Robotic Intracellular Manipulation: 3D Navigation and Measurement inside a Single Cell

Magnetic micromanipulation is an untethered technique and has enabled numerous applications in the scale of millimeters to micrometers from the tissue level to cell level. However, existing systems are not capable of maneuvering a sub-micrometer object for precise force control, preventing the realization of intracellular manipulation or 'fantastic voyage' inside a single cell. The magnetic micromanipulation task achieved in this work is sub-micrometer position control and piconewton force control of a sub-micron (0.7 mu m) magnetic bead inside a single human bladder cancer cell (RT4). The magnetic bead was 3D positioned in the cell using a generalized predictive controller that effectively tackled the control challenge caused by the slow visual feedback (1 Hz) from high-resolution confocal microscopy. The average positioning error was quantified to be 0.43 mu m, which is slightly larger than Brownian motion-imposed constraint (0.31 mu m). The system is capable of three-dimensionally applying a maximum force of 60 pN with a resolution of 4 pN. In experiments, a 0.7 mu m magnetic bead was controlled to move from an initial position in a cell to target positions on the cell nucleus. Force-displacement data were obtained from multiple locations along the cell nucleus' major and minor axes. The results revealed, for the first time, significantly higher stiffness exists in the cell nucleus' major axis than the minor axis. This stiffness polarity was likely attributed to the aligned stress fibers of actin filament inside the cells.