Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 212

It should be mentioned that strength and flexibility are not the only characteristics
of synthetic materials that can be improved by bioinspired designs. The mechanical
sensing mechanism of low-frequency vibrations in spiders in the presence of
significant background noise, and their ability to distinguish between air flow, for
example, and signals from prey can inspire an efficient mechanical high-pass filter
( Fratzl and Barth 2009 ). The sensory hairs on the spider's legs have both high
sensitivity to small deflections and associated protection against overload. Other
biological structures for interest in designing actuators and mechanical sensors are
discussed in Fratzl and Barth ( 2009 ).
Structures with controlled and tunable mechanical properties often follow
nature's design. For instance, scaffolds with controlled gradation of mineral
contents, in particular calcium phosphate, which resemble the tendon-to-bone
attachment and hence are invaluable in prosthetics and tissue regeneration, have
been reported by Li et al. ( 2009 ). These structures are important since they avoid
stress development at the interface between soft materials, like tendons, and stiff
materials, such as bones. Calcium phosphate incorporates easily into nonwoven
mats of electrospun nanofibers, which provide a large surface area and high porosity
that resemble those in the extracellular matrix, the mineralization gradient being
obtained by varying the immersion time of the mats into a concentrated simulated
body fluid. The latter task can be achieved simply by introducing the mat into
a vial and pouring the mineral solution at constant rate in the vial; the bottom
part of the mat will present a larger calcium phosphate concentration because it is
immersed longer in the mineral solution, the concentration decreasing linearly from
the bottom to the top of the mat. The morphology of different mat regions is also
different: a nanotextured mineral coating is observed near the bottom of the vial,
while the porous structure of the mat is still preserved in regions near the top of
the mat. Mouse preosteoblast cells deposited on regions with higher concentration
of calcium phosphate showed higher levels of cell density, as expected from this
functional graded material. From the point of view of material engineering, the
graded scaffolds show also graded mechanical properties: the local strain under
uniaxial tensile deformation was higher at the unmineralized end of the scaffold
than at the mineralized end, whereas the Young modulus increased twice as the
calcium phosphate covering increased by 23%. The results suggest that mineral
coating stiffened the nanofibers.
Among the few bioinspired hierarchically organized materials are the scaffolds
fabricated from bioactive nanotitanate grown on a three-dimensional microporous
NiTi/Ti orthopedic scaffold, which are detailed in Wu et al. ( 2008 ). The NiTi/Ti
scaffold is submerged in a 10-M NaOH aqueous solution and then heated, such
that a titanate nanoskeleton layer appears on the surface of the scaffold, on
which titanate nanowires and nanobelts nucleate in time. Such metallic Ti-based
hierarchical scaffolds have great importance in tissue regeneration since, in addition
to resembling the hierarchical organization of the main bone constituents collagen
and hydroxyapatite, they are superhydrophilic and so enhance bone proliferation.
Light manipulation is an active direction of research, which profit from bioin-
spired structures to produce pigment-free colors, for example. Few life-forms have