A team led by Joseph Wang of the University of California, San Diego, has now developed microrockets that use strong acids—already present in extreme environments such as the stomach—as fuel. The new versions have a tubular zinc core and an outer layer of polyaniline. The zinc reduces acids to produce hydrogen bubbles, which serve as the propellant. The researchers showed that the speed of the acid-fueled rockets depends on solution pH and that by adding layers of titanium and nickel to the outside of the tubes they can control rocket motion with a magnetic field. Wang says his group will be working on extending the lifetime of the rockets, which are currently consumed by strong acid after two minutes.
An acid-propelled microrocket (left) moves at about 500 microns per second in 1 M HCl, leaving a trail of H2 bubbles behind (right). Credit: J. Am. Chem. Soc.
Tubular polyaniline (PANI)/Zn microrockets are described that display effective autonomous motion in extreme acidic environments, without any additional chemical fuel. These acid-driven hydrogen-bubble-propelled microrockets have been electrosynthesized using the conical polycarbonate template. The effective propulsion in acidic media reflects the continuous thrust of hydrogen bubbles generated by the spontaneous redox reaction occurring at the inner Zn surface. The propulsion characteristics of PANI/Zn microrockets in different acids and in human serum are described. The observed speed–pH dependence holds promise for sensitive pH measurements in extreme acidic environments. The new microrockets display an ultrafast propulsion (as high as 100 body lengths/s) along with attractive capabilities including guided movement and directed cargo transport. Such acid-driven microtubular rockets offer considerable potential for diverse biomedical and industrial applications.