Researchers have been designing a wide variety of self-propelled micromotors, many which operate using an oxygen-bubble propulsion mechanism that requires a high concentration of hydrogen peroxide as fuel, which in turn is hazardous at high concentrations, hindering its usefulness in biomedical applications. In a new study, scientists have created a new type of micromotor that can propel itself through acidic environments using only hydrogen bubbles. At low pH levels, the micromotors can travel at speeds of up to 100 body lengths per second, prompting the colloquial use of microrockets to designate these devices.
Wei Gao, Ayesegul Uygun and Joseph Wang from the University of California, San Diego, published their study on hydrogen-bubble-powered mircorockets in the most recent issue of the Journal of the American Chemical Society. They state that this is the first reported use of chemically-powered microrockets that can be self-propelled without any external fuel. These acid-powered microrockets could expand the scope of nano- and microscale motor applications in extreme environments, such as the human stomach or silicon wet-etching baths. This could lead to targeted drug delivery systems or nano-imaging.
The microrockets in questions are the shape of tiny tubes, measuring about 10 micrometers in length, with diameters of 2 to 5 micrometers, and were fabricated out of common polymer polyaniline using templates, then electro-deposited on a thin layer of zinc on the inner surface. When the rockets are immersed in a highly acidic solution, the zinc loses its electrons, and due to having a more negative redox potential than hydrogen, promotes the production of hydrogen bubbles. Experimentally, zinc produced more bubbles than iron or lead.
The speed of the microrockets seems to increase as the pH of the solution decreases, and the fastest speed of 1,050 micrometers per second was achieved by a 5-micrometer-diameter microrocket at a pH of 0.2. Tests show that the speed decreased about 10 micrometers per second at a pH of 1.3. Researchers noted that these microrockets could be used safely in human stomachs and in some types of human serum. The overall lifetime of the microrockets varies from 10 seconds to 2 minutes, depending on the rate of zinc dissolution.
The microrockets can move around autonomously, but they have been controlled by depositing a magnetic layer on the microrocket’s outer surface, allowing a magnet to guide the device into the right direction. The researchers showed that a microrocket could magnetically capture a polystyrene cargo, transport it on a predetermined path, and then release it.
In acid rich environments, the scope of the micromotors could be further expanded but researchers hope to optimize and improve the microrockets so that they will function in milder settings, as well as extending their lifetime.