Biomedical Engineering Reference
In-Depth Information
acids such as glycine, alanine and serine in different amounts. Fibroins
are core proteins coated by sericin, an amorphous hydrophilic glue-
like protein with a molecular weight of 300,000 g.mol
−1
, which
maintains the physical structure of fibroins. Although present in
silkworm silk, sericin is absent in spider silk. The presence of sericin
is a main concern since it is associated with hypersensitivity reactions
and poor biocompatibility. When sericin is removed, the immune
response is similar to that of other biomaterials [MON 11]. Fibroins
are highly crystalline macromolecules (crystallinity around 70%) with
two types of molecular weight (325,000 or 25,000 g.mol
−1
), and
containing the recurrent amino acid sequences Glycine-Serine-
Glycine-Alanine-Glycine-Alanine. Fibroins are natural block
copolymers constituted of hydrophobic and hydrophilic blocks which
confer to silk its unique properties including high elastic modulus (5-
17 GPa) and tensile strength (500-740 MPa), mechanical stability,
and elongation at break around 4-20%. Silk fibroin has hemostatic
properties, non-cytotoxicity, low antigenicity and non-inflammatory
characteristics. Because of its crystallization and compact structure,
silk degrades over a time period of several months
in vivo
and is
slowly absorbed. The degradation occurs through proteolytic enzymes
such as chymotrypsin, actinase and carboxylase, and therefore the rate
of degradation depends on many factors, such as the silk processing
conditions, the physical characteristics of the material and the
implantation site. Silk can be designed in various forms and was
frequently used as sutures. Recently, silk fibroin found applications in
burn-wound dressings, enzyme immobilization matrices, vascular
prostheses and scaffolds for tissue engineering [CAO 09].
3.4.3.
Biomedical elastomers
Elastomers are made up of long and highly flexible polymeric
chains that are cross-linked to obtain enough mechanical strength.
They can withstand large deformation and recover from deformation.
Their mechanical properties are tailored according to the intended
application by varying different factors such as chain length, cross-
linking density, nature of the polymeric chain and cross-linker. Lately,
recent advances in tissue engineering have demonstrated that the
