Bioadhesives

Bioadhesives are natural polymer compounds that act as adhesives. The term covers a broad range of substances: some, such as gelatine, have been used by man for many years (and in many cases have been superseded by synthetic alternatives); others are not yet in commercial use.

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Bioadhesives are of commercial interest because they tend to be biocompatible for applications involving skin or other body tissue, they work in wet environments and under water, and they will stick to low surface energy/ nonpolar surfaces like plastics. In nature, bioadhesives are typically generated by a "mix to activate" process, and are used:

• for colonization of surfaces (e.g. mussels, bacteria, algae)
• for tube building by worms and leeches (which live in these structures)
• as silks for pupae, insect nests, and prey capture by larval insects

Temporary Adhesion

Organisms such as barnacles use linear-chain polymer slimes to create Stefan Adhesion, which makes pull-off much harder than lateral drag, allowing the organism mobility over a surface. Example compounds include glycoproteins (used to stick macroalgae spores to surfaces) and mucopolysaccharides (used by marine invertebrates).

Barnacles achieve pull-off forces as high as 400 000 N/m² through this mechanism.

Permanent Adhesion

Permanent bioadhesion is thought to occur via van der Waals force. On polar surfaces, hydrogen bonding and "cation bridging" allow higher sticking forces to be achieved.

• microorganisms use acidic polysaccharides (molecular weight around 100 000)
• marine bacteria use carbohydrate exopolymers to achieve bond strengths to glass of up to 500 000 N/m²
• marine inverterbrates commonly employ polysaccharide/protein mixes: some mussels achieve 800 000 N/m² on polar surfaces and 30 000 N/m² on nonpolar surfaces
• algae and invertebrates also use polyphenolic proteins containing DOPA to promote hardening of the bioadhesive. The hardening is a tanning reaction catalysed by phenol oxidase, also present in the natural bioadhesive.

Commercial Applications

Bioadhesives are presently expensive, but some commercial applications now exist, with others in development. The most promising materials are those employed by marine invertebrates.

• Columbia Labs' Crinone [1] is a vaginal progesterone supplement in a polycarbophil bioadhesive gel, used to achieve a more effective drug delivery at low drug doses.
• Other drug delivery applications are in development
• Dental adhesives are believed to be in development as a result of work by Washington University
• Wound dressings have been investigated.

Several commercial methods of production are being researched:

• direct chemical synthesis (practical only for smaller molecules)
• farming of transgenic bacteria or yeasts (using mussel genes)
• farming of natural organisms (small and large)
• in-vitro cell cultures of mussels and other invertebrates.