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arxiv: 2604.05260 · v2 · submitted 2026-04-06 · 💻 cs.RO · cond-mat.soft· cs.HC

ZipFold: Modular Actuators for Scaleable Adaptive Robots

Niklas Hagemann , Daniela Rus This is my paper

Pith reviewed 2026-05-10 18:38 UTC · model grok-4.3

classification 💻 cs.RO cond-mat.softcs.HC
keywords ZipFold actuatormodular deployable beamsshape-changing robotsstiffness transformationfolding and zipping3D-printed actuatorsadaptive roboticsreversible expansion
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The pith

ZipFold actuators use compound folding and zipping of 3D-printed strips to create beams that switch between flexible and quasi-rigid states for modular robots.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper introduces a compact actuator built from folding and zipping flexible 3D-printed plastic strips into square-section beams. These units transition smoothly and continuously between a compact flexible condition and an expanded quasi-rigid condition through simple actuation. When several modules are assembled, the system supports robots that alter overall shape, size, and stiffness to match changing tasks and surroundings. The work characterizes the mechanical behavior and shows the approach in a four-module walking robot.

Core claim

The ZipFold actuator 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.

What carries the argument

The compound folding and zipping of 3D-printed strips that forms square-section deployable beams capable of reversible expansion and stiffening.

If this is right

  • Multiple modules can be assembled into robots capable of continuous shape and stiffness adaptation.
  • Shape-changing systems become easier to scale and reconfigure across different applications.
  • The actuator supports smooth control of transitions between flexible and rigid states.
  • Practical integration is shown by the four-module walking robot demonstration.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Robots using these modules could adjust rigidity on the fly to handle varied surfaces or loads without redesign.
  • Because the parts are 3D-printed, the approach may allow rapid customization of module sizes for specific tasks.
  • Combining the actuators with embedded sensors could enable closed-loop adaptation to environmental changes.

Load-bearing premise

Repeated folding, zipping, and unzipping of the 3D-printed strips will preserve consistent stiffness, deployment force, and structural integrity over many cycles without fatigue or slippage.

What would settle it

A durability test in which the beams lose more than a specified percentage of their expanded stiffness or fail to reach full deployment after a fixed number of actuation cycles would disprove reliable long-term reversibility.

Figures

Figures reproduced from arXiv: 2604.05260 by Daniela Rus, Niklas Hagemann.

Figure 1
Figure 1. Figure 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) [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 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. to combine and integrate into a range of deployable and shape- or stiffness-adaptive robotic systems. To address this gap, we here propose a new class of compact deployable actuator that can enable reversible shape- and stiffness￾change thr… view at source ↗
Figure 3
Figure 3. Figure 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 [PITH_FULL_IMAGE:figures/full_fig_p002_3.png] view at source ↗
Figure 4
Figure 4. Figure 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 exoskele￾ton, (d) scaled-down modules as actuators within a smart articulated surface. compact storage, but helically interloc… view at source ↗
Figure 5
Figure 5. Figure 5: ZipFold module design and fabrication. The ZipFold actuator makes use of the Fold-and-Zip principle illustrated in [PITH_FULL_IMAGE:figures/full_fig_p003_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Control architecture for an n-module robot [PITH_FULL_IMAGE:figures/full_fig_p003_6.png] view at source ↗
Figure 7
Figure 7. Figure 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… view at source ↗
Figure 8
Figure 8. Figure 8: Experiments with the four-module walking robot (a) A basic walk (compare to crawl gait sequence depicted in [PITH_FULL_IMAGE:figures/full_fig_p004_8.png] view at source ↗
Figure 9
Figure 9. Figure 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. B. Mechanical characterization a) Compressive strength: As expected, the compressive strength of the zipper rapidly decreases as it is extended, more o… view at source ↗
read the original 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper introduces ZipFold, a compact modular actuator fabricated from flexible 3D-printed plastic strips that uses compound folding and zipping to form square-section deployable beams. It claims this enables smooth, continuous, reversible transitions between a compact flexible state and an expanded quasi-rigid state, supporting diverse shape and stiffness transformations in larger assemblies. Mechanical performance is stated to be characterized, and the approach is demonstrated via a four-module adaptive walking robot.

