cs.ROApr 6, 2026

ZipFold: Modular Actuators for Scaleable Adaptive Robots

Authors: Niklas Hagemann, Daniela Rus

Organizations: Computer Science and Artificial Intelligence Laboratory, MIT, Cambridge, MA 02139 USA

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Abstract

There is a growing need for robots that can change their shape, size and mechanical properties to adapt to evolving tasks and environments. However, current shape-changing systems generally utilize bespoke, system-specific mechanisms that can be difficult to scale, reconfigure or translate from one application to another. This paper introduces a compact, easy-to-fabricate deployable actuator that achieves reversible scale and stiffness transformations through compound folding and zipping of flexible 3D-printed plastic strips into square-section deployable beams. The simple actuation method allows for smooth, continuous transitions between compact (flexible) and expanded (quasi-rigid) states, facilitating diverse shape and stiffness transformations when modules are combined into larger assemblies. The actuator's mechanical performance is characterized and an integrated system involving a four-module adaptive walking robot is demonstrated.

Figures & tables

Fig. 1: Walking robot composed of four ZipFold modules . The robot can expand and contract to adapt to changes in its environment (pictured: expanded, crouching and walking into a confined space).
Fig. 2: Extension of a single ZipFold actuator. Two flexible strips are folded along their longitudinal axis and zipped at their edges to form a square-section deployable beam.
Fig. 3: The square Fold-and-Zip principle. Coupled counter-rotating rollers pull the flat (flexible) zipper strips through two guides, folding and zipping the strips together in a single continuous step and deploying a square-section beam. A simple passive pin ensures reversibility by unzipping the strips when the rollers are driven in reverse.
Fig. 4: Application areas for ZipFold modules. Modules could be combined to form a variety of adaptive soft-rigid systems: (a) a deployable, impact-resistant robot that starts off soft, (b) a soft adaptive (e.g. aquatic) robot, (c) adjustable joints within a hybrid soft/rigid robot or soft exoskeleton, (d) scaled-down modules as actuators within a smart articulated surface.
Fig. 5: ZipFold module design and fabrication. The ZipFold actuator makes use of the Fold-and-Zip principle illustrated in Figure 3 and is quick to assemble from minimal components (a single geared DC motor, 3D printed gears and enclosure, a steel dowel pin and rubber rollers). The zipper strips are designed with oblique inwards-facing zipper features and a central hinge. They can be printed flat on a desktop 3D printer (e.g. Bambu X1, with a 0.4 mm nozzle) from PLA plastic.
Fig. 6: Control architecture for an n-module robot
Fig. 7: Overview of the four-module robot walker and crawl gait sequence. Four ZipFold modules are mounted to a simple central chassis. The flexible zipper strips are ’collected’ in an ad-hoc fashion at the top of the robot chassis. For locomotion, a simple ’crawl’ gait is implemented. Each ‘step’ involves retracting an actuator, tilting the servo motor forward, and re-extending the zipper to a new position on the ground. ‘Push’ involves all modules pushing backwards.
Fig. 8: Experiments with the four-module walking robot (a) A basic walk (compare to crawl gait sequence depicted in Figure 7 ), (b) stepping over a small obstacle (four stacked module shells, 4 x 6 x 5 cm), (c) expanding and reaching over an obstacle (15 cm height). A fourth experiment is shown in Figure 1 , with the robot starting in an expanded state and crouching to crawl into a 15 cm pipe.
ZipFold Module  
Footprint (w x d x h)3 x 4 x 2.5cm
Mass, actuator block28g
Mass, two zipper strips (per 30 cm length)4g
Peak axial force at 28 cm extension12N
Current draw, unloaded extension @6V50mA
Extension speed @6V10mm/s
TABLE I: Overview: Metrics for the ZipFold actuator module
Four-module Robot  
Footprint, compact (w x d x h)9 x 10 x 11cm
Mass270g
Standing height, compact11cm
Standing height, expanded32cm
Max reach (standing + one arm extended)55cm
Power draw, upwards expansion2.2W
TABLE II: Overview: Metrics for the four-module walking robot
Fig. 9: Mechanical characterization . (a) Bending-stiffness tests for a single flexible strip and a deployed/zipped beam (a 36x increase in stiffness) (b) peak compressive loads at different deployment lengths, (c) bending stiffness for different deployment lengths.
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