Biological Materials in Engineering Mechanisms - Biomimetics: Biologically Inspired Technologies - page 370

layer of soluble and insoluble proteins (predominately) that act as the ''mortar.'' Soluble, aspartic
acid-rich glycoproteins direct crystal formation by binding with calcium leading to the formation of
aragonite crystals. Depending on the species, the organic matrix also consists of insoluble b-chitin
and silk-like proteins that do not contribute to the mineralization process directly, but function by
establishing a more rigid scaffold in part responsible for the superior fracture toughness and
strength associated with these shells. In recent studies, some of these concepts were studied through
the isolation of specific proteins or groups of proteins from native mollusks and utilized in studies of
controlled mineralization, such as for the formation of biopearls (Zaremba et al., 1996). It is also
worth noting that other organic-inorganic composites in nature, such as bone and tooth enamel, are
similarly organized in terms of organic templates and molecular scale interactions, although the
specific organic components are different. For example, collagens represent the bulk of the organic
matrix in these composites and hydroxyapatite is the major inorganic component.
Mechanism — Unlike the mortar in a brick wall, the organic matrix of the nacreous layer in
mollusk shells is flexible and contributes to the strength and toughness of the shell by absorbing and
displacing stress applied to the aragonite tablets. Insoluble fibers bind to the aragonite tablets at
the optimized inorganic interface, acting as a natural adhesive between the layers. In addition, the
organic matrix acts to dissipate crack propagation (Pokroy and Zolotoyabko, 2003). The fracture
toughness of these types of structures is directly related to the presence of specific proteins in
the organic matrix with domains characteristic of elastic behavior based on the amino acid sequence
chemistry. For example, studies of nacre with Atomic Force Microscopy illustrated stepwise
unfolding of the associated proteins, reflective of this elastic behavior (Smith et al., 1999). Specific
proteins have been isolated and ascribed with these features, such as Lustrin A, which contains
cysteine- and porline-rich domains (Zhang et al., 2002). Other domains in this protein also appear to
provide regions with direct interactions with the aragonite component of nacre (Wustman et al.,
2002).
Biomimetics — To form relatively inexpensive materials that mimic the mechanical features of
nacre, weak interfaces have been layered between sheets of ceramics. In this method each layer of
ceramic is approximately 200 mm thick and is cut from a larger sheet made by treating silicon
carbide powder with boron to create a pliable material. The silicon carbide layers are coated with
graphite and subsequently pressed together and sintered at 2000 8 C. The resulting material has a
fourfold increase in fracture toughness when compared to monolithic silicon carbide, requiring 100
times the amount of work to break the layered ceramic (Clegg et al., 1990). This ceramic material
also offers increased heat resistance, which contributed to its successful testing as a combustion
liner for gas turbine engines. Although this ceramic mimics nacre, it does not harness the strength
and toughness associated with the nano-scale architecture found in biocomposites such as nacre,
bone, and coral. The ability to manufacture materials that resemble nacre structure and properties
has been approached using alternating layers of clay and polymer. Unlike the self-assembling
components of nacre, this artificial nacre is prepared by physically applying sequential layers of
negatively charged clay, montmorillonite, and positively charged polyelectrolytes, poly(diallydi-
methylammonium) chloride. The high affinity between the two components induces a strong
inorganic/organic interface. Under stress, these sacrificial bonds are broken to allow platelet
movement, which results in displacement of force much like the organic matrix of the nacreous
layer. Two hundred sequential clay and polymer layers resulted in the formation of a film with a
thickness of 4.9 mm, thus each clay or polymer layer had an average thickness of 250 nm (Tang
et al., 2003). This film exhibited similar mechanical properties to nacreous layers.
The self-assembly of the shell components is an attractive feature because this ensures highly
specific spacing, alignment, and placement of material components at small length scales, a feature
more difficult to attain by current synthetic fabrication methods. Molecular erector sets have been
proposed to mimic this self-assembly process by employing cell-surface and phage display tech-
nologies, which can produce polypeptide sequences that specifically interact with an inorganic
surface with high affinity (Sarikaya et al., 2003). The development of molecular erector sets is