Biodegradable magnesium fuel-based Janus micromotors with surfactant induced motion direction reversal

The speed and motion directionality of bubble-propelled micromotors is dependent on bubble lifetime, bubble formation frequency and bubble stabilization. Absence and presence of bubble stabilizing agents should signif-icantly influence speed and propulsion pattern of a micromotor, especially for fast-diffusing molecules like hydrogen. This study demonstrates a fully biodegradable Janus structured micromotor, propelled by hydrogen bubbles generated by the chemical reaction between hydrochloric acid and magnesium. Six different concen-trations of hydrochloric acid and five different concentrations of the surfactant Triton X-100 were tested, which also cover the critical micelle concentration at a pH corresponding to an empty stomach. The Janus micromotor reverses its propulsion direction depending on the availability and concentration of a surfactant. Upon surfactant-free condition, the Janus micromotor is propelled by bubble cavitation, causing the micromotor to be pulled at high speed for short time intervals into the direction of the imploding bubble and thus backwards. In case of available surfactant above the critical micelle concentration, the Janus micromotor is pushed forward by the generated bubbles, which emerge at high frequency and form a bubble trail. The finding of the propulsion direction reversal effect demonstrates the importance to investigate the motion properties of artificial micro -motors in a variety of different environments prior to application, especially with surfactants, since biological media often contain large amounts of surface-active components.

Automated summary

The speed and motion directionality of bubble-propelled micromotors depend on bubble lifetime, bubble formation frequency, and bubble stabilization. This study presents a fully biodegradable Janus structured micromotor driven by hydrogen bubbles. The presence and concentration of a surfactant reversibly change the micromotor's propulsion direction. Without a surfactant, the micromotor is pulled backward by bubble cavitation, while with a surfactant above the critical micelle concentration, it is pushed forward by the generated bubbles.