No motor, no gears: Princeton engineers build robot inspired by ketchup bottle cap

Princeton engineers have built a small robot that rolls, crawls and changes shape using geometry and magnets instead of motors. The work shows how curved structures inspired by a ketchup bottle cap can create multiple stable states for simpler machines. Kevin Liu displays a robot that uses geometry to crawl, roll and shapeshift without internal controls or engine. (Image: engineering.princeton.edu/ photo by Aaron Nathans)What if a robot could move and change shape without carrying a motor or a complicated mechanical system inside it? Engineers at Princeton University have developed a new approach that could make this possible by using geometry to control movement.A team led by Princeton engineer Glaucio Paulino discovered that curved, folded structures can naturally settle into multiple stable shapes. Using this principle, the researchers built a small robot that can roll, crawl and change shape using magnets, without a conventional motor. The research was published in Proceedings of the National Academy of Sciences on June 15, 2026.HOW THE RESEARCHERS MADE THE DISCOVERYThe work began with a simple question about the familiar flip-top cap found on ketchup bottles: why does the cap snap open and remain open instead of simply falling back?The answer lies in the cap's curved hinge. A thin, flexible shell is connected to a thicker, rigid section along a curved edge. This creates two stable positions, or states, separated by an energy barrier. The cap therefore remains open until enough force is applied to make it switch back. Princeton researchers wanted to know whether the same principle could be used to create structures with more than two stable configurations.They turned to curved-crease origami, a branch of origami that studies structures formed using curved rather than straight folds. The team developed mathematical rules to predict how these curved shells would behave.RESEARCHERS FOUND SIX STABLE SHAPES Their initial mathematical analysis suggested that fixing a curved edge of a shell would allow two configurations without stretching the surface.But physical prototypes made using laser cutting and 3D printing produced a surprise.The structures could settle into six or more stable configurations, rather than just two. The researchers found that the structures naturally concentrated deformation into a thin band, effectively creating a new crease. They called this a pseudocrease.The team then incorporated these pseudocreases into its mathematical models and simulations, allowing it to better understand and reproduce the additional stable states.FROM GEOMETRY TO A MOVING ROBOTThe researchers used the principle to design a small robot whose shape and movement are controlled by magnets.Instead of using motors, gears or complicated internal controls, the robot exploits its different stable configurations. By changing its shape, it can perform different movements, including rolling and crawling.The design also allows multiple robots to operate under a common magnetic field while retaining independent active and inactive states.“Geometry is the real actuator here,” Paulino said, explaining that mathematical principles can be used to build multiple stable configurations directly into a structure.The significance of the research goes beyond a motorless robot. The researchers say their approach does not depend on specialised manufacturing or unusual materials. Instead, the behaviour comes from geometry and can potentially be applied to different materials, sizes and surface shapes.The team has already explored other possible uses, including reconfigurable architecture, snapping boxes and electrical switches.The broader idea is to make structures that can change their configuration without requiring a separate locking mechanism or a complex collection of moving parts.The researchers say the same mathematical principles connect seemingly unrelated things — from curved-crease origami and everyday bottle caps to robotic movement and deployable structures.For robotics, the approach could offer a route towards lighter and simpler machines in which the structure itself performs part of the work normally handled by mechanical components.- EndsPublished By: Mridusmita DekaPublished On: Aug 28, 2026 11:00 IST