Magnetic Soft Materials and Robots

Abstract

In conventional classification, soft robots feature mechanical compliance as the main distinguishing factor from traditional robots made of rigid materials. Recent advances in functional soft materials have facilitated the emergence of a new class of soft robots capable of tether-free actuation in response to external stimuli such as heat, light, solvent, or electric or magnetic field. Among the various types of stimuli-responsive materials, magnetic soft materials have shown remarkable progress in their design and fabrication, leading to the development of magnetic soft robots with unique advantages and potential for many important applications. However, the field of magnetic soft robots is still in its infancy and requires further advancements in terms of design principles, fabrication methods, control mechanisms, and sensing modalities. Successful future development of magnetic soft robots would require a comprehensive understanding of the fundamental principle of magnetic actuation, as well as the physical properties and behavior of magnetic soft materials. In this review, we discuss recent progress in the design and fabrication, modeling and simulation, and actuation and control of magnetic soft materials and robots. We then give a set of design guidelines for optimal actuation performance of magnetic soft materials. Lastly, we summarize potential biomedical applications of magnetic soft robots and provide our perspectives on next-generation magnetic soft robots.

A reduced-order, rotation-based model for thin hard-magnetic plates

“…Soft active materials are compliant to mechanical deformations and responsive to external stimuli, making them ideal candidates for smart structures and systems in applications ranging from sensors and actuators to tissue engineering and energy harvesting (Kim and Zhao, 2022;Wang et al, 2022). This family of materials includes liquid crystal polymers (Wang et al, 2022), shape-memory polymers (Ze et al, 2020), magnetorheological elastomers (MREs) (Kim et al, 2018;Wu et al, 2020;Kim and Zhao, 2022), etc. In particular, MREs, consisting of a polymeric matrix embedded with magnetic particles, can change their elastic modulus and morph their shape under magnetic actuation (Danas et al, 2012;Kim et al, 2018).…”

“…In particular, MREs, consisting of a polymeric matrix embedded with magnetic particles, can change their elastic modulus and morph their shape under magnetic actuation (Danas et al, 2012;Kim et al, 2018). When these materials are embedded with hard-ferromagnetic particles (hard-MREs), the interaction between the applied field and the remnant magnetization of the particles can impose significant torques in the material, leading to large deflections or rotations (Kim et al, 2018;Wu et al, 2020;Kim and Zhao, 2022). The extreme, reversible deformation of structures made of hard-MREs combined with a fast and remote magnetic actuation have recently been leveraged to demonstrate novel functionalities in soft robotics (Kim et al, 2018;Hu et al, 2018;Gu et al, 2020), biomedical engineering devices (Kim et al, 2019;Zhang et al, 2021), flexible electronics (Yan et al, 2021c;Kim and Zhao, 2022), and metamaterials (Chen et al, 2021).…”

“…When these materials are embedded with hard-ferromagnetic particles (hard-MREs), the interaction between the applied field and the remnant magnetization of the particles can impose significant torques in the material, leading to large deflections or rotations (Kim et al, 2018;Wu et al, 2020;Kim and Zhao, 2022). The extreme, reversible deformation of structures made of hard-MREs combined with a fast and remote magnetic actuation have recently been leveraged to demonstrate novel functionalities in soft robotics (Kim et al, 2018;Hu et al, 2018;Gu et al, 2020), biomedical engineering devices (Kim et al, 2019;Zhang et al, 2021), flexible electronics (Yan et al, 2021c;Kim and Zhao, 2022), and metamaterials (Chen et al, 2021). The functionality of those devices can also be customized through the design of a heterogeneous magnetization profile of the MRE (Kim et al, 2018;Lum et al, 2016;Alapan et al, 2020).…”

“…The aforementioned reduced-order models for beams, rods, and shells were all developed using the torquebased theory of hard-MREs proposed by Zhao et al (2019), ought to its simplicity and proven validity in several configurations (Kim et al, 2018(Kim et al, , 2019Kim and Zhao, 2022). In this existing torque-based 3D theory, as mentioned above, the magnetization of the MRE body in the deformed configuration is related to that in the initial configuration through the deformation gradient, including both stretching and rotational contributions.…”

“…Even if several promising applications involving magnetic plates have been proposed in the literature (Kim and Zhao, 2022;Hu et al, 2018;Tang et al, 2018;Gray, 2014), their design has been primarily driven by intuition. Thus, there is a timely need for a plate theory to help understand the mechanics of plates under magnetic actuation and support the predictive and rational design of plate-like magneto-elastic structures.…”