Spinning-enabled Wireless Amphibious Origami Millirobot

In plain words

This paper builds a tiny robot out of a folded paper-like cylinder, the kind of pattern used in origami. A small flat magnet is stuck to one end. Nothing is inside the robot: an outside magnetic field spins the magnet, and the robot follows the spin. Depending on how the spinning field is aimed, the robot rolls along the ground, flips over obstacles, or spins like a propeller to swim.

The same shape does more than move. Its hollow body can be squeezed like a pump, pushing out liquid that was stored inside, and its spinning end can pull small objects toward an opening, like a suction cup. The authors want one simple body to handle getting around, carrying things and delivering medicine, which would suit tiny devices inside the body.

TL;DR

A magnetically actuated Kresling-origami cylinder with one magnetic plate, driven by a rotating external field, that rolls and flips on ground, swims by spinning in water and at the air-water surface, pumps liquid by folding and sucks in cargo by spinning. Results are author-reported and come from bench tests and ex vivo pig stomach experiments, not in-vivo or clinical tests.

Problem

Millimeter-scale origami robots are used for biomedical tasks, but existing designs usually need separate parts for locomotion and for their function, and most have limited locomotion modes. The authors state that none of them can move both on ground and in water p. 2.

Rotation-based locomotion (rolling, flipping, spinning) is fast and efficient on ground and in water, but it normally needs a different structure for each environment p. 2. The paper aims to get both from one body, using the geometry of a Kresling origami cylinder p. 2 fig. 1A-B.

Method

Structure. The robot is a Kresling origami, a triangulated hollow cylinder folded from a flat pattern cut from thin polypropylene film p. 17. Its cross-section diameter in the shown prototype is 7.8 mm p. 3. A thin magnetic plate is attached to one hexagonal end p. 3 fig. 1D. The robot is monostable: after the magnetic field is removed it unfolds by itself p. 4-5.

Actuation. A uniform field is generated by three-dimensional Helmholtz coils over a 160 x 120 x 80 mm workspace p. 17. The plate has volume V and in-plane magnetization M. A field B exerts a torque on it, T = V(M x B), which turns the robot so that M lines up with B p. 4. A continuously rotating field therefore produces continuous rotation of the robot p. 4 fig. 1E-G.

Locomotion modes. The mode depends on the axis of field rotation p. 4 fig. 1E-G:

• Rolling on ground: the field rotates in the plane perpendicular to the robot's long axis p. 4 fig. 1E.
• Flipping on ground: the field rotates in a plane parallel to the long axis p. 4 fig. 1F.
• Swimming: the robot spins about its long axis in water, and its tilted triangular panels act like propeller blades p. 3 fig. 1G fig. 3B.
• Steering: changing the rotation axis of the field turns the robot, since M follows B in the rotation plane p. 4 fig. 1H.

Self-adaptive switching. With one fixed field (B = 10 mT, f = 4 Hz) the robot rolls, and when an obstacle blocks it, it switches to flipping without a change to the field, then returns to rolling p. 6 fig. 2A-B. The authors also report climbing spaced stairs, a 30 degree incline, a column array and a cylinder obstacle, each of about robot size p. 6-7 fig. 2C fig. S7.

Jumping. An instant field of strength B pulls the plate around; the jump height and distance depend on B and on the angle theta between the field and the plate's magnetization p. 7 fig. 2D.

Pumping and drug delivery. Two plates are attached to the two hexagonal ends. Their magnetizations M1 and M2 have equal magnitude and are 90 degrees apart p. 8. Their vector sum is the net magnetization M_net. An external field along M_net gives two opposite torques (+T and -T) that fold the robot; the inner cavity shrinks and pushes liquid out through the radial cuts p. 7-8 fig. 2G-H. A needle on one end punctures a dye container at the other end, releasing the stored liquid p. 7 fig. 2G. Repeated fold and unfold cycles release the liquid in controlled doses p. 8.

Swimming and air-water interface. The authors compare a robot with a frontal hole and six radial cuts against one without them p. 10 fig. 3C. The hole takes in water that is thrown out through the cuts, reducing the high-pressure region in front of the robot p. 11 fig. 3E. The robot can also float: a captured air bubble lowers its effective density, and a field perpendicular to the surface releases the bubble p. 12 fig. 3I.

Cargo transport. When the robot spins in front of a cargo, the low pressure at its hole draws the cargo in. The cargo is released by gravity when the hole faces the ground p. 14 fig. 4B.

