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. Polyvinyl alcohol gels upon the addition of borax through hydrogen bonding between boric acid and the polymer's alcohol groups. Other examples of materials which form physically cross-linked gels include gelatin, collagen, agarose, and agar agar.