Many permanent bioadhesives (e.g., the oothecal foam of the mantis) are generated by a "mix to activate" process that involves hardening via covalent cross-linking. On non-polar surfaces the adhesive mechanisms may include van der Waals forces, whereas on polar surfaces mechanisms such as hydrogen bonding and binding to (or forming bridges via) metal cations may allow higher sticking forces to be achieved.
* Microorganisms use acidic polysaccharides (molecular mass around 100 000 Da)
* Marine bacteria use carbohydrate exopolymers to achieve bond strengths to glass of up to 500 000 N/m
* Marine invertebrates commonly employ protein-based glues for irreversible attachment. Some mussels achieve 800 000 N/m on polar surfaces and 30 000 N/m on non-polar surfaces these numbers are dependent on the environment, mussels in high predation environments have an increased attachment to substrates. In high predation environments it can require predators 140% more force to dislodge mussels
* Some algae and marine invertebrates use lecproteins that contain L-DOPA to effect adhesion
* Proteins in the oothecal foam of the mantis are cross-linked covalently by small molecules related to L-DOPA via a tanning reaction that is catalysed by catechol oxidase or polyphenol oxidase enzymes.
L-DOPA is a tyrosine residue that bears an additional hydroxyl group. The twin hydroxyl groups in each side-chain compete well with water for binding to surfaces, form polar attachments via hydrogen bonds, and chelate the metals in mineral surfaces. The Fe(L-DOPA) complex can itself account for much cross-linking and cohesion in mussel plaque, but in addition the iron catalyses oxidation of the L-DOPA to reactive quinone free radicals, which go on to form covalent bonds.