Hydrogels Based on Natural Polymers

Yu Chen

• Publisher: Elsevier
• Year: 2019
• Print ISBN: 9780128164211
• eBook ISBN: 9780128166185
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Hydrogels Based on Natural Polymers presents the latest research on natural polymer-based hydrogels, covering fundamentals, preparation methods, synthetic pathways, advanced properties, major application areas, and novel characterization techniques. The advantages and disadvantages of each natural polymer-based hydrogel are also discussed, enabling preparation tactics for specific properties and applications. Sections cover fundamentals, development, characteristics, structures and properties. Additional chapters cover presentation methods and properties based on natural polymers, including physical and chemical properties, stimuli-responsive properties, self-healing properties, and biological properties. The final section presents major applications areas, including the biomedical field, agriculture, water treatments, and the food industry. This is a highly valuable resource for academic researchers, scientists and advanced students working with hydrogels and natural polymers, as well as across the fields of polymer science, polymer chemistry, plastics engineering, biopolymers and biomaterials. The detailed information will also be of great interest to scientists and R&D professionals, product designers, technicians and engineers across industries. - Provides systematic coverage of all aspects of hydrogels based on natural polymers, including fundamentals, preparation methods, properties and characterization - Offers a balanced assessment of the specific properties and possibilities offered by different natural polymer-based hydrogels, drawing on innovative research - Examines cutting-edge applications across biomedicine, agriculture, water treatments, and the food industry

Part I

Fundamentals of Natural Polymers and Their Hydrogels

Outline

• Chapter 1 Properties and development of hydrogels
• Chapter 2 Natural polymers and the hydrogels prepared from them

Chapter 1

Properties and development of hydrogels

Yu Chen*, School of Material Science and Engineering, Beijing Institute of Technology, Beijing, China

Abstract

Hydrogels are cross-linked polymeric networks prepared from hydrophilic polymers, which can absorb and retain a high water content while maintaining their three-dimensional integrity. Hydrogels are very important in medical and pharmaceutical applications, personal care products, environmental protection, the food industry, and agriculture. The diverse properties of natural polymers mean that they are an ideal skeleton for the preparation of hydrogels. In the last 2–3 decades there have been many reported papers that highlight hydrogels based on natural polymers due to their excellent properties—biodegradable, biocompatible, environmentally friendly, and flexible, similar to natural tissue. The aim of this chapter is to give a brief introduction to the structure, properties, classification, and development of hydrogels.

Keywords

Hydrogels; cross-linking; polymers

Hydrogels are a type of polymer network that can absorb and store large amounts of water [1]. The hydrophilic groups or segments of the polymer network can be hydrated under neutral conditions to produce a gel structure [2]. The cross-linked network structure prevents the dissolution of the hydrophilic polymer chains or segments. Hydrogels are solid-like and show nonflowing properties as inverted. They can also be described by their fluidics [3]. In general, a low-concentration aqueous hydrophilic polymer solution with no entangled structures exhibits Newtonian mechanical behavior. Cross-linked hydrogels possess rheological properties with extremely high viscosity (>10⁵ Pa s) and high elasticity (shear yield stress >2000 Pa), similar to solids. Due to their water insolubility, hydrogels are the research hotspots of swollen polymer networks, as well as their application studies. Hydrogels have been applied to a variety of fields, such as drought resistance in dry areas [4], masks in cosmetics [5], antipyretic stickers [6], analgesic stickers [7], agricultural films [8], dew prevention agents in construction [9], humidity control [10], water-blocking agents in the petroleum industry [11], dehydration of crude oil and refined oil [12], dust suppressants in mining [13], food preservatives [14], thickeners [15], pharmaceutical carriers in medical applications [16], tissue engineering scaffolds [17], wound dressings [18], etc. It is worth noting that different application fields require different polymer materials.

In 1960, Wichterle and Lim synthesized a cross-linked HEMA hydrogel for the first time [19]. Its high hydrophilicity and biocompatibility attracted great interest in the biomaterial field and it has been widely used in contact lenses since then. The most important and influential discovery was the successful embedding of cells in an alginic acid microcapsule by Lim and Sun in 1980 [20]. Later, Yannas and his coworkers prepared hydrogels using natural polymers, collagen and shark cartilage, and explored their applications as burn wound dressings [21]. The natural polymer hydrogels have become the research hotspot for cell embedding. Later, with the rapid development of tissue engineering, hydrogels were used as the matrix for cell growth and carrier of growth factors to repair and re-elevate organs of various tissues [22–24]. With the changes in the application purpose and requirement, the research and development of hydrogel have shifted from traditional hydrogels to intelligent hydrogels.

