At the current scale, the MR-LF central compartment has an internal volume of 300 mm3 (length: 7.8 mm, diameter: 7 mm) which comprises 17% of the robot’s volume (1725 mm3). In comparison, we estimate that the compartment in a recent ingestible magnetic origami crawler comprises approximately 7% of the robot’s volume \cite{Ze2022}. Critically, in contrast to our work, the cylindrical compartment is not centralized (3.5% on each robot end), has a lower volume (24 mm3, length: 7.6 mm, diameter: 2 mm), and requires fixed-free mounting to preserve actuation. Other recent work demonstrated a soft magnetically-controlled millirobot that carried cargos up to 20 times its body weight and three times its body volume while maintaining its multimodal locomotion \cite{Wu2022}. However, leveraging the soft millirobot design to integrate electronics for ingestible systems remains challenging. In the published work, additional loads were attached to the exterior of the robot and were smaller than a typical modular electronic assembly (approx. volume: 3.6 mm3). In contrast, the centralized compartment of MR-LF enables the integration of modular electronics and functional components into an ingestible crawling robot.
2.3. Effect of geometry on speed
To investigate the effect of localizing flexibility on locomotion speed, experiments were performed using an MR-DF and an MR-LF of equal mass (2.55 g). In each experiment, the robot was placed in a confined channel and actuated by a rotating actuator magnet at a vertical offset ya relative to the robot’s initial position (x = 0, y = 0; Figure \ref{709773}D). Rotation of the actuator magnet (ω = 2 Hz) induced body bending, causing the robot to take steps at the same frequency and move in the +x direction (sign convention in Figure \ref{709773}D). A range of ya values was studied because existing literature reported a relationship between ya and locomotion \cite{Pham2020}.
Results show that the average initial speed (average speed for the first ten steps of locomotion) of MR-LF was faster than the MR-DF control at every ya offset (Figure \ref{709773}F). At ya = 11 cm, the robots had the closest speeds (difference of 3%) and exhibited their fastest average initial speed (MR-DF: 6.61 mm/rev, MR-LF: 6.82 mm/rev). The largest difference (299%) and slowest average initial speed for both designs were at ya = 15 cm (MR-DF: 0.34 mm/rev, MR-LF: 1.37 mm/rev). As anticipated, the closeness in robot speeds is likely due to the comparable foot flexion between the designs (0% difference in minimum, 10% difference in maximum foot flexion). The superior performance of MR-LF, which had an average initial speed of 0.21 to 2.27 mm/rev faster than MR-DF across all ya, may be due to an expected difference in mass distribution between the robots or from the 10% reduction in maximum foot flexion. The closeness in locomotion performance between the MR-DF and MR-LF designs, and the superiority of MR-LF across all ya is exciting because it demonstrates that localizing flexibility yielded a 3-299% increase in speed while also freeing up space for an internal compartment (300 mm3) for functional integration.
In the experiments, the robot’s motion in the +x direction (i.e., away from the actuator magnet) was consistent with prior literature \cite{Steiner2021} to demonstrate the feasibility of locomotion against the attraction forces between the robot and actuator. In practice, robot speed and endurance can be improved by having the robot travel toward the actuator magnet and actively modulating the separation between the actuator and robot.
2.4. Effect of robot mass on speed
To investigate how the increased mass of functional components and payloads within the compartment affects locomotion speed, experiments were performed using MR-LF with varying mass (2.55, 2.87, 3.5, 4.43 g). Comparison between plots shows that, in general, increasing MR-LF mass increases the initial speed at smaller ya values and lowers the initial speed at larger ya values within the studied ya range (Figure \ref{402200}A). At the smallest offset (ya = 9 cm), the heaviest MR-LF (4.43 g) exhibited the fastest average initial speed (9.01 mm/rev), while the other MR-LFs were unable to exhibit sustained locomotion (discussed in the next section). Conversely, at the largest offset (ya = 15 cm), the heaviest MR-LF (4.43 g) exhibited low average speed (0.02 mm/rev), while the 2.55 g and 2.87 g MR-LFs had average speeds greater than 1.0 mm/rev. Our results have also demonstrated that MR-LF was able to achive locomotion even with a mass greater than existing capsule endoscopy devices (PillCam™ SB 3: 3.0 g \cite{medtronic}, PillCam™ Colon 2: 2.9 g \cite{medtronica}). Indeed, the heaviest robot in our experiments exhibited the fastest average initial speed (9.01 mm/rev), and results show that increasing robot mass can improve locomotion speed and change the ya for the fastest locomotion. To further investigate this mechanism, we study the effect of magnetic field strength on locomotion as described in the next section.