General Aspects of Biomimetic Materials - Biotechnologies and Biomimetics for Civil Engineering - page 63

scale. The tensile strain of human cortical bone is 2 % while for elk antler it can
reach 12 % (Launey et al. 2010 ). In contrast to nacre and enamel, bone is a dynamic
material with anisotropic distribution of mass, where the regions subjected to higher
loads are reinforced in comparison to the overall bone. Besides reducing the den-
sity, bone porosity grants the elasticity necessary to competently deflect impacts
(Young's modulus of bone is between 8 and 24 GPa) (Meyers et al. 2008 ). This
property has been explored by Wilfredo Méndez Vázquez (from Escuela de
Arquitectura de la Pontificia Universidad Católica de Puerto Rico) to create Stick.S,
a more sustainable concrete associated with a design less vulnerable to natural
disasters (earthquakes, hurricanes, etc.). 1
Silk. In addition to mineralization, another strategy used in the natural world to
produce strong tissues like silk, arthropods' exoskeleton, hoofs, and horns is to
increase the cross-linking of polymeric or fibrous materials. Spider silk possesses a
remarkably high tensile strength and extensibility (the strain at failure can exceed
1,600 %), which evolved in order to quickly absorb the kinetic energy of insects
trapped in webs (Meyers et al. 2008 ). The viscoelastic behavior of silk fibers is
attributed to the entropic unfolding of its constitutive proteins and to the transfer of
load to protein nanocrystals embedded in the disordered matrix (Meyers et al.
2013 ). The properties of each silk thread are determined by the proportion of each
component, the flexible glycine-rich matrix and the strong anti-parallel b-sheets
crystals, as well as by the entanglement of the fibers. The published literature on
the production of artificial silk suggests that the spinning process is crucial to
obtain fibers with similar characteristics (Eadie and Ghosh 2011 ). Another
remarkable property of silk is its ability to retain 10,000 times its weight in water,
which can be used to create humidity-responsive fibers. Clearly, silk is an out-
standing material that can serve as an inspiration to produce stronger and lighter
cables, textiles, or protective clothing.
Wood. The mechanical properties of cellular matrixes deserve some credit as
well. Wood, for example, has been used by humankind since its rise in applications
ranging from buildings to tools. Its success as a structural material is explained by
the high specific strength (comparable to steel) provided by its hierarchical
structure and optimized orientation of fibrils reinforcement (Meyers et al. 2008 ).
Wood is mainly composed of cellulose, hemi-cellulose, and lignin polymers
arranged in parallel hollow tubular cells (Eadie and Ghosh 2011 ). The optimiza-
tion of the number of layers and biopolymer orientation in some specific types of
woods, like bamboo, can double their tensile strengths up to 520 MPa (Li et al.
1995 ). The mimicking of these reinforcement patterns may be applied when
designing the structural support of buildings and in the manufacturing of com-
posite materials (Li et al. 1995 ; Banthia et al. 2012 ).
Pummelo. Using the hierarchical structure of cellular matrices, nature has found
an ingenious solution to resist impact in the form of the Citrus maxima fruit (also

1

STICK.S lightweight structural system, Ask Nature, Biomimicry 3.8. Accessed on the 14th
March 2014, http://www.asknature.org/product/8779abb44e4bcd7a9e8f3bf18a2c0a89 .