General Aspects of Biomimetic Materials - Biotechnologies and Biomimetics for Civil Engineering - page 59

Hierarchy. Hierarchical structures are complex systems made up by a combi-
nation of interconnected parts across the nano-, micro- and millimeter scales (and
beyond), which have evolved to display physical properties that solve specific
functional challenges (Stratakis and Zorba 2010 ; Bae et al. 2014 ). The majority of
these structures contain a limited number of elements arranged and oriented in a
controlled way as to produce materials that are most often heterogeneous and
anisotropic. Furthermore, they comply with the fundamental biological principle
of minimizing the amount of matter in noncritical areas, a characteristic which
leads to materials that are lightweight. Materials displaying these hierarchical
structures are extremely resilient and will typically display ability to self-repair
cracks and fractures caused by usage (self-healing). Two well-known examples of
biological materials displaying hierarchical structure are bone (Oyen 2008 ) and
hair (Popescu and Höcker 2007 ).
Composite nature. Nearly all biomaterials are composites made up of a rela-
tively small number of polymeric and mineral building blocks (Wegst and Ashby
2004 ). These composites will typically arrange themselves by a process of self-
assembly into complex hierarchical structures (see above). Natural composites
contain a considerable proportion of mineral material without becoming brittle.
The combination with biological polymers confers strength and toughness values
superior to man-made ceramics. In comparison with the thousands of compounds
used by humankind, living organisms rely on a smaller range of molecules to
produce materials with disparate properties. Examples of minerals widely present
across living organisms include (i) calcium carbonate (e.g., in mollusk shells, eggs,
crustaceous exoskeletons), (ii) calcium phosphate (e.g., in teeth, bones) and
(iii) amorphous silica (e.g., in diatoms and sponges). On the other hand, proteins
like collagen (tendons, muscle, bones), keratin (hair, horns, beaks), and elastin
(skin) and carbohydrates like cellulose (wood) and chitin (arthropods exoskele-
ton) provide a soft matrix for elastic behavior and create the support required for
the self-assembly and stacking up of the mineral building blocks in biomaterials.
Multifunctionality. Multifunctionality is a wide spread characteristic of
biological materials and structures. This means that a given architecture and
composition typically originates a material that displays a variety of functions. For
example, the microstructure of the scales that cover butterfly wings is responsible
for functionalities which range from super-hydrophobicity and directional self-
cleaning to structural color and chemical sensing (Liu and Jiang 2011 ).
Self-healing. Self-healing is one of the secrets of biological materials. The
toughness behind many materials is translated, at the molecular level, by the
fracture of sacrificial bonds (Bhushan 2009 ). Whenever the load stops, the material
has the ability to regenerate. Most biomaterials have this ability, which, besides
allowing its recovery, opens the possibility for adaptation (Meyers et al. 2008 ).
Some biomaterials, like bone, are dynamic, continuously restructuring themselves
in response to the applied load. The increment in life-time attributed by self-
healing is a valuable characteristic, which many researchers are trying to replicate
(Wool 2008 ).