Hydrogels have three-dimensional network structures, high water content, good flexibility, biocompatibility, and stimulation response, which have provided a unique role in many fields such as industry, agriculture, and medical treatment. Poly(vinyl alcohol) PVA hydrogel is one of the oldest composite hydrogels. It has been extensively explored due to its chemical stability, nontoxic, good biocompatibility, biological aging resistance, high water-absorbing capacity, and easy processing. PVA-based hydrogels have been widely investigated in drug carriers, articular cartilage, wound dressings, tissue engineering, and other intelligent materials, such as self-healing and shape-memory materials, supercapacitors, sensors, and other fields. In this paper, the discovery, development, preparation, modification methods, and applications of PVA functionalized hydrogels are reviewed, and their potential applications and future research trends are also prospected.
Polymer hydrogel networks are low crosslinking materials, which can be formed by chemical or physical crosslinking methods and form covalent or noncovalent crosslinking points. Such networks can expand or contract, absorbing and retaining large amounts of water while insoluble in water [
Design strategies of functional hydrogels have been extensively studied. Hydrogels prepared
As one of the old polymer hydrogel materials, poly(vinyl alcohol) (PVA) has shown new vitality in recent years. Many studies have shown that it has the value of the further study. PVA (Figure
Molecular sketch (a) and chemical structure (b) of PVA chain.
In this review, we summarized the preparation, modification, and applications of PVA hydrogels, especially focused on the frontier works, and a prospect of future development was given at last.
In the early 1970s, frozen gelled PVA was proposed for biomedical applications [
At room temperature, a solution of highly alcoholysis PVA (>98%) can autonomously and slightly gelated between molecular chains into hydrogels. However, such hydrogel has poor mechanical properties and applications due to the low gelation density [
At present, the repeated “freezing-thawing method” is the most commonly used physical crosslinking process [
The gelation mechanism (c.f. Figure
Gelation schemes of PVA are formed by: (a) physically crosslinking (freezing-thawing) [
Chemical crosslinking is the most commonly used preparation method of hydrogels, and the hydrogel properties are affected by the concentration of monomer, crosslinker, and reaction conditions. For PVA, chemical crosslinkers could form chemical crosslinking points between PVA molecules to form gels (Figure
Radiation crosslinking utilizes high-energy rays, such as gamma-ray, electron beams, and X-ray, that directly radiate PVA solutions. As shown in Figure
Neat PVA hydrogels are not sensitive to environmental stimuli. Thus, functional monomers/polymers are used to prepare hybrid hydrogels with modified properties [
Modification of PVA by: (a) CBA-PVA (chemically) [
By utilizing the intermolecular force between polymer chains to form molecular aggregates, the composite system with excellent properties could be achieved. Physical crosslinking chitosan (CS)/PVA blend hydrogels with only PVA gelated networks have been widely used in many fields [
Composite with inorganic fillers or organic small molecules can also enhance the PVA hydrogels [
Biomedical polymer materials should have a series of characteristics such as nontoxic, good biocompatibility, degradability, and processability. The excellent properties of PVA hydrogel meet the requirements of biomedical polymers. Therefore, PVA has been widely used in drug delivery carriers [
PVA hydrogels are used as drug carriers while small drug molecules are embedded, and the stability of the drug can be increased [
The biomedical applications: (a) controlled drug releasing from PVA/PEG hydrogels [
Although the traditional gauze dressings have a wide range of uses, there are many disadvantages in their application due to their small amount of absorption of excess wound exudate, frequent replacement, and adhesion with the wound [
However, the PVA hydrogels themselves do not have antibacterial properties; in order, it is necessary to enhance the antibacterial performance of PVA hydrogels as dressing materials. Antibacterial modification mainly includes inorganic, organic, and natural antibacterial molecules. Silver nanoparticles are commonly used inorganic antibacterial agents, which are considered efficient and broad-spectrum antibacterial, but the main barrier for industry application is the high cost [
The antibacterial materials of the PVA hydrogel systems are generally expensive, and the modification methods are complicated. Therefore, the preparation of antibacterial PVA hydrogels with low cost and simple preparation, large-scaled production method is the research focus [
Scaffold material plays an important role in tissue engineering. An ideal scaffold material should have the following characteristics: nonimmunogenicity, nontoxicity, good biocompatibility, high porosity, degradability, appropriate degradation rate, and easy manufacturing [
Hydrogel elastomers have similar tribological properties and physical properties such as high water content as articular cartilage [
PVA hydrogels materials are potential candidates for artificial joints, artificial muscles, artificial vitreous body, artificial cornea, and iris. PVA hydrogel has porous, permeable structures that are similar to natural cartilage and contain much water. Liquid can be infiltrated and extruded under a certain load, which can be entrained as a lubricant. Hydrogels’ high water content and surface structure are very similar to natural cartilage fabrics [
Intelligent hydrogels can respond promptly to subtle changes or stimuli in the environment, such as temperature, pH value, electric field, and pressure. For instance, the thermosensitive hydrogel can shift its swelling degree according to the temperature of the environment to control drug release at desirable conditions. In recent years, more attention has been paid to self-healable, shape memory, and stimulus responsive materials, which can be made from PVA hydrogels and be implemented in intelligent devices. However, the structures and functionality of hydrogels can be damaged by external mechanical or chemical erosion, especially under complex internal environment conditions. Therefore, designing and synthesizing hydrogels with self-healing/shape memory ability is widely concerned by researchers.
