Bioadhesive

Bioadhesives are natural polymeric materials that act as adhesives. The term is sometimes used more loosely to describe a glue formed synthetically from biological monomers such as sugars, or to mean a synthetic material designed to adhere to biological tissue. Bioadhesives are natural polymeric materials that act as adhesives. The term is sometimes used more loosely to describe a glue formed synthetically from biological monomers such as sugars, or to mean a synthetic material designed to adhere to biological tissue. Bioadhesives may consist of a variety of substances, but proteins and carbohydrates feature prominently. Proteins such as gelatin and carbohydrates such as starch 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 but not yet in widespread commercial use. For example, bioadhesives secreted by microbes and by marine molluscs and crustaceans are being researched with a view to biomimicry. Bioadhesives are of commercial interest because they tend to be biocompatible, i.e. useful for biomedical 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 like plastic. In recent years, the synthetic adhesives industry has been impacted by environmental concerns and health and safety issues relating to hazardous ingredients, volatile organic compound emissions, and difficulties in recycling or re mediating adhesives derived from petrochemical feedstocks. Rising oil prices may also stimulate commercial interest in biological alternatives to synthetic adhesives. 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 2 MPa (2 N/mm2).Silk dope can also be used as a glue by arachnids and insects. The small family of proteins that are sometimes referred to as polyphenolic proteins are produced by some marine invertebrates like the blue mussel, Mytilus edulis by some algae', and by the polychaete Phragmatopoma californica. These proteins contain a high level of a post-translationally modified—oxidized—form of tyrosine, L-3,4-dihydroxyphenylalanine (levodopa, L-DOPA) as well as the disulfide (oxidized) form of cysteine (cystine). In the zebra mussel (Dreissena polymorpha), two such proteins, Dpfp-1 and Dpfp-2, localize in the juncture between byssus threads and adhesive plaque. The presence of these proteins appear, generally, to contribute to stiffening of the materials functioning as bioadhesives. The presence of the dihydroxyphenylalanine-moiety arises from action of a tyrosine hydroxylase-type of enzyme; in vitro, it has been shown that the proteins can be cross-linked (polymerized) using a mushroom tyrosinase. Organisms such as limpets and sea stars use suction and mucus-like slimes to create Stefan 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 (often glycoproteins) to secure their initial attachment to surfaces favorable for colonization. Tacky and elastic secretions that act as pressure-sensitive adhesives, forming immediate attachments on contact, are preferable in the context of self-defense and predation. Molecular mechanisms include non-covalent interactions and polymer chain entanglement. Many biopolymers – proteins, carbohydrates, glycoproteins, and mucopolysaccharides – may be used to form hydrogels that contribute to temporary adhesion. 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. 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-DOPA3) 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.