To create an ingestible bidirectional robot with a centralized compartment for functional integration, we build upon the design of recent work by Abbot et al. and Leang et al., where they demonstrated soft endoluminal robots with impressive bidirectional locomotion in confined lumens \cite{Pham2018a,Pham2020,Steiner2021}. The robots were actuated by a rotating permanent magnet above the robot’s path. The rotating nonuniform (dipole) magnetic field rotates the robot’s feet and flexes the body to generate a periodic gait. The actuator magnet rotation direction dictated the robot locomotion direction \cite{Pham2018a}. However, the magnetic robot in the previous work does not have a centralized compartment for functional integration (hereafter referred to as "magnetic robot with distributed flexibility" [MR-DF]) as its gait motion relies on the flexibility of its entire body.
Here, we create a centralized compartment by localizing the body flexibility of the soft robot and demonstrate the MR-LF design preserves the bidirectional locomotion characteristics and ingestible form factor (Figure \ref{709773}). Specifically, we modified the robot geometry to localize bending to small regions, or flexures, near each foot to convert the remaining central body length into a compartment for functional integration. We actuate the robot using a cylindrical permanent magnet at a fixed position rotating at an angular velocity ω (Figure \ref{709773}D). The locomotion of MR-DF was shown to primarily depend on body flexibility and foot rotation induced by the magnetic field of the actuator magnet \cite{Steiner2021}. Thus, to preserve locomotion, the MR-LF flexure geometry was designed to yield the same foot flexion angle as the MR-DF control under an applied torque. The length of the MR-LF flexure was selected as 2 mm to provide sufficient length for bending while preventing contact between the foot and compartment, and the diameter was calculated to be 3.6 mm (details in Experimental Section).