Spider silk has some amazing material properties, so there’s lots of enthusiasm for the prospect of using it to make something useful. Unfortunately, spiders aren’t domesticated, and attempts to make the silk proteins in other organisms haven’t been entirely successful. And then there’s the matter of what to do with silk once you have it. It doesn’t always cooperate with modern manufacturing techniques.

But some researchers in India figured out a way to get spider silk to play nicely with lasers. Under the right conditions, the silk itself helps amplify a laser’s power, to the point where it can either cut the silk in specific locations, or soften it to the point where it can be bent or welded.

The work relies on a physics effect termed “nonlinear multiphoton interactions.” In the simplest terms, the effect allows two photons of a given energy to act as a single photon of twice the energy (higher combinations are also possible). It’s a nonlinear effect, since it involves a sudden jump in energy; you don’t end up with any photons in between, at 1.5x the original energy.

The multiphoton effect can be significant when a molecule wouldn’t normally absorb the low-energy photon but can absorb the higher energy one. In this case, light that would otherwise pass through a material is suddenly absorbed and excites it to a high-energy state. These higher energy states can lead to broken or rearranged chemical bonds, fluorescence, or just an absorption of energy that leads to localized melting.

In this case, the researchers used it to direct energy to specific locations in spider silk. They combined it with the extremely short pulses of a femtosecond laser (that’s 10^-15 of a second, in contrast to what they call “long picosecond pulses” at 10^-12 of a second). This keeps the energy deposited in the silk strand relatively localized, instead of allowing it to build and diffuse to the point where it causes widespread structural changes.

Tests at various wavelengths identified several wavelengths where spider silk would double, triple, or quadruple the energy of incoming photons. Nothing much happened below a certain power threshold (80 GigaWatts per square centimeter). At 300GW per square centimeter, the laser simply cut the spider silk. The team showed that the laser at this energy level could be used to place precise cuts and chop up the silk into pieces of precisely defined lengths.

But in between the two, the silk partially melted, creating a bit of a bulge and, if focused off-center, a bend in the silk. The degree of bending varied based on how long the silk was exposed to laser pulses. By moving the focus of the laser and making repeated bends, the authors managed to make solenoids, springs, and more complicated knots.

But the melting proved useful in other ways, too. When the spider silk was placed against a material, melting it would act a bit like welding it to that material. The team managed to weld spider silk to metal, glass, and even kevlar. Stress tests showed that the weld was roughly equivalent in strength to the spider silk itself. In one case, they even welded some silk to four corners of a tiny mirror, then welded it to a square frame, allowing them to suspend the mirror using nothing but spider silk.

The authors speculate on all sorts of potential applications for fine-tooled spider silk. Of course, those all depend on our ability to produce lots of silk threads, which is still an unsolved problem. But they also suggest we should look at the prospects for using the same approach to craft structures from a variety of other biomaterials—shells, exoskeletons, hair, and fur—that are far easier to obtain in bulk. As they suggest, applying this approach to these materials is “worth exploring.”

Source: Ars Technica