OSCAR: An Ovipositor-Inspired Self-Propelling Capsule Robot for Colonoscopy

TL;DR

OSCAR, an ovipositor-inspired self-propelling capsule robot that translates the transport strategy of parasitic wasps into a propulsion mechanism for colonoscopy, is presented, enabling safer and more robust self-propelling locomotion for robotic capsule colonoscopy.

Abstract

Self-propelling robotic capsules eliminate shaft looping of conventional colonoscopy, reducing patient discomfort. However, reliably moving within the slippery, viscoelastic environment of the colon remains a significant challenge. We present OSCAR, an ovipositor-inspired self-propelling capsule robot that translates the transport strategy of parasitic wasps into a propulsion mechanism for colonoscopy. OSCAR mechanically encodes the ovipositor-inspired motion pattern through a spring-loaded cam system that drives twelve circumferential sliders in a coordinated, phase-shifted sequence. By tuning the motion profile to maximize the retract phase relative to the advance phase, the capsule creates a controlled friction anisotropy at the interface that generates net forward thrust. We developed an analytical model incorporating a Kelvin-Voigt formulation to capture the viscoelastic stick--slip interactions between the sliders and the tissue, linking the asymmetry between advance and retract phase durations to mean thrust, and slider-reversal synchronization to thrust stability. Comprehensive force characterization experiments in ex-vivo porcine colon revealed a mean steady-state traction force of 0.85 N, closely matching the model. Furthermore, experiments confirmed that thrust generation is speed-independent and scales linearly with the phase asymmetry, in agreement with theoretical predictions, underscoring the capsule's predictable performance and scalability. In locomotion validation experiments, OSCAR demonstrated robust performance, achieving an average speed of 3.08 mm/s, a velocity sufficient to match the cecal intubation times of conventional colonoscopy. By coupling phase-encoded friction anisotropy with a predictive model, OSCAR delivers controllable thrust generation at low normal loads, enabling safer and more robust self-propelling locomotion for robotic capsule colonoscopy.

Figures (11)

• Figure 1: Ovipositor-Inspired Capsule Robot for Colonoscopy (OSCAR). (a) Parasitic wasps use reciprocating sliding valves that alternately anchor and release to transport eggs. (b) OSCAR translates this principle into a compact capsule with reciprocating sliders that generate self-propelling motion.
• Figure 2: Bio-inspiration and conceptual locomotion principle of OSCAR. (a) Schematic representation of a parasitic wasp transporting eggs through coordinated stick–slip interactions generated by three sliding valves in their ovipositor. During each cycle, one valve retracts while the other two remain stationary, holding the egg in place because of the higher friction. When all valves reach their retract position, they advance together, producing a net forward displacement of the egg. Repeating this sequence enables controlled transport of the egg through the lumen till its deposition. (b) OSCAR translates and extends this principle into a capsule architecture using circumferential sliders driven by a rotating cam profile. Unlike the biological mechanism, where one valve moves at a time while the rest remain stationary, OSCAR drives a slider forward while driving the remaining sliders in the backward during each stroke. This continuous, phase-shifted reciprocation can yield a steady and continuous steadier average forward thrust suitable for self-propelling locomotion in colonoscopy.
• Figure 3: Design and prototype of the OSCAR capsule robot. (a) The design of OSCAR consists of 12 spring-loaded sliders distributed evenly around the circumference of the capsule body and driven by a central cam mechanism to realize the bio-inspired motion cycle described in the working principle. Each slider is guided by a cylindrical rod to ensure purely axial motion and is covered by a grooved slider cap to enhance frictional contact with the mucosa. The cam is mounted concentrically on a motor and enclosed by a back cover. Different cam profiles with varying numbers of motion reversals (jumps) were designed and fabricated to study the influence of motion pattern on thrust. (b) The spring-loaded cam system enables the use of steep cam angles while maintaining continuous contact between the sliders and the cam profile. This configuration allows maximum number of sliders to remain engaged in retraction, maximizing the potential net thrust. During the advance (fall) phase, the spring-loaded force of the advancing slider acting on the cam profile recycles the stored spring energy by generating a torque that assists the motor, thereby reducing the required actuation load. (c) Assembled and disassembled prototypes of OSCAR, showing all key components and the compact integration of the actuation and transmission mechanisms.
• Figure 4: Single--slider kinematics and thrust force generation. (a) Kinematic description of OSCAR’s slider actuation. A cam rotating with angular velocity $\omega$ drives each slider through a prescribed linear trajectory $x(t)$. The cam law for the single--jump profile consists of a slow rise phase (slider retraction), a steep fall phase (slider advance), and a short dwell. The interval asymmetry between rise and fall sets the duty--cycle imbalance that underpins net thrust generation. (b) Thrust force model based on frictional interaction with the colonic wall. During retraction (top), the slider moves backward relative to the capsule, generating positive thrust as the wall traction opposes the slider motion. During advance (bottom), the slider moves forward, producing negative thrust as the wall reaction reverses direction. Superposition of these phase--shifted contributions of all sliders yields the average net thrust force.
• Figure 5: Simulation of kinematics and thrust force of OSCAR. (a) Single-slider displacement, velocity, and instantaneous traction force for 1-, 2-, and 3-jump cam geometries. Each cam produces distinct rise–return sequences corresponding to retracting (green) and advancing (red) phases, with normalized slider velocity given by $\dot{x}_{slider}^*/(\omega h_{\max}) = \pm 1/(2\pi d_{r,a})$. (b) Instantaneous and cycle-averaged capsule thrust force obtained by superposing the phase-shifted contributions of $n=12$ sliders. The upper plots show the total normalized thrust $\tilde{F}_{\mathrm{capsule}}(\theta)$ and its mean value $\bar{\tilde{F}}_{\mathrm{capsule}}$, while the lower plots show the instantaneous number of retracting $n_{ret}(\theta)$ and advancing $n_{adv}(\theta)$ sliders. Increasing the number of jumps compresses the individual stroke, increases switching frequency, and reduces the average thrust, while amplifying ripple due to increased overlapping between slider transitions. Refer to Supplementary Video.1 for a demonstration of the motion cycles of the three cam profiles.