Abstract

Microrobots navigating the human body require biocompatible actuators capable of functioning in physiological fluids. The microalga Euglena gracilis offers precise phototactic control; however, its operational stability in simulated physiological environments remains unproven. Here, we report that a stepwise acclimation process preserves the robotic functionality of E. gracilis in 100% phosphate-buffered saline (PBS), 100% fetal bovine serum (FBS), and a NaCl solution at a concentration equivalent to PBS (137 mM). We compared direct transfer against a graduated adaptation protocol, evaluating morphology, swimming speed, motility rate, and phototaxis. Direct transfer to each medium caused near-total immobilization, whereas stepwise acclimation retained motility. Acclimated cells exhibited size reduction (miniaturization) while maintaining their characteristic eccentricity. These adapted cells sustained a negative phototactic response among the remaining motile population, supporting optical controllability despite reduced swimming speed. These results indicate that stepwise acclimation allows E. gracilis to retain partial motility and phototactic controllability under simulated physiological saline conditions, and that the observed miniaturization and preserved photo-responsiveness may be useful features for future biohybrid microrobotics.

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