Biomedical Engineering Reference
In-Depth Information
TABLE 7.1
Mechanical Properties of Silk and Other Biodegradable Polymeric Materials
Modulus
(GPa)
UTS
(MPa)
Strain (%)
at Break
Source of Biomaterial
References
B. mori
silk (with sericin)
5-12
500
19
[138]
B. mori
silk (without
sericin)
15-17
610-690
4-16
[138]
B. mori
silk
10
740
20
[139]
N. clavipes
silk
11-13
875-972
17-18
[139]
Collagen
0.0018-0.046
0.9-7.4
24-68
[140]
Cross-linked collagen
0.4-0.8
47-72
12-16
[140]
Polylactic acid
1.2-3.0
28-50
2-6
[141]
the less ordered hydrophilic blocks give rise to the combined elasticity and toughness of silk fibroin
materials, such as the native fibers [3,10,15].
Mechanistic insight has been developed into how silk fibroin solutions are processed to insoluble
fibers by various organisms. The process involves spinning a highly concentrated silk fibroin solution in
a non-Newtonian liquid crystalline state, where the silk fibroins are lubricated and stabilized by water
through micelle-like structures as a result of phase separation due to silk fibroin's intrinsic hydrophilic-
hydrophobic blocks [3,9]. The process is known to be mediated by the content and location of water. The
silk fibroin concentration in the gland gradually increases, leading to the formation of micelles and gels
[9]. Also, the silk fibroin protein organizes into a metastable state that maintains sufficient water content
to avoid premature conversion to insoluble β-sheet structures. Upon spinning via the figure eight head
movement of the silkworm or the pulling by the legs of spiders, chain alignment leads to the final assem-
bly of β-sheet crystalline blocks [9]. In the final stages of spinning the fibers in silkworms, hydrophilic
proteins, termed sericins, form composite matrices by coating the core fibroin fibers [3,9]. Once formed,
silk fibers are insoluble in most organic solvents, water, and dilute acids and bases; hexafluoroisopro-
panol (HFIP), calcium nitrate, lithium bromide, and lithium thiocyanate can be used to solubilize silk
[16,17]. The repetitive peptide domains in the silk fibroin sequence form the core basis for genetically
engineered silk-like polymers, in cassette-like approaches, in host systems such as bacteria, yeast, mam-
malian cells, and plants [11,18,19].
7.2 Sources of Silk Proteins
Many animals produce silk proteins for various different needs [20]; however, silkworm silk proteins are
the most extensively studied and abundant due to the 5000 years of domestication known as sericulture
for textile-related needs. A wash after harvesting cocoons removes the sericin coating on the silk fibers
yielding
B. mori
silk fibroin [21]. Similar processes are applicable to silk fibers produced by wild silk-
worms (such as
Antheraea pernyi
,
Antheraea mylitta
, or
Samia cynthiaricini
[21]) providing silk proteins
with differing amino acid sequences. Spider silk is also widely studied due to its mechanical properties;
however, its limited availability from native sources requires biotechnology-generated sources via clon-
ing and expression in a range of heterologous hosts as listed earlier. Although it is possible to harvest
milligrams of spider silk from fibers of egg cocoons or webs, or directly from the animal via controlled
silking, this is time-consuming and inefficient. For example, a 3.4 m
2
rug produced from major ampul-
late silk fibers of
Nephila madagascarenis
spiders took 70 people 4 years to complete, consisted of silk
collected from over 1 million spiders, and cost of over half a million US dollars [22]. Unfortunately,
attempts to farm spiders on an industrial scale have been unsuccessful due to their cannibalistic nature.
Progress in the production of spider silk-like proteins using recombinant DNA technology continues to
progress to provide alternative silk options [23]. Recombinant DNA technology has also been used to
