Chemists can learn from some shellfish. Mussels, for example, produce an
adhesive that sticks strongly to metal and stone, even under water. Chemists
have reproduced the protein responsible for this in a synthetic material that
contains the same adhesive elements. Irrespective of whether the adhesive is
completely made up of these elements or whether they represent just a tenth
of its make-up, adhesion is equally good. These findings were made by researchers
at the Max Planck Institute for
Polymer Research and at the Johannes Gutenberg University in Mainz. It might
be possible to use the 90% of the polymers that are not necessary to create
a good bond for other functions by providing them with chemical adjuncts which
will allow them to adhere to surfaces other than metal or stone. (Advanced Materials,
October 2008)
 | | Holding on tightly: Some shellfish attach themselves to a foundation with proteins containing the amino acid dopa. Adhesion is equally good whether the protein contains a high proportion or a low proportion of the substance. Credit: Creative Commons / Andreas Trepte, Marburg |
Some shellfish have a hard life: when they settle at the bottom of the sea
close to the coast, the constant surging to and fro of the surf pulls at them.
So that they are not washed away by the waves, the shellfish use special proteins
to attach themselves firmly to a foundation - an ability that engineers still
find difficult to achieve: adhesion under water. The shellfish can do this thanks
to the amino acid dihydroxyphenylalanine, also known as dopa. Its chemical structure
allows it to form very stable bonds with metals and minerals and is contained
in the adhesion proteins with which shellfish attach themselves to the sea bed.
Scientists working with Hans-Jürgen Butt, Director at the Max Planck Institute
for Polymer Research in Mainz, and Professor Wolfgang Tremel from the University
of Mainz, have now reproduced the adhesive shellfish proteins with artificial
polymers. These consist of long chains of molecules and carry the same chemical
adjuncts that make the shellfish proteins adhesive. As the researchers in Mainz
have now discovered, the number of the links in the chain carrying the binding
dopa adjuncts has no overall relevance for the chain's adhesiveness, provided
it is not less than 10% of the total.
The researchers measured the force which allowed them to detach different polymer
chains from a surface. They tested polymers that consisted completely of links
with the binding dopa adjunct and some where it was only present on a fifth
or a tenth of the links. The force required to pull a single polymer from the
surface was always the same: 67 piconewtons. This is equivalent to a millionth
of the weight force of a flea. This force alone could not keep a shellfish on
the bottom of the sea. However, the creatures attach themselves firmly with
a dab containing innumerable polymer chains, which allows them to brave the
movement of the waves.
"The fact that the adhesive effect is, to a certain extent, independent
on the number of binding sites could be used to give the other links in the
polymer other functions," says Hans-Jürgen Butt. For example, chemists
could manufacture a polymer that adheres equally to different materials. Dopa
bonds predominantly with metals and minerals. Chemists could provide other links
in the polymer chain with adjuncts that adhere to wood, glass or bone. Adhesives
which bond metal and bone would be interesting for securing artificial joints,"
says Wolfgang Tremel.
At first, the researchers in Mainz were puzzled as to why the adhesive strength
of the polymer chains was largely independent of the number of adhesive links.
"Normally, we imagine that an adhesive polymer is like a strip of scotch
tape that adheres over the whole of its length," says Hans-Jürgen
Butt. However, the more an adhesive strip bonds to a surface, the harder it
is to pull it off. This model, which describes the adhesiveness of a polymer
as a continuous force, does not apply to shellfish proteins and their artificial
counterparts.
"We see our polymers as chains of single binding sites linked with very
loose springs," says Wolfgang Tremel. When they peel them off, he and his
team measure only the force with which a single binding site is anchored to
the surface. How closely the adhesive links in the chain follow each other is
then irrelevant.
The density of the binding sites would have an effect if a weight was pulling
evenly across the whole length of the polymer and not from one end. "In
practice, this only plays a part when the surface is completely level,"
explains Butt. "Most surfaces are very rough at nano level, so that a weight
on one end always pulls more strongly there than on the other."
The scientists have designed their experiment to correspond to this detachment
process. They apply a single layer of the polymer to a titanium surface. Using
the titanium tip on an atomic force microscope, which only measures a few nanometers,
they pick up a single chain of the polymer in the same way someone would pick
up a thread from a table with their finger. Then they pull the tip away from
the surface and measure the force required. They need 67 piconewtons to break
the bond between the titanium surface and a dopa group on the polymer. As the
polymer itself behaves like a loose spring, the force hardly falls before the
next bond is broken, but remains almost constant.
The researchers now want to use the findings from this experiment to manufacture
polymers with binding sites for different materials. The newly established Max
Planck Graduate Center will be particularly suitable in future for pursuing
this area of research as it will specialize in interdisciplinary projects of
this nature.
Posted November 18th, 2008
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