Enhanced ion tolerance of electrokinetic locomotion in polyelectrolyte-coated microswimmer

Over the last decade, researchers have endeavored to mimic the naturally motile microorganisms and develop artificial nano/microswimmers, which propel themselves in aqueous media. However, most of these nano/microswimmers are propelled by the self-electrophoretic mechanism, which has one critical incompetency: the inability to operate in a high concentration electrolyte solution, such as the most important body fluid, blood. This ionic quenching behavior is well backed by the classical Helmholtz–Smoluchowski theory and seems to be an insurmountable challenge which has shadowed the otherwise promising biomedical applications for artificial nano/microswimmers. Here, we propose that the active nano/microswimmer’s self-electrophoresis is fundamentally different from the passive nanoparticle electrophoresis. By significantly increasing the Dukhin number with polyelectrolyte coating and geometry optimization, a favorable deviation from the Helmholtz–Smoluchowski behavior can be realized, and ion tolerance is enhanced by over 100 times for a visible light-powered self-electrophoretic microswimmer. Electrophoretic microswimmers aren’t well suited to function in vivo because electric-field screening reduces their speed in higher ionic-strength solutions. Here the authors propose a microswimmer that exploits high surface conductance to improve this ion tolerance to enable operation in a high salt environment.