Researchers at the University of California have shown that a micromotor fueled by stomach acid can take a bubble-powered ride inside a mouse. These tiny motors, each about one-fifth the width of a human hair, may someday offer a safer and more efficient way to deliver drugs or diagnose tumours. The experiment is the first to show that these micromotors can operate safely in a living animal.
Previously the team have experimented with different designs and fuel systems for micromotors that can travel in water, blood and other body fluids in the lab. But this is the first example of loading and releasing a cargo in vivo. The team felt it was the logical extension of the previous work, to see if these motors might be able to swim in stomach acid.
Stomach acid reacts with the zinc body of the motors to generate a stream of hydrogen microbubbles that propel the motors forward. In their study published in the journal ACS Nano, the researchers report that the motors lodged themselves firmly in the stomach lining of mice. As the zinc motors are dissolved by the acid, they disappear within a few days leaving no toxic chemical traces.
When they loaded up the motors with a test ‘payload’ of gold nanoparticles the team found that more of these particles reached the stomach lining when carried by the motors, compared to when the particles alone were swallowed. The motors delivered 168 nanograms of gold per gram of stomach tissue, compared to the 53.6 nanograms per gram that was delivered through the traditional oral route.
The team state that this initial work verifies that this motor can function in a real animal and is safe to use.
In the experiment, the mice ingested tiny drops of solution containing hundreds of the micromotors. The motors become active as soon as they hit the stomach acid and zoom toward the stomach lining at a speed of 60 micrometers per second. They can self-propel like this for up to 10 minutes.
This propulsive burst improved how well the cone-shaped motors were able to penetrate and stick in the mucous layer covering the stomach wall. It’s the motor that can punch into this viscous layer and stay there, which is an advantage over more passive delivery systems.
The researchers found that nearly four times as many zinc micromotors found their way into the stomach lining compared with platinum-based micromotors, which don’t react with and can’t be fueled by stomach acid.
The team state that it may be possible to add navigation capabilities and other functions to the motors, to increase their targeting potential. Now that the researchers have demonstrated that the motors work in living animals, similar nanomachines soon may find a variety of applications including drug delivery, diagnostics, nanosurgery and biopsies of hard-to-reach tumours.