Fig. 5. Super-compliant picospring to actuate microrobots.

a, Microoscillator oscillation driven by the magnetic field. The deflection decreases from left to right as the cantilever picospring fabricated at higher laser powers becomes less compliant. The microoscillators oscillate over 100 times with no observable fatigue, revealing the structural stability of the picosprings. Scale bar, 30 μm. b, Single- or multiple-exposure method for fabricating micromachines with homogeneous or inhomogeneous magnetization distributions. c, Schematic illustration of the micropenguin propulsion generated by the magnetic torques (τ) that close the flippers. d, Closing moment of the micropenguin flippers. The picospring linkages store energy during the closing of the flippers. Under a magnetic actuation field (for example, 16 mT) along its axial direction (+x), the micropenguin closes its flippers and deforms the picospring linkages, achieving a forward movement by pushing the fluid backwards. The driving force is provided by the magnetic torque aligning the magnetic easy axis of each geometrically anisotropic flipper towards the magnetic field direction. When the magnetic field decreases to zero, the picosprings relax and drive the micropenguin back to its original position. Scale bar, 10 μm. e, Rotation angle (Δθ) of the micropenguin flipper relevant to the magnetic field. ${\theta }_0^{\mathrm{A}}$, ${\theta }_0^{\mathrm{B}}$, θ^A and θ^B represent the angles between the axis of the micropenguin and its left or right wing at theinitial state (${\theta }_0^{\mathrm{A}}$ or ${\theta }_0^{\mathrm{B}}$) and under specific magnetic fields (θ^A or θ^B), respectively. The flipper rotation and the related picospring bending are positively related to the applied field. An approximately constant bending stiffness can be achieved in accordance with the bending model of a coil spring. f, Opposite magnetic torques acting on the microturtle flippers under orthogonal magnetic fields. The soft magnetic flippers can be aligned towards the orientation of the magnetic field but without direction selectivity. Each flipper has two rotation directions under different magnetic fields, whereas the non-magnetic torso’s orientation is not changed by the magnetic fields. Scale bar, 20 μm. g, Rotation angles of the microturtle flipper towards two magnetic field directions. For e and g, n = 3 microrobots for each group, mean ± s.d.

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