Poikittaissidonta

Poikittaissidonta (cross-linking) is a chemical process by which bonds form between polymer chains or biomolecules, linking them into a three-dimensional network. The bonds can be covalent, ionic, hydrogen-bonded, or physical in nature, and they alter the mobility and properties of the material.

Covalent crosslinks involve bonds that share electrons and tend to produce durable networks, as seen in vulcanized rubber where sulfur bridges connect polyisoprene chains or in cured resins where multifunctional agents create rigid linkages. Ionic crosslinks involve metal ions bridging functional groups, a common mechanism in alginate gels with calcium ions. Non-covalent crosslinks, including hydrogen bonds and metal-ligand interactions, can form reversible or dynamic networks and are important in certain hydrogels. Physical crosslinking may arise from crystallites, phase separation, or entanglements and can contribute to material stability without changing chemical composition.

Cross-linking can be achieved chemically using bifunctional or multifunctional crosslinkers, enzymatically with specific enzymes, or physically by changing temperature, pH, or ionic strength. Photoinitiated crosslinking uses light to generate reactive species that form bonds, a technique widely used in polymers and hydrogel systems.

The degree of cross-linking, or cross-link density, controls mechanical properties, swelling behavior, permeability, and thermal stability. Higher density generally yields stiffer, less soluble materials with reduced swelling, while lower density produces more compliant networks with greater solvent uptake.

Applications span materials science, dentistry, and biomedicine. Examples include vulcanized rubber, epoxy and polyurethane networks, hydrogel matrices for tissue engineering, contact lenses, wound dressings, and controlled-release systems. In biology, poikittaissidonta stabilizes protein structures and extracellular matrices and can influence molecular stability and interactions.