Unlike traditional hydrogels, self-healing hydrogel forms a spatial network through dynamic connectivity. Recently, self-healing hydrogels have attracted extensive research interest and have been applied to constructing various new intelligent materials [
PVA contains large amounts of hydroxyl groups that can form intermolecular hydrogen bonds
Schemes of PVA showing: (a) self-healing [
Self-healing properties of some reported PVA materials.
Synthetic strategy | Mechanical property | Healing time | Healing efficiency | Ref. |
---|---|---|---|---|
Hydrogen bond | Fracture stress ~105 kPa | 10 s to 48 h | Up to 70% | 15 |
Synergistic crosslinking of bimetallic ions | <60 s | Up to 95% | 98 | |
Hydrogen bond | Compression modulus is 39.6 kPa~74.0 kPa, | ≥4.68 min | 63.21% | 101 |
Reversible hydrogen bond | ~10 min | / | 102 | |
Hydrogen bond | Tensile stress ≈60 kPa | <24 h | ~74% | 134 |
In addition, the physically crosslinked PVA was introduced into the chemically crosslinked PEG network to form a double crosslinking network, such PEG/PVA hydrogel not only showed self-healing property but also available of shape memory [
An early electrochemical supercapacitor was assembled with glutaraldehyde crosslinked PVA hydrogel as electrolyte and activated carbon fiber cloth as electrode [
As functional materials, thermotolerance is also a very important characteristic. Similar to self-healing, acid/alkali resistance, antifreeze, and compression resistance can bring more possibilities for intelligent materials. As functional devices, frozen resistance property is also very important [
Based on excellent physical and chemical properties, other functions such as antifreeze, self-healing, and stimuli response of PVA-based hydrogels can also be applied to act as specific sensing applications. The hydrogels respond to changes in the external environment, causing a swelling/shrinkage behavior. The change of the environment can be speculated by measuring the volume change of the hydrogels.
A PVA hydrogel-based temperature sensor with frost resistance, water retention, and moldability was prepared by introducing glycerol and silver nanofibers (AgNWs) into PVA hydrogel [
Another sensing mechanism utilizes the so-called photonic crystals as colorimetric elements [
The properties of typical PVA-based hydrogel sensors.
Sensing target | Response time | Mechanical property | Characteristic | Ref. |
---|---|---|---|---|
Temperature | ~1 s | Good elastic and mechanical properties | Moisture retention under normal temperature and moderate conditions (water retention 37% ~ 95%) | [ |
Strain | 0.32 s | High tensile strain up to 500%, | [ | |
Strain | 170 ms | Low detection strain (0.25%), wide sensing range (0.8 kPa~ 50 kPa), durable operation | [ | |
pH and Pb+2 | / | / | Capable of dehydration and rehydration | [ |
Glucose concentration | 180 s | / | Low detection limit (glucose concentration: 0 ~ 20 mM) | [ |
The long-term stability and sensitivity of flexible sensors designed with injectable, antifreeze, and intelligent sensing hydrogels as electrolytes at low temperatures remain urgent challenges. In addition, compared with traditional electronic materials, the disadvantages of hydrogels include low modulus and high water percentage, which may lead to poor integration of hydrogels and devices. Therefore, optimizing and improving the integration process of hydrogel-based sensors are urgent challenges.
In addition to the intensive studies and reports described above, there are more interesting studies showing specific properties of PVA materials that deserve attention. Such as an ultrastrong nacre-inspired double network PVA hydrogel with the impact-resistance property was reported [
PVA-based functional hydrogels with their applications in: (a) remote triggered liquefaction [
The future design of hydrogels should have the following advantages: (1) functionalized hydrogel materials should utilize nontoxic, biodegradable macromolecules, that is, good biocompatibility and biodegradability, and hydrogels can be self-healed to extend their lifecycles; (2) for tissue engineering, the mechanical properties (such as hardness and elasticity) and structure (such as porosity and density) of the hydrogels should be highly matched with the embedded tissues; (3) the injected or implanted hydrogels could be tracked
The improvement of the preparation and processing technology of hydrogels also played a very positive role in improving their performance. During the past decade, 3D printing technology has developed rapidly, and 3D printing devices with different principles and materials have been developed one after another. Among them, smart materials are a kind of material that has attracted much attention recently. In 2013, the importance of developing 3D printing for multifunctional intelligent, responsive materials was emphasized, and the concept of 4D printing was proposed [
With the continuous development of hydrogel design and continuous improvement, as well as the optimization of processing and molding methods, the traditional PVA hydrogel materials keep revealing more possibilities, and the future potential of PVA is enormous as illustrated in Figure
PVA-based hydrogels as multifunctionalized materials.
The authors declare no conflict of interest.
M.W, J.B., and W. T. wrote the paper under the supervision of C.C.; and Z.D., K.S., D.L., J.Y., and S.H. revised the review paper. Menghan Wang and Jianzhong Bai contributed equally to this work.
This research was funded by Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the Sailing Project from Science and Technology Commission of Shanghai Municipality (17YF1406600), the Chenguang project supported by Shanghai Municipal Education Commission (18CG68), the Fundamental Research Funds for the Central Universities (2232020G-07), the key subject of Shanghai Polytechnic University (Material Science and Engineering, XXKZD1601), and Gaoyuan Discipline of Shanghai-Materials Science and Engineering.