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Biological Materials in Engineering Mechanisms 369

significant sequence (chemistry) variability permitted in the hydrophobic or dominant regions of
the protein which give rise to the variations in functional properties of the different silks. In the final
step of the process of formation of silk fibers, the assembled proteins are spun through orifices in the
abdomen of spiders or mouth of silkworms, inducing the structural and morphological transition of
the gel state from the gland into the b-sheet structure and fiber morphology. The spun fibers are
insoluble in water.
In general, the combined control of sequence chemistry and processing conditions is central to
the successful formation of high-performance fibers based on silk proteins. Included in this is the
presence of suitable blocks and chemical features of these blocks to deal with the processing
environment and the appropriate mechanical properties. To date, no successful example has been
reported of spinning recombinant or reconstituted silk proteins that emulate the full range of novel
mechanical properties of silkworm or spider silk fibers.
Mimetic Systems — Many aspects of the silk spinning process can be mimicked in vitro and the
all-aqueous environment used is instructive as a model for polymer processing, in general. Employ-
ing new insights into silk fibroin solubility and assembly has permitted new forms of these protein-
based materials to be generated. For example, porous 3-D sponges have been formed from
regenerated silk fibroin (Nazarov et al., 2004). Blending silk fibroin and polyethylene oxide to
obtain sufficient solution viscosity suitable for electrospinning to generate nanoscale-diameter
fibers has also been reported (Jin et al., 2002). The insights into silk protein assembly have been
utilized toward the formation of patterned peptide multilayer thin films with nanoscale order
(Valluzzi et al., 2003). This engineered liquid crystallinity to form smectic layers was generated
with thin films and bulk materials, with tunability of the layer thickness and patterning based on
sequence design and chain length. In addition to recapitulation of the silk assembly process in vitro
as outlined above based on new insight into the process of silk protein–protein interactions, efforts
are underway in many laboratories to develop synthetic analogs of silk. For example, copolymers of
glycine–alanine repeats (hard segements, b-sheet formation) with polyethylene glycol (soft seg-
ments) have been studied to provide fundamental insight into control of polymer assembly and
structure (Rathore and Sogah, 2001).
With continuing advances in genetic engineering, improved quantities and the availability of
additional sequence variants of silk proteins can be anticipated. These materials, combined with
new understanding of aqueous processing of these polymers, should help with continued improve-
ments in the ability to generate silk-based materials for a wide range of potential utility. At the same
time, novel polymer mimics of these systems in which the key design rules are considered will
continue to emerge. Finally, with the wealth of silk protein sequence–structure–function data being
generated from the study of different silks, this family of unusual protein polymers can serve as a
blueprint for future designs and engineering to develop synthetic analogs using more traditional
synthetic polymer approaches.

14.2.2 Seashells — High Performance Organic–Inorganic Composites from Nature

Background — The shells of mollusks exhibit exceptional toughness, despite compositions of
predominately brittle inorganic salts. Calcium carbonate (CaCO 3 ), or ‘‘chalk’’ comprises up to 98%
of the content of many of these shells, with the remaining few percent consisting of proteins and
glycoproteins (Levi-Kalisman et al., 2001). The organic components create a molecular-level
template for control of nucleation and crystal growth. The mechanical strength of these shells
can be attributed to these molecular interfaces as well as the complex hierarchal structure consisting
of ordered lamellae or layers (Song et al., 2003). The nacreous layer, the interior lining of seashells,
exhibits three orders of magnitude increased mechanical strength when compared to pure aragonite
crystals (Almqvist et al., 1999). Aragonite, calcite, and vaterite are polymorphs of these calcium
salts with the same chemical composition (CaCO 3 ), but different crystal shape, size, and symmetry.
In the nacreous layer, these inorganic ‘‘bricks’’ are polygonal aragonite tablets separated by a thin