Cheryl Hayashi: The magnificence of spider silk

I'm here to spread the word about the

magnificence of spiders

and how much we can learn from them.

Spiders are truly global citizens.

You can find spiders in nearly

every terrestrial habitat.

This red dot marks

the Great Basin of North America,

and I'm involved with an alpine biodiversity

project there with some collaborators.

Here's one of our field sites,

and just to give you a sense of perspective,

this little blue smudge here,

that's one of my collaborators.

This is a rugged and barren landscape,

yet there are quite a few spiders here.

Turning rocks over revealed this crab spider

grappling with a beetle.

Spiders are not just everywhere,

but they're extremely diverse.

There are over 40,000 described species

of spiders.

To put that number into perspective,

here's a graph comparing the 40,000

species of spiders

to the 400 species of primates.

There are two orders of magnitude more

spiders than primates.

Spiders are also extremely old.

On the bottom here,

this is the geologic timescale,

and the numbers on it indicate millions

of years from the present, so the zero here,

that would be today.

So what this figure shows is that spiders

date back to almost 380 million years.

To put that into perspective, this red

vertical bar here marks the divergence time

of humans from chimpanzees,

a mere seven million years ago.

All spiders make silk

at some point in their life.

Most spiders use copious amounts of silk,

and silk is essential to their survival

and reproduction.

Even fossil spiders can make silk,

as we can see from this impression of

a spinneret on this fossil spider.

So this means that both spiders

and spider silk have been around

for 380 million years.

It doesn't take long from working with spiders

to start noticing how essential silk is

to just about every aspect of their life.

Spiders use silk for many purposes, including

the trailing safety dragline,

wrapping eggs for reproduction,

protective retreats

and catching prey.

There are many kinds of spider silk.

For example, this garden spider can make

seven different kinds of silks.

When you look at this orb web, you're actually

seeing many types of silk fibers.

The frame and radii of this web

is made up of one type of silk,

while the capture spiral is a composite

of two different silks:

the filament and the sticky droplet.

How does an individual spider

make so many kinds of silk?

To answer that, you have to look a lot closer

at the spinneret region of a spider.

So silk comes out of the spinnerets, and for

those of us spider silk biologists, this is what

we call the "business end" of the spider. (Laughter)

We spend long days ...

Hey! Don't laugh. That's my life.

(Laughter)

We spend long days and nights

staring at this part of the spider.

And this is what we see.

You can see multiple fibers

coming out of the spinnerets, because

each spinneret has many spigots on it.

Each of these silk fibers exits from the spigot,

and if you were to trace the fiber back

into the spider, what you would find is that

each spigot connects to its own individual

silk gland. A silk gland kind of looks like a sac

with a lot of silk proteins stuck inside.

So if you ever have the opportunity to dissect

an orb-web-weaving spider,

and I hope you do,

what you would find is a bounty

of beautiful, translucent silk glands.

Inside each spider, there are hundreds

of silk glands, sometimes thousands.

These can be grouped into seven categories.

They differ by size, shape,

and sometimes even color.

In an orb-web-weaving spider,

you can find seven types of silk glands,

and what I have depicted here in this picture,

let's start at the one o'clock position,

there's tubuliform silk glands, which are used

to make the outer silk of an egg sac.

There's the aggregate and flagelliform silk

glands which combine to make the sticky

capture spiral of an orb web.

Pyriform silk glands make the attachment

cement -- that's the silk that's used to adhere

silk lines to a substrate.

There's also aciniform silk,

which is used to wrap prey.

Minor ampullate silk is used in web construction.

And the most studied silk line

of them all: major ampullate silk.

This is the silk that's used to make the frame

and radii of an orb web, and also

the safety trailing dragline.

But what, exactly, is spider silk?

Spider silk is almost entirely protein.

Nearly all of these proteins can be explained

by a single gene family,

so this means that the diversity of silk types

we see today is encoded by one gene family,

so presumably the original spider ancestor

made one kind of silk,

and over the last 380 million years,

that one silk gene has duplicated

and then diverged, specialized,

over and over and over again, to get

the large variety of flavors of spider silks

that we have today.

There are several features that all these silks

have in common. They all have a common

design, such as they're all very long --

they're sort of outlandishly long

compared to other proteins.

They're very repetitive, and they're very rich

in the amino acids glycine and alanine.

To give you an idea of what

a spider silk protein looks like,

this is a dragline silk protein,

it's just a portion of it,

from the black widow spider.

This is the kind of sequence that I love

looking at day and night. (Laughter)

So what you're seeing here is the one letter

abbreviation for amino acids, and I've colored

in the glycines with green,

and the alanines in red, and so

you can see it's just a lot of G's and A's.

You can also see that there's a lot of short

sequence motifs that repeat over and over

and over again, so for example there's a lot of

what we call polyalanines, or iterated A's,

AAAAA. There's GGQ. There's GGY.

You can think of these short motifs

that repeat over and over again as words,

and these words occur in sentences.

So for example this would be one sentence,

and you would get this sort of green region

and the red polyalanine, that repeats

over and over and over again,

and you can have that hundreds and

hundreds and hundreds of times within

an individual silk molecule.

Silks made by the same spider can have

dramatically different repeat sequences.

At the top of the screen, you're seeing

the repeat unit from the dragline silk

of a garden argiope spider.

