We also demonstrate that the centralized compartment does not compromise the robot’s bidirectional locomotion in water (Movie S7) and when moving between water and air environments, even on a slope with an angle of 8° (Figure \ref{570124}C-D, Movie S5 and S8). Bidirectional motion is critically useful in a confined region where reversing direction by turning in place is challenging such as in the lumens of the human body. In addition, the MR-LF design is able to overcome obstacles (Movie S9) and push obstacles in the lateral direction (Movie S10) as may be necessary in confined and complex environments.
We also demonstrate that the MR-LF rigid compartment can be entirely replaced with a soft material (“MR-LF-S”) without a loss of locomotion characteristics (details in Supporting Information). The design of MR-LF-S can incorporate an internal lumen, similar to what was exhibited for an MR-DF design in a previous study \cite{Pham2020}. Indeed, tethering a continuum device (e.g., catheter) to the MR-LF-S internal lumen can enable novel treatment and diagnosis in a highly-confined region. For instance, MR-LF-S can guide and transport diagnostic tools (e.g., endoscope) or enable local delivery of drugs (Figure \ref{570124}D, Movie S8). Further, the entirely soft compartment suggests the future possible integration with prior works in 3D printing of soft magnetic materials to create an entirely-soft robot \cite{Kim2018,Wu2020}.
Lastly, we demonstrate MR-LF’s ability to navigate turns and bifurcations in confined channels (Figure \ref{570124}E-F and Movie S11-S12). The turning is achieved by controlling the position and orientation of the actuator magnet in the x-z plane at ya = 11 cm such that the rotation axis is approximately perpendicular to the robot’s path. Importantly, MR-LF is also capable of bidirectional locomotion in a confined lumen where reversing direction by turning in place is challenging. Specifically, as shown in Figure \ref{570124}E, the robot starts at location 1 and moves forward around the 180° bend. At location 2, the actuator magnet rotation direction is reversed such that the robot reverses direction and moves backward around the bend to location 1. Similarly, in Figure \ref{570124}F, the robot moves forward from location 1 to 2, backward to location 1, then forward through the other side of the bifurcation to location 3. As demonstrated in the added data, the turning and bidirectional locomotion capabilities can be combined to enhance the navigation ability of MR-LF. Indeed, building on these demonstrations, we anticipate that more complicated robot trajectories could be achieved by controlling the actuator magnet’s position, orientation, and rotation direction.