Gel

Gel

A gel, derived from the Latin "gelu" meaning freezing, cold, ice, or "gelatus" meaning frozen, immobile, is a solid, jelly-like material with a spectrum of properties ranging from soft and pliable to hard and resilient. Gels are characterized as substantially diluted cross-linked systems, maintaining their shape without flow when in a steady state.

Despite being predominantly liquid by weight, gels exhibit solid-like behavior due to a three-dimensional network of cross-links within the liquid. These cross-links provide the structure and contribute to the stickiness of gels. Essentially, gels are a dispersion of liquid molecules within a solid, where the solid forms the continuous phase and the liquid forms the discontinuous phase.

Gels comprise a solid three-dimensional network spanning a liquid medium, entrapping it through surface tension effects. This internal network structure may arise from physical bonds (physical gels) or chemical bonds (chemical gels), as well as crystallites or other junctions that remain intact within the expanding fluid.

Virtually any fluid, including water (hydrogels), oil, and air (aerogel), can be utilized as an extender. Both in terms of weight and volume, gels predominantly consist of fluid, thereby exhibiting densities akin to those of their constituent liquids. A common example of a hydrogel is edible jelly, which possesses a density similar to water.

Cationic polymers, characterized by positive charges, inhibit the formation of coiled polymers. This allows them to contribute significantly to viscosity in their elongated state, as the stretched-out polymer occupies more space. Gel constitutes a colloid solution, with the dispersion phase being liquid and the dispersion medium being solid.

Types of Gels

Hydrogels

Hydrogel, also known as an aquagel, constitutes a network of polymer chains that are hydrophilic, sometimes forming a colloidal gel where water serves as the dispersion medium. Hydrogels, whether natural or synthetic polymers, are highly absorbent, with some containing over 99.9% water. They possess a degree of flexibility akin to natural tissue due to their significant water content. Various applications of hydrogels include:

- Serving as scaffolds in tissue engineering, often incorporating human cells for tissue repair.
- Employed in hydrogel-coated wells for cell culture.
- Utilized in environmentally sensitive hydrogels, also known as 'Smart Gels' or 'Intelligent Gels,' capable of sensing changes in pH, temperature, or metabolite concentration and releasing their load accordingly.
- Acting as sustained-release drug delivery systems.
- Providing absorption, desloughing, and debriding of necrotic and fibrotic tissue.
- Serving as biosensors, responsive to specific molecules such as glucose or antigens, and employed in drug delivery systems.
- Used in disposable diapers and sanitary napkins for urine absorption.
- Applied in contact lenses and medical electrodes for EEG and ECG measurements.
- Used in water gel explosives and rectal drug delivery.

Other less common applications include breast implants, glue formulation, soil moisture retention in arid regions, and dressings for wound healing, particularly for burns or hard-to-heal wounds.

Common ingredients include polyvinyl alcohol, sodium polyacrylate, and acrylate polymers with abundant hydrophilic groups. Natural hydrogel materials, such as agarose, methylcellulose, hyaluronan, and other naturally derived polymers, are under investigation for tissue engineering applications.

Organogels

Organogel is a non-crystalline, thermoreversible solid material composed of a liquid organic phase trapped in a three-dimensionally cross-linked network. The liquid phase can be an organic solvent, mineral oil, or vegetable oil. The solubility and particle dimensions of the structurant molecules are crucial for the elastic properties and firmness of the organogel. These systems often rely on the self-assembly of structurant molecules. Organogels find applications in pharmaceuticals, cosmetics, art conservation, and food industries.

Xerogels

A xerogel is a solid formed from a gel through drying, resulting in unhindered shrinkage. Xerogels typically maintain high porosity and enormous surface area, along with small pore size. When solvent removal occurs under supercritical conditions, aerogels, highly porous, low-density materials, are produced. Heat treatment of xerogels at elevated temperatures leads to viscous sintering, transforming the porous gel into a dense glass.

Properties

Many gels exhibit thixotropy, becoming fluid when agitated and resolidifying when at rest. Generally, gels appear as solid, jelly-like materials. By replacing the liquid with gas, aerogels, with exceptional properties including very low density, high specific surface areas, and excellent thermal insulation, can be produced.

Some species secrete gels effective in parasite control. For instance, the long-finned pilot whale secretes an enzymatic gel that prevents other organisms from establishing colonies on the surface of its body.

Applications

A wide range of substances can form gels when a suitable thickener or gelling agent is added. This approach is prevalent in the manufacturing of various products, from foods to paints and adhesives.

In fiber optics communications, a soft gel is used to fill plastic tubes containing fibers, preventing water intrusion and buffering fibers against mechanical damage. Hydrogels naturally existing in the body.