# Locomotion from the field's rotation axis  [p. 4] [fig. 1E-G]
# Input:  plane_normal (unit vector normal to the field's rotation plane),
#         long_axis (unit vector along the robot body), medium
# Output: locomotion mode

def locomotion_mode(plane_normal, long_axis, medium):
    # Spinning about the long axis in water gives propulsion  [fig. 1G]
    if medium == "water":
        return "swimming"
    # Rotation plane perpendicular to the body axis gives rolling  [fig. 1E]
    # (the 0.99 threshold is an implementation choice, not from the paper)
    if dot(plane_normal, long_axis) > 0.99:
        return "rolling"
    # Rotation plane parallel to the body axis gives flipping  [fig. 1F]
    return "flipping"

# Torque on a magnetized plate in an applied field  [p. 4]
# Input:  volume (scalar), magnetization (3,), field (3,)
# Output: torque (3,)

def plate_torque(volume, magnetization, field):
    # Torque is volume times the cross product of M and B  [p. 4]
    torque = volume * cross(magnetization, field)     # (3,)
    return torque

Note: the paper contains no numbered equations. The torque formula is inline in the text at p. 4, and the pumping torque magnitude is at p. 7.

Hyperparameters

Not stated in the main text or the PDF: the magnetic moment of the plate, the coil current-to-field relation, the field profile over time (in SI Fig. S6), the robot mass or density, and the exact frequency steps in the swim sweep.

Results

Author-reported. Setup: bench tests on ground and in water, and ex vivo pig stomach with a viscous fluid. The only explicit baseline is the same robot without the frontal hole and radial cuts, for swimming p. 10.

The paper reports no numerical cargo size, cargo mass, pumping volume per cycle or number of repeats. Error bars appear in fig. 2F and fig. 3D. The number of trials behind them is not stated in the PDF.

Paper vs code

No official code.

We searched the paper's text, the arXiv abstract page and the GitHub repository search for the title and method words ("amphibious origami millirobot", "Kresling origami millirobot", "Kresling"). No repository could be tied to the authors. The closest hit, PARSES-Lab/kresling-structure (described as "Code for the Kresling origami project", with no license), belongs to an organisation named "PARSES at Northeastern", and we could not confirm a link to the paper's authors, so it was not used.

Pitfalls

• Body length is ambiguous. The paper gives speed in both mm/s and body lengths per second: 81.2 mm/s = 11.9 body lengths/s and 66.0 mm/s = 9.7 body lengths/s. Both ratios imply a body length near 6.8 mm, which differs from the 7.8 mm cross-section diameter p. 3 p. 10. The paper does not state which length it uses, so use one definition consistently when comparing speeds.
• Field definition. The field is given in mT, but the paper does not state how it was measured, where in the workspace, or how it maps to coil current. The field's time profile is in SI Fig. S6, which is not in the PDF p. 6.
• Jump is an impulse. Jumping needs an "instant" field, and its duration is not stated p. 7. The jump height and distance depend strongly on the angle theta, so the single 120 degree number is not the robot's general jump performance fig. 2F.
• Pumping is cyclic and dose-free. The paper shows that cyclic folding releases liquid but gives no volume per cycle, no dose accuracy and no residual volume p. 8 fig. 2G-H. "Controllable dose" is a claim without a number.
• Swimming depends on the hole and cuts. The hole and six cuts raise speed in the paper's tests (81.2 vs 66.0 mm/s) p. 10. The hole comes from a Mylar cap with a 3 mm opening on a 0.05 mm film body p. 17; the effect of fabrication tolerances is not reported.
• Density matching is not described. Swimming trials used a robot density close to water to reduce gravity effects p. 11. How this was achieved is in the SI, not the PDF.
• Air-water floating relies on surface treatment. Floating uses an air bubble held by a hydrophobic outer and hydrophilic inner surface p. 12 p. 17. The treatment details are in the SI.
• Viscosity was tested once. Swimming in the viscous fluid uses a single viscosity (12 mPa·s) and one stomach p. 14. The authors say the effect of viscosity needs more study p. 14.
• Ex vivo, not in vivo. All biomedical demonstrations are ex vivo pig stomach experiments with a surgically extracted organ p. 8 p. 14 p. 18. No in-vivo result is reported.
• Intro claim. "None of them can achieve both on-ground and in-water locomotion" is the authors' statement about the literature p. 2. It is not checked here.

Lineage

• Builds on: Kresling origami (folding of triangulated cylinders) p. 3 (refs 30 and 31: Guest and Pellegrino 1994; Hunt and Ario 2005). Also builds on magnetic actuation of small robots (refs 32 to 35), and on the twist-induced contraction of a foldable shell (ref 38, Novelino et al. 2020) p. 4. The paper gives no arXiv ids for these.
• Compared against: existing millirobots and origami robots in refs 10 to 16 and 39 to 42, described qualitatively p. 2 p. 6. The paper does not run any experiment against another robot.
• Superseded by: not stated.