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Symmetry-breaking Actuation Mechanism for Soft Robotics and Active Metamaterials
Shuai Wu, Qiji Ze, Rundong Zhang, Nan Hu, Yang Cheng, Fengyuan Yang, Ruike Zhao
TL;DR
Existing magnetic-responsive soft materials commonly provide only single-mode deformation, limiting tunable behavior. This paper introduces programmable composites with asymmetric joints that shift the neutral axis and produce multimodal actuation, demonstrated in active structures, crawling and swimming robots, and tunable metamaterials.
Problem
Conventional magnetic-responsive soft materials commonly achieve only single-mode deformation, limiting their tunable properties.
Method
The paper designs programmable composites with asymmetric joints that shift the neutral axis between magnetic-cell and film mid-planes to combine bending and folding.
Results
The asymmetric multimodal mechanism enables asymmetric shape-shifting structures, crawling and swimming robots, and 2D metamaterials with tunable properties.
Takeaways & Limitations
The mechanism broadens the design space for multifunctional soft robots, biomedical devices, and acoustic metamaterials.
Takeaways & Limitations
The paper demonstrates only two simple metamaterial designs while envisioning additional systems.
Abstract
from arXiv · showhide
Magnetic-responsive composites that consist of soft matrix embedded with hard-magnetic particles have recently been demonstrated as robust soft active materials for fast-transforming actuation. However, the deformation of the functional components commonly attains only a single actuation mode under external stimuli, which limits their capability of achieving tunable properties. To greatly enhance the versatility of soft active materials, we exploit a new class of programmable magnetic-responsive composites incorporated with a multifunctional joint design that allows asymmetric multimodal actuation under an external stimulation. We demonstrate that the proposed asymmetric multimodal actuation enables a plethora of novel applications ranging from the basic 1D/2D active structures with asymmetric shape-shifting to biomimetic crawling and swimming robots with efficient dynamic performance as well as 2D metamaterials with tunable properties. This new asymmetric multimodal actuation mechanism will open new avenues for the design of next-generation multifunctional soft robots, biomedical devices, and acoustic metamaterials.
INTRODUCTION
Hard-magnetic soft materials enable field-driven deformation, but conventional designs exhibit mirror-symmetric single-mode actuation. An asymmetric joint shifts the neutral axis, combining bending and folding in one programmable material system.
- Conventional actuation: Under switching magnetic fields, conventional hard-magnetic soft materials show mirror-symmetric, single-modal deformation.A downward field bends the material downward, while switching the field produces mirror symmetry; finite-element simulations can predict the deformation.
- Motivation: Symmetry breaking is challenging because smart materials are commonly homogeneous and isotropic, despite asymmetric motions being important for net propulsion.The paper motivates asymmetric responses using examples such as bird flight and frog swimming, where asymmetric wing or leg motions generate net propelling.
- Joint design: The proposed material uses two magnetic unit cells bonded by a thin soft film, with either a symmetric or asymmetric joint.The symmetric design includes a gap to prevent interference, whereas the asymmetric design places adjacent unit cells in direct contact.
- Multimodal mechanism: Shifting the neutral axis between the film and magnetic-cell mid-planes enables multimodal deformation combining elastic bending with rigid-body folding.For the asymmetric joint, field switching moves the neutral axis to the magnetic-cell mid-plane, making bending mainly dependent on magnetic-unit-cell elasticity.
RESULTS AND DISCUSSION
Asymmetric joints enable magnetic-responsive systems to switch between folding and bending modes, producing reconfigurable shapes, locomotion, swimming, and tunable mechanical properties. Across 1D and 2D designs, multimodal actuation supports controlled deformation with demonstrated translational and swimming performance.
- 1D multimodal deformation: Symmetric joints produce a near-mirror-symmetric folding mode, whereas asymmetric joint combinations induce distinct bending and folding modes under field reversal.S-A-S systems form M and arc modes, while A-S-A systems form two-arc and W modes.
- 1D multimodal deformation: At -150 mT, bending stores 9.0 mJ, 26 times the 0.35 mJ stored during folding with the same amplitude, yielding a much stiffer system.The energy-storage difference enables similar overall geometries with tunable physical properties.
CONCLUSION
The asymmetric joint enables multimodal deformation by shifting the neutral axis under directional actuation, supporting soft robots, active metamaterials, and programmable systems.
- The asymmetric joint breaks material symmetry by shifting the neutral axis under directional actuation.
- Multimodal deformation is harnessed to create soft robots with effective motion and active metamaterials with tunable properties.
- The multimodal concept can extend from simple geometrical forms to complex 3D geometries by strategically assembling basic structures.
- The approach is envisioned to create routes for designing programmable machines and systems as demand for reconfigurable materials grows.
- Integrated design can tailor properties at the local or mechanism level to enhance global performance or enable unprecedented functionalities.
Materials and Methods
The methods fabricate magnetic and nonmagnetic soft-material components, characterize their mechanical and magnetic properties, and simulate magnetic-actuated deformation.
- Material shear moduli are obtained by fitting uniaxial stress-stretch data with a neo-Hookean model.The measured shear moduli are 164 kPa for magnetic PDMS and 19 kPa for nonmagnetic Ecoflex.
- Magnetization is measured with a vibrating sample magnetometer using magnetic hysteresis loops.For PDMS 20:1 with 20 vol% NdFeB, the measured magnetization is 109 kA m-1.
- Finite element simulations use a user-defined 8-noded element subroutine with a neo-Hookean energy function coupled to magnetic potential.The subroutine uses shear modulus, magnetization vector, and external field vector to calculate magnetic deformation.
Supporting Information
The Supporting Information provides analytical, simulation, characterization, and experimental materials accompanying the study. It includes videos documenting multimodal deformations, crawling and swimming robots, and magnetic-actuated metamaterial systems.
- Access: The Supporting Information is available free of charge through the ACS Publications website.
- Analytical and simulation materials: The supplementary materials include analytical solutions for asymmetric and symmetric joint bending and folding behavior.
- Analytical and simulation materials: They report calculations of effective magnetic-actuated metamaterial stiffness and simulations of bending in a two-layer composite beam.
- Experimental setup and characterization: The materials document single-axis Helmholtz coils for generating a controllable one-dimensional magnetic field and characterize the materials used in demonstrations.
- Supplementary demonstrations: Supplementary videos show magnetic-actuated one-dimensional multimodal deformation in four-cell and eight-cell systems and magnetic-actuated crawling robots.
- Supplementary demonstrations: Additional videos show a four-leg biomimetic swimming robot, its comparison with a bending-only control, two-dimensional multimodal deformation, and magnetic-actuated metamaterial systems.