Multifunctional magnetic soft composites: a review

Abstract

Magnetically responsive soft materials are soft composites where magnetic fillers are embedded into soft polymeric matrices. These active materials have attracted extensive research and industrial interest due to their ability to realize fast and programmable shape changes through remote and untethered control under the application of magnetic fields. They would have many high-impact potential applications in soft robotics/devices, metamaterials, and biomedical devices. With a broad range of functional magnetic fillers, polymeric matrices, and advanced fabrication techniques, the material properties can be programmed for integrated functions, including programmable shape morphing, dynamic shape deformation-based locomotion, object manipulation and assembly, remote heat generation, as well as reconfigurable electronics. In this review, an overview of state-of-the-art developments and future perspectives in the multifunctional magnetically responsive soft materials is presented.

Keywords: configurable structures; magnetic soft materials; soft robotics; stimuli-responsive materials.

Figure 1. Magnetic particle options commonly used for magnetic soft composites.

(a) Magnetic flux density B with respect to magnetic field H curves of soft-magnetic and hard-magnetic particles. (b) The actuation mechanism of the soft-magnetic composites. (c) The actuation mechanism of the hard-magnetic composites.

Figure 2. Soft matrix types commonly used for magnetic soft materials with various functionalities.

Magnetic hydrogels with (a) low stiffness (Adapted from [61]. Copyright 2011, National Academy of Sciences) and (b) good biocompatibility (Adapted with permission from [64]. Copyright 2010, Springer Nature). Magnetic elastomers with (c) reconfigurable actuation (Reproduced from [48]. Copyright 2016, National Academy of Sciences) and (d) tunable stiffness (Top: Reproduced with permission from [30]. Copyright 2008, Elsevier. Bottom: Reproduced with permission from [67]. Copyright 2018, Elsevier). Magnetic shape memory polymers to achieve (e) shape locking (Adapted with permission from [23]. Copyright 2019, Wiley) and (f) reprogrammability (Adapted with permission from [68]. Copyright 2018, Royal Society of Chemistry).

Figure 3. Various manufacturing methods for magnetically responsive soft materials.

(a) Molding of silicone rubber embedded with hard-magnetic particles with programmable magnetization (Reproduced from [48]. Copyright 2016, National Academy of Sciences). (b) Two-photon polymerization for the fabrication of small-scale magnetic robots (Adapted with permission from [94]. Copyright 2018, American Chemical Society). (c) Direct ink writing (Adapted with permission from [49]. Copyright 2018, Springer Nature) and (d) digital light processing (Adapted with permission from [95]. Copyright 2019, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science) of magnetic soft robots with programmable magnetization. (e) Fused filament fabrication of PLA filament embedded with Fe₃O₄ particles (Reproduced with permission from [96]. Copyright 2019, Elsevier). (f) Microfabrication of configurable magnetic structure by e-beam lithography (Adapted with permission from [24]. Copyright 2019, Springer Nature). (g) Field-directed self-assembly of magnetic microactuator by dielectrophoretic forces (Reproduced with permission from [97]. Copyright 2019, Springer Nature).

Figure 4

Multifunctionality of magnetically responsive soft materials, including (a) shape morphing (Reproduced with permission from [114]. Copyright 2019, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science), (b) dynamic motion (Reproduced with permission from [15]. Copyright 2018, Springer Nature), (c) object manipulation (Reproduced with permission from [115]. Copyright 2017, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science), (d) heat generation (Reproduced with permission from [74]. Copyright 2010, Royal Society of Chemistry), and (e) signal sensing (Reproduced with permission from [117]. Copyright 2019, Wiley) capabilities.

Figure 5. Programmable shape morphingand tunable properties of magnetic soft composites.

(a) Actuator with predesigned particle alignment distribution (Reproduced with permission from [108]. Copyright 2011, Springer Nature). (b) Magnetic soft materials programmed with continuous magnetization for undulatory wave configurations (Reproduced with permission from [45]. Copyright 2014, AIP Publishing). (c) Magnetic auxetic metamaterial with encoded discrete magnetization and reprogrammable deformations (Reproduced with permission from [118]. Copyright 2020, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science). (d) An array of Janus microplates with controllable wettability (Reproduced with permission from [119]. Copyright 2019, Wiley). (e) Auxetic metamaterials with actively tunable stiffness (Left: Adapted with permission from [46]. Copyright 2019, American Chemical Society) and acoustic bandgaps (Right: Adapted with permission from [120]. Copyright 2020, Wiley). (f) Active metamaterial with shiftable mechanical behaviors under the cooperative thermal and magnetic stimuli (Adapted with permission from [107]. Copyright 2020, American Chemical Society).

