Anisotropy, topography and non-newtonian properties of cellular interiors probed by helical magnetic nanobots.

Cells are building blocks of living systems. Spatio-temporal mapping of local biophysical changes within cells can lead to novel insights into various biological events. As demonstrated in previous works, successful internalization, controlled manipulation, bio-compatibility, and surface-functionalization capabilities make the helical magnetic nanobots, an ideal candidate for local intracellular measurements. In this work, we focus on both qualitative and quantitative understanding of the mechanical properties of the intracellular medium based on intriguing new observations that emerge in the dynamics of the helical nanobots, driven inside cells. Our studies show that orientational changes in the nanobots can be an important measure of the underlying anisotropy and local topographical confinements in the cell cytoplasm. Inside cells, the orientational differences (from the intended direction fixed by the magnetic drive) can sometimes be as high as 70-80 degrees, significantly higher than those expected for homogeneous Newtonian media. We find that correlating these orientational changes to the corresponding velocities of the nanobots can enable us to sense local confinements and boundaries in the cellular interiors. Also, the hydrodynamic pitch during propulsion significantly depends on the nanobot position inside cells. At times, the pitch can get as high as 700 nm (about 3-4 times higher than the hydrodynamic pitch in a Newtonian medium), showing the presence of local solid-like (elastic) behavior of the cell cytoplasm. Interestingly, the signature of intermittencies in dynamics and backward motion also shows up in the pitch measurements, highlighting the presence of local confinements and topographical variations. These studies demonstrate how the dynamics of the helical nanobots can be utilized to develop novel metrics for spatio-temporal mapping of mechanical variations inside cells.

Supplementary information: The online version contains supplementary material available at 10.1007/s12213-024-00176-x.