Prepared by Max Darnell - AP225 Fall 2011
Crosslinking is the process of connecting two polymer strands. This is an important industrial process, exemplified by the vulcanizaiton of rubber, as well as an important biological process, evidenced by the crosslinking of DNA and ECM fibers. The actual crosslinking molecules can form ionic or covalent connections with the polymer chains and can constist of a single atom (Ca2+) or a complex (sulfur in vulcanization).  Below is a figure depicting the crosslinking of poly(isoprene), which is a rubber. This process is instrumental in increasing the durability of car tires. 
Applications/Connections to Soft Matter
A recent area of research has involved studying the effects of crosslinking in biology and biomaterials. Many biomaterials are hydrogels, so many of their properties are highly tunable by crosslinking. For example, in alginate, one of the most commonly used biomaterials for wound healing, tissue regeneration, and drug delivery applications, is crosslinked by divalent cations such as calcium and magnesium. The degree of crosslinking effects the stiffness of the gel, which can be sensed by cells to influence behavior or degrade at a specified rate to release a drug along a desired profile. 
Another classic use of crosslinking is formaldehyde, which is used in fixing tissue specimens for microscopy and examination. Formaldehyde forms -CH2- linkages between amino acids, halting the degradation and increasing the shelf-life of these tissues. 
Thermo Scientific. (2010). Chemistry of Crosslinking. PierceNet. Thermo Fisher Scientific Inc. Retrieved from http://www.piercenet.com/browse.cfm?fldID=CE4D6C5C-5946-4814-9904-C46E01232683
Levental, K. R., Yu, H., Kass, L., Lakins, J. N., Egeblad, M., Erler, J. T., Fong, S. F. T., et al. (2009). Matrix crosslinking forces tumor progression by enhancing integrin signaling. Cell, 139(5), 891-906.
Morita, E., Sullivan, A., & Coran, A. (1985). Vulcanization Chemistry. Rubber Chemistry and Technology, 58, 284-294.