A future populated by tiny nanobots seems like a foregone conclusion. They'll be at work mostly within medicine, blasting cancer cells, scraping the foul shit from our arteries, delivering pharmaceuticals cellular door to cellular door, and really whatever we can imagine. This will happen sooner rather than later because... technology. As you read this, there's a full-on nanorobot race underway between technology heavyweights like General Electric, Synopsys, Northrop Grumman, Siemens, and Hewlett-Packard.
One of the challenges between us and that future is how to power these nanobots. After all, we're talking about machines at the molecular scale—it's not like we can strap batteries on to the things. Approaches to the problem have mostly involved phoretic motion, which can viewed as the motion or transmission of some particle in response to something in its environment. It might be induced by different pressures, temperatures, electrical fields, and even pH.
We can add light to that list now, according to a paper posted to the arXiv preprint server. The paper, which comes courtesy of a team drawn from New York University and South Korea's Sungkyunkwan University and led by physicist Paul Chaikin, describes a self-propulsion scheme used by materials known ascolloids, a general category referring to an insoluble particle or particles suspended within some other substance—imagine tapioca beads, or whatever.
The current work involves suspensions of titanium oxide and hematite, two materials with photocatalytic properties; light makes them do stuff, and creates conditions in which a certain reaction is more likely to happen (the definition of a catalyst). In the case of titanium oxide and hematite, we get two different effects: self-propulsion and attraction among individual particles.
With those two effects, it becomes possible to create a transportation system of sorts, with the ability to target and transport passive microscale materials. Together, the colloids, "are able to "form 'living crystals' which form, break apart, heal and reform," according to the paper.
There's a third material at work in the Chaikin team's set-up: a bath of hydrogen peroxide. This acts as the fuel for the desired reactions, decomposing in the presence of UV light and changing the concentration of dissolved oxygen in the solution. This creates a chemical gradient (imagine a hill) that the colloids respond to. They become attracted to their neighboring colloids and the final result observed by the group was raspberry-like structures of clustered particles.
In the same set of experiments, the team arranged hematite particles along a glass substrate within the same bath of hydrogen peroxide. In the presence of a chemical gradient, they respond with a form of self-propulsion via osmosis (something moving in response to a gradient). This happens collectively, and the result are the mentioned crystals above.
It's possible to tune the overall environment so that the crystallizing effects and the attractive effects become disentangled. This is done by applying a magnetic field. When the researchers removed both the light and the magnetic field from the solution, the crystals "melted." But if the magnetic field was turned back on before the light, the particles didn't attract each other or form crystals, but rather they marched along a single line in the direction of the magnetic field.
So, we're left with a toolkit of sorts from which it should be possible to build systems that do different things, like transport materials (drugs, likely) to specific locations (cancer cells, likely).
"We show that a collection of these particles spontaneously assemble in mobile living crystals, which form, heal, break apart and reform, and could reproduce this with numerical simulations of a simple model," Chaikin et al conclude. "Using a magnetic field to direct the particles with a iron oxide, hematite, component, we show that the collisions are central in the observed formation of the crystals. This self-trapping mechanism is a non-equilibrium effect and points towards the emergence of novel organization principles in non-equilibrium systems."
Particles surrounded by chemical fields isn't exactly the image conjured by "nanobot," but the control is impressive. It's much easier to introduce and target UV light within, say, the human body than it is with pressure or pH or even temperature.
"Shrinking people down to the micron is a classical science-ﬁction premise in which the agents could manipulate tiny objects as in a microscopic factory," the paper notes. "Ultimately, they may be injected in the body and repair disfunctional organs or carry drugs to the appropriate cells. Beyond the limitless imagination of writers, this points towards a challenging question for the scientiﬁc community: how can we design populations of artiﬁcial micro-agents capable of moving autonomously in a controlled fashion while performing complex tasks?"
The answer: Chemistry, of course.