Spinning-enabled wireless amphibious origami millirobot

Millimeter-scale origami robots offer significant potential for biomedical applications such as disease diagnosis, targeted drug delivery, and minimally invasive surgery. However, existing designs often require separate components for locomotion and functions, increasing complexity. Furthermore, most lack amphibious capabilities (on-ground and in-water locomotion). This limitation hinders adaptability in complex biomedical environments. Rotation-based locomotion (rolling, flipping, spinning) is efficient for both terrestrial and aquatic movement. While effective, it usually necessitates different structural designs for each environment. This work introduces a novel amphibious origami millirobot leveraging rotation-based locomotion for multimodal operation and integrated functionality, addressing the shortcomings of previous designs. The robot integrates multiple functions within a single, simple structure, using the origami's geometry for omnidirectional movement in diverse environments, folding/unfolding for controlled drug delivery, and spinning for cargo transport. This multi-functional design is proposed as a minimally invasive tool for biomedical applications.

Existing literature showcases the potential of wireless millimeter-scale origami robots in biomedical applications. However, a key limitation is the separation of locomotion and functional components, adding to system complexity. Furthermore, most existing robots demonstrate limited locomotion modes and lack amphibious capabilities (movement on land and in water). This paper addresses these shortcomings by introducing a robot that integrates multiple functions within a single structure.

The amphibious millirobot is designed based on the Kresling origami pattern, a triangulated hollow cylinder with high geometrical symmetry (aspect ratio ≈ 1). This symmetry facilitates rolling and flipping locomotion. The tilted triangular panels, resembling propeller blades, generate propulsion when spinning about the longitudinal axis for in-water movement. Magnetic actuation enables wireless control: a magnetic plate attached to the robot allows remote manipulation through a rotating magnetic field. The rotation axis of the magnetic field determines the locomotion mode (rolling, flipping, or spinning). Omnidirectional steering is achieved by altering the rotation axis of the magnetic field. The Kresling's folding/unfolding capability serves as a pumping mechanism for liquid medicine delivery. Two magnetic plates attached to the ends generate torques that fold the robot, releasing the liquid upon unfolding. The monostable design ensures autonomous return to the unfolded state. The spinning motion also facilitates cargo transportation via a suction mechanism: spinning creates a low-pressure area to capture and transport cargo. The robot's locomotion and functionalities were characterized experimentally and computationally. Experiments were conducted on various terrains and in water, including an ex vivo pig stomach to mimic the biomedical environment. Computational fluid dynamics (CFD) simulations were used to analyze the swimming mechanism, particularly the effects of a frontal hole and radial cuts on propulsion efficiency. The magnetic actuation system consisted of 3D Helmholtz coils generating a three-dimensional magnetic field. A permanent magnet was used to actuate folding for liquid medicine delivery. Fabrication involved assembling the Kresling origami with magnetic plates from polypropylene film and a silicone-NdFeB composite.

The Kresling origami millirobot demonstrated effective multimodal locomotion on diverse terrains and in water. Self-adaptive locomotion was observed on unstructured ground, with the robot autonomously switching between rolling and flipping modes to navigate obstacles. In water, the robot achieved a maximum swimming speed of 81.2 mm/s (with modifications including a frontal hole and radial cuts), significantly faster than the unmodified design (66.0 mm/s). CFD simulations revealed that the frontal hole and cuts decreased swimming resistance. The robot successfully performed controlled delivery of liquid medicine through the folding/unfolding pumping mechanism. A spinning-enabled suction mechanism allowed for targeted cargo transportation in both terrestrial and aquatic environments. Amphibious locomotion was demonstrated in an ex vivo pig stomach environment, showing adaptability on complex, uneven surfaces and in viscous fluids mimicking gastric juice. The robot exhibited agile swimming in various trajectories (straight line, 2D ∞ path, 3D spiral path) through magnetic field manipulation. Furthermore, the robot successfully navigated at the air-water interface by trapping an air bubble to adjust its buoyancy.

The results demonstrate a significant advancement in miniaturized amphibious robots. The integration of multiple functionalities within a single structure, combined with adaptable locomotion, overcomes the limitations of previous designs. The self-adaptive locomotion on unstructured ground, efficient swimming, controlled drug delivery, and cargo transport showcase the robot's potential for various applications. The ex vivo experiments validate the robot's functionality in a relevant biomedical environment. The findings contribute to the development of minimally invasive medical devices, capable of navigating complex environments and performing diverse tasks.

This work successfully demonstrated a wireless amphibious origami millirobot with integrated multifunctional capabilities. The robot's adaptable locomotion, controlled liquid delivery, and cargo transport capabilities highlight its potential for minimally invasive biomedical procedures. Future research can focus on integrating additional functionalities (e.g., sensors, tools) and exploring applications in different biomedical settings. Scaling the design to smaller sizes for applications within blood vessels or ureters is also a promising avenue for future development.

The current study used an ex vivo pig stomach model; in vivo experiments are needed to fully assess the robot's performance in a living organism. The magnetic field strength required for actuation might need optimization for specific biomedical environments. The long-term stability and biocompatibility of the materials used need further investigation before clinical applications.