Significance. If the performance claims hold with supporting data, the modular, easy-to-fabricate design could provide a scalable alternative to bespoke shape-changing mechanisms in adaptive robotics, facilitating reconfigurable systems. The integration into a multi-module walker demonstrates practical assembly potential.

major comments (3)
  1. [Characterization section] Characterization section (referenced in abstract): The mechanical performance characterization is described but supplies no quantitative metrics such as force-displacement curves, measured stiffness values, expansion ratios, transition forces, or error bars. Without these, the central claims of smooth continuous transitions and quasi-rigid behavior cannot be evaluated.
  2. [Walking robot demonstration section] Walking robot demonstration section: The four-module walker is presented as an integrated system, yet no performance data (e.g., walking speed, load capacity, transition cycle times, or stability metrics) are reported. This leaves the claim that modules facilitate diverse transformations unsupported by evidence.
  3. [Reversibility and durability discussion] Reversibility and durability discussion: The weakest assumption—that repeated folding, zipping, and unzipping maintains consistent geometry, friction, and structural integrity—is not addressed with high-cycle fatigue tests (e.g., stiffness retention or slippage after hundreds of cycles). This is load-bearing for the reversibility claim.
minor comments (2)
  1. [Figures] Figure captions and labels could more explicitly link visual results to the claimed smooth transitions and quasi-rigid states.
  2. [Introduction] The abstract and introduction would benefit from a brief comparison table to existing deployable actuators to clarify the novelty of the zipping approach.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and constructive suggestions. We will revise the manuscript to provide the requested quantitative data and address the concerns regarding characterization, demonstration performance, and durability.

read point-by-point responses

  1. Referee: [Characterization section] Characterization section (referenced in abstract): The mechanical performance characterization is described but supplies no quantitative metrics such as force-displacement curves, measured stiffness values, expansion ratios, transition forces, or error bars. Without these, the central claims of smooth continuous transitions and quasi-rigid behavior cannot be evaluated.

    Authors: The referee is correct that the current version lacks explicit quantitative metrics in the characterization section. While the manuscript includes some experimental observations and figures illustrating the behavior, we did not provide the detailed curves and values. In the revised manuscript, we will add force-displacement data, stiffness measurements with error bars, expansion ratios, and transition forces to substantiate the claims of smooth, continuous, and reversible transformations. revision: yes

  2. Referee: [Walking robot demonstration section] Walking robot demonstration section: The four-module walker is presented as an integrated system, yet no performance data (e.g., walking speed, load capacity, transition cycle times, or stability metrics) are reported. This leaves the claim that modules facilitate diverse transformations unsupported by evidence.

    Authors: We agree that additional performance metrics for the walking robot would strengthen the demonstration. The current manuscript focuses on the qualitative integration and functionality of the four-module system. We will include quantitative data such as walking speeds under different configurations, load capacities, transition times, and stability assessments in the revised version to better support the claims. revision: yes

  3. Referee: [Reversibility and durability discussion] Reversibility and durability discussion: The weakest assumption—that repeated folding, zipping, and unzipping maintains consistent geometry, friction, and structural integrity—is not addressed with high-cycle fatigue tests (e.g., stiffness retention or slippage after hundreds of cycles). This is load-bearing for the reversibility claim.

    Authors: This is a valid point; the manuscript does not include high-cycle fatigue testing. We will conduct additional experiments to test durability over hundreds of cycles, reporting on stiffness retention, geometric consistency, and any slippage or wear. These results will be added to the revised manuscript, and if limitations are found, they will be discussed. revision: yes

Circularity Check

0 steps flagged

No circularity: purely descriptive hardware paper with no equations, derivations or fitted predictions

full rationale

The manuscript introduces a deployable actuator based on compound folding and zipping of 3D-printed strips, characterizes its mechanical performance through experiments, and demonstrates a four-module walker. No mathematical models, first-principles derivations, parameter fitting, or predictions appear in the abstract or described content. The work contains no equations that could reduce to their own inputs by construction, no self-citation chains supporting uniqueness theorems, and no renaming of known results as novel derivations. The derivation chain is therefore empty; the paper is self-contained descriptive engineering whose claims rest on fabrication and testing rather than any closed logical loop.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical model, free parameters, axioms, or invented physical entities are introduced; the contribution is an engineering prototype and assembly demonstration.

pith-pipeline@v0.9.0 · 5428 in / 1234 out tokens · 34308 ms · 2026-05-10T18:38:08.630794+00:00 · methodology

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Reference graph

Works this paper leans on

22 extracted references · 22 canonical work pages

  1. [1]

    Robotic locomotion through active and passive morphological adaptation in extreme outdoor environments,

    M. Polzin, Q. Guan, and J. Hughes, “Robotic locomotion through active and passive morphological adaptation in extreme outdoor environments,”Science Robotics, vol. 10, no. 99, p. eadp6419, Feb. 2025. [Online]. Available: https://www.science.org/doi/10.1126/ scirobotics.adp6419

    work page 2025

  2. [2]

