Everything about Bioadhesives totally explained
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been used as general-purpose glues by man for many years, but typically their performance shortcomings have seen them superseded by synthetic alternatives.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, for example 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
plastics. In recent years, the synthetic adhesives industry has been impacted by
environmental concerns and health/safety issues relating to hazardous ingredients,
volatile organic compound emissions, and difficuties in recycling or remediating adhesives derived from
petrochemical feedstocks. Rising
oil prices may also stimulate commercial interest in biological alternatives to synthetic adhesives.
Examples of bioadhesives in nature
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/mm
2).
Silk dope can also be used as a glue by
arachnids and
insects.
Temporary Adhesion
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.
Permanent Adhesion
Many permanent bioadhesives (for example, 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 [[Atomicmass unit]Da]])
Marine bacteria use carbohydrate exopolymers to achieve bond strengths to glass of up to 500 000 N/m2
Marine inverterbrates commonly employ protein-based glues for irreversible attachment. Some mussels achieve 800 000 N/m2 on polar surfaces and 30 000 N/m2 on non-polar surfaces
Some algae and marine invertebrates use polyphenolic proteins containing L-DOPA
Proteins in the oothecal foam of the mantis are cross-linked covalently by small molecules related to L-DOPA via a tanning reaction that's 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-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.
Commercial Applications
Some commercial applications now exist, with others in development.
Commodity wood adhesive based on a bacterial exopolysaccharide
USB PRF/Soy 2000, a commodity wood adhesive that's 50% soy hydrolysate and excels at finger-jointing green lumber
Mussel adhesive proteins can assist in attaching cells to plastic surfaces in laboratory cell and tissue culture experiments (see External Links)
The Notaden frog glue is under development for biomedical uses, for example as a surgical glue for orthopedic applications or as a hemostat
Mucosal drug delivery applications. For example, films of mussel adhesive protein give comparable mucoadhesion to polycarbophil, a synthetic hydrogel used to achieve effective drug delivery at low drug doses.
Several commercial methods of production are being researched:
direct chemical synthesis, for example incorporation of L-DOPA groups in synthetic polymers
fermentation of transgenic bacteria or yeasts that express bioadhesive protein genes
farming of natural organisms (small and large) that secrete bioadhesive materialsFurther Information
Get more info on 'Bioadhesives'.
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