fluid penetration, such as medical personnel and firefighters, protective clothing requires a higher degree of waterproofing
[24]. Because of the long-term use of surgical gowns, it is important to ensure the safety and comfort of doctors during an operation
[43]. Therefore, WBMs play a crucial role not only in preventing a patient’s blood from infecting the doctor but also in maintaining the doctor’s comfort and dryness during surgery. Excellent WVTR is also important for wound healing
[44] as it controls the moisture content of the wound to promote the proliferation of epidermal cells and fibroblasts. Therefore, one way to create the ideal microenvironment around a wound is to apply WBMs as a wound dressing. In addition, patients often need to recover in bed for a long time after receiving treatment. Traditional hospital sheets and hospital gowns are made of cotton fabric, but cotton fabric generally has good hydrophilicity and poor sweat release
[32], which can easily cause bacteria and viruses to multiply on the sheets and hospital gowns, which is also not conducive to the postoperative recovery of patients. Consequently, the development of medical supplies with good protection and water vapor permeability to protect the common health of medical workers and patients is an urgent demand in the medical health field.
Yu et al.
[45] mixed fluorosilane-modified silica nanoparticles (F-SiO
2) with synthetic polyurethane (PU) solution, then compounded it with polyacrylonitrile (PAN) solution, and obtained a new hydrophobic microporous nanofiber membrane by electrospinning. The obtained PAN/(F-SiO
2/PU) nanofiber microporous membrane achieved strong tensile strength (19.5 MPa), good WVRT (10.3 kg m
−2 d
−1), good water contact angle (137.2°), and excellent thermal stability and mechanical properties. It is believed that the enhanced PAN/(F-SiO
2/PU) nanofiber composite membranes have potential application prospects in medical products such as chemical protective clothing, military combat clothing and self-cleaning materials.
In view of the green manufacturing method, Guo et al.
[24] developed a skin-like waterproof breathable polyvinyl butyral (PVB) embedded polydimethylsiloxane (PDMS) fiber membrane. The addition of hydrophobic agent PDMS enhanced the surface hydrophobicity of the fiber membrane, with a maximum hydrostatic pressure of 54.32 kPa, a WVRT of 8.98 kg m
−2 d
−1 and an increase in mechanical strength of 4.95 MPa. The developed fibrous membrane provided functions similar to human skin and allowed for adequate stretching at the joint location, enabling better design of waterproof, breathable and stretchable wound dressings. Of course, there are also many studies on cellulose membranes in the field of wound healing. Shi et al.
[46] synthesized a cross-linked cellulose membrane (CEM) constructed with epichlorohydrin (EP) crosslinking agent, which has high fracture strength (137.4MPa), adaptive permeability and excellent biocompatibility. Compared with the original cellulose membrane, the light transmittance and water repellency of the crosslinked membrane have been improved to some extent.
2.4. Bionic Textiles
In recent years, bionic intelligent textiles have gradually entered the vision of people with the development of bionics
[47]. Because the damage caused by washing to a fabric’s waterproof and breathable properties is permanent, self-cleaning waterproof breathable textiles have become a popular research direction
[48]. Researchers have noticed that the waxy surface structure of lotus leaves has a low adhesion to water droplets, thus possessing good waterproofing and self-cleaning capabilities, which is known as the “lotus effect”
[34][49]. In addition, the wings and scales of many animals, as well as the leaves of plants, exhibit cleaning properties (
[50]). By controlling the assembly of carbon nanotubes, silicon and polymer
[51], the special rough structure of the lotus surface is imitated, so that the water contact angle is greater than 150° and the purpose of self-cleaning is achieved. Refs
[35] demonstrate the stomatal structure of simulated leaves, as well as the WBMs that can automatically adjust water vapor permeability by closing or opening their stomata according to the prepared environment.
Introducing photochromic microcapsules (PM) into an electrospinning thermoplastic polyurethane (TPU) membrane, a new waterproof and breathable membrane with good photochromic properties was prepared by Liu et al.
[52]. Compared with the original TPU samples, the composite TPU/PM membrane had reversible photochromic properties. In addition, the composite membrane not only had a water contact angle of 137° and milk contact angle of 130°, but also had moderate comprehensive properties: for instance, WVRT of 19.3 kg m
−2 d
−1, high air permeability of 962 mm s
−1, low water resistance of 2.813 kPa and tensile strength of 12.08 MPa. The convenience and efficiency of the manufacturing process will allow the mass production of multifunctional fibrous membranes, which can also be applied to photochromic skin, “chameleon” tablecloths and protective clothing, among other materials. Furthermore, Liu et al.
[40] created a wearable multifunctional textile by combining antibacterial material with waterproof, breathable membranes. In consequence, the combination of waterproof, breathable, photochromic, antibacterial, heat insulation, electronics and other aspects provides new inspiration for the structural design of intelligent textiles, which will also be the development trend of the textile industry in the future.
This entry is adapted from the peer-reviewed paper 10.3390/ma16155339