Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 211

Fig. 8.2 Bioinspired
composite material exhibiting
( a ) plastic flow and ( b )
catastrophic rupture

a b

invertebrates, for instance, in the nacreous layer of mollusk shells or in the mineral-
ized tissues of vertebrates, exemplified by teeth or bones. These biological materials
have a layered structure, which consist of an organic, soft, and ductile matrix that
incorporates inorganic, strong platelets. Although the inorganic materials such as
silica, phosphates, or calcium carbonates are weak, the resulting platelets are strong
due to the nanoscale confinement of at least one of their dimensions. Moreover, the
platelets are often arranged in a hierarchical structure over a wide range of length
scales. Such hybrid structures can be described by a model ( Bonderer et al. 2008 )
which predicts that the tensile strength of the material, , is a weighted average of
the tensile strengths of the platelets, p , and of the organic matrix, m :

D ˛V p p C .1 V p / m : (8.1)

In ( 8.1 ), V p is the volume fraction of platelets and ' is a constant that depends
on the aspect ratio of the platelets, r, on their bonding to the matrix, expressed
by the interfacial strength i , as well as on the shear strength of the matrix m .
If r<r c , with r c a critical aspect ratio, the organic matrix breaks first, and
the platelets are pulled out (see Fig. 8.2 a), the composite becoming tougher and
ductile and exhibiting a matrix plastic flow before rupture. This is the case of
biological materials discussed above. In this situation, ˛ D m r=.2 p / or ˛ D
i r=.2 p / if the fracture appears first at the organic-inorganic interface and then
in the organic matrix. On the contrary, when r>r c , the platelets are fractured
first and the composite undergoes a brittle catastrophic rupture, regime for which
˛ D 1 p =.2 m r/. Synthetic materials with the same structure as nacres have
been fabricated using a sequential deposition of organic and inorganic layers at
ambient conditions and colloidal-based techniques for the assembly of the platelets.
These artificial structures used a chitosan polymer, with a similar m value as in
nacre, of 40 MPa, but alumina platelets instead of the aragonite platelets in nacre,
allowing p to become 2 GPa instead of the 360-500-MPa range of values in nacre
( Bonderer et al. 2008 ). Thus, r c increased from the biological value of 9-12.5 to
50 so that 200-nm-thick platelets could be used in the synthetic material, value
comparable to that in nacre. The fabricated structures had a V p value of up to 0.15,
range in which a load applied parallel to the ordered platelets increases the elastic
modulus of the composite from 2 to 10 GPa, and the tensile stress for plastic yielding
(regime in which the mechanical behavior deviates from the linear elastic regime)
augments from 50 MPa to 300 MPa. Although artificial composites have less-
elaborate structure compared to their biological counterparts, synthetic materials
can outperform their biological counterparts in terms of strength and ductility since
the range of available materials is larger than in the natural environment.