Y-zipper: A Variable Stiffness Mechanism for Robotic Locomotion and Manipulation

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A patent for a zipper that was rejected 40 years ago has become the skeleton of a robot.

Recently, while scrolling through short videos, I saw a robot with a very unique visual style.

It doesn't have traditional mechanical joints; the body of its four legs is actually a modified

three-sided zipper.

The motor pushes the slider to engage and lock, causing the legs to gradually tighten, harden, and extend to lift the body; when the slider slides in the opposite direction and the teeth disengage, the stiffness instantly decreases, and the legs soften and shorten.

It can easily adjust to uneven terrain and low, narrow spaces, effortlessly crossing or crawling through them.

At first glance, I thought it was a hobbyist's fun craft project, but after looking through some resources out of curiosity, I found out it has a big background.

It's called Y-zipper, an official study included in CHI 2026, developed by the MIT Computer Science and Artificial Intelligence Laboratory (MIT CSAIL).

This clever idea was actually a concept that had been dormant for exactly forty years, until this year it finally came to life.

01.

In 1985, a strange zipper that was rejected by the industry

Time goes back to 1985.

Bill Freeman was then an electrical engineer at Polaroid, later becoming a member of the US National Academy of Engineering. The image algorithms in our phones and the underlying technology for the first image of a black hole are all from his research.

At that time, during a creative contest, everyone was thinking about better-selling consumer products, but he asked two questions that defied common sense: why can a zipper only be two-sided? Can we make a three-sided zipper?

He envisioned a three-sided zipper as three independent flexible strips. When it is spread out, it is soft, bendable, androllable; when the slider is pushed, the three strips' teeth synchronize to engage and lock into a triangular hollow rigid rod. Even, he used wooden strips and belts, manually cut and polished,

to make the first prototype, proving the principle was entirely feasible.

However, this proposal was directly rejected by the design company. Nevertheless, Bill Freeman applied for a patent for the prototype, and the related documents were stored in his garage.

Nearly forty years later, as a professor at MIT, Freeman contacted Li Jiaji, who was researching variable stiffness structures,

（Jiaji Li）

and handed this long-forgotten three-sided zipper concept back to this team.

The first author of the study, Li Jiaji's Xiaohongshu profile.

The paper's authors also include researchers from Tianjin University, the University of Munich, and

MIT CSAIL

several other institutions.

02.

What exactly is Y-zipper?

Upon receiving this long-forgotten concept, the team advanced at an astonishing pace!

The first prototype was completed in three days with all 3D printing; the entire project was completed in four months

. This once-rejected idea, after being officially published at CHI 2026, even attracted attention from brands like YKK and Rimowa.

Y-zipper working process: first 3D print three zipper tapes, then assemble them into a Y-shaped zipper, using a Y-shaped zipper slider to switch between flexible and rigid states.

Many people at first glance might think, isn't it just a zipper with an extra row of teeth?

Far from that simple. The core of Y-zipper is a reversible, controllable flexible-rigid switching mechanism. The paper's experimental results show that after closing, the bending stiffness increases by 160 times

The entire structure is composed of three toothed tapes, a dedicated bidirectional slider, and ball-and-socket interlocking nodes. Each zipper tape consists of teeth, a flexible bridge, a ball-shaped node + ball socket, and a guide rail.

In the open state, the TPU or fabric flexible bridge remains relaxed, the three tapes are not constrained, and can be freely twisted, rolled, and stored; in the locked state, the slider gathers the three tapes, the wavy teeth interlock, and the ball-shaped nodes lock into the grooves, preventing lateral sliding. The three tapes form a stable triangular prism, with pressure distributed among the three faces, instantly becoming a load-bearing rod.

The slider is the soul of the entire design, with a dual-module for gathering and separating, featuring a rotating lock. When engaged, it locks in order, and when reversed, it smoothly separates, ensuring smooth opening and closing without jamming teeth.

In addition, the team developed three driving schemes: manual push-pull, dynamic driving with a motor moving the slider, and static driving with a fixed motor feeding the material. It can be adapted for small wearable wrist supports and large tent frames up to several meters in size.

