On-chip enucleation using an untethered microrobot incorporated with an acoustically oscillating bubble

This paper presents a novel on-chip enucleation method using a magnetically driven microrobot incorporated with an acoustically excited bubble. The proposed microrobot mainly consists of a compressible bubble for the manipulation of a cell and twin neodymium magnets for the motion control of the microrobot in an aqueous medium. First, the two-dimensional (2D) motion control of the microrobot - horizontal, vertical, and rotational motions - is demonstrated by using an external magnetic controller attached beneath the bottom of a chip owing to the interaction forces induced by twin magnets installed inside both the microrobot and controller. Second, the enucleation of a cell is separately investigated using an acoustically oscillating bubble. When a bubble is acoustically excited at its natural frequency, it oscillates and simultaneously generates cavitational microstreaming and radiation forces around it. The flow pattern and strength of the microstreaming in different frequencies and voltages are studied using a microscopic PIV system. The flow strength is proportional to the voltage and strongly dependent on the frequency and maximum at its natural frequency. To investigate the effects of the microstreaming on the inside of a cell through a narrow slit, particle extraction test is conducted using a small PDMS cylinder chamber. It shows that most of the particles initially filled in the chamber are extracted by an acoustically excited bubble and the chamber becomes empty within 160 seconds. Finally, as proof of concept, the embryo extraction from a fish egg (1.7 mm diameter) dyed by methylene blue is successfully achieved using the proposed microrobot. This on-chip enucleation technique may improve the efficiency of enucleation processes with minimizing the contact damage between a cell and a micromanipulation tool.