Bioadhesives are naturalpolymeric materials that act asadhesives. The term is sometimes used more loosely to describe a glue formedsynthetically from biologicalmonomers such assugars, or to mean a synthetic material designed to adhere to biologicaltissue.
Bioadhesives may consist of a variety of substances, butproteins andcarbohydrates feature prominently. Proteins such asgelatin and carbohydrates such asstarch have been used as general-purpose glues by man for many years, but typically their performance shortcomings have seen them replaced by synthetic alternatives. Highly effective adhesives found in the natural world are currently under investigation. For example, bioadhesives secreted by microbes and by marinemolluscs andcrustaceans are being researched with a view tobiomimicry.[1] Furthermore, thiolation of proteins and carbohydrates enables these polymers (thiomers) to covalently adhere especially to cysteine-rich subdomains of proteins such askeratins or mucus glycoproteins viadisulfide bond formation.[2] Thiolatedchitosan and thiolatedhyaluronic acid are used as bioadhesives in various medicinal products.[3][4]
Organisms may secrete bioadhesives for use in attachment, construction and obstruction, as well as in predation and defense. Examples include their use for:
Some bioadhesives are very strong. For example, adult barnacles achieve pull-off forces as high as 2MPa (2 N/mm2). A similarly strong, rapidly adhering glue - which contains 171 different proteins and can adhere to wet, moist and impure surfaces - is produced by the very hard[5][6] limpet speciesPatella vulgata; this adhesive material is a very interesting subject of research in the development of surgical adhesives and several other applications.[7][8][9]Silk dope can also be used as a glue byarachnids andinsects.
The small family of proteins that are sometimes referred to as polyphenolic proteins are produced by somemarine invertebrates like the blue mussel,Mytilus edulis[10] by somealgae'[citation needed], and by the polychaetePhragmatopoma californica.[11] These proteins contain a high level of a post-translationally modified—oxidized—form of tyrosine,L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA)[11] as well as the disulfide (oxidized) form of cysteine (cystine).[10] In the zebra mussel (Dreissena polymorpha), two such proteins, Dpfp-1 and Dpfp-2, localize in the juncture betweenbyssus threads and adhesive plaque.[relevant?][12][relevant?] The presence of these proteins appear, generally, to contribute to stiffening of the materials functioning as bioadhesives.[13][citation needed] The presence of the dihydroxyphenylalanine-moiety arises from action of atyrosine hydroxylase-type of enzyme;[citation needed] in vitro, it has been shown that the proteins can be cross-linked (polymerized) using a mushroomtyrosinase.[relevant?][14]
Organisms such aslimpets andsea stars use suction andmucus-like slimes to createStefan adhesion, which makes pull-off much harder than lateral drag; this allows both attachment and mobility. Spores, embryos and juvenile forms may use temporary adhesives (oftenglycoproteins) to secure their initial attachment to surfaces favorable for colonization. Tacky andelastic secretions that act aspressure-sensitive adhesives, forming immediate attachments on contact, are preferable in the context of self-defense andpredation. Molecular mechanisms includenon-covalent interactions and polymer chain entanglement. Manybiopolymers – proteins,carbohydrates,glycoproteins, andmucopolysaccharides – may be used to formhydrogels that contribute to temporary adhesion.
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Many permanent bioadhesives (e.g., theoothecal foam of themantis) are generated by a "mix to activate" process that involves hardening viacovalent cross-linking. On non-polar surfaces the adhesive mechanisms may includevan der Waals forces, whereas onpolar surfaces mechanisms such ashydrogen bonding and binding to (or forming bridges via)metalcations may allow higher sticking forces to be achieved.[citation needed]
L-DOPA is atyrosine residue that bears an additionalhydroxyl group. The twin hydroxyl groups in eachside-chain compete well with water for binding to surfaces, form polar attachments viahydrogen bonds, andchelate themetals inmineral surfaces. The Fe(L-DOPA3) complex can itself account for much cross-linking and cohesion inmussel plaque,[16] but in addition theiron catalysesoxidation of the L-DOPA[17] to reactivequinonefree radicals, which go on to form covalent bonds.[18]
Bioadhesives are of commercial interest because they tend to be biocompatible, i.e. useful forbiomedical applications involving skin or other body tissue. Some work in wet environments and under water, while others can stick to low surface energy –non-polar surfaces likeplastic. In recent years,[when?] the synthetic adhesives industry has been impacted byenvironmental concerns and health and safety issues relating to hazardous ingredients,volatile organic compound emissions, and difficulties in recycling or re mediating adhesives derived frompetrochemical feedstocks. Risingoil prices may also stimulate commercial interest in biological alternatives to synthetic adhesives.
Shellac is an early example of a bioadhesive put to practical use. Additional examples now exist, with others in development:
Several commercial methods of production are being researched:
A more specific term than bioadhesion ismucoadhesion. Most mucosal surfaces such as in the gut or nose are covered by a layer ofmucus. Adhesion of a matter to this layer is hence called mucoadhesion.[24] Mucoadhesive agents are usually polymers containing hydrogen bonding groups that can be used in wet formulations or in dry powders for drug delivery purposes. The mechanisms behind mucoadhesion have not yet been fully elucidated, but a generally accepted theory is that close contact must first be established between the mucoadhesive agent and the mucus, followed by interpenetration of the mucoadhesive polymer and the mucin and finishing with the formation of entanglements and chemical bonds between the macromolecules.[25] In the case of a dry polymer powder, the initial adhesion is most likely achieved by water movement from the mucosa into the formulation, which has also been shown to lead to dehydration and strengthening of the mucus layer. The subsequent formation of van der Waals, hydrogen and, in the case of a positively charged polymer, electrostatic bonds between the mucins and the hydrated polymer promotes prolonged adhesion.[citation needed][24]
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