Synthesis, properties and applications of highly stretchable hydrogels
Introduction
Hydrogels can be characterised as three-dimensional polymeric networks that are able to efficiently absorb water (Matsumoto, et al., 2021). The respective polymeric formations that participate in the formation of hydrogels are hydrophilic with cross-linked individual chains. Typical hydrophilic groups that may be found in hydrogels can be either acid (-COOH, -SO3H) or basic (-NH2, -OH, -CONH2) (Matsumoto, et al., 2021). The ability to effectively absorb water along with other polar molecules allows hydrogels to change their shape while sustaining a solid formation. Hydrogels were synthesised and characterised in the middle of the 20th century. Research in this area allowed the development of advanced materials for biomedical applications, drug delivery and tissue engineering. The increasing popularity of hydrogels is also connected with their ability to show tissue-like flexibility. In particular, hydrogels show tissue properties in cases when the amount of absorbed water is above 10% by mass (Matsumoto, et al., 2021).
Considering properties of these highly advanced materials, literature in the area suggested that physical, biological and chemical stimuli promoted volume phase (shrinking or swelling of the material due to changes in the environment) or gel-sol phase (a gradual transformation of the material into a gel-like formation with both solid and liquid phases present) transitions (Zhang, et al., 2021). Typical chemical and biological stimuli are presented by changes in the pH of the environment as well as changes in the ionic strength of the solution interacting with the hydrogel (Zhang, et al., 2021). Physical stimuli are alterations in the electric or magnetic field, temperature, pressure and light intensity. It should also be indicated that the level of cross-linkage, chemical nature of the corresponding polymeric formations and charge density dictate the level and the nature of hydrogel responses to changes in the environment. These changes may be different, reversible or irreversible. For instance, interactions of individual chains, leading to the formation of new covalent bonds are irreversible, while conformational changes are reversible (Zhang, et al., 2021).
Despite the possibility of application in a considerable number of branches of the modern industry, the use of hydrogels is limited by their weak and brittle nature (Zhou & Rajeev, 2021). For instance, the reduced mechanical strength of hydrogels makes it challenging for producing hydrogel-based implants and drug delivery materials. Nevertheless, by changing the chemical nature of individual chains it was possible to increase the toughness, resilience, elasticity and flexibility of hydrogels (Zhou & Rajeev, 2021). A range of various synthesis techniques was developed; however, it was determined that covalent-ionic bonds hybrid formations offered the most effective compromise between toughness and flexibility. As a result, based on a high level of covalent-ionic bonds integration it was possible to generate long-chain covalently linked polymeric materials with adequate stretchability and enhanced toughness due to efficient dissipation of mechanical energy-generating as a result of physical deformations (Zhou & Rajeev, 2021).
The aim of the essay is to outline current preparation methods, properties and most important applications of highly stretchable hydrogels using literature from scientific databases.
Synthesis of highly stretchable hydrogels
A literature search in ScienceDirect suggests that the number of publications on highly stretchable hydrogels was increasing on yearly basis. For instance, looking at publications outlining synthesis methods of highly stretchable hydrogels a publication by Sun and co-workers can be put forward (Sun, et al., 2012). In the respective literature source, it was highlighted that highly stretchable hydrogels could be generated using a combination of alginate and polyacrylamide. In particular, the research group indicated that both stretchable and tough hydrogels could be generated using crosslinked alginate and ionically crosslinked polyacrylamide (Sun, et al., 2012) (Figure 1).
Figure 1: Synthesis of alginate-polyacrylamide hybrid hydrogel, adapted from (Sun, et al., 2012)
Alginate chains were produced by mannuronic acid units (M groups) and guluronic acid units (G groups) (Sun, et al., 2012). The corresponding groups are interlinked to produce three-dimensional blocks. G blocks can support the generation of alginate-based formations in water as a result of interaction with a range of divalent ions. The outlined types of interactions and structural formations were presented in figure 1 (Sun, et al., 2012).
In addition to alginate and polyacrylamide-based hydrogels, a series of industrially important highly stretchable hydrogels are synthesised using polyethene glycol (Huang, et al., 2019). For instance, it was shown that to generate hydrogels with the necessary mechanical properties it was required to combine chemicals that were able to efficiently dissipate mechanical energy with long-chain polyethene glycol formations (Huang, et al., 2019). As a result, it was shown that polyacrylamide/polyethene glycol networks could be as effective as alginate-based highly stretchable hydrogels in terms of mechanical deformations and water absorption abilities. Looking at specific examples, N, N-methylenebisacrylamide can be put forward as a crosslinking agent, while hyaluronan or chitosan can be used as the mechanical energy dissipation compound. To generate crosslinks, calcium sulphate, iron (III) chloride or sodium triphosphate should be added to the final stage of the hydrogel synthesis (Huang, et al., 2019).
