Wireless actuation of soft material is vital in fields such as biomedical engineering and soft robotics. Magnetic fields are particularly attractive because they can safely penetrate the human tissue and allow remote actuation in specific spaces. Previous studies showed promise for magnetically sensitive soft devices, but establishing multistability is a significant challenge. Earlier magnetic soft structures required a sustained field, an optical or thermal trigger, or a stiff component, which limited their biomedical applications and necessitated flexibility, safety, and robustness.
This study aimed to design and demonstrate a fully soft, magnetically sensitive, and multistable metamaterial that can reversibly shift from programmable stable states in remote magnetic actuation. The primary aim was to surpass the limitation of earlier designs by integrating the soft silicone material, magnetic domain programming, and programmable unt cell geometrics to produce strong biostability and transformation capabilities in a variety of environmental conditions.
The researchers created bistable unit cells made completely from silicone elastomers with magnetic microparticles to develop programmable domains. Geometric parameters like beam thickness-to-length ratio (t/L), rotation angle (θ), and radius (R) were systematically varied to tune energy barriers of the unit cells. Reduced-order simulations were used to predict energy barrier trends in compression and tension, which guided experimental designs.
The metamaterial structures were assembled in configurations such as 2 × 8 and 4 × 8 cylindrical architectures. These were tested for selective actuation by using nonuniform magnetic fields and enabling row-by-row triggering. Complex assemblies, like a wirelessly actuated valve and a peristaltic pump, were fabricated to showcase functionality.
Robustness was assessed under dynamic environmental stressors (temperature fluctuations, water jets, and air currents) and extreme adverse conditions like high strain, blunt impacts, simulated gastric fluid exposure, and fire. Performance metrics included transformation speed, retention of multistability, and energy barrier changes.
The metamaterial showed several essential capabilities. Adjustment of /L, θ, and R allows for accurate control of energy barriers, and the force enables the selective actuation of individual rows in the large assembly. The structures rapidly transitioned between stable states (below 0.15 s) in nonuniform magnetic fields and maintained their configurations without continuous field input. Demonstrated complex functionality, including twisting and bending assembly, a reconfigurable light-emitting device, and a magnetically operated peristaltic pump, which can withstand fluid pressure of up to 18.5 kPa while maintaining multistability.
The metamaterial worked at −20° to 100°C temperature, in high mechanical strain (ε ≈ 1.40), dynamic fluid stresses, and extreme impacts (~87 kN), even after immersion in the simulated gastric fluid for 7 days. Transformation ability was maintained with a below 15% change in energy barriers. Fire exposure caused partial demagnetization, but reprogramming restored the actuation.
The study shows that a soft, magnetically actuated metamaterial may shift between stable states in a programmed and reversible manner. This silicone-based design is resistant to mechanical, chemical, and thermal stress while remaining functional, which is critical for implantable and ingestible devices. The capacity to selectively program unit cells enables complicated actions like bending, twisting, and pumping. Demonstrations with programmable LEDs and peristaltic pumps highlight the possibility for deployable biomedical devices and soft robotic systems. The scalability of the design makes it applicable to both microscale biomedical devices and larger soft robotic platforms.
A limitation was observed under fire exposure, where magnetic microparticles lost their magnetization because of the Curie temperature. However, the metamaterial maintained multistability, and reprogramming restored the performance, which underscores the robustness of soft architecture.
This study established a new pathway for multistable, soft, magnetically actuated metamaterials that can bridge the critical gap in biomedical device design and soft robotics. The combination of soft silicone construction, tunable bistability, and wireless magnetic control makes this technology suitable for transformative applications in the reconfigurable implantable and ingestible systems and adaptive for robotics platforms.
Reference: Greenwood TE, Zhang Y, Lee J, et al. Soft multistable magnetic-responsive metamaterials. Sci Adv. 2025;11(36):eadu3749. doi:10.1126/sciadv.adu3749
