Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 208

8.1 Bioinspired Materials

Bioinspired nanoscale surfaces and structures can lead to the development of new
materials with amazing properties. In particular, spiders inspired the synthesis of
flexible hair, fire beetles inspired infrared sensors based on flexible membranes
( McConney et al. 2009 ), whereas hydrogel networks are bioinspired dynamic mate-
rials that can change their volume and shapes and can assemble and disassemble
in response to external stimuli ( Mohammed and Murphy 2009 ). Nature is also
an endless source of inspiration of superantiwetting interfaces with controlled
liquid-solid adhesion through micro- and nanostructures ( Liu et al. 2010 ). For
instance, lotus flowers, which have the ability to maintain their cleanliness even
in swampy waters due to a low-adhesion superhydrophobic surface, have inspired
the production of self-cleaning paints. On lotus leaves, composed of papillae that
are covered with nanohairs, the almost spherical water droplets easily roll off the
surface at small tilt angles and wash away any small particles of dirt. Similar to the
self-cleaning effect of lotus flowers, fishes are able to resist pollution of oil in water
due to the superoleophobic water/solid interface of their rough scales. Rose petals,
which consist of arrays of micropapillae, and gecko feet, covered with millions of
nanohairs, are high-adhesion superhydrophobic surfaces, on which water droplets
are pinned at any tilt angles, although the static contact angle is larger than 150 ı .
On superhydrophobic surfaces, for which the static contact angle is higher than
150 ı , the liquid-solid adhesion can be high or low, depending on the contact mode,
which is determined by the composition and geometrical structure of the surface,
and the triple-phase liquid/air/solid contact line. High adhesion corresponds to a
continuous and stable triple-phase contact line and a wet contact mode, in which
the water droplet penetrates the valleys of a patterned surface, while low adhesion is
associated with a discontinuous triple-phase contact line and a composite contact
mode, in which the droplet is suspended by air pockets trapped on a textured
surface. Intermediate states also exist, in which the water droplet partially wets a
superhydrophobic surface when air pockets are partially trapped in the valleys of the
textured surface ( Liu et al. 2010 ). In butterflies, for example, the surface adhesion of
the wings is not fixed but can be controlled by their posture (upward or downward)
and the airflow direction across the surface. Examples of synthetic bioinspired
superhydrophobic surfaces with different liquid-solid adhesion are described in Liu
et al. ( 2010 ).
Bioinspired surface micro- and nanostructuring changes the hydrophobicity of
metallic alloys and prevents them from corrosion. For example, the ultralight, rigid,
strong, and highly ductile Mg-Li alloy, with potential applications in electronics and
aviation, becomes stably superhydrophobic after modifying its surface in a peony-
like hierarchical structure as that in the petals of Paeonia roses. In fact, to obtain a
static contact angle of 160 ı and a sliding angle as small as 5 ı , plates of Mg-Li alloys
are ultrasonically cleaned in ethanol and doubly distilled water at room temperature,
dried in N 2 , immersed in 0.1 M HCl aqueous solution, rinsed in doubly distilled
water and dried again in N 2 , exposed to a 1% ethanol solution of fluoroalkylsilane