Hydrogels are cross-linked three-dimensional polymeric networks which can absorb a great quantity of water and keep mechanically stable without dissolution. Due to the biocompatibility and biodegradability, biological hydrogels have been wildly investigated and used in various fields, such as adsorption materials, shape memory materials, self-healing materials, sensor units, super capacitor, drug carriers, and so on. In this chapter, we would focus on some of the upper aspects and give a brief introduction.
KeywordsHydrogels Adsorption Stimuli-responsive Self-healing
Hydrogels are three-dimensional hydrophilic polymeric networks and are typically soft and elastic, owing to their compatibility with water. Cross-links and interconnections, which make polymer chains get together, can be formed by physical entanglements or chemical bonds, leading to physical and chemical hydrogels. Chemical hydrogels can be formed by chemical reactions such as radical polymerization , photopolymerization , high-energy radiation , and covalent conjugation . These hydrogels generally show good physical stability and mechanical strength. Physical hydrogels are composed of polymer self-aggregation via non-covalent interactions such as hydrogen bonds , hydrophobic interactions , electrostatic interactions , inclusion complex , π–π stack , ionic interactions , crystallinity , and other affinity interactions . These hydrogels exhibit excellent swelling and absorption capacities. Based on the raw materials and synthetic methods, the hydrogels can be classified to be petroleum-based hydrogels or bio-based hydrogels, covalent or physical hydrogels, copolymer networks or interpenetrating networks, degradable or nondegradable hydrogels, and so on. Due to their unique characteristics, hydrogels have been extensively studied in bioscience and material science and widely applied as functional materials, for example, contact lenses , disposable diapers , wastewater treatments , and moist pads for healing wounds  or burns . In this chapter, we would like to introduce the advanced research and applications as follows: (1) adsorption hydrogels, (2) stimuli-responsive hydrogels, and (3) self-healing hydrogels.
2 Adsorption Hydrogels
Attributed to the network structure, hydrogels have excellent adsorption capacity to absorb large quantity of water and keep stability. Through designing functional molecular or modifying natural products, the hydrogels can provide complexing sites for the templates as adsorption materials. Recently, lots of researches are dealing with the use of hydrogels for adsorption materials, and also many researchers are investigating the hydrogels for wastewater treatments.
2.1 Wastewater Treatments
With the development of industry, environmental pollution has received great attention and became the focus of research. Water pollution, especially heavy metal ions and organic dye, is a menace to health. Many methods are used to purify water, such as chemical separation, electrochemical separation, adsorption, and cation exchange. Among these methods, adsorption is the most high-efficient method with high adsorption capacity, selectivity, and reusability. In recent years, many researchers pay attention to natural materials and polymer composites for the removal of pollutants from water, which have received excellent achievements.
2.1.1 Broad Adsorption of Heavy Metal Ions
2.1.2 Selective Adsorption of Metal Ions
2.1.3 Adsorption of Organic Dyes
2.2 Drug-Selective Adsorption, Delivery, and Release
Hydrogels are hydrophilic polymer networks that can absorb more than 100 times their dry weight in water, giving the physical characteristics like soft tissue. In addition, hydrogels are highly permeable which facilitate exchanges of oxygen, nutrient, and other water-soluble metabolites. Thus, hydrogels are being investigated as drug delivery system due to their potential which can control the transport and release of macromolecular drugs such as pesticides [27, 28, 29], proteins [30, 31, 32], and nucleotides . The diffusion mechanism of solute molecules within hydrogels is of great interest for a wide variety of industrial applications.
3 Stimuli-Responsive Hydrogels
Stimuli-responsive hydrogels are a broad class of hydrogels whose swelling or deswelling processes, gel-to-solution or gel-to-solid transitions, and shapes can respond to the physical or chemical external stimuli, such as temperature, pH, magnetic, ultrasonic, electrochemistry, or light. Different kinds of stimuli-responsive hydrogels are used in various areas, like sensors [42, 43] and actuators, display and image devices [44, 45, 46], conditional controlled drug delivery [27, 28, 29, 30, 31, 32, 33], and so on. There are many different synthetic methods and sources to prepare stimuli-responsive hydrogels and exploit different applications.
