Editing Cross-link (section)

===Physical cross-links===
In contrast to chemical cross-links, physical cross-links are formed by weaker interactions. For example, sodium [[alginate]] gels upon exposure to calcium ions, which form ionic bonds that bridge between alginate chains.<ref>{{cite journal |doi= 10.1021/acs.biomac.6b00378|pmid= 27177209|title= Structural Characterization of Sodium Alginate and Calcium Alginate|journal= Biomacromolecules|volume= 17|issue= 6|pages= 2160–2167|year= 2016|last1= Hecht|first1= Hadas|last2= Srebnik|first2= Simcha}}</ref> [[Polyvinyl alcohol]] gels upon the addition of [[borax]] through hydrogen bonding between [[boric acid]] and the polymer's alcohol groups.<ref>{{cite web |title=Experiments: PVA polymer slime |url=https://edu.rsc.org/experiments/pva-polymer-slime/756.article |website=Education: Inspiring your teaching and learning |publisher=Royal Society of Chemistry |access-date=2 April 2022 |date=2016 |quote=A solution of polyvinyl alcohol (PVA) can be made into a slime by adding borax solution, which creates cross-links between polymer chains.}}</ref><ref>{{cite journal |doi= 10.1021/ed063p57|title= The gelation of polyvinyl alcohol with borax: A novel class participation experiment involving the preparation and properties of a "slime"|journal= Journal of Chemical Education|volume= 63|issue= 1|pages= 57|year= 1986|last1= Casassa|first1= E.Z|last2= Sarquis|first2= A.M|last3= Van Dyke|first3= C.H|bibcode= 1986JChEd..63...57C}}</ref> Other examples of materials which form physically cross-linked gels include [[gelatin]], [[collagen]], [[agarose]], and [[agar agar]].