Robust Autonomous Control of a Magnetic Millirobot in In Vitro Cardiac Flow

Abstract

Untethered magnetic millirobots offer significant potential for minimally invasive cardiac therapies; however, achieving reliable autonomous control in pulsatile cardiac flow remains challenging. This work presents a vision-guided control framework enabling precise autonomous navigation of a magnetic millirobot in an in vitro heart phantom under physiologically relevant flow conditions. The system integrates UNet-based localization, A* path planning, and a sliding mode controller with a disturbance observer (SMC-DOB) designed for multi-coil electromagnetic actuation. Although drag forces are estimated using steady-state CFD simulations, the controller compensates for transient pulsatile disturbances during closed-loop operation. In static fluid, the SMC-DOB achieved sub-millimeter accuracy (root-mean-square error, RMSE = 0.49 mm), outperforming PID and MPC baselines. Under moderate pulsatile flow (7 cm/s peak, 20 cP), it reduced RMSE by 37% and peak error by 2.4$\times$ compared to PID. It further maintained RMSE below 2 mm (0.27 body lengths) under elevated pulsatile flow (10 cm/s peak, 20 cP) and under low-viscosity conditions (4.3 cP, 7 cm/s peak), where baseline controllers exhibited unstable or failed tracking. These results demonstrate robust closed-loop magnetic control under time-varying cardiac flow disturbances and support the feasibility of autonomous millirobot navigation for targeted drug delivery.

Figures (8)

• Figure A1: Conceptual illustration of targeted cardiac drug delivery using magnetically actuated millirobot to treat infective endocarditis.
• Figure A2: (a) Experimental system: (i) Twelve-coil electromagnetic actuation setup surrounding the heart phantom, with a top-mounted camera for vision-based feedback control; (ii) Enlarged view of the heart phantom showing the internal flow circuit, where a peristaltic pump recreates right atrial and ventricular flow; (iii) Magnetic millirobot capsule containing an embedded permanent magnet, a drug chamber, and micro-holes for controlled drug release. (b) Finite element simulations of the electromagnetic system used to characterize magnetic flux density and field gradients under different coil activation patterns.
• Figure B1: (a) Computational mesh of the heart phantom model with two flow inlets and an outlet and (b) velocity magnitude contour plot for 4.3 cP fluid at the plane of robot control (i.e., $z$ = 8 mm, where $z$ = 0 mm is the plane passing through the centerline of the inlets and outlets) and $0.295$ cm/s flow speed at each inlet.
• Figure B2: A* planned trajectory (yellow) within the user-defined canal mask with 5 mm wall clearance and 50% centerline bias. Start (blue star) and end (red star) points are connected through resampled waypoints (magenta), and the green spline indicates the user-selected canal boundary.
• Figure C1: Trajectory tracking under moderate flow conditions (20 cP; 7 cm/s peak speed). Representative video snapshots show the millirobot’s path, color gradient from start ($t_i$) to end ($t_f$), inside the heart phantom using (a) PID and (b) SMC–DOB controllers. (c) Tracking error over time. SMC–DOB maintained a tighter trajectory tracking and reduced error spikes compared to PID, demonstrating improved robustness to flow disturbances.