Gels

Gels are semi-solid materials in which a liquid is immobilized within a three-dimensional solid network, producing a soft, jelly-like substance. The liquid phase can be water in hydrogels, an organic solvent in organogels, or, in the case of aerogels, a dried, highly porous solid obtained after solvent removal. The network provides shape and mechanical integrity, while the trapped liquid enables swelling, diffusion, and transport of solutes.

Structure and behavior: The network is typically formed by polymers, inorganic frameworks, or a combination of both, held together by chemical covalent bonds or physical associations such as ionic interactions, hydrogen bonding, or entanglements. The gel point marks the transition from a sol (fluid) to a gel (solid-like) state as crosslink density increases. Gels are usually highly swollen and exhibit viscoelastic properties; the balance between the storage modulus (G') and the loss modulus (G'') describes their solid-like and liquid-like responses over different timescales and frequencies.

Classification and examples: Common types include hydrogels (high water content), organogels (non-aqueous solvents), cryogels (formed at subzero temperatures), and aerogels (dry, highly porous networks). Supramolecular and stimuli-responsive gels use reversible interactions and exhibit changes in properties in response to pH, temperature, ions, or light.

Applications: Gels are used in foods for texture (gelatin desserts, pectin jellies), biomedicine for drug delivery, wound dressings, and tissue engineering, cosmetics, agriculture, environmental remediation, and sensors. Representative materials include gelatin, agar, alginate, collagen, agarose, polyacrylamide, and silica-based gels. Limitations can include mechanical fragility, dehydration (syneresis), and potential toxicity of certain crosslinking agents.