Hydrogels can be classified by different methods.

1. 1. Hydrogels can be classified into natural hydrogels and synthetic hydrogels based on their sources.
   1. a. Natural polymers, such as collagen, gelatin, hyaluronic acid, fibrin, sodium alginate, agarose, chitosan, dextran, cyclodextrin, etc. are from natural sources with good biocompatibility and biodegradability [25–30]. The hydrogels prepared with these abundant polymers inherit these excellent properties and are sensitive to the external environment. Therefore they have become the research hotspot of hydrogel and are the focus of this book. In addition, most animals and plants contain a large number of natural polymer gels. The study of the structure and properties of such hydrogels is also an important way to study the physiological mechanisms of tissues for biomimetic applications [31–33].
   2. b. Synthetic hydrogels are cross-linked polymers prepared by the addition reaction or ring-opening polymerization under artificial conditions. Polyacrylic acid and its derivatives [34], polyvinyl alcohol [35], polyethylene glycol and its copolymers [36], and polyvinylpyrrolidone [37], etc. are usually used as the skeletons to prepare synthetic hydrogels. Synthetic hydrogels have the advantages of easy industrial production and chemical modification and precisely controllable properties, yet with poor biocompatibility, bioactivity, and biodegradability, as compared with natural polymer hydrogels.
2. 2. Based on the formation mechanism of three-dimensional network structure, hydrogels can be divided into chemical hydrogels and physical hydrogels.
   1. a. Chemical hydrogels are formed by the chemical cross-linking between molecules, and this cross-linking is irreversible. Chemical hydrogels usually have the advantages of stable properties, tunable structures, good mechanical properties, etc. [38–40].
   2. b. Physical hydrogels are mainly the three-dimensional networks formed by noncovalent bonds (secondary bonds), such as electrostatic interaction, hydrogen bonding, chain entanglement, and hydrophobic interaction, between linear molecules that form physical cross-linking joints [34,41,42]. Physical hydrogels usually exhibit reversible sol–gel conversion because very low energies are required to break the physical interactions between the molecules [43]. No chemical reaction is involved in their preparation and the preparation conditions are relatively mild, which is favorable to their applications in the biomedical field [44].
3. 3. Based on their degradability, hydrogels can be divided into biodegradable hydrogel and nonbiodegradable hydrogel.
   1. a. Most of the natural polymer hydrogels are biodegradable hydrogels [45]. The three-dimensional structures of these hydrogels can be destroyed by the actions of microorganisms and enzymes under natural conditions. The bonding between the molecular chains and within the molecular chains is then broken and the strength of the hydrogel is reduced. Eventually, the hydrogel is degraded into small molecules.
   2. b. Nonbiodegradable hydrogels are a class of hydrogels that are insensitive to environmental stimuli and can maintain stable structural, physical, and chemical properties for a long time. Most synthetic hydrogels prepared by chemical cross-linking are nonbiodegradable hydrogels [46].
4. 4. Based on their responsiveness to external stimuli, hydrogels can be divided into environmentally responsive hydrogels (intelligent hydrogels) and environmentally unresponsive hydrogels (regular hydrogels).
   1. a. Environmentally responsive hydrogels, also known as intelligent hydrogels or smart hydrogels, can reversibly respond to external stimuli. When a hydrogel is exposed to environmental stimuli, such as temperature, pH, ion strength, electric field, light, stress, magnetic field, etc., its three-dimensional network structure changes, swells or shrinks, or transits between the dense phase and the dilute phase, showing dramatically altered shape, mechanical and optical properties, etc. in response to the stimuli. The hydrogel will automatically return to a lower steady state of internal energy as the external stimuli disappear.
      Thermo-responsive hydrogels are a type of hydrogel showing temperature-induced volume shr...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of contributors
• Part I: Fundamentals of Natural Polymers and Their Hydrogels
• Part II: Preparation Methods for Hydrogels based on Natural Polymers
• Part III: Properties of Hydrogels Based on Natural Polymers
• Part IV: Applications of Hydrogels based on Natural Polymers
• Part V: Characterization Tools and Techniques of Hydrogels
• Index