Y-zipper driving methods: (a) manual slider; (b) dynamic driver with a motor gear set; (c) static driver with three motors, gears, and springs.

They also defined four basic motion primitives, linear, bending, spiral, and curling. You can select the desired shape and input parameters in the Grasshopper design tool, and the software automatically generates the zipper structure, directly outputting printable files. Long structures are automatically segmented and joints are reserved for connections.

In other words, it is not a fixed-size part, but a programmable language of deformable structures.

Y-zipper's four basic motion primitives: linear motion, bending motion, curling motion, and spiral motion.

Design for connectors to link multiple Y-zippers.

03.

Beyond robotic legs, the application imagination space continues to expand

This out-of-the-box four-legged prototype is just Y-zipper's most direct answer.

Each of its legs is an independent 360mm Y-zipper rod. N20 motor drives the slider, hip reel stores the excess flexible strip.

Zipping the zipper extends the leg, raising the robot's body to step over a stone; unzipping the zipper makes the rod soft and contract, lowering the overall height to crawl through narrow gaps.

A four-legged robot with active adjustable leg length realized by Y-zipper: low and high configuration, single-leg module, low gap crawling and obstacle crossing.

Compared to industry mainstream solutions, its advantages are very prominent.

Self-locking and shape retention, no need for continuous power supply to maintain stiffness. Inflatable soft bodies require continuous air supply, and granular blockage relies on continuous negative pressure; once locked, the shape remains stable and stands firmly;

Switching is reversible and controllable. A single slider movement completes the soft-hard switch, compared to origami and shape memory materials, the control freedom is significantly higher;

The retracted shape is extremely compact, retracted as a soft strip, with full portability;

Modular assembly, connector interconnection, not only single robot legs, but also capable of building large-scale reconfigurable trusses.

Of course, it also has its shortcomings. The current 3D printed plastic version has limited load capacity, fatigue life only about ten thousand cycles, still far from industrial-grade robots for continuous operation.

However, Its application imagination space has already exploded.

TFC rehabilitation wrist support, daily soft and conforming to wrist free movement. When sleeping, zip and lock to become a rigid protective gear, blocking excessive stretching and impact, single-handed operation, no need to repeatedly remove.

There is also a dynamic device simulating vine growth, as well as a high-speed tent, which can be assembled in 1 minute and 20 seconds, disassembled in 40 seconds, and can be directly shrunk into a soft strip for packing and taking away.

Dynamic art installation using Y-zipper: three zipper strips pulled together form a continuously growing vine, its flowers open during the extension process.

Y-zipper tent assembly: (a) initial state; (b) completed tent structure; (c) single-person manual closing; (d) dynamic actuator closing with four actuators in parallel operation.

04.

A different approach to the variable stiffness track

These diverse applications all rely on the same core principle: zipper-based variable stiffness.

Adjustable stiffness is a core challenge in the field of deformable robots that has persisted for years.

Traditional rigid robots have strong power and high precision, but are stiff and cold, collisions easily cause self-damage or damage the environment; soft robots are safe and adaptable to complex terrains, but have weak stiffness and are difficult to output powerful operations. Past solutions are either bulky and complex, with long control chains, or cannot achieve fast and reversible switching, with poor generality.

Granular blockage and layer blockage are the most mature routes currently, relying on squeezing medium friction to lock the shape, good at local small range gripping. However, granules are prone to wear, sealing is easy to fail, and it is also difficult to make complex configurations such as spirals and long curves.

Y-zipper has paved a new path by using a ``chain tooth'' structure to freely switch between softness and hardness

It can be extended over long distances step by step, creating a large-scale, deformable framework that can be repeatedly reshaped.

Engineers can directly reuse this zipper unit, combined with parametric design and 3D printing, to rapidly iterate on deformable, easy-to-store structures, which have potential applications in emergency support and outdoor equipment scenarios.

It is not a disruptive black technology for the industry, but it provides a very simple new idea: making deformable robots does not necessarily require stacking hydraulics, pneumatics, and a large number of servomotors. Simply starting from the logic of gear engagement, one can create a low-cost, clean, and replicable variable stiffness skeleton.

In the future, a large number of lightweight deformable robots may grow along this zipper interlocking concept.

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END

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