Microcannons firing nanobullets

Sometimes I read papers that enhance my understanding of how the universe works, and sometimes I read papers about fundamental research leading to promising new technologies. Occasionally though, I read a paper that is just inherently cool. The paper by Fernando Soto, Aida Martin, and friends in ACS Nano, titled "Acoustic Microcannons: Toward Advanced Microballistics" is such a paper.

The grand scheme of this research is developing a tool that can selectively shoot drugs into cells at a microscopic level. This is hard because everything happens really slowly at the  microscopic scale in a liquid, in ways that meter-sized beings who live in air would not necessarily expect. For example, it is impossible for small organisms to move through a fluid using a repetitive motion that looks the same in reverse. For example, the way we move our feet back and forth to walk would not work for a tiny aquatic human, because the forward motion in the first phase of movement would be nullified by backwards motion in the second phase. This is why bacteria use things like rotating flagella to move*. Digressions aside, if you tried to shoot a tiny bullet through a cell wall, it would halt really quickly and diffuse away. Soto, Martin, and collaborators wanted to beat this.

They developed a "microcannon," starting with a thin layer of polycarbonate  plastic studded with small pores, which is a thing you can buy and don't have to make. They deposited graphene oxide onto the inside of pores in polycarbonate using electrochemistry, and then sputtering gold onto the inside of the graphene layer. The polycarbonate could be washed away with acid, leaving free-floating carbon and gold cannon barrels a few microns in size. While they were still in plastic membrane, the cannon pores were filled with a gel (literally gelatin from the supermarket) loaded with micron-sized plastic beads to act as bullets, and the "gunpowder."

The microcannons, loaded with nanobullets before and after firing.

Regular readers of my blog will remember that bubbles are somewhat of an exception to the small+water=slow rule, and that when they collapse it can lead to very fast motion on very small scales. So, the authors of the paper used perfluorocarbon (same structure as a hydrocarbon but with fluorine instead of hydrogen) droplets as a propellant, which they turned into bubbles with an ultrasound-induced phase transition. The bubbles collapse, leading to a pressure wave which drives the nanobullets out of the barrel towards their target**.

Composition and operation of the microcannons.

The authors wanted to characterize how powerful these things were, so they did two relevant tests. First, they embedded the cannons in an agar gel and loaded them with fluorescent beads. They looked at where the beads were before firing the ultrasound trigger at the cannon, and after. It was observed that they penetrated an average of 17 microns through the gel. I don't have much context to gauge whether this is a lot or a little.

The bullets were too fast to record with a microscope camera, so their second test involved recording the motion of the cannon after it fired the bullets. Naively one would expect to be able to calculate the bullet speed with conservation of momentum from knowing the cannon's speed, but momentum isn't conserved in a noisy viscous environment. They modeled the fluid dynamical forces acting on the system, measured that the terminal speed of the cannon was about 2 meters per second, and concluded that the initial speed of the bullets is 42 meters per second or 150 kilometers per hour. Pretty fast, especially for something so small in a draggy environment.

I don't know if this technology will succeed in the authors' goal of localized drug delivery to cells, but I think it's awesome that they made a functioning microscale cannon.

Oh the humanity.

*I recommend reading Life at Low Reynolds Number if this interests you.
**Or just in whatever direction it was pointing, I guess.