    Modular Self-Reconfigurable Robot Systems [Grand Challenges of Robotics],

    M. Yim, W.-m. Shen, B. Salemi, D. Rus, M. Moll, H. Lipson, E. Klavins, and G. S. Chirikjian, “Modular Self-Reconfigurable Robot Systems [Grand Challenges of Robotics],”IEEE Robotics & Automation Magazine, vol. 14, no. 1, pp. 43–52, Mar. 2007. [Online]. Available: https://ieeexplore.ieee.org/document/4141032/

    work page arXiv 2007

  3. [3]

    Reconfiguration Motion Planning for Variable Topology Truss,

    C. Liu and M. Yim, “Reconfiguration Motion Planning for Variable Topology Truss,” in2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Nov. 2019, pp. 1941–1948. [Online]. Available: https://ieeexplore.ieee.org/document/8967640/

    work page arXiv 2019

  4. [4]

    Modular shape-changing tensegrity-blocks enable self-assembling robotic structures,

    L. Zhao, Y . Jiang, M. Chen, K. Bekris, and D. Balkcom, “Modular shape-changing tensegrity-blocks enable self-assembling robotic structures,”Nature Communications, vol. 16, no. 1, p. 5888, Jul. 2025. [Online]. Available: https://www.nature.com/articles/ s41467-025-60982-0

    work page 2025

  5. [5]

    Strength and efficiency of deployable booms for space applications,

    R. Crawford, “Strength and efficiency of deployable booms for space applications,” in12th Structures, Structural Dynamics and Materials Conference. American Institute of Aeronautics and Astronautics, 1971. [Online]. Available: https://arc.aiaa.org/doi/abs/ 10.2514/6.1971-396

    work page doi:10.2514/6.1971-396 1971

  6. [6]

    Miura and S

    K. Miura and S. Pellegrino,F orms and Concepts for Lightweight Structures. Cambridge University Press, Mar. 2020, google-Books- ID: U5fYDwAAQBAJ

    work page 2020

  7. [7]

    Deployable and retractable telescoping tubular structure development,

    M. W. Thomson, “Deployable and retractable telescoping tubular structure development,” May 1994, nTRS Author Affiliations: Astro Aerospace Corp. NTRS Document ID: 19940028811 NTRS Research Center: Legacy CDMS (CDMS). [Online]. Available: https://ntrs.nasa.gov/citations/19940028811

    work page arXiv 1994

  8. [8]

    Advanced Deployable Shell-Based Composite Booms for Small Satellite Structural Applications Including Solar Sails,

    J. M. Fernandez, “Advanced Deployable Shell-Based Composite Booms for Small Satellite Structural Applications Including Solar Sails,” Kyoto, 2017, nTRS Author Affiliations: NASA Langley Research Center NTRS Report/Patent Number: NF1676L-25486 NTRS Document ID: 20170001569 NTRS Research Center: Langley Research Center (LaRC). [Online]. Available: https: //...

    work page arXiv 2017

  9. [9]

    Multistable inflatable origami structures at the metre scale,

    D. Melancon, B. Gorissen, C. J. Garc ´ıa-Mora, C. Hoberman, and K. Bertoldi, “Multistable inflatable origami structures at the metre scale,”Nature, vol. 592, no. 7855, pp. 545–550, Apr. 2021. [Online]. Available: https://www.nature.com/articles/s41586-021-03407-4

    work page 2021

  10. [10]

    KinetiX - designing auxetic-inspired deformable material structures,

    J. Ou, Z. Ma, J. Peters, S. Dai, N. Vlavianos, and H. Ishii, “KinetiX - designing auxetic-inspired deformable material structures,”Computers & Graphics, vol. 75, pp. 72–81, Oct. 2018. [Online]. Available: https://linkinghub.elsevier.com/retrieve/pii/S0097849318301006

    work page 2018

  11. [11]

    Origami tubes assembled into stiff, yet reconfigurable structures and metamaterials,

    E. T. Filipov, T. Tachi, and G. H. Paulino, “Origami tubes assembled into stiff, yet reconfigurable structures and metamaterials,” Proceedings of the National Academy of Sciences, vol. 112, no. 40, pp. 12 321–12 326, Oct. 2015. [Online]. Available: https: //www.pnas.org/doi/full/10.1073/pnas.1509465112

    work page doi:10.1073/pnas.1509465112 2015

  12. [12]