It's short. And on the bottom,

this is the repeat sequence for the

egg case, or tubuliform silk protein,

for the exact same spider. And you can see

how dramatically different

these silk proteins are -- so this is

sort of the beauty of the diversification

of the spider silk gene family.

You can see that the repeat units differ

in length. They also differ in sequence.

So I've colored in the glycines again

in green, alanine in red, and the serines,

the letter S, in purple. And you can see

that the top repeat unit can be explained

almost entirely by green and red,

and the bottom repeat unit has

a substantial amount of purple.

What silk biologists do is we try to relate

these sequences, these amino acid

sequences, to the mechanical properties

of the silk fibers.

Now, it's really convenient that spiders use their silk

completely outside their body.

This makes testing spider silk really, really

easy to do in the laboratory, because

we're actually, you know, testing it in air

that's exactly the environment that

spiders are using their silk proteins.

So this makes quantifying silk properties by

methods such as tensile testing, which is

basically, you know, tugging on one end

of the fiber, very amenable.

Here are stress-strain curves

generated by tensile testing

five fibers made by the same spider.

So what you can see here is that

the five fibers have different behaviors.

Specifically, if you look on the vertical axis,

that's stress. If you look at the maximum

stress value for each of these fibers,

you can see that there's a lot of variation,

and in fact dragline, or major ampullate silk,

is the strongest of these fibers.

We think that's because the dragline silk,

which is used to make the frame and radii

for a web, needs to be very strong.

On the other hand, if you were to look at

strain -- this is how much a fiber can be

extended -- if you look at the maximum value

here, again, there's a lot of variation

and the clear winner is flagelliform,

or the capture spiral filament.

In fact, this flagelliform fiber can

actually stretch over twice its original length.

So silk fibers vary in their strength

and also their extensibility.

In the case of the capture spiral,

it needs to be so stretchy to absorb

the impact of flying prey.

If it wasn't able to stretch so much, then

basically when an insect hit the web,

it would just trampoline right off of it.

So if the web was made entirely out of

dragline silk, an insect is very likely to just

bounce right off. But by having really, really

stretchy capture spiral silk, the web is actually

able to absorb the impact

of that intercepted prey.

There's quite a bit of variation within

the fibers that an individual spider can make.

We call that the tool kit of a spider.

That's what the spider has

to interact with their environment.

But how about variation among spider

species, so looking at one type of silk

and looking at different species of spiders?

This is an area that's largely unexplored

but here's a little bit of data I can show you.

This is the comparison of the toughness

of the dragline spilk spun

by 21 species of spiders.

Some of them are orb-weaving spiders and

some of them are non-orb-weaving spiders.

It's been hypothesized that

orb-weaving spiders, like this argiope here,

should have the toughest dragline silks

because they must intercept flying prey.

What you see here on this toughness graph

is the higher the black dot is on the graph,

the higher the toughness.

The 21 species are indicated here by this

phylogeny, this evolutionary tree, that shows

their genetic relationships, and I've colored

in yellow the orb-web-weaving spiders.

If you look right here at the two red arrows,

they point to the toughness values

for the draglines of nephila clavipes and

araneus diadematus.

These are the two species of spiders

for which the vast majority of time and money

on synthetic spider silk research has been

to replicate their dragline silk proteins.

Yet, their draglines are not the toughest.

In fact, the toughest dragline in this survey

is this one right here in this white region,

a non orb-web-weaving spider.

This is the dragline spun by scytodes,

the spitting spider.

Scytodes doesn't use a web at all

to catch prey. Instead, scytodes

sort of lurks around and waits for prey

to get close to it, and then immobilizes prey

by spraying a silk-like venom onto that insect.

Think of hunting with silly string.

That's how scytodes forages.

We don't really know why scytodes

needs such a tough dragline,

but it's unexpected results like this that make

bio-prospecting so exciting and worthwhile.

It frees us from the constraints

of our imagination.

Now I'm going to mark on

the toughness values for nylon fiber,

bombyx -- or domesticated silkworm silk --

wool, Kevlar, and carbon fibers.

And what you can see is that nearly

all the spider draglines surpass them.

It's the combination of strength, extensibility

and toughness that makes spider silk so

special, and that has attracted the attention

of biomimeticists, so people that turn

to nature to try to find new solutions.

And the strength, extensibility and toughness

of spider silks combined with the fact that

silks do not elicit an immune response,

have attracted a lot of interest in the use

of spider silks in biomedical applications,

for example, as a component of

artificial tendons, for serving as

guides to regrow nerves, and for

scaffolds for tissue growth.

Spider silks also have a lot of potential

for their anti-ballistic capabilities.

Silks could be incorporated into body

and equipment armor that would be more

lightweight and flexible

than any armor available today.

In addition to these biomimetic

applications of spider silks,

personally, I find studying spider silks

just fascinating in and of itself.

I love when I'm in the laboratory,

a new spider silk sequence comes in.

That's just the best. (Laughter)

It's like the spiders are sharing

an ancient secret with me, and that's why

I'm going to spend the rest of my life

studying spider silk.

The next time you see a spider web,

please, pause and look a little closer.

You'll be seeing one of the most

high-performance materials known to man.

To borrow from the writings

of a spider named Charlotte,

silk is terrific.

Thank you. (Applause)

(Applause)