Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 227

assuring the pressure-driven stretching of the thin wall that mimics the intrapleural
pressure in lungs.
Bioinspired nanoengineering of surfaces can also reduce the thermal shock
resistance in ceramic materials ( Song et al. 2010 ). The poor resistance of ceramics
to thermal shock, which decreases their strength with respect to the melting
point value, is the main disadvantage of this otherwise high-temperature-resistant
material. The failure mechanism in ceramics subjected to thermal shocks is crack
initiation due to the thermal gradient and stresses on the surface. However, inspired
by dragonfly wing membranes, ceramics can be rendered insensitive to thermal
shock up to the melting temperature by roughening their surface, such that thin
air layers can envelope the nanofinned surface and increase the heat transfer
resistance with several orders of magnitude. Surface roughening is achieved by
high-temperature plasma etching and chemical corrodation in a mixed solution of
HF and HNO 3 , the result being the appearance of randomly distributed circular
nanoscale rod fins that are stand up nearly vertically. These nanofins occupy an
areal fraction of 0.37 with a density of 71m 2 and have average thicknesses and
diameter values of 375 nm and 81 nm, respectively. Their action is to dramatically
increase the heat transfer resistance for heated ceramics quenched in water and
thus to prevent crack appearance, by rendering the surface hydrophobic and hence
able to trap a thin air layer between nanofins when quenched. The heat transfer
resistance at the interface increased from about 10 4 m 2 KW 1 , without nanofins,
to 0:63 m 2 KW 1 with nanofins for the refractory ceramic ZrB 2 -20%SiC p -5%AlN
because the steep temperature difference at quenching acts on the nanofins and no
longer on the ceramic.
Bioinspired artificial bacterial flagella, which could be used as micro-
/nanomanipulators with six degrees of freedom or as in vivo medical micro- or
nanorobots, which can sense and deliver chemical and biological substances, can be
fabricated by self-scrolling of helical nanobelts and Cr/Ni/Au soft-magnetic heads
( Zhang et al. 2009 ). The flagella swimming mechanism depends on the cell type:
eukaryotic flagella create paddling motions, while prokaryotic (bacterial) flagella
turn the base or bundle of the helical flagella with the help of a molecular motor.
Self-propelled artificial bacterial flagella can be controlled by a weak rotating
magnetic field, the frequency of which is directly proportional to the translational
velocity of the device, as long as it is lower than the step-out frequency. A velocity
of 18ms 1 , comparable to that of the E. coli bacteria, can be obtained with
a magnetic field of 2 mT rotating at a frequency of 30 Hz. Such swimmers are
fabricated by patterning a 1:8-m-wide ribbon-like InGaAs/GaAs/Cr trilayer, with
11/16/15 nm thicknesses, terminated with a 10/180/10-nm Cr/Ni/Au soft-magnetic
head on a sacrificial layer, which is wet etched releasing the 2D mesa, which self-
organizes as a tethered artificial flagella with a diameter of 2:8m. The artificial
flagella with smaller heads swim faster at low frequency due to the smaller viscous
drag, but swimmers with larger heads have larger maximum velocity and step-out
frequency since the exerted magnetic torque is larger. The swimming direction,
upward or downward, which depends on the sum of the gravitational, propulsive,