Snap-through effect helps engineers solve soft material motion trade-off

Everyday occurrences like snapping hair clips or clicking retractable pens feature a mechanical phenomenon known as “snap-through.” Small insects and plants like the Venus flytrap cleverly use this snap-through effect to amplify their limited physical force, rapidly releasing stored elastic energy for swift, powerful movements.

Inspired by this natural mechanism, researchers from Hanyang University have developed a polymer-based jumper capable of both vertical and directional leaps, triggered simply by uniform ultraviolet (UV) light irradiation.

Published in Science Advances, this study tackles a classic engineering dilemma: how to make soft materials produce strong, rapid motions.

Much like pulling back a bow to launch an arrow, material needs to bend and store energy before releasing it to produce a sudden, powerful motion. Too much stiffness limits the bending, whereas too much softness weakens the stored force.

Led by Jeong Jae (JJ) Wie, Professor at Hanyang University, the team cleverly addressed this “trade-off” by introducing patterns of varying stiffness within a single polymer film, combining the strengths of both stiff and soft materials.

The key to achieving rapid, powerful jumping motion lies in snap-through, a phenomenon driven by nonlinear transitions in bistable structures. Imagine bending an elastic strip or string by slowly bringing its ends closer together. Initially, it curves smoothly upward, but suddenly, it snaps into a completely reversed curvature.

This rapid transition converts stored elastic energy into kinetic energy, propelling the material upward as it pushes against the ground, following Newton’s third law of action and reaction.

Translating this snap-through mechanism into controlled motion, the team engineered stiffness variations into a jumper made of liquid crystalline polymer networks. Soft regions allowed the material to bend easily, while rigid regions stored and released elastic energy efficiently.

By placing a rigid area at one corner, the polymer jumper achieved precise directional jumps and rapid spins. Conventionally, controlling direction of motion in polymeric soft robots requires external cues, like angled light or geometric tweaks.

In this study, however, the researchers used patterned stiffness variation to build in asymmetry. This strategy allowed the material to bend unevenly under uniform light and release energy in one direction, creating rapid spins.

Meanwhile, placing a rigid area at the center enabled record-setting vertical jumps, reaching 49 mm, or about 25 times the jumper’s length. The stiff center acted as a solid anchor, forcefully pushing against the ground, while the surrounding soft regions bent more easily to build up curvature and store energy.

This combination allowed for stronger energy accumulation and subsequent release, overcoming the typical trade-off between stiffness and flexibility to achieve a powerful vertical leap.

Further expanding their design, the team extended the jumper’s body length and adapted a soft-rigid alternating pattern to achieve a “dual-mode” jumper.

Depending on the deformed shape, this single polymer film could either jump vertically or directionally, demonstrating remarkable versatility. When the film was rolled into a tighter curve, it acted like a finger flick, snapping upward for a strong vertical leap.

In contrast, a more gently arched, elongated shape allowed the snapping motion to travel gradually from one end to the other, resulting in directional movement.

Despite its greater body mass, this dual-mode jumper performed on par with single-mode jumpers specialized for vertical or directional leaps. It even demonstrated rapid switching between the two jumping modes in a continuous sequence, which is a behavior rarely achieved in polymer-based jumping soft robots.

Looking ahead, this team’s design strategy could lead to soft actuators that not only move with precision but also deliver powerful, amplified motion.