Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 210

The fibers are made from polyurethane, a material with high strain before failure,
high tear strength, and high Young modulus, of about 3 MPa, using lithography and
molding techniques. An array of such fibers can support, through shear adhesive
strength, loads of about 1 kg cm 2 if loaded in the gripping direction and self-
releasing behavior if loaded in the opposite, shear direction, when it supports only
aloadof200 gcm 2 . The shear anisotropy, caused by the stresses originating in
the moment of the sheared tip, is controlled by the tip angle, while the interfacial
shear strength is determined by the tip area. The tip angle can vary between 0 ı and
90 ı , whereas the base fiber angle can be varied between 0 ı and 33 ı . Combining the
controlled fabrication of arrays of micrometer-sized angled polymer stalks with a
strategy of controlling the toes' smooth attachment and detachment and the balance
forces among the feet allowed the fabrication of the bioinspired Stickybot robot.
Stickybot uses a hierarchy of conformable structures to climb glass, ceramic tiles,
or smooth plastic surfaces at a speed of 4 cm s 1 ( Kim et al. 2008 ).
Gecko-inspired synthetic materials consisting of angled nanostructures have been
extensively studied. It was shown that for nanohairs fabricated from both soft and
hard polymers, an increase in the leaning angle enhances significantly the shear
adhesion and the adhesion hysteresis, the effect being more pronounced for soft
materials ( Jeong et al. 2010 ). The reason is that the hard polymer nanohairs make
a tip contact with the substrate, while soft polymer nanohairs make a side contact.
However, nanohairs with a high leaning angle have also increased difficulty to attach
to rough surfaces and tend to clump on the substrate, phenomena that are avoided
in gecko lizards by a multiscale, hierarchical organization of foot hairs. Bioinspired
design strategies for smart multiscale interfacial materials are exemplified in Xia
and Jiang ( 2008 ).
Often, gecko-inspired synthetic materials with high adhesion do not maintain this
property in wet environments. To create an artificial material with comparable high
adhesion in air and water, we have to look to both gecko and mussels for inspiration.
Mussels are able to cling to wet surfaces due to the secretion of a specialized
adhesive protein containing a catecholic amino acid. The resulting material is called
geckel ( Lee et al. 2007 ) and consists of an array of poly(dimethylsiloxane) elastomer
nanopillars fabricated by electron-beam lithography coated with an ultrathin layer
(with a thickness of less than 20 nm) of a synthetic polymer with similar properties
as the wet adhesive proteins in the holdfasts of mussels. The geckel has superior
adhesion force per pillar than gecko-inspired material in both air and water
environments: 120 nN for geckel in air compared to about 40 nN for gecko-like
material in air and 86.3 nN for geckel in water with respect to only about 6 nN for
gecko-inspired adhesive in water. The force per pillar in water increased about 15
times, the wet adhesion properties maintaining over more than 1,000 contact cycles.
It is important to emphasize that the synthetic polymer used for coating peels off
quite easily if applied on flat substrates.
Nature's sophisticated architectures are also visible in the creation of hybrid
flexible and strong structures starting from materials with poor qualities. Humans
have long since fabricated strong and stiff clay-based nanocomposites, which
lack, however, flaw tolerance and ductility encountered in the outer skeleton of