Enzymatic-Crosslinking
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Enzymatic crosslinking

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Enzymatic crosslinking is the way in which our blood clots during wound healing – cells secrete an enzyme, transglutaminase, which crosslinks fibrin, forming an insoluble protein polymer, or clot. By using this enzyme, along with the appropriate functional groups, hydrogels can be crosslinked in much the same way.
Alginate_Hydrogrels
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Ionically crosslinked hydrogels

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Some of the earliest hydrogels used for cell encapsulation were, (not ironically) naturally derived, ionically crosslinked materials, such as alginate, chitin and agarose. Although somewhat low-tech, these were a natural place to start.