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Developments in recombinant silk and other elastic protein fi bers 255

formed in vivo (Williams et al., 1978). The recombinant collagen expressed
in yeast formed these fi brillar structures at neutral pH in phosphate buffer

(Huang et al., 2001a, 2001b) and were the first to be electrospun as collagen
scaffolds for wound dressing. Shortly thereafter, spun non-crosslinked col-

lagen fibers exhibited a concentration dependence on the fi nal fi ber diam-
eters produced, and showed suitable biological properties in preliminary in

vitro tests (Matthews et al., 2002). Recently, collagen fibers have been pro-
duced without either organic solvents or blend formation with any synthetic
and natural polymers, to provide new electrospinning experimental condi-
tions to obtain biomimetic collagen self-assembled nanofi bers (Foltran et
al., 2008).

10.7.3 Elastin

Native elastin is one of the most abundant ECM proteins. Working in part-
nership with collagen, elastin allows the body organs to stretch and relax.
Thus, while collagen provides rigidity, elastin allows the recoil of elastic
tissues. In its natural state, elastin is an insoluble protein owing to the
presence of crosslinks. However, its soluble forms, such as tropoelastin
(Mithieux et al., 2004) and α-elastin (Annabi et al., 2009), are frequently
used as biomaterials. Natural elastin undergoes a self-aggregation process

in its natural environment, leading to the formation of nanofibrils from a
water-soluble precursor called tropoelastin (Urry, 2005). This ability resides
in certain relatively short amino acid sequences, which are known to coac-

ervate and form as fibrillar aggregates with a high degree of β-structure
(Yang et al., 2002). Fibrous matrices as scaffolds for tissue engineering have
been formed by α-elastin and tropoelastin fi bers. The electrospinning
process was optimized to provide uniform fibers in the range of microns

showing, especially with tropoelastin, ‘quasi-elastic’ wave-like patterns at
increased solution delivery rates (Li et al., 2005).
Elastin-like polymers (ELPs) are a promising model for biocompatible
protein-based polymers. The basic structure of ELPs is a repeat sequence
found in the mammalian elastic protein elastin, or a modifi cation thereof
(Miao et al., 2003). Some of their main characteristics are derived from
those of the natural protein. For example, the crosslinked matrices of these
polymers retain most of the striking mechanical properties of elastin (Di
Zio and Tirrell, 2003), which becomes important when this behavior is
accompanied by other interesting properties, such as biocompatibility (Urry
et al., 1991), stimuli-responsive behavior, and the ability to self-assemble.
These properties are based on a molecular transition of the polymer chains
called the inverse temperature transition (ITT). This transition is the key

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