Conventional pharmaceuticals aren't always the best way to treat an ailment. Drugs are often imprecise, unpredictable, or come along with tricky side effects. Medicine is always trying to move on to more targeted treatments. And soon, robots will be one of those options: small and mobile, they could theoretically deliver pharmaceuticals right where they’re needed, tear through tumors, or rebuild broken bits of your body. Of course, these kinds of treatments are decades away—which might not be a problem, depending on how you feel about maggots.
See, the hitch with robots is getting them to move. For obvious reasons, it’s especially crucial that the borgs creeping through your body aren’t complete klutzes. And as far as modes of transportation go, few are as gentle as the scootch. That’s why mechanical engineer Franck Vernerey started modeling his machines after maggots' squirming movement.
These aren’t your typical robots. Really, at this point, Vernerey's creations aren't even really robots at all: They're just tiny cylinders of hydrogel, a synthetic material that sucks up or spits out water, depending on its temperature. But Vernerey, whose lab is at the University of Colorado Boulder, was able to induce these makeshift medicinal maggots to creep through tubes by cycling them through warm, then cool water.
Maggots move through a combination of two mechanical processes—extending and contracting their bodies. "For motion to happen however, sliding has to be easier in one direction than another," says Vernerey. So, larvae have tiny, rearward facing spikes on their body. When the maggot extends its body, the spikes’ backside brushes over whatever piece of rotten food the maggot is trying to squirm through. When it contracts, the sharp end of the spikes bite into the surface, creating friction, and the creature’s body moves forward.
Vernerey thought to reproduce this motion with hydrogels. They're sort of like rubbery sponges, and his expand or contract depending on their temperature. "The specific hydrogel used in this research displays a dramatic and reversible phase transition at a temperature around 32 ˚C," he says. Below that temperature, the hydrogel becomes hydrophilic, absorbing water. Above that temperature, the reverse happens. So, Vernerey made little cylinders of the stuff, about 3 centimeters long by 1.3 centimeters in diameter. I measured, and that’s about the length and width of my thumb from the tip to the first knuckle. So, imagine a thumb, except blue, somewhat translucent, and gelatinous.
But no spikes. It’s pretty tough to put those on a hydrogel, so Vernerey cheated a little by printing out 3-D tubes with scaly textures on the inside. He made four of these, the largest about two-thirds the diameter of his hydrogel at normal temperature, and the other three smaller by one centimeter increments. He stuffed the hydrogel inside each, and submerged them in a fluid that he could heat and then cool.
If this were a real medical robot, it'd be crawling through flesh, aiming to deliver its medicine, shred apart a tumor, or help heal some tissue. But before it can crawl, it has to ... crawl. So, Vernerey went about testing his proof of concept in tubes of various sizes to see which mechanical forces optimized the hydrogel's ability to squirm forth. At play here are several factors. First, they had to figure out the best angle of the scales inside the tube. Then they found out that longer bits of hydrogel were more efficient crawlers than shorter ones. "This might explain why maggots are longer than they are wide," says Vernerey.
And finally, the shape of the tube itself presents competing forces for movement. First is confinement: In a smaller tube, the hydrogel would have more surface area to grasp against. However, this also inhibits the hydrogel’s ability to extend itself. After repeated trials, he found that a tube that is between 20 and 50 percent of the hydrogel’s diameter was most efficient. Then he used a computer model to generalize his results to smaller scales—after all, a thumb-size robot is much too big to be putting inside anybody’s body.
This all might sound exciting, but remember that Verneney is still just learning how to crawl. Astute readers might point out that the inside of our bodies are not lined with scales, so a hydrogel might have trouble finding a grip. Fabricating hydrogels with scales of their own is still an open question, says Vernerey. He also has to teach these things how to home in on their target, and figure out locomotion itself. He envisions embedding the hydrogels with magnetic nanomaterials, and then exciting them through warm and cool cycles through an EMP field. But maybe you’re still grossed out by the thought of maggot-like robots creeping through your body. Well, the medical grade hydrogels will be micron scale. Out of sight, out of mind, perhaps?