The thread that shouldn't exist…maybe?

Have you ever tried holding a piece of silk to the light? Go ahead and try if you haven’t. You will witness it shift from that luminous ivory to gold, shadow to shimmer, as your hand moves. It feels like the fabric is doing something, doesn’t it? It almost looks alive. In a sense, it is. Or at least, it was at one point in the past. What you're holding is the product of one of the most sophisticated manufacturing processes on Earth, and it didn't happen in a factory. It happened inside a caterpillar.

To spell out the strange thing here: we have known how to use silk for five thousand years, and we're only now beginning to understand what it actually is!

A protein with a plan

At its very core, silk is primarily made of a protein called fibroin. That word  ‘protein’ tends to make people's eyes glaze over, so let's just skip past it for a second and break down what fibroin actually is. 

Imagine a brick wall, but built differently from any brick wall you have seen. Instead of cement holding the bricks together, the whole thing is threaded through with a stretchy rubber mesh. The bricks are rigid, locked in place. The rubber can flex, absorb, and give way without breaking. Now shrink that wall down to a scale you cannot see, cannot even really imagine, perhaps a few billionths of a meter, and string billions of them end to end. That's a silk fibre for you. 

The "bricks" here are crystalline regions where fibroin chains pack together so tightly that it can be hard to pull them apart. The "rubber" is the amorphous regions between them, loose and flexible, which allows the fabric to absorb energy rather than crack under it. This two-part architecture is the reason silk is an incredible fabric.

Stronger than steel?

On a weight-for-weight basis, silk fibre has a tensile strength comparable to high-grade steel. When engineers first measured this properly, it was the kind of result that makes you run the test again.

But tensile strength, which is the resistance to being pulled apart, is only half the story. Toughness is the amount of energy a material can absorb before it fails. And here, silk doesn’t just match steel, but some silks can significantly outperform many of them.

Spider silk, in particular, takes this even further. It may look delicate, but in truth, it behaves like a highly optimized energy-absorbing system.

All that glitters are indeed not gold

Now back to that shimmer. You might assume silk shines because it's smooth, the way a polished floor reflects light. But that's not exactly right.

If you cut a silk fiber in cross-section, you would find that it isn’t round, like a hair. It’s closer to a triangular shape. Each fiber acts like a tiny prism. When light hits it, the fiber doesn’t just bounce it straight back—it redirects it, scattering it at multiple angles at once.

Move the fabric and those angles shift, so the light seems to move through the material rather than simply off its surface. That’s the shimmer. It’s not just a surface effect. It’s geometry.

This is also why cheap synthetic silk so often looks flat and unconvincing. Well, you can copy the chemistry of silk more easily than you can convincingly replicate the shape of it.

“The sound of silk can tell [the spider] what type of meal is entangled in their net... by plucking the silk like a guitar string and listening to the echoes."

             — Fritz Vollrath 

(Vollrath is a leading expert on spider silk biomechanics at Oxford University and often uses this concept to describe natural engineering.)

Goats, genes, and what’s next for silk?

Let’s turn the strangeness up a notch.

Here’s something unexpected: researchers have taken the genes responsible for producing silk proteins and inserted them into yeast, into bacteria—even into goats. Actual goats, on actual farms, producing milk that contains silk proteins.

The goats, of course, are unaware. They are simply following instructions written into their DNA. The instructions that evolution perfected long before we knew how to read them.

The milk is harvested, the proteins extracted, and the result is biosynthetic silk: fibroin without the worm.

Early commercial production has already begun. And the goal isn’t just fabric. It’s sutures that dissolve cleanly in the body. Drug delivery systems. High-strength, lightweight materials with potential protective uses.

Five thousand years ago, someone unravelled a cocoon and made a thread. We’re still pulling on it.