Gecko-Mussel Mimetic Adhesive
Geckos can easily walk on surfaces with any roughness, climb
vertical walls, and even cling to the ceilings. This superior
adhesion power of geckos has fascinated a lot of researchers.
In 2000, a research team at University of California at Berkerly
revealed the secrets of an adhesion strategy of geckos that
relies on the tiny hairs found at foot skins and also measured
the detaching forces (K. Autumn et al., Nature,
405, 681, 2000). Geckos have morphologically hierarchical micro-
and nano-structures on their toes. Each toe is packed with similarly
oriented millions of foot-hairs called setae, which are approximately
110um in length and 4.2um in diameter and primarily consisted
with tough beta-keratins. Each seta is subdivided into hundreds
to thousands of thin and triangular shape of tips, called spatulae,
with a diameter of approximately 200nm (R. Ruibal et al., J.
Morphol. 117, 271, 1965). The spatulae make van der Waals
contacts with opposing surfaces, enabling geckos to hold themselves
on vertical surfaces (K. Autumn et al., PNAS, 99, 12252,
2002).
The adhesion force generated by a single seta is 40µN
or more so that each spatula can exert 10nN of adhesive force
(K. Autumn et al., PNAS, 99, 12252, 2002 and G. Huber
et al., Biol. Letter. 1, 2, 2005). Depending on a pulling angle,
the adhesive force differs by two orders of magnitue, indicating
the importance of the engaging mechanism of gecko's feet so
when interacting properly with surfaces, the resulting adhesive
force is strong enough to hang dozens of geckos at once. In
addition to its superior strength, gecko's adhesion is temporary,
repeatable, and directional because of the anisotropic structures
of setae. Setae arrays are slightly inclined so that when a
force is loaded in a desired direction, the contact area and
adhesion force can be maximized with minimum loading stress
(K. Autumn et al., J. Exp. Biol, 209, 3569, 2006). On
the other hand, if a preload is applied to against curvature
of the setae (increase the angle of the setal shaft to be above
30°), van der Waals force between individual spatula and
the surface reduces greatly, resulting in the setae to return
to the default state and detach the toes from the surface. (K.
Autumn et al., Nature, 405, 681, 2000)
Gecko uses dry adhesion strategy, and its enormous strength
vanishes in a wet environment like other adhesives. The gecko
adhesion becomes stronger as humidity increases by the contributions
of capillary force and the changes of contact geometry and van
der Waals Hamaker constant (G. Huber et al. PNAS, 102,
16293, 2005). Few synthetic mimics have succeeded in reproducing
the reversible property over repeating contact/release cycles
and none of them worked underwater.
 
Thus, we have developed nanoadhesive called geckel that is
able to show reversible adhesion up to a thousand times without
compromising strength in the air and even underwater by merging
the geckos' dry adhesion strategy with the mussels' wet adhesive
properties (H.
Lee et al., Nature, 448, 338, 2007). An array of
PDMS pillars with 600nm in height and 400nm in diameter was
fabricated by electron-beam lithography and subsequently coated
with a thin layer of mussel-mimetic polymer called poly(dopamine
methacrylamide-co-methoxyethylacrylate), p(DMA-co-MEA). The
synthetic polymer has high content of 3,4-L-dihydroxyphenylalanine
(DOPA), found in mussel adhesive proteins, which has been extensively
studied in our group (More information on this project can be
found in the section on Mussel Adhesive
Protein Mimetics). The adhesion performances evaluated using
atomic force microscopy (AFM) showed that the polymer coating
improved wet adhesion 15 times over uncoated arrays, and the
wet adhesion strength is similar to that in air. Without DOPA
residues in polymer, the adhesion strength dropped significantly
which proves that DOPA is critical to polymer adhesion to the
interacting surfaces.
Our next step is to scale up the geckel adhesive for mass production,
optimize the pillar geometry, and engineer mussel-mimetic polymers.
We believe that geckel adhesives will be used in many applications
such as medical, industrial, and military settings in the future.
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