Figure 6. Magnetic soft composites for functional navigation.

(a) Bio-inspired swimming robots embedded with aligned magnetic particles for self-folding (Reproduced with permission from [109]. Copyright 2016, Springer Nature). (b) Biomimetic scallop swimming robot (Reproduced with permission from [129]. Copyright 2014, Springer Nature). (c) Multifunctional bio-inspired magnetic jellyfish robot (Reproduced with permission from [69]. Copyright 2019, Springer Nature). (d) Magnetic swimming robot with asymmetric actuation for combined folding-bending deformation (Adapted with permission from [46]. Copyright 2019, American Chemical Society). (e) Biomimetic dog trot gait of a magnetic robot with synergistically controlled legs (Adapted with permission from [130]. Copyright 2020, Wiley). (f) Steering and navigation of a magnetically responsive catheter in a vascular model (Reproduced with permission from [22]. Copyright 2019, the Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science). (g) Magnetic microgripper with controlled navigation and drug release (Reproduced from [63]. Copyright 2016, IOP Publishing). (h) Magnetically controlled capsule robot with the fine-needle biopsy capability (Reproduced with permission from [131]. Copyright 2020, Mary Ann Liebert, Inc.). (i) Bio-hybrid swimming robot guided by the magnetic field (Reproduced with permission from [132]. Copyright 2018, The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science).

Figure 7. Manipulation of magnetic soft composites for functional assembly.

(a) Part assembly of soft pneumatic robots by magnetic dipole interaction (Reproduced with permission from [150]. Copyright 2013, Wiley). (b) Magnetically and thermally controlled soft gripper grasping and cutting a cell cluster (Adapted with permission from [66]. Copyright 2015, American Chemical Society). (c) Capturing, transporting, and releasing of an immotile sperm cell via a magnetic robot (Reproduced with permission from [151]. Copyright 2016, American Chemical Society). (d) Different 2D structures assembled by the same building module via a magnetic robot (Reproduced with permission from [152]. Copyright 2010, Wiley). (e) Assembly of 3D structures with different shapes of building modules via a magnetic robot (Reproduced with permission from [153]. Copyright 2014, Springer Nature). (f) Magnetic field-assisted self-assembly and biomimetic swimming motions (Reproduced with permission from [154]. Copyright 2020, American Chemical Society). (g) Three-dimensional assembly of magnetic hydrogels embedded with cells (Adapted with permission from [65]. Copyright 2011, Wiley). (h) Cooperative behaviors of magnetic pillars to spray liquid (Reproduced with permission from [155]. Copyright 2020, American Chemical Society). (i) A swarm of robots to move an object cooperatively under the application of magnetic field (Adapted with permission from [156]. Copyright 2019, Springer Nature).

Figure 8. Heat generation and energy output of magnetic soft composites.

(a) Localized hyperthermia guided bythe magnetic particle imaging technique (Reproduced with permission from [54]. Copyright 2018, American Chemical Society). (b) Double network magnetic hydrogel for hyperthermia treatment (Adapted with permission from [169]. Copyright 2019, Royal Society of Chemistry). (c) Controlled drug release by inductive heating of magnetically responsive hydrogel (Adapted with permission from [60]. Copyright 2008, Elsevier). (d) Magnetic robot swarm with controlled locomotion and on-demand drug-releasing (Reproduced with permission from [170]. Copyright 2020, Wiley). (e) Magnetic shape memory polymer actuator with remote activation by inductive heating (Reproduced from [28]. Copyright 2006, National Academy of Sciences). (f) Remotely controlled self-healing of magnetic composite with dynamic bonds (Reproduced with permission from [171]. Copyright 2015, Elsevier).

Figure 9. Configurable electronics and sensors via multiphysics coupling of magnetic soft composites.

(a) Configurable antenna with an actively tunable resonant frequency (Adapted with permission from [23]. Copyright 2019, Wiley). (b) Multifunctional Magneto-mechano-electric origami assembly for digital computing. (Adapted from [188]. Copyright 2020, National Academy of Sciences). (c) Magnetically responsive soft material sensor by embedding silver nanowires in MRE (Adapted with permission from [189]. Copyright 2018, Elsevier). (d) Self-sensing magnetic graphene aerogel with strain-dependent resistance (Adapted with permission from [182]. Copyright 2015, American Chemical Society). (e) Magnetically responsive soft material sensor embedding liquid metal in MRE (Adapted with permission from [190]. Copyright 2019, Springer Nature). (f) Deformation sensor mimicking cilium structure (Adapted with permission from [191]. Copyright 2015, Wiley).

Figure 10. Future perspectives to make magnetic soft composites intelligent and autonomous.