127
in specialized sets of abdominal glands. The spider then completes the outer ring and
spokes, and finally builds a spiral.13
Dragline silk is several times stronger than steel, on a weight-by-weight basis, but
a spider’s dragline is only about one tenth the diameter of a human hair. The movie
“Spider-Man” drastically underestimates the strength of silk real dragline silk would not
need to be nearly as thick as the strands deployed by the web-swinging hero.14
The silk of a spider’s web is much stronger than the silk of a silkworm, and more
variable. Spider silks each have different functions, which have led to the evolution of
a stunning array of web designs. Even though spiders and silkworms are not closely
related, their silks are quite similar. Both use keratin, a protein that is also found in hair,
horn and feathers.
Silkworms and spiders have evolved a pump and valve pressure system that enables
them to make threads (filaments) from proteins.15 In spiders, the filament emerges from
batteries of spigots arrayed atop three sets of paired spinnerets. Because the silk emerges
from paired sources, each filament is composed of two strands.16 The strands, only a few
micrometers thick, are surrounded by a coat of highly viscous liquid. The liquid originates
in special glands that lie next to those producing the fibers. It is applied to the outgoing
core fibers as a continuous film, which, after uptake of atmospheric water, quickly sepa-
rates into tiny droplets.
Stickiness is provided by microscopic glycoprotein doughnuts that straddle the core fibers
like beads on a string. The glycoproteins are bathed in a solution of five major compounds
closely related to amino acids that serve as the spider’s neurotransmitters.17 These amino acids
are highly hygroscopic, which means they have the chemical ability to attract water.
Dragline silk is a composite material comprised of two different proteins, each con-
taining three types of regions with distinct properties. One of these forms an amorphous
(noncrystalline) matrix that is stretchable, giving it elasticity. When an insect strikes the
web, the stretching of the matrix enables it to absorb the kinetic energy of the insect’s flight.
Embedded in the amorphous portions of both proteins are two kinds of crystal-
line regions that toughen the silk. Although both kinds of crystalline regions are tightly
pleated and resist stretching, one of them is rigid. The resulting composite is strong (has a
high breaking stress or force per cross-sectional area), tough (has a high breaking energy
in joules absorbed per unit volume) and yet elastic.18 Spiders are able to produce several
mechanically distinct fibers from different silk glands. These include dragline silk, which
is stiff and strong, and capture silk, which acts like a very stretchy rubber.19 The capture
spiral in an orb web is stretchy and can triple in length before breaking.
Although important insights into silk protein assembly by spiders have been
achieved over the past 10 years, the mechanisms by which these proteins achieve meta-
stable states in the glands of the spinning organisms remain unclear. What is truly
remarkable is that the process allows the concentration of protein in the glands to reach
more than 30 weight percent of water, whereas at this concentration most proteins would
aggregate and precipitate.20 The lack of full comprehension of these processing steps has
limited the ability to spin reconstituted silk solutions into fibers with properties compa-
rable to native ones.
Spiders use silk for a number of activities central to their survival and reproduc-
tion, including wrapping of egg sacks, preparing safety lines, lining retreats and, most
famously, capturing insects.21
Current and Future Uses
Silk sutures have been used for thousands of years, along with other natural materials
like animal gut, cotton and linen. Most of these natural materials were replaced by nylon
starting in the 1930s, and later with degradable polymer systems from polyesters (e.g.,
polylactic acid and polylactic-co-glycolic acid).22
Spiders: Vile, But Valuable
in specialized sets of abdominal glands. The spider then completes the outer ring and
spokes, and finally builds a spiral.13
Dragline silk is several times stronger than steel, on a weight-by-weight basis, but
a spider’s dragline is only about one tenth the diameter of a human hair. The movie
“Spider-Man” drastically underestimates the strength of silk real dragline silk would not
need to be nearly as thick as the strands deployed by the web-swinging hero.14
The silk of a spider’s web is much stronger than the silk of a silkworm, and more
variable. Spider silks each have different functions, which have led to the evolution of
a stunning array of web designs. Even though spiders and silkworms are not closely
related, their silks are quite similar. Both use keratin, a protein that is also found in hair,
horn and feathers.
Silkworms and spiders have evolved a pump and valve pressure system that enables
them to make threads (filaments) from proteins.15 In spiders, the filament emerges from
batteries of spigots arrayed atop three sets of paired spinnerets. Because the silk emerges
from paired sources, each filament is composed of two strands.16 The strands, only a few
micrometers thick, are surrounded by a coat of highly viscous liquid. The liquid originates
in special glands that lie next to those producing the fibers. It is applied to the outgoing
core fibers as a continuous film, which, after uptake of atmospheric water, quickly sepa-
rates into tiny droplets.
Stickiness is provided by microscopic glycoprotein doughnuts that straddle the core fibers
like beads on a string. The glycoproteins are bathed in a solution of five major compounds
closely related to amino acids that serve as the spider’s neurotransmitters.17 These amino acids
are highly hygroscopic, which means they have the chemical ability to attract water.
Dragline silk is a composite material comprised of two different proteins, each con-
taining three types of regions with distinct properties. One of these forms an amorphous
(noncrystalline) matrix that is stretchable, giving it elasticity. When an insect strikes the
web, the stretching of the matrix enables it to absorb the kinetic energy of the insect’s flight.
Embedded in the amorphous portions of both proteins are two kinds of crystal-
line regions that toughen the silk. Although both kinds of crystalline regions are tightly
pleated and resist stretching, one of them is rigid. The resulting composite is strong (has a
high breaking stress or force per cross-sectional area), tough (has a high breaking energy
in joules absorbed per unit volume) and yet elastic.18 Spiders are able to produce several
mechanically distinct fibers from different silk glands. These include dragline silk, which
is stiff and strong, and capture silk, which acts like a very stretchy rubber.19 The capture
spiral in an orb web is stretchy and can triple in length before breaking.
Although important insights into silk protein assembly by spiders have been
achieved over the past 10 years, the mechanisms by which these proteins achieve meta-
stable states in the glands of the spinning organisms remain unclear. What is truly
remarkable is that the process allows the concentration of protein in the glands to reach
more than 30 weight percent of water, whereas at this concentration most proteins would
aggregate and precipitate.20 The lack of full comprehension of these processing steps has
limited the ability to spin reconstituted silk solutions into fibers with properties compa-
rable to native ones.
Spiders use silk for a number of activities central to their survival and reproduc-
tion, including wrapping of egg sacks, preparing safety lines, lining retreats and, most
famously, capturing insects.21
Current and Future Uses
Silk sutures have been used for thousands of years, along with other natural materials
like animal gut, cotton and linen. Most of these natural materials were replaced by nylon
starting in the 1930s, and later with degradable polymer systems from polyesters (e.g.,
polylactic acid and polylactic-co-glycolic acid).22
Spiders: Vile, But Valuable