3.1 pH- and Temperature-Sensitive Hydrogels
3.2 Light-Sensitive Hydrogels
3.3 Electric-Sensitive Hydrogels
3.4 Dissolving Hydrogels
3.5 Shape Memory Hydrogels
3.6 DNA Hydrogels
Except normal materials, DNA, RNA, or nucleic acids also can be introduced into composite hydrogels as functional components. DNA can be precisely designed with specific sequences and self-assemble into two- or three-dimensional structure to assemble nanoparticles. Through the cross-linked networks of DNA assembly, DNA hydrogels have great potential applications in biomaterials, such as drug and gene delivery, biosensing, and tissue engineering. In the past few years, many DNA hydrogels have been reported. The driving forces of the DNA hydrogels are physical interaction and chemical interaction. For physical interaction, DNA directly extracted from the nucleus in nature, like a long linear polymer, and formed a hydrogel via physical entanglement or by chemical cross-linking of small molecules. For chemical modification, DNA could be covalently grafted onto synthetic polymers and served as a cross-linker. The recognition of complementary DNA strands led to cross-linking of polymer chains and caused hydrogel formation.
3.7 RNA Hydrogels
4 Self-Healing Hydrogels
Self-healing is one of the most remarkable properties of biological materials. The special ability of natural materials to heal cracks often involves an energy dissipation mechanism due to the so-called sacrificial bonds that break and reform dynamically before the fracture of the molecular backbone. Numerous studies have been conducted in recent years to improve the mechanical performance of hydrogels. The self-healing materials which are capable of recovering their original mechanical performance after fracture require intermolecular non-covalent interactions. Although synthetic hydrogels are very similar to biological tissues, they are normally very brittle and lack the ability to self-heal, which hinders their use in any stress-bearing applications.
As new functional materials, self-healing hydrogels are under vast research nowadays for drug delivery, 3D cell proliferation, and tissue engineering. The broken self-healing hydrogels after damaged can regenerate the integral network and stay at the target position, enhancing the medicine delivery efficiency. This property can be introduced into macromolecules by the hydrogen bond driven through the creation of specific non-covalent intermolecular interactions. Moreover, self-healing systems can repair themselves autonomously, restoring the initial structures and functions without external stimulus after the interior or exterior cracks, which is similar to the ability of some living organisms (e.g., human skin). Constructing a non-covalently bonded system is an efficient approach to preparing self-healable polymeric hydrogels. Thus, introducing the self-healing feature to the hydrogel can improve the functionality and extend the application range of the hydrogel.
4.1 Self-Healing Hydrogels (Imine Bonds)
4.2 Self-Healing Hydrogels (Host–Guest)
4.3 Self-Healing Hydrogels (Hydrogen Bonds)
4.4 Self-Healing Hydrogels (Hydrophobic–Hydrophilic)
This chapter delivers a brief overview at functional hydrogels, especially adsorption hydrogels, stimuli-responsive hydrogels, and self-healing hydrogels. Due to the unique structure and properties, hydrogels can be used as adsorption materials for water treatment, drug separation and condition-controlled delivery system, sensors and actuators, and many other smart materials.
6 Future Scope
After decades of investigation, significant accomplishments in the development and characterization of hydrogels have been achieved, and the application of hydrogels has been spread to many aspects, such as green materials, new energy, biomedicine, etc. The future hydrogels must be more and more smart. The trends of the hydrogel application would be on-demand deformation, artificial organs, wearable devices, etc. Meanwhile, more fundamental researches are necessary to make clear the mechanism of the unique properties of hydrogels.
The authors acknowledge East China Normal University for providing research facilities and platform.
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