Yup, those are silkworms! Bear with me here.
Let’s get right to the point first: some very bright researchers, at a place aptly called Silklab at Tufts University, have invented a material whose shape can be controlled only by light. The silkworms are here with us because natural silk played a large role in this. (Thank you again, Nature!) It’s reported today (March 12) in Nature Communications.
If you do nothing else, check out the two short videos below to see it in action.
To give you a quick and very compelling glimpse at what this material can do, here is a pair of butterfly wings made out of it. They fold up when hit by light, then they stretch back out when the light is turned off:
Now to show the control aspect a bit, and to illustrate how this material can follow the light, they made a small flower out of it, and here they show how it can rotate toward the light:
You can imagine how much better a solar cell might do if it were attached to such a thing. Actually, you don’t have to imagine, because they attached a solar cell to one of these little flowers and measured its energy output as the angle of the light changed. Compared to a plain old static solar cell, it was superb!
In the graph below, you’ve got two things going on: 1) The filled-in shapes (⚫) go with the “self-tracking” solar cell, and the unfilled shapes (⚪) go with a regular solar cell. 2) Orange is power conversion efficiency, and purple is current generated. These things are proportional to each other, so you can just pick your favorite color and look at that.
You see that the regular solar cell, of course, doesn’t do too well as the angle of the light hitting it gets larger and larger, but the self-tracking solar cell, stuck to the morphing flower, just churns out the same amount of energy no matter the angle:
That’s a huge increase in efficiency, accomplished with the addition of no external energy at all.
It’s not that nobody has ever made a tracking solar cell before, but you either have to supply energy externally, or you need a bunch of moving mechanical parts that make it clumsy. I’ll let the EnergySage explain that one:
Most tracking systems out there are active systems – this means that the tracking system is provided with energy to run a motor or other mechanical device that tilts the attached solar panels the right way. On the other hand, passive solar trackers also track the sun, but without any added energy source. They move by using the heat from the sun to warm a gas. When that gas expands, it causes a mechanical movement of the solar panels. When the sun moves and the gas cools, it compresses again and the panels move back.
So this strikes me as a really elegant way to pull off passive tracking.
But now for the silkworms!
Here’s how this material works, very generally: you have two layers stuck together, back to back, one of silk and one of a polymer.
Silk is a peculiar material in many ways. One standout feature is its strength, but another is that unlike most other materials, it shrinks when it is heated. So that’s one of your layers.
The other layer is a polymer called polydimethylsiloxane, or PDMS, that expands a good deal when you heat it, and it’s also very good at bending many times without losing its shape, and it’s optically clear, to let light through.
When you put these layers back-to-back and shine light on them, they heat up a bit. One layer expands while the other contracts, and that gives you a bending motion. It’s kind of like the closing of Venus flytrap leaves, except the flytrap does the expansion and contraction by quickly moving water around.
So what about that silk? How did they cast it into this material? Well, Silklab has been at this sort of thing for a while. Back in 2012 they described what they called a “silk inverse opal”. That’s the silk layer of this new material.
You stack up some tiny spheres of a polymer called poly(methyl methacrylate), or PMMA, then pour a solution of silk extracted from Bombyx mori cocoons over it, let it set, dissolve the polymer spheres away using acetone, and then you end up with a “silk inverse opal”, kind of like a Scrub Daddy made of silk:
You get a pattern with a repeating unit that’s in the range of visible light, so it diffracts certain colors of light better than others, and that gives you “structural color” — color made not with pigment but by the contours of the material itself — like a real butterfly wing.
So while you do get a material with a pretty opalescent sheen, what this also does is it holds up certain wavelengths of light within the material. If you intersperse the silk with particles that absorb light at a matching wavelength, this causes light to heat up the material more effectively. So you get a bigger bend when light hits it.
We still have so much to learn from Nature, and this is one of the many reasons we can’t let natural habitats get destroyed by our greed. You think iron ore is valuable? Palm oil? Nature’s innovations are worth far, far more to us, if we just look carefully.