Magnetic soft micromachines made of linked microactuator networks

Abstract

Soft untethered micromachines with overall sizes less than 100 μm enable diverse programmed shape transformations and functions for future biomedical and organ-on-a-chip applications. However, fabrication of such machines has been hampered by the lack of control of microactuator's programmability. To address such challenge, we use two-photon polymerization to selectively link Janus microparticle-based magnetic microactuators by three-dimensional (3D) printing of soft or rigid polymer microstructures or links. Sequentially, we position each microactuator at a desired location by surface rolling and rotation to a desired position and orientation by applying magnetic field-based torques, and then 3D printing soft or rigid links to connect with other temporarily fixed microactuators. The linked 2D microactuator networks exhibit programmed 2D and 3D shape transformations, and untethered limbless and limbed micromachine prototypes exhibit various robotic gaits for surface locomotion. The fabrication strategy presented here can enable soft micromachine designs and applications at the cellular scales.

Fig. 1. Soft micromachine fabrication strategy.

(A) Fabrication process schematics of soft micromachines using a two-photon polymerization (3D microprinting) system with an integrated electromagnetic coil setup. Microactuators (soft magnetic, monodisperse, and spherical Janus microparticles) are manipulated magnetically to reach a desired position and orientation and fixed there temporarily using 3D microprinting. Soft and rigid materials can be 3D-printed on a fixed microactuator to link it with other fixed microactuators. (B) Schematic details of the fabrication process for an example two-particle chain: (i) The first microactuator is positioned to a specific location by surface rolling. (ii) The microactuator’s orientation is controlled by a rotating magnetic field H. (iii) The oriented microactuator is anchored temporarily on the glass substrate. (iv) A ring-shaped holding structure is 3D-printed to be able to bond other 3D-printed microstructures at the microactuator’s metallic site. The above four steps are repeated for the second microactuator. (v) A soft (i.e., gelatin hydrogel) or rigid link between the microactuators is 3D-printed. (vi) The final soft micromachine is released (with laser assistance) from the glass substrate by soaking it in deionized (DI) water. Then, the released device is actuated by external magnetic fields generated by the electromagnetic coils in the given operation space.

Fig. 2. Characterization of the fabricated hydrogel cantilever beams.

(A) The particle orientation and cantilever geometry determine bending directions under an applied field. (i) Rectangular and ladder hydrogel beams are printed on microactuators with approximate 45° and 90° angles to the easy axis, respectively. (ii) Under the applied field in the x axis (9 mT), the rectangular beam presents an in-plane bending to the right, but the ladder beam does not have bending owing to the particle already in the m_c state. (iii) Under the applied field in the y axis, the rectangular beam presents an in-plane bending to the left, and the ladder beam presents an out-of-plane bending to lift the particle. (B) Main magnetic states of microactuators, m_a (90°), m_b (+45° or − 45°), and m_c (0°). Magnetic energy density on microactuators (10-μm diameter) are performed under an applied field (10 mT). The multiplication factor for the color bar is 110 J/m³. (C) Magnetic hysteresis loops of microactuators (10-μm diameter) with different easy-axis orientations, 90°, 45°, and 0°, respectively, to the applied field. (D) Maximum bending angles of the particle cantilever under in-plane rotating fields in clockwise direction at 0.5-Hz frequency. (E) Oscillation of different-sized particle cantilevers [schematic (i)] under an oscillating field (from +9 to −9 mT at 1.2-Hz frequency). The particle diameters are 10 μm (ii) and 3 μm (iii).

Fig. 3. Soft robotic particle chains.

(A and B) Magnetic energy density over the oriented particles (i), schematic of the oriented particles (ii), and the optical microscopy image of hydrogel-linked particles (iii) from the left to the right side, respectively. The red arrow represents the magnetic field direction. The multiplication factor for the color bar is 41 J/m³. After applying an in-plane magnetic field in the y axis, the chain with two microactuators at the m_c and m_a states (A) rotates 90° in the yz plane to the state of both at the m_c state (B). (C) Hydrogel-linked particles with the specific orientations (i) show a surface locomotion (ii and iii) under an oscillation field between H_x and H_y (9 mT at 3-Hz frequency). (D) Hydrogel-linked particles with the specific orientations (i) show locomotion (ii and iii) under an oscillating field along one direction (amplitude 9 mT at 2-Hz frequency). (E) Gripping performance of the hydrogel-linked particles under an applied static field (9 mT). (F) A triangle structure with three encoded particles at the apexes can stand up (i and ii) under an applied in-plane field. The standing structure exhibits walking locomotion (iii and iv) with two particles as one pair of legs and the third particle (at the m_c state) controlling the balance, under an oscillating field between H_x and H_y (9 mT at 0.5-Hz frequency).

Fig. 4. Soft robotic 2D particle networks having 2D and 3D shape deformations.

(A) An auxetic microstructure with negative Poisson’s ratio can be observed in a 3 by 3 array microactuator network. The easy axis of each microactuator is encoded with alternating 45° along the beams indicated by the double-sided arrows (i and ii). The cross-sectional dimensions of the rectangular hydrogel beam are 1-μm width and 4-μm height. By applying a magnetic field (10 mT) on and off, the network presents programmable shape transformations (iii and iv). (B) A microstructure of a 3 by 3 array network with ladder-like hydrogel beams presents a wobbling motion under a rotating field. The easy axis of each microactuator is encoded orderly perpendicular and parallel to the beams indicated by the double-sided arrows (i and ii). The cross-sectional dimensions of the ladder-like hydrogel beam are 1-μm ladder beam width, 4-μm overall width, and 0.5-μm height. By applying a rotating 3D field H in clockwise direction (8.1-mT field strength, tilting angle in the +z axis from the xy plane is 8.5°, and 0.6-Hz frequency), the encoded network presents periodic 3D shape deformations (iii and iv).

Fig. 5. Two-legged soft microrobot demonstrations and fabrication of a tetrahedron array toward future 3D soft micromachine configurations.

(A to D) A lizard-like walking robot with four rigid legs presents a surface locomotion under an oscillating field (from +9 to −9 mT at 1-Hz frequency). (E to H) Soft-legged robot presents programmable robotic gaits under an alternating field (9 mT). (I) Tetrahedron array of four oriented particles with 7-μm diameter. Particle levitation is controlled by the applied magnetic gradient fields in the vertical (z) direction. Schematic (i), printed 3D hydrogel structures on three encoded particles (ii), the optical microscopy image of focusing on the three particles (iii), and the optical microscopy image of focusing on the lifted/levitated particle (iv). (J) 3D configuration of a particle with 10-μm diameter by rolling motion along the hydrogel beam. Schematic (i), the optical microscopy image of focusing on the particle (ii), the optical microscopy image of focusing on the hydrogel beams (iii), and actuation (iv) under a rotating in-plane field (12.5 mT). (K) Two oriented particles show 3D shape changes of hydrogel beams under an applied field. Schematics (i and ii), the optical microscopy image of focusing on the hydrogel beams without applied fields (iii), and with actuation (iv) under an applied field (12.5 mT).