Artificial and micro/nano biohybrids have emerged as an excit¬ing branch of research at the interface of materials engineering and biological science. People have found vast potential for ap¬plications ranging from nanomedicine to environmental reme¬diation.1 Among the biohybrids, self-propelled artificial micro¬motors have been extensively investigated in the last few years, showing promise for controlled drug delivery, sensors, environ¬mental remediation, and micromanipulation. However, strate¬gies for developing methods to achieve precise and corporative autonomous directional movement of these nanomachines in an isotropic solution (e.g. without chemical, physical gradient or any form of external manipulation) is not yet achieved. As we advance toward real-world applications, steering of the mo¬tors to a specific destination and with speed regulation will be required.
Here, we report for the first time the design of a novel subma¬rine-like micromotor that is capable of regulating its buoyan¬cy force to achieve reversible, corporative directional vertical motion in centimeter-scale2. Guided by density functional the¬ory (DFT) calculations, we synthesized a composite metal-or¬ganic framework (MOF)-based micromotor system containing a bioactive enzyme as the engine for gas bubble generation and a pH-responsive, hydrophilic/hydrophobic phase-shifting polymer as the gear to tune the micromotor buoyancy force through modulated interaction with the produced gas bubbles. We show that the gas bubbles produced by the micromotor can be reversibly retained/expelled from the micromotors, leading to the buoyancy-controlled ascending or descending vertical motion. Importantly, anti-cancer drug-loaded micromotors showed directional cytotoxicity to the three-dimensional cell cultures, depending on the pH of the cellular environment. We found that such facile and versatile method for exploring novel driving forces for motion manipulation could be further applied to colloidal science and electrochemistry, showing po¬tential as smart cargo transport microsystems that could accom-plish more challenging tasks by exploiting the complex biologi¬cal environment.
