Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 225

Fig. 8.13 Flexible polymer
matrix with muscle-like
properties

actuated

not actuated

be differently functionalized with a Pd catalyst. In the latter case, the aroma from
the vapors of different alcoholic beverages was sensed, and the beverages were
identified. The response of electronic noses can be so reliably and sensitive that
it can even be used to detect chronic renal failure in rats from the analysis of their
exhaledbreath( Haick et al. 2009 ). A random network of single-walled CNTs coated
with different organic materials was used in this case.
Muscles have the ability to repeatedly stretch and exert mechanical power.
Therefore, artificial muscles could generate force and move objects. An electrically
powered artificial muscle fabricated from carbon nanotube aerogel sheets has been
reported in Aliev et al. ( 2009 ) and is illustrated in Fig. 8.13 . The artificial muscle is
actuated by applying a positive voltage to the nanotube sheet electrode with respect
to a distant ground plane. It works in a temperature range that extends between
80 and 1,900 K and shows huge elongation rates, of .3:7 10 4 /% per second, and
giant elongations, of 220%, which can be frozen by van der Waals forces acting
between the expanded sheet and a substrate. The observed giant elongations are
in the width direction and are caused by the periodic corrugation in this direction
during cycling of the nanotube sheet, with similar actuator strokes of 200% in
the thickness direction, which implies that the sheet contracts a few percent in
length when actuated. In fact, in this composite material, the Poisson's ratios have
opposite signs along the width and length directions and lead to a negative linear
compressibility and densification of the actuated artificial muscle. Such densified
sheets have a stress-generation capability 32 times larger than the natural skeletal
muscle.
The skin is a very sensitive tissue, able to sense pressure and temperature.
An artificial skin has been reported in Graz et al. ( 2009 ), which consists of a
flexible 30-m-thick foil of ferroelectric polymer matrix that incorporates piezo-
electric ceramic lead titanate nanoparticles with an average diameter of 100 nm.
Both pressure and temperature can be sensed since the polarization directions
of the ferroelectric polymer and nanoparticles can be adjusted independently by
an area-selective, sequential two-step poling technique. Thus, the same material
accommodates two sensory modes, the electronic skin consisting of arrays of
bifunctional sensory cells composed of two subcells, each reacting to changes
in pressure or temperature. Temperature is measured in the pyroelectric subcell,
obtained when the polarizations of polymer and ceramic nanoparticles are parallel,
whereas an antiparallel polarization orientation corresponds to a piezoelectric
subcell, which senses pressure. A controlled manipulation of the polarization
directions of the polymer and nanoparticles is possible since their piezoelectric
coefficient has a different sign and their Curie temperatures are different. Then,
rendering the material pyroelectric or piezoelectric implies a sequential poling