Microrockets Fizz Along
Tiny devices push themselves forward on a stream of hydrogen bubbles
In an acidic solution, a microrocket zips around at speeds of up to 1,050 micrometers a second—not bad for something only about 8 micrometers long, too small to be seen by the human eye. The minuscule cone-shaped devices, created by nanoengineer Joseph Wang and his colleagues at the University of California, San Diego, work best in the kinds of harsh, acidic environments that are usually detrimental to most machines and sensors. As the team reported in the January 18 issue of the Journal of the American Chemical Society, the device is powered by a reaction commonly known in high school chemistry.
Wang and his colleagues fabricate the rockets with a tubular template that has an opening pore that is only 1 to 2 micrometers wide. The template’s inner wall is first coated with a polymer film, then deposited with a layer of zinc. After the microrocket is complete, the template is dissolved away.
When placed in an acidic solution, the zinc undergoes oxidation, meaning it loses electrons to the surrounding fluid. Hydrogen ions in the acid take up the electrons and transform to a gaseous state, producing bubbles on the zinc surface. The bubbles are emitted from the larger end of the conical rocket, thrusting the rocket forward. “Now it can swim by itself,” Wang says. “Every time a bubble detaches from the back surface, it moves the microtubular engine continuously.”
The team can also coat the outer polymer surface of the microrockets with a metal such as nickel, allowing the rockets to be steered with an external magnetic field (such as from an MRI machine) from up to about 20 centimeters away. Using this system, Wang and his colleagues have demonstrated that the microrocket can pick up a small sphere, transport it some distance, then can be made to release the sphere by a rapid change in the direction of the external magnetic field. In later versions, Wang thinks that cargo could be attached chemically or with a link that can be cleaved by exposure to light.
The speed of the rocket is highly dependent on the acidity of the solution it is in, with speed increasing with decreasing pH. This correlation allows the microrockets to be used to visually sense and monitor the pH of the solution. “The thing that is amazing is the more acidic the environment, the better it works, and it’s normally the opposite,” Wang says. “It’s nice that you go to the extreme and you get better performance.”
Wang says the microrockets have potential applications in both biological and industrial environments. “Zinc itself is a nutrient, it’s nontoxic, so there’s no issue of zinc being dissolved in the body,” he notes. The team has tested the devices in human serum, where the rockets move at a somewhat slower pace due to the higher viscosity of the fluid. The rockets eventually might be used to monitor stomach pH or deliver a medication. They could also keep track of acidity in the harsh etching baths used in the manufacture of semiconductors for computer electronics, or they might monitor the environment for acid rain. Even some carbonated beverages are acidic and need to be checked during production, so the potential applications could range widely.
Right now the zinc in the microrockets lasts only a couple of minutes at most before the oxidation reaction dissolves it away completely. That’s still enough time for the rocket to do some useful work, such as take a biopsy or record an image, but Wang and his group are hoping to develop ways to expand the operation lifetime. “We are all dreaming that in the future, we’ll be able to use a biological fuel like glucose in the blood,” he says.
Research into microrockets such as these has only been going on for a couple of years, and previous versions have all needed some kind of additional “fuel” (primarily hydrogen peroxide) in order to function. “In this case we demonstrated for the first time the ability of a microrocket to swim in its own environment, without needing the addition of external fuel,” Wang says.
“We made the breakthrough by eliminating the fuel requirement at all. This hydrogen-zinc reaction is very old, but nobody was implementing it into microrockets. We used the simple chemistry just to be able to connect the dots and put the pieces together.