Properties of highly stretchable hydrogels
The main properties of highly stretchable hydrogels are presented by the ability to retain the initial form once the mechanical forces are removed and high fracture energy (energy that is necessary to tear the initial structure and produce several pieces of the starting material) (Shih, et al., 2019). The fracture energy of highly stretchable hydrogels is above 9000 J/m2, which allows these materials to be stretch by approximately 170% of the size of the initial sample. The outlined properties of highly stretchable hydrogels are determined by the possibility to realise two mechanisms namely hysteresis through the reformation of ionic crosslinks and reduced level of crack bridging through the introduction of covalent bonding (Shih, et al., 2019). The initial forms of hydrogels are also maintained by ionic crosslinks. As a result, the described properties of highly stretchable hydrogels make them excellent candidates for the analysis of energy dissipation and deformation mechanisms in solid objects (Shih, et al., 2019).
Chemical composition and the type of chain-chain interactions determine the mechanical properties of the synthesised highly stretchable hydrogels (Liu, et al., 2020). For instance, it was shown that hydrogels that contain elevated levels of slide-ring polymeric chains may be stretched by more than 10 times the initial length. The indicated formations are generated by using tetra/poly alcohols mixtures combined with ethylene glycol. As a result, the produced hydrogels can withstand mechanical strength of up to 2.6 MPa. As in the case of other hydrogels, elastic deformations are common for ethylene glycol-based hydrogels. However, elastic hydrogels are both brittle and highly notch-sensitive. The outlined features have detrimental effects on the range of applications that hydrogels may be used in. Because high mechanical strength, as well as elevated stretchability, are required for the dominating number of deformations ethylene-glycol based hydrogels are enriched with cross-links to enhance their physical transformations (Liu, et al., 2020).
Applications of highly stretchable hydrogels
Unique properties of highly stretchable hydrogels make them excellent candidates for the production of novel devices in electronics, biocompatible materials and optical fibres (Zeng, et al., 2020). Synthesis of highly stretchable, strain sensitive optic fibres as described in the literature. In particular, it was shown that hybrid polyacrylamide-alginate hydrogels were able to generate excellent optically active materials what was essential in the generation of fibres. Also, with the introduction of the described hybrids, the stretching of the respective material could be increased to up to 700% (Zeng, et al., 2020).
The optical activity of highly stretchable hydrogels could be changed by altering interactions between the core of the material and the clad (Sun, et al., 2020). Also, with changes in the polyacrylamide-alginate levels in hydrogels, it is possible to change the optical attenuation of the final product. For instance, optical attenuation drops with increasing levels of acrylamide. These materials are characterised by high stretchability combined with elevated transparency. By using unique physical and optical properties of the produced hydrogels it is possible to generate new strain sensitive compounds for applications in various areas of the industry, including areas where highly stretchable compounds are necessary. For instance, high flexibility, stretchability and biocompatibility made it possible to use highly stretchable hydrogels in the manufacturing of wearable sensors and implantable devices (Sun, et al., 2020).
In addition to the production of materials with optical properties, highly stretchable hydrogels are used in the manufacturing of novel electronic devices (Wang, et al., 2021). By using a combination of polymer/elastomer matrices it is possible to generated microchips, conductors, resistors, capacitors and transducers with different properties. Polydimethylsiloxane is used in the described materials to reduce interaction with water and resulting degradation. In addition to the outlined applications, drug delivery channels may also be generated by using highly stretchable hydrogels. In this case, it was established that to produce materials with the necessary functionality it was necessary to use tough and stretchable hydrogel matrices (Wang, et al., 2021).
Conclusion
In summary, it is clear that highly stretchable hydrogels presented innovative and advantageous materials that can be used in various areas of the modern industry. It is clear that changes in the levels of ethylene glycol, alginate and polyacrylamide in the hydrogel organic chains will result in changes in the stretching abilities and optical properties. Also, by using hydrogels with different levels of the indicated monomers it will be possible to alter their levels of biocompatibility. The area of highly stretchable hydrogels is expanding at a rapid pace and synthesis methods, properties and applications of these innovative materials are not limited to the ones discussed. However, a full description of every possible aspect of highly stretchable hydrogels is beyond the goal of the essay.
Reference list
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