Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 209

Fig. 8.1 Gecko-inspired
tipped fiber with anisotropic
adhesion

self-releasing
direction

gripping
direction

for 12 h, and finally heated at 100 ı C for 2 h. After developing random papillae with
peony-like morphology, the surface energy is lowered by an additional coating with
fluoroalkylsilane. The papillae are 2-3m in diameter, and the 30-50-nm-thick
petal-like structures are separated by interspaces with a thickness of 40-300 nm.
The acquired surface roughness traps air and so minimizes the contact area between
alloy and water droplets ( Liu et al. 2008 ).
Controlled anisotropic/directional wetting, as in butterfly wings, duck feathers,
or shark skin, can be achieved in periodic structures with different heights, periods,
and linewidths, obtained by laser interference lithography ( Wu et al. 2010 ). It was
found that the height of the grooves mainly determines the anisotropic wetting,
due to the dependence of free energy barrier on this parameter, the linewidths and
periods having a much weaker influence. For example, the anisotropy of a 2-m-
period grating, defined as the difference of contact angles in the perpendicular
and parallel directions to the longitudinal axis of the grating, increases from 9 ı
to 48 ı when the height of the grooves increased from 100 nm to 1:3m. The
height of the grooves can be controlled by varying the thickness of the resin, the
period ƒ D =.2 sin / depends on the laser wavelength and incident angle ,
while the width of the grooves is determined by the laser exposure dosage. An
increase in contact angles on both directions to values higher than 100 ı (131 ı and
108 ı ) is achieved by fluoroalkylsilane surface modification. High contact angles are
beneficial for water roll down on grass leaves, for example. In addition, the wetting
anisotropy is accompanied by iridescent diffraction patterns.
The design of strong adhesive materials can get inspiration from gecko lizards,
which can climb easily on vertical or even inverted surfaces, due to the attachment
mechanism of their setae. (Setae are specialized keratinous foot hairs on the foot
pads of geckos.) This mechanism of high interfacial shear adhesion is determined by
intermolecular surface forces acting at the tips of the microscale angled keratinous
fibers on the lizard feet. In fact, the adhesion strength is the result of the contribution
of millions of contact points with nanometer scale. The angled fibers determine a
significant frictional anisotropy, with high adhesion at dragging along the gripping
direction and only weak Coulomb friction along the opposite, releasing direction,
which allows also an easy detachment of gecko setae from smooth surfaces.
Synthetic adhesives that mimic the behavior of gecko setae use 100-m-long angled
fibers with diameters of 35 m terminated with flat, mushroomed-shaped tips to
increase the contact area ( Murphy et al. 2009 ). A singular tipped fiber is represented
in Fig. 8.1 .