    Flexible mechanical metamaterials,

    K. Bertoldi, V . Vitelli, J. Christensen, and M. van Hecke, “Flexible mechanical metamaterials,”Nature Reviews Materials, vol. 2, no. 11, p. 17066, Oct. 2017. [Online]. Available: https://www.nature.com/articles/natrevmats201766

    work page 2017

  13. [13]

    DropPop: Designing Drop-to-Deploy Mechanisms with Bistable Scissors Structures,

    Y . Fu, E. Guan, J. Gu, D. K. Patel, J. U. Soza Soto, Y . Luo, C. Majidi, J. Hester, and L. Yao, “DropPop: Designing Drop-to-Deploy Mechanisms with Bistable Scissors Structures,” inProceedings of the 38th Annual ACM Symposium on User Interface Software and Technology, ser. UIST ’25. New York, NY , USA: Association for Computing Machinery, Sep. 2025, pp. 1...

    work page doi:10.1145/3746059.3747657 2025

  14. [14]

    Vine Robots,

    M. M. Coad, L. H. Blumenschein, S. Cutler, J. A. R. Zepeda, N. D. Naclerio, H. El-Hussieny, U. Mehmood, J.-H. Ryu, E. W. Hawkes, and A. M. Okamura, “Vine Robots,”IEEE Robotics & Automation Magazine, vol. 27, no. 3, pp. 120–132, Sep. 2020. [Online]. Available: https://ieeexplore.ieee.org/abstract/document/8917931

    work page arXiv 2020

  15. [15]

    Task-Driven Manipulation with Reconfigurable Parallel Robots,

    D. Morton, M. Cutkosky, and M. Pavone, “Task-Driven Manipulation with Reconfigurable Parallel Robots,” in2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Oct. 2024, pp. 9924–9930, arXiv:2403.10768 [cs]. [Online]. Available: http://arxiv.org/abs/2403.10768

    work page arXiv 2024

  16. [16]

    Spiralift System Platform

    “Spiralift System Platform.” [Online]. Available: https://www.sofito. com.tr/en/l/14/Stage-Mechanics/159/Spiralift-System-Platform.html

    work page

  17. [17]

    Design of a spherical robot arm with the Spiral Zipper prismatic joint,

    F. Collins and M. Yim, “Design of a spherical robot arm with the Spiral Zipper prismatic joint,” in2016 IEEE International Conference on Robotics and Automation (ICRA). Stockholm, Sweden: IEEE, May 2016, pp. 2137–2143. [Online]. Available: http://ieeexplore.ieee.org/document/7487363/

    work page arXiv 2016

  18. [18]

    A Highly Compact Zip Chain Arm with Origami-Inspired Folding Chain Structures,

    D.-K. Kim and G.-P. Jung, “A Highly Compact Zip Chain Arm with Origami-Inspired Folding Chain Structures,”Biomimetics, vol. 8, no. 2, p. 176, Jun. 2023. [Online]. Available: https: //www.mdpi.com/2313-7673/8/2/176

    work page 2023

  19. [19]

    Designing Expandable-Structure Robots for Human- Robot Interaction,

    H. Hedayati, R. Suzuki, W. Rees, D. Leithinger, and D. Szafir, “Designing Expandable-Structure Robots for Human- Robot Interaction,”Frontiers in Robotics and AI, vol. 9, Apr. 2022. [Online]. Available: https://www.frontiersin.org/journals/ robotics-and-ai/articles/10.3389/frobt.2022.719639/full

    work page doi:10.3389/frobt.2022.719639/full 2022

  20. [20]

    On the stability properties of quadruped creeping gaits,

    R. B. McGhee and A. A. Frank, “On the stability properties of quadruped creeping gaits,”Mathematical Biosciences, vol. 3, pp. 331–351, Aug. 1968. [Online]. Available: https://www.sciencedirect. com/science/article/pii/0025556468900904

    work page arXiv 1968

  21. [21]

    R. G. Budynas, K. J. Nisbett, J. K. Nisbett, and J. E. Shigley,Shigley’s mechanical engineering design, 10th ed., ser. Mcgraw-Hill series in mechanical engineering. New York, NY: McGraw-Hill Education, 2015

    work page 2015

  22. [22]

    ReachBot: A Small Robot for Large Mobile Manipulation Tasks,

    S. Schneider, A. Bylard, T. G. Chen, P. Wang, M. Cutkosky, M. Lap ˆotre, and M. Pavone, “ReachBot: A Small Robot for Large Mobile Manipulation Tasks,” in2022 IEEE Aerospace Conference (AERO), Mar. 2022, pp. 1–12. [Online]. Available: https://ieeexplore.ieee.org/document/9843346

    work page arXiv 2022