Magnetically Actuated Fiber-Based Soft Robots

Abstract

Broad adoption of magnetic soft robotics is hampered by the sophisticated field paradigms for their manipulation and the complexities in controlling multiple devices. Furthermore, high-throughput fabrication of such devices across spatial scales remains challenging. Here, advances in fiber-based actuators and magnetic elastomer composites are leveraged to create 3D magnetic soft robots controlled by unidirectional fields. Thermally drawn elastomeric fibers are instrumented with a magnetic composite synthesized to withstand strains exceeding 600%. A combination of strain and magnetization engineering in these fibers enables programming of 3D robots capable of crawling or walking in magnetic fields orthogonal to the plane of motion. Magnetic robots act as cargo carriers, and multiple robots can be controlled simultaneously and in opposing directions using a single stationary electromagnet. The scalable approach to fabrication and control of magnetic soft robots invites their future applications in constrained environments where complex fields cannot be readily deployed.

Keywords: fibers; magnetic actuation; magnetic composites; soft robots; thermal drawing.

Figure 1.. Fabrication and characterization of magnetically-actuated fibers.

(a) Schematic of the fabrication process of 3-D fiber-based magnetic soft robots. Scale bars represent 1 cm. (b) Cross-sectional image of the preform. Scale bar represents 1 cm. (c) Bundles of thermally drawn fibers. The scale bar represents 10 cm. (d) Cross-sectional image of thermally drawn fiber following the removal of the PMMA cladding. Scale bar represents 500 μm. (e) Cross-sectional image of a fiber following injection and photo-curing of the magnetic composite. Scale bar represents 500 μm. The inset shows an SEM image of NdFeB particles embedded in 4T₄2T₉₆2A₁₀₀ polymer and the scale bar is 20 μm. (f) Pictures of strain engineering process to create the helical structure from an as-drawn fiber. The red arrow indicates a perversion. The scale bar represents 1 cm. The inset shows the helical fiber after removal of the perversion and its scale bar is 1 mm in length. (g) Stress-strain curves of the fibers and individual components. The black, red, and blue lines indicate 10, 15, and 20%COCe integrated fibers with magnetic composite, respectively. Orange, green, purple, and gray lines represent COCe, SEBS, polymer matrix, and magnetic composite, respectively. The inset shows the elongation of the 4T₄2T₉₆2A₁₀₀ polymer under 250% tensile strain and its scale bar is 1 cm in length. (h) Measured bending stiffness for fibers with various compositions and sizes. The black, red, and blue curves represent the stiffness of 10, 15, and 20%COCe fibers, respectively. Solid and dashed lines indicate fibers with cross-sectional area of 0.5 and 0.1 mm², respectively. Shaded areas correspond to the standard errors (n=5 samples). (i) Coil diameters of the helical fibers depending on compositions, sizes, and strain. The black, red, and blue curves indicate the coil diameters of 10, 15, and 20%COCe fibers, respectively. Solid and dashed lines represent fibers with cross-sectional area of 0.5 and 0.1 mm², respectively. Shaded areas show standard deviations (n=5 samples). (j-k) Magnetic actuation of (j) linear and (k) helical fibers using a permanent magnet. Scale bars represent 1 cm.

Figure 2.. Structural and magnetic properties of 3-D magnetic soft robots.

(a and c) Schematic of (a) the crawler and (c) the walker at the initial and actuated states. Red arrows indicate the expected magnetic moment direction following magnetization. The magnetic field actuating the robot is applied along the direction of the blue arrow. (b and d) Positions of (b) the crawler and (d) the walker on their alignment fixtures (in yellow) during magnetization. (b) The colored arrows indicate defining the length parameters for crawlers and (d) the colored arcs indicate defining the magnetization angle parameters for walkers. The external magnetizing field is applied along the direction of the blue arrow. Scale bars are 5 mm.

Figure 3.. Crawling motion of 3-D magnetic soft robots.

(a) Photographs of the left-to-right motion of the crawler under an oscillating, unidirectional, sinusoidal 2 Hz magnetic field along the z-direction. The scale bar is 1 cm. (b and c) Motility study of the crawler depending on (b) length and (c) curvature magnetization parameters (see insets). Blue circle and red cross represent motion and no motion, respectively. (d) Measured crawling velocity in y-direction depending on the magnetic field frequency. Black line and shaded area present the mean and standard deviation of the crawling velocity, respectively (n ≥ 5 samples). (e, f) Photographs of a crawler used as a cargo carrier. The cargo is circled in yellow. The magnitude of the magnetic fields applied along the z-direction for transporting (e) and releasing (f) modes are plotted on the left. Scale bar = 1 cm. All crawler parameters are summarized in Table S3.

Figure 4.. Walking motion of 3-D magnetic soft robots.

(a) Photographs of the right-to-left motion of two walkers under a 2 Hz oscillating, unidirectional sawtooth magnetic field. Each walker has different coil numbers at each foot (different foot coil condition). Scale bars represent 1 cm. (b) Relationship between the angle parameters (see inset) and motility of the walker, which has the same coil number in their feet (same foot coil condition). Blue and red triangles represent LLD (long-leg direction) and SLD (short-leg direction) walking motion, respectively. Triangle size is proportional to the walking velocities. The blue and red shaded areas represent direction of motion as predicted by a hybrid statics-dynamics model; blue and red correspond to LLD and SLD, respectively. The purple region represents an area where the movement direction is opposite during folding and unfolding motion, therefore the net motion of the walker is ambiguous based on the hybrid statics-dynamics model. (c) Distance travelled along the y-direction by the walker depending on foot conditions. Blue and red lines represent LLD and SLD walking motion, respectively. Solid and dashed lines indicate movement of walkers with different and same foot coil conditions, respectively. (d) Measured walking velocity along the y-direction dependent on the magnetic field frequency. Blue and red lines represent the mean walking velocities of the LLD and SLD modes, respectively. Shaded areas correspond to the respective standard deviations of walking velocity (n ≥ 5 samples). (e) Photographs of a walker used as a cargo carrier. The walker has different foot coil condition. Cargo is marked in yellow. The magnetic fields applied along the z-direction for transporting (left) and releasing (right) modes are plotted under each mode. Scale bar represents 1 cm. (f) Photographs of multiple walkers moving along different directions. The walkers have different magnetization and foot coil conditions. The magnetic field oscillates at 2 Hz along the z-direction. Scale bars represent 1 cm. All walker parameters can be found in Table S4.