DESIGNING NANOFIBROUS SCAFFOLDS FOR TISSUE ENGINEERING - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 448

13.4.1.1.3 SILK FIBROIN. Conventional silk is a natural protein derived from
the cocoon of Bombyx mori (silk worms) [126]. Other forms of silk such as
dragline silk are obtained from N. clavipes spiders. Silk obtained from cocoons as
well as N. clavipes spiders has a desirable combination of biocompatibility, high
mechanical strength, and even compressibility [127]. Silk obtained from all
sources has been electrospun into nanofi bers. Although silk fi bers provide
improved strength, the immune response elicited—due to sericin protein present
on the outer layer of silk protein—has limited its use in tissue engineering appli-
cations. Purifi ed silk fi broin, after the removal of sericin, retains all the attributes
of silk fi ber, while reducing its immunogenicity.
Zarkoob et al. [127] reported the electrospinning of silk obtained from both
N. clavipes spider and the B. mori silkworm using HFP as the solvent. The electro-
spun silk nanofi bers obtained were smaller in diameter than the whole fi bers pro-
duced by nature ( B. mori , 10 - 20

m) as well as from
conventional spinning techniques. Silk fi bers obtained by electrospinning can be
observed by electron diffraction without the need for microtome or other destruc-
tive sample preparation procedures. Sukigara et al. [128] reported the use of
formic acid as the solvent system for the fabrication of nanofi bers of silk fi broin.
The use of formic acid helps does not affect the structure of fi broin (which imparts
mechanical strength to the silk fi ber) that can potentially be lost when HFP is
used as the solvent [128]. In continuation to this study, Ayutsede et al. studied the
infl uence of the process of electrospinning on the structure, morphology and
properties of silk fi ber assemblies. This study demonstrated that the dissolution of
silk fi broin in formic acid enhances

μ m and N. clavipes , 2 - 5 μ

-sheet crystallization and hence reduces the
hydrodynamic radius of the fi broin molecules that result in an increase in intra-
molecular hydrogen bonding of the fi broin, which in turn facilitates

β

- sheet for-
mation in the electrospun fi ber [129]. Therefore, formic acid might be a desirable
solvent for electrospinning of silk.
Other fi brous proteins that have been electrospun into nanofi bers include
fi brin, elastin, and fi brinogen [129-133]. Successful attempts have also been
made towards electrospinning of globular proteins such as hemoglobin and
myoglobin [134] .

β

13.4.1.2 Carbohydrates. Carbohydrates are biologically - produced mate-
rials, with carbon, hydrogen and oxygen [C x (H 2 O) y ] as the building blocks.
Polysaccharides are long chains of simple sugar units bonded together. Com-
mercially available products include starch, cellulose and its derivatives (such
as cellulose acetate, carboxymethyl cellulose, and methyl cellulose), sodium
alginate, xanthan gum, dextran, carrageenan, and hyaluronic acid. The syn-
thesis of carbohydrate nanofi bers using electrospinning has been extensively
explored.
Glycosaminoglycans (GAGs) are long unbranched polysaccharides con-
taining a repeating disaccharide unit. They are negatively charged polycarbox-
ylated molecules that are located primarily on the surface of cells and in the
ECM [135]. The majority of GAGs in the body are linked to core proteins,