What are Hydrogels?

Hydrogels are three-dimensional networks of hydrophilic polymer chains that have the ability to absorb and retain a significant amount of water. These materials are known for their high-water content and unique physical properties, they have a strong affinity for water, which is called “hydrophilicity”. This property allows hydrogels to absorb and retain large amounts of water, often many times their dry weight, which makes them valuable in a wide range of applications, particularly in the fields of biomedicine, drug delivery, tissue engineering, and various industrial uses[i].Hydrogels, being versatile materials have remarkable water-absorbing properties. They can be classified into two broad categories: natural and synthetic hydrogels. Each type offers unique characteristics, advantages, and applications, making them pivotal in various fields ranging from medicine to environmental science.

These hydrogels are naturally occurring and are found in living organisms. They are often composed of biopolymers such as proteins, polysaccharides, or nucleic acids. Examples of natural hydrogels include the extracellular matrix in tissues, cartilage, and certain plant-based materials. These hydrogels form organically within biological systems through complex biological processes[ii].

Synthetic hydrogels, on the other hand, are engineered and produced through scientific or synthetic methods. They are created in laboratories using various chemical processes. Common synthetic hydrogel materials include polyacrylamide, polyvinyl alcohol, and polyethylene glycol. Researchers can precisely control the composition, structure, and properties of synthetic hydrogels, allowing for customization to meet specific requirements for various applications[iii].

Hydrogels are everywhere in the world, particularly within living organisms[iv], where their hydrophilic polymer networks enable them to absorb significant amounts of water[v]. These versatile materials find applications as scaffolds in tissue engineering[vi], providing temporary support for cells[vii], and serving as carriers in drug delivery systems[viii][ix].

Properties of Hydrogels:

  • Biocompatibility: Many hydrogels are biocompatible, meaning they are well-tolerated by living tissues without causing significant adverse reactions. This property is essential for their use in biomedical applications such as wound healing, contact lenses, and drug delivery systems[x]. Biological hydrogels originate from existing constituents within body tissues, namely collagen, hyaluronic acid (HA), or fibrin. These substances, collagen, HA, and fibrin, are naturally present in the extracellular matrix of mammals. Collagen, as a primary structural element in tissues, inherently possesses cell-signaling domains that facilitate cell growth. To transform collagen into a hydrogel with enhanced mechanical properties, it necessitates chemical crosslinking, crosslinking through UV light or temperature, or a combination with other polymers. Resulting collagen hydrogels exhibit non-toxicity and biocompatibility[xi].
  • Mechanical Properties: The mechanical properties of hydrogels can be tailored to mimic the characteristics of natural tissues. This tunability is essential for applications in tissue engineering, where hydrogels are used as scaffolds to support cell growth and regeneration[xii].
  • Permeability: Hydrogels can be designed to have controlled permeability, allowing for the selective passage of molecules based on size, charge, or other specific characteristics. This property is exploited in drug delivery systems to control the release of therapeutic agents[xiii].
  • Swelling Capacity: One of the most distinctive features of hydrogels is their ability to swell in the presence of water. This swelling behavior is crucial for their applications in drug delivery and tissue engineering, where the hydrogel can absorb and release water, drugs, or other bioactive molecules[xiv].

Applications of Hydrogels:

  • Agriculture: Hydrogels are employed in agriculture to improve water retention in soil, promoting plant growth and reducing water usage[xv].
  • Biological and Chemical Sensors: The high water content of hydrogels makes them suitable for biological and chemical sensors. Changes in the hydrogel structure in response to specific stimuli can be utilized for sensing applications[xvi].
  • Contact Lenses: Soft contact lenses often utilize hydrogel materials because of their high water content, which enhances comfort and allows oxygen to pass through the lens to the cornea[xvii].
  • Drug Delivery: Hydrogels are extensively used in drug delivery systems due to their ability to encapsulate and release drugs in a controlled manner. The swelling and de-swelling behavior of hydrogels can be engineered to match specific drug release profiles[xviii]. These attributes render hydrogels highly suitable for applications as controlled drug delivery devices. The site of hydrogel adhesion within the body is influenced by its chemical composition and interactions with surrounding tissues. When administered orally, the hydrogel may adhere to various locations in the gastrointestinal tract, encompassing the mouth, stomach, small intestine, or colon. Targeted adhesion to a particular region leads to localized drug delivery, resulting in an elevated concentration of the drug absorbed by the surrounding tissues. Responsive to stimuli such as temperature or pH variations, smart hydrogels undergo changes in their swelling characteristics, consequently influencing the release of the drug embedded within the fibers, either enhancing or diminishing it[xix].
  • Hygiene Products: Hydrogels are used in hygiene products such as diapers and sanitary pads due to their high absorbency and ability to retain liquids[xx].
  • Tissue Engineering: Hydrogels serve as scaffolds in tissue engineering to support cell growth and tissue regeneration. Their tunable mechanical properties and biocompatibility make them suitable for mimicking the extracellular matrix of various tissues[xxi].
  • Wound Healing: Hydrogels are employed in wound dressings to maintain a moist environment, facilitate healing, and protect the wound from external contaminants. Some hydrogels also possess antimicrobial properties, contributing to infection prevention[xxii].

Researchers are also exploring hydrogels that respond to specific stimuli, such as changes in pH, temperature, or the presence of certain ions. These stimuli-responsive hydrogels can have applications in controlled drug release and smart materials[xxiii]. These “smart hydrogels” have the ability to respond to temperature, light, electromagnetic fields/frequencies, pressure, and ultrasound (US) radiation, chemical (pH, glucose, and ionic strength, or biological, such as enzymes and antigens/antibodies stimuli[xxiv][xxv][xxvi][xxvii].

Aerogels share a fundamental similarity with hydrogels, but instead of water, they incorporate air. This substitution imparts remarkable mechanical characteristics, positioning them as a potential choice for industrial applications that demand an exceptionally lightweight material (comprising 99.8% air)[xxviii].

Aerogels excel as thermal insulators by nearly negating two out of the three heat transfer methods – conduction (predominantly comprised of insulating gas) and convection (microstructure inhibits significant gas movement). Their effectiveness as conductive insulators arises from their composition primarily consisting of gases, inherently poor conductors of heat. Notably, silica aerogel stands out as an exceptional insulator due to silica’s inherent low thermal conductivity; conversely, metallic or carbon aerogels would be comparatively less effective. Acting as convective inhibitors, aerogels impede air circulation through their lattice structure. However, they exhibit limitations as radiative insulators, as infrared radiation, responsible for heat transfer, can permeate through them.

Hydrogels represent a versatile class of materials with a broad range of applications across various industries. Ongoing research and advancements in polymer chemistry and material science are expected to further enhance the properties and functionalities of hydrogels, expanding their use in diverse fields, from medicine to agriculture.


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Hussein M. El-Husseiny, Eman A. Mady, Lina Hamabe, Amira Abugomaa, Kazumi Shimada, Tomohiko Yoshida, Takashi Tanaka, Aimi Yokoi, Mohamed Elbadawy, Ryou Tanaka, Smart/stimuli-responsive hydrogels: Cutting-edge platforms for tissue engineering and other biomedical applications, Materials Today Bio,Volume 13,2022,100186,ISSN 2590-0064
[xxviii] https://www.sciencedirect.com/topics/materials-science/aerogels