Four-Dimensional-Printed Microrobots and Their Applications: A Review

Abstract

Owing to their small size, microrobots have many potential applications. In addition, four-dimensional (4D) printing facilitates reversible shape transformation over time or upon the application of stimuli. By combining the concept of microrobots and 4D printing, it may be possible to realize more sophisticated next-generation microrobot designs that can be actuated by applying various stimuli, and also demonstrates profound implications for various applications, including drug delivery, cells delivery, soft robotics, object release and others. Herein, recent advances in 4D-printed microrobots are reviewed, including strategies for facilitating shape transformations, diverse types of external stimuli, and medical and nonmedical applications of microrobots. Finally, to conclude the paper, the challenges and the prospects of 4D-printed microrobots are highlighted.

Keywords: 4D-printed; applications; medical; microrobots; nonmedical; shape reversible; stimuli-responsive.

Figure 6

Magnetic-responsive microrobots: (a) Schematics of the magnetic configurations and shape morphing behaviors of the micromachines upon application of the controlling magnetic field B = 15 mT; the optical microscopy images depict the four different conformations of the fabricated devices. Scale bar is 10 μm. Reprinted with permission from Ref. [89]. Copyright 2019, Springer Nature. (b) (i) six-armed magnetic shape-morphing material film. Schematic illustrations of magnetization pattern, finite-element simulations, and optical images of six-armed magnetic shape-morphing material depending on magnetization patterns can be transformed to (ii) Chair. (iii) Spider. (iv) Bud. Scale bar is 5 mm. Reprinted with permission from Ref. [92]. Copyright 2020, Springer Nature.

Figure 7

Medical applications of 4D-printed microrobots: (a) (i) Soft robot navigating across a synthetic stomach phantom by using a combination of locomotion modes. (ii) Ultrasound-guided locomotion whereas the robot rolls within the chicken tissue and visualized by ultrasound. (iii) Robot approaching a cargo item by walking on a flat rigid surface, picking up the cargo by curling into the C-shape, transporting the cargo away and maintaining the shape, and releasing the cargo at targeted area. (iv) Dynamic and selective cargo release. Scale bars are 1 mm. Reprinted with permission from Ref. [100]. Copyright 2018, Springer Nature. (b) (i) Artificial networks fabricated by photolithography and PDMS molding. (ii) Magnetic shape-morphing microfish swimming in these channels. (iii) Snapshots of the HeLa cells viability in the doxorubicin-release area. Reprinted with permission from Ref. [49]. Copyright 2021, American Chemical Society. (c) (i) Demonstration of cell biopsy from a cell cluster by gripping the cell. (ii) Immunofluorescence images of cells captured by the grippers. Reprinted with permission from Ref. [102]. Copyright 2015, American Chemical Society. (d) Schematic illustration of heparin release and sperm hyperactivation. Reprinted with permission from Ref. [103]. Copyright 2022, Wiley-VCH. (e) (i) Ex vivo model of a rat intestine. (ii) Demonstration of ex vivo transformation and transportation of ionic shape-morphing microrobotic end-effectors carrying MNPs and fluorescent microbeads through external magnetic field control. Scale bar is 2 cm. Reprinted with permission from Ref. [104]. Copyright 2021, Springer Nature.

Figure 1

Overview of 4D-printed microrobots based on their stimulations and applications.

Figure 2

Temperature-responsive microrobot: (a) Shape change of a gripper in response to changing of temperature. Reprinted with permission from Ref. [46]. Copyright 2019, American Chemical Society. (b) Shape transformations of various printed structural configurations. (i) A leptasteria-like gripper with six identical deformable directional lappets. (ii) A dual-head gripper with two opposite deformable directional lappets. (iii) A gripper simulating shellfish with different cross-linking densities in the vertical direction. Reprinted with permission from Ref. [75]. Copyright 2022, Elsevier.

Figure 3

Chemical-responsive microrobot: (a) Illustration depicting the mechanism of hydrogel expansion. Reprinted with permission from Ref. [48]. Copyright 2023, Wiley-VCH. (b) Schematic diagram depicting optimization of the proposed system. (c) Showcase experiments conducted to test the reversibility of shape morphing under stimulation. Reprinted with permission from Ref. [18]. Copyright 2021, American Chemical Society.

Figure 4

pH-responsive microrobots: (a) schematic diagram illustrating pH-responsive swelling and shrinking behaviors of various hydrogel architectures; (b) optical images depicting shape transformations of different hydrogel architectures: cubic (top), octagonal (middle), and inverted microclaw (bottom). Reprinted with permission from Ref. [38]. Copyright 2022, American Chemical Society.

Figure 5

Light-responsive microrobots: (a) (i) Scanning electron microscopy image of microwalker lying upside down. Scale bar is 10 μm. (ii) Side view of the microwalker. Scale bar is 10 μm. (iii) Actuation of microwalker under laser beam radiation with 532 nm. Scale bar is 50 μm. Reprinted with permission from Ref. [55]. Copyright 2015, Wiley-VCH. (b) Design of light-propelled rolling robot. (c) Photograph of robot illuminated on one side of the body to induce tilt (angle φ). The inset shows the same robot when light is off. (d) Images showing multigait rolling motion along a straight line under irradiation through the bottom substrate from the direction of the violet arrow. Reprinted with permission from Ref. [53]. Copyright 2019, Wiley-VCH.

Figure 8

Nonmedical applications of 4D-printed microrobots: (a) (I–IV) After being exposed to the NIR light for two seconds, the gripper shrank and captured the target. Reprinted with permission from Ref. [14]. Copyright 2022, IOP. (b) (i) Geometry, magnetization profile, and working mechanism of a magnetic microgripper. The black arrows indicate the direction of local magnetization of each part; meanwhile, the blue arrows indicate the actuating magnetic field. (ii) Illustration of gripper to perform cargo delivery. (iii) Top and side-view images of cargo delivery of gripper in silicon oil. Scale bar is 5 mm. Reprinted with permission from Ref. [91]. Copyright 2019, Science. (c) Dynamic locomotion and on-demand cargo release by magneto-origami quadruped robot. Reprinted with permission from Ref. [90]. Copyright 